Drainage

Notice:

This document is issued for the sole purpose of stakeholder engagement in advance of a stakeholder workshop to be held on 18 and 19 June 2013.

The contents of the document are draft and must not be used for any purpose other than preparation for the said workshop.

Note that the document is a work-in-progress and as such may contain inaccuracies in cross referencing, table and figure referencing and page numbering. It may also contain rough drafts of figures and could include inconsistencies and areas of text yet to be developed.

Drainage

Contents

Glossary ............................................................................................................................................... 1

1 Introduction ................................................................................................................................. 5

1.1 Overview ........................................................................................................................... 5

1.2 Scope ................................................................................................................................. 5

1.2.1 Scope of manual ................................................................................................... 5

1.2.2 Responsibilities ..................................................................................................... 6

1.3 Drainage design philosophy .............................................................................................. 6

1.3.1 Minor system ........................................................................................................ 6

1.3.2 Major system ........................................................................................................ 7

1.4 Functions of Highway Drainage ........................................................................................ 7

1.5 Climatic and physical considerations ................................................................................ 9

1.5.1 Resilience and urban creep .................................................................................. 9

1.5.2 Climate change ................................................................................................... 11

1.6 Policies and environmental controls ............................................................................... 11

2 Project Initiation ........................................................................................................................ 13

2.1 Gateway 1 summary ....................................................................................................... 13

2.2 Data gathering ................................................................................................................. 15

2.3 Catchment assessment ................................................................................................... 16

2.3.1 Overview ............................................................................................................ 16

2.3.2 Flood Risk Assessment ....................................................................................... 18

2.4 Consideration of road geometry ..................................................................................... 21

2.5 Determine viable outfalls ................................................................................................ 22

2.6 Consideration of treated sewage effluent (TSE) ............................................................. 22

2.7 Identify pollution control requirements ......................................................................... 22

2.7.1 Background ......................................................................................................... 22

2.7.2 Scope .................................................................................................................. 23

2.7.3 Purpose ............................................................................................................... 23

2.8 Develop Strategy ............................................................................................................. 23

2.9 Initial stakeholder engagement ...................................................................................... 23

3 Pre-design work ......................................................................................................................... 25

3.1 Gateway 2 summary ....................................................................................................... 25

3.2 Design criteria ................................................................................................................. 26

3.2.1 Flood return periods ........................................................................................... 26

3.2.2 Acceptable highway flood standards ................................................................. 26

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3.3 Hydrological data ............................................................................................................ 27

3.3.1 Rainfall characterisation .................................................................................... 27

3.3.2 Run-off coefficients (C) ...................................................................................... 30

Figure 3.43: Run-off coefficients for Urban Catchments ............................................... 31

3.3.3 Catchment area (A) ............................................................................................ 31

3.3.4 Surface run-off (Q) ............................................................................................. 32

3.3.5 The Colebrook-White equation ......................................................................... 32

3.3.6 Manning's equation ........................................................................................... 33

3.3.7 Time of concentration (Tc) ................................................................................ 33

3.3.8 Hydrogeology (P) ............................................................................................... 34

3.4 Hydraulic analysis processes .......................................................................................... 35

3.4.1 Overview ............................................................................................................ 35

3.4.2 The rational method .......................................................................................... 35

3.4.3 Mathematical models ........................................................................................ 37

3.5 Pollution control process ................................................................................................ 38

3.5.1 Method .............................................................................................................. 38

3.6 Review process ............................................................................................................... 46

4 Detailed design work ................................................................................................................ 47

4.1 Gateway 3 summary ....................................................................................................... 47

4.2 Design in urban areas ..................................................................................................... 48

4.2.1 Urban catchments ............................................................................................. 48

4.2.2 Positive drainage ............................................................................................... 49

4.2.3 Drainage of the carriageway ............................................................................. 49

4.2.4 Design of traditional drainage capture techniques ........................................... 53

4.2.5 Drainage of medians, foot-ways and verges ..................................................... 64

4.2.6 Emergency flood areas (EFA) ............................................................................. 65

4.2.7 Swales -sustainable urban drainage systems (SUDS) ........................................ 66

4.2.8 Retention of storm-water .................................................................................. 66

4.3 Design in rural areas ....................................................................................................... 67

4.3.1 Rural catchments ............................................................................................... 67

4.3.2 Drainage of the carriageway ............................................................................. 67

4.3.3 Drainage of medians and verges ....................................................................... 68

4.3.4 Natural surface drainage ................................................................................... 69

4.4 Junction Drainage ........................................................................................................... 70

4.4.1 Considerations for drainage at junctions .......................................................... 70

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4.4.2 T – junctions ....................................................................................................... 71

4.4.3 Roundabouts ...................................................................................................... 73

4.4.4 At grade junctions .............................................................................................. 74

4.4.5 Grade separated junctions ................................................................................. 74

4.5 Sustainable Drainage Systems (SuDS) ............................................................................. 74

4.5.1 Introduction to SuDS .......................................................................................... 74

4.5.2 Infiltration Guidance .......................................................................................... 78

4.5.3 Specification of Suitable Vegetation .................................................................. 79

TBC 79

4.5.4 Pervious Surfaces ............................................................................................... 80

4.5.5 Soakaways .......................................................................................................... 83

4.5.6 Swales ................................................................................................................. 86

4.5.7 Filter Trenches and Drains .................................................................................. 89

4.5.8 Bioretention Areas ............................................................................................. 92

4.5.9 Sand Filters ......................................................................................................... 96

4.5.10 Basins ................................................................................................................ 100

4.6 Pollution control ............................................................................................................ 105

4.7 Maintenance strategies................................................................................................. 105

4.7.1 Planned maintenance ....................................................................................... 106

4.7.2 Re-active Maintenance..................................................................................... 106

5 Subsurface Drainage ................................................................................................................ 107

5.1 Introduction .................................................................................................................. 107

5.2 Subsurface drainage methods ...................................................................................... 108

5.2.1 General design considerations ......................................................................... 108

5.2.2 Considerations for fin drains and narrow filter drains: .................................... 108

5.2.3 Considerations for combined carrier filter drains: ........................................... 108

5.2.4 Special considerations for coastal areas .......................................................... 109

6 Appendix A .............................................................................................................................. 110

6.1 Doha .............................................................................................................................. 111

6.2 Al Ruwais ....................................................................................................................... 112

6.3 Al Saliyah ....................................................................................................................... 113

6.4 Dokhan .......................................................................................................................... 114

6.5 Abu Samra ..................................................................................................................... 115

6.6 Umm Bab ....................................................................................................................... 116

7 Appendix B ............................................................................................................................... 117

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7.1 Doha ............................................................................................................................. 118

7.2 Al Ruwais ...................................................................................................................... 119

7.3 Al Saliyah ...................................................................................................................... 120

7.4 Dokhan ......................................................................................................................... 121

7.5 Abu Samra .................................................................................................................... 122

7.6 Umm Bab ...................................................................................................................... 123

Tables

Table 1.1: Considerations for the design life of roads...................................................................... 10 Table 2.1: Typical permeability values by soil type (From QHDDM) ................................................ 17 Table 2.2: Flood risk classification .................................................................................................... 18 Table 2.3: Flood risk vulnerability classification (based on NPPF Technical Guide – P9) ................. 21 Table 3.1: Required levels of flood protection for each road classification ..................................... 26 Table 3.2: Guidelines for flood standards on Qatar roadways ......................................................... 27 Table 3.3: Likely viable software packages for mathematical modelling ......................................... 38 Table 3.4: Pollution control requirements ....................................................................................... 41 Table 3.5: Levels of treatment options for pollution control ........................................................... 42 Table 3.6: Spillage rates (SS) ............................................................................................................. 43 Table 3.7: Probability of a serious pollution incident occurring as a result of a serious spillage .... 44 Table 3.8: Spillages and risk reduction factors (indicative) .............................................................. 45 Table 4.1: Design flow widths on various road types (typical values of B) ...................................... 57 Table 4.2: Design flow widths for special situations (typical values of B) ........................................ 58 Table 4.3: Typical values of Mannings ‘n’ for various surfaces ........................................................ 59 Table 4.4: Maintenance factor ‘m’ ................................................................................................... 59 Table 4.5: Grating type design values .............................................................................................. 60 Table 4.6: Grating bar pattern coefficient ........................................................................................ 61 Table 4.7: Limiting parameters to equation for design of level of nearly level roads ..................... 63 Table 4.8: Values for index W ........................................................................................................... 63 Table 4.9 - Potential suitability for SUDS installation on Urban Roads ............................................ 76 Table 4.10 - Potential Suitability for SUDS Installation on Rural Roads ........................................... 77 Table 4.11: Potentially suitable locations for a pervious surface ..................................................... 80 Table 4.12: Pervious surface maintenance requirements ............................................................... 82 Table 4.13: Potentially suitable site locations for soakaway installation ......................................... 84 Table 4.14: Soakaway maintenance requirements .......................................................................... 86 Table 4.15: Potentially suitable locations for installing a swale ....................................................... 87 Table 4.16: Swale maintenance requirements ................................................................................. 89 Table 4.17: Potentially suitable locations for trenches .................................................................... 90 Table 4.18: Potential suitable locations for bioretention areas ....................................................... 93 Table 4.19: Maintenance requirements for bioretention areas ...................................................... 94 Table 4.20: Potential site locations for sand filters .......................................................................... 97 Table 4.21: Sand filter maintenance requirements ........................................................................ 100 Table 4.22: Potentially suitable locations for a basin ..................................................................... 102 Table 4.23: Maintenance requirements for basins ........................................................................ 105

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Figures

Figure 1.1: Design gateways process overview .................................................................................. 5 Figure 1.2: Functions of highway drainage ....................................................................................... 8 Figure 2.1: Project initiation process steps ...................................................................................... 14 Figure 2.2: Data gathering process steps ......................................................................................... 15 Figure 2.3: Catchment assessment process steps ............................................................................ 16 Figure 2.4: Stakeholder engagement process steps ........................................................................ 24 Figure 3.1: Pre-design process steps ................................................................................................ 25 Equation 3.1: IDF relationship equation (from Section 11.1, study of regional design rainfall, Qatar, 2013) ................................................................................................................................................ 28 Figure 3.2: Intensity-Frequency-Duration relationship for Al Ruwais (current climatic condition) (from Study of Regional Design Rainfall, Qatar- Volume 1, Chapter 11, Figure 11-6) ..................... 29 Figure 3.3: Intensity-Frequency-Duration relationship for Al Ruwais (2070-99) (from Study of Regional Design Rainfall, Qatar- Volume 1, Chapter 12, Figure 12-16) ........................................... 30 Figure 3.43: Run-off coefficients for Urban Catchments ................................................................. 31 Figure 3.4: Rational method process ............................................................................................... 36 Figure 3.5: Pollution control procedure flowchart .......................................................................... 40 Figure 4.1: Detailed design process steps ........................................................................................ 47 Figure 4.2: Typical Road Cross-section ............................................................................................. 50 Figure 4.3: Typical Detail of Rolling Crown across a Single Carriageway ......................................... 51 Figure 4.5: Combined kerb drainage ................................................................................................ 52 Figure 4.6: Side outlet outfall unit ................................................................................................... 53 Figure 4.7: Gully design parameters ................................................................................................ 57 Figure 4.8: Effective catchment width ............................................................................................. 59 Figure 4.9: Terminal gully design parameters .................................................................................. 62 Figure 4.10: Typical median ditch cross-section .............................................................................. 68 Figure 4.11: Permissible depths of flow for unlined channels ......................................................... 69 Figure 4.12: Typical drainage at T Junctions .................................................................................... 72 Figure 4.13: Large signalised junction drainage ............................................................................... 73 Figure 4.14: Roundabout Drainage .................................................................................................. 74 Figure 4.15: Sustainable drainage objectives ................................................................................... 75 Figure 4.16: Consideration of infiltration ......................................................................................... 78 Figure 4.17: Pervious surface design steps ...................................................................................... 82 Figure 4.18: Soakaway design steps ................................................................................................. 85 Figure 4.19: Diagram of typical swale .............................................................................................. 88 Figure 4.20: Infiltration trench design steps .................................................................................... 91 Figure 4.21: Infiltration trenches maintenance requirements ........................................................ 92 Figure 4.22 - Typical Cross Section through a Bio-retention Area ................................................... 94 Figure 4.23: Diagrams of a typical surface sand filter (left) and a typical underground sand filter (right) ................................................................................................................................................ 96 Figure 4.24: Typical sand filter bed construction ............................................................................. 98 Figure 4.25: Sand filter design steps ................................................................................................ 99 Figure 4.26: Plan view of a typical basin ........................................................................................ 101 Figure 4.27: Typical cross section of a detention basin. ................................................................ 103

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Figure 4.28: Basin design steps ...................................................................................................... 104 Figure 4.29: Typical vortex grit remover ........................................................................................ 106

Equations

Equation 3.1: IDF relationship equation (from Section 11.1, study of regional design rainfall, Qatar, 2013) ................................................................................................................................................. 28 Equation 3.2: Surface run-off (Q) for catchments <50 Ha ................................................................ 32 Equation 3.3: The Colebrook-White equation ................................................................................. 33 Equation 3.4: Manning’s Equation ................................................................................................... 33 Equation 3.5: Time of concentration (Tc) ......................................................................................... 34 Equation 3.6: Annual probability of a spillage with the potential to cause a major pollution event .......................................................................................................................................................... 44 Equation 3.7: Probability of spillage event resulting in serious pollution event .............................. 44 Equation 4.1 .................................................................................................................................... 55 Equation 4.2 .................................................................................................................................... 55 Equation 4.3 .................................................................................................................................... 55 Equation 4.4 .................................................................................................................................... 56 Equation 4.5 .................................................................................................................................... 56 Equation 4.6 .................................................................................................................................... 56 Equation 4.7 ..................................................................................................................................... 60 Equation 4.8 ..................................................................................................................................... 60 Equation 4.9 ..................................................................................................................................... 61 Equation 4.10 ................................................................................................................................... 63 Equation 4.11 ................................................................................................................................... 64 Equation 4.12: Filter area size calculation ........................................................................................ 98

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Glossary

Term Description

Capillary break layer Spacing left wide enough between two layers to prevent water moving through capillary action.

Capillary rise The upwards movement of water molecules along the surface of a solid.

Carrier drain (CD) A sealed pipe for the conveyance of surface water

Carrier filter drain A (half or fully) perforated pipe used in order to collect and convey both surface water and subsoil water to the outfall.

Catchment area (A) A defined area, determined by topographic features, drainage patterns and land use, within which all rain will contribute run-off to a specific point or system.

CFD Computational fluid dynamics.

Colebrook-White Equation

A method for determining flows in conduits (pipes or open channels). This method is the most appropriate for flows in smoother bore pipes.

Collecting system A system of conduits that collects and conveys surface water.

Concentration (points of/ time of) The spatial or temporal point of peak flow within a drainage system.

Collector channel System of channels which collects and conveys surface water.

Control structures Devices designed to control the outflow from an attenuation or storage facility, such as sluice gate valve, aperture or vortex flow control device.

Crown When considering the high point of a road in either long section or cross section, the point at which water will run in opposite directions.

Culvert A circular, ovoid, arched or rectangular closed conduit used to convey water from one area to another, usually from one side of a road to the other side.

DA Public Works Authority, Drainage Affairs.

Detention tank A tank built to store runoff and release it at a controlled rate so that the peak flow is reduced and the flow is spread over a longer period.

Discharge rate Volume of water per second passing out of the system at a specific point.

Domestic waste water Waste water from residential settlements and services which originates predominantly from the human metabolism and from household activities.

Drainage basin An extent of land where water from precipitation drains into a body of water. Drainage routes (natural)

The route flow will naturally take if not intercepted or diverted by drainage measures.

EFA Emergency flood area used to contain safely large and sudden accumulations of storm water. This is additional to any sustainable drainage systems.

Effluent Storm or foul water discharge.

Filter drain A linear drainage feature consisting of a trench filled with permeable materials, designed to capture and convey surface water.

Filter membrane A type of geotextile that allows the unimpeded flow of water through its surface but prevents the onward passage of silt and other small particles. This prolongs the life of drainage medium, such as in filter drains.

Final effluent The treated liquid resulting from a waste water treatment process, at the point of discharge to a watercourse. See also treated sewage effluent (TSE).

Flood Inundation of the road and surrounding areas with storm water or effluent from

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Term Description

a burst pipe.

Ford A shallow place where a river or stream formed by storm water may be crossed by wading or in a vehicle.

Foul sewer A sewer conveying waste water of domestic and/or industrial origin and groundwater infiltration.

GRC Glass reinforced concrete.

Groundwater Water located beneath the earth’s surface in soil pore spaces and in the fractures of rock formations.

Groundwater table The level below which the ground is saturated with water. The elevation of this layer will vary seasonally and spatially.

Gully An opening, usually covered by a grate, which allows surface water to enter a drainage system.

Hydraulic controls Factors which control fluid mechanics; namely conveyance.

IDF Intensity – Duration – Frequency relationship.

Impermeable surface Surfaces or ground unable to absorb rainfall; e.g. concrete, most asphalt surfaces and hardstandings.

Infiltration The process whereby water seeps into the ground or a part of the drainage system.

InfoWorks CS Urban drainage network modelling software.

Irrigation The artificial application of water to land to assist in the production of crops and other plants. Treated sewage effluent is commonly used for irrigation in Qatar and as such there is a low risk of contact with pathogens.

Major system Surface water trunk sewer network, surface water pumping stations, groundwater control networks and surface water storage and retention areas/tanks.

Mannings equation A method for determining flows in conduits (pipes or open channels). This method is the most appropriate for flows in rougher open channels.

Median (carriageway) The piece of land on a roads project that sits between the carriageways. On smaller roads medians are usually omitted.

Minor system Road drainage, comprising gullies, soakaways, ditches, connecting pipework and storage areas required prior to connection to the major system.

Outfall Point of discharge from a pipe or channel system. Pathogens Disease-causing organisms.

Peak flow The most voluminous period of flow at a location during a set time period, usually in the period during or directly after a storm.

Penstock A sliding plate which regulates flow through movement to vary the size of an opening.

Percolation The downwards movement of water into or through a permeable layer.

Permeable surface Surfaces or ground able to absorb rainfall; e.g. open textured ground, soil, grassed areas, open spaces.

Ponding The accumulation of surface water.

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Term Description

Pore water pressure

Positive drainage A piped system operating by gravity flow associated with an urban situation and used in conjunction with gullies and kerbs/footways.

Public sewer A sewer that is owned and maintained by one of the UK water and sewerage undertakers.

Pumping station Facility including pumps, power supply and control equipment for pumping liquids from one place to another.

Real time control The control or management of flow within a piped system.

Return period An estimate of the likelihood of an event occurring. This is a statistical measurement, typically based on historical data, denoting the average reoccurrence interval over an extended period of time.

Roundabout A type of circular intersection or junction in which road traffic is slowed and flows almost continuously in one direction around a central island to several exits onto the various intersecting roads.

ROW Rights of way (e.g. of utilities).

Run-off (surface) Water which flows over the ground following rainfall. This occurs when the ground is saturated or impermeable, and during intense rainfall, where rainfall exceeds the rate of infiltration.

Run-off coefficient (C) A measure of the amount of rainfall which is converted to run-off. This varies depending on factors such as the roughness and permeability of a surface.

Sabkha A salt flat, characteristic of saline intrusion in coastal areas of Qatar.

Saline A liquid mixture of salt and pure water; or in the context of soils those which contain or are impregnated with salt.

Sewerage A system of pipes and drains for the collection and transportation of domestic and industrial waste water.

Soakaway A sub-surface drainage feature which water is conveyed to, designed to facilitate infiltration.

Source control Methods of managing and reducing storm water runoff at site level. Storage Water which is strategically held in a specific area.

Storm hydrograph A graph which records the rate of flow through a catchment during a storm event.

Storm water Surface water resulting from precipitation during a storm event.

Subgrade Material, usually natural in situ, but may include a capping layer (ie below the formation level of a pavement.

Sustainable urban drainage systems (SuDS or SUDS)

Techniques to try to replicate natural systems that use cost effective solutions with low environmental impact to drain away dirty and surface water run-off through collection, storage, and cleaning before allowing it to be released slowly back into the environment..

Surface water Water that travels across the ground and hard surfaces rather than seeping into the soil, e.g., from paved roads and buildings.

Swale A shallow vegetated channel designed to capture, retain and encourage the infiltration of surface water. The vegetation also works to filter out particulate matter.

Treated sewage effluent (TSE)

Effluent treated to a standard suitable for plant irrigation under controlled conditions, which minimises or eliminates contact with humans.

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Term Description

Trunk sewer A sewer that receives many tributary sewers and serves a large area.

Urbanisation Agglomerations of buildings and infrastructure for human accommodation and commercial, social and industrial activities leading to the reduction in permeable areas.

Urban drainage Removal of surface water within a city or settlement. Utility corridor

Wadi Wadi is the Arabic term traditionally referring to a valley. In some cases, it may refer to a dry riverbed that contains water only during times of heavy rain.

Waste water Domestic waste water or the mixture of domestic waste water with industrial waste water, infiltration and/or run-off rain water.

Verge The strip of land at the edge of the carriageway that separates the carriageway from the earthworks / tie in to surrounding ground.

Vortex grit remover

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1 Introduction

1.1 Overview This document sets out the standard approach required to identify and provision adequate surface and sub-surface drainage measures when designing highways within Qatar. This approach details the relevant methodologies and specifications required to meet standards. The approach is divided into 3 gateways, which reflect the three key design stages. Each of these stages includes a number of process steps designed to meet appropriate standards. The process is outlined within Section 1.2.1.

1.2 Scope

1.2.1 Scope of manual This document will outline the process required to meet drainage standards for all roads within Qatar. This will be presented as an approach split into three key design stages; this is outlined within Figure 1.1 (below).

Figure 1.1: Design gateways process overview

GATEWAY 1 Project initiation

Section 2, Page 13

GATEWAY 3 Detailed design work

Section 4, Page 47

GATEWAY 2 Pre-design work

Section 3, Page 25

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1.2.2 Responsibilities

Drainage of highways is the joint responsibility of the Civil Engineering Department's Roads Division and Drainage Division. Each Division has defined responsibilities and procedures which shall be adhered to when designing highway drainage. These are explained in the following diagram: (Illustrate with diagram showing forward links to other sections).

1.3 Drainage design philosophy When designing drainage measures it is important to consider the size and type of system which will be best suited to the conditions and characteristics of the project area.

A more sustainable approach to drainage needs to be considered in an effort to minimise the impact of future road construction. Such techniques are becoming common place globally and are referred to as Sustainable Drainage Systems (SuDS). SuDS mimic natural drainage processes to reduce the effect on the quality and quantity of runoff from developments and provide amenity and biodiversity benefits. When specifying SuDS, early consideration of the potential multiple benefits and opportunities will deliver the best results.

Where it is not possible to accomodate drainage using only SUDS, ‘traditional’options can be applied either to complement SUDS measures or to provide standalone solutions where necessary. Assistance on viable SuDS options can be found in Error! Reference source not found. and Error! Reference source not found..

1.3.1 Minor system The Roads Division is responsible for the design of the Minor System, namely the road drainage comprising gullies, soakaways, connecting pipework, SUDS features and storage areas required prior to discharge into the Drainage Division Network. The highway drainage system shall be designed using parameters defined in this section. The point of discharge and discharge parameters listed below will be provided by the CED Drainage Division:

• Diameter of trunk sewer • Allowable discharge volume • Invert level of trunk sewer • Location of trunk sewer • Acceptable method of discharge into the trunk sewer • Consideration of SUDS features

It is also important to consider flow constraints within a minor system. Predominant limiting factors are likely to be:

• Permeability of ground • Underlying geology (including issues with contamination and saline intrusion) • Rates of evaporation • 48 hour drain down of storage • Allowable discharge rate into trunk sewers

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1.3.2 Major system CED Drainage Division is responsible for the Major System which comprises all the drainage components beyond the agreed interface point with the minor system. This includes:

• Trunk sewers, surface water sewer network. • Surface water pumping stations • Ground water control networks • Surface water storage retention areas/tanks.

The preferred drainage method is by a positive system. However should this not be practical due to distance from a suitable discharge point or economics, agreement to discharge water to the ground or adjacent areas may be sought from the Director of the Civil Engineering Department.

1.4 Functions of Highway Drainage The requirement for satisfactory road drainage has a direct bearing on (a) the ability to use the road during and after a rainfall event, (b) the long-term serviceability of the road structure, (c) the provision of an acceptable urban environment and (d) minimising health risk caused by long term surface ponding. The construction of a highway shall not be allowed to increase the risk of flooding to properties.

The highway drainage system must therefore be considered as providing four primary functions which, due to land use constraints, are usually dealt with differently in urban and rural situations, namely:

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Figure 1.2: Functions of highway drainage

•Collect storm and surface water run-off from the highway, adjacent side roads and wider catchment and convey to a suitable outfall: •Reduces the danger of standing water to traffic •Maintains the use of all trafficked lanes •Reduces sediment build up at the road side, this can be further controled using swales

•SuDS features can provide effective flow control

Collection and control

•Remove water percolating through the pavement, lower ground water and prevent capillary rise: •Reduces the damaging affect of pore water build up in the pavement, formation or subgrade

•Prevents pavement weakening due to ingress of salt lenses from the lower subgrade layers

Sub-surface drainage

•Convey surface water run-off safely across or under roadways: •Minimises disruption to traffic •Minimises damage to the pavement or embankment structure

•Guides surface water run-off to suitable discharge points •Minimises road impact on the natural surface hydrology in rural areas

Conveyance

•In the case of exceptional rainfall events the road surface itself can be used as a storm carrier: •Prevents damage to property in flood prone areas •Concentrates flood water to discharge basins for easy removal

•Drainage should be designed to route flows and assist in mitigation in 'above the design events'

Flood risk management

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1.5 Climatic and physical considerations The topography of Doha is relatively flat but undulating, and thus catchment boundaries and natural drainage routes are often poorly defined. Recent extensive development has caused flooding to become more problematic, especially in the Greater Doha area, due to:

• Changing rainfall patterns with more intense rainfall becoming more frequent; • Increased roofed and paved areas producing greater and quicker surface water

runoff flows; • Reduced permeable areas for surface waters to soak into the ground; • Interference with natural flood paths by urban development and road

construction; • Little provision within the roads services hierarchy for surface or groundwater

drainage systems; • Development becoming increasingly distant from natural drainage outlets on the

coast; • Greater public awareness of flooding; • High groundwater tables which are rising to close to ground level in places, due to

the impact of irrigation, reducing the ability of surface waters to soak into the ground;

• Nature of the groundwater, which is variously saline and formerly treated sewage effluent (TSE);

• High and saline groundwater can affect foundations and the stability of highway sub-grades;

• Development within dry valleys (wadis) reducing the extent and availability of natural water courses required during storms;

• Development of underpasses, over-bridges and large grade-separated junctions which interrupt natural water courses;

• Underpasses which require separate drainage arrangements and pumping stations.

1.5.1 Resilience and urban creep

1.5.1.1 Background In Qatar urban expansion is taking place at a significant rate at present, and is likely to continue for the foreseeable future. In order to guard against infrastructure being outdated shortly after its construction it is essential to build in resilience by accounting for urban creep.

Urban creep is a term used to describe the phenomenon of where developments are constructed and then at a later date additional impermeable area is added to the existing thus increasing surface water runoff.

1.5.1.2 Scope This section of the manual provides guidance on how to take account of urban creep within roads projects.

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1.5.1.3 Purpose By incorporating an allowance for urban creep within the road drainage design will ensure the long term resilience to future urban expansions.

1.5.1.4 Method Urban creep should be allowed for within a road drainage design by applying a percentage increase to rainfall intensities during the design phase. The proportion rainfall intensities should be increased by is directly related to the road project’s design life. The values to be applied are given in Table 1.1: Considerations for the design life of roadsTable 1.1below;

Design life of road

10 years 25 years >50 years

Rainfall intensity (% increase for urban creep) +10% +20% +30%

Table 1.1: Considerations for the design life of roads

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1.5.2 Climate change

1.5.2.1 Background Within Qatar, climate change is likely to result in increased variability within weather patterns, both spatially and temporally. This polarisation within weather is expected to manifest itself in both more extreme droughts and more extreme rainfall events. It is therefore vital to provision for these effects in order to ensure the long term effectiveness of drainage measures.

Within this approach the effects of climate change have been assessed using two climate scenarios for Qatar (‘global dry’ and ‘global wet’), according to results from four emissions scenarios. This approach originates from the ‘study of regional design rainfall , Qatar (2013)’.

1.5.2.2 Scope This section of the manual provides guidance for the consideration of the effects of climate change within the design of highways drainage.

1.5.2.3 Purpose Through incorporating an allowance for climate change within drainage design the engineer will provision the long term resilience of drainage solutions.

1.5.2.4 Method Climate change should be considered with reference to Chapter 12 of the ‘study of regional design rainfall, Qatar- Volume 1 (2013)’. This states that to account for climate change, engineers should use corrected IDF relationships, which represent changes to rainfall events over an extended time period, within drainage design. This is outlined within Section 3.3.1.1. For a detailed explanation of the limitations associated with this specific modelling approach refer to the Study of Regional Design Rainfall, Qatar- Volume 1 (2013).

1.6 Policies and environmental controls The difficulty in draining catchments that have no natural outlet to the sea or to low-lying inland areas is recognised. The advantages of controlling surface runoff at source are also accepted. The policy principles for design of surface water and groundwater control systems are:

• Surface water and groundwater systems should use common facilities where possible;

• Where stormwater discharges above ground level, such as from bridges and flyovers, runoff control systems (i.e. source control) should be installed;

• Runoff control systems should be installed at source to regulate discharge to the public infrastructure drainage systems;

• Where development is likely to be slow, soakaway systems and / or use of retention areas should be used as an interim solution.

• Positive drainage systems should be provided to drain flows to the sea or other approved discharge areas;

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• Where a stormwater system is planned or already exists, the permissible peak flow from the new sub-catchment into the stormwater system will be determined by DA. If the calculated peak flow exceeds this figure, the difference must be catered for by a combination of attenuation tanks and soakaways;

• Rate of runoff should be attenuated by the use of short-term flooding of roads, storage areas or tanks;

• Soakaways to drain surface waters may be required to attenuate runoff to positive drainage systems or retention areas;

• Flood plains and routes are to be identified and kept clear of development to facilitate runoff of surface waters;

• Positive drainage systems, using pipes and culverts should be constructed where possible in carriageways in accordance with the agreed services hierarchy. The designer should note that there is currently no allowance for positive drainage systems within the road hierarchy and therefore the location of all drains must be agreed with the Public Works Authority, Drainage Affairs (DA).

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2 Project Initiation

2.1 Gateway 1 summary In order to ensure effective and reliable design standards it is important to establish a consistent and well documented approach for determining design criteria. The following section details the method required to calculate and identify appropriate criteria by establishing standard methodological requirements for drainage requirements. This process corresponds to Gateway 1 of the summary process diagram (Figure 1.1) this is expanded in Figure 2.1 (below). The steps outlined within this figure are then expanded upon and explained in detail in the following sections.

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Figure 2.1: Project initiation process steps

The purpose of this design stage is to assemble the raw data required for a preliminary design, and to produce a strategy which will facilitate drainage requirements.

Data gather Section 2.2, Page 15

Catchment assessment Section 2.3, Page 16

Determine viable outfalls Section 2.4, Page 21

Liaise with others re constraints Section 2.4, Page 23

Road geometry Section 2.4, Page 21

Consider TSE Section 2.6, Page 2215

Pollution control requirements Section 2.7, Page 22

Develop strategy and submit for review

Approval

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2.2 Data gathering It is imperative that the engineer has a comprehensive understanding of the drainage requirements for each project. In order to achieve this it is important to collect high quality data which can be relied upon to represent the characteristics of the catchment. These data requirements are outlined below within

Figure 2.2, and are expanded upon within the relevant sections of this chapter.

Figure 2.2: Data gathering process steps

Existing 3D topographic data – to review catchment extent

Existing drainage asset data

Existing utilities data

Proposed road horizontal & vertical alignment

Proposed utilities and 3rd party assets

Site investigation • Permeability • Groundwater • Sensitive geology • Contaminated land • Flood risk

Records of flooding

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2.3 Catchment assessment

2.3.1 Overview Once data has been gathered, this should be used to identify and assess the catchment(s) and sub-catchments relevant to the area which requires drainage planning. This process is outlined within Figure 2.3.

Figure 2.3: Catchment assessment process steps

The engineer must first determine the catchment extent of the entire project. This can be calculated using the 3D topographic data gathered within the previous step of the initiation process. Once this is complete the engineer should then utilise the same data to identify high and low points across the catchment, this will then be used in conjunction with the locations of outfalls to define sub-catchments appropriate for the project.

The scale of the catchment will determine the modelling approach required. For smaller simple catchments, defined for this purpose as less than 50 Ha, programmes such as ‘Microdrainage’, ‘Inforworks’ and ‘Isis’ could be used. Where catchments are

Once suitable catchments have been identified the engineer should calculate runoff rates (refer to Section 3.3.2), which will be used to calculate provisional drainage volumes required. In some locations local flood assessments may already be undertaken and run off volumes may already be available; where this is the case these should be reviewed as outputs may be suitable for use as part of the design initiation stage.

During project initiation it may be useful to use preliminary values for permeability. To do this reference can be made to Table 2.1 and to records held by CEO Roads and Drainage

Determine catchment extent (whole project)

Breakdown into sub-catchments based on high and low points and available

outfalls

Calculate existing run-off per catchment for a range of return periods

Scale sets modelling approach

See text below

Agree/ seek approval for discharge rates

Flood risk assessment

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Divisions. However, it should be noted that this is only sufficient for the initial scoping of the project, and that detailed site investigation (as detailed above) is required for all projects.

Soil Type Permeability (m/s)

Clean gravels 1

10-1

Desiccated and fissured clays

Clean sands and sand-gravel mixtures

10-2

10-3

10-4

Very fine sands, silts and clay-silt laminate

10-5

10-6

Unfissured clays and clay-silts (>20% clay)*

10-7

10-8

10-9

10-10

*Special measures required in this range

Table 2.1: Typical permeability values by soil type (From QHDDM)

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2.3.2 Flood Risk Assessment

2.3.2.1 Background It has been common practice in Qatar to construct roads without the provision of a surface water drainage system, which has in some instances lead to severe flooding, damage to buildings and other important infrastructure, and on occasion loss of life. For these reasons during the road design consideration of flooding related to new roads both within the design criteria and beyond the design event (exceedance) are of paramount importance.

2.3.2.2 Purpose This section of the manual focuses on directing those preparing the road design towards minimizing the risk of flooding to the road user, adjacent land owners, critical infrastructure and the surrounding environment.

2.3.2.3 Scope Inappropriate road construction within areas at risk of flooding should be discouraged with lower risk areas being favoured, but where this is unfeasible the road should be made safe (in accordance with the guidance set out in table 3.2) without worsening flooding elsewhere.

The term “areas at risk of flooding” means land that is prone to flood during more frequently experienced storm events (high and medium risk), such as the 2 - 10yr return period (high risk areas), and the 10 - 25yr return period (medium risk areas). Areas outside of these parameters are classed as low risk (i.e. greater than 25yr return period)

The term “flood risk” means the risk from all sources of flooding including surface water runoff, surcharged sewers systems, groundwater, and the sea.

Risk level Return periods

High 2 year – 10 year

Medium 10 year – 25 year

Low > 25 year

Table 2.2: Flood risk classification

In addition to the consideration of whether the road corridor falls within an area at risk of flooding, the purpose and hence vulnerability classification of the road should also be borne in mind. For example if the road was serving a hospital then it would have a higher vulnerability classification than say a road serving a supermarket car park. (See Please note : Roads that combine a mixture of uses should be placed into the higher of the relevant classes of flood risk sensitivity. Table 2.3 below).

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By considering the flood risk area and the flood risk vulnerability together the design should be amended and / or mitigation measures proposed to manage the residual risk. The output from this process should be a flood risk assessment report which clearly sets out the existing flood risk for the land proposed for the road construction, together with the subsequent risk (incorporating any mitigation measures proposed and exceedance flow routes) post road construction.

2.3.2.4 Method Obtain flood records from the Drainage Division for sewerage infrastructure, roads, groundwater and the sea where appropriate. Following receipt of these flooding records determine the risk category (high, medium, or low). Based upon primary road usage determine the vulnerability classification from the Please note : Roads that combine a mixture of uses should be placed into the higher of the relevant classes of flood risk sensitivity.

Table 2.3 below;

Essential Infrastructure

Essential transport infrastructure (including mass evacuation routes) which has to cross the area at risk

Essential utility infrastructure which has to be located in a flood risk area for operational reasons, including electricity generating power stations and grid and primary substations; and water treatment works/ desalinisation plants that need to stay operational in times of flood.

Highly vulnerable

Police stations, ambulance stations and fire stations and command centres and telecommunications installations required to be operational during flooding.

Emergency dispersal points.

Basement dwellings.

Installations requiring the use, storage or onward transmission of hazardous substances. (Where there is a demonstratable need to locate such instaltions for bulk storage of materials with port or other similar facilities, or such installations with energy infrastructure or carbon capture and storage installations, that require coastal or water-side locations, or need to be located in other high flood risk areas, in these instances the facilities should be classified as “essential infrastructure”).

More vulnerable

Hospitals

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Residential institutions such as residential care homes, childrens homes, social services homes, prisons and hostels.

Buildings for dwelling houses, students halls of residence and hotels.

Non-residential uses for health services, nursaries and educational establishments.

Landfill and sites used for waste management facilities for hazardous waste.

Sites used for camping, subject to a specific warning and evacuation plan.

Less vulnerable

Police, ambulance or fire stations which are not required to be operational during flooding.

Buildings used for shops, financial, professional and other services, restaurants and cafes, offices, general industry, storage and distribution, non-residential institutions not included in “more vulnerable”, and assembly and leisure.

Land and buildings used for agriculture.

Waste treatment (except landfill and hazardous waste facilities).

Mineral working and processing (except for sand and gravel working).

Water treatment works which do not need to remain operational during times of flood.

Sewage treatment works (if adequate measures to control pollution and manage sewage during flooding events are in place).

Water compatable development

Flood control infrastructure.

Water transmission infrastructure and pumping stations.

Sewage transmission infrastructure and pumping stations.

Sand and gravel working.

Docks, marinas and wharves.

Navigation facilities.

Defence installations.

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Ship building, repairing and dismantling, dockside fish processing and refrigeration and compatable activities requiring a waterside location.

Water-based recreation (excluding sleeping accommodation).

Lifeguard and coastguard stations.

Amienity open space, nature conservation and biodiversity, outdoor sports and recreation and essential facilities such as changing rooms.

Essential ancillary sleeping or residential accomodation for staff required by uses in this category, subject to a specific warning and evacuation plan.

Please note : Roads that combine a mixture of uses should be placed into the higher of the relevant classes of flood risk sensitivity.

Table 2.3: Flood risk vulnerability classification (based on NPPF Technical Guide – P9)

Propose mitigation measures based on flood risk area, vulnerability classification, and surrounding land use and submit to overseeing organization for comment / approval. On larger scale roads projects, or for special corridors the overseeing organization may require more comprehensive hydrological studies and drainage investigations to support the design. This requirement should be discussed with the overseeing organization at gateway 1 project initiation to ensure enough time is allowed to undertake the work prior to commencing pre-design work (gateway 2).

Once discharge rates have been calculated stakeholder engagement should take place in order to approve/ re-evaluate acceptable rates. Contacts for the correct approval procedure are laid out within Section 1.2.2.

2.4 Consideration of road geometry Drainage design is intrinsically linked to road type and classification, and therefore requires an understanding of road geometry and footprint in order to develop effective drainage measures.

During project initiation geometric design of carriageway and road type should be identified in order to ensure appropriate drainage measures are developed. This should be an iterative process where highways and drainage engineers work together to develop a robust solution. This information should then be taken forward to ensure correct flood return period and depth standards are used (see Sections 3.2.1 and 3.2.2), and that appropriate pollution control requirements are applied.

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2.5 Determine viable outfalls As part of the project initiation it is important to develop a strategy for placing outfalls. Outfalls should be positioned in a location where they pose no negative effects upon the local community, highway or environment. Because of the range of parties who may potentially be affected by outfall location stakeholder engagement should take place to approve outfall locations.

When determining provisional outfall locations it is important to consider a range of factors, including:

• Outfall method (to watercourse, sewer or through infiltration). • High and low points of topography and road geometry. • Access for maintenance. • Safety screens (required if there is a risk of children/ animals gaining access to

large diameter pipes). • Velocity of water (control this to prevent scour or damage to system). • Pollution risk (build up of particulates in dry conditions could be washed through

the system in a rainfall event).

2.6 Consideration of treated sewage effluent (TSE) Within Qatar’s cities It is commonplace to find TSE used for the irrigation of planted flowerbeds. When designing drainage features it is therefore important to consider whether spills or run-off from this could enter SuDS systems and either contribute to dry weather flow or leach and potentially contaminate groundwater supplies. This is of particular importance where groundwater is extracted for use as potable supplies.

Consideration of this should be made through consultation with overseeing organisations and through the investigation of local aquifers. Where the potential for groundwater contamination is identified this should be taken forward as a key design consideration.

2.7 Identify pollution control requirements

2.7.1 Background Qatar is characterised by its arid climate, with infrequent high intensity storms during the rainy season (October - May), which when considered in conjunction with the absence of watercourses and high levels of salt in the ground can lead to serious pollution incidents emanating from roads if surface water runoff is not appropriately controlled.

Due to the long dry periods pollutants build up on the road surface which leads to the first storms of the rainy season being the most severe from a pollution perspective. This is particularly the case on rural roads that are not prone to runoff created from irrigation by treated sewage effluent (TSE). However, in urban locations TSE can itself run into the surface water drainage system and during the dry periods can result in potential pollution.

For these reasons it is important to provide pollution control measures for all roads schemes. The extent and type of control measures to be provided will be selected

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dependent upon the risk posed by the location type and the numbers of vehicles using the given road.

2.7.2 Scope This section of the manual gives guidance on the selection of pollution control measures dependent upon the road location and the risk associated with the number of vehicles.

The road location type relates to the risk level posed by the given location, for example, over-turning or collision at junctions, roundabouts etc.

The number of vehicles criteria has been simply linked to the type of road in question as certain types of road by their very nature will convey higher numbers of vehicles and as such pose a far greater risk than smaller roads.

2.7.3 Purpose By providing guidance on the selection of pollution control measures for new roads it is envisaged that pollution incidents will be minimized and the frequency of groundwater pollution from roads sources reduced.

2.8 Develop Strategy At this stage the engineering team responsible for planning drainage measures should incorporate the findings from all previous sections in order to outline a high level drainage strategy which suitably meets stakeholder and overseeing authorities expectations. It is expected that this strategy will put forward the main reccommendations for further work.

2.9 Initial stakeholder engagement Ongoing internal consultation within design teams (highways, geotechnical, structures, utilities, etc) should take place throughout all stages as a matter of course. However, on completion of a draft strategy it is important to re-engage with stakeholders to ensure the strategy is robust and meets all necessary requirements.

The purpose of this stakeholder engagement will be to outline the provisional drainage plan and iteratively refine this based on stakeholder feedback concerning possible constraints (for example, inappropriate discharge rates/ outfall locations) and potential opportunities (for example, possibilities of collaborative work with other nearby developments or schemes).

The process is shown below in Figure 2.4. Ultimately this step will result in approval of a strategy, which will then be taken forward into the design work at Gateway 2.

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Figure 2.4: Stakeholder engagement process steps

This step is included at the end of the gateway 1 process in order to highlight the necessity to obtaining sign off approval from relevant parties regarding the strategy; however, in practice ongoing stakeholder engagement throughout the entire process, through regular constructive dialogue, is likely to be the most efficient and effective method of ensuring the strategy remains consistent with the expectations of all parties. Section 1.2.2 details the relevant parties responsible for approvals; however, it should be noted that it is likely that smaller, location specific parties will also be important stakeholders to involve; this should be assessed on a project by project basis, and is supplementary to the standard parties required for approval.

Discuss opportunities/ constraints with client

Liaise with external parties

Develop and agree/ seek approval for strategy

Ong

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3 Pre-design work

3.1 Gateway 2 summary The following section outlines the process and standards required in order to develop the pre-design information necessary to define detailed design criteria. Pre-design work primarily focuses on a detailed and specific determination of catchment hydrology, hydraulics and flow, and follows on from the strategy outlined within the project initiation stage. The overall process is outlined below within Figure 3.1.

Figure 3.1: Pre-design process steps

Determine site hydrological data and design criteria Sections 3.2 and 3.3, Pages 26 and 27

Assess highway design/ geometry • High and low points • Develop typical cross sections • Geotechnical

Section 2.4, Page 20

Develop pre-design strategy SuDS → SuDS and ‘traditional’ → traditional

Determine pollution control requirements Section 3.5, Page 38

Undertake hydraulic analysis Section 3.4, Page 35

Stakeholder engagement and review Section 3.6, Page 46

Seek approval

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3.2 Design criteria

3.2.1 Flood return periods The levels of flood protection required by the DA, arranged by road classification, are shown in Note: it is likely that in some instances the overseeing organisation may require higher return periods for these road types. This should be discussed at gateway 1, project initiation. Table 3.1 below. For a full description of highway classifications please refer see QHDM Part 3.3 – 3.6 and Tables 3.1 – 3.2.

Special corridors, as characterised in QHDM Part 1 Section 3.5, should have their level of storm return period resilience agreed by the overseeing organisation at gateway 1.

Storm event return period Location and road type

Urban Rural

1 in 2 Years Storm Local roads Service roads

Local roads Collectors

Arterials

1 in 5 Years Storm Collectors Freeways

1 in 10 Years Storm Arterials Expressways

Note: it is likely that in some instances the overseeing organisation may require higher return periods for these road types. This should be discussed at gateway 1, project initiation.

Table 3.1: Required levels of flood protection for each road classification

When using this classification, where an urban road becomes a rural road the point of transition shall be defined as the limit of the settlement or city boundary. It should also be noted that where multiple road types (ie urban to rural) exist in one drainage catchment, then the worst case return period should be selected.

Due to the intense rainfall within Qatar it is acceptable to temporarily flood highways to the depths and extents set out in Table 3.2.

3.2.2 Acceptable highway flood standards DA guidelines for acceptable flooding levels are laid out below within Table 3.2. It should be noted that, as with acceptable flood return periods, where an urban road becomes a rural road the point of transition shall be defined as the limit of the settlement or city boundary; and that where multiple road types (ie urban to rural) exist in one drainage catchment, then the lesser level of acceptable flooding outlined within Table 3.2 should be selected.

Special corridors, as characterised in QHDM Part 1 Section 3.5, should have their level of acceptable flooding agreed by the overseeing organisation at gateway 1.

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Road location and type Acceptable Flooding

Urban Areas

Local Roads Flood depth of 0.15m maximum depth and duration of 1 hour

Service Roads Flood depth of 0.15m maximum depth and duration of 1 hour

Collectors Flood depth of 0.10m maximum and duration of 30 minutes

Arterials Flood depth of 0.10m maximum and duration of 10 minutes

Expressways Flood depth of 0.10m maximum and duration of 10 minutes

Rural Areas

Local Roads Flood depth of 0.15m maximum depth and duration of 2 hours

Collectors Flood depth of 0.15m maximum and duration of 1 hour

Arterials Flood depth of 0.10m maximum and duration of 30 minutes

Freeways Flood depth of 0.10m maximum and duration of 10 minutes Table 3.2: Guidelines for flood standards on Qatar roadways

It should be noted that acceptable flood depths and times are only acceptable where they exist as a direct result of periodic surface water inundation at the return periods specified within Note: it is likely that in some instances the overseeing organisation may require higher return periods for these road types. This should be discussed at gateway 1, project initiation. Table 3.1. It is not acceptable for groundwater flow to collect within drainage measure and thus contribute to flooding.

3.3 Hydrological data The Qatar Sewerage and Drainage Design Manual (QSDDM), State of Qatar - Public Works Authority, Drainage Affairs (DA) must be referred to for the design of highway drainage, in particular Section 1.5 - Design Storms (Rainfall, Intensity & Rainfall Depth), for the information on hydrological data and design methodology.

3.3.1 Rainfall characterisation It is important to accurately characterise rainfall and storminess in order to quantify the drainage capacity required to compensate for runoff.

Qatar lies in an arid region and annual rainfall may vary from 20mm to over 200mm per annum. Individual storms occasionally as intense as 124mm in a 24 hour period and 54mm in a 3 hour period have been recorded. Rainfall is therefore characterised by:

• High variability • Severe thunderstorms of limited geographical extent

For the purpose of highway drainage design the country shall be considered as having the same rainfall characteristics for all regions.

For design purposes reference should be made to the study of regional design rainfall, Qatar (2013). IDF relationships from this study are shown in Section 3.3.1.1 (below).

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3.3.1.1 Intensity-Duration-Frequency (IDF) It is important to establish the relationship between rainfall event intensity, duration and frequency in order to reliably identify the drainage capacity required for each design.

Current IDF values should be extracted from relevant figure dependent upon the location of the proposed road. Where a road passes between two cities the figures for the city each catchment is closest to should be used, and if there is any doubt the worst case scenario should be selected. The figures referenced in this section are taken from the recent study of regional design rainfall, Qatar (2013). Figure 3.1 shows the IDF relationship for Al Ruwais, which is located on the north coast of Qatar. Specific IDF relationships for 5 other areas within Qatar are also available within the study of design rainfall for:

• Doha • Al Saliyah • Dokhan • Abu Samra • Umm Bab • Al Ruwais

All six cities demonstrate a close correlation within IDF relationships. Data from Al Ruwais has been presented within this section as this represents the most extreme rainfall intensity. The remaining five cities curves can be viewed in Appendix A.

Equation 3.1: IDF relationship equation (from Section 11.1, study of regional design rainfall, Qatar, 2013)

𝑖(𝑇, 𝑡) = 𝑙1𝛼𝐷24 �11.182𝐿𝑁𝑒(𝑇) + 11.267

𝑡0.8477 + 7.0636�

Where:

T = return period (years)

t = duration of rainfall event (minutes)

𝑖(𝑇, 𝑡) = rain intensity (mm/hour)

𝑙1 = 24 hour average rainfall at site

𝛼𝐷 = discretization adjustment factor

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Figure 3.2: Intensity-Frequency-Duration relationship for Al Ruwais (current climatic condition) (from Study of Regional Design Rainfall, Qatar- Volume 1, Chapter 11, Figure 11-6)

In order to ensure future resilience of drainage solutions climate change should be incorporated within drainage design. This should be undertaken with reference to the relevant IDF relationship presented in the curves for the 2070-2099 period (Appendix B) as illustrated in Figure 3.33 also from the study of regional design rainfall (chapter 12) (2013). As with relationships for current day IDF, this document also describes the likely effects of climate change in Doha, Al Saliyah, Dokhan, Abu Samra and Umm Bab. For the methodology for devising these relationships and the specific limitations of the approach please refer to this document.

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Figure 3.3: Intensity-Frequency-Duration relationship for Al Ruwais (2070-99) (from Study of Regional Design Rainfall, Qatar- Volume 1, Chapter 12, Figure 12-16)

3.3.2 Run-off coefficients (C) Typically, for densely built up areas, there is a high run-off for all rainfall intensities. However, as development becomes more sparse or ground conditions more pervious the total runoff will reduce. Run-off is also affected by storm intensity.

Calculation of surface water run-off shall be made using Figure 3.4 which gives values for run-off coefficients which reflect the above situations

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Figure 3.44: Run-off coefficients for Urban Catchments

3.3.3 Catchment area (A) Both rural and urban catchments will exhibit different constraints and opportunities for drainage design. As such it is important to define the catchment area and treat it accordingly.

3.3.3.1 Rural The area to be considered shall incorporate two parts:

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• The area of the road corridor subject to direct precipitation • The broader natural catchment area within which the road runs. Specifically, the

effect the road may have on the natural surface and sub-surface drainage of the area.

Reference to topographic mapping should be made to assess the catchment area.

3.3.3.2 Urban The area to be considered shall incorporate two parts:

• The area of the road corridor subject to direct precipitation • The additional adjacent area assessed by reference to the Development Plans and

topographic mapping for the area.

The additional area will be dependent on factors such as intensity of development, provision of flood storage areas, and contribution from adjacent roads and developments.

3.3.4 Surface run-off (Q) Highway drainage areas to be considered in Qatar are typically less than 50 Hectares. For these areas surface run-off (Q litres/second) shall be calculated using the formula:

Equation 3.2: Surface run-off (Q) for catchments <50 Ha

For areas larger than 50 Hectares, mostly rural conditions, consideration should be given to assessment of run-off by a combination of historic observation and generation of storm hydrographs. The method used shall be agreed with CED.

3.3.5 The Colebrook-White equation The Colebrook-White equation allows calculation of velocity of flow (v) in a gravity drain flowing full for any given gradient, diameter, and roughness coefficient, as follows;

Q = 2.78 C I A

Where:

C = Run-off coefficient I = Rainfall intensity (mm/h) A = Area (ha)

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Equation 3.3: The Colebrook-White equation

When considering short duration storms the rainfall intensity changes rapidly with only a small change in storm duration, (QSDDM). Therefore it is important that for small drainage areas an accurate assessment of Time of Concentration is made. However, due to the necessity for the surface to receive rainfall and reach a flowing condition the Time of Concentration shall not be reduced to less than 3 minutes.

3.3.6 Manning's equation A number of equations have been developed for computation of the Time of Concentration for various methods of flood analysis. However, it is recommended that where the Rational Method is employed, Manning's equation is used for the calculation of flow velocity in gutters, drainage channels or pipes.

Equation 3.4: Manning’s Equation

3.3.7 Time of concentration (Tc) The engineer wishing to size a drainage system must ascertain the peak rainfall run-off from the catchment under consideration for the designated design storm return period.

V = R2/3 S1/2

n

Where:

V = Mean velocity of flow (m/s) n = Manning's coefficient of roughness R = Hydraulic radius (m) S = Slope (percent)

𝒗 = −𝟐�(𝟐𝒈𝑫𝑺) log� 𝒌𝒔𝟑.𝟕𝑫

+ 𝟐.𝟓𝟏𝑽𝑫�(𝟐𝒈𝑫𝑺)

� Where :

g = Acceleration due to gravity (m/s2) D = Diameter, (m) S = Slope or headloss per unit length (UNIT?) ks = Roughness coefficient, (mm) V = Kinematic viscosity of water (m2/s)

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At a chosen point the peak flow generally occurs at the instant all parts of the catchment are contributing to the flow.

The Time of Concentration (Tc) is defined as the interval in time from the beginning of the rainfall to the time when water from the most remote part of the catchment reaches the point under consideration by the engineer. The Time of Concentration is a function of the average slope, length and roughness of the catchment.

Equation 3.5: Time of concentration (Tc)

3.3.8 Hydrogeology (P) It is important to investigate hydro-geological conditions at each site as part of the pre-design stage. To investigate site hydrogeology, local aquifer types and extents should first be identified in order to ensure testing is undertaken at suitable locations, and that consultation takes place with the correct stakeholders. It should be possible to determine local aquifers through the investigation of geological maps as part of a preliminary desk study.

Once aquifers have been identified stakeholder consultation should be undertaken in order t ensure drainage design poses no risk to water quality within groundwater supplies, this is particularly important where aquifers are, or are planned to be, used for drinking water abstraction .It is necessary that approval is obtained for all projects as part of stakeholder engagement; this approval process is outlined within Section 1.2.2.

It is also necessary to measure permeability at sites in order to identify appropriate drainage measures. For the project design stage it is necessary that site testing is undertaken to determine hydraulic conductivity across the site. Hydraulic conductivity should be investigated through permeability testing undertaken at multiple borehole locations.

Testing can be undertaken through a variety of methods which all relate the rate of flow to differences in hydraulic pressure between the borehole and ground levels. These methods include: ‘falling head’, where water is first increased by pumping into the borehole; ‘rising head’, where water is pumped out of the borehole; or through ‘variable’ or ‘constant head’ testing. These will be applied as appropriate by the site investigation contractor.

Tc = L V

Where:

Tc = Time of concentration (s) V = Mean velocity of flow (m/s) L = Length of flow path from the point of consideration

to the furthest catchment extremity (m)

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In some areas point testing may be weak as a result of predominantly impermeable geology underlying the site. Where this is the case it may be more suitable to determine permeability using an extended linear infiltration trench. This method will cover a larger area and will be likely to identify where small fissures in the geology or localised soil types and features facilitate a higher hydraulic conductivity. Once identified, these areas will be more suitable for infiltration features such as soakaways, or trenches can be filled with gravel and be converted to permanent linear infiltration features.

3.4 Hydraulic analysis processes

3.4.1 Overview [Explain and introduce decision making process regarding appropriate methods to use, and what data (of what quality) is required]

3.4.2 The rational method The ‘rational method’ for hydraulic analysis has the following aspects: • It depends on a thorough knowledge of the local rainfall characteristics; • It requires accurate IDF curves from which rainfall intensities can be deduced for

different storm durations for the design return period; • It assumes that, for a given return period, longer storms have lower intensities and

shorter storms have higher intensities; • It assumes that rain falls uniformly across the catchment; • Contributing impermeable areas are required for each pipe length; • A time of entry must be determined in order to avoid unrealistically high rainfall

intensities; • Base flows from groundwater can be included in the design; • Iterative process for design; • Pipe sizes and gradients are adjusted to provide appropriate self-cleansing

velocities; • Half pipe flow velocity is numerically equal to full pipe flow velocity; • The user must be aware of the limitations of this method of design; • The Modified Rational Method is suitable for catchments up to 150ha. Over this

area detailed hydraulic computer modelling should be undertaken

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Figure 3.5: Rational method process

Determine and confirm with DA: • Design rainfall return period (T) • Pipe roughness (ks) • Time of entry (te) • Run-off coefficient (C)

Prepare preliminary layout of drains and inlet locations

Mark pipe numbers on plan in accordance with numbering

Estimate impervious areas for each pipe

Assume approximate gradients and pipe diameters for each pipe

Calculate pipe full velocity (vf) and pipe full capacity or discharge rate:

Qf = ND2vf/4

Calculate time of concentration from time of entry and time of flow:

tc = te + tf For downstream pipes compare alternative feeder branches

and select the branch resulting in the maximum tc

Read rainfall intensity from the IDF curves for t = tc for design storm return period T

Estimate the cumulative contributing impervious area

Calculate Qp from the formula: Q = 2.78 C I A

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The following aspects need to be considered: • Pipes should be of sufficient size to carry maximum design flows at a depth D, i.e.

at pipe full condition; • Surface water drains require higher velocities than foul sewers for self-cleansing

purposes because of the higher density of solid material to be transported; • Surface water drains should not be less than 300mm in diameter; • Self-cleansing velocities increase with pipe size; • Pipe sizes should not decrease downstream even when the calculations indicate

that this would be hydraulically satisfactory; • Pipes should be designed to run parallel to the ground surface wherever possible.

3.4.3 Mathematical models Sewerage and drainage models use construction record data to build representations of the system as linked pipes and nodes, with specific modules for ancillaries such as pumping stations and overflows. Inflows from connected developments and contributing areas are directed to the nodes, and a computerised hydraulic engine simulates the hydraulic performance of flows around the system.

The veracity of the model is established by verifying flows and depths predicted by the model against actual measurements taken by flow monitors temporarily installed at hydraulically significant points around the system. After the model has been verified, then simulations of future changes and system modifications are run to check the effect on the system and the effectiveness of proposed upgrading.

It should be noted that the rainfall characteristics in Qatar will not make it possible to verify models in accordance with common practice. The WRC Guide to Short Term Flow surveys recommends minimum survey duration of five weeks; however, surveys in Qatar should be planned to commence in October and may need to last up until April to capture a sufficient number of discrete rainfall events. Should these occur early in the survey, then it can be curtailed before the forecasted completion date, but conversely the survey may need to be extended for another rain season if insufficient rainfall occurs.

Sensitivity analysis may be performed on the verified model by varying some of the input parameters to indicate their impact on the theoretical outcomes. This is used to determine more cost effective and / or efficient design options.

Mathematical models used for hydraulic analysis should be agreed with the relevant approving authority (see Section 1.2.2). Discussion should be made as to the benefits of certain modelling software relevant to the particular project scale and scope.

Hydraulic models shall be constructed, verified (where possible) and reported in accordance with the Code of Practice for the Hydraulic Modelling of Sewer Systems, as published by the Waste Water Planning Users Group (WaPUG).

Models shall be retained electronically by the designer for a minimum period of 12 years from the date on which the last modifications for which the model was used have been

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commissioned and taken over by the DA. DA proposes to model all of the trunk mains, and follow with infill models of local areas during the coming years.

Current software packages which are likely to be viable for mathematical modelling are laid out within Table 3.4. The final selection of which model to use is likely to be dependent on the scale of each project and the availability of relevant expertise.

The following table suggests several options for hydraulic models and suggests likely project scales for which they would be suitable. This list is neither exhaustive nor specific, as modelling software is continually updated and new products made available. When deciding which model to use the engineer should appraise the benefits and limitations associated with the selection and confirm choices with the overseeing authority (see Section 1.2.2).

Software package Likely scale best suited to

ISIS > 50 Ha or by agreement with overseeing

organisation (sites </> this area may be suitable)

Infoworks >50 Ha or by agreement with overseeing

organisation (sites </> this area may be suitable)

Microdrainage WinDES <50 Ha or by agreement with overseeing

organisation (sites up to 100Ha may be suitable)

Table 3.3: Likely viable software packages for mathematical modelling

3.5 Pollution control process

3.5.1 Method As a general rule Sustainable Drainage Systems (SuDS) should always be the preferred approach unless there are particular reasons for their exclusion. Justification for the exclusion of SuDS will need to be provided to the overseeing organization at approvals stage gateway 2 and again at gateway 3. Please see Section 4.5 with respect to the SuDS management train, SuDS selection, and SuDS limitations.

Table 3.4 (below) details the levels of treatment to be provided, dependent upon the road type and AADT (Average Annual Daily Traffic). Once the number of levels of treatment has been derived from Table 3.4 for each sub-catchment, Table 3.5 should be used to select the combination of types of treatment for each catchment.

The final step in deriving the pollution control required for each catchment is the consideration of pollution impacts from spillages. The methodology used is based upon that set out in the UK DMRB Volume 11, Section 3, Part 10 HD45/09. Annex I of HD45/09 sets out a method (Method D) for determining the pollution impact on receiving water bodies which in this case will be groundwater. Furthermore, it then provides a means of taking into

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account the pollution control devices already provided for general pollution control due to their mitigating effects on any given spillage. By applying the risk reduction factor a corrected risk can be calculated; to be acceptable the annual probability predicted needs to be less than 1%. If the consequence of such a spillage occurring is severe, i.e. drinking water abstraction could be affected, then a higher standard of protection could be stipulated so the annual probability of such an occurrence happening reduces to <0.5%. The acceptable level of risk should be approved by the overseeing organization at gateway 1.

The flow chart below shows the process of determining the levels of treatment required for roads projects in Qatar, the flow chart also details when guidance should be sought from the overseeing organization.

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Figure 3.6: Pollution control procedure flowchart

Ascertain proposed road data

Determine catchments for road drainage based on road H & V alignment or

topographical survey

From data gathering define key constraints for selection of pollution control feature(s) including level of

protection for groundwater from overseeing organisation

Using Error! Reference source not found. and previous steps determine number of

treatment levels required

Using Error! Reference source not found. and Section 4.5 (SuDS) determine

pollution control features to be provided

Calculate risk of spillage and apply mitigation where required*

• Road types • Junctions • AADT • % HGVs

• H &V alignment • Topographic survey • LiDAR

• Ground contamination

• Salt • Sand • Water abstraction

*Where risk is > 1% apply additional mitigation

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Location and road type

AADT traffic flow* Pollution control – levels of treatment required

1 level 2 levels 3 levels

Urban

Local Roads <5000

Service roads <5000

Collector roads (minor)

5000 – 20000

Collector roads (major)

10000 – 50000 >20000

Collector roads (distributor)

5000 – 50000 >20000

Arterials (minor) 20000 – 50000

Arterials (major) 30000 – 60000

Arterials (boulevard)

30000 – 60000

Expressways 50000 – 80000

Rural

Local roads <1000

Collector 1000 – 2000

Arterial 2000 – 8000

Freeway >8000 >20000

* Figures for AADT have been extracted from the QHDM Part 1, Section 3, Tables 3.1 & 3.2.

Table 3.4: Pollution control requirements

Once the level of treatment has been established using Error! Reference source not found. adequate control measures should be identified using Error! Reference source not found.. SuDS options should be used in preference over traditional options; however, where justification is supplied, it is acceptable to use combinations of both techniques. To prevent mosquitoes SuDS features which retain water at surface level should drain within 48 hours.

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Where proprietary systems are used for a level of treatment they should not be used for subsequent levels of treatment.

Levels of treatment

Level 1 Level 2 Level 3

SuDS

Permeable pavement

Filter strip

Bio-retention

Filter drain

Infiltration trench

Swale

Permeable pavement

Filter strip

Bio-retention

Filter drain

Infiltration trench

Swale

Detention basin

Sub-surface storage/ infiltration

Soakaway

Detention basin

Sub-surface storage/ infiltration

Traditional

Modified gully (Funkee Gruppe)

Downstream defender

Downstream defender

Class 1 bypass separator

Class 1 bypass separator

Up-flow filter (Hydro International)

Storm treat (Storm Treat Systems)

Storm x4 (polypipe)

Table 3.5: Levels of treatment options for pollution control

Having determined the number of levels of treatment required from Error! Reference source not found. and subsequently selecting the types of treatment for each level usingError! Reference source not found., the probability of spillage should now be calculated. In order to calculate the annual probability of spillage for each section of road it is first necessary to gather the following data:

• Length of road for each category in Error! Reference source not found. below • AADT two way flow for each section of road (other than slip roads) identified in

step 1 • Percentage of Heavy Goods Vehicles (HGVs) as a proportion of AADT

When considering the length of road in each category the risk factor from Error! Reference source not found. applies to all lengths of road within 100m of these junctions types. To

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demonstrate how the designer has determined this a sketch of the road types in relation to junctions should be provided to the overseeing organisation at gateway 2.

Junction type

Road type No junction Slip road Roundabout Cross road Side road

Urban

Local roads 0.29 0.83 3.09 0.88 0.93

Service roads 0.29 0.83 3.09 0.88 0.93

Collector roads

Minor

Major

Distributor

0.31 0.36 5.35 1.46 1.81

Arterials

Minor

Major

Boulevard

0.36 0.43 3.09 1.46 1.81

Expressways 0.36 0.43 3.09

Rural

Local roads 0.29 0.83 3.09 0.88 0.93

Collector roads 0.29 0.83 3.09 0.88 0.93

Arterial 0.29 0.83 3.09 0.88 0.93

Freeway 0.36 0.43 3.09

Table 3.6: Spillage rates (SS)

Using the data gathered above the annual probability of spillage for each section of road can now be calculated using the following formula:

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Equation 3.6: Annual probability of a spillage with the potential to cause a major pollution event

Having calculated the probability of a spillage event the probability of that spillage resulting in a serious pollution incident should be determined by using the following equation:

Equation 3.7: Probability of spillage event resulting in serious pollution event

Receiving water body

Urban (response time to site <20 minutes)

Rural (response time to site <1 hour)

Remote (response time to site >1 hour)

Groundwater 0.3 0.3 0.5

Table 3.7: Probability of a serious pollution incident occurring as a result of a serious spillage

If the proposed levels of treatment determined from Table 3.5 include any of the systems shown in Table 3.8: Spillages and risk reduction factors (indicative) below then a further risk reduction factor should be applied before determining the final risk. So, if the risk of a serious pollution event without the mitigation provided by the pollution control level of

PINC = PSPL x PPOL

Where:

PINC = The probability of spillage event resulting in a serious pollution event

PPOL = The probability that once a spillage has occurred that it will result in a serious pollution event. This value should be selected Error! Reference source not found.

PSPL = RL x SS x (AADT x 365 x 10-9) x (%HGVs/100)

Where:

PSPL = Annual probability of a spillage with the potential to cause a serious pollution event

RL = Road length in kilometers

SS = Spillage rates from Error! Reference source not found.

AADT = Annual average daily traffic (based upon design year for a new road)

%HGV = Percentage of heavy goods vehicles

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treatment is PINC, then the adjusted risk incorporating the levels of treatment is determined by PINC x RF, where RF is the risk reduction factor for that system. For values of RF, see below (based on table 8.1 from HD45/09)

Pollution control Risk reduction factor RF (%)

SuDS

Filter drain 0.6 (40%)

Swale 0.6 (40%)

Detention basin 0.6 (40%)

Infiltration trench 0.6 (40%)

Bio-retention 0.7 (30%)

*Sub-surface storage/ infiltration 0.6 (40%)

Traditional

Penstock/ valve 0.4 (60%)

Bypass separator (Class 1)

(or other proprietary system delivering water quality filtering over and above class 1 from BS EN

858)

0.5 (50%)

Table 3.8: Spillages and risk reduction factors (indicative)

Note: In some situations a higher factor signifying a lower risk reduction may be more appropriate due to the limited extent of the given type of pollution control type selected. An example of this would be where a short length of swale is proposed which only serves a proportion of the road under assessment. In such instances it would be appropriate to use a lesser reduction factor, say 20% or a factor of 0.8.

After calculating the adjusted figure for PINC add the annual probabilities for each section of road discharging to an outfall. If this figure is greater than the figure agreed with the overseeing organization (default value 1%) then look at each section of road to determine the highest risk. Consider whether any of the factors need amending, or if an additional form of mitigation can be included to reduce the risk to an acceptable level. Recalculate the risk using this iterative process until an acceptable level is reached.

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3.6 Review process After design criteria have been established it is necessary to review findings with appropriate stakeholders. Engineers should continue to engage with relevant consultations originating from the project initiation stage as well as following the standard approvals procedure as laid out within Section 1.2.2.

As with the previous approval process, this stage of design is likely to be inherently iterative in order to include and balance the interests and feedback of key stakeholders within the approval list.

To best achieve the required outputs and to facilitate an efficient approval process continued engagement should be undertaken throughout the process.

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4 Detailed design work

4.1 Gateway 3 summary

Within the detailed design phase the comprehensive design criteria, as established within the two previous stages of work, are utilised to develop a final working design. The process for this is outlined within Figure 4.1

Figure 4.1: Detailed design process steps

As Stage 3 +

Design calculations • Hydraulic modelling • Drainage asset sizing • Load capacity • Pump/ STW • Floatation • Structural

Drawings • Plans • Cross-sections/ Long sections • STD details and schedules • M & E (pump station)

Review and amend

Specification

Apply for approval

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As each design will vary according to the location and drainage requirements specific to each project, not every stage within this chapter will necessarily be relevant for all designs. The engineer using this guidance should utilise

Figure 4.1 to follow the correct procedure, and then reference the standards laid out within this chapter as appropriate.

4.2 Design in urban areas The term “Urban Drainage” is the terminology used for road drainage for all roads, where major or minor (as defined in Section 7.1.3) in built up urban areas. These are roads maintained by the Civil Engineering Department (CED) for Roads and Drainage.

The objective of providing highway drainage for this area is to collect precipitation fallen on to the impervious road areas and semi-impervious verges and direct flows to an approved outfall. Surface water runoff shall not be allowed to stand within the highway reservation for an extended period of time so as to cause public nuisance or a health hazard. Surface water discharges from the highway will not be allowed to be disposed of onto private land outside the highway curtilage. The exception to this is when land has been purchased for the purpose of storage / attenuation and controlled release of water. There are situations where the highway curtilage has no available space for storage / attenuation and in that situation extra land will need to be purchased outside of the existing highway land.

Early planning of highway drainage in the design process is essential to establish drainage corridors and ROW’s which will not clash with other planned utility corridors. (See Section 5) . [discuss early stakeholder participation]

Good highway planning and drainage considerations, which take into account any natural topographical restraints, will resolve many of the drainage problems associated with highway design. Highways have the following advantages when it comes to provisioning drainage measures:

• Provides a natural drainage area to a discharge point. • Will direct surface water run-off flows to discharge points. • Provide a lengthened drainage path and increased the time of concentration. • Provide extra surface water storage area within the highway curtilage. • Provide isolated drainage catchment areas. • Potential to reroute flood flows to relieve / reduce flood risk in other key areas.

At the planning and design stage it is important to carefully consider other amenity areas such as parks and car parks, which may have the potential to be used for strategic storage. There is also the potential to incorporate SuDS options, such as run-off collection, at nearby public realm sites to alleviate surface water flows.

4.2.1 Urban catchments The increase in housing, roads and amenities is regarded as urban development and as such this changes the nature of the existing land and the drainage surface water runoff process.

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When considering new roads for urban development the engineer should consider the total catchment area either side of the road. An assessment of the existing surface water runoff from the catchment needs to be established to ascertain if this runoff will contribute to the overall highway drainage system. Part of the assessment process is to establish the availability of discharge points for the collection of water and any potential pollution problems caused by the intake of surface water runoff into the highway drainage system. What is of great concern is the intake of blown sand into the drainage system causing blockages and potential flooding problems. The highway designer will need to provide suitable strategies to reduce this potential hazard.

4.2.2 Positive drainage Positive drainage is the term used to describe surface water being collected by gullies and piped or channeled directly to a low point in the network system, before being discharged away from the highway to an approved discharge point.

4.2.3 Drainage of the carriageway It is important to remove rainfall off road surfaces as efficiently as possible to reduce the risk of aquaplaning to road vehicles. Aquaplaning occurs when a vehicle travelling at a certain speed hits standing water on a road and the wheels are then lifted from the road by a thin layer of water. In this situation, which is similar to skidding on ice, the vehicle loses control which can result in a serious accident.

The typical road cross-section is as shown in Figure 4.2, although this does not indicate a means of SuDS collection for surface water which should be considered in preference to traditional techniques such as gullies where site constraints allow. For a balanced road section the road falls away from the centre line and crown of the road to the channel and kerb line. This fall is known as the transverse gradient and 2% is considered as normal for drainage design.

Longitudinal gradient for the channel line to the discharge point (SuDS technique or traditional technique such as a gully pot), can be a minimum of 0.3%. This reduces the peaking of the vertical alignment of the road. However, a desirable minimum longitudinal gradient of 0.5% is to be provided, where practical.

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Figure 4.2: Typical Road Cross-section

The road designer needs ensure that no flat zones are created at road junctions. Where conflicting longitudinal gradients occur then to avoid a flat zone a “Rolling Crown” can be used. The length of the rolling crown is determined using the same formula as that for applying super elevation. (See Figure 4.3)

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Figure 4.3: Typical Detail of Rolling Crown across a Single Carriageway

For the purpose of collection of surface water from road areas at low points gullies should be provided along the kerb line or gutter. On slack longitudinal road gradients, typically less than 0.5% channel blocks can be used to facilitate the surface water to the gully (see Figure 4.4).

Figure 4.4: Kerb and block channel arrangement

Gully spacing is a function of grating size, road gradient and cross-fall and acceptable flow width at the channel. Standard gully spacing and criteria are given in Error! Reference source not found..

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To maintain gully performance under the influence of wind borne debris and dust and to improve collection under the effect of high rainfall intensity, it is preferred that gullies are constructed as pairs. It is also important to design an appropriate maintenance schedule, which will prevent the build up of sand causing sub-optimal operation of drainage measures (See Section X).

Other forms of road surface water drainage are the combined kerb drainage systems. Where the kerb is the conduit for the transfer of water and the top section of the kerb allows free passage of surface water into the conduit. (See Figure 4.5).

Figure 4.5: Combined kerb drainage

Connections from the conduit to the discharge surface water system is made using a side outlet outfall unit (Figure 4.6).

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Figure 4.6: Side outlet outfall unit

Utilities shall be located so as not to provide a hindrance to the drainage system installation and maintenance or increase the chance of damage during utility maintenance works.

Storm sewer design shall be in accordance with CED Roads and Drainage Divisions' design guides and specifications. Storm sewers shall cater for the flows computed from the design criteria in this Section and any additional flows advised by CED Roads or Drainage Divisions at the project initiation stage (gateway 1).

4.2.4 Design of traditional drainage capture techniques This section of the manual covers the methods for determining the sizing /spacing of ‘traditional’ roadside surface water drainage capture techniques. It also describes the limitations to each approach to enable the designer to select the most appropriate method. These techniques fall within the 'traditional' approach to drainage, but can legitimately be considered in conjunction with the SuDS techniques detailed in section XXX, or with appropriate modification could be considered as SuDS in their own right.

INCLUDE EXAMPLES??? Funkee Gruppe gully insert / Permavoid bio filter channel and crate....

4.2.4.1 Gully design: The purpose of this section is to allow designers to space road gullies at a distance which provides the best balance between adequate drainage and minimising the number of gullies required.

The design is based upon the equations provided within the UK Design Manual for Roads and Bridges, volume 4, section 2, part 3, HA102/00, “Spacing of Road Gullies”.

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There are two primary equations, the choice of which is most applicable to a design is based upon whether or not the gradient of a road is uniform. These equations contain several variables which also need to be derived the process for which is also included.

In order to complete this design the following inputs shall be required: The longitudinal gradient of a road (as a fraction); the cross fall of a road (as a fraction); the acceptable flow width (m) to be agreed with the overseeing organisation; Manning’s roughness coefficient for the surface of the road to be drained (typical road surface values are provided in this guidance); the maintenance factor (the selection of which is outlined later in this guide) which is dependent on the roads future maintenance; the rainfall intensity (see section 3.2 for details on its derivation); the width of catchment (m) for the area which drains to the road kerb; the grating parameter (the calculation of which is outlined further in this section); the grating’s waterway and the grating’s slot dimensions and pattern must be known, these may be provided by the manufacturer. All other variables are calculated using these inputs with equations provided in the following section.

a) Limitations of Design Method: • The slots in the grating may not have a total waterway area which is less

than 30% of the grating’s clear area. • The distance between the edge of kerb face and the first slot of a gully

grating may not be greater than 50mm. • The portion of the total waterway area within 50mm of the kerb face may

not be less than 45cm2. • Gullies should be either rectangular or triangular (rectangular preferred)

with one side of the frame positioned hard against the kerb face. Circular gullies, and other shapes which cannot for-fill this will not be accepted.

• Where a pedestrian crossing exists, a gully must be placed directly upstream to minimise flow and therefore minimise disturbance to pedestrians.

• On steep sections of road, the maximum allowable spacing between gullies may not be determined by the collection efficiency of the grating but by the flow capacity of the gully pot beneath it. Generally a gully pot can accept about 10 litres/s without surcharging if the outlet pipe has a diameter of 100mm, and 15 litres/s if it has a diameter of 150mm.

• The design method given in this section is appropriate for the range of longitudinal gradients between 1/300 (0.33%) and 1/15 (6.67%) and can reasonably be extended to a gradient of 1/12.5 (8.00%). For gradients flatter than 1:300 this approach is not applicable and alternative methods should be applied such as Whiffin AC and Young CP. “Drainage of level or nearly level roads”. TRRL Report LR 602, 1973.

• Road gullies have an advantage over surface water channels since the gradient to carry the road runoff from the gully to the outfall is not dependent on the gradient of the road. They do not however usually provide the best drainage solution for long lengths of flat gradients; this should be considered during design.

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b) Intermediate Gratings/inlets: For intermediate gratings/inlets where there is a uniform gradient throughout the section to be drained then the maximum allowable spacing between adjacent gratings (Sp) may be calculated from the equation below:

𝑆𝑃 =�3.6 × 106𝑄 𝑚𝜂

100�𝑊𝑒𝐼

Equation 4.1

Where there is a non-uniform gradient between gullies, the spacing’s are calculated (starting with the upstream gully) with the equation below:

𝑆𝑃 =�3.6 × 106 �𝑄 − 𝑄𝑢𝑠 �1 −𝑚𝑢𝑠𝜂𝑢𝑠

100 ���𝑊𝑒𝐼

Equation 4.2

c) Calculating Flow Rate: The flow rate, Q (in m3/s) approaching the grating is calculated from Manning’s equation:

𝑄 =�𝐴𝑓𝑅

23𝑆𝐿

12�

𝑛 (9)

Equation 4.3

Where: Q = Flow Rate (m3/s) 𝑚 = Maintenance Factor 𝜂 = Flow Collection Efficiency (%) 𝐼 = Design Rainfall Intensity (mm/hr) We = Effective Catchment Width (m)

Where: Q = Flow Rate (m3/s) 𝑚 = Maintenance Factor 𝜂 = Flow Collection Efficiency (%) 𝐼 = Design Rainfall Intensity (mm/h) We = Effective Catchment Width (m) Where Qus, mus and ηus refer to the upstream grating, calculations using this equation should commence at the upstream end. If the upstream end is at the top of a crest with no gully, Qus becomes zero.

Where: Af = Cross-sectional area of flow (m2) 𝑆𝐿 = Longitudinal gradient (fraction) R = Hydraulic radius (m) 𝑛 = Manning’s roughness coefficient

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d) Hydraulic Radius and Cross Sectional Area: In order to calculate the hydraulic radius and cross sectional area, the depth of water against the kerb must first be calculated using the equation below:

𝐻 = 𝐵𝑆𝑐

Equation 4.4

The cross sectional area can then be calculated using the equation below:

𝐴𝑓 =𝐵𝐻2

Equation 4.5

This now allows for the calculation of the hydraulic radius using the equation below:

𝑅 =𝐴𝑓

𝐻 + √𝐵2 + 𝐻2

Equation 4.6

e) Maximum Allowable Flow Width: The flow of water parallel to the kerb should not exceed an allowable width as shown in the figure below:

Where: B = Maximum allowable flow width (m) SC = Cross fall (fraction)

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Figure 4.7: Gully design parameters

An allowable width of flow may be designated by the overseeing organisation, below is a table of typical values of B.

Road type: Flow width (m): Note:

Expressway 1.0 Where there is a hard shoulder the flow width may be extended to 1.5m

Arterial Roads 1.5

Collector Roads 2.0

Service Roads 2.5

Local Roads Half the width of the total road.

This width of flow is only acceptable where there are low volumes of traffic, not travelling fast.

Table 4.1: Design flow widths on various road types (typical values of B)

Note that these are typical values and the actual value for B should be site specific and dependent on the speed and volume of traffic, rainfall intensity, road maintenance and gradients. The following should also be considered:

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Situation: Flow width (m): Note:

Pedestrian crossings or bus stops

0.45

A gully should be placed directly upstream of these points to ensure this is not exceeded

Kerb returns 1.0

Other As determined by local authority

As determined by overseeing organisation

Table 4.2: Design flow widths for special situations (typical values of B)

f) Manning’s coefficient (n): Manning’s roughness coefficient or Manning’s ‘n’ value is related to the roughness of a surface and different values will need to be applied depending on the surface of the road. The table below provides typical values of ‘n’ for commonly used surface materials for flow in triangular channels:

Surface type n

Concrete gutter (trowelled finish) 0.012

Asphalt pavement:

smooth texture

rough texture

0.013

0.016

Concrete pavement:

float finish

broom finish

0.014

0.016

Brick and Pavement Blocks 0.016

Gutter with vegetation and cracks 0.020

Sprayed Seal 0.018

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Table 4.3: Typical values of Mannings ‘n’ for various surfaces

g) Maintenance Factor (m): Reduced maintenance and the build up of debris will lower the efficiency of an inlet or grating as the hydraulic area is reduced. The maintenance factor ‘m’ is introduced to allow for this effect. The higher the level of maintenance and condition of the road, the closer the value of ‘m’ tends to 1.0. Below is a table showing suggested values for ‘m’:

Situation Maintenance factor (m)

Well-maintained urban roads 1.0

Roads subject to less frequent maintenance 0.9

Roads subject to substantial leaf falls or vehicle spillages (eg at sharp roundabouts)

0.8

Sag points on road gradients 0.7

Table 4.4: Maintenance factor ‘m’

h) Design Rainfall Intensity: The design rainfall intensity should be given in mm, refer to section 3.2 in this manual for how to calculate it.

i) Effective Catchment Width: The effective catchment width of a gully should be given in m and represents the width of area that shall be draining to the grating/inlet and all paved and un-paved areas should be included.

Figure 4.8: Effective catchment width

Where un-paved areas are being included, provided the un-paved area does not exceed the paved area then it can be assumed that the contribution of unpaved areas is 20% of paved areas.

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j) Flow Collection Efficiency:

4.2.4.2 Kerb Inlets: The flow efficiency (as a %) is calculated using the equation below:

𝜂 = 100 −36.1𝑄𝐿𝑖𝐻1.5

Equation 4.7

4.2.4.3 Gully Gratings: The flow efficiency (as a %) is calculated using the equation below:

𝜂 = 100 − 𝐺𝑑 �𝑄𝐻�

Equation 4.8

If the grating efficiency η is less than about 80% for an intermediate gully, the most effective solution is likely to be redesign with an improved grating type.

a) Determining the Grating Type: The design value of Gd is based upon the grating type and can be read from the table below:

Grating type P Q R S T

Range of G (s/m2)

≤30 30.1 - 45 45.1 - 60 60.1 - 80 80.1 - 110

Design value Gd (s/m2)

30 45 60 80 110

Table 4.5: Grating type design values

As there are a large number of possible grating designs that could be manufactured, these 5 grating types were created in order to allow designers to specify specific hydraulic requirements for the gratings to the contractor. The grating types are as per the HR Wallingford report, “Spacing of Road Gullies”, report SR 533.

The value of G can be calculated from the equation below:

Where: Q = Flow Rate (m3/s) H = Water Depth against Kerb (m) Li = Length of the Opening in the Line of the Kerb Provided by the Inlet (m) Gd = The Grating Parameter (Value is Determined by the Grating Type)

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𝐺 =69𝐶𝑏𝐴𝑔0.75�𝑝

Equation 4.9

The grating bar pattern coefficient can be found using the table below:

Grating Bar Pattern Cb

Transverse bars 1.75

Other bar alignments – (i.e. longitudinal, diagonal and bars in curve plan)

1.5

Table 4.6: Grating bar pattern coefficient

b) Terminal gullies: A terminal gully is required at the end of drainage runs or low points, they differ from intermediate gullies as it is important for them to have a high flow collection efficiency in order to collect a high percentage of the water and thus prevent the build up of water interfering with traffic. This is usually done with a double gully.

Kerb inlets are not advised for use as terminal gullies unless used in conjunction with a grating.

Equation 4.3 should be used to determine which side of a sag point will provide the greater flow (if using a single terminal gully this flow should be doubled), and Equation 4.1

or Equation 4.2 used to determine the flow collection efficiency η, (for a terminal gully to be effective, then each gully to be used should have a value of η ≥ 95%).

If the grating efficiency η of a terminal grating is less than 95%, redesign is essential and an improved grating type should be used. If the required efficiency is still not achieved then the permitted width of flow (B) should be reduced. This will decrease the design flow approaching the grating and increasing the grating efficiency, however it may result in additional intermediate gullies being needed.

Where: Cb = Bar pattern coefficient Ag = The area of the smallest rectangle parallel to the kerb that includes all the slots (m2) p = The waterway area as a % of the grating area (Ag)

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Figure 4.9: Terminal gully design parameters

4.2.4.4 Design of surface water capture on flat or shallow gradients: The process described in the sections above is only suitable within the limitation described. Designers may be faced with scenarios which fall outside of these limitations. The design outlined in this section is a guide to gully spacing along roads which are level or nearly level.

This section is based on the design approach set out in “Drainage of level or nearly level roads”, by Whiffin AC and Young CP, TRRL Report LR 602, 1973. This report should be referred to for any queries extending beyond the scope of this section. This study was carried out using the following parameters:

Parameter Minimum Maximum

Road width (m) 5.43 14.00

Crossfall (%) 0.5 5.0

Longitudinal gradient (%) 0.00 0.50

Rainfall intensities (mm/hr)

38.1 57.0

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Maximum width of flow along the edge of road kerb (m)

0.5 3.0

Table 4.7: Limiting parameters to equation for design of level of nearly level roads

Use of this approach beyond these parameters cannot be guaranteed to provide a suitable design and an alternative approach may be considered.

𝐽 = 545 �𝑁3

𝐼𝑊�

34𝐶2316 �1 +

𝐵𝑁74𝑌𝑊

(𝐼𝑊)78�

Equation 4.10

𝑤 = 2.32 − 0.1𝐶

Crossfall, % Coefficient, B Index, w

0.5 117 2.26

1.0 190 2.19

1.5 265 2.125

2.0 326 2.06

2.5 380 1.995

s3.0 416 1.93

3.5 448 1.80

4.0 448 1.67

Table 4.8: Values for index W

4.2.4.5 Linear Drainage Design In places, highway geometry may be such that the longitudinal gradient of the carriageway is such that there is minimal or no longitudinal fall and a linear drainage system may be

Where: J = Outlet spacing (m) N = Maximum flow width (m) I = Rainfall intensity (mm/hr) W = Carriageway & hard shoulder (m) C = Crossfall (%) Y = Longitudinal gradient (%) W = Index depending on crossfall B = Coefficient depending on crossfall

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required. These systems can be provided in many forms, the most common of which is an combined kerb drainage unit or CKD where a series of holes or a continuous grating is used as the inlet.

Whilst linear drainage can be a useful solution to overcoming flat or nearly flat longitudinal gradients, a higher frequency of maintenance can be required to retain the self cleansing conditions and to ensure the units operate as designed.

This design process is based on upon the HR Wallingford report “Hydraulic Capacity of Drainage Channels with Lateral Inflow”. Report SR581.

𝑄 = 2.66𝐴1.25 �6.74𝑆0.7 + 0.4 +𝐿ℎ𝑏�

Equation 4.11

4.2.5 Drainage of medians, foot-ways and verges

4.2.5.1 Medians The median of a road system is the middle area of the road, which is usually paved or landscaped in urban areas. As such these areas need to be contained with kerbing to contain any soil within this area and have a cross-fall for the finished level to ensure any surface run-off is directed to the carriageway and the surface water drainage system.

4.2.5.2 Foot-ways & cycle-ways Foot-ways and cycle-ways are usually paved areas adjacent to the carriageway in urban areas and should be designed to have a 2% cross-fall towards the carriageway. The cross-fall will allow surface water run-off to be discharged onto the carriageway and into a surface water system.

For highway construction in existing urban areas the highway designer must take into account the finished road levels in relation to property owner’s access to the pavements and road areas. Where there are wide pavements extra pavement drainage may be required away from the carriageway and the finished pavement levels must be suitable for pedestrian access and maintenance of the drainage system.

Where: Q = Hydraulic capacity (m3/s) L = Channel length (m) A = Channel cross-sectional area (m2) h = Design depth of water (m) S = Longitudinal slope (expressed as a fraction) with: 𝑏 = 0.132𝑆 − 0.00022 for 𝑆 ≤ 1

200

𝑏 = 0.00044 for 1200

< 𝑆 ≤ 130

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Adjacent property owners to the carriageway need to be advised that it is their responsibility to ensure that excessive run-off from their properties does not run across the footway, inconveniencing the passage of pedestrians. Such flows should be suitably channelled into collector channels and then directed to a surface water system. Any run-off from forecourts areas of petrol stations in to the highway drainage system needs to have oil/petrol interceptors installed and also grit traps.

4.2.5.3 Verges Verges with hard landscaping shall be sloped to shed water towards the carriageway. Where soft- landscaping is provided then it shall be edged and sloped to prevent run-off from depositing soil and plant debris onto the adjacent pedestrian or trafficked surfaces, or into property thresholds. Areas of raised planting which incorporate drain holes shall incorporate a filter membrane to prevent washout of soil onto adjacent areas.

4.2.6 Emergency flood areas (EFA) EFA’s are areas of land either external to the carriageway or where space is available, within the highway curtilage that are used for the storage of rain water which has surcharged the normal drainage system and has backed flowed into a EFA. They can also be used to catch and store water from the external catchment area to the carriageway to prevent flooding of road areas and damage to properties. There are a number of design consideration required for large storage areas which are:

• Water should not be allowed to pond for extended periods so as to cause a health hazard.

• Water should be stored in a location where it can be easily pumped by tanker or temporary pumping station.

• Borehole soakaways to aid discharge to the ground water table, where investigation has shown this is achievable.

• Permanent surface water pumping station and rising main connected to the trunk sewer system.

• Where there is a gravity connection to the main surface water system some form of control mechanism is required. Vortex spinners as flow control mechanisms are recommend as they give a constant discharge flow rate under variable head conditions and have no moving parts for maintenance issues.

In order to make the best use of land in developed areas it is normal practice to design EFA's as sports fields, parks, playing fields, car parks etc. EFA's that are not landscaped or utilised for other purposes have a tendency to collect rubbish and become an eyesore.

EFA's should be considered a potential drowning and disease hazard. Where possible they should be kept shallow and spread over a large area. This helps evaporation and dissipation and presents a less deep water hazard. Side slopes should be gentle to allow easy exit and marker posts should be located around the rim to identify the deeper area in times of heavy flooding.

Prior to designing EFA's the prevailing groundwater table should be ascertained to ensure the excavation does not allow standing water to remain. Soakaways or boreholes can be

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constructed in the base of the EFA to encourage water dissipation. Discharging run-off water to lower aquifers is subject to Ministry of Environment (MOE) approval.

4.2.7 Swales -sustainable urban drainage systems (SUDS) Swales are shallow extended ditches, which run in verge areas usually trapezoidal in cross-section. These features are designed to collect run-off water from the carriageway store and control a flow back to the main surface water system. The storing and delaying the flow of water and controlled discharge back to the surface water system, has the effect of reducing the peaking factor of high intensity storms and ultimately this reduces the effects of road flooding. Again the preferred controlling mechanism is the vortex spinner.

4.2.8 Retention of storm-water

4.2.8.1 Minimising or eliminating mosquitoes Mosquitoes lay their eggs on fresh or stagnant water, although some species are able to lay their eggs on damp soil and salt water tides. In usual conditions it takes 10 days for an egg to develop into an airborne adult, these eggs typically take 48 hours to hatch into larvae.

a) Standing water To minimise the presence of Mosquitoes, manmade temporary sources of surface water must not be left to stand for longer than 48 hours.

This includes but is not limited to: subsurface storage, temporary wetlands, detention basins, conveyance swales, wet swales and within rainwater harvesters.

b) Water conveyance Water conveyance systems must be designed to minimise the potential for allowing Mosquitoes to hatch.

Conveyance structure gradients must be such that water is not allowed to stand for more than 48 hours. Routine maintenance should occur to ensure that this gradient is maintained throughout the life of the structure.

Conveyance structures should be designed to ensure that scour does not create depressions that may hold standing water.

Electric pumps should not be used as these systems are prone to failure and as such may cause standing water.

Where a large ditch or swale is to be designed slopes of at least 3:1 or steeper should be used along with a minimum of 1.2m wide bottom. This will discourage burrowing animals, seepage and vegetation growth problems.

c) Access Sealed manhole covers should be used where possible to prevent mosquitoes accessing below ground structures; this is especially important in places where a sump or basin may be used as these are ideal situations for Mosquitoes to lay eggs.

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Where the sump or basin is sealed it is important to remember that female Mosquitoes can fly through pipes, as such, where it is possible the inlet and outlet should be submerged.

Where possible below ground sumps should be designed with the equipment necessary to allow for the unit to be dewatered.

d) Inspections In order to ensure that drainage systems can drain freely they must be kept clear of debris and vegetation. Channels, gutters, ditches and drainage facilities should be inspected regularly to ensure they remain clear. Inspections should also be carried out to ensure that no standing water has developed and that immature Mosquitoes aren’t developing.

As such it is important that maintenance access is considered during the design.

4.3 Design in rural areas The designer for highway drainage needs to consider two basic requirements which are:

• The precipitation falling on to the harden areas of the road and reservation areas. • Drainage flows from the full drainage catchment area.

Surface water run-off from rural roads is normally achieved by dispersion to road verges.

4.3.1 Rural catchments The highway drainage for rural roads does not normally require the storage of run-off water during times of high intensity rainfall events as required in the urban situation. The general principle is to allow surface water run-off from the road areas to the verge areas and into the natural drainage paths. However rural catchment can be considerable areas and generate vast quantities surface water run-off during storms of high intensity precipitation. The highway drainage designer needs to ensure that any over land flooding does not impede traffic flow and it may be appropriate to install culverts under rural roads to link natural drainage paths.

4.3.2 Drainage of the carriageway Drainage of the carriageway for the rural situation amounts to basic vertical alignment of finished road levels allowing surface water run-off to natural verge areas. There are two requirements for this provision, which are:

• Transverse gradients of 2% are provided as normal for drainage of the travelled way.

• Longitudinal gradients are not considered for drainage purposes on un-kerbed roads. However, care must be taken during the design of super-elevated sections to avoid flat zones in the carriageway.

Any areas on route of a rural road where surface water could damage embankments then kerbing and positive drainage may be required.

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4.3.3 Drainage of medians and verges

4.3.3.1 Medians Medians in rural areas will normally be open land with no paving and should be sloped away from the carriageway to avoid soil washing on to the road in times of rainfall. Where run-off is collected from long sections of gradient, median outlets should be provided at Wadi and valley points. This is to prevent water ponding and flooding on to the carriageway. Alternatively the median may be broken into individual catchment segments and surface water allowed to percolate into the ground or evaporate. Median ditches, if required, should have a maximum side slope of 1 in 6 and shall be designed such that water in the ditch cannot percolate into the road construction. (See Error! Reference source not found.).

Figure 4.10: Typical median ditch cross-section

4.3.3.2 Verges and ditches Verges adjacent to rural roads sloped to direct surface water away from the carriageway. At the back of the verge a shallow ditch may be provided to both collect and transport carriageway run-off and catch minor area run-off for disposal to natural drainage paths along the route of the road.

The highway drainage designer shall ensure that ditches are located such that surface water is introduced into the pavement construction. Normal practice is to provide a ditch with ditch invert 0.3m below carriageway formation at the edge of the carriageway.

Rural ditches generally will be unlined and the shape will be dependent on highway safety issues and the following hydraulic considerations:

• Contributing catchment area. • Appropriate storm duration. • Gradient.

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• Roughness coefficient of lining/surface.

Permissible depths of flow for unlined channels are given in Error! Reference source not found.. Shallow side ditches are not normally graded to provide a fall but follow the road profile.

Figure 4.11: Permissible depths of flow for unlined channels

Ditch slopes should present a significant hazard to traffic leaving the road during an accident. Slide slopes of 1 in 6 or shallower should suffice for this. In areas of steep cutting, ditches should be located so they are not filled with loose debris from the cutting. In areas where natural surface run-off is high it may be necessary to install a ditch setback from the top of the cuttings to prevent rainfall damaging the cutting face.

4.3.4 Natural surface drainage Where a highway crosses a wadi, the wadi catchment characteristics, design storm and class of road will determine the type of road crossing required. It is normal practice to allow run-off even from small catchments, to cross under the road so as to minimise disruption to the natural surface flow.

4.3.4.1 Culverts A culvert is a covered channel or pipeline used to convey a watercourse under the road. It consists of an inlet, one or more barrels and an outlet.

Typically, culvert barrels will be constructed from concrete or steel pipes or boxes. Inlets and outlets may be constructed with gabions, mattresses, stone pitching or concrete.

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The hydraulic characteristics of a culvert are complex due to the number of flow conditions that can occur. The highway engineer shall consult specialist literature in his design of culverts and shall choose the most appropriate culvert for the specific purpose considering the following general constraints:

• Preferred minimum pipe culvert diameter 800mm. • Minimum pipe culvert diameter 450mm. • Flooding against embankments is acceptable short term. Freeboard to edge of

carriageway to be a minimum of 0.5m for the design storm. • Embankment slopes of 1 in 6 or greater do not normally require protection against

washout due to short term ponding. Long term ponding may require embankment slopes of 1 in 10.

The engineer shall balance embankment height with culvert height to provide a satisfactory technical and economic solution.

4.3.4.2 Fords Where wadi flows are exceptionally high or the road requires a low storm design return period and is lightly trafficked, culverts may prove impractical. The engineer may therefore consider incorporating a dry ford or vented dry ford. In designing a dry ford, care must be exercised to ensure driver awareness of the potential hazard. Guide posts should be positioned adjacent to the carriageway to assist traffic positioning and advance signing should be used to indicate the dry ford to approaching drivers.

Specific attention must be paid to minimising scour and the prevention of carriageway surfacing and edge loss. Verges, medians and embankment slopes should be protected by impervious layers or rock. Washout of embankment fines should be prevented by the use of filter layers or impermeable membranes.

4.4 Junction Drainage

4.4.1 Considerations for drainage at junctions Effective drainage of the carriageway at junctions is particularly necessary for two reasons:

• The need to retain surface grip to enable the safe stopping, starting and turning manoeuvres routinely undertaken by vehicles at these locations.

• The need to maintain the traffic system capacity, particularly at major junctions makes it essential that flooding of lanes and reduction in junction capacity is avoided.

The following criteria must be considered to satisfy the above requirements:

• Satisfactory transverse gradients must be maintained, particularly on the approach to "Stop" or "Give Way" lines

• Longitudinal gradients must be carefully chosen to keep slack sections of channel to a minimum

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• Where slack gradients are unavoidable the transverse gradient should be a minimum of 2%

• Collection points must be carefully sited to avoid ponding or run-off across carriageways from one channel to another

• Collection points must link to an easily maintainable disposal system with adequate capacity.

Junctions should preferably be situated away from valley points for large catchments to prevent flood concentration at these points. Locating junctions adjacent to trunk sewers or EFA's to provide additional drainage facilities should also be considered.

Urban junctions should always be kerbed and are therefore drained by gullies to the disposal system.

Rural junctions would normally be kerbed however an economic collection and disposal method may be achieved by flush kerbs located at collection points with shallow lined channels removing the water to the adjacent ground.

Lightweight Glass Reinforced Concrete (GRC) embankment channels are easily installed to prevent washout of embankment slopes at areas of run-off concentration such as at kerb ends.

Carriageway cross-falls and longitudinal gradients at junctions are used to channel water to collection points. The following are examples of satisfactory cross-fall layouts with typical collection points:

4.4.2 T – junctions The following features are required for effective drainage design at T junctions (also see Error! Reference source not found.):

• Constant camber maintained on major road. • Longitudinal gradient on major road maintained across minor road throat. • Longitudinal gradient maintained on minor road to major road channel line. • Constant transverse gradient on minor road maintained to radius tangent points. • Gully positions chosen to prevent flow crossing the minor road entry/exit. • It is preferred to maintain the major carriageway transverse gradients through

cross roads or small signalized junctions.

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Figure 4.12: Typical drainage at T Junctions

4.4.2.1 Large signalized junctions The following features are required for effective drainage design at Large Signalized Junctions (also see Figure 5.2):

• Transverse gradients to be maintained at approach to "Stop" lines & pedestrian crossings.

• Longitudinal gradients to be satisfactory to prevent a large flat area being created at the intersection point.

• Transverse gradients on right turn slips to provide super-elevation. • Valleys created in slips to have adequate collection and disposal points. • Additional gullies placed at collection points serving a large surface area. • Gully positions chosen to prevent flow crossing carriageways.

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Figure 4.13: Large signalised junction drainage

4.4.3 Roundabouts The following features are required for effective drainage of roundabouts (also see Error! Reference source not found.):

• Transverse gradients maintained at approach to "Give Way" lines. • Longitudinal gradients to continue to be maintained on approaches and

departures. • Channel of Central Island to fall to one collection point. • Transverse gradients provide super elevation for right turners or those circulating. • Gullies positioned to prevent cross carriageway run-off.

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Figure 4.14: Roundabout Drainage

4.4.4 At grade junctions Ongoing.

4.4.5 Grade separated junctions Ongoing.

4.5 Sustainable Drainage Systems (SuDS)

4.5.1 Introduction to SuDS Drainage systems which mimic the natural drainage process within a site before development and which promote sustainable development are collectively referred to as sustainable drainage systems (SuDS).

Within a given location these sustainable drainage systems are designed so that environmental risks resulting from urban surface water runoff are managed in a way which will also promote environmental enhancement.

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So SuDS objectives are to minimise the impacts from the development on the quantity and quality of the runoff, and maximise amenity and biodiversity opportunities. This three-way concept, as shown in Figure 4.15, highlights that these main objectives should all take equal precedence and the ideal solution will achieve benefits in all three categories, however this may be achieved to varying degrees depending on site characteristics and constraints.

Figure 4.15: Sustainable drainage objectives

SuDS designers should always endeavor to reduce runoff by integrating multiple stormwater controls throughout a site in small, discrete units. Through effective control of runoff at the source, the need for large flow attenuation and flow control structures may be minimised.

Advice on applicable SuDS techniques for each road type can be found below in Error! Reference source not found. and Error! Reference source not found..

Quantity Quality

Amenity and biodiversity

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Table 4.9 - Potential suitability for SUDS installation on Urban Roads

Source Control

Site Control

Regional Control

Urban Road Classification

Tech

niqu

e

Pervious surfaces

Filter drains

Filter Strips Soakaways Swales Infiltration

Trenches

Bio -retention

Areas

Sand Filters

Pipes, Subsurface

Storage Ponds Detention

Basin Infiltration

Basins

Expressways

N Y Y P Y Y P Y Y N Y Y

Major Arterial Roads

N Y Y

P Y Y P Y Y N Y Y

Minor Arterial Roads

N Y Y

P Y Y Y Y Y N Y Y

Boulevard Arterial Roads

N Y Y

P Y Y P Y Y N Y Y

Major Collector Roads

N Y Y

P Y Y P Y Y N Y Y

Minor Collector Roads

N Y Y

P Y Y Y Y Y N Y Y

Distributor Collector Roads

N Y Y

P Y Y P Y Y N Y Y

Service Roads

Y Y Y

P Y Y P Y Y N Y Y

Local Roads

Y P Y P

Y P Y P Y N P P

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Table 4.10 - Potential Suitability for SUDS Installation on Rural Roads

Source Control

Site Control

Regional Control

Rural Road Classification

Tech

niqu

e

Pervious surfaces

Filter drains

Filter Strips Soakaways Swales Infiltration

Trenches

Bio -retention

Areas

Sand Filters

Pipes, Subsurface

Storage Ponds Detention

Basin Infiltration

Basins

Freeway

N Y Y Y Y Y P Y Y N Y Y

Arterial Roads

N Y Y Y Y Y P Y Y N Y Y

Collector Roads

Y Y Y Y Y Y P Y Y N Y Y

Local Roads

Y P Y Y Y P Y P Y N P P

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4.5.2 Infiltration Guidance Infiltration of storm water runoff into the surrounding soil is a useful way to help reduce the volume of runoff at or close to source. It can help to promote groundwater restore, improve water quality through physical filtration and absorption, and reduce the need for further drainage to be installed downstream to convey flows away from the site.

However, infiltration is not appropriate in all circumstance and care is needed to ensure that the infiltration device is suitable for the specific site location.

As a general guide, infiltration is advisable where:

• pre treatment and emergency control, such as an oil interceptor or being situated offline, can be used to prevent groundwater from becoming polluted.

• where the groundwater table is more than 1m below the base of the proposed infiltration device.

• the infiltration rate of the surrounding soil is greater than 0.001mm/hr (e.g. clay and most rocky soils are not generally appropriate).

• the structure of the soil is suitably stable to support large volume of runoff infiltrating into it without the risk of failure.

To aid decision making it is advisable to consult with a geotechnical engineer to determine whether infiltration is likely to be appropriate. The outline process in Table 4.22 helps highlight the main steps which should be considered when specifying an infiltration device.

Figure 4.16: Consideration of infiltration

Consult a geotechnical engineer to discuss whether an infiltration device is likely to be suitable based on

a high level review of available site data.

Conduct an onsite trial pit investigation to determine the actual infiltration rate of the soil and take soil

samples for analysis.

Determine the type of device suitable for the site and design the outline of the system based on default

values.

Discuss findings with a geotechnical engineer and feed back into the design.

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4.5.3 Specification of Suitable Vegetation

TBC

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4.5.4 Pervious Surfaces Pervious surfaces allow rainfall to infiltrate through their surface and into the sub base. This provides a level of attenuation, water quality treatment and provides point of source collection which reduces the need for other drainage systems to be constructed.

The sub base layers can either be drained via infiltration and/or a piped drainage system.

Porous asphalt and porous concrete are the most suitable forms of pervious surface for use on highways.

4.5.4.1 Location Setting Careful attention needs to be paid to vehicular loading and road speeds as due to the more permeable nature of the sub base compaction of material can lead to depressions and damage to the functionality of the system.

Also, where high sedimentation loads are expected, and unlikely to be regularly dispersed or cleaned, then these surfaces may prove less effective and may be subject to failure.

Road location and type Potential Suitability

Urban Areas

Local Roads Y

Service Roads Y

Collectors Y

Arterials Y

Expressways N

Rural Areas*

Local Roads N

Collectors N

Arterials N

Freeways N

Table 4.11: Potentially suitable locations for a pervious surface

* Please note that these locations are only likely to be suitable where frequent sweeping or dispersion of the sediment along the road can be maintained.

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4.5.4.2

4.5.4.3 Benefits and limitations

4.5.4.4 Key Design Elements • The pervious surface and sub-base should be structurally designed for the specific

site and excepted vehicular loading. This should be done inline with the manufactures recommendations.

• Guidance should be sort by a suitably qualified geotechnical engineer as to whether infiltration should be allowed or whether a liner should be installed to provide an impermeable barrier.

• Surface infiltration rates should normally be an order of magnitude greater than the design rainfall intensity. Manufactures guidelines should be referred to in order to clarify this.

• The subsurface storage volume should be adequate to ensure that the infiltration and/or discharge rate through the continuation pipe will not become a limiting factor and create surface ponding during an event equivalent to its design event.

• Angular, crushed material with high surface friction should be used for sub base construction. Sand and gravel with rounded particles should not be. This is in order to maintain voids and limit compaction of the material which would reduce permeability. Guidance should be sort by a geotechnical engineer about suitable, local material where possible.

• The sub-base should usually be laid in 100–150 mm layers and lightly compacted to ensure that required void ratio is achieved for the particular material used. Guidance should be sort by a geotechnical engineer and/or the surface manufacture about a suitable sub base construction.

• A geotextile may be specified as a filtration treatment component near the top of the sub base. Where this is done, care should be taken to ensure that the rate of infiltration is greater than the rate through the pervious road surface.

Limitations • Require regular maintenance

where sediment loads are likely to high in order to prevent clogging.

• Vehicular loading, volumes and road speed need careful consideration to avoid failure of the road surface.

• If the geotextiles becomes blinded with fine silt it will adversely affect the infiltration rate into the sub base.

Benefits • Aids the removal of rainfall at

source.

• Can help to attenuate flows and improve water quality.

• Dual purpose and reduces the need for additional installation space e.g. drainage and road surface in one.

• Reduces surface ponding on the road surface.

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• Soil and other material must be prevented from contaminating the pavement surface and sub-structure.This can be achieved by ensure adjoining land slopes away from the road surface.

4.5.4.5 Design Stages

Figure 4.17: Pervious surface design steps

4.5.4.6 Maintenance requirements Maintenance Type Action Frequency

Regular

Inspection of pervious surfaces for evidence of structural defection or reduced infiltration efficiency.

Quarterly/ as the manufacture recommends.

Cleaning/ sweeping of the pervious surface

As stated by manufactures instructions.

Occasional

Remediation of any surface depressions

As required

Reconstruction of the pervious surface and sub base

When total failure of the structural integrity or ability to absorb runoff occurs.

Table 4.12: Pervious surface maintenance requirements

4.5.4.7 Construction Advice • Care needs to be taken to suitably prepare the formation level inline with

manufacture guidelines. • Any noticeable soft spots in the formation level should be excavated and back-

filled with suitable well compacted material prior to laying the sub base layers. • Impermeable membranes must be treated with care during installation to ensure

that it is not damaged. • To maintain permeable properties the sub base must not be compacted. This with

reduce the void ratio and effectiveness of the system.

Use manufactures instructions to design the pavement and use the key design criteria in section 4.5.3.3 as a guide of

consideration.

Is a pervious surface suitable?

Are high sediment

loads expected?

Will regular cleaning be possible?

Is vehicle loading

expected to be high?

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• Geotextiles should be laid in accordance with manufacturers’ instructions and with overlaps between adjacent strips without any folds or creases to ensure full coverage.

• Contaminants should be prevented from entering the pavement surface and sub-base both during and after construction. This is in order to ensure that the pavement remains permeable throughout its design life.

• To do this, silt fences and temporary drainage which diverts runoff away from the area should be considered to manage these risks during construction and landscaping should be carefully designed to prevent deposition of materials from adjacent land e.g. sloping sides away from the highway.

4.5.5 Soakaways Soakaways are excavated pits or chambers usually filled with rubble. They can be lined with brickwork, pre-cast concrete or polyethylene rings which are perforated storage structures surrounded by granular backfill.

They can be grouped and linked together to drain large areas including highways.

Soakaways provide stormwater attenuation, stormwater treatment and groundwater recharge.

4.5.5.1 Location Setting Soakaways can be designed to fit with site constraints, but an important feature is that infiltration into the ground is acceptable at this should be considered at an early stage.

Table 4.13 highlights potentially suitable site locations.

Road location and type Potentially Suitability

Urban Areas

Local Roads Y

Service Roads Y

Collectors Y

Arterials Y

Expressways Y

Rural Areas

Local Roads Y

Collectors Y

Arterials Y

Freeways Y

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Table 4.13: Potentially suitable site locations for soakaway installation

4.5.5.2 Benefits and limitations

4.5.5.3 Key Design Elements • Field investigations are required at an early stage to confirm the infiltration rates

and acceptability of infiltration to the soil. A geotechnical engineer should be consulted during this assessment.

• Inspection of the soakaway should be made possible during the design phase. This can either be through an inspection well or opening in the cover.

• A perforated pipe can be incorporated to provide a point of discharge to drain small soakaway. This should be visible and access should be provided to allow debris and sediments to be cleared from the pit.

• Soakaways can simply be built as simple excavations that are backfilled with high voids media, or supported perforations pre-cast concrete or plastic chambers which can help improve stability and maximise infiltration to the surrounding ground.

• A suitable geotextile lining should be specified to ensure that granular material can be separated from the surrounding soil and to prevent migration of fines into the soakaway.

• Soakaways must be of sufficient strength to cater for the loads acting on them, especially where they are required to be traffic bearing. A structural engineer should be consulted to assess this.

• Storm events in excess of the design return period will need to be considered to ensure that floodwater can safely be conveyed downstream. This may require additional drainage components to be specified as the infiltration rate will present a limiting factor.

Benefits • Minimal land take is needed for

installation.

• Can be designed in series or individually for the needs of the site.

• Help to provide groundwater recharge.

• Good volume reduction and peak flow attenuation.

• Easy to construct and operate.

Limitations • Not suitable for poor draining soils. • Not suitable where infiltration is

not acceptable. • Not appropriate for draining

polluted runoff. • There is some uncertainty over

long-term performance as continual observations are limited.

• Reduced performance during long wet periods is likely as the ground becomes heavily saturated.

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• The soakaway should be designed to discharge from full to half-volume within 24 hours so that sufficient capacity is available to receive runoff from subsequent storms.

• The soakaway should fully discharge within 48 hours so that mosquito breeding is minimised.

4.5.5.4 Design Stages

Figure 4.18: Soakaway design steps

* a model is recommended for use as can provide a better understanding of the likely response of drainage features at the time design and during its lifetime.

Hand calculations for design purposes can be used where the drainage catchment is small and a model would be inappropriate for the size of the scheme. The storage required will be a function of inflow minus outflow over time and can be determined using the level pool routing technique.

4.5.5.5 Maintenance requirements

Maintenance Type Action Frequency

Regular Inspect and identify areas that are not operating as designed and remediate them.

Monthly for the first 3 months, then six monthly

Consider if a soakaway is suitable for the site location? (e.g. is infiltration appropriate)

Use a model* to determine the volume of storage required to attenuate a design event in accordance

with Table 3.1.

Determine whether the infiltration rate is sufficient to ensure the soakaway will be drain fully in 48 hours?

(use the model or convert the infiltration rate to a volume over 48 hours and subtract this from the

design volume)

Ensure the key design considerations in section 4.5.4.3 are considered during design.

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Maintenance Type Action Frequency

Remove debris and blockages. Monthly

Occasional Remedial

Remove sediment from pre-treatment structures.

Annually (or as required)

Detailed inspection of all structural elements

Annually and after large storms

Repairs to structure elements As required

Table 4.14: Soakaway maintenance requirements

4.5.5.6 Construction Advice • Untreated drainage from construction sites should not be discharged into

soakaways during construction as this can cause the soakaway to become ineffective.

• It is recommended that the exposed surface of the soil should be manually cleaned to ensure the geotextile and granular fill surrounding the chamber are installed in optimal conditions.

4.5.6 Swales Swales are wide, shallow, gently sloping channels usually covered by grass or other suitable vegetation, although reinforced earth or rip rap be can also used.

Standard swales are designed to incept flow from the highway and convey runoff slowly along their surface. Infiltration can be encouraged through the use if check dams or berms installed across the flow path.

Infiltration can be conducted in standard swales or by the use of a dry swale. These incorporate a filter bed and under drain system under the base of the swale to improve the capacity of the system.

Where infiltration is not permitted, the swale can be lined with an impermeable membrane.

4.5.6.1 Location Setting Swales are usually situated parallel to the highway, but need to be kept relatively wide so that shallow depths can be maintained. This usually makes them most suited to areas where there are wide verges available, but it is still useful to consider them where there are only narrow corridors, such as in urban areas.

Road location and type Suitability

Urban Areas

Local Roads Y

Service Roads Y

Collectors Y

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Road location and type Suitability

Arterials Y

Expressways Y

Rural Areas

Local Roads Y

Collectors Y

Arterials Y

Freeways Y

Table 4.15: Potentially suitable locations for installing a swale

4.5.6.2 Benefits and limitations

4.5.6.3 Key Design Elements • Vegetation in the flow channel should typically be maintained at a height of 100–

150 mm. • The shape of a swale should be trapezoidal or parabolic in cross section as these

are easiest to construct and maintain, and offer good hydraulic performance. • Swale side slopes should be no greater than 1 in 4 to promote sheet flow and low

velocities, and to maximise the wetted perimeter, promote filtration and minimise erosion.

• The normal maximum dry swale depth is between 400 and 600 mm (providing all technical and safety issues have been considered)

• A freeboard of 150 mm should be provided over the design flow depth to allow for blockages.

• Conveyance swales should have a minimum longitudinal slope of 1 in 300. • The base width of the swale should preferably be between 0.5 and 2 m.

Limitations • Not suitable for where the

longitudinal gradient of the highway is steep.

• Not suitable where vehicles may park on the swale and damage their construction.

• Potential risk of blockages when connecting swales with pipe work.

• Vegetation requires regular maintenance.

Benefits • Help to reduce urban pollutants

in runoff.

• Can help reduce runoff rates (and volumes where infiltration is permitted).

• Pollution and blockages are visible and easily dealt with.

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• The design event runoff volumes should half empty within 24 hours to ensure runoff from further storms can be accommodated.

• The design event runoff volumes should fully empty within 48 hours to reduce stagnation and mosquito breeding.

• The maximum flow velocity in the swale for events below a 1 year return period should be 0.3 m/s to promote settlement.

• Flow velocities for extreme events should be kept below 1.0 m/s to prevent erosion.

• Check dams and appropriate pre-treatment systems should be used to improve both the hydraulic and water quality performance of a swale system by reducing velocities, increasing residence time, increasing infiltration and promoting storage.

• Where required, check dams are typically provided at 10 – 20 m intervals and the water level at the toe of the upstream dam should be the same level as the crest of the downstream dam.

• Check dams should be constructed into the sides of the swale to ensure that water does not bypass the structure and a small orifice or pipe at the base of the dam will allow low flows to be conveyed downstream.

Figure 4.19: Diagram of typical swale

4.5.6.4 Maintenance requirements Maintenance Type Action Frequency

Regular Litter and debris removal Monthly/ as required.

Watering of vegetation. Daily

Occasional Remedial

Check for areas of poor vegetation growth and re-seed accordingly.

Half yearly/ as required

Inspect inlets, outlets, overflows, check dams for signs of erosion, silting, blockages also check for areas of ponding.

Half yearly

Repair structure As required

Remove pollutants and sediment build up

As required

Re-level uneven surfaces. As required

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Table 4.16: Swale maintenance requirements

4.5.6.5 Construction Advice • Swales should not receive any storm runoff until vegetation in the system is fully

established and construction at the site has reached a state where sediment from the site will not cause siltation of the swale.

4.5.7 Filter Trenches and Drains Filter trenches are shallow excavations, lined with a geotextile and usually back filled with stone to create an underground reservoir to drain runoff from small catchment areas. Runoff can then infiltrate into the surrounding soil.

Filter drains are similar, but use a porous or perforated pipe placed at the base of the trench to allow flow to pass forward into a further drainage system. Infiltration can be permitted, or an impermeable liner used to omit this.

These systems are designed to provide attenuation by promoting slow infiltration through their fill material and into the ground where infiltration is allowed. They also provide storage in the trench itself.

4.5.7.1 Location Setting Filter drains and trenches are suitable for installation at the edge of highways and can be designed to site characteristics.

Care is needed to ensure high speed vehicles do not veer onto the drain or park on top of them. This can cause stone scatter which will be dangerous to other road users and heavy loading on the trench median which can cause compaction and is not advisable.

Care should also be taken to ensure that they are not located where stones are likely to be scattered by human intervention.

Road location and type Potential Suitability

Urban Areas

Local Roads N

Service Roads Y

Collectors Y

Arterials Y

Expressways Y

Rural Areas

Local Roads N

Collectors Y

Arterials Y

Freeways Y

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Table 4.17: Potentially suitable locations for trenches

4.5.7.2 Benefits and limitations

4.5.7.3 Key Design Elements • To maximise the life time effectiveness of these systems, it is recommended that

runoff be pre treated to remove heavy solids before arrival at the trench as it is difficult to restore the operation of trenches once clogged.

• • Trenches should not be constructed on steep slopes where there is an increased

risk of stability concerns. • Where designed to allow for infiltration, the maximum groundwater level should

be greater than 1m below the trench to avoid continued saturation of the filter median and risk of groundwater infiltrating into the drainage network.

• A full geotechnical assessment should be conducted to ensure that ground conditions are suitable for the trench excavation and that infiltration is appropriate (see section 4.5.2).

• The trench should be designed to by half emptied within 24 hours from being full to ensure capacity is made available for subsequent events and reduce the risk that the trench will be waterlogged for long periods.

• Trench depths should normally be between 1 and 2m. • Locally available granular stone or rock fill should be specified where possible. • The geotexile should have greater permeability than the surrounding soil where

infiltration is allowed to ensure the geotexile doesn’t present a barrier to flow. Manufactures guidance should be used to assess this.

Benefits • Help slow the flow rate and

provide attenuation. • Where infiltration is

permitted, runoff volumes will be significantly reduced and can aid groundwater recharge.

• Filtration aids treatment of contaminated runoff leading to improved water quality.

• Can be easily installed close to the highway to reduce the need for further drainage.

• Relatively low land take and cost of installation.

Limitations • Stable subsurface material is

essential to reduce the risk of trench collapse.

• Pipe collapses in filter drains are very common due to the nature of the pipe material.

• Not suitable for sites where filter clogging is likely (e.g clay/silt in the upstream catchment).

• Failure of the system is difficult to see in the early stages.

• Regular maintenance is required to reduce the risk of failure due to clogging.

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• Measures should be put in place to ensure stone scatter due to vehicles or human interaction does not occur. This could simply include locating the trench slightly away from the highway and not constructing this type of system in highly urban areas or close to settlements.

• Infiltration trenches should be constructed with a high level outlet for exceedance conditions, and appropriate drainage designed to convey flows safety downstream.

• The base of the trench should be gently sloping to encourage flow movement and avoid ponding.

• Adequate access to the trench should be provided for maintenance as this will include the need for washing and replacement of the top layers regularly.

4.5.7.4 Design Stages

Figure 4.20: Infiltration trench design steps

* a model is recommended for use as can provide a better understanding of the likely response of drainage features at the time design and during its lifetime.

Hand calculations for design purposes can be used where the drainage catchment is small and a model would be inappropriate for the size of the scheme. The storage required will be a function of inflow minus outflow over time and can be determined using the level pool routing technique.

A trench which can be accommodated along the highway based on site constraints should be

modelled* and design to accommodate a design storm in accordance with Error! Reference

Determine whether the trench can be drained by half via the outlet pipe or infiltration in 24 hrs

Use the key design consideration laid out in section 4.5.6.3 to ensure the trench is designed

inline with best practice.

Is a trench suitable for the site location?

Is stone scatter likely?

Is the filter likely to get

clogged

Are ground conditions suitable?

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4.5.7.5 Maintenance requirement

Figure 4.21: Infiltration trenches maintenance requirements

4.5.7.6 Construction • Trenches should be constructed after highways have been installed to minimise

compaction of surrounding soil. Where soil has been compacted during previous construction activity, this needs to be improved to ensure that the effectiveness of the infiltration properties of the ground are maintained.

• Stone fill and geotextile should be clean before installation and care should be taken to ensure the geotexile covers all sides of the trench and is not damaged during installation.

• Health and safety consideration should be paid close attention to during construction in order to ensure trench collapse does not occur during excavation or prior to installation of fill material. This is of particular importance when the trench depth is greater than 1.2m or ground conditions are weak. Support should be used to stabilise the trench in both cases.

• After installation, an infiltration test should be performed to ensure that the trench is operating as expected.

4.5.8 Bioretention Areas Bio-retention areas are shallow landscaped depressions which comprised of several components which act together in order to improve water quality.

Grass filter strips or channel help to reduce runoff velocities; a ponding area is normally incorporated to provide temporary storage; an organic layer and planting is provided to encourage filtration and pollutant uptake; and a sand bed is provided to promote aerobic conditions.

They are aimed at managing and treating runoff from frequent rainfall events. Excess runoff from extreme events passed forward to other drainage facilities.

Maintenance Type Action Frequency

Regular

Removal of litter and debris from the trench surface.

Monthly

Inspection of trench surfaces for evidence of ponding and silt accumulation.

At least quarterly.

Washing of exposed stone. Annually

Occasional

Remediation of filter median and geotextiles when clogging occurs.

As required

Inspection of pipe work for evidence of operation and failure.

As required/ half the expected asset life

Re-excavation of trench walls As required

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4.5.8.1 Location Setting As vegetation is incorporated, daily watering will be required to maintain their efficiency. As a result they are perhaps only suitable for installation where this is likely.

Road location and type Suitability

Urban Areas

Local Roads Y

Service Roads Y

Collectors Y

Arterials Y

Expressways Y

Rural Areas*

Local Roads Y

Collectors Y

Arterials Y

Freeways Y

Table 4.18: Potential suitable locations for bioretention areas

* Only suitable where regular watering can be conducted.

4.5.8.2 Benefits and limitations

4.5.8.3 Key Design Criteria • Typically the system is drained via an under drain and relies on engineered soils

and enhanced vegetation. Individual units can be supplied for site needs and a suitable manufacture should be consulted during the design phase.

• The storage volume of the system should be designed so that it will be half emptied within 24 hours. This can either be via infiltration or through the outlet pipe.

Benefits • Provide aesthetic appeal and

can be incorporate as a landscape feature.

• Very effective in removing urban pollutants.

• Can reduce volume and rate of runoff.

• Flexible layout to fit into landscape.

• Good retrofit capability.

Limitations • Requires landscaping and

management.

• Susceptible to clogging if surrounding landscape is poorly managed.

• Not suitable for areas with steep slopes.

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• The shape of a bio-retention area is not a critical feature in design, but a minimum width of 3 m and length to width ratios of 2:1 is recommended to aid planting.

• The soil bed should have a minimum depth of 1 m. Where trees are planted, the depth should be 1.2–1.5 m.

• The soil should be a sandy loam mixture with a permeability of at least 12.6 mm/h and pH ranging between 5.2 and 7.

• The sand filter should have a minimum thickness of 0.3 m and consist of sand with a grain size of 0.5 to 1 mm.

• The gravel around the under drain should comprise 20 mm to 5 mm aggregate. A typical cross section is presented in Figure 4.22.

Figure 4.22 - Typical Cross Section through a Bio-retention Area

4.5.8.4 Maintenance requirement

Maintenance Type Action Frequency

Regular

Watering of plants Daily

Litter and debris removal Monthly or as required

Weeding As required

Occasional Remedial

Replacing mulch and spiking of soil

Annually

Inspection of inlets, outlets and overflows

Half yearly

Restoration of vegetation / eroded areas

As required

Clearing of blockages and removal of silt and built up vegetation.

As required

Repair to structure As required

Table 4.19: Maintenance requirements for bioretention areas

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4.5.8.5 Construction • Bioretention areas should ideally be constructed at the end of development, to

minimise erosion and sediment generation. • Care should be taken not to compact the soils below the bioretention area, and

particularly the filter and soil planting bed, as this will reduce infiltration capacities.

• To excavate a bioretention area, a backhoe excavator should be used and construction plant should avoid running over the bioretention area.

• If soil for the filter layer is imported, soil testing should be carried out: the test should include a particle size distribution, pH and organic matter test for each retention area.

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4.5.9 Sand Filters Sand filters are designed to treat surface water runoff through filtration and can be used to significantly improve water quality.

The filter median is usually sand, but gravel, peat and compost also be used.

Surface sand filters (as shown on the left of Figure 4.23) are above ground structures, usually constructed as offline facilities which incorporate a sand filter bed at the base of the excavation. The filters can be designed with an impervious lining and drained via a drainage system, or can be designed to allow infiltration into the surrounding soil.

Underground sand filters, as shown on the left of Figure 4.23, can also be used where space is limited. These are essentially chambers which are improved to help to aid water quality treatment prior to flow passing forward through the drainage system

Figure 4.23: Diagrams of a typical surface sand filter (left) and a typical underground sand filter (right)

4.5.9.1 Location Setting The design of sand filters is flexible and allows the opportunity for incorporation into multiple locations. However, care should be taken to ensure adequate pre-treatment or emergency control can be made such as installing them offline.

Table 4.20 below highlights road types where sand filters could potentially be incorporated.

Road location and type Suitability

Urban Areas

Local Roads Y

Service Roads Y

Collectors Y

Arterials Y

Expressways Y

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Rural Areas

Local Roads Y

Collectors Y

Arterials Y

Freeways Y

Table 4.20: Potential site locations for sand filters

4.5.9.2 Benefits and limitations

4.5.9.3 Key Design Elements Surface Sand Filter

• Pre-treatment is required to remove debris and heavy sediment prior to entering the filter bed.

• A flow separating device should be used to ensure flow is distributed evenly across the filter bed to avoid disruption of the bed material.

• A series of under drains should be installed parallel to the direction of flow to promote flow transfer. These should be 150 mm diameter perforated PVC pipes installed in a gravel layer. They must have a minimum slope of 1 per cent and spacing should not exceed 3 m.

• The filter bed should consist of a 0.45–0.6 m layer of washed medium sand. Topsoil or gravel can be installed on the top to prevent erosion. Where this is done this layer should be approximately 75mm deep. Figure 4.24shows the typical construction of the bed layers.

Benefits • Flexibility of design.

• Efficient in removing a range of urban runoff pollutants.

• Suitable for retrofits and in tightly constrained urban locations.

Limitations • Not recommended for areas

with high debris content in runoff.

• Waterlogged conditions can support algae growth, filter clogging and mosquito breeding.

• Negative aesthetic appeal/possible odour problems.

• Not suitable for large catchment areas.

• High capital cost and maintenance burden.

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Figure 4.24: Typical sand filter bed construction

• A permeable filter fabric should be placed both above and below the sand bed to prevent clogging of the filter and under drain system.

• The length to width ratio of the filer should ideally be a minimum of 2:1. • The sides of the excavation should be a minimum of 1:6 • The design should allow for easy access for maintenance. There may be a need to

remove heavy wet sand from the system, usually by hand, and the access should be designed to facilitate this.

• Inspection/cleanout wells should be provided to the under drain.

The filter area should be sized to completely drain in 48 hours or less. This area can be determined using the principles of Darcy’s Law and the Equation 4.12

𝐴𝑓 = 𝑉𝑡(𝐿)𝑘(ℎ+𝐿)𝑡

Equation 4.12: Filter area size calculation

Where: Af Surface area of filter bed (m2) Vt Water quality treatment volume (m3) L Filter bed depth (m) typically 0.45 – 0.6m) k Coefficient of permeability of filter medium for water (m/s) = 0.001 (approx) for 0.5mm sand = 0.006 (approx) for 1.0mm sand h Average height of water above filter bed (half maximum height, where hmax is typically ≤2m) (m)

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Under drain Filters

• A sedimentation chamber should be incorporated into the design to promote settling of heavier solids to avoid clogging the sand filter.

• To prevent backflow of water in the system the maximum head of water that can develop in the sedimentation must be at least be twice the average height of water above the filter device.

• These systems should be set offline and/or an overflow should be provided to allow for exceedance conditions.

4.5.9.4 Design Stages

Figure 4.25: Sand filter design steps

* a model is recommended for use as can provide a better understanding of the likely response of drainage features at the time design and during its lifetime.

Hand calculations for design purposes can be used where the drainage catchment is small and a model would be inappropriate for the size of the scheme. The storage required will be a function of inflow minus outflow over time and can be determined using the level pool routing technique.

Determine the volume of storage required to retain runoff from a critical duration design

event in line with Error! Reference source not

Can the basin be drained via piped outlet or infiltration within 48 hours? (use the model or

Equation 4.12 )

Use the key design consideration laid out in section 4.5.8.3 to ensure the basin is designed

inline with best practice.

Is a sand filter suitable for the site location?

Will runoff have a high debris

content?

Can the filter be designed to be

offline?

Can the basin be accommodated in the space available on site?

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4.5.9.5 Maintenance requirements

Maintenance Type Action Frequency

Regular

If vegetated, watering of plants. Daily

Check for odors indicating presence of anaerobic conditions. Monthly.

Monitoring of sediment accumulation and vegetation, as appropriate.

Monthly.

Removal of sediment, litter and debris from the inlet and outlet.

As required/ after a storm event

Check that the sedimentation chamber is <50% full.

As required/ at least annually

Check that filter bed has <15 mm surface sediment accumulation. Quarterly

Washing of the top layers of the sand to maintain efficiency

As required/ at least annually

Remedial

Repair of eroded surfaces. As required/ after a storm event

Realignment of erosion protection.

As required/ after a storm event

Repair of inlet/ outlet. As required

Table 4.21: Sand filter maintenance requirements

4.5.9.6 Construction Advice • Filters should not receive any runoff until vegetation in the system is fully

established and construction at the site has reached a state where sediment concentrations in the runoff will not cause clogging.

• It is important that the top of the filter bed is constructed completely level, otherwise filtration will be localized and early failure may occur.

• In areas where groundwater protection is a concern, the completed tank structure (concrete or membrane) should be filled with water for 24 hours to ensure that there is no leakage.

4.5.10 Basins Detention and infiltration basins are surface storage basins that are designed to detain a certain volume of runoff for a particular attenuation period. They also provide a level of water quality treatment through settling of particulate pollutants. A typical layout is shown in Figure 4.26.

Basins are dry during normal conditions and either lined to prevent infiltration (detention basin), or unlined to encourage infiltration (infiltration basin). Rip rap or geotextile matting protections could potentially be used to improve the soil structure, and reduce or replace the need for a fully vegetated system.

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Figure 4.26: Plan view of a typical basin

4.5.10.1 Location Setting The size of the basin depends on the catchment being served and can be designed to accommodate varying volumes. They are therefore suitable for a range of situations.

Opportunities to incorporate basins in narrow corridors or in densely urbanized zones may be challenging, but they offer benefits from a water quality and ease of maintenance perspective that should be considered when selecting an appropriate technique.

Table 1 shows where a detention pond is potential suitable, but ultimately it will depend on site constraints.

Road location and type Potential Suitability

Urban Areas

Local Roads Y

Service Roads Y

Collectors Y

Arterials Y

Expressways Y

Rural Areas

Local Roads Y

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Road location and type Potential Suitability

Collectors Y

Arterials Y

Freeways Y

Table 4.22: Potentially suitable locations for a basin

4.5.10.2 Benefits and limitations

4.5.10.3 Key Design Elements • The basin should be sized to delay peak runoff. It should however be designed to

be drained within 48 hours to avoid stagnation and minimize the opportunity for mosquitoes to breed.

• Pre-treatment should be provided where possible and especially where infiltration is allowed. This should be done to avoid reducing the infiltration properties of the soil and to ensure heavily polluted accidental spills can be captured prior to infiltrating into the ground.

• Basins should usually be implemented as off line systems, but where they are on-line, an emergency spillway should be designed to safety convey exceedance flow.

• The maximum depth of water in the basin should not normally exceed 1.5m due to health and safety concerns. Adequate protection should also be provided around the pond to prevent the risk of motorists or other road users from accidently entering.

Benefits • Can cater for a wide range of

rainfall events.

• Detention basins can be used where infiltration is inappropriate or groundwater could become polluted as a result of contaminated runoff.

• Infiltration basins can contribute to groundwater recharge and reduce the need for further drainage.

• Simple to maintain, design and construct.

• Safe and visible capture of accidental spillages.

Limitations • Little reduction in runoff volume

where detention basins are used or infiltration rate into the soil is low.

• Detention depths are limited by the drain down time and health and safety considerations.

• Likely to be subject to heavy siltation, particularly in rural areas, and require regular maintenance.

• Vegetation , where applicable, will require regular watering and maintenance to be undertaken.

• Structural improvement is likely to be required where the soil is loose and not structurally stable.

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• Adequate access must be provided to the detention basin for inspection and maintenance, including for appropriate equipment.

• Side slopes should not usually exceed 1 in 4 unless site conditions and/or safety arrangements allow for steeper slopes.

• The bottom of the basin should be gently sloping towards the outlet to prevent standing water. It should have a gradient shallower than 1 in 100.

• A minimum length/width ratio of 2:1 is recommended. • A liner may be specified to prevent infiltration in unstable locations e.g. infiltration

rates < 50mm/hr or where groundwater could become contaminated by polluted runoff.

• If soil conditions are unsuitable and an embankment is required to impound the water, the embankment fill material should use inert natural soil that will not leach contaminants into the stored runoff.

• Rip rap or other scour protection should be used to dissipate energy of incoming flows. Velocities of incoming flows should be < 1 m/s.

• Outlet flows should be controlled via a control device. This could be a v-notch weir, orifice plate or vortex flow control device. The device should be built into a dyke or berm with easy access for maintenance.

• The bottom and side slopes should be structurally stable and where soil conditions do not allow for this, an embankment from improved material should be constructed.

• Consideration should be given to how flows in exceedance of the design event will be accommodated. If there is adequate space then consideration could be given to providing volume in the basin, or could include bypassing the basin and routing the flow safely a downstream.

• An emergency overflow should be designed to safely convey excess flows in emergency situations. This is will need to be designed specifically for the basin.

Figure 4.27: Typical cross section of a detention basin.

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4.5.10.4 Design Steps

Figure 4.28: Basin design steps

* a model is recommended for use as can provide a better understanding of the likely response of drainage features at the time design and during its lifetime.

Hand calculations for design purposes can be used where the drainage catchment is small and a model would be inappropriate for the size of the scheme. The storage required will be a function of inflow minus outflow over time and can be determined using the level pool routing technique.

4.5.10.5 Maintenance requirements

Model* the basin to determine the volume of storage required to retain runoff from a critical

design storm in line with Error! Reference

Can the basin be drained via piped outlet or infiltration within 48 hours? (use the model or Equation 4.12 if specifying an infiltration basin)

Use the key design consideration laid out in section 4.5.9.3 to ensure the basin is designed

inline with best practice

Is a basin suitable for the site location?

Is pre-treatment

of flow possible?

Is there likely to be adequate

space?

Are ground conditions suitable?

Can the basin be accommodated in the space available on site?

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Maintenance Type Action Frequency

Regular

If vegetated, watering of plants. Daily

Monitoring of sediment accumulation and vegetation, as appropriate.

Monthly.

Removal of sediment, litter and debris from the inlet and outlet.

As required/ after a storm event

If vegetated, maintenance of vegetation e.g. cutting, pruning.

As required/ at least annually

Remedial

Repair of eroded surfaces. As required/ after a storm event

Realignment of erosion protection.

As required/ after a storm event

Repair of inlet/ outlet. As required

Table 4.23: Maintenance requirements for basins

4.5.10.6 Construction Advice • Where a liner is to be used, care should be taken to avoid damage during

construction activities. • Construction should be timed to avoid periods of heavy rainfall to ensure stability

of the basin is maintained. • Care should be taken to ensure the slopes are stable and that material is unlikely

to fall away during storm conditions. • Side slopes should be kept shallower than 1:4 whilst constructing the pond to

avoid collapse during excavation. • The base of the basin should be carefully prepared to an even grade with no

significant undulations. • All excavation and levelling should be performed by equipment with tracks

exerting very light pressures to prevent compaction of the basin floor, which may reduce infiltration capacity.

• Construction of the infiltration basin should take place after the site has been stabilised in order to minimise the risk of premature failure of the basin.

4.6 Pollution control

4.7 Maintenance strategies All drainage systems require planned and re-active maintenance in order to perform in an efficient manner. The following section outlines the required steps required to develop an effective maintenance strategy. This should be a balanced strategy which takes into account both planned and reactive maintenance requirements.

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4.7.1 Planned maintenance Planned maintenance for highway drainage takes into account activities which will ensure the drainage system is operating when it’s needed. In Qatar rain events are in-frequent and are usually not severe. However the highway designer needs to consider worse case events. One of the major problems of maintenance is blown sand into the drainage system. Catch-pits and vertical vortex spinners should be employed to remove grit from the highway drainage system. (See Figure 4.29). From a planned maintenance perspective maintenance activities should be increased before the on-set of the rainy period. Catch pits and pipe systems should be de-silted to ensure the free passage of disposable water.

[Consider detailing maintenance requirements when out of season rain events are forecast]

Figure 4.29: Typical vortex grit remover

4.7.2 Re-active Maintenance Re-active maintenance is non-planned activities usually associated with blockages in highway drainage systems. It is important to optimise this type of activity to a minimum to avoid maintenance budget over-spend. Some re-active maintenance will always be necessary and so it is important to accommodate this, especially within the short rainy season.

CEO Highway Maintenance Section are the responsible authority for the maintenance of the highway drainage system, including EFA's and storage areas not in the Trunk Storm Sewer System.

CEO Drainage Division is the responsible authority for maintenance of the Trunk Storm Sewer System.

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5 Subsurface Drainage

5.1 Introduction Historically sub-surface drainage was not detailed in Qatar as it was not considered to be a problem, with low lying areas being filled prior to construction to raise them above the groundwater table. However, with the rapid urbanization of Qatar in recent years, groundwater levels have been noted as rising significantly. As such it is essential for ground investigations to be undertaken to establish the extent of sub-surface drainage necessary for new roads throughout Qatar, but especially within the urban environment.

Sub-surface drainage is provided to allow the removal of any water that permeates through the pavement layers, and also to control groundwater where it is sufficiently high so as to have a negative effect on the pavement design life.

Sub-surface drainage can take a number of forms but is typically longitudinal drains located at the low edges of the road pavement, which serve to drain the pavement layers and the pavement foundation as well as controlling ingress of water from road verges. Adequate drainage of these layers and of formation and sub-formation can be achieved by the shaping of each so as to direct flows to the sub-surface drainage in the verge or median, and to prevent the creation of low points elsewhere for water to collect.

There are five primary types of sub-surface drainage which include fin drains, narrow filter drains, filter drains (essentially a large narrow filter drain), combined carrier filter drains, and drainage blankets. This section focuses on the first four; the design of drainage blankets should be undertaken in close consultation with a geotechnical engineer and approval sought from the overseeing organisation in advance of developing such a solution. It is likely that drainage blankets will be required in locations that have high groundwater levels and / or are in deep cuttings.

The following measures are key factors when developing the design of sub-surface drainage;

• Slope the formation to drain away from the carriageway to the verge or median. • Avoid steps in the formation that could lead to water concentration points. • Keep planting areas separated from the pavement construction to prevent

moisture transfer. • Ensure planting area watering is effectively controlled to prevent over watering. • Utilise surface water drainage details that will reduce the chance of accidental

damage and maintenance problems. • Ensure soakaways do not introduce water to the pavement construction.

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5.2 Subsurface drainage methods

5.2.1 General design considerations Due to seasonal variations groundwater will vary overtime in relation to rainfall, underlying strata permeability, gravity, capillary action and proximity to the coast. In order to develop a suitable approach to sub-surface drainage, it is essential to gain a thorough understanding of ground conditions by undertaking robust ground investigations.

If ground conditions are not established and as a result suitable sub-surface drainage proposals are not developed it is likely that failure of the road structure will occur before the design life is reached. Furthermore, it is likely that any corrective action at a later stage will be more costly as full reconstruction could be required. By allowing the build up of water in the pavement layers, formation and sub-formation, pore water pressure will increase and can result in the pavement being weakened by;

• Washout of fines by movement of pore water • Increase in salt content in pavement layers resulting in swelling due to capillary

rise when significant concentrations of salt are present in the underlying material • Swelling of susceptible material followed by shrinkage or drying out

During the design of the road, pavement engineers will base their calculations on CBR values for the subgrade, these CBR values will be affected by any increase in groundwater levels. Where groundwater is allowed to rise unchecked by sub-surface drainage the bearing capacity of the formation and sub-formation will be diminished.

5.2.2 Considerations for fin drains and narrow filter drains: Fin drains and narrow filter drains should be installed at a minimum depth determined by the nominal pipe diameter (DN) + 50mm to invert beneath sub-formation level, or 600mm to invert below formation level. If no capping layer is present the drains should be laid to the greater of the two depths. If groundwater is within 300mm of the sub-formation level these minimum depths will be insufficient, and the fin or narrow filter drain should be installed at a greater depth. A geotechnical engineer should advise on the design depth of such drainage. Where large quantities of groundwater are encountered a filter drain is likely to provide a better solution than either the fin or narrow filter drain options.

As the topography of Qatar is typically gently undulating a further consideration is to ensure that sub-surface drainage can discharge from all low points to a suitable outfall.

STANDARD DETAIL OF FIN DRAINS AND NFDS TO BE BASED UPON UK DMRB TYPES 5-10.

5.2.3 Considerations for combined carrier filter drains: It is usual practice to keep surface water drainage separate from sub-surface drainage to attempt to prevent large volumes of water entering into the road foundation and pavement layers. Where this approach is not viable such as in cuttings an alternative philosophy to use combined carrier filter drains is acceptable.

The use of combined filter drains offers a number of benefits including;

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• removal of groundwater to a greater depth than possible using fin or narrow filter drains due to their comparatively large hydraulic capacity

• simpler construction than having to lay both a carrier drain and fin or narrow filter drains

• easier access for inspection and maintenance than possible with both fin and narrow filter drains

Where combined carrier filter drains are to be constructed they should consist of half perforated or slotted pipes laid with their perforations or slots face up, with sealed joints to minimise the loss of water through the trench base. The base of the trench should also be lined with impermeable membrane up to pipe soffit to reduce loss of water to the soil below that is likely to be dry otherwise. The trench is backfilled with permeable material which is wrapped in a geotextile to prevent ingress of fines.

As already mentioned it is viewed as best practice where possible to keep surface and sub-surface drainage separate due to the potential problems in performance with the combined approach. It has been noted in the UK that issues with stone scatter, pavement failure, earthworks failure and maintenance problems have arisen where the combined approach has been adopted. However, these issues can be mitigated by appropriate maintenance of the system, and implementing design measures to control stone scatter as detailed below;

• Either spraying the exposed filter medium with bitumen, or using a bitumen bonded filter material for the top 200mm of the trench.

• Use of geogrids to reinforce the top layer of filter medium • Use of lightweight aggregate for filter material in the top 200mm of the trench

A further design consideration with the combined approach is where attenuation is required at the downstream end of a system to meet discharge rate constraints. Such attenuation requirements can result in the surcharging of the surface water drainage network, and where this is formed of a combined system any surcharging of the carrier pipe can cause backflow into the filter medium. Therefore where combined systems are proposed the designer must demonstrate that surcharging of pipe due to downstream flow control will not occur.

5.2.4 Special considerations for coastal areas In tidal coastal areas, ‘sabkha’ is likely to be present as an indication of a high groundwater table. In these situations capillary rise of up to 1.0m can draw saline water up to the road formation level, depositing salt lenses and increasing pore pressure.

This is generally prevented by:

• Construction of high embankments • Introduction of a granular capillary break layer below the formation.

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6 Appendix A

Current climatic condition design IDF curves

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6.1 Doha

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6.2 Al Ruwais

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6.3 Al Saliyah

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6.4 Dokhan

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6.5 Abu Samra

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6.6 Umm Bab

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7 Appendix B

Future climatic condition (2070 – 2099) design IDF curves

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7.1 Doha

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7.2 Al Ruwais

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7.3 Al Saliyah

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7.4 Dokhan

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7.5 Abu Samra

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7.6 Umm Bab

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