Pluvial Flooding and Surface Water Management · What is Surface Water Flooding? Combination of...

5th EWA Brussels Conference, 6th November 2009European Water Management and Implementation of the

Floods Directive

Pluvial Floodingand Surface Water Management

Ronnie FalconerSenior Consultant Jacobs Engineering UK Ltd

Member of the EWA European Technical and Scientific CommitteeChairman of the EWA Sustainable Flood Management Working Group

Overview

• The problem• Legislative drivers• SEPA - Improved Understanding of Pluvial Flood Risk in

Scotland• DEFRA – Richmond and Kingston First Edition Surface Water

Management Plan• Common outcomes and lessons for future SWMPs• Outcomes from the EWA Expert Meeting on Pluvial Flooding

Photograph of the Boscastle floodcourtesy the Environment Agencyand Marc Hill.

Elmvale Row - courtesy ScottishWater

Glasgow : August 2002

Shettleston Pluvial Flood Aug 2002Images courtesy Scottish Water

Hull and Sheffield : Summer 2007

London area and Berkshire : July 2007

Images courtesy BBC News Website

Victoria Station Kensal Green

Croydon

Berkshire

East Belfast : June 2007On 12th June 2007, a total of 98.3mm of rain fell in 1 hour in the East andSouth Belfast catchments.

This resulted in both fluvial flash flooding and pluvial flooding which causedmajor disruption throughout Belfast with over 400 properties affected.

• Higher energy storm events with near ‘tropical’ rainfall intensitiesproducing ‘pluvial’ surface water flooding and flash floods.• EU White Paper on Adapting to Climate Change – Annual precipitationcould increase more than 40% in some parts of northern Europe by 2100.• UK Defra Guidance (Oct 2006) – 30% increase in peak rainfall intensity2085 -2115.• Potentially >100% increase in properties affected and 2 - 20 timesincrease in flood damage

Climate Change

What is Surface Water Flooding?Combination of pluvial flooding, sewer flooding, flooding fromsmall urban watercourses, and groundwater flooding.

What is Pluvial Flooding?“ Flooding as a result of rainfall when water ponds or flowsover the ground before it enters a natural or man-madedrainage system or watercourse, or when it cannot enterbecause the system is already full to capacity.”(SEPA ‘Improved Understanding of Pluvial Flood Risk in Scotland’ June 2009)

- Surface and below ground drainage systems often totallyoverwhelmed.

Pluvial Flooding Characteristics� Usually associated with short duration (up to 3hrs) high intensity

rainfall exceeding 20mm/hr but can also occur with lower intensity rainfall(~ 10mm/hr) over longer periods or melting snow and can be worse whenthe ground is saturated, frozen, developed or otherwise has lowpermeability.(SEPA ‘Improved Understanding of Pluvial Flood Risk in Scotland’ June 2009)

Difference between Pluvial and Flash Flooding� Same extreme rainfall.� Flash flooding usually arises from a watercourse.� Pluvial flooding arises from rainfall runoff before it reaches a watercourse

or drainage system.

Legislative DriversFloods Directive– all sources of flooding

Boscastle flood courtesy the Environment Agency andMarc Hill. Isla flood (top) courtesy SEPA

• Fluvial Floods• Coastal Flooding• Pluvial and Flash Floods• Sewer Derived• Tsunami• Dambreak• Canal Breach

Floods Directive• Establish a framework for the assessment and management offlood risks• Aim is to reduce/mitigate the adverse consequences of flooding on:

� human health,� the environment,� cultural heritage and� economic activity

2009 2010 2011 2012 2013 2014 2015 2016 …….. 2021

PreliminaryFlood Risk

Assessment

Flood Riskand Hazard

Maps

FloodManagement

Plans

Transposition

PRFA

Mapping

Planning

Implementation andreview every 6 years

Floods Directive – Key dates

Flood Risk Management (Scotland) Act 2009• Transposes Floods Directive, reforms existing flood legislation andclarifies roles and responsibilities.• SEPA have strategic overview role and responsibility for Scotland-wide flood risk assessment, mapping and flood risk management planpreparation.• Local authorities responsible for Local Flood Risk Management Plansincluding surface water management.• Creates a framework for sustainable catchment focused approachesto flood risk management.

Act aims to deliver Sustainable Flood Management

England and Wales - Flood and Water Management Bill……… or equivalent legislation.Aims to …….� provide updated legislation which reflects current and future

approaches to flood and coastal erosion risk management andorganisational structures

� implement a number of recommendations made by the Pitt Reviewinto the Summer 2007 flooding

� facilitate a strategy for adapting to the increased flood and coastalerosion risk through climate change

� implement the Floods Directive

greater security……better service….future sustainability

Flood and Water Management Bill - DeliveryRegional Flood &

Coastal CommitteesAdvise the EnvironmentAgency on how Flood andCoastal Risk Managementshould be pursued in theRegions.

Environment AgencyContinue delivery and operation role for flood risk managementworks on main rivers and the sea and provide flood warnings forall sources of flood risk and…..• Produce National Strategy for Flood and Coastal RiskManagement – full strategic overview role for all sources offlooding.• Support lead local authorities and others in Flood and CoastalRisk Management by providing information and guidance on fulfillingtheir roles.

Lead (County or Unitary) Local AuthoritiesLeadership role for local flood risk management:• Setting local strategy for local flood risk management.• Leadership and accountability for management of local flood risk fromordinary watercourses, surface run-off and groundwater.• Production of local flood risk assessments, maps and plans includingan asset register, Surface Water Management Plans andStrategic Flood Risk Assessments.• Adopting and maintaining new build SUDS.

District Authorities,Internal Drainage Boards,Highways bodies, Water

Companies• Undertake flood risk functionsin accordance with local andnational strategies.• Cooperate and shareinformation.

ADVICE

SUPPORT & GUIDANCE

LEADERSHIP

SEPA StudyImproved Understanding ofPluvial Flood Risk in ScotlandUndertaken with HR Wallingford and Met OfficeRemit:� Collection of flood hazard information

on past pluvial flood events in Scotland.� Review of the processes involved in pluvial flooding.� Development of screening tool to identify pluvial flood

susceptibility.� Application of the screening tool – Edinburgh used as pilot study

area.� Production of technical guidance for further, more detailed pluvial

modelling.

SEPA Study - Pluvial Flood Event Database� Past flood events and facility to add data for future events.� >100 events already on database including recent Edinburgh events.

SEPA Study – Review of Pluvial Flooding Processes� Rainfall

� thresholds� spatial extent

– when >5 km2

� storm movement– direction - speed

� climate change

Duration 15m 30m 1hr 2hr 3hr 4hr 6hr

Trigger (mm) 15 20 30 35 40 45 50

Pluvial Flooding Processes� Topography and infrastructure

� flowpaths and ponding� steepness� barriers to flow

� Permeability and underlying ground� runoff threshold� rate of dissipation.

� Antecedent soil conditions� wetness� frozen ground.

Pluvial Flooding Processes� Surface and underground Drainage Capacity

� totally overwhelmed?� contribution? or drawdown?� dynamic exchange� tide-locking

Pluvial Flooding Processes� Doorway threshold levels

� can be a critical factor.

� Sensitivity of Land Use� permeability� damage consequences.

� Interaction with other types of flooding� fluvial� flash flooding from watercourses� tidal� sewerage� groundwater.

Screening Tools – ‘Dry’� GIS based Contour Polygon Screening – simple high level

determination of ponding depressions.� GIS Based ‘rolling ball’ Flowpath Generation – bare earth

DTM with buildings superimposed.

Screening Tools – ‘Wet’� Rapid Flood Spreading Model (RFSM) pluvial mapping:

� Rainfall generation based on FEH – total rainfall depth for givenprobability and duration e.g. 1:200 3hr event on a 1km gridinterpolated to a 2m grid then fitted to FEH storm profile.

� Final depth calculation – impact zones (IZs) – filling/spilling processuntil equilibrium reached – final level and volume in each IZ.

� Maximum velocity calculation – estimated from maximum dischargebetween IZs.

� Outputs depth and velocity maps – shallow depths and lowvelocities screened out.

Pilot Study Application� Trial areas - Edinburgh Central and

Leith - Edinburgh West and Rural.� Followed PEEPs process.� Incident records from June/July

2007and August 2008.� Desktop review of mapping.� Site inspections to incident locations

and mapped hotspots.� 35 locations – 25 Central and Leith –

11 West and Rural.� Verification of mapping.� Preliminary risk assessment.

Mapping:1:200 3hrRFSM andflowpaths

EdinburghCentralandLeith

Mapping:1:200 3hrRFSMFinal depths

EdinburghWest and Rural

Mapping:RFSM1:200 3hrVelocities

EdinburghCentral

Overall Hazard RatingHazard Rating = ponded volume (m3) / area (hectare)

Scoring Matrix for Preliminary Risk Assessment

SevereHighModerateLowNot SignificantOverall Preliminary Risk Rating>109 to 106 to 83 to 50 to 2Total Score

43210Score

Most below ground levelMost at ground level.Some below ground

levelMost 0m to 0.2m

above ground levelMost above 0.2m but

some 0m to 0.2mabove ground level

Most above 0.2mabove ground levelDescription

Doorway ThresholdLevels

43210Score

Critical infrastructurepresent.

Critical transportationlinks present.

Basement flats present.

Central urban or towncentre residential /commercial / retail /

industrial areas whereflooding would havehigh consequence.

Suburban residential /commercial / retail /

industrial areas whereflooding would have

moderateconsequence.

Parkland, openground or farmland

where flooding wouldhave some

consequence.

Open areas that canbe flooded without

significantconsequence.

Description

Sensitivity of Land Use43210Score

Velocity more than 1m/s(generally steeply

sloping terrain) anddepth of flow more than

0.5m.

Velocity more than1m/s (generally steeply

sloping terrain)

Velocity 0.5 -1m/s(generally moderately

sloping terrain)

Velocity 0.1 to 0.5m/s(generally gentlysloping terrain)

Still Water or velocity< 0.1m/s

(generally flat terrain)Description

Velocity of Flow32100Score

Yes. Depth >2mYes. Depth 1-2mYes. Depth 0.5-1mYes. Depth <0.5mNo or Flowpath only.DescriptionFlowpath feedstopographicdepression ?

43210Score

More than 100properties potentially at

risk. Widespread(>10ha)

Up to 100 propertiespotentially at risk.

Extensive (up to 10ha)

Up to 10 propertiespotentially at risk

Moderate (up to 1ha)

1 property potentiallyat risk

Localised <0.1ha

No propertiespotentially at riskLocalised <0.1ha

DescriptionProperties at Risk andExtent of Flooded Area

43210Score>2m1m to 2m0.5m to 1m0.2m to 0.5m<0.2mDescriptionDepth of Flooding

Very HighHighModerateLowVery LowHazard LevelAttribute

Site Inspection Record

Preliminary Risk AssessmentEdinburgh Central and Leith

� Steep flowpaths, criticalinfrastructure andbuildings, and basementflats.

� Justifies furtherassessment.

Pilot Study Application – Specific Observations� Rail and road embankments can

act as a barrier – ponding atunderpasses.

� Critical infrastructure andbuildings of national importance.

� Doorway thresholds – often<200mm – access to public buildingsand retail premises.

� Basement flats a particular issue.� Flowpaths – often steep –

sometimes along frequently usedstreets and confined narrow streets.

Hybrid Composite of techniques best……..CPS LIDAR with flowpaths…..can highlight detail.+

RFSM 1:200 3hr eventretaining flowpaths.

Indicative max velocities.

SEPA Pilot Study Outcomes� Urban Areas - hybrid approach – derivation of final depth in ponding

areas using ‘wet’ approach but overlain with flowpaths derived fromGIS rolling ball analysis. ‘Dry’ methods with LIDAR can provide detail.

� Value of site inspections proven – verification / source of flooding /land use / preliminary risk assessment / mitigation measures.

� Desktop review – mapping checks – augment with web-basedimagery - maximises benefit from targeted site inspections.

� Hazard Index – useful indication of relative susceptibility but based onstandard event.

� Preliminary Risk Assessment based on site inspections - convenient,easily applied and rapid method of assessing potential severity ofpluvial risk.

Why SWMPs?� Overall purpose is to identify sustainable management

responses to surface water flooding.� PPS25 introduced the idea.� Making Space for Water identified the need for integrated

urban drainage management approaches in complex urbanareas.

� Pitt Review (rec. 18) recommended that local authorities shoulduse SWMPs as basis for managing all local flood risk.

� Draft Flood & Water Bill places responsibility for SWMPs underthe lead local authority.

Surface Water Management Plans

� Preparation� Risk assessment� Options appraisal� Implementation &

review

Draft DEFRA Guidance

Key SWMP Elements

First Edition SWMPs� DEFRA produced ‘living draft’ SWMP guidelines� London Borough of Richmond and Royal Borough of Kingston one of

six local authority groups to test and refine the guidance ( togetherwith Gloucestershire, Hull, Leeds, Warrington and Thatcham )

� Key criteria:� history of surface water flooding� benefit from integrated drainage approach� partners agree to share information

6 months£50k1st Edition Proposed

9-18 monthsTimescales> £100kFunding

What is a Surface Water Management Plan?� “a framework through which key local partners … work together to

understand the causes of surface water flooding and agree themost cost effective way of managing surface water flood risk”.

� Overall output envisaged to be an Action Plan to deliver the surfacewater management strategy.

� Elements can include:− Location, ownership, status and purpose of surface water

drainage infrastructure− Current and future risks of surface water flooding, including

maps− Options appraisal considering feasibility, cost, effectiveness and

public acceptability of mitigation measures− Identification of a preferred option in each ‘at risk’ area

Richmond and Kingston methodology included elementsfrom Defra RF5 Study approach to Pluvial Flood Warning

Promoted the concept of bringing forecasting and mapping together…..• Met Office provide targeted forecasts as Extreme Rainfall Alerts for high

intensity rainfall to act as trigger for action in identified vulnerable areas.• Improve preparedness through Pluvial Extreme Event Planning system –

PEEPs (identifying vulnerable areas based on rapid screening, site visits,prioritising actions).

PreparationSelected approaches:

• Risk based tiered approach due to large (~100km2)geographical area.

• Initial screening followed by desktop review to identifysusceptible areas based on:�historic records of flooding�EA Areas Susceptible to Surface Water Flooding maps�rapid GIS screening

• Site inspections and preliminary risk assessment to identifyhighest risk areas.

• More detailed hydraulic modelling in identified high risk areas.�buildings stamped onto bare earth DEM�range of rainfall scenarios�removal of a proportion of rainfall to underground system

Screening Tools‘Dry’ method:� GIS based Contour Polygon Screening – simple high level

determination of ponding depressions.� GIS Based ‘rolling ball’ Flowpath Generation

– bare earth DEM with buildings superimposed.

‘Wet’ method:� Rainfall generation based on FEH – total

rainfall depth for givenprobability and duration.

� Final depth calculation.

Overall Hazard RatingNational Mapping (%of total land area)Richmond: 16% Less 10% Intermediate 1% MoreKingston: 11% Less 9% Intermediate 2% More

GIS Based Mapping

Risk Assessment� Site inspections for preliminary risk assessment:

− 49 locations in Richmond and Kingston visited over 4 days− simple scoring system to rate perceived overall risk

Inspection Schedule : Richmond

Risk Assessment

� Summary results of site inspections:- table shows results as percentage of sites visited, not land area.

14%7%28%41%24%Kingston (29)10%5%20%50%25%Richmond (20)

-> 108 - 105 - 70 - 4Points Score

PotentialRisk to

Life

SevereRisk

High RiskModerateRisk

Low orInsignificant

Risk

Risk Assessment – Specific Observations

Doorway thresholds

Pluvial / fluvial interaction Schools : over-reporting?

Railway and road underpasses

Some very flat areas.

Risk Assessment – Specific Observations

Steep flowpaths

High value property

Basement flats

Strawberry Hill Drain

Acre Road

Worcester Park

Teddington Latchmere Road

Kingsway

River LaneMarlow Crescent

Kingston Road

Risk Assessment – Critical Areas

Risk Assessment – Critical Areas� Approach taken to more detailed risk assessment:

� 2D hydraulic model (TuFLOW) of specific locations, built usingDEM with buildings ‘stamped’ on

� determine rainfall input forvarious scenarios (e.g. 4.5hrduration event with a 1:200chance of occurring)

� assume proportion of rainfallremoved by undergroundsystems (information providedby Thames Water)

� model depths and velocitiesof excess water at the surface

Detailed Risk Assessment : Acre Road to Petersham

Acre Road to PetershamMaximum velocities

Damage Assessment:12,400 propertiesAnnual Average Damages £3m

Source Control and SUDS:� Fringe interception: control runoff from urban/rural

fringe (e.g. Richmond Park)� Detention basins: large storage areas (normally

dry) e.g. car parks, recreational areas� Ponds and wetlands: incorporate in existing open

spaces (permanent standing water)� Swales: surface water runoff channels / storage� Permeable paving: permits infiltration into the

ground and/or temporary storage (depends onpermeability of underlying strata – poor if clay)

� Green roofs: roofs covered with vegetation tocontrol runoff - large flat roofs best e.g. industrial /commercial buildings or schools

� Rainwater harvesting: temporary storage e.g.household water butts

Options

Design for exceedance:� Surface flow routes: divert or channel water

flow along designated routes using roadprofiling, kerb height, speed bumps etc.

� Storage schemes: designated flood storagein open spaces – often involves landscapingand control works

� Threshold raising: raising doorwaythresholds, particularly along identifiedflowpaths

Options

Increasing capacity:� Increase network capacity: adding

more and/or larger sewers� Culvert daylighting: opening up

culverted channels� Improve channel capacity: urban

watercourse widening/deepening� Improve floodplain storage: usually

associated with fluvial measures� Improve gullies: regular clearance� Improve maintenance: checking/

clearance of trash screens etc� Obstruction removal: clearance of any

flow obstructions e.g. trees� Desilting: clearance of silted

watercourses

Options

Non-structural measures:� Flood warning: raise awareness and new forecasting

techniques� Development control: control development within

identified risk areas� Building control: specific measures including basement

flats� Land management: natural measures to slow runoff e.g.

contour ditching, forestation� Building resilience and resistance (retrofit): various

improvements e.g. raised wiring, water resilient floors andwalls; door and vent seals / boards / guards to keep waterout (undertake during post flooding repairs or proactivelyin identified risk areas)

� Building resilience and resistance (newdevelopment): as above but ensure for new developmentin identified susceptible areas

Options

Option Evaluation� Options workshop held to agree simple MCA scoring of

suggested options to identify a feasible subset.� Technical feasibility assessed from site inspections and

collective experience from workshop.

Option EvaluationAcre Road toPetershamSurface Storage Options

Options Appraisal

Summary – Defra First Edition SWMPs� Overall purpose is to identify sustainable

management responses to surface water flooding.� Richmond and Kingston are one of six local authority

groups to test the living guidance.� SWMP phases:

� Preparation: establish partnership, collate data and selectapproach.

� Risk assessment: identify high risk areas throughscreening, targeted site inspections and detailed modelling.

� Options appraisal: identify potential measures through siteinspections and workshop to refine and select options.

� Implementation & review: First Edition SWMP report(August 2009).

Common outcomes and lessons for future SWMPs� Screening – hybrid approach using composite of wet and dry

techniques – overlay flowpaths.� Value of Site Inspections – verification / source of flooding /

land use / preliminary risk assessment / mitigation measures.� Hazard Index – useful indication of relative susceptibility.� Preliminary Risk Assessment based on site inspections -

convenient, easily applied and rapid method of assessingpotential severity of surface water flood risk.

� Option Evaluation Matrix coupled with StakeholderWorkshop – MCA scoring approach to identify mostappropriate options.

Common Outcomes- Staged Approach

SEPA Technical Guidance:� Level of assessment should

be proportionate toperceived level of pluvial floodrisk.

� Tiered approachcommencing with screening- then targeting progressivelymore refined modelling andassessment at the areas ofgreatest assessed risk.

EWA Expert Meeting on Pluvial Flooding28 October 2008 - Brussels

Objective: To prepare a situation paper on Pluvial Flooding in Europewhich can be presented to WGF and to the proposed WGF Workshop onFlash and Pluvial Flooding planned for May 2010 in Cagliari, Sardinia.Topics Considered� Definition and Understanding of Relevant Processes.� Identifying the Problem - Extent of Pluvial Flood Risk across Europe

– current and future.� Approaches to Mapping and Risk Assessment.� Potential Mitigation Measures.� Aspects Requiring Guidance and Further Research.

EWA Expert Meeting on Pluvial Flooding28 October 2008 - Brussels

Expert Attendance (14)Member States (7)� Western Seaboard - Ireland, UK� Central Europe - Poland, Hungary (Danube River Basin Authority)� Southern Europe - Italy, PortugalEuropean Research Programmes (3)� FloodResilienCity� IMPRINTSEuropean Commission and EC Joint Research Centre (2)European Water Association (2)

Outcomes from EWA Expert Meeting on Pluvial FloodingDefinitionsPluvial FloodingPluvial Flooding is flooding as a result of heavy rainfall when water which

does not infiltrate the ground ponds in natural or artificial hollows or flowsover the ground as overland flow, before it enters a natural or man-madedrainage system or watercourse …… or when it cannot enter because thesystem is already full to capacity.

Usually associated with short duration high intensity rainfall but can alsooccur with lower intensity rainfall over longer periods, or melting snow, andcan be worse when the ground is saturated, frozen, compacted, developedor otherwise has low permeability. High velocity overland flow anddeep ponding and pose a particular hazard.

Pluvial Flooding is in some countries referred to as Excess Water Floodingand in this case includes Groundwater Flooding.

Outcomes from EWA Expert Meeting on Pluvial FloodingDefinitionsFlash FloodingA Flash Flood is rapid flooding in urban or rural areas arising from a small

river, stream or watercourse. It is caused by high intensity heavyrainfall and is often associated with thunderstorms. Its time to peakgenerally ranges from a few minutes to a few hours.

It can occur with debris flow and the rapid high velocity nature of theflooding can be a hazard to life.

Outcomes from EWA Expert Meeting on Pluvial FloodingDefinitionsSurface Water FloodingSurface Water Flooding arises from a combination of flooding from surface

runoff, sewers, drains, small watercourses and ditches that occurs duringheavy rainfall. The impact is often greatest in urban areas. It includes:- Pluvial flooding (as defined above)- Sewer or drainage system flooding (when the capacity of undergroundsystems is exceeded due to heavy rainfall – not due to blockage orstructural failure of the system)- Flooding from small open-channel or culverted watercourses(usually in urban areas)- Flooding from groundwater

Outcomes from EWA Expert Meeting on Pluvial FloodingExtent of Pluvial Flood Risk across Europe� “Pluvial flooding can happen anywhere at any time”� Climate change likely to make worse. Land-use also a factor.� Is there a greater risk in western, northern and central

Europe?..... but areas of increased risk elsewhere also.� Severity of impact? Do flash floods have a greater impact?� Questionnaire to determine extent of problem

across Member States- definitions need to be clear- do differences in organisational

responsibilities affect understandingof the problem?

Outcomes from EWA Expert Meeting on Pluvial FloodingApproaches to Mapping and Risk Assessment� Evidence base will be required to demonstrate assessed level of

significance.� Observed incidents of pluvial flooding very valuable – use as much as

possible.� Top down risk based approach with progressively more detailed

assessment where justified.� Fixed methodologies should not be

imposed on Member States –provide good practice examples.

� Methodologies evolving rapidly- provide guidance includingexamples of application.

Outcomes from EWA Expert Meeting on Pluvial FloodingPotential Mitigation Measures� Solutions likely to comprise of a mix of measures:

- Source control and overland flow routing/storage- Improved conveyance (larger sewers, drains and urban channels)- Resistance, resilience and non-structural measures

� Adaptable solutions – climate change uncertainty.� Gather examples to prepare a catalogue of good practice and win-win

measures. Catalogue existing guidance documentation.� Include examples of good planning control – national legislation could assist.� Public education is important – source control measures.

Outcomes from EWA Expert Meeting on Pluvial FloodingResearch Initiatives� Categorisation of ‘magnitudes’ of flooding rather than specific flood

probabilities.� Categorisation of sources of flooding also helpful.� Some extensive records of past flooding (for example Excess Water Flooding

in Hungary) – need to recognise value and gather.� Harmonising land use and rural development.

Outcomes from EWA Expert Meeting on Pluvial Flooding

Way Forward� Questionnaire to determine extent of the problem.� Informal ‘situation paper’ for EC Working Group F and WGF

Workshop on Flash and Pluvial Flooding in Sardinia 26-28 May2010.

� Develop key issues at WGF Workshop in Sardinia.� Identify needs for further guidance and research.

Questions?

Thank you or yourattention

Ronnie Falconer : Senior Consultant : Jacobs : ronnie.falconer@jacobs.com : www.jacobs.com