RESEARCH REPORT FOR FLOOD PROTECTION STRUCTURES · YEAR 2018 Researched By: Thinley Choden, Principal Engineer, FEMD, DES, MoWHS RESEARCH REPORT FOR FLOOD PROTECTION STRUCTURES FLOOD

YEAR 2018

Researched By: Thinley Choden, Principal Engineer, FEMD, DES, MoWHS

RESEARCH REPORT FOR

FLOOD PROTECTION

STRUCTURES

FLOOD ENGINEERING AND MANAGEMENT DIVISION,

DEPARTMENT OF ENGINEERING SERVICES

MINISTRY OF WORKS AND HUMAN SETTLEMENT

ACKNOWLEDGEMENT

Flood Engineering and Management Division under Department of Engineering Services,

Ministry of Works and Human Settlement have taken immense initiative and efforts in carrying

out this study to assess the effectiveness of the past interventions carried out for reducing the

vulnerability of communities to flooding in Bhutan. This is done through site investigations,

present conditions of the existing structures and interactions amongst the engineers. Further, the

study has also emphasized on the structural and non-structural measures used in other countries

for flood management. The Division has successfully completed this project on time.

The Flood Engineering and Management Division would also like to acknowledge and thank all

those who have contributed and willingly helped us with their abilities towards carrying out this

study.

ACRONYMS:

FEMD Flood Engineering and Management Division.

HEC-RAS

The Hydrologic Engineering Center, River Analysis System is a computer

program that models the hydraulics of water flow through natural rivers

and other channels. The program is one-dimensional, meaning that there is

no direct modeling of the hydraulic effect of cross section shape changes,

bends, and other two- and three-dimensional aspects of flow. The program

was developed by the US Department of Defense, Army Corps of

Engineers in order to manage the rivers, harbors, and other public works

under their jurisdiction; it has found wide acceptance by many others since

its public release in 1995.

GIS Geographical Information System is a computer based method for

analyzing geographical information and maps.

FHM Flood Hazard Map.

PFHA Preliminary Flood Hazard Assessment.

AB Articulating Block

MoWHS Ministry of Works and Human Settlement.

DHS Department of Human Settlement.

RRM Random Rubble Masonry

PCC Plain Cement Concrete

RCC Reinforced Cement Concrete

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CONTENTS

1. INTRODUCTION ........................................................................................................................... - 1 -

1.1 Background .......................................................................................................................... - 1 -

1.2 Objective of the Study ......................................................................................................... - 2 -

1.3 Problem Statement .............................................................................................................. - 2 -

2. PAST FLOOD INTERVENTIONS IN BHUTAN ............................................................................. - 3 -

2.1 Structural Measures .................................................................................................................. - 3 -

2.1.1 Gabion Walls ..................................................................................................................... - 3 -

2.1.1.1 Reasons for failure of Gabion walls in the past .......................................................... - 5 -

2.1.1.2 Methods to enhance the life of the Gabion walls ...................................................... - 7 -

2.1.2 Random Rubble Masonry (RRM) Walls .............................................................................. - 7 -

2.1.2.1 Reasons for failure of Random Rubble Masonry (RRM) walls in the past ............... - 9 -

2.1.2.2 Methods to enhance the life of Random Rubble Masonry (RRM) walls ................ - 10 -

2.1.3 Plain Cement Concrete (PCC) Walls .................................................................................. - 11 -

2.1.3.1 Reasons for failure of Plain Cement Concrete (PCC) walls in the past ................... - 12 -

2.1.3.2 Methods to enhance the life of Plain Cement Concrete (PCC) walls ..................... - 12 -

2.1.4 Reinforced Cement Concrete (RCC) Walls ........................................................................ - 13 -

2.1.4.1 Reasons for failure of Reinforced Cement Concrete (RCC) walls in the past ......... - 14 -

2.1.4.2 Methods to enhance the life of Reinforced Cement Concrete (RCC) walls. .......... - 15 -

2.1.5 Gabion Revetment ............................................................................................................. - 16 -

2.1.5.1 Reasons for failure of gabion revetment in the past................................................. - 17 -

2.1.5.2 Methods to enhance the life of gabion revetment ................................................... - 19 -

2.1.6 Plain Cement Concrete (PCC) Revetment ......................................................................... - 20 -

2.1.6.1 Reasons for failure of Plain Cement Concrete (PCC) revetment in the past .......... - 21 -

2.1.6.2 Methods to enhance the life of Plain Cement Concrete (PCC) revetment ........... - 22 -

2.1.7 Articulating Block Mattresses ............................................................................................ - 23 -

2.1.7.1 Reasons for failure of Articulating Block Mattresses in the past ............................ - 24 -

2.1.7.2 Methods to enhance the life of Articulating Block Mattresses ...............................- 25 -

2.1.8 Detention Basin .................................................................................................................- 25 -

2.1.8.1 Reasons for failure of Detention Basin in the past .................................................. - 27 -

2.1.8.2 Methods to enhance the life of Detention Basin .................................................... - 28 -

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2.1.9 Embankment with gabion revetment ................................................................................. - 28 -

2.1.10 Embankment with gabion revetment and spurs ............................................................... - 30 -

2.1.11 RIP RAP ......................................................................................................................... - 32 -

2.1.12 Dredging ......................................................................................................................... - 33 -

2.1.13 Stone Spur ...................................................................................................................... - 34 -

2.1.14 Jetty Jacks/Porcupines .................................................................................................... - 35 -

3. FLOOD CONTROL MEASURES .................................................................................................. - 36 -

3.1 Structural Measures ................................................................................................................ - 36 -

3.1.1 Dams ............................................................................................................................... - 36 -

3.1.2 Sabo Dams ........................................................................................................................ - 37 -

3.1.3 Wooden revetment ............................................................................................................ - 39 -

3.1.4 Tetrapods ......................................................................................................................... - 40 -

3.1.5 Rock Spurs or Groynes ...................................................................................................... - 41 -

3.1.6 Jetty Jacks ........................................................................................................................ - 42 -

3.1.7 Retention Basin ................................................................................................................ - 43 -

3.1.8 Flood-proofing ................................................................................................................. - 44 -

3.2 Non-structural Measures ......................................................................................................... - 45 -

3.2.1 Restoring Natural River Functions ................................................................................... - 46 -

3.2.2 Flood Hazard Map/ Land use zoning ................................................................................ - 47 -

3.2.3 Afforestation .................................................................................................................... - 49 -

3.2.4 Relocation. ....................................................................................................................... - 50 -

3.2.5 Flood Warning System ...................................................................................................... - 51 -

3.2.6 Emergency preparedness, response and recovery .............................................................. - 51 -

3.2.7 Flood Management Plans ..................................................................................................- 52 -

4. RECOMMENDATIONS ................................................................................................................. - 53 -

5. LIMITATIONS .............................................................................................................................. - 54 -

6. REFERENCES .............................................................................................................................. - 55 -

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Figure 1: Gabion walls used as flood protection structure. ............................................................. - 4 -

Figure 2: Stolen gabion wire. ........................................................................................................... - 5 -

Figure 3: Scouring at the base. ......................................................................................................... - 5 -

Figure 4: Missing stone filling in the gabion box. ........................................................................... - 6 -

Figure 5: Irregular shaped stones..................................................................................................... - 6 -

Figure 6: Hand woven gabion boxes. .............................................................................................. - 6 -

Figure 7: Small size stone fillings. .................................................................................................... - 7 -

Figure 8: RRM walls in (a) Trashigang (b) Samdrupjongkhar. ....................................................... - 8 -

Figure 9: Bulging of the wall due to hydrostatic pressure. ............................................................. - 9 -

Figure 10: Scouring from the base. ................................................................................................... - 9 -

Figure 11: Structurally weak (stagnation of water). ........................................................................ - 10 -

Figure 12: Insufficient weep holes. .................................................................................................. - 10 -

Figure 13: Bonding stone in RRM walls. .......................................................................................... - 11 -

Figure 14: Plain cement concrete walls used as flood protection structure along Dungsumchu. - 11 -

Figure 15: Failure due to scouring. .................................................................................................. - 12 -

Figure 16: Drainage system to avoid hydrostatic pressure. ........................................................... - 13 -

Figure 17: Reinforced cement concrete walls along Dungsumchu. ............................................... - 13 -

Figure 18: Scouring from the side. .................................................................................................. - 14 -

Figure 19: Scouring from the base .................................................................................................. - 15 -

Figure 20: Crack in RCC wall. ......................................................................................................... - 15 -

Figure 21: Providing filter material to drain water efficiently. ....................................................... - 16 -

Figure 22: Gabion revetment along (a) Shetikheri Stream (b) Aipoly stream. ............................. - 16 -

Figure 23: Scouring from the base. ................................................................................................. - 18 -

Figure 24: Filling up of floodway with debris. ................................................................................ - 18 -

Figure 25: Debris falling the gabion walls. ..................................................................................... - 18 -

Figure 26:Longer panels of gabion boxes ....................................................................................... - 19 -

Figure 27: Concrete revetment along Chukarpa in Jomotshangkha. ........................................... - 20 -

Figure 28: Scouring at the base of the apron.................................................................................. - 21 -

Figure 29: Failure due to the scouring. ........................................................................................... - 21 -

Figure 30: Failure due to rigid launching apron. ......................................................................... - 22 -

Figure 31: Failure at the end of the wall ......................................................................................... - 22 -

Figure 32: AB Mattresses used along Phochu( Left) and Haachu (Right) as flood protection

structure. ......................................................................................................................................... - 23 -

Figure 33: Subsidence. .................................................................................................................... - 24 -

Figure 34: Scouring from the base. .................................................................................................- 25 -

Figure 35: Detention pond along Aipoly stream. .......................................................................... - 26 -

Figure 36 : Debris collected near the hume pipe. ......................................................................... - 27 -

Figure 37 : Location where floodway meets the main storm water drain. ................................... - 27 -

Figure 38: The outlet of the detention basin. ................................................................................ - 28 -

Figure 39: Downstream of bridge .................................................................................................. - 29 -

Figure 40: Downstream of bridge .................................................................................................. - 29 -

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Figure 41: Upstream of bridge ........................................................................................................ - 29 -

Figure 42: Upstream of bridge ....................................................................................................... - 29 -

Figure 43: Initial site condition ...................................................................................................... - 30 -

Figure 44: After construction of revetment with spurs. ............................................................... - 30 -

Figure 45: Deposition of sand between the spurs. ......................................................................... - 31 -

Figure 46: Spur deflecting the water current. ................................................................................ - 31 -

Figure 47: Rip-rap used along Mochu (Left) and Phochu (Right). ................................................ - 32 -

Figure 48: Dredging used as a short term measure to prevent flooding. ..................................... - 33 -

Figure 49: Filling up after dredging works. ....................................................................................... - 34 -

Figure 50: Stone spurs used along Mochu. ........................................................................................ - 34 -

Figure 51: Deposition between the spurs. .......................................................................................... - 34 -

Figure 52: Wooden porcupines used to divert river. ..................................................................... - 35 -

Figure 53: Dam in Bhutan for electricity generation (Source: Google). ....................................... - 36 -

Figure 54: Sabo dam in Indonesia. .................................................................................................... - 38 -

Figure 55: Sediment deposition on the upstream portion. .................................................................. - 38 -

Figure 56: Sabo dam with steel rods. ............................................................................................. - 38 -

Figure 57: Sabo dam with concrete slits. (Source: Google) .......................................................... - 38 -

Figure 58: Wooden revetment along the banks. ........................................................................... - 39 -

Figure 59: The worn-out planks in wooden revetment. (Source: Google ) .................................. - 39 -

Figure 60: Tetrapod used on breakwaters. ..................................................................................... - 41 -

Figure 61: Tetrapod with identification numbers. ......................................................................... - 41 -

Figure 62: Types of spurs/groynes. (Source: Google) .................................................................... - 41 -

Figure 63: Groynes used for river training..................................................................................... - 42 -

Figure 64: Rock spurs. (Source: Google image) ............................................................................ - 42 -

Figure 65: Jetty jacks used for flood protection. ........................................................................... - 43 -

Figure 66: Jetty jacks along river bank. ......................................................................................... - 43 -

Figure 67: Retention Basin. (Source: Google) ................................................................................... - 44 -

Figure 68: Periodic maintenance of retention basin required. (Source: Google) ................................. - 44 -

Figure 69: Dry flood-proofing. (Source: Google) .......................................................................... - 45 -

Figure 70: Wet flood-proofing. (Source: Google) ......................................................................... - 45 -

Figure 71: Levee Breach. (Source: Google) ..................................................................................... - 46 -

Figure 72 : Bypass Channel. (Source: Google) .................................................................................. - 46 -

Figure 73: Flood Hazard Mapping for different flood profiles. .................................................... - 48 -

Figure 74: Afforestation for flood management. ........................................................................... - 49 -

Figure 75: The processes that reduce the flooding risk. (Source: Google) ................................... - 49 -

Figure 76: Structures that can be relocated. ................................................................................. - 50 -

Figure 77: Flood warning systems. (Source: Google) ..................................................................... - 51 -

Figure 78: Flood warning stations in Bhutan. ................................................................................ - 51 -

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1. INTRODUCTION

1.1 Background

Bhutan has a history of loss of life and damage to property due to flooding. Rivers are generally

characterized by steep slopes in the upper catchment, which are subject to intense seasonal

rainfall and high rates of erosion. As the rivers flow towards the southern foothills, the transition

from mountainous areas to flat plains typically occurs and is accompanied by extensive flooding.

On the other hand, owing to Climate Change, the rainfall pattern has become erratic with

prolonged drought period followed by unusually high precipitation which causes flash floods all

over the country. Climate change and variability has resulted in changing rainfall and

temperature patterns, thereby aggravating these disaster risks, leading to higher risks, especially

for the poor and vulnerable.

In the year 2011, the Government of Bhutan expressed concern for damages caused by floods

and had instructed the MoWHS to establish an institution to oversee all the flood management

works in the country. So in the following year 2012, a new Division named 'Flood Engineering

and Management Division' (FEMD) under the Department of Engineering Services (DES) was

created.

The mandates of Flood Engineering and Management Division are listed below, but are not

limited to:

Identification of flood prone areas

Carry out Preliminary Flood Hazard Assessment Studies

Design and Construction of Appropriate River Training Measures

Fortification of towns and communities from flood

Reclamation of land from flood plains

Provide assistance to Local government in Implementation of Flood Alleviation

Projects

Planning and design of storm water drains.

One of the most important mandates for FEMD, DES, MoWHS is to identify flood prone areas

and design appropriate flood protection structures to manage the flood in all the 20 Dzongkhags

in Bhutan. After the design, necessary mitigation works are planned and implemented in the

flood prone areas.

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1.2 Objective of the Study

To study the effectiveness of the past interventions under taken and also to identify an

appropriate up-to-date flood control measures that can be adopted in Bhutan for preparedness

and adaptation of local communities to climate change induced disasters.

1.3 Problem Statement

Bhutan is prone to multiple natural hazards that pose varying degrees of risk to the lives and

livelihoods of its population. Flash floods and landslides pose an annual threat to human lives,

properties and livelihoods, especially in the southern parts of the country. Sarpang Dzongkhag

located in the southern part of Bhutan has a history of flooding due to erratic rainfall pattern. The

flooding causes damage to the lives (people and cattle), properties and agricultural lands. The

problem of flooding is aggravated by high rates of erosion due to fragile geology resulting in

rivers changing its course towards the settlements, debris flow, flashflood etc. Geologically,

southern Bhutan falls under Siwalik Zone, where soil predominantly consists of sandstones,

siltstones, clay shale and boulder beds. These types of soils are very susceptible to erosion.

Therefore, the flash flood in southern Bhutan usually overtops the banks and erodes sediments

from upstream. The sediments are then deposited in large scale in the downstream portion of the

river and finally a new river course is developed over once fertile agricultural land. Some of the

best potential agricultural land is found in the Southern Dzongkhag and yield per acre is high

where irrigation facilities exist. The favorable terrain and climatic conditions combined with

fertile agriculture land offer tremendous opportunity for farm mechanization and commercial

horticultural development in the southern Bhutan. However, the floods in summer make the once

fertile land infertile and unsuitable for agricultural use. Like any other country in the world, the

flood in Bhutan is caused by many factors such as heavy rain, blockage of storm water drains,

failure of dams, levees or any other water retention structures. The flooding in the country is also

aggravated by the developmental activities whereby, leading to increased impervious area.

Further, infrastructure like roads, houses, bridges, flood protection structures etc. are also being

washed away by the flood every year as seen in and hence, the Government continue to invest

huge amount of money for reconstruction works every financial year. Huge investment has also

been made in the past for flood protection works. Further, due to climate change, the rainfall

pattern and rainfall intensity has changed increasing the vulnerability of the community to

flooding in the country. Therefore, to reduce the vulnerability of the community to flood, this

research is carried out to critically study the pros and cons of past intervention carried out in our

country. In addition to this, best practices of the flood protection measures used in other parts of

the world also will be studied for recommendation to be used in our country.

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2. PAST FLOOD INTERVENTIONS IN BHUTAN

Floods are natural processes occurring throughout the world, difficult to prevent but can be

managed in a proper way to reduce its impacts. Flooding is a threat to life, properties and the

environment in the face of climate change. It is therefore, very important that flooding risks be

accessed to identify any flood risk to the area and accordingly plan mitigation measures to

reduce its impacts on the most vulnerable. In Bhutan, the flood risks to the vulnerable

communities are managed or reduced by planning and implementing appropriate flood control

measures. The flood control measures are the methods used to reduce or control the damaging

effects of flood waters. The flood control measures can be divided into two parts such as

structural measures and non-structural measures.

2.1 Structural Measures

The structural measures are the measures that reduce the flood hazards by controlling the flow of

water in rivers and streams. It involves the construction of artificial structures to reduce and

manage the flooding risk. Each artificial structure has its own advantages and disadvantages.

Managing the flood risk through construction of artificial structures such as gabion walls,

detention basins etc. are cost intensive both during the construction and maintenance phase.

Further, it requires high technology and also impacts environment by disturbing its ecological

system. These are some of the structural measures implemented in Bhutan at present to manage

flood risk in flood prone areas.

2.1.1 Gabion Walls

The gabion walls are retaining walls made of stacked stones filled in gabion boxes which are

either hand woven or mechanically woven by using wire meshes such as galvanized steel wire

and stainless steel as given in Figure 1. The stone fill should be of hard and durable material. To

reinforce the structure, all the mesh panel edges are selvedge with a wire of greater diameter than

the wire mesh. The mesh panel is divided into cells by providing diaphragm at every 1 meter

interval.

- 4 -

Figure 1: Gabion walls used as flood protection structure.

Advantages

1) The construction materials for the gabion walls are easy and cheaper to transport and use at

site. (Stones and gabion boxes)

2) The flexibility of the wire mesh and the stones results in their modularity and ability to be

stacked in various shapes.

3) It can conform to subsidence as it can move with the earth and also dissipate energy from

flowing water.

4) In some cases, strength of gabion walls may increase with time as silt and vegetation fill the

voids and reinforce the structure.

5) Their permeability allows the gabion baskets to drain water easily preventing buildup of water

pressure behind them.

6) They are environmentally friendly (green alternative) and requires no special masonry or

skilled labor to construct it.

7) In some areas, gabions might be the only practical choice, particularly in remote sites that are

off limit or inaccessible to heavy machinery.

Disadvantages

1) The life expectancy of gabions depends on the lifespan of the wire, not on the contents of the

basket.

2) Aesthetically not pleasing to sight.

3) When the velocity of the streams and rivers are high, the gabion mesh baskets can tear open,

spilling the rock fill.

4) The gabion baskets are easily damaged by corrosion and also debris floating in the water.

5) The damaged gabions baskets are hazardous to public safety. 6) The gabion walls on failing will result in releasing non-indigenous stones in that area.

https://en.wikipedia.org/wiki/Subsidence

- 5 -

2.1.1.1 Reasons for failure of Gabion walls in the past

The gabion walls have been constructed along the river banks in Bhutan for decades to reduce

the vulnerability of the communities to flooding. However, it‟s effectiveness in managing the

floods in flood prone areas has become a concern in recent times. The most basic causes of

failure of the gabion walls are scouring and erosion of the foundation of the structure. Further,

compromising on the quality of the stones and gabions wires during implementation are also

some of the factors leading to the failure of the wall. The following are some of the causes of

failures of gabion walls in our country:

The Figure 2 shows typical example of a

situation leading to failure of gabion walls

used for the flood protection structure. The

gabion wire meshes are stolen from the site

to be sold as a scrape resulting in reduction

of overall structural integrity of the gabion

wall.

Figure 2: Stolen gabion wire.

The scouring at the base of the flood

protection structure is one of the main

causes of failure. It is the result of the

erosive action of running water resulting in

excavation and transportation of material

away from the banks of streams as given in

Figure 3. The rate of scouring depends on

the type of materials, i.e. loose granular

soils would scour more rapidly compared to

cohesive soils.

Figure 3: Scouring at the base.

- 6 -

The Figure 4 illustrates a situation whereby,

the gabion baskets weren‟t adequately filled

with stones during construction of the walls.

This has led to failure as the rocks have

shifted in place to fill in the voids within the

structure.

Figure 4: Missing stone filling in the gabion box.

The stones in the gabion box should be

arranged in such a way to minimize the

voids as the structural integrity of a gabion

wall depend on its ability to function as a

unit. The Figure 5 gives an example of a

wall with less structural integrity due to use

of irregular stones resulting in more voids.

Figure 5: Irregular shaped stones.

Figure 6 shows that in Bhutan, the gabion

boxes are woven by the people at site and

not factory manufactured. This results in

failure of the walls due to poor quality of

wire mesh used. The function of the mesh is

not only to contain the stone fill but also to

provide a comprehensive reinforcement

throughout the structure.

Figure 6: Hand woven gabion boxes.

- 7 -

In the exposed side of a gabion box, the

stones of nearly equal size should be neatly

packed with a flat side up against the mesh.

Whereas, in Figure 7, it can be clearly seen

that small stones are filled in the gabion

box. Over time, the small size stones will be

displaced leading to failure of the wall due

to settlement.

Figure 7: Small size stone fillings.

2.1.1.2 Methods to enhance the life of the Gabion walls

A hexagonal-shaped gabion displays better strength capabilities as opposed to the

conventional rectangular-shaped gabion.

The gabion boxes used should be made from a good quality material as per the

specifications.

While filling the gabion baskets with stone, it is very important to minimize voids within

the stones to prevent settlement in future. If there are large voids between the stones, as

the wall settles due to heavy rain fall etc., the stones will begin to move and fall into the

open voids reducing the structural integrity of the wall.

It is recommended to not fill gabions with extracted stone or rock from the existing site as

it may not possess the correct qualities. The stones used for filling should be properly

selected as per the specifications provided by the engineers. The stones that are smaller

than the mesh will not be contained by it. Non-frost susceptible quarried stone, which is

normally angular, is the preferred fill as the interlock is very good and voids are minimal.

Proper apron for the flood protection structure should be provided to prevent scouring at

the base of the structure.

2.1.2 Random Rubble Masonry (RRM) Walls

Random rubble masonry walls are also used in Bhutan as flood protection structures. The stones

of different sizes are bonded together with a mortar as seen in Figure 8. The random rubble

masonry walls may be coursed or uncoursed depending upon the quality of stones used in the

walls. In the coursed RRM, the stones used are either hammer dressed or chiseled dressed laid at

regular courses with non-uniform joints. In the uncoursed RRM, the courses are not maintained

regularly. The bigger stones are laid first and then, spaces in-between them are filled with spalls.

- 8 -

The stones are not properly dressed for this kind of walls resulting in irregular shaped stones

with non-uniform joints. The strength of RRM wall depends on the bonds between the stones.

Figure 8: RRM walls in (a) Trashigang (b) Samdrupjongkhar.

Advantages:

1. The stones used for the RRM walls are strong, durable and weather resistance increasing the

life span of walls.

2. The stone masonry walls don‟t suffer from dampness.

3. The walls constructed from the stones are aesthetically appealing.

4. Stones used for RRM walls are natural material, therefore leading to environmental friendly

construction.

5. The stones used for RRM are easily available in various shapes and sizes.

6. The cost of constructing RRM is cheaper than other walls.

Disadvantages:

1. Size of stones are not uniform thereby, dressing work consume more time.

2. Dead load of stone masonry is more leading to premature sinking of the foundation.

3. Stones are heavy in weight and handling them is difficult.

4. Stones are to be brought from quarries which are located only at few places, increasing the

transportation cost.

5. Thinner walls can be constructed with bricks but it is not so with stones.

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2.1.2.1 Reasons for failure of Random Rubble Masonry (RRM) walls in the past

In the past, Random Rubble Masonry walls have been constructed along the banks of river to

protect the river bank from erosion. However, it has failed due to scouring at the base and poor

drainage condition of soil behind the walls. The following are some of the causes of failures of

RRM walls in our country:

The main cause of the failure of the

random rubble masonry walls is the

poor drainage, without proper drainage

the hydrostatic pressure builds up

behind the walls resulting in the

bulging of the walls as illustrated in

Figure 9.

Figure 9: Bulging of the wall due to hydrostatic

pressure.

When RRM walls are constructed along

the river banks as flood protection

structure, scouring at the base of the

walls mostly accounts for failure of the

walls as given in Figure 10.

Figure 10: Scouring from the base.

- 10 -

The Figure 11 presents a scenario

whereby, undressed stones are piled up

randomly without mortar (binder) to

function as a flood protection wall. This

has led to failure of the wall.

Figure 11: Structurally weak (stagnation of water).

To prevent the failure of the RRM walls

due to buildup of hydrostatic pressure

behind the walls, weep holes are

provided to drain the water from

behind. However, insufficient weep

holes are responsible for failure of the

walls due to poor drainage as seen as

Figure 12.

Figure 12: Insufficient weep holes.

2.1.2.2 Methods to enhance the life of Random Rubble Masonry (RRM) walls



The RRM walls should have adequate drainage to drain water from behind the wall to

minimize hydrostatic pressure buildup. There should be a proper drainage system

consisting of a perforated pipe (to channel the water), gravel (to maintain porosity at the

wall), and geotechnical fabric (to separate the gravel from the soil).

To ensure proper drainage of the surface water, the surface of the backfill should be

graded in such a way to direct water away from the wall.

For increased strength and stability, the stones in the RRM wall do not require laying

stones in courses but each layer must contain bonding stone that extend through the wall

to tie the walls together.

- 11 -

The Figure 13 shows bonding

stones used in RRM walls to

increase the strength of the wall.

Figure 13: Bonding stone in RRM walls.

2.1.3 Plain Cement Concrete (PCC) Walls

These walls are made from plain cement concrete, a mixture of cement, sand, aggregates and

water in the right proportion as shown in Figure 14. The PCC walls made of concrete has high

compressive strength (capacity of a material or structure to withstand loads tending to reduce

size). In the PCC walls, no reinforcements are used. A concrete wall is constructed whenever the

compressive strength and weight is the only requirement. The tensile strength of PCC is about

1/10th of its compressive strength. For this, fibers and polymers are introduced to increase its

tensile strength.

Figure 14: Plain cement concrete walls used as flood protection structure along Dungsumchu.

- 12 -

Advantages

1) The ingredients of concrete (cement, sand, aggregates) are easily available in most of the

places in Bhutan.

2) The concrete can be prepared at site with desired strength.

3) The durability of concrete is very high and it can be cast in any shape.

4) The concrete can withstand high temperatures, wind and water.

5) Concrete wall is stronger than a gabion wall.

Disadvantages

1) The tensile strength of plain cement concrete is very low resulting in cracks of walls.

2) The weight of PCC is high compared to its strength.

3) It isn't flexible and therefore, it will not retain its structural integrity if there is any earth

movement.

4) The PCC walls are not aesthetically pleasing to eyes and its strength will decreases with time.

2.1.3.1 Reasons for failure of Plain Cement Concrete (PCC) walls in the past

The flood protection walls made of plain cement concrete are constructed along the banks of

river to protect it from erosion in Bhutan. However, it has failed due to scouring at the base and

poor drainage condition of soil behind the walls. The following are some of the causes of failures

of PCC walls in our country:

The Figure 15 shows the failure of the

PCC wall due to scouring at the base of

the flood protection walls.

Figure 15: Failure due to scouring.

2.1.3.2 Methods to enhance the life of Plain Cement Concrete (PCC) walls

The weep holes should be provided with filter material between the walls and the backfill

to immediately drain the area behind the wall. The filter material will prevent clogging,

loss of backfill and caving.

- 13 -

If the length of the wall is long, expansion joints are required to be provided in a place

with large variance in temperature.

Construction joints should be provided in the wall to prevent cracking of concrete due to

drying, shrinkage and moisture expansion.

It is recommended to keep deep footing enough to resist the weight of saturated soil and

also to prevent scouring at the base of the foundation.

The concrete mix should be of good quality materials as per the specifications to enhance

the strength of the PCC walls.

Figure 16 shows an example of a

drainage system used to drain the water

from the backfill behind the retaining

wall.

Figure 16: Drainage system to avoid hydrostatic

pressure.

2.1.4 Reinforced Cement Concrete (RCC) Walls

RCC walls are the walls made from the mixture of concrete and reinforcements (steel bars) as

presented in Figure 17. This type of walls can withstand both the compressive strength and

tensile strength (capacity of the material to withstand loads tending to elongate).

Figure 17: Reinforced cement concrete walls along Dungsumchu.

- 14 -

Advantages

1) The reinforced concrete has a high compressive strength and tensile strength.

2) It is also fire and weather resistance.

3) The reinforced concrete walls are more durable than any other materials.

4) The maintenance cost of reinforced concrete is very low.

5) The reinforced concrete requires less skilled labor during construction.

6) The walls can be constructed in a place with limited space.

Disadvantages

1) The final strength of reinforced concrete is affected by steps used during its preparation

(mixing, casting, and curing).

2) The cost of the formworks used for casting RCC is high.

3) The shrinkage of the reinforced concrete causes crack development and strength loss.

4) It is expensive depending on the amount of steel bars used and skilled labor involved.

5) The construction time for RCC walls are more.

2.1.4.1 Reasons for failure of Reinforced Cement Concrete (RCC) walls in the past

The flood protection walls made of reinforced cement concrete are constructed along the banks

of river to protect it from erosion in Bhutan. However, it has failed mostly due to scouring at the

base of the wall. The following are some of the causes of failures of RCC walls in our country:

The Figure 18 shows a standing RCC walls

failing in its function to prevent erosion of

the bank behind it. During the high flood

water level, the water has entered behind the

wall from the side and eroded the whole

bank.

Figure 18: Scouring from the side.

- 15 -

The RCC wall in Figure 19 has failed

because of scouring at the base of the wall.

Scouring is caused by the erosion of the

sediment around a structure.

Figure 19: Scouring from the base

The RCC walls might fail due to cracks in

the wall as seen in Figure 20. The cracks are

caused due to shrinkage, corrosion of

reinforcing steel, freeze-thaw disintegration

etc.

Figure 20: Crack in RCC wall.

2.1.4.2 Methods to enhance the life of Reinforced Cement Concrete (RCC) walls.

The weep holes with adequate size and spacing should be provided with filter material

between the walls and the backfill to immediately drain the area behind the wall. The

filter material will prevent clogging, loss of backfill and caving.






also to prevent scouring at the base of the foundation.

The concrete mix and steel bars should be of good quality materials as per the

specifications to enhance the strength of the RCC walls.

- 16 -

The main cause of retaining wall failure is

poor drainage caused due to hydrostatic

pressure buildup behind the retaining wall. To

overcome this problem, retaining walls should

be provided with weep hole and filter material

as illustrated in Figure 21.

Figure 21: Providing filter material to drain

water efficiently.

2.1.5 Gabion Revetment

Figure 22: Gabion revetment along (a) Shetikheri Stream (b) Aipoly stream.

The revetments are sloping structures placed on the bank of the river to protect it from erosion by

absorbing the energy of the incoming water. Prior to revetment construction, the existing ground

should be stabilized by grading to an appropriate slope to prevent slide failure of the revetments

after construction. If required, fill material should be added to achieve uniformity and it should

also be free of large stones. Finally, it should be firmly compacted before the construction of

revetment begins. The revetments are made of different materials such as plain cement concrete,

articulating block mattress, gabion mattress etc. The Figure 22 shows a gabion revetment used

to reduce the impact of flood water during flooding in southern part of Bhutan. Gabion

revetments are generally preferred to gabion walls along the river bank as they are more stable.

Further, there is possibility of revetments being covered by deposition from the flowing water

- 17 -

blending it with the natural environment. They are also porous and will trap windblown soil

allowing the growth of vegetation under favorable conditions.

Advantages

1) The construction materials for the gabion revetments are easy and cheaper to transport and use

at site. (Stones and gabion boxes) 2) The flexibility of the wire mesh and the stones results in their modularity and ability to be

stacked in various shapes.


flowing water.

4) In some cases, strength of gabion walls may increase with time as silt and vegetation fill the

voids and reinforce the structure.





7) In some areas, gabions might be the only practical choice, particularly in remote sites that are

off limit or inaccessible to heavy machinery.

Disadvantages

1) The life expectancy of gabions depends on the lifespan of the wire, not on the contents of the

basket.





5) The damaged gabions baskets are hazardous to public safety. 6) The gabion revetment on failing will result in releasing non-indigenous stones in that area.

2.1.5.1 Reasons for failure of gabion revetment in the past

The gabion revetment has been constructed to reduce the impacts of flooding to the community

because of its affordability and availability in Bhutan. The PVC coated gabion wire also has been

used along with the Zinc coated gabion wires. The PVC coasted gabions wires are not

recommended to be used in areas with lots of debris problem. The gabion revetment in the past

has failed mainly due to scouring, usage of longer panels, filling up of floodway etc. as given

below:


- 18 -

One of the major problems of the

gabion revetment is the scouring at the

base of the revetment. The launching

aprons provided to prevent scouring at

the base has worked as given in Figure

23. However, it has still failed in some

areas.


In places with a debris problem,

floodway constructed with gabion

revetment can only serve as temporary

measures for alleviation of floods in

that area. Therefore, after every rainy

season the floodway gets filled up with

debris as given in Figure 24. This

renders the floodway useless after one

rainy season.

Figure 24: Filling up of floodway with debris.

One of the major disadvantages of the

gabion revetment is that during high

flood event, the gabion wires catch hold

of debris resulting in its failure. The

Figure 25 shows an example of such

failure.

Figure 25: Debris falling the gabion walls.

- 19 -

While constructing the gabion

revetment, shorter panel of gabion

mattresses should be used. Figure 26

presents a situation whereby, longer

panel of gabion mattresses used has

cause failure of the whole structure.

Figure 26:Longer panels of gabion boxes

2.1.5.2 Methods to enhance the life of gabion revetment

A hexagonal-shaped gabion displays better strength capabilities as opposed to the

conventional rectangular-shaped gabion.

The gabion mattresses used should be made from a good quality material as per the

specifications.

While filling the gabion baskets with stone, it is very important to minimize voids within

the stones to prevent settlement in future. If there are large voids between the stones, as

the wall settles due to heavy rain fall etc., the stones will begin to move and fall into the

open voids reducing the structural integrity of the wall.

It is recommended to not fill gabions with extracted stone or rock from the existing site as

it may not possess the correct qualities. The stones used for filling should be properly

selected as per the specifications provided by the engineers. The stones that are smaller

than the mesh will not be contained by it. Non-frost susceptible quarried stone, which is

normally angular, is the preferred fill as the interlock is very good and voids are minimal.



The existing ground should be stabilized by grading to an appropriate slope for long-term

durability of the mat.

The voids or other soft areas in the ground should be filled with suitable materials and

firmly compacted before the placement of revetment.

The geotextile fabric should be placed on top of the prepared ground surface to avoid

leaching of the fine soils from the prepared ground.

- 20 -

2.1.6 Plain Cement Concrete (PCC) Revetment

The plain cement concrete revetments are used as flood protection structure at a location where

strength and durability are important factors. The PCC revetments as presented in Figure 27 are

used along the left bank of Chukarpo in Jomotshangkhag Drungkhag to reduce the vulnerability

of the community to flooding.

Figure 27: Concrete revetment along Chukarpa in Jomotshangkha.

Advantages

1) The plain concrete revetment has a high compressive strength.


3) The plain concrete revetment is durable compared to other materials.

4) The ingredients of concrete (cement, sand, aggregates) are easily available in most of the

places in Bhutan.

Disadvantages

1) The concrete revetment is impermeable leading to buildup of hydrostatic pressure in the

backfill behind it.

2) The shrinkage of the concrete revetment causes crack development and strength loss.

3) The maintenance cost of concrete revetment is very high.

4) The installation of concrete requires more skilled labor and formworks during construction

and thus, it is expensive.

5) The concrete revetment is rigid and doesn‟t conform to ground movement leading to failure of

the walls.

6) It is an eye sore and not environmentally friendly.

- 21 -

2.1.6.1 Reasons for failure of Plain Cement Concrete (PCC) revetment in the past

In Bhutan, concrete revetment is also used as flood protection structures. It is mainly constructed

to prevent the erosion of the river banks and also to prevent inundation of the nearby areas from

rivers during extreme flood events. During site investigations, it has been observed that

revetment has failed mostly due to scouring at the base of the wall. The following are some of

the causes of failures of concrete revetments:

The Figure 28 shows that it is important

to find out the scour depth while

designing the revetment structure. The

height of the revetment should start

from the top of the water level to the

bottom of the river to prevent scouring

of the foundation.

Figure 28: Scouring at the base of the apron.

The failure of the concrete revetment in

Figure 29 is due to scouring at the base

of the apron. The sediments from the

base of the apron are eroded by the fast

flowing river and hence, the apron has

collapsed. The failed aprons are still

functioning by diverting the water

current away from the revetment.

Figure 29: Failure due to the scouring.

- 22 -

A flexible launching apron should be

used at the foot of the revetment. This

is to ensure that when the scour occurs,

the apron will launch and cover the

scour hole created. The apron should be

wide enough to cover the deepest scour

expected after launching. From Figure

30, it can be seen that failure of the

apron is due to the rigidity of the

launching apron.

Figure 30: Failure due to rigid launching apron.

The subsoil below the revetment is

displaced and hence, it has failed as per

Figure 31. The concrete revetment is

not flexible and therefore, it cannot

deform to varied surface geometry of

the backfill.

Figure 31: Failure at the end of the wall

2.1.6.2 Methods to enhance the life of Plain Cement Concrete (PCC) revetment

Cast in situ plain cement concrete is impermeable and therefore, weep holes should be

provided at certain locations to reduce the hydrostatic pressure in the backfill behind the

wall.






also to prevent scouring at the base of the footing.


the strength of the PCC walls.

- 23 -







2.1.7 Articulating Block Mattresses

Figure 32: AB Mattresses used along Phochu( Left) and Haachu (Right) as flood protection

structure.

Articulating Block Mattresses given in Figure 32 are used as revetments to protect river bank

from erosion. The AB fabric form consists of a series of compartments. The compartments are

interconnected by grout ducts and further, high-strength cables are also installed between and

through the compartments and grout ducts. Once filled with concrete, the AB Mats become

pillow-shaped rectangular concrete blocks with cables embedded in the concrete to link the

blocks together.

Advantages

1) The AB mat is flexible revetment conforming to any ground movement without failure.


3) The AB mats are more durable than any other materials.

4) Enhanced stability due to interconnectivity provided between the blocks by high strength

cables.

- 24 -

Disadvantages

1) It is highly expensive.

2) The quality of the revetment is affected by the availability of the highly skilled labor.

3) High capacity pump and concrete mixer is required during the installation of the AB mat.

4) It is not easily available in the market.

5) It is not environmentally friendly.

6) Possibility of concrete cracking/fracturing under extreme weather conditions and when

exposed for long-term. 2.1.7.1 Reasons for failure of Articulating Block Mattresses in the past

The articulating block mattresses constructed in the past has been functioning very well in our

country. However, issues like subsidence of the soil in the embankment and scouring at the base

of the apron may result in failure of the revetment.

The Figure 33 shows that subsidence might

result in failure of the revetment to control the

flood due to reduced height of the revetment.

Further, it might also create tension in the

high strength cables connecting the concrete

blocks resulting in failure of the AB mat to

function as a monolithic unit.

Figure 33: Subsidence.

- 25 -

The Figure 34 shows that scouring at the base

of the launching apron has resulted in

deformation of the AB mats at the base of the

revetment. Even if there is scouring at the

base, the flexible apron has conform to the

ground contour and is still functioning well.


2.1.7.2 Methods to enhance the life of Articulating Block Mattresses







It is recommended to provide wider apron to cover the expected scour depth while

launching.


the strength of the concrete blocks.

2.1.8 Detention Basin

A detention basin is an excavated area installed on rivers or streams to protect the vulnerable

community against flooding by storing water for a limited period of time. Detention basins are

storm water best management practices that prevent flooding in a flood prone area. The basins

allow large volume of water to enter it and then, limit the outflow of the water from it depending

upon the capacity of the downstream culverts or storm water drains.

In Bhutan, detention basin is constructed on Aipoly stream as a pilot project. The Aipoly stream

is a seasonal stream which causes flooding near the international check post every rainy season

in Gelephu Municipality. The stream brings in huge quantities of debris during the rainy season

and also keeps on changing its course. Therefore, to reduce the impacts of flooding near the

international check post, FEMD had proposed a detention pond with a capacity to store the

runoff of 38,100cum for a certain period of time. The water is to be stored and discharged in a

- 26 -

controlled way through the outlet within the capacity of the storm water drain near the culvert.1

The Figure 35 shows the detention basin along Aipoly stream.

Figure 35: Detention pond along Aipoly stream.

Advantages

1) The empty pond can be used for recreational activities in dry season.

2) The construction materials for the gabion detention basin are easy and cheaper to transport

and use at site. (Stones and gabion boxes)

3) The flexibility of the wire mesh and the stones results in their modularity and ability to be

stacked in various shapes. 4) It can conform to subsidence as it can move with the earth and also dissipate energy from

flowing water.

5) In some cases, strength of gabion detention basin may increase with time as silt and vegetation

fill the voids and reinforce the structure.

6) Detention basin constructed with gabion box and stones are environmentally friendly (green

alternative) and requires no special masonry or skilled labor to construct it.

1 Post Flood Report (August, 2016)


- 27 -

Disadvantages

1) The dry detention basin is not very effective in removing soluble pollutants from water.

2) The filled pond is not safe for people living in the area.





5) The construction of detention basin requires large areas.

6) The maintenance cost of dry detention basins is high.

2.1.8.1 Reasons for failure of Detention Basin in the past

The failure of the detention basin is due to the

deposition of debris in the detention basin

thereby, clogging the hume pipe used to

discharge off the water from the basin with

thrash and debris. The debris collected near

the hume pipe is as shown in Figure 36.

Figure 36 : Debris collected near the hume pipe.

The water from the detention basin

should be discharged in such a way to

not exceed the discharge capacity of the

culvert or the storm water drain at the

downstream location. The Figure 37

shows the confluence of the floodway

with the main storm water drain near

international check post.

Figure 37 : Location where floodway meets the main

storm water drain.

From detention

basin

- 28 -

The wall at the downstream portion of the

detention pond as shown in Figure 38

should be strengthened to prevent failure

due to the hydrostatic pressure in the

detention basin.

Figure 38: The outlet of the detention basin.

2.1.8.2 Methods to enhance the life of Detention Basin

The inflow area should be protected with concrete blocks or some other structure from

incoming high speed flood water.

The inflow area should have debris drop vaults (deep holes at the entrance) to collect

large rocks before it enters the basin.

The site for construction of detention basin should be properly selected and it should be

far away from the settlement.

The sites must be easily accessible for the equipment to routinely maintain and clean the

basin.

2.1.9 Embankment with gabion revetment

The earthen embankments are constructed along the river banks within the flood plains of a river.

The embankments are constructed to confine the river flood water within the cross- section

available between the embankments preventing it from spilling over to the flood plains. This

type of flood protection against flooding has been provided in some flood prone rivers with low

banks in Bhutan. To prevent the erosion of the earthen embankments, it is further protected by

constructing revetments on the riverside of the embankment. When the revetment is constructed

with gabion mattress filled in with stones, it is called gabion revetment.

Embankment with gabion revetment is used as a flood protection structure along Haa River in

Yangthang village at Haa Dzongkhag. The Figure 39 and Figure 40 show the initial condition of

the river bank and completed embankment with revetment along the right bank of Haa River

respectively (Downstream of the bridge).

- 29 -

Figure 39: Downstream of bridge

(Initial condition).

Figure 40: Downstream of bridge

(After construction).

Similarly, Figure 41 and Figure 42 show the initial condition of the river bank and completed

embankment with revetment along the right bank of Haa River respectively (Upstream of

bridge). The flood protection work was implemented in 2016.

Figure 41: Upstream of bridge

(Initial condition).

Figure 42: Upstream of bridge

(After construction).

Advantages

1) Can be used as path by the pedestrian beside river.

2) The construction materials for this type of flood protection structure are easy to transport and

use at site. (Stones, soils and gabion mattresses)


flowing water.




- 30 -



Disadvantages





4) It is expensive.

5) Upon failure of the gabion revetment, the earthen embankment can be easily eroded.

2.1.10 Embankment with gabion revetment and spurs

Along Sunkosh River in Lhamoizingkha, combination of embankment with revetment and spurs

are provided for protecting the bank against erosion and reclamation of land. The groynes or

spurs are constructed transverse to the rivers extending from the bank into the river. This spurs

perform different functions such as training the river, silting up the area and protecting the bank.

They are used to attract, deflect (or repel) and hold the flow in a channel or river.

The Figure 43 and Figure 44 show the initial condition of the site before construction of the

flood protection structure and the site after completion of the embankment.

Figure 43: Initial site condition

Figure 44: After construction of revetment

with spurs.

- 31 -

The Figure 45 and Figure 46 show an area whereby, deposition of the sand has taken place

reclaiming the area near the spur.

Figure 45: Deposition of sand between the

spurs.

Figure 46: Spur deflecting the water current.

Advantages

1) The construction materials for this type of flood protection structure are easy to transport and

use at site. (Stones, soils and gabion mattresses)


flowing water.





5) Can be used as path by the pedestrian beside river.

Disadvantages





4) It is expensive.

5) Upon failure of the gabion revetment, the earthen embankment can be easily eroded.


- 32 -

2.1.11 RIP RAP

Rip rap is the name given to loose rock armor used for river bank protection. The rip rap covers

the bank with a layer of stones of varying sizes. The sizes of the stones used depend upon the

velocity of the flowing river. The stones can be hand placed or machine installed depending upon

the size of area to be stabilized, accessibility of the machines for placing the stones and

requirement of more natural arrangement of stones. The Figure 47 shows the use of rip rap in

certain places in Bhutan to prevent the erosion of the river banks from fast flowing rivers.

Figure 47: Rip-rap used along Mochu (Left) and Phochu (Right).

Advantages

1) Easy to install in general and it can even be placed under water.

2) High hydraulic roughness to reduce the force of waves and current.

3) The maintenance cost of riprap is low and easy to repair.

4) It is durable and highly flexible allowing it to settle into underlying soil contour.

5) It is aesthetically pleasing to look at blending in with the natural environment.

6) It is resistance to scour and there is no issue of hydrostatic pressure.

7) It can experience minor damage and still continue to function.

Disadvantages

1) The rip rap requires skilled labor during construction.

2) It is expensive to use rip rap as the large stones have to be quarried and transported from long

distances.

3) Large stones used for rip rap on steeper slopes will be prone to falling down and thus,

dangerous.

4) Smaller rip rap stones has the chances of being vandalized.

- 33 -

2.1.12 Dredging

In Bhutan, dredging is an activity used to prevent future flooding impacts. The dredging activity

involves the removal of sediment from the bottom and sides of river channels resulting in

straightening of channels and deepening of the river bottom. The dredging activities can be used

to manage flood with other flood management tools as given in Figure 48.

Figure 48: Dredging used as a short term measure to prevent flooding.

Advantages

1) Dredging reduces the water level of the river system.

2) It is one of the flood management tools.

3) It is one of the most effective short term measures for flood reduction.

4) The dredged materials can be reused as backfilling materials.

Disadvantages

1) It requires long term maintenance.

2) Dredging can disturb the ecological balances within the river system.

3) The dredging work on the upstream portion of the river can sometimes result in increasing the

flood risk at the downstream portion of the river.

4) It cannot prevent flooding during extreme event.

5) It is not sustainable.

6) Dredging creates more problems than it solves.

- 34 -

The Figure 49 shows that the river

system is dynamic and it will adjust

itself to restore its dynamic equilibrium

after disturbances.

Figure 49: Filling up after dredging works.

2.1.13 Stone Spur

Spurs are stone, rock, gravel or piled structure built at an angle or transverse to a river bank to

deflect the flowing water away from the critical area. The length, location, angle and material of

the spur vary from place to place. Stone spur has also been used in Bhutan in few places to divert

the water from the vulnerable banks, thus reducing the bank erosion. Figure 50 and Figure 51

show the use of rocks as spurs along Mochu in Punakha.

Figure 50: Stone spurs used along Mochu.

Figure 51: Deposition between the spurs.

- 35 -

2.1.14 Jetty Jacks/Porcupines

Wooden porcupines are also use to divert river in some places in Bhutan as given in Figure 52.

Figure 52: Wooden porcupines used to divert river.

- 36 -

3. FLOOD CONTROL MEASURES

3.1 Structural Measures

3.1.1 Dams

The dams are built across the river to store the excess water from the river and help control

devastation flood. It then either releases flood water in a controlled manner to the river below the

dam or divert it for other uses as given in Figure 53 . There is risk associated with the dams as it

can break and cause more destruction than the natural flood. In addition to this, the dam also

traps sediments disturbing the erosion and deposition process in the downstream part of river

thus, reducing the efficiency of the dam in controlling flood. Further, the storage capacity of the

dam is also reduced by the sediment deposition.

Figure 53: Dam in Bhutan for electricity generation (Source:

Google).

Advantages

The dam can be used for electricity production.

It retains hazardous materials and sediments

The water from the dam can be used by other water users (irrigation, recreation, water

supply etc.)

It reduces flooding in the downstream portion of the river.

- 37 -

Disadvantages

The effectiveness of the dam can be reduced by sedimentation.

The dam construction leads to displacement of the people from floodplains.

The dam construction interferes with natural wildlife migration pattern of fishes.

The failure of the dam will cause more destruction.

It requires regular maintenance and is expensive.

3.1.2 Sabo Dams

Sabo dam (also called check dam) is a structure built around the stream with a metal grate as

depicted in Figure 56 or a concrete structure with a slit as given in Figure 57. The Figure 54 and

Figure 55 give an example of stepped type Sabo dam collecting sediments behind the dam. The

Sabo dams are most commonly used in Japan to prevent flooding due to debris flows and

volcanic lahars. A debris flow is a fast-flowing mixture of rock, water, mud, and sand that flows

rapidly downhill under the force of gravity. Debris flows can engulf villages and cities rapidly,

sometimes with little to no warning and usually happens in a mountainous region.

Under normal condition, the Sabo dam allows the stream to flow normally carrying its sediment

load. The processes such as erosion and deposition occur normally. However, during a flood

event with debris flow or lahar, the presence of dam is felt. The speed of the flowing water is

reduced by the dam and deposition of the sediment take place behind. Furthermore, if the river

begins carrying large objects such as boulders or felled trees, those objects will be fully trapped

behind the dam and will actually interrupt the flow even more resulting in deposition of

sediment.

Advantages

It retains hazardous materials and sediments

The water from the dam can be used by other water users (irrigation, hydropower etc.)

It reduces flooding in the downstream portion of the river.

Disadvantages

The dam construction leads to displacement of the people from floodplains.

The dam construction interferes with natural wildlife migration pattern of fishes.

The failure of the dam will cause more destruction.

It requires regular maintenance and is expensive.

- 38 -

Figure 54: Sabo dam in Indonesia.

Figure 55: Sediment deposition on the

upstream portion.

Figure 56: Sabo dam with steel rods.

(Source: Google)

Figure 57: Sabo dam with concrete slits.

(Source: Google)

- 39 -

3.1.3 Wooden revetment

Figure 58: Wooden revetment along the banks.

(Source: Google )

Figure 59: The worn-out planks in wooden

revetment. (Source: Google )

Wooden revetments have been used for protecting the river bank from soil erosion as shown in

Figure 58 and Figure 59. It is made of planks laid against wooden frames so that they disrupt the

force of the water. The revetment is designed in such a way to break the force of the wave

without reflecting the energy. The sediments such as sand, shingle or pebbles are carried through

or over the revetment structure by an incoming wave allowing longshore drift (transport of sand

and pebbles along the coast). However, the sediment cannot easily return seaward as the force of

the wave has been dissipated. This action results in a build-up of material behind the revetment

and the increased depth of material protects the foot of the cliff from being attacked by the sea.

Advantages

Low cost flood protection structure.

Effectively reduces the wave energy thus, preventing erosion of river banks.

Disadvantages

The use of wooden revetments has been replaced by modern concrete-based defense

structures.

The wooden revetment does not last long and has to be regularly replaced.

The wooden revetment does not protect against big storms.

Not aesthetically pleasing.

- 40 -

3.1.4 Tetrapods

A tetrapod is a tetrahedral concrete structure used on coasts as part of coastal management as

shown in Figure 60. It is placed on the breakwaters (a barrier built on the sea to protect the

shore). A tetrapod's shape is designed to dissipate the force of incoming waves by allowing water

to flow around rather than against it and to reduce displacement by allowing a random

distribution of tetrapod to interlock. The weights of the tetrapods vary worldwide depending

upon the local weather conditions. In the olden days, boulders and concrete blocks were used but

they ended up becoming dislodged over time by the force of the ocean. The tetrapod are often

numbered as presented in Figure 61 for easy monitoring of the displaced structures from

photographs. They are no longer protected by a patent, and are widely used all over the world in

different shapes. Tetrapod are pre-casted and therefore, they can be placed quickly even when

coast are being attack by the waves.

Advantages

Their displacement can be easily monitored.

The shape of tetrapod ensures reduction in their displacement.

It easily dissipates the force of incoming waves by allowing water to flow around them.

Due to their weight and design, they can remain stable during the extreme weather

conditions.

Good protection measure for hard bottom substratum.

Disadvantages

Even with their weight and design, they are displaced due to strong currents of the oceans

and seas.

They pose danger to swimmers, surfers, and boaters.

They are not aesthetically appealing.

They are not environmentally friendly.

There is also an assumption that wave action on tetrapod removes the sand faster than the

natural process from the shore.

They often fail as protection measures in soft bottom areas.

Very expensive to construct

Altered sediment transport processes can cause erosion downstream of the sea or ocean.

- 41 -

Figure 60: Tetrapod used on breakwaters.

(Source: Google)

Figure 61: Tetrapod with identification

numbers.

(Source: Google)

3.1.5 Rock Spurs or Groynes

The spurs (groynes) are structures constructed transverse to the river bank to protect it from

erosion. It is mainly used for river training and can have different function based on how they are

constructed. It can be impermeable, permeable, deflecting, repelling and attracting spur. The

spurs can be used singly or in series. It can be aligned either perpendicular to the bank or at an

angle pointing upstream or downstream. Further, it can also be used in combination with other

river training structures for flood protection. The Figure 62 shows different types of spurs.

Figure 62: Types of spurs/groynes. (Source: Google)

When the spur is constructed at an angle pointing upstream, it is called repelling spur. The spurs

constructed in this way repel the river flow away from it and is usually used where major

channel changes are required. The repelling spurs are usually constructed in a group to repel the

water current. When the upstream angled spur is of short length and changes only the direction

of flow without repelling it, it is called a deflecting spur. It gives local protection only.

Likewise, a spur inclined towards downstream reach, is called an attracting spur as it attracts the

river flow towards it. The spur constructed perpendicular to the stream is usually the shortest

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possible and thus, most economic. The Figure 63 and Figure 64 give an example of groynes used

in training the rivers.

Advantages

Spurs can be constructed using different materials as per the site conditions.

Environmentally friendly spurs can be constructed using local materials.

The construction of the spurs doesn‟t require skilled personal.

Very efficient in trapping sediments.

It builds up beaches.

Disadvantages

High river bank should be available to anchor (or tie) the spur back.

Extra protection should be given to nose/head of the spur against anticipated scour.

If river reach to be protected is long, spurs should be used in series.

The first spur on the upstream portion of the river is more vulnerable and thus, should be

given special care during design and construction.

No single type of spur is suitable for all locations.

The position, length and shape of spurs depend on site conditions.

Single spurs are neither strong enough to deflect the current nor as effective in causing

silt deposition upstream and downstream.

Figure 63: Groynes used for river training.

(Source: Google image)

Figure 64: Rock spurs. (Source: Google image)

3.1.6 Jetty Jacks

Jetty jack is the permeable form of bank protection that performs the bank protection works at a

low cost. It was invented by H. F Kellner and the first jetty jack was made with three willow

poles tied together at the mid-point. Further, the willow poles were laced with wire to keep it

extended. He later replaced the willow trees with a steel angle and it is still used nowadays. If

properly installed, the jetty jacks will trap sediments and debris during flooding event and

- 43 -

reclaim the area by formation of its own levee. The Figure 65 and Figure 66 shows the

functioning of the jetty jacks in preventing erosion of the vulnerable river banks.

Figure 65: Jetty jacks used for flood protection.


Figure 66: Jetty jacks along river bank.


Advantages

It riparian ecosystem provided habitat for a variety of wildlife species.

Cost effective river training measures in alluvial rivers rich in sediment.

Disadvantages

Jetty jacks field become a non-functional eyesore and redundant after sometime.

The performance of the jetty jacks fails with reduction in sediment concentration of the

river.

It is not environmentally friendly.

It is not available in Bhutan.

3.1.7 Retention Basin

Flood retention basins as seen in Figure 67 also called wet detention basin are designed to

capture storm water runoffs to prevent flooding downstream. It has a permanent pool of water in

the pond.

Advantages

The retention basin can be used for recreation.

It will recharge the ground water.

It improves the water quality.

Reduce the risk of flooding in the vicinity.

- 44 -

Disadvantages

It will require periodic maintenance to remove sediments collected at the bottom of the

basin and also vegetative management as seen in Figure 68.

Requires inspection of inlet and outlet pipes periodically to check the clogged pipes.

Safety issues when the pond is filled with water.

Not suitable for steep areas as there are requirement of high embankment.

Figure 67: Retention Basin. (Source: Google)

Figure 68: Periodic maintenance of retention

basin required. (Source: Google)

3.1.8 Flood-proofing

The flood-proofing is done to reduce or avoid the impacts of flooding on the structures. This is

mainly done by elevating the structures above the floodplain, using designs and building

materials to ensure that the structures become more resilient to flood damage. The flood-proofing

also prevents floodwaters from entering structures in the flood prone areas. Two types of flood-

proofing such as dry flood-proofing and wet flood-proofing are used. The wet flood-proofing

reduces the flood damage by: (1) allowing flood waters to easily enter and exit a structure by

using openings or breakaway walls to minimize the structural damage; (2) by using flood

resistant materials below the flood depth; (3) by properly anchoring structures against flood

flows; and (4) by elevating important utilities in the flood zone. On the other hand, dry flood-

proofing is the practice of making a building watertight or substantially impermeable to

floodwaters up to the expected flood height (FEMA, 2008). An example of dry flood-proofing

and wet flood-proofing is as shown in Figure 69 and Figure 70 respectively.

- 45 -

Figure 69: Dry flood-proofing. (Source: Google)

Figure 70: Wet flood-proofing. (Source:

Google)

Advantages

The flood proofing measures are cheaper than construction of flood protection structure.

The flood proofing measures doesn‟t require additional land as compared to flood

protection structure.

The structures are less likely to fail during flood for wet flood-proofing as the external

and internal hydrostatic pressure equalizes during a flood.

The flood proofing measures will make it quicker for the people to clean-up and repair

after the flood.

Flood-proofing can be done by individual house owner.

Disadvantages

The risk of flooding should be communicated in advance to the residents for any flood

proofing measures to function.

There is an additional cost for providing flood proofing as flood hazard mapping and

early warning systems are required to disseminate flood risk information to public.

The residents are not allowed to live in the flood proof houses during flooding.

In case of dry flood-proofing, the flood shields will not be aesthetically pleasing.

3.2 Non-structural Measures

Non-structural measures are very effective for both short and long-term flood damage reduction.

Non-structural measures for preventing flood damage are based on the acceptance of flooding as

a natural process which cannot be completely controlled. They are also cost effective and

sustainable when compared to structural measures. The following non-structural measures are

used in reducing flood risk and damages associated with flooding:

- 46 -

3.2.1 Restoring Natural River Functions

The river in the natural state provides a live storage for managing flood. Along with the rivers,

the adjacent floodplains also help in storage and release of floodwaters slowly by reducing the

peak flood downstream. Further, the wetland adjacent to the rivers also acts as a large sponge by

soaking up floodwaters. It also filters water and recharges the groundwater supplies.

However, due to structural measures constructed in the past, the natural live storage capacity of

the rivers is reduced leading to flooding issues on the downstream portion of the reach. When the

meanders are cut off to straighten the rivers for flood management purposes, the storage capacity

provided by the longer, meandering river channel is reduced. Likewise, when the levees are

constructed to keep rivers within their channels, the floodplains doesn‟t receive floodwater from

the rivers to store and slowly release the flood water. As a result, in some cases the peak flood

increases and cause greater flood risk downstream. Therefore, we have to restore the natural

flood-carrying capacity of rivers by recommending the following ways:

Breaching levees-temporary flooding old floodplains to restore natural floodplain forming

process and store excess water.(Refer Figure 71)

Setting back levees (Moving levees away from rivers) - to increase the floodplain area to

store floodwaters and also to restore habitat characteristic of natural rivers.

Restoring meanders- it restores storage capacity of river and also reduces the peak flow

downstream.

Constructing bypass channels-Alternate channels designed to mimic natural channels will be

used by the river or stream above certain flow levels.(Refer Figure 72)

Restoring vegetated banks and wetlands.

Figure 71: Levee Breach. (Source: Google)

Figure 72 : Bypass Channel. (Source: Google)

- 47 -

Advantages

It is cheaper as compared to structural measures.

The maintenance required is quite low.

They are more environmentally friendly and thus, sustainable.

They have lower education & technology requirements and can be implemented by local

people in remote parts of poor countries.

Disadvantages

This kind of non-structural measures cannot always be used in areas where the

floodplains are already built-up.

The techniques are not necessarily reliable and may not always be effective in reducing

the vulnerability of communities to flooding.

These measures focus more on reducing the impacts of a flood rather than preventing it.

3.2.2 Flood Hazard Map/ Land use zoning

Flood hazard mapping is an exercise to define flood prone areas under extreme weather

conditions. Its primary objective is to reduce the impact of flooding and also to enhance our

understanding and awareness on the risk of flooding among the public, local authorities and

other organizations. Thus, it is a very important component required for appropriate land use

planning in flood-prone areas. It also helps us in prioritizing mitigation and response efforts

based on the urgency of the situation.

Flood hazard mapping typically provides a „snapshot‟ of flood risk at a given point in time. Due

to climate change, the flood hazard maps will require periodic updates in order to reflect the

changing risk of flooding. Figure 73 shows a typical example of Flood Hazard Map. Based on

the flood hazard maps, urban planners can place restrictions on land usage in the areas

surrounding a river. Usually, all constructions are allowed outside of the floodplain but only

construction of public outdoor facilities like playing fields and parks are allowed within the flood

plains. In this way, the planners can reduce the risk and impact of flooding by using the maps.

Further, such type of floodplain zoning also ensures that land on the floodplain isn‟t urbanized

allowing easy infiltration. In this way, the surface run-off is reduced limiting the flooding in

those areas.

- 48 -

Figure 73: Flood Hazard Mapping for different flood profiles.

Advantages

The flood hazard map will identify those areas at risk of flooding and will help prepare

emergency responses and location of shelters for evacuees.

The flood hazard maps will allow planners to locate critical infrastructure, such as

electricity supplies, sewage treatment, emergency services etc. in a low risk area for it to

function during a flood event.

The flood hazard maps will ensure greater awareness of the risk of flooding to the general

public.

In the longer-term, flood hazard maps can be integrated into planning procedures for

sustainable development by identifying high risk locations and steering development

away from these areas.

Disadvantages

If the flood hazard mapping is not integrated into emergency response planning and town

planning procedures, it does not cause a reduction in flood risk.

The development of flood hazard maps requires technical capacity and expertise.

Data such as topographic data, hydro-meteorological data are required to prepare flood

hazard maps.

The flood hazard maps may increase fear and anxiety as residents are more aware of the

risk of flooding.

The costs of flood hazard mapping depend on the requirement of data.

A lack of public understanding about the benefits of flood hazard mapping may also

provide a barrier to implementation.

Limits development in certain areas.

Possibility of relocation of people in already developed floodplains.

- 49 -

3.2.3 Afforestation

The Figure 74 and Figure 75 show how afforestation can be used for flood management. It

involves planting of trees in a drainage basin to increase interception and storage while reducing

surface run-off. This reduces a river‟s discharge and so makes it less likely to flood.

Afforestation could also help towns downstream reduce the “peak height” of floods. When

combined with floodplain zoning, afforestation can be very effective at reducing the risk of

flooding. Afforestation has the benefit of creating new habitats for animals and improving water

quality by filtering pollutants out of rainwater.

Figure 74: Afforestation for flood management.

(Source: Google)

Figure 75: The processes that

reduce the flooding risk. (Source:

Google)

Advantages

Managed rivers are returned to natural state with trees planted and old channels restored.

Improved environment, sustainable, habitats created/restored, can be low cost.

Reduces the runoffs in the area.

Disadvantages

If not managed correctly, invasive and alien species can be introduced accidentally.

Requires lot of space to be effective.

Trees take most of the nutrients from the soil.

- 50 -

3.2.4 Relocation.

Figure 76: Structures that can be relocated.

(Source: Google)

The structure in the high flood risk area is removed from the floodplain and transported to a new

location entirely outside the floodplain. Thus, relocation from the floodplain to a safer area

results in significant reductions of flood risk. This measure can only be used in places where

houses can be easily relocated as given in Figure 76. It mitigates the flood problem as the

structure is located outside the flood prone area. However, adequate land is required for

relocation and can be costly depending upon the structures to be relocated.

Advantages

The risk of flooding is reduced as they are not located on flood prone areas.

The flood plains are converted to open spaces allowing it to revert to its natural state to

reduce the flood risk naturally.

Disadvantages

Can be costly depending on the structures to be relocated.

Not possible in Bhutan, since most of the structures constructed along the floodplains are

permanent.

Relocation is a disruptive mitigation method for the occupants of the structure.

There is restricted use of the floodplains after relocation.

- 51 -

3.2.5 Flood Warning System

Figure 77: Flood warning systems. (Source:

Google)

Figure 78: Flood warning stations in Bhutan.

( Source: NCHM)

It is a non-structural measure with mandates under National Centre for Hydrology and

Meteorology (NCHM). The location of the flood warning stations in Bhutan is given in

Figure 78. The flood warning system detects the flood hazard in advance enabling the public to

be warned on time resulting in reduction of flooding impacts as depicted in Figure 77.

Advantages

The people can be warned and thus, the impact of flooding is reduced.

Warnings give people time to move to a safer location with their possessions.

Disadvantages

Can be difficult to predict flash floods.

People may not hear the warning.

The installation of flood warning systems are expensive.

3.2.6 Emergency preparedness, response and recovery

The community should be made aware of the risk of flooding in their area and should be trained

on their role in responding to emergency situation when it arises. They should also be given

capacity development in organizing coordinated evacuation from the affected area , prevent

pollution in flooded areas, maintain healthy environment and cleaning operations after the flood.

This mandate is under Department of Disaster Management.

- 52 -

Advantages

Having an evacuation plan in place before a flood reduces injuries and property damage.

The training provided will make sure that all workers and people know what to do in case

of an emergency.

The communities are trained to respond to and recover from a disaster.

Disadvantages

The officials responsible should create awareness on the emergency preparedness,

response and recovery to reduce the flood implications.

Funding required for creating awareness.

The evacuation plans should be practice on a regular basis to be effective.

Regular training should be provided for both response personnel and concerned citizens

to reduce the impacts during emergency.

3.2.7 Flood Management Plans

Flood Management Plans highlight the hazards and risks of flooding from rivers, lakes,

groundwater etc. and discusses on how the flood management agencies would work together

with communities to manage flood risk. The main objective of the flood management plans are

to reduce the vulnerability of individuals, communities, infrastructure and the environment to

flooding , to raise awareness amongst people, to engage people in effective response and

prioritize investment in the most risk areas.

The Flood Management Plans must include:

• Flood risk assessment report

• A map showing the boundaries of the Flood Risk Area

• Recommendations from the flood hazard and risk maps

• Proposed measures for managing the flood risk

• Identify the implementers

Note: The flood management plan is the requirement of flood risk regulations in various

countries.

Advantages

It is an important tool to identify and manage the flooding risk.

With the flood management plan in place, better decision and investment can be made for

managing flooding risk.

- 53 -

4. RECOMMENDATIONS

Effective management of flood risks requires a balance of both structural and non-

structural flood damage prevention measures.

Another way to reduce the severity of flooding is to manage water where it falls with

stormwater management practices that slow it down, spread it out, and soak it in.

The quality of construction for any structural measures should be monitored strictly

during the construction phase by the engineer.

Efforts should be made to restore rivers' natural flood zones to revive the ability of

natural wetlands and floodplains to retain water and lessen flood impacts.

Once the structural measures are constructed, it should not be neglected; there should be

periodic maintenance to increase its live span in reducing the flood risk.

There should be proper technical studies before the implementing phase.

The engineers should be trained and given capacity development on a regular basis for

effective flood management in the country.

- 54 -

5. LIMITATIONS

Although this study is carried out to assess the effectiveness of the past interventions carried out

for reducing the vulnerability of communities to flooding in Bhutan, there are some unavoidable

limitations such as:

The study is solely carried out through desktop studying and experience of the author

without involvement of any flood management experts.

The appropriateness of the structural and non-structural measures should be recommended

only after discussing amongst the flood engineers comparing its pros and cons.

The reliability of the information contained in this study should be validated from other

external sources.

The structural measures for flood management alter the dynamics of hydrologic systems and

can have adverse impacts in other areas.

- 55 -

6. REFERENCES

1. Department of Engineering Services, MoWHS, June 2016, Preliminary Flood Hazard

Assessment for Dungsumchu in Samdrupjongkhar Dzongkhag, MoWHS, Thimphu,

Bhutan.

2. Asian Disaster Preparedness Centre, 2014, Coursework book for Training in Flood Risk

Assessment and Planning of Mitigation Measures, ADPC, Bangkok, Thailand.

3. Engineering Service, Office of Public Works, National Preliminary Flood Risk

Assessment (PFRA), Ireland.

4. Grassel, K. July 2002. Issues of Jetty Jack removal in Bosque and river restoration

planning. Publication No. WRP-6. The University of New Mexico, Water Resources

Program. Accessed online on 11 October 2017.

5. Floodplain definition and flood hazard assessment,

http://www.oas.org/CDMp/document/NHP/oea66e/ch08.htm (16th October, 2017)

6. ClimateTechWiki, A clean technology platform,

http://www.climatetechwiki.org/technology-information (16th October,2017)

7. Gabion walls, https://en.wikipedia.org/wiki/Gabion (15th August, 2017)

8. Spurs or Groynes, http://www.aboutcivil.org/spurs-groynes-requirements-geometry.html

(19th September, 2017)

9. Articulated concrete block mattresses, https://www.maccaferri.com/products/articulated-

concrete-block-mattresses/ (20th September, 2017)

10. Articulating block mattresses, http://www.synthetex.com/articulating-block (20th

September, 2017)

11. Tetrapod (structure), https://en.wikipedia.org/wiki/Tetrapod_(structure) (3rd

October,

2017)

12. Coastal defences in Norfolk, Timber revetment,

http://www.geocases1.co.uk/printable/Coastal%20defences%20in%20Norfolk.htm (11th

October, 2017)

13. Benefits of dam, https://www.fema.gov/benefits-dams (12th October, 2017)

14. Sabo Dam, http://the-earth-story.com/post/121410105520/sabo-dams-mountainous-areas-

of-the-world-have (12th October, 2017)

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May, 2018)

16. In-river flood management, http://www.appropedia.org/In-river_flood_management (31st

May, 2018)

http://www.oas.org/CDMp/document/NHP/oea66e/ch08.htm

http://www.climatetechwiki.org/technology-information

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https://www.maccaferri.com/products/articulated-concrete-block-mattresses/

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Documents

RESEARCH REPORT FOR FLOOD PROTECTION STRUCTURES · YEAR 2018 Researched By: Thinley Choden, Principal Engineer, FEMD, DES, MoWHS RESEARCH REPORT FOR FLOOD PROTECTION STRUCTURES FLOOD