Flood Risk Analysis in Ganges Basin

Fall 2014

Thesis Student:

Mufazzal Hossain

REG NO: 09205046

Department of Civil Engineering

University of Asia Pacific (UAP)

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Dedicated to

My Parents, Teachers, Friends and Family

And

To the Flood Affected People of Bangladesh

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Flood Risk Analysis in Ganges Basin

Thesis Student

Mufazzal Hossain

Reg no: 09205046

Department Of Civil Engineering

University of Asia Pacific (UAP)

Fall 2014

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DEPARTMENT OF CIVIL ENGINEERING

UNIVERSITY OF ASIA PACIFIC (UAP)

We hereby recommend that the thesis prepared by Mufazzal Hossain entitled “Flood Risk

Analysis in Ganges Basin” is accepted as fulfilling the part of the requirements for the

degree of Bachelor of Science in Civil Engineering.

M.R. Kabir, Ph.D.

Professor

Department of Civil Engineering

University of Asia Pacific (UAP)

Dhaka, Bangladesh.

Chairman of the Committee

(Supervisor)

Muhammad Mizanur Rahaman, D.Sc.

Associate Professor

Department of Civil Engineering

University of Asia Pacific (UAP)

Dhaka, Bangladesh.

(Member)

Alamgir Habib, Ph.D.

Professor & Head of the Department

Department of Civil Engineering

University of Asia Pacific (UAP)

Dhaka, Bangladesh.

(Ex-officio)

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ACKNOWLEDGEMENT

I would like to avail this opportunity to express my sincere gratitude and respect to

my honorable supervisor and Head of the Department, Professor M. R. Kabir, Department

of Civil Engineering, University of Asia Pacific, for granting me this rare opportunity to work

under his direct supervision. It is his untiring effort, invaluable suggestions and constant

guidance throughout the research period that inspired me to gain the level of understanding of

the subject that enabled me to present this thesis. I am particularly grateful to my honorable

supervisor, Professor M. R. Kabir for the time he has spared out of his busy schedule. With

the continuous help of Professor M. R. Kabir from the beginning to the end of this study,

the work has finally been successfully complete.

Sincere appreciation and whole hearted thanks to Muhammad Mizanur Rahaman,

Associate Professor, Department of Civil Engineering, University of Asia Pacific and

Emtazul Haque, Assistant Professor, Department of Civil Engineering, University of Asia

Pacific, for their valuable discussions and kind suggestions on this study. I am very grateful

for their helping attitude in every phase of this thesis.

I am very thankful to Mr. MD. Shah Alam, Director, Dipak Chandra Nath,

Deputy Director and Naima Baten, Weather Specialist of Bangladesh Meteorological

Department to provide me with the required data.

I am solely indebted to my parents and to my family & friends for their patience and

efforts that helped me to be what I am today.

Lastly, I would like to pay my utmost respect to the Almighty Allah, for granting us

this wonderful opportunity to work and to live.

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Table of Contents Abstract ...................................................................................................................................... 1

Chapter 1: Introduction .............................................................................................................. 2

1.1. Background ................................................................................................................. 2

1.2. Objectives .................................................................................................................... 5

Chapter 2: Literature Review and Methodology ....................................................................... 6

2.1 Introduction to Ganges Basin ...................................................................................... 6

2.2 Introduction to Flood Risk Analysis ........................................................................... 8

2.2.1 The Causes of Flooding ..................................................................................... 10

2.2.2 Risk Based Approach to Managing Floods ....................................................... 10

2.2.3 Strategies for Flood Risk Analysis and Management ........................................ 12

2.3 Methodology ............................................................................................................. 16

2.3.1 Data Analysis Methods ...................................................................................... 16

2.3.1.1 Rational Method ............................................................................................. 16

2.3.1.2 Flood Frequency Analysis .............................................................................. 17

2.3.1.3 Snyder’s Synthetic Unit Hydrograph ............................................................. 20

2.3.2 Risk Analyzing Methods.................................................................................... 22

2.3.2.1 Risk Identification Method............................................................................. 22

2.3.2.2 Risk Rating Method ....................................................................................... 23

2.3.2.3 Method of Determining the Risk Level.......................................................... 24

2.4 Determine Prevention and Emergency Response Measures ..................................... 26

Chapter 3: Data Analysis and Discussion ................................................................................ 27

3.1 Data Analysis ............................................................................................................ 27

3.1.1 Determining Average Rainfall and Discharge per Day ..................................... 27

3.1.2 Flood Frequency Analysis ................................................................................. 29

3.1.3 Formation of Hydrographs ................................................................................. 32

3.2 Risk Analysis............................................................................................................. 35

3.2.1 Identified Risks and Discussion ......................................................................... 35

3.2.2 Risk Rating......................................................................................................... 41

3.2.3 Risk Levels and Remarks................................................................................... 42

3.3 Discussion on Prevention and Emergency Response Measures ............................... 43

3.3.1 Current Mitigation Measures ............................................................................. 43

3.3.2 Suggested Mitigation Measures ......................................................................... 46

Chapter 4: Conclusions and Recommendation ........................................................................ 50

4.1 Limitations of This Study .......................................................................................... 51

4.2 Recommendation ....................................................................................................... 52

References ................................................................................................................................ 53

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List of Figures

Figure 1: Riverside Settlement in Flood Plain (Fleming, 2002) ................................................ 2

Figure 2-1: Ganges, Brahmaputra & Meghna River Watershed (Rahaman, 2006) ................... 7

Figure 2-2: Ganges Basin in Bangladesh (Source: Banglapedia, 2014) .................................... 8

Figure 2-3: Snyder's Synthetic Unit Hydrograph .................................................................... 20

Figure 3-1: Discharge Frequency Curve .................................................................................. 31

Figure 3-2: Synthetic Unit Hydrograph ................................................................................... 33

Figure 3-3: Hydrograph for Each Return Period ..................................................................... 34

List of Tables

Table 2-1: Managing Flood Risk - Addressing Likelihood & Impacts (Environment Agency,

2009) ........................................................................................................................................ 11

Table 2-2: Likelihood Categories for a Risk Matrix (Source: Ayyub, 2003) .......................... 23

Table 2-3: Consequence Categories for a Risk Matrix (Source: Ayyub, 2003) ...................... 24

Table 2-4: Risk Matrix (Source: Ayyub, 2003) ....................................................................... 25

Table 2-5: Risk Level and Remarks......................................................................................... 25

Table 3-1: Area & Weighted Area for Polygons Surrounding Each Rain Gauge Station ....... 27

Table 3-2: Runoff Coefficients of Various Drainage Complex (Source: Sir William Halcrow

and Partners Ltd., 1993) ........................................................................................................... 28

Table 3-3: Discharge Frequency Table .................................................................................... 29

Table 3-4: Table for Discharge for Corresponding return Period by Using Chow’s Equation

and Gumbel’s Method.............................................................................................................. 30

Table 3-5: Calculated Discharge for Different Return Period ................................................. 31

Table 3-6: Rainfall Excess for Each Return Period ................................................................. 33

Table 3-7: Probability of Various Flood Events ...................................................................... 35

Table 3-8: Risk Rating Table ................................................................................................... 41

Table 3-9: Risk Level of Identified Risk and Remarks ........................................................... 42

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Abstract

Bangladesh is a low lying country which was created due to the sedimentation

process because of the flow of rivers such as Ganges, Brahmaputra, Jamuna, Surma etc.

which ultimately gave Bangladesh the recognition of the largest delta in the world. Now,

as Bangladesh is a riverine country, it is likely to be a very much fertilized land. This is

the advantage of Bangladesh when it comes to growing crops. But there is a major

disadvantage as well which makes Bangladesh face the loss of agricultural goods,

habitats, and man power etc. resources every year. Rain-fed flood has been a problem in

this region for a very long time and during the rainy season, it becomes quite impossible

for people to live in the flood prone areas. This thesis concentrates on the Ganges Basin

of Bangladesh, where due to flood, not only loss of crop is a problem but also a lot of

people are affected by flood which spreads diseases rapidly and in extreme cases, death

occurs. Because of flood, people who have domestic animals, also has to go through a lot

of sufferings.

Keywords: Development, Ganges River Basin in Bangladesh, Risk Analysis,

Mitigation & Flood.

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Chapter 1: Introduction

1.1. Background

Even though there’s always a risk of periodic flooding, settlements still was

witnessed on flood plains. Historically it was worth taking the risk of living on a

riverside location in order to gain the social, economic and environmental benefits of

living beside a river. (Figure 1.3).

The social and economic benefits mainly were taken due to the advantage of

trading sites at river crossing points, along the valley routes or at transshipment points

between river or sea transport and inland routes. Not only that but also riversides places

were often defensible by the military which is why riversides were always a good place

for building castles and other fortification.

From the environmental point of view flood plains are known to be very fertile

and workable land for agricultural use due to the amount of sediments being transferred

Figure 1: Riverside Settlement in Flood Plain (Fleming, 2002)

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by the rivers flow. They also offer sustainable sources of water power for economic

activities, such as milling, tanning and brewing.

When water overflows a river and covers land that is not normally submerged

then that land is said to be flooded. Being an important occurrence in the hydrologic

cycle, back in the good old days flood cycles were taken to be blessings because they

were persistent to the riverine ecosystems and the economies of the floodplain depended

on it. Floods became a social problem only when they did not occur. But as time passed

by more people started to settle in the floodplain for which reason the agricultural

production was disrupted to an unavoidable extent and therefore it caused high amount

damage to the settlement. When this started, flood became more of a problem rather than

a blessing for the society living in the floodplains and dwellers soon expected

governments to reduce or prevent flood damages through hydrologic engineering. The

construction of dykes in order to stop the spread of flood waters and to increase the

current to flush silt downstream dates back to at least the eighth century CE, in China

(Dodge, 2001). Roman-raised embankments in the Fens lasted until the eighteenth

century in England (Purseglove, 1988)

Until the mid-twentieth century, the story of modern flood control was the

transition from adaptation to the inevitable to an expectation that government would

provide maximum flood prevention and generous post-disaster relief for floodplain

dwellers. For the last sixty years or so, the story has been the growing recognition,

especially as the understanding of climate change has increased, that the goal of

maximum protection is unobtainable because flood damage is an inevitable risk that can

only be managed, but never totally avoided. Thus, we are now making the transition to

the idea that we must manage floodplains through a combination of structural defenses,

upstream storage, and land-use controls. (APFM, 2006)

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According to WMO/GWP Associated Programme on Flood Management’s case

study Bangladesh: Flood Management (September 2003), Bangladesh normally

experiences 4 types of flood and these are flash flood, rain-fed flood, river flood and

flood due to cyclonic storm surges. When it comes to flood within the Ganges Basin in

Bangladesh, rain-fed flood is the kind of flood which is discussed.

Rain-fed flood usually occurs at the termination point of Gangetic Deltas in the

south-western part of the country where most of the drainage systems are being

deteriorated because of the fall in up-land inflow from the main river Ganges. Human

interferences, mainly illegal occupation of river courses and construction of unplanned

rural roads are the main reason for the natural drainage systems to be disturbed in flood

plains where rain-fed flood also happens. For the disturbance in the natural drainage

system, the runoff generated by intense rainfall can’t be carried hence temporary

inundation occurs in many places. This kind of flood is also now happening in the urban

areas too. With the increase of population in urban areas, new habitats are being

developed as well and in the process of this development low lying areas and natural

drainage systems are being filled up continuously.

Professor Nazrul Islam of Emory University, Atlanta, GA, USA stated in one of

his publications ‘Flood Control in Bangladesh: Which Way Now?’, “Bangladesh’s flood

control program has been so far dominated by the embankment approach”. He also

stated, “According to this approach, it is necessary to cordon off areas in order to protect

them from flooding”. Rajshahi city protection embankment is one of the embankments

done to protect the city from flooding. But according to The News Today’s 23rd July,

2014 edition, the city protection embankments are at threat due to the river erosion in

Ganges.

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1.2. Objectives

From the above discussion it can be summarized that this research has the

objectives written below;

1. To calculate the surface discharge due to precipitation by flood frequency

analysis, generate hydrographs by using the concept of synthetic unit

hydrograph for 5, 10, 15, 25, 50 and 100 years return period.

2. Calculate the probability of occurrence at least once of such floods within

2063.

3. To identify the various risks such floods poses in the study area and discuss

them.

4. To present the flood effect mitigation measures and emergency response for

planning purpose at the study area.

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Chapter 2: Literature Review and Methodology

2.1 Introduction to Ganges Basin

The Ganges rises in the Gangotri glacier which is located in the Uttar Kashi

district of Uttar Pradesh Province in India where its elevation is about 3,139m above sea

level (Encarta, 2001). From there the Ganges enters Bihar and then it enters West Bengal

and flows south. Almost 40 km below Farakka it divides into two arms. The left arm

flows eastwards into Bangladesh and the right arm which is known as Bhagirat, flows

south through West Bengal. The Bhagitri is known as Hooghly River when it flows west

and south west of Kolkata. It flows into the Bay of Bengal after being divided into two

streams after reaching Diamonds Harbor. The right arm known as Haldi River enters the

Bay of Bengal as well. (Rahaman, 2009)

A number of big and small tributaries have joined the Ganges in the middle of its

flow towards the easterly direction. These tributaries join the Ganges on the northern

side from the Himalaya sub-basin. They are named as Ramaganga, Gomati, Ghagra,

Gandak and Kosi which are originated from the mountains of Himalaya. Another

tributary, Mahayana joins the massive river in Bangladesh. The Yamuna tributary, joins

the Ganges on the southern side at Allahabad. There are other major and minor

tributaries as well which originates in the peninsular sub-basin. These are named as

Kehtons, Sone, Kiul and Punpun. The average annual runoff of the Ganges below

Allahabad is about 150,000 million cubic meters with the ration of the contribution

between the Ganges and the Yamun as 2:3.

Ganges enters Bangladesh about 50 km below Farakka. The Ganges has a total

length of about 2,600 km and a catchment area of approximately 907,000 sq km. Within

Bangladesh, the Ganges is divided into two sections - first, the Ganges (258 km long)

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starting from the western border with India to its confluence with the Jamuna at

Goalandaghat, some 72 km west of Dhaka. Second one is the Padma, about 120 km long,

running from the Goalandaghat confluence to Chandpur where it joins the Meghna. The

total drainage area of the Ganges is about 1,087,400 sq km, of which about 46,300 sq.

km. lies within Bangladesh. After approximately another 90 km of flow, the combined

discharge falls into the Bay of Bengal. (Source: Banglapedia, 2014)

Figure 2-1: Ganges, Brahmaputra & Meghna River Watershed (Rahaman, 2006)

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Figure 2-2: Ganges Basin in Bangladesh (Source: Banglapedia, 2014)

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Introduction to Flood Risk Analysis

According to EU Directive (COM, 2006) for flood management, "flood risk" is

the likelihood of a flood event together with the actual damage to human health and life,

the environment and economic activity associated with that flood event.

Risk analysis refers to the evaluation of the tolerability of the estimated risks

based on the local society’s acceptability criteria. The comparison of the estimated risks

with acceptable ones results in the decision of what risk will be acceptable in the

particular affected system and what risk reduction measures will be applied; if needed.

(Pistrika & Tsakiris, 2007)

In another publication the authors (Thieken, Merz, Kreibich & Apel, 2006)

defined Flood Risk Analysis as written below:

Flood risk is defined as the probability that floods of a given magnitude and a

given loss will occur within a given time span. A complete flood risk analysis has to

identify possible damage scenarios (What can go wrong?), to estimate scenario

probabilities (How likely is it that it will happen?), and to assess the consequences (If it

does happen, what are the consequences?). The results of flood risk analysis should be

the basis for identifying locations with significant flood risk, and for deciding between

flood mitigation options.

From the explanations written above, Flood Risk Analysis can be defines as the

process of analyzing the risk due to flood in order to find out the probable impact level

and then preventing irreversible changes and mass damages from happening or mitigate

with the adverse impacts.

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In order to carry out a successful Flood Risk Analysis, it is required to know the

causes of flooding and to have a risk based approach to managing floods.

2.1.1 The Causes of Flooding

In the Ganges Basin portion of Bangladesh the common forms of flooding are:

1. River Flooding: River flooding occurs when a watercourse cannot cope

with the water draining into it from the surrounding land. This can happen, for example,

when heavy rain falls on an already waterlogged catchment.

2. Surface Water Flooding: Surface water flooding which occurs when

heavy rainfall overwhelms the drainage capacity of the local area. It is difficult to predict

and pinpoint, much more so than river or coastal flooding.

3. Coastal Flooding: Coastal flooding results from a combination of high

tides and stormy conditions. If low atmospheric pressure coincides with a high tide, a

tidal surge may happen which can cause serious flooding.

2.1.2 Risk Based Approach to Managing Floods

A risk-based approach is required to achieve the best results possible by using the

budgets and resources available because floods are a part of nature which is neither

technically feasible nor is it economically affordable to prevent all properties from

flooding. A flood risk analysis's aim is to minimize the harm caused by flooding, which

involves reducing the likelihood of flooding and reducing the impacts when flood occurs

for the benefit of the people, the economy and the environment. While aiming at

minimizing the harm caused by flooding, there are some underlying pressures that are

increasing risks, such as climate change, housing development or changes in land use.

These drivers can sometimes be affected, for example, by influencing planning and land

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development. But, there still are some drivers which are beyond direct influence, such as

climate change impact on the weather and sea level rise. There are various factors which

affects the risks of flooding. Figure 2-3 shows some of such factors.

Current Flood

Risk

Higher Likelihood of

Flooding

More frequent and more severe extreme

weather.

Deterioration of defenses.

Hardening of surfaces due to development.

Deforestation and loss of land over

Soil degradation

Lower Likelihood of

Flooding

New improved and well maintained

defenses.

Steer development away from flood risk

areas.

Diverting flood water to unpopulated land.

Better land management.

More Serious Impacts of

Flooding

More severe floods due climate change.

Damage to higher value property and

contents.

Loss of insurance cover.

New development in high flood risk areas.

Reduction in property value.

Less Serious Impacts of

Flooding

Good flood forecasts and widely received

warnings.

Improved property level protection.

Effective preparation and emergency

response.

Help with recovery.

Table 2-1: Managing Flood Risk - Addressing Likelihood & Impacts (Environment

Agency, 2009)

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2.1.3 Strategies for Flood Risk Analysis and Management

Not only flood itself is responsible for its risks, but also the vulnerability of an

individual, property or environment exposed to the event is responsible. Therefore the

risk of flooding is a product of both the flood event and the vulnerability of the affected

individual, property and the environment exposed to it. As the main objective of flood

risk analysis is to reduce the likelihood and the effect of flooding, the strategies and steps

for the risk analysis and management should be as follows:

1. Strategy and Policy Framework: First of all there should be a policy

framework that sets the principle and responsibilities of various organizations and

individuals during a flood period, post-flood period and pre-flood period.

2. Flood Risk Assessment and Flood Planning: Afterwards flood risk

assessment and flood mapping is required in order to find out the population within the

risk zone so that they can be categorized into high risk, medium risk and low risk

categories depending on the available historical data. This assists identifying various risk

that can affect a society and also helps in understanding the various circumstances in

which the risks are effective.

3. Planning and Development: After identifying risks, it is crucial to

control the development through planning system. For example, development of new

structures can be done in risk free zone. If that is not possible then the development can

be continued in zones having the least risk. It should be also noted that development in

one place does not cause problems in another place.

4. Protecting Communities in Floodplain: While new zones with no risks

or least risk are given attention for flood effect free development, it is also important to

place vast importance on places in which the risk of flooding is high. In such places there

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are structures, properties and humans which requires a lot of attention so that the effects

of risks can be minimized. To do such, construction and maintenance of flood defense

and using other techniques for the control or containment the flow of water from entering

an area is required. For example, after the devastating 1988 flood, Government of

Bangladesh has built a primary and secondary embankment around Dhaka city so that

during flood seasons, water will be restricted from entering the main city. The

embankment has been serving as a crucial safety structure ever since its establishment.

The embankment might have been built, but it should also be maintained so that it does

not become vulnerable to flood, from which it is suppose to protect the city from

5. Defending Individual Properties by Resistance and Resilience: It is

impossible and impractical to reduce all flood risk or to defend all possible floods in all

places. However, if an individual take responsibilities of taking care of one's own

properties through resistance and resilience measures, then the flood risks can be reduced

to a significant level. Examples of resistance and resilience measures are door guards

which helps by not letting the floodwater get into a property and water resistant wall

plaster which minimizes the damage when floodwater is already in a property,

respectively.

6. Forecast and Warning Systems: Even after taking all kinds of pre-flood

measures, it is required to establish a very strong flood forecast and warning system.

Bangladesh has two types of warning systems which are flood warning system and

cyclone warning system.

7. Flood Response and Recovery: When a flood occurs, various emergency

services, NGOs, local authorities etc. takes responsibilities and distributes the

responsibilities among themselves for a unified response to help the affected people in

the floodplain. By working together, other organizations are involved, such as water and

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energy companies, treatment facilities, relief sponsors etc. Such unified response gives

the responders a legal duty to work together to assess the risks and then make plans to

prevent, reduce, control, and mitigate the effects of an emergency. Post flood periods can

be considered to be the hardest time as this is when flood recovery becomes a great

challenge so that the economy, health, environment of the affected people in the

floodplain can go back to the way it was before the occurrence of the flood.

8. Insurance: Insurance industry plays a great role in managing the recover

from flooding. Through paid premiums, policy holders pool their risks, gaining access to

the funds when they need to replace and repair their property. For example, in cases of

households, an important contribution to recovery comes from insurance claims that pay

for rebuilding and repairs, and may contribute to temporary accommodation costs while

a property dries out.

9. Funding to Support Flood Risk Analysis and Management: It is very

important to have a managerial plan in order to tackle the risks posed by floods and for

that reason a vast amount of funding is required. As the people of the floodplain are the

ones who will be benefitted by such management, the funding can be made from the tax

they pay. Also, every year when the annual budget is announced, a great deal of fund

should be kept for flood risk analysis and management.

10. Protecting Communities at Risk in the Longer Term: The previous

mentioned steps were designed for a single immediate event of flood. But a floodplain

requires a management plan with much broader affects for a long term solution. Such

management plan should have the purpose of spreading the previously mentioned steps

in all of the flood prone areas in Bangladesh for a much stronger and integrated flood

risk management. In order to imply such process in the various areas in Bangladesh, a

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consistent method can be used which is followed by national guidance to provide a broad

assessment and plan. This method includes the processes mentioned in the next page.

i. Area Overview: A thorough overview of the hydrology, land use

and other characteristics of the catchment.

ii. Current Flood Risks and Management: A broad account of the

existing flood risks and the protective regime already in place to manage them.

iii. Future Flood Risk: An assessment of how the risks may evolve,

for example through climate change.

iv. Policy Appraisal: An examination of what actions are possible to

meet the objectives in each specific area within the area.

Bangladesh has various existing structural, nonstructural measures, policies &

legislation and warning systems which has been described later in chapter 3.

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2.2 Methodology

2.2.1 Data Analysis Methods

2.2.1.1 Rational Method

The method used in this research is quite similar to the data analysis method of

the Flood Risk Analysis of Godo River Basin. The data required for the first step of this

research is the daily rainfall data from 1984 to 2013 for the rain gauges located in

Chuadanga, Madaripur, Faridpur, Jessore, Barisla, Khulna, Satkhira, Patuakhali, Bhola,

Mongla and Ishurdi which has been obtained from Bangladesh Meteorological

Department. From these data the average rainfall per day will be determined by using

Thiessen Polygon Method. The parameters used in this analysis are written below.

R =∑ RiAi

A ………………………………………….(2.1)

I = R

tc ..……...…………………………...…….(2.2)

tc = 0.76 × A0.38 ....…………………………...…………..(2.3)

Q = CIA …………………………….……………(2.4)

Where,

Ri = Rainfall for rain gauge no. – i

A = Polygon area for the corresponding rain gauge no. – i

R = Average rainfall (mm)

I = Rainfall intensity (m/day)

tc = Concentration time (day)

A = Catchment area (km2)

C = Runoff coefficient.

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In the obtained data has daily rainfall for 11 stations starting from 1984 to 2013

and in some cases for some stations, the rainfall data is missing. To solve this problem,

equation (2.5) is used.

Rx = Nx

M [∑

Ri

Ni

M1 ] ……………………………………....…(2.5)

Where,

Rx = Rainfall for the rain gauge ‘x’ which’s data is missing,

Nx = Normal daily rainfall for rain gauge ‘x’ obtained from Bangladesh

Meteorological Department website,

Ni = Normal daily rainfall for rain gauge no – i,

M = Number of rain gauge with available data from.

Afterwards the daily average rainfall is used to find out the daily discharge by

logically assuming that the runoff coefficient, C is equal to 0.43. The catchment area, A

is equal to 46,300 km2 (Source: Joint River Commission). The daily discharge of month

May to October is summed to find out the flood discharge for each year.

2.2.1.2 Flood Frequency Analysis

The next step is to plot the Gumbel Probability Paper on a semi-log graph paper,

where discharges for 5, 10, 15, 20, 25 and 30 years return periods are plotted on the y-

axis and return periods are plotted on the x-axis. The discharge for the previously

mentioned return periods can be determined by ranking the flood discharge of each year

from one to thirty in a descending order. The return periods for the flood discharge of

each year has to be calculated and the required equations are presented in the next page.

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P = m/(N + 1) ..…………………………………………(2.6)

T = 1/P ………………………………………….(2.7)

Where,

P = Probability,

N = Numbers of years,

m = Rank,

T = Return period.

Afterwards the discharges for each return period can be determined by the

General Equation of Hydrologic Frequency Analysis given by Chow (1951).

XT = X + K …………………………………………. (2.8)

In order to plot the Gumbel Probability Paper by Gumbel Method, equation 2.9

can be re-written as written below.

XT = X + K )1( N ………………………………………….(2.9)

Where,

XT = Value of the variate X of a random hydrological series with a return period

T,

X = Mean of the variate,

K = Frequency factor,

s(N-1) = Standard deviation of the sample size of N.

The equations for standard deviation and frequency factor are written below.

1

)( =

2

)1(

N

XXN ……….………………………………...(2.10)

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N

NT

S

yyK

...………………………………………(2.11)

Where,

ny = Reduced mean & a function of sample size N,

nS = Reduced standard deviation & a function of sample size N,

Ty = Reduced variate, a function of return period, T.

The equation of Ty is given by,

1lnln

T

TyT ………………………………………….(2.12)

Once the discharges for 5, 10, 15, 25, 30 years return periods are determined, the

Gumbel Probability Paper can be plotted on a semi-log paper. The discharge is plotted on

the y-axis and return period on the x-axis. The equation of the best fitted straight line of

the Gumbel Probability Paper is then used to find out the flood discharges for return

periods of 50 and 100 years. By dividing the discharges of 5, 10, 15, 25, 50 and 100

years return periods with the total area of the catchment the total effective rain from the

month of May to October for each year can be calculated. This effective rainfall is used

to determine the depth of flood per unit area and to plot the hydrographs for the flood for

the aforementioned return periods. In order to plot such hydrographs, the Synthetic Unit

Hydrograph is used. Snyder presented a method in 1938 to plot the Synthetic Unit

Hydrograph.

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2.2.1.3 Snyder’s Synthetic Unit Hydrograph

tp = Ct(LLca)0.3 ……………..…………………………(2.13)

Where,

tp = time from mid-point of tr to the time of peak discharge (hour),

L = basin length measured along the water course from the basin divide to the

gauging station in km,

Lca = distance along the main water course from the gauging station to a point

opposite the watershed centroid in km,

Ct = a regional constant representing watershed slope and storage.

The value of Ct in Snyder’s study ranged from 1.35 to 1.65. In this study, Ct is

assumed to be 1.5. Snyder adopted a standard duration tr in hour(s) of effective rainfall

given by,

tr = tp/5.5 ...……………………………………...(2.14)

The peak discharge Qps (m3/s) of a unit hydrograph of standard duration tr in

hour(s) is given by Snyder as,

Qps = (2.78CpA)/tp ………………………………………...(2.15)

Figure 2-3: Snyder's Synthetic Unit Hydrograph

~ 21 ~

Where,

A = catchment area in km2 and

Cp = a regional constant. (In this study regional constant is considered to be 0.43)

If a non – standard rainfall duration tR in hour(s) is adopted then,

t’p = tp + (tR – tr)/4 ………………………………………...(2.16)

tb = (72 + 3 t’p) hours ………………………………………...(2.17)

Taylor and Schwartz recommended,

tb = 5 (t’p + tR/2) hours ………………………………………...(2.18)

W50 = 5.87/q1.08 ………………………………………...(2.19)

W75 = W50/1.75 ………………………………………..(2.20)

Where,

t’p = basin lag in hours for an effective duration of tR in hour(s) and tp.

tb = time basin.

W50 = width of unit hydrograph in hour(s) at 50% peak discharge

W75 = width of unit hydrograph in hour(s) at 75% peak discharge

q = Qp/A = peak discharge per unit catchment area in m3/s/km2

~ 22 ~

2.2.2 Risk Analyzing Methods

The probability of 5, 10, 15, 25, 50 and 100 years flood happening at least once

within 1963 can be determined by using the equation (2.15).

Pr = 1 − [1 − 1/T]N ………………………………………..(2.21)

Where,

Pr = Probability

T = Return Period

N = Number of years starting from 2014 to 2063 = 50 years

2.2.2.1 Risk Identification Method

The various strengths, weaknesses, opportunities and threats of the study area can

be useful tools for the strategic planning process and environmental management. This

method is widely recognized and it constitutes an important basis for learning about the

situation and for designing future procedures, which can be seen necessary for thinking

in a strategic way (Lozano & Valles, 2007). Such analysis can be used in the Ganges

River Basin to decrease the impact due to the risk implied by flood. The Strengths,

Weaknesses, Opportunities and Threats can be determined based on the current condition

of the study area and also by studying the previous studies done on the condition of the

study region.

By studying the Weaknesses and Threats, the possible risks, their impacts and

consequences for each flood events can be realized. The impact consequences can later

be used to find out the type of risks the impacts may implement in that area which will

ultimately assist the process of risk rating by determining the category of consequences.

~ 23 ~

By studying the Strengths and Opportunities the possible mitigation measures can be

determined.

2.2.2.2 Risk Rating Method

The risk matrix is an effective tool to determine the risk rating of the risks of each

flood events. In order to carry out the task of risk rating the likelihood category and the

consequence category has to be determined. To determine the likelihood category the

probability of occurrence of each flood event is used. The consequence category can be

determined by assessing the risk impacts of each flood event by studying the previous

impacts of flood in the study area. The likelihood categories and consequence categories

have been explained in table 2-1 and table 2-2. A slight modification has been done to

the original table by designating each category with numbers so that the level of

likelihood and consequence can be further understood.

Category Level Description Probability Range

A 6 Likely Pr ≥ 0.1

B 5 Unlikely 0.1≥ Pr ≥ 0.01

C 4 Very Unlikely 0.01≥ Pr ≥ 0.001

D 3 Doubtful 0.001≥ Pr ≥ 0.0001

E 2 Highly Unlikely 0.0001≥ Pr ≥ 0.00001

F 1 Extremely Unlikely 0.00001≥ Pr

Table 2-2: Likelihood Categories for a Risk Matrix (Source: Ayyub, 2003)

~ 24 ~

Category Level Description Example

I 6 Catastrophic Larger number of fatalities, and/or major term impact.

II 5 Major Fatalities, and/or major short term impact.

III 4 Serious

Serious injuries, and/or significant environmental

impact.

IV 3 Significant Minor injuries, and/or short term impact.

V 2 Minor First aid injuries only, and/or minimal impact.

VI 1 None No significant consequences.

Table 2-3: Consequence Categories for a Risk Matrix (Source: Ayyub, 2003)

After determining the likelihood and consequence category of each risk for each

flood event, the corresponding levels are multiplied in order to find out the risk rating of

each impact. The following equation is used for such purpose.

Risk Rating = Consequence Level × Likelihood Level……………………………..(2.22)

2.2.2.3 Method of Determining the Risk Level

After computing the Risk Rating for each impact, the ratings are used to

determine the risk level. For this purpose the risk matrix is used. In the 13th page of

‘Common Methodology for Risk Assessment & Management’ by the Royal Academy of

Engineering of United Kingdom it is stated, “The Risk Control matrix is a powerful tool

for displaying different levels of information on relations between risks and controls”.

~ 25 ~

Lik

elih

ood

A (6) L (6) M (12) M (18) H (24) H (30) H (36)

B (5) L (5) L (10) M (15) M (20) H (25) H(30)

C (4) L (4) L (8) M (12) M (16) M (20) H (24)

D (3) L (3) L (6) L (9) M (12) M (15) M (18)

E (2) L (2) L (4) L (6) L (8) L (10) M (12)

F (1) L (1) L (2) L (3) L (4) L (5) L (6)

VI (1) V (2) IV (3) III (4) II (5) I (6)

Consequence

Table 2-4: Risk Matrix (Source: Ayyub, 2003)

After the risk rating is done, it is required to find out whether the risks are within

the tolerable limit or not, which is done by observing the place of the risk rating in the

risk level table. In the risk matrix shown above L (Low), M (Medium) and H (High) are

the three levels of risks which are recognized by the risk rating of an impact which are

the products of consequence and likelihood level. The products of such elements are

within the bracket beside the levels of risk. The level of risk of an impact is determined

by using the Risk Matrix in table 2-4.

Table 2-5: Risk Level and Remarks

Risk Level Risk Rating Remarks

Low 1 – 10 Requires minimum amount of adjustments for safety and

precaution.

Medium 11 – 20 Requires a significant amount of adjustment, mitigation,

management and back up security.

High 21 – 36

Requires emergency prevention measures, mitigation and

management measures, evacuation of people along with

food supply and treatment facility supplies.

~ 26 ~

2.3 Determine Prevention and Emergency Response

Measures

After the risk level is estimated, the existing prevention and controlling measures

has to be documented. Then additional risk controlling measures has to be introduced to

reduce the risk level as much as possible.

According to the Royal Academy of Engineering of United Kingdom, the

Prevention and Emergency Response Measure can be implemented in the 5 following

ways.

1. Avoid: To make a fundamental change so that the risk is no longer an issue

2. Reduce: Reduce the likelihood of the incident to happen.

3. Mitigate: To decrease the consequences of the incident if it should happen.

4. Transfer: To transfer the risk to an organization more capable of handling it.

5. Do Nothing: To realize the risk and accept the consequences.

~ 27 ~

Chapter 3: Data Analysis and Discussion

3.1 Data Analysis

3.1.1 Determining Average Rainfall and Discharge per

Day

In the first part of the data analysis the Thiessen Polygon method is used to

determine the average rainfall per day for every year from 1984 to 2013. For this purpose

equation no. 2.1 is used. Note that the catchment area, A is equal to 46300 km2. The data

of rainfall per day for every year was collected for eleven rain gauge stations, each of

them having a corresponding weighted area for the polygon surrounding them, which

was determined by using Autodesk AutoCad. The weighted area for each rain gauge

station is shown in table 3-1 below.

Thiessen Polygon Method

Station Polygon Area (km2) Total Area (km2) Wt. Average

Chuadanga 3,428.93

46,300

0.0741

Faridpur 3,997.32 0.0863

Madaripur 4,170.66 0.0901

Jessore 3,971.30 0.0858

Barisal 2,108.91 0.0455

Khulna 3,095.08 0.0668

Satkhira 3,199.08 0.0691

Patuakhali 5,126.09 0.1107

Bhola 2,108.97 0.0456

Mongla 6,108.01 0.1319

Ishurdi 8,985.63 0.1941

Table 3-1: Area & Weighted Area for Polygons Surrounding Each Rain Gauge

Station

~ 28 ~

Discharge per day is calculated by using equation no. 2.4, for which equation no.

2.2 is used to calculate rainfall intensity. The rainfall intensity is calculated by using

equation no. 2.3 to determine the time of concentration. The runoff coefficient has been

logically assumed to be 0.43 by assessing table 3-2.

After the daily discharge is calculated, the discharge per day from May to

October is summed to find out the flood discharge for every year. By using the flood

discharge per year, the Discharge Frequency Curve is plotted to calculate the flood

discharge for 5, 10, 15, 25, 50 and 100 years return period.

Drainage Complex Area (km2) Runoff Coefficient

Upper Bhairab - Mathabanga 1,510 0.46

Kaliganga - Kumar - Nabaganga - Chitra 2,813 0.43

Begabati - Chitra - Bhairab 2,142 0.38

Gorai - Madhumati - Chandana - Barasia 2,068 0.46

Betna - Kobadak 1,462 0.4

Total 9,995 0.43

Table 3-2: Runoff Coefficients of Various Drainage Complex (Source:Sir William

Halcrow and Partners Ltd., 1993)

~ 29 ~

3.1.2 Flood Frequency Analysis

The table 3-3 shows the flood discharge of each year in a descending manner

according to their magnitude, the calculated probability and return period of each flood

discharge.

Years Discharge (m3/Flood Period) Rank (m) Probability (Pr) Return Period (T)

1984 22,508,071,407.36 1 0.032 31

2004 21,951,689,874.73 2 0.065 15.50

1999 21,419,567,841.28 3 0.097 10.33

2002 21,039,607,753.24 4 0.129 7.75

1986 20,568,601,625.84 5 0.161 6.20

2005 20,232,416,410.05 6 0.194 5.17

2007 20,044,623,708.56 7 0.226 4.43

1991 19,896,987,883.67 8 0.258 3.88

1988 19,712,823,890.56 9 0.290 3.44

2001 19,553,483,504.41 10 0.323 3.10

2011 19,519,654,089.26 11 0.355 2.82

1987 19,420,540,072.29 12 0.387 2.58

2013 18,732,636,493.40 13 0.419 2.38

2000 18,247,302,727.93 14 0.452 2.21

1990 18,229,677,313.51 15 0.484 2.07

2006 18,057,244,848.19 16 0.516 1.94

1996 17,948,767,145.92 17 0.548 1.82

1995 17,794,899,581.57 18 0.581 1.72

2008 17,604,309,803.24 19 0.613 1.63

1993 17,537,748,085.90 20 0.645 1.55

1997 17,526,722,274.52 21 0.677 1.48

2009 17,090,502,990.80 22 0.710 1.41

1998 17,066,775,665.58 23 0.742 1.35

2003 16,431,286,349.10 24 0.774 1.29

1985 15,872,089,850.65 25 0.806 1.24

1989 14,969,923,147.08 26 0.839 1.19

2010 14,648,286,081.60 27 0.871 1.15

1992 14,237,410,917.13 28 0.903 1.11

1994 14,231,401,251.34 29 0.935 1.07

2012 13,693,472,680.37 30 0.968 1.03

Table 3-3: Discharge Frequency Table

~ 30 ~

From table 3-3 the following data can be inferred.

Sample size, N = 30,

Mean of the variate, X = 18192950842.303 m3/Flood Period,

Standard deviation, s(N-1) = 2364171490.43,

Reduced mean for sample size 30, Ny = 0.5362,

Reduced standard deviation for sample size 30, NS = 1.1124

By using the above values in equation 2.9, 2.11 and 2.12 the following table is

developed.

Return

period,

T (years)

Reduced

variate, Ty

Frequency

factor, K

Discharge, QT (m3/Flood period)

(Value of variate X corresponding

to T, XT)

5 1.50 0.87 20,240,964,312

10 2.25 1.54 21,835,839,280

15 2.67 1.92 22,735,654,274

20 2.97 2.19 23,365,681,657

25 3.20 2.39 23,850,968,050

30 3.38 2.56 24,245,762,229

Table 3-4: Table for Discharge for Corresponding return Period by Using Chow’s

Equation and Gumbel’s Method

~ 31 ~

Table 3-4 is used to plot the Gumbel’s Probability Paper in a semi-log graph in

which the discharge is aligned with the y-axis and return period is aligned with the x-

axis. The graph is shown in figure 3-1.

Figure 3-1: Discharge Frequency Curve

From Chow’s equation and by using the best fitted straight line’s equation the

discharge on surface due to precipitation for 5, 10, 15, 25, 50 and 100 years return period

which is shown in the table 3-5.

Return Period in Years Discharge (m3/Flood Period)

5 20,240,964,312

10 21,835,839,289

15 22,735,654,274

25 23,365,681,657

50 27,824,046,011

100 29,210,340,372

Table 3-5: Calculated Discharge for Different Return Period

y = 2E+09ln(x) + 2E+10

R² = 0.9998

200.00

205.00

210.00

215.00

220.00

225.00

230.00

235.00

240.00

245.00

1 10 100

Dis

charg

e, Q

(m

3/F

lood

per

iod

)

x 1

00000000

Return Period (T)

Gumbel’s Probability Paper

~ 32 ~

3.1.3 Formation of Hydrographs

In order to plot hydrographs for various return periods, the synthetic unit

hydrograph has to be plotted. For the purpose equation 2.8 to 2.14 are used.

If,

L = 468 km

Lca = 262 km

Ct = 1.5

tR = 45 hrs

Cp = 0.43

A = 46300km2 = 46300×106 m2

Then,

tp = 37.82 hours

tr = 6.88 hours

Qps = 1463.4 m3/s

t’p = 47.35 hours

W50 = 256.27 hours

W75 = 146.44 hours

tb = 214.05

Taylor and Schwartz recommended,

tb = 349.25 hours

By using the data provided above, the synthetic unit hydrograph for 1mm rain has

been plotted in figure 3-2.

~ 33 ~

Figure 3-2: Synthetic Unit Hydrograph

After plotting the synthetic unit hydrograph, the rainfall excess is calculated by

dividing the flood discharge for 5, 10, 15, 25, 50 and 100 years return period by the

catchment area. The rainfall excess for each return period is shown in the table below.

Return Period in Years Rainfall excess per flood period

(mm)

5 437.17

10 471.62

15 491.05

25 515.14

50 600.95

100 630.89

Table 3-6: Rainfall Excess for Each Return Period

0

400

800

1200

1600

0 100 200 300 400

Synthetic Unit Hydrograph

Dis

char

ge,

Q (

m3/s

)

Time (Hours)

~ 34 ~

The rainfall excess is the used in the synthetic unit hydrograph to plot

hydrographs for flood for 5, 10, 15, 25, 50 and 100 years return period.

Figure 3-3: Hydrograph for Each Return Period

In figure 3-3, it can be observed that floods with a 5, 10, 15, 25, 50 and 100 years

return periods has the peak flow of 639775.8 m3/s, 690186.5 m3/s, 718627.8 m3/s,

753880.7 m3/s, 879461.6 m3/s and 923279.5 m3/s, respectively.

0

10

20

30

40

50

60

70

80

90

100

0 100 200 300 400

x 1

0000

Flood Hydrograph for the Return Period of 5 Years

Flood Hydrograph for the Return Period of 10 Years

Flood Hydrograph for the Return Period of 15 Years

Flood Hydrograph for the Return Period of 25 Years

Flood Hydrograph for the Return Period of 50 Years

Flood Hydrograph for the Return Period of 100

Years

Time (Hours)

Dis

char

ge,

Q (

m3/s

)

~ 35 ~

3.2 Risk Analysis

The table 3-7 shows the probabilities of 5, 10, 15, 25, 50 and 100 years return

period floods occurring at least once within the timeframe of 2014 to 2063. The

probability of each event was calculated by using equation 2.21.

Flood Events Probability

5 years return period flood 1

10 years return period flood 0.99

15 years return period flood 0.96

25 years return period flood 0.87

50 years return period flood 0.64

100 years return period flood 0.39

Table 3-7: Probability of Various Flood Events

3.2.1 Identified Risks and Discussion

By studying and analyzing the discussions in the dissertation of Saleh Ur Rahman

(2014) and term paper of Md. Abdulla Hel Kafi & Abu Sale Md. Tofahel Chowdhury

(2010), the followings were obtained as Strengths, Weakness, Opportunities and Threats.

Strengths:

1. There are available agricultural land and manpower for increased

agricultural work. The land and manpower can be used to produce food at the time of

flood by growing crops and harvesting them in the floodless seasons and storing

them afterwards till the flood occurs.

2. Some infrastructural control of flood was identified such as

embankments, evacuation measures, shelter homes for the flood affected people.

~ 36 ~

3. Institutions such as the Ministry of Disaster Management and Relief,

Bangladesh Water Development Board (BWDB), Institute of Water Modeling

(IWM), Water Resource Planning Organization (WARPO), BUETS’s Institute of

Water and Flood Management (IWFM) etc. has taken the authority of flood

management in the study area as well as the other flood prone area of Bangladesh.

4. Control of Floodplain Development by controling the location of

development (i.e., land uses, buildings, infrastructure, etc.) on identified floodplains.

Techniques include: floodplain regulations, zoning and land use by-laws, building

codes, development policies, and tax adjustments. Government of Bangladesh passed

legislation for conservation of floodplains and wetlands.

Weaknesses

1. The houses in the rural areas of the study region are mostly built up by

using bamboos and mud with tin as roofs. The RCC houses often tend to be

structurally weak due to building more stories than the foundation allows. Not only

that, but also, many of these houses are un-elevated which will allow the flood water

to get into them easily.

2. There are drainage systems that are either damaged or blocked due to lack

of maintenance. The damages and blocks often occurs because of improper design,

exceed of lifetime, less sustainability and other reasons such as accumulation of

leaves, mud, tree branches etc.

3. A lot of the road network systems are not as elevated as they should be.

Also, these road networks are either made out of clay and mud which are vulnerable

to flood or are made out of improperly prepared bitumen and asphalt. Water was

always a threat for bitumen and asphalt. That is why weak bitumen and asphalt are

much more vulnerable to flood water.

~ 37 ~

4. The study region is mostly a low lying area and the whole of it is the

catchment of Ganges in Bangladesh. These reasons pose a greater threat as the region

is flood prone.

5. Above all of the threats, the intensity of rainfall in the rainy season is

high.

Opportunities

1. The food storage facilities can be increased by utilizing the manpower and

available agricultural area.

2. Various codes and regulations are available which are verified by the

Government of Bangladesh, which may be useful in cases of modifying the existing

structures, embankments, road networks etc

3. Foreign and domestic reliefs are available for the help of flood affected

people in the study region.

4. National and international NGOs and researchers are constantly working

to minimize the impact of flood in the study area.

Threats

1. Climate change, a global problem, poses a great to the study region as it is

close to the coast. Due to the climate change not only the sea level is rising but also,

the pattern of weather is changing. The coastal areas of this region have even lower

elevation. For this reason these areas are very vulnerable when it comes to flood. The

farmers of this region are accustomed to grow crops in a pattern corresponding to the

flood pattern. But due to the climate change, the flood pattern is changing. To adjust

with the flood patterns, the farmers will have to change their farming pattern which

may take some while and therefore there might a problem with the crop based food

supply.

~ 38 ~

2. Trees always play a significant environmental role in the world. The

roots of trees absorb the water of flood and a bit of the rainfall stays in the leaves of

trees, which gets evaporated directly. These contribution of trees helps by reducing

the flood level a little bit if not significantly. But in Bangladesh deforestation is

occurring at an increasing pace.

3. Human population increase has been a problem of great significance in

Bangladesh. The study region also has a significant population increase rate, which if

continues for the next 50 years then one might dare say that there will not be enough

food, housing facilities, health facilities etc. to provide to the vast population at a

time of flood.

4. Inundation, sedimentation and erosion may have an adverse impact on

agriculture by destroying the crops and by erosion of agricultural land.

5. Loss of livestock may occur due to flood, if not re-located properly. This

loss will severely affect the people of the rural areas of the study region as most of

them are dependent on the income made by using the live stocks as an income source

after agriculture.

6. Floods have a huge impact on the human health as during the flood

period, most of the health institutes remain closed because of entrance of water.

Apart from that, flood also spreads water borne and water related diseases such as

cholera, diarrhea, typhoid etc to which the elderly and children are most vulnerable.

~ 39 ~

From the findings of the previous discussion, the following risks have been

identified.

1. Damage of Houses and Household Products: Houses may get washed

away, demolished or water may enter the premises of low lying houses and damage

the household products

2. Inundation of Road Networks and Their Damage/Destruction: Rural

road networks are mostly built out of mud and clay. These roads may get washed

away. Also there are road networks built by asphalt and bitumen, which my get

damaged due to flood. Due to the elevation of the road networks, it is very likely that

they will get inundated in the flood period.

3. Damage of Crops and Losses of Livestock: As the study region is a low

lying region of Bangladesh where most of its dwellers are dependent on agricultural

goods and livestock, the inundation during flood period may result in damage of

crops and livestock, which will affect the income and livelihood of the people of this

region.

4. Erosion of Agricultural Land and Problems in Relocation of People:

Due to being a part of the Ganges river basin, the huge flow of the river during the

flood period may result in erosion of agricultural land, making it more difficult to

gain sustainable livelihood by growing crops after the flood period. Moreover, the

constant increase of population is not helping. Because of the lacking in land due to

erosion, not only will it be hard to supply agricultural goods, but also relocation of

the flood affected people will become a problem.

5. Spread of Diseases and Effects on Health: The affected people specially

the elderly and children will get affected by various water-borne and water related

diseases such as typhoid, cholera, diarrhea etc. There is another disease which can be

~ 40 ~

added to this list. Even though it did not strike Bangladesh yet, Ebola continues to

pose a great threat. Our neighboring country India is already dealing with the

consequences of Ebola and it is only a matter of time before it strikes Bangladesh. In

the flood period, it may strike and spread with rapid speed as it has the capability of

spreading by water, if water comes in contact with the Ebola affected people. Also, in

the flood periods, the hospitals, nursing homes, clinics and other health complex

remains close due to the entrance of water in their premises. So, at the time of the

wild spread of diseases, there will be few medical attentions to give.

~ 41 ~

3.2.2 Risk Rating

By using table 2-1 and 2-2 the likelihood and consequence categories can be

determined for each risk. Afterward equation 2.22 is used to find out their risk rating. All

of the flood events have a probability more than 0.1 of occurring at least once from 2014

to 2063. For this reason, it can be realized that all of the identified risks has a probability

of more than 0.1, which means that the risks are likely to occur having likelihood

category of A with the likelihood level of 6. The table below shows the risk rating of

each risk.

Risk Likelihood Consequences Risk

Rating Category Level Description Category Level Description

Damage of

Houses and

Household

Products

A 6 Likely II 5 Major 30

Inundation

of Road

Networks

and Their

Damage &

Destruction

A 6 Likely II 5 Major 30

Damage of

Crops and

Losses of

Live Stocks

A 6 Likely III 4 Serious 24

Erosion of

Agricultural

Land and

Problems in

Relocation

of People

A 6 Likely I 6 Catastrophic 36

Spread of

Diseases

and Effect

on Health

A 6 Likely I 6 Catastrophic 36

Table 3-8: Risk Rating Table

~ 42 ~

3.2.3 Risk Levels and Remarks

The risk levels of the various risks can be obtain by using table 2-4 and the risk

matrix in table 2-3.

Risks Risk

Rating

Risk

Level

Color

Code

Remarks on Prevention and

Emergency Response Measures

Damage of Houses

and Household

Products

30 High Red

Requires emergency prevention

measures, mitigation and

management measures, evacuation of

people along with food supply and

treatment facility supplies.

Inundation of Road

Networks and Their

Damage &

Destruction

30 High Red

Damage of Crops

and Losses of Live

Stocks

24 High Red

Erosion of

Agricultural Land

and Problems in

Relocation of People

36 High Red

Spread of Diseases

and Affect on Health 36 High Red

Table 3-9: Risk Level of Identified Risk and Remarks

~ 43 ~

3.3 Discussion on Prevention and Emergency Response

Measures

Previously in chapter two, it was discussed that according to the Royal Academy

of Engineering of United Kingdom, the Prevention and Emergency Response Measure

can be implemented by avoiding, reducing, mitigating, transferring, and by doing

nothing. The risk levels of the identified risks are high. This indicates that avoiding,

reducing, transferring and doing nothing about the impacts of these risks is undesired.

The one option that is available is that the impacts of such risks can be mitigated and

managed so that the effects can be reduced to a tolerable extent.

3.3.1 Current Mitigation Measures

The Asian Disaster Reduction Center (January, 2005) has described the on-going

mitigation practices in Bangladesh. In recent years, the Government of Bangladesh

(GoB) has been placing increased emphasis on reduction of the human, economic and

environmental costs of disasters, through enhancing the national capacity for disaster

mitigation. The programs on disaster management in Bangladesh focus equally on

structural and non-structural practices intended for disaster mitigation:

1. Structural Mitigation: GoB has so far constructed 1,841 cyclone shelters

and 200 flood shelters. During the last four decades 482 water and flood control projects

have been implemented, through which flood protection embankments totaling about

8,200 km, drainage channels of total length 3,400 km and 9,000 sluice gates and

regulators on different rivers and canals have been constructed as safety measures

against inundation by tidal waves, storm-surges and flooding.

~ 44 ~

2. Non-Structural Mitigation: Non-structural mitigation practices pursued

by the GoB focus on (1) preparedness and possibilities for action to reduce risks and

losses, and (2) better coordination mechanisms between all actors involved (GoB, NGO

and community people at the grass-roots level) during all phases of disaster. Such

practical measures under the recently completed project: "Support for Disaster

Management" involve the following.

i) Legislation, Policy and Planning: The Disaster Management Legislation

(Act) and the Clear and Comprehensive National Policy on Disaster Management and

National Disaster Management Plan was prepared.

ii) Training and Public Awareness: The Disaster Management Bureau

(DMB) has conducted 447 training courses, workshops and seminars attended by

government and semi-government officials of different levels, as well as various

stakeholders. In addition, the DMB has developed Disaster Management training

modules and has supported the arrangement of disaster management training workshops.

iii) Institutional Arrangements: The GoB has taken a number of significant

steps for building up institutional arrangements from national to union levels for

effective and systematic disaster management.

1. Naming of the Ministry of Relief and Rehabilitation as the

Ministry of Disaster Management & Relief (MDMR).

2. Establishment of Disaster Management Bureau (DMB) in 1993.

3. Establishment of Council and Committees at the national, district,

upazilla and union levels.

4. Establishment of Emergency Operations Centre (EOC).

iv) Warning Systems: In Bangladesh there are two warning systems: flood

warning and cyclone warning.

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1. Flood Warning System: Flood warning has been in a state of

continuous development since 1972. There was a significant improvement in the

Flood Forecasting and Warning system (FFWS) with the introduction of the

hydrodynamic super model MIKE 11 and an increase in the number of real-time

monitoring stations to 30. With such modernization, FFWS yielded a successful

result during the devastating flood of 1998.

2. Cyclone Warning System: The existing cyclone warning has two

signal numbers inherited from British India. In line with growing need, the

warning signals were simplified and made specific and easily understandable in

June 2002.

v) Local Disaster Action Plans: The Disaster Management Bureau (DMB)

drafted a model action plan at local levels. It was modified after discussion at a

workshop attended by eminent experts involved in disaster-related activities. Based on

that, the DMB has been able to finalize the Local Disaster Action Plan (LDAP) with the

help of national consultants. The main purpose of the LDAP is to mobilize local

communities in the most disaster-prone areas to increase their own capacities to cope

with and recover from a disaster.

3. Coordination Mechanisms: The high-powered National Disaster

Management Council (NDMC) and Inter-Ministerial Disaster Management Coordination

Committee (IMDMCC) meet twice and four times a year respectively. While the NDMC

formulates and reviews disaster management policies and issues directives to all

concerned, the IMDMCC plays the key role in implementing the directives maintaining

inter-ministerial coordination, supervising the services of the Armed Forces as well as

NGOs working in the field of disaster management.

~ 46 ~

4. CDMP: In mid-1999 the GoB, together with UNDP and other

development partners, agreed to address the issue of risk reduction in a more

comprehensive programmatic approach. As agreed, the Comprehensive Disaster

Management Program (CDMP) has been formed.

3.3.2 Suggested Mitigation Measures

Even though the GoB has introduced numbers of mitigation measures, from the

discussion of this thesis, a few more mitigation measures can be proposed for the flood

affected people in the Ganges basin of Bangladesh. These mitigation measures are

described below.

1. Environmental Management:

i) Mangrove Preservation: Mangrove may help reduce the impacts

of risks and damages in three ways. First mangroves can reduce storm surge

levels by up to half a meter for each kilometer of mangrove that the storm surge

passes through. Secondly, the height of wind and swell waves is reduced by 13 to

66 per cent within the first 100m of mangroves. These waves can be

superimposed on top of storm surges, so their reduction can make a critical

difference to storm impacts. Even during ‘everyday’ conditions, when such

waves are small, they still contribute to coastal erosion if their energy is not

dissipated by a dense tangle of mangrove roots and branches. Their devastating

impact is felt for example in parts of Central Java, where the coastline retreated

hundreds of meters in a decade due to a mix of mangrove deforestation,

aquaculture development, soil subsidence and sea level rise. Thirdly, mangroves

can help to stabilize sediments, both through their active growth and deposition

~ 47 ~

of organic matter and by capturing sediments. (Tonneijck, van Wesenbeeck &

Spalding, 2013)

ii) Afforestation: It is said that trees are friends of mankind. The

people of Bangladesh are suffering from vast environmental disadvantages due to

the reduction of the trees. Trees absorbs water through their root and they also

have the capability of holding the soil together by using their roots & therefore

reduce the amount of erosion of land. They also contribute by capturing some of

the rain fall which decreases the intensity as well as sends the water back to the

environment through direct evaporation. Hence such friends are required to

decrease the magnitude of flood and reduce its effects thereby.

iii) Waste Management: It has been discussed previously that the

wastes such as leaves, tree branches, papers etc. are the reason for water to get

logged in the drainage system. Also human wastes are not managed properly and

therefore they remain exposed by mixing with the logged water in the drains. Due

to floods, these wastes get mixed with the flood water which spreads diseases.

Therefore, waste management mitigation measures are vital for health hazard

mitigations.

2. Water Resource Management:

i) Drainage: Drainage facilities need to be improved and increased.

Proper maintenance and cleaning of drains will ensure a reduced danger of health

by removing the gray and black water.

ii) Rain Water Harvesting: Rain water harvesting is a very efficient

way of collecting and storing rainwater so that in times of flood, pure drinking

water can be available. Now-a-days, rain water harvesting has become a very

popular method of collecting water and various researches are being done on this.

~ 48 ~

3. Infrastructural Measures:

i) Road Networks: Road networks need to be more elevated. Also

they require stronger materials so that the road networks do not get damaged at

the period of flood. For stronger material and their correct use, trained and highly

motivated supervisors and workers are needed. The foundation of such road

networks also has to be strong enough to withstand the force of discharge of

flood.

ii) Buildings and Houses: Current buildings and houses has to be

modified as per BNBC code so that they can resist the flow force of flood to

some extent. The new houses and structures should be built by elevating their

foundations as well as by following the BNBC codes to avoid the damages that

may occur due to floods. Also shelter homes should be built at an elevation where

flood water cannot reach.

4. Agricultural Mitigation Measures: The available agricultural land and

manpower should be utilized in order to produce maximum food from crops. In seasons

when flood is less likely to occur, food production from crop should be maximized so

that enough food can be stored in order to supply them readily at the time of flood when

there may be a scarcity of food.

5. Mitigation Measures Related to Fisheries:

i) Development of Storm Resistant Boats: Many of the fishermen

of the study region fishes in the Ganges and its distributaries. At times of storms

which causes flood, the safety of these fishermen becomes threatened due to the

weakness of their boats. For this reason, storm resistant boats are required.

ii) Conflict Resolution between Shrimp & Rice Farmer: As found

by Hossain, Uddin & Fakhruddin (2013), many shrimp ponds have been

~ 49 ~

constructed on ex-agricultural land, especially rice paddy. Here, although land is

often privately held, a transfer of land-use (if not ownership) has occurred from

the former small-scale farmers to larger private shrimp farming concerns. The

means of transfer reported have covered legal sale of deed, forced sale through

harassment, sale following degradation of the land by shrimp farm pollution, sale

after inundating and degrading the land with salt water, and evictions of tenants.

The effects are much the same as for mangroves—an established livelihood

support system is broken down as land with multiple agricultural uses is turned

over to mono-crop production. The earnings from gher construction through

conversion of rice fields are temporary (Ito, 2002). As soon as the construction of

a gher is completed, this employment opportunity as a laborer is gone. Rice

farming is more labor-intensive than shrimp farming (Barraclough and Finger-

Stich, 1996). These types of land conversion are the following results: rural

peasants become detached from reliance on the land and are effectively made

available to the wage labor market; food production (and other products) for

own-consumption and local markets declines; food security worsens as

livelihoods more dependent on both the market for shrimps and the market for

foodstuffs; changes in land use and ownership lead to conflict sometimes violent

between dispossessed groups and new capitalist owners (Neiland et al. 2001). In

addition, the conversion of rice fields into shrimp ponds has reduced the

opportunities for other traditional dry season activities, such as grazing cattle and

homestead gardening (Alam et al., 2005). Conflict resolution between rice and

shrimp farmers is required in order to produce secure food in the flood period.

~ 50 ~

Chapter 4: Conclusions and Recommendation

This study attempts to understand the characteristics of floods of various return

periods in the Ganges Basin of Bangladesh and the risk they pose. In this study the

existing mitigation measures and the future mitigation measures which can reinforce the

current mitigation measures were discussed.

By using the flood frequency analysis the flood discharge for 5, 10, 15, 25, 50

and 100 years return period has be calculated to be 20.24 × 109 m3/Flood period, 21.84 ×

109 m3/Flood period, 22.74 × 109 m3/Flood period, 23.37 × 109 m3/Flood period, 27.82

× 109 m3/Flood period and 29.21 × 109 m3/Flood period respectively and by using the

synthetic unit hydrograph the peak discharge during each event has been calculated to be

639775.8 m3/s, 690186.5 m3/s, 718627.8 m3/s, 753880.7m3/s, 879461.6 m3/s and

923279.5 m3/s respectively. The probability of the floods with the various return period

stated above has found to be 1, 0.99, 0.96, 0.87, 0.64 and 0.39 respectively.

The identified risks which is posed by various flood events are Damage of

Houses and Household Products, Inundation of Road Networks and Their

Damage/Destruction, Damage of Crops and Losses of Livestock and Spread of

Diseases and Effects on Health.

The effects of these risks can be reduced to a tolerable amount by the

implementation of various mitigation and control measures. The Asian Disaster

Reduction Center (January, 2005) has described the existing mitigation measures that are

being practiced in Bangladesh and these measures can be divided into categories such as

Structural Measures, Non-Structural Measures, Coordination Mechanism and

Formation of Comprehensive Disaster Management Program (CDMP).

~ 51 ~

This study also identified a few mitigation measures which can be useful to

strengthen the flood risk effect mitigation practice in the study area. These mitigation

measures are Environmental Management, Water Resource Management,

Infrastructural Measures, Agricultural Mitigation Measures and Mitigation

Measures Related to Fisheries.

The figure of the hydrographs for each return period, the discussion on the

identified risks and their current & suggested mitigation measures has been shown and

discussed in chapter 3.

The findings of this study are based on the precipitation characteristics and

magnitude in the Ganges Basin of Bangladesh, but not limited to the study area. The

mitigation measures that are discussed in this study is applicable to every flood prone

region in Bangladesh.

4.1 Limitations of This Study

Even though this study covers the whole aspect of Flood Risk Analysis, there are

some limitations in it which are discussed below.

1. Due to insufficient amount of information on the whole study regions land

covers and uses the exact runoff coefficient could not be used.

2. As there was a disadvantage of not having high technology software, most

of the calculations were done manually. Also due to the same reason flood mapping

could not be done to estimate which of the areas, the amount of areas and how many

people may be affected.

~ 52 ~

3. Because of the unavailability of economic and financial information, the

amount of financial and economic losses for each flood event could not be

calculated.

4. The study could be more accurate if the precipitation data for the basin of

the upstream of Ganges River was available, but unfortunately, the upstream basin

lies in India.

4.2 Recommendation

If one has the wish to advance this study, then the boundaries of this study can be

further expanded if one can obtain the unavailable data & information and overcome the

disadvantage of not having high technological software. One can also strengthen this

study by completing the Flood Frequency Analysis by Log-Pearson Type III distribution

and Log Normal Distribution in order to compare with the Gumbel’s Extreme-Value

Distribution method which has been used in this study. Also the same study can be done

in any part of Bangladesh in order to manage and mitigate the flood risks in this country.

~ 53 ~

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