Module-04:

Disasters of Bangladesh: Floods

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January 29, 2018

URP 4243: Hazards and Disaster Management

Course Teacher: Md. Esraz-Ul-Zannat Assistant Professor Dept. of URP, KUET

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ACKNOWLEDGEMENT

These slides are aggregations for better understanding of the topic

mentioned in the previous slide . I acknowledge the contribution of

all the authors and photographers from where I tried to accumulate

the info and used for better presentation.

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Topics to be Covered by this Presentation

Disasters of Bangladesh

Flood Definition

Types of Floods

Causes of Flood in Bangladesh

Flood Hazard Parameters

Flood Hazard Mapping

Flood types in Bangladesh

Approaches of Flood Damage Assessment

Flood Forecasting and Warning Services, by FFWC

Flood Forecasting and Warning Services Measure

Flood Preparedness

Flood Shelters

Flood Embankment

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Common disasters of Bangladesh are Floods, Earthquakes, Cyclone and Storm Surges, Landslide, River Bank Erosion, Drought, Fire, Building collapsed, Arsenic Contamination.

Salinity, thunderstorm and extreme temperature on the rise.

DISASTERS OF BANGLADESH

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FLOODS

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FLOOD

A simple definition of flooding is water where it is not wanted. According to GeoScience, Australia, Comprehensive definition of a flood is

A general and temporary condition of partial or complete inundation of normally dry land areas from overflow of inland or tidal waters from the unusual and rapid accumulation or runoff of surface waters from any source.

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Majorly four types of floods:

Climate Change Induced Flood

Flash Floods

Riverine floods

Coastal floods

TYPES OF FLOODS

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CLIMATE CHANGE INDUCED FLOODS

Bangladesh is a very low lying country. Floods are exacerbated by global warming in the last few years has set in motion the rise in sea water levels.

Flood magnitude and frequency could increase in many regions as a consequence of increased frequency of heavy precipitation events as a result of climate change, which can increase runoff in most areas as well as groundwater recharge in some floodplains.

The shallow sea bed and the fact that water coming down from the rivers Ganges and Brahmaputra cannot escape when the water level raises all contribute to the severe flooding of the coastline in Bangladesh.

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FLASH FLOODS

Most floods develop slowly with some early warning

Flash floods occur and dissipate suddenly, and the water rises and falls rapidly

Flash floods are rapid onset floods that occur 6-12 hours or less after the associated rainfall. Flash floods near the foothills of mountains after intense rainfall on higher grounds

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FLASH FLOODS They can also be triggered by the failure of hydraulic

infrastructure such as dams or levees, outburst of glacier lakes, ice-jams or log-jams.

Due to the force of rapidly flowing water, they pose serious threats to the structural integrity of buildings and infrastructure.

Damage with flash floods is unpredictable due to having difficulties with predicting the phenomenon and with the short warning lead times. In matter of damages there is less time for preparatory action, so that potential losses are closer to actual losses than with other flood types.

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RIVERINE FLOODS

Riverine flooding, runoff from heavy rainfall or rapid snow melts due to higher temperature exceeding the capacity of a river's channel due to heavy rains from monsoons, hurricanes and tropical depressions, winds, warm rain affecting snow pack and unexpected drainage obstructions such as landslides, ice, or debris.

Factors like obstructions to flood flow, dam failures etc. can aggravate damage

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COASTAL FLOODS

Coastal flooding is occurred in coastal areas due to severe sea storms, or as a result of another hazard (e.g. tsunami or hurricane). Coastal flooding includes a storm surge as well, from either a tropical cyclone or an extra-tropical cyclone. The combination of damages induced by contact with flood waters, and wind damage to structures is characteristic for this type of floods.

Factors contributing to coastal floods:

Atmospheric pressure drop

Effect of wind

Influence of sea bed

Funnel effect

Angle and speed of storm movement

Tides

Earthquakes

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CAUSES OF FLOOD IN BANGLADESH

A. Heavy monsoon rainfall in the catchment areas of the Brahmaputra, the Ganges and the Meghna;

B. Snow melting in the Himalayan Mountains;

C. Deforestation and flood protection projects in the upper catchments outside Bangladesh;

D. Low general topography;

E. Heavy localized rainfall within Bangladesh;

F. Backwater effects of river at their points of confluence;

G. Shifting of the major river courses;

H. Rise of mean sea level;

I. Flow tide in the Bay of Bengal.

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CAUSES OF FLOOD IN BANGLADESH

A. Heavy monsoon rainfall in the catchment areas of the Brahmaputra, the Ganges and the Meghna;

Due to simultaneous heavy rainfall in three major basins, there will be:

Simultaneous rise in river water;

Simultaneous peak flow;

Huge backwater effect at the points of confluence;

Rivers swell, overspill and food vast tract of land.

Heavy rainfall within Bangladesh at the same time will aggravate flood situation.

Out of the total volume of flood water in Bangladesh, around 93% is generated in the catchment areas outside Bangladesh and only 7% is generated inside the country.

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CAUSES OF FLOOD IN BANGLADESH B. Snow melting in the Himalayan Mountains;

Melting of snow occurs due to

Temperature rise;

Monsoon rainfall.

During the months of July, August, and September there is heavy melting of snow and as a result it constitutes:

about 30% of the Ganges flow;

about 30% of the Brahmaputra flow.

With increasing global warming due to green house effect, flood situation is expected to worsen further on account of

More melting of snow;

Due to rise in mean sea level;

Less gravity drainage at the outflow;

More drainage congestion in land.

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CAUSES OF FLOOD IN BANGLADESH C. Deforestation and flood protection projects in the

upper catchments outside Bangladesh;

CC--11..

Due to increase in population, more and more people have started living in the catchment areas of the rivers. These people cut down trees in the forest and land to ---

Make new habitants;

Make new agricultural land;

Make grazing land for cattle;

Meet increasing demand for wood and timber;

Due to the above mentioned changes in the land use pattern in the upper catchment, surface runoff and top soil erosion have increased resulting in the possibilities of more severe floods.

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CAUSES OF FLOOD IN BANGLADESH C. Deforestation and flood protection projects in the

upper catchments outside Bangladesh;

CC--22.. Flood protection measures in the upper catchment:

The Brahmaputra, the Ganges and the Meghna and their tributary pass through several countries such as China, India, Nepal and Bhutan.

Previously flood water used to overspill riverbanks and spread over vast tract of land in the above mentioned countries, due to the construction of flood protection embankment along these rivers in the upper catchment, flood water has been confined within the rivers thereby raising the water level and velocity of flow.

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CAUSES OF FLOOD IN BANGLADESH D. Low General topography; Bangladesh is a flat low lying deltaic flood plain. 50% area

of Bangladesh is within 7 meter from the mean sea level. 80% area of Bangladesh is within 16 meter from the mean sea level. This low topography makes Bangladesh vulnerable to flooding. Besides, there are some saucer shaped large depressions in many parts of the country, such as:

Chalan Beel;

Haors in greater Sylhet district;

Beels and Baors in greater Faridpur, Jessore and Khulna districts.

Water accumulates into these depressions and gravity drainage is not possible. For draining water from these depressions, canals are to be excavated up to the intended outfall from the center of depressions and pumps are to be used to drain excess water.

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CAUSES OF FLOOD IN BANGLADESH E. Heavy localized rainfall within Bangladesh;

Average annual rainfall in Bangladesh is equal to 2500 mm.

But average rainfall for during the months of July, August and September is almost 1200 mm.

Continuous rainfall for several days creates drainage congestion and floods vast tract of land.

Average rainfall in south-eastern part and north-eastern part of Bangladesh is equal to 500 mm. besides, Cherapunji, which receives world’s highest rainfall (13500 mm) is only about 30 Km from Bangladesh border and lies in the Meghna basin.

In 1987 and 1988 floods were aggravated due to heavy localized rainfall within Bangladesh.

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CAUSES OF FLOOD IN BANGLADESH F. Backwater effects of river at their points of

confluence;

As the water levels in the rivers rise almost simultaneously, there is serious backwater effect at their points of confluence.

High water levels in the major rivers slow down flow of their tributaries, thereby forcing the tributaries to swell, overtop the banks and flood vast areas of low lying lands.

Example 1: Teesta and Mohananda rivers which are tributaries to the Brahmaputra and Ganges respectively can not discharge fully into the main rivers and thereby flood vast area in the upstream.

Example 2: high water level in the Ganges retards the flow of the Jamuna at their points of confluence near Goalanda as such there is backwater effect both in the Ganges and the Jamuna thereby flooding vast tract of low-lying land in the north-western part of Bangladesh (Chalan Beel).

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CAUSES OF FLOOD IN BANGLADESH

G. Shifting of the major river courses;

G-1. Shifting of the Ganges:

About one thousand years ago the Ganges was flowing along the Hughly River in the West Bengal. Almost four hundred years ago the Ganges was flowing along the Gauri-Modhumati rivers in Bangladesh. About two hundred years ago, the Ganges was flowing along the Arialkha River.

G-2. Shifting of the Brahmaputra:

In 1787 due to a big earthquake in north-eastern India, the Brahmaputra changed its course and started flowing along the present Jamuna river making confluence with the Ganges. Before the earthquake, Brahmaputra was flowing along the present old Brahmaputra river and Jamuna was a small distributory.

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CAUSES OF FLOOD IN BANGLADESH H. Rise of mean sea level;

Mean Sea Level in the Bay of Bengal varies between dry and wet seasons. Mean Sea Level in the Bay of Bengal rises by about 60 cm during wet season due to---

heavy river flow into the Bay of Bengal;

Strong monsoon wind flowing from south-western direction

Pushes sea water along the coastal belt.

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CAUSES OF FLOOD IN BANGLADESH I. Flow tide in the Bay of Bengal.

Tide in the seas and oceans occur due to the attraction/ gravitational pull of moon on earth. Sea water being liquid swells in one part and depresses in another part. The swelling of sea water causes flow tide and depression of sea water causes ebb tide. Normally river water flows into the sea. But during flow tide direction of river flow reverses and sea water flows towards upland thereby blocking the discharge of river water which causes drainage congestion and hence flooding in low lying areas upland.

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Depth of water

Duration

Velocity

Rate of rise

Frequency of occurrence

Seasonality

FLOOD HAZARD PARAMETERS

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Normally the job of hydrologists

Determination of probable future discharges and associated characteristics

Estimation of peak discharges (and river levels) and their frequency of occurrence at given river cross sections.

Estimates are often intuitive and always uncertain

FLOOD HAZARD PARAMETERS

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Flood frequency analysis

Analysis on historical data on floods

Frequency of floods equaling or exceeding a given magnitude

Return Period Highest Flood over a particular period

Average interval for flood of given depth

FLOOD HAZARD PARAMETERS

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Geomorphological map

Sequential inundation map

FLOOD HAZARD MAPPING

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FLOOD TYPES IN BANGLADESH

Non Flood (F0) 0.0 m < Depth < 0.3m

Shallow Flood (F1) 0.3 m < Depth < 0.9 m

Medium Flood (F2) 0.9 m < Depth < 1.8 m

Deep Flood (F3) 1.8 m < Depth < 3.6 m

Very Deep Flood (F4) 3.6 m < Depth

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FFWC (FLOOD FORECASTING AND WARNING CENTRE )

Flood Forecasting and Warning Centre of Water Development Board provides flood warnings

Shallow/Normal Flood 0.5 m below DL

Moderate Flood Up to 0.5 m above DL

Severe/Deep Flood > 0.5 m above DL

DL = Danger Level

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APPROACHES OF FLOOD DAMAGE ASSESSMENT

There are basically two methods in carrying out flood damage estimations.

One is to carry out a thorough questionnaire survey of affected population and properties to estimate the incurred loss.

The other is to use what are known as stage-damage functions which describe the damage extent to different types of property for a given inundation depth and inundation duration

In simplest word, approach to flood damage is generally of two types viz. parcel-based approach and GIS grid-based approach.

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The parcel-based approach keeps detail information of the socio-economic activities, e.g. the family housing, factories and stores, etc. at parcel level.

The respective stage-damage curve is then applied to each parcel with the estimated flood depth at that point for its damage assessment.

The regional damage is the loss sum of all parcels for the entire flooding area.

The data needed for this approach including land parcel maps and socio-economic activities on each parcel. These data are multifarious and difficult to establish and maintain. This is especially true in the developing countries that suffer most often from the natural hazards like flood.

This is comparatively difficult due to collecting and maintaining gigantic database of different types of data.

PARCEL BASED (UNIT LOSS) APPROACH

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GIS grid-based Flood Loss Estimation approach is comparatively newer, which is used for loss estimation of post and future floods as well as for real-time loss estimation.

Although a parcel-based flood damage assessment is more accurate but is also not very practical because it needs gigantic and detailed information at the parcel level. The grid-based approach requires data that are comparatively easier to collect.

The grid-based approach is based on a grid data model which divides the region into grid cells of equal area. The socio-economic activities are considered to be homogeneous within each cell and are aggregated into a single value and assigned to that cell.

GIS GRID-BASED APPROACH

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The data needed for this model can be derived from the aggregated census data that is more available than the individual ones. The flood damage is then estimated from the average flood depth and the aggregated census data for each cell using the stage-damage curve. The regional flood damage is calculated from the summation of loss estimation of each cell.

GIS GRID-BASED APPROACH

Current Services/practices

54 points on 29 rivers with lead time up to 5 days

Only in North East region on pilot basis flush flood

forecasting with lead time only 2 days

Urban flood forecasting not available

Gaps??

Services need to be expanded for more points on more rivers

with up to 10 days lead time

Services need to be expanded on floodplains and community

levels with much lead time (5-10 days)

More important projects/structures can be taken

Need flush flood forecasting on more areas (north west, south

east), up to 5 days lead time

Should we address urban flooding??

FLOOD FORECASTING AND WARNING SERVICES, BY FFWC

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Measures/Programmes: Strengthening and

Improvement of FFWS

Tasks: Capacity building – two (2) ways

Increasing manpower

Technology and Infrastructure development

Total Officers Employees

Setup 22 6 16

Currently available 10 5 5 (including drivers)

Man power (source FFWC):

Task: Data need - LIDAR survey

Task: Field level dissemination of flood forecasting –

flood preparedness volunteers

FLOOD FORECASTING AND WARNING SERVICES MEASURE

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Flood preparedness volunteers

They play role in 2 ways:

In disseminating flood forecasting to community

In actual preparedness

– taking people to flood shelters, prevent breaching of

embankment, evacuate/rescue people particularly from

charlands, outside embankment, discourage people not living

on most vulnerable areas

Flood Shelters

Planning, design and implementation of flood shelters in flood

prone areas. For example, Kurigram, Gaibandha, Sirajganj based

on hazard maps

Note: Policy may be formed like cyclone shelters (Cyclone Shelters

Construction and Maintenance and Management Policy 2011, Sec 3.3

involves)

FLOOD PREPAREDNESS

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Flood Shelters (source: MoDMR)

Construction of Flood Shelters in Flood Prone and River

Erosion Areas (99 completed, 156 being completed)

What can be done??

Road Swells (newly coined!!) as Emergency and Temporary

Flood Shelters. As seen usually people take temporary shelter on

high roads. Keeping wider width on high roads intermittently

with mild slope with vegetative cover.

Some khas land can be made high enough so that people can go

in emergency

FLOOD SHELTERS

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Flood Embankment

Weak points, holes, cut, breaks etc causes embankment

breaching is the main cause of flooding and damage for

embanked areas.

What can be done??

Improvement of construction quality.

Monitoring and maintenance is a must. A routine monitoring

and maintenance work

No monitor/operator for regulating structures

Allocating adequate budget. Some minimum budget must be

there so that shape, size and strength of embankment can be

maintained.

FLOOD EMBANKMENT

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WHAT WE HAVE COVERED TODAY

Disasters of Bangladesh

Flood Definition

Types of Floods

Causes of Flood in Bangladesh

Flood Hazard Parameters

Flood Hazard Mapping

Flood types in Bangladesh

Approaches of Flood Damage Assessment

Flood Forecasting and Warning Services, by FFWC

Flood Forecasting and Warning Services Measure

Flood Preparedness

Flood Shelters

Flood Embankment

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WHAT WE LEARNT

Understanding of flood scenario in Bangladesh in terms of its causes, consequences, types and management as well.

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What Next?

Lecture 5: Disasters of Bangladesh: Cyclones

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Sample Questions

1. Briefly explain the major types of floods normally occurs and it’s behind

causes in Bangladesh.

2. Write down the flood hazards parameters. Briefly explain the

approaches of flood damage assessment.

3. Classify floods based on inundation depth in context of Bangladesh.

What are the existing deficiencies available in managing flood and

what measures can be taken up in this regards?

4. Explain the Causes of Flood in Bangladesh.

5. Write down the strengths and weakness for Flood Forecasting and

Warning Services by FFWC.