Effect of Climate Change on Floods of Bangladesh: Learning from the past

Dr. Md. Shahjahan Ali , Associate Professor

Bangladesh

Khulna University of Engineering & Technology (KUET), Bangladesh

Location of Bangladesh in the Indian Ocean

Bangladesh

Area of the country = 144,000 sq. kmPopulation =160 millionDensity = 1150/sq. km.

Water Resources

River System

River System :

24,000 km (about 500 rivers)

3.3 % of the total landmass

Still Water Bodies:

7,400 km2

5.3% of total landmass

Annual Average Rainfall:

2300 mmTrans-boundary Flow:

57 rivers

Brahmaputra (Inflow 626 BCM)

Ganges

(Inflow 380 BCM)

Meghna (Inflow 195 BCM

Lower Meghna

B A Y OF B E N G A L

GBM Basin

Meghna Basin82,000 sq.km

Brahmaputra Basin552,000 sq.km

Ganges Basin1,087,000 sq.km

B A Y O F B E N G A L

BHUTAN

I N D I A

C H I N A

BANGLADESH

I N D I A

Drainage basin inside Bangladesh 7.5 %Drainage basin outside Bangladesh 92 %

• Geographically, Bangladesh is situated at the tip of a funnel, through which huge amount of water discharged in monsoon

• Depression on the ocean, Cyclones, tides and storm surges can easily affect the country through the unprotected shore

• Therefore, Bangladesh is a disaster prone country. Effect of Climate Change magnifies the governing factors of disaster.

• Sea level rise• Natural disasters:

• Floods• cyclones• droughts

• Saline water intrusion

Major Effects of Climate Change in Bangladesh

• Economy loses •Human casualty• Lose of traditional lifestyles.• Biodiversity losses• Diseases spreads• Famines etc.

Outline of the Presentation

1. Disaster Profile of Bangladesh

2. Impact of CC on the Return period and frequency of floods Historical Time-series data of Rainfall Historical Time-series data of river discharge and Historical Time-series data of Floods Change of Return Period and probability of flood

3. Impact of CC on the intensity of Flood How CC impact magnifies the intensity of flood Analysis of Two Mega Floods (1988 and 1998) Temporary Sea Level Rise in the Bay of Bangle and Its Impact on Flooding

4. Impact of CC on other Disasters (brief) Cyclone & Tornadoes Salinity Intrusion Ecosystem of Sundarban

Outline of the Presentation

1. Disaster Profile of Bangladesh

2. Impact of CC on the Return period and frequency of floods Historical Time-series data of Rainfall Historical Time-series data of river discharge and Historical Time-series data of Floods Change of Return Period and probability of flood

3. Impact of CC on the intensity of Flood How CC impact magnifies the intensity of flood Analysis of Two Mega Floods (1988 and 1998) Temporary Sea Level Rise in the Bay of Bangle and Its Impact on Flooding

4. Impact of CC on other Disasters (brief) Cyclone & Tornadoes Salinity Intrusion Ecosystem of Sundarban

Vulnerability Profile of Bangladesh

Disaster Type Population exposed World ranking

Flood 19,279,660 1st out of 162 countries

Cyclone 4,641,060 6th out of 89 countries

Drought 642227 63rd out of 184 countries

(b) Economic exposure

Disaster Type GDP exposed World ranking

Flood 9.74 bill. USD 3rd out of 162 countries

Cyclone 2.36 bill. USD 12th out of 89 countries

(a) Human exposure

(Source: Global Assessment Report, UNDP, 2009)

drought6%

Flood20%

Storm74%

No. of Disaster for 1900-1981(Total = 74, Avg. = 0.91 nos/yr.)

drought79%

Flood2%

Storm19%

No. of People Killed for 1900-1981(Total = 2.4 mil., Avg. = 29600/yr.)

drought2%

Earthquak3%

Extreme Temp.

9%

Flood33%

Storm51%

Landslide1%

Tsunami0.5%

No. of Disaster for 1982-2011(Total = 202, Avg. = 6.7 nos./yr.)

Extreme Temp.

1% Flood7%

Storm92%

No. of People Killed for 1982-2011(Total = 0.18 mil., Avg. = 6000/yr)

drought8%

Flood75%

Storm17%

Affected People nos. for 1982-2011(Total = 308 mil., Avg. =10.3 mil./yr.)

drought12%

Flood67%

Storm21%

Affected People nos. for 1900-1981(Total = 119 mil., Avg. = 1.5 mil./yr)

Disaster Year 1900-1981

Disaster Year 1982-2011

Flood65%

Storm32%

Tsunami3%

Economic Loss for 1982-2011(Total USD = 16.9 Billion, Avg. = 563 mil./yr.)

Flood77%

Storm23%

Economic Loss for 1900-1981(Total USD = 1.25 Billion, Avg. = 15 mil./yr )

Disaster Year 1900-1981 Disaster Year 1982-2011

Disaster Parameters 1900-1981 1982-2011Events/Year 0.91 6.7

No. of people killed/Year 29,600 6,000Affected people/Year 1.5 Million 10.3 MillionEconomic loss/year 15 Mil. USD 563 Mill USD

Disaster nos., no. of affected people & economic loss increases with time in an alarming rate.

Climate Change is not a future threat for Bangladesh any more. It has already affected the country, it is the reality !

Bangladesh is one of the most vulnerable country in terms of inherent natural calamities, which is already facing the challenges of climate change

so

Is the Climate Change a future Threat for Bangladesh?

Drought

Flood

High saline zone

Cyclone prone(surge ht. >1m)

Location of Disaster-prone Areas

North

East

West

South

Outline of the Presentation1. Disaster Profile of Bangladesh

2. Impact of CC on the Return period and frequency of floods Historical Time-series data of Rainfall Historical Time-series data of river discharge and Historical Time-series data of Floods Change of Return Period and probability of flood

3. Impact of CC on the intensity of Flood How CC impact magnifies the intensity of flood Analysis of Two Mega Floods (1988 and 1998) Temporary Sea Level Rise in the Bay of Bangle and Its Impact on Flooding

4. Impact of CC on other Disasters (brief) Cyclone & Tornadoes Salinity Intrusion Ecosystem of Sundarban

0

500

1000

1500

2000

2500

1958 1960 1962 1964 1966 1968 1970 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006

Year

Mon

soon

Rai

nfal

l (m

m)

From the trend line for 50 years of rainfall data, it is found that the monsoon rainfall increases very gently as 2.65 mm/year.These changing phenomena in rainfall in Bangladesh can be

explain as the probable impact of climate change (IPCC,2007).

2.1 Climate Change Impacts on the Trend of historical rainfalls in Bangladesh:

Rainfall Anomaly

- 30

- 20

- 10

0

10

20

30

1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008

Yea r

An

om

aly

(%) Moderate risk

Moderate risk

Severe risk

Severe risk

Rainfall Anomaly (%) = (P – Pavg)* 100/ Pavg

Low risk

Flood

Drought

Drought events are in decreasing and wet events are found in increasing trend.

2.2 Increase of Brahmaputra River Discharge

• Yearly increasing rate about 54 cumec

• About 7.5% increased in 50 years• Fluctuation in the variation of

monsoon discharge is increased

Brahmaputra River Discharge is Increasing

0

10000

20000

30000

40000

50000

60000

Janu

ary

Februa

ryMarc

hApri

lMay

June Ju

ly

Augus

t

Septem

be

Octobe

r

Novembe

r

Decembe

r

25 y

ears

Avg

. Mon

thly

Dis

char

ge (c

umec

)

1956-1980

1981-2005

The Averaged monthly Brahmaputra discharge in Bangladesh is increased about 8% in 50 years.

0

20,000

40,000

60,000

80,000

100,000

120,000

1954 1956 1961 1963 1965 1967 1969 1971 1973 1975 1977 1980 1983 1985 1987 1989 1991 1993 1995 1998 2000 2002 2004 2006 2008

Year

Area

(sq.

km

)

0

10

20

30

40

50

60

70

80

% o

f tot

al A

rea

2.3 Climate Change Impacts on the Trend of Historical Floods:

The top 5 floods in terms of inundated area occurred in last 20 years in 60 years of history.

24% of the country inundated in a normal flood, the highest flood inundates 67% in 1998

Inundation to the extent of 24% area of the country is beneficial for crops and ecological balance (AFR, 2008). But the flood more than 24% cause direct and indirect damages.

Lets define, Moderate flood as (24±5)% of area inundated, Extreme high flood as >29% and Extreme low flood as < 19% area inundated flood, then…

0

20,000

40,000

60,000

80,000

100,000

120,000

1954 1956 1961 1963 1965 1967 1969 1971 1973 1975 1977 1980 1983 1985 1987 1989 1991 1993 1995 1998 2000 2002 2004 2006 2008

Year

Area

(sq.

km

)

0

10

20

30

40

50

60

70

80

% o

f tot

al A

rea

Climate Change Impacts on the Trend of Historical Floods:

Time-spanAverageFloodingarea (%)

Standarddeviation

(%)

Nos. ofNormalFloods

Nos. ofextreme

high floods

Nos. oflow floods

1954-1972 23.6 5.1 13 01 021973-1990 16.9 16.3 04 03 091991-2007 20.7 17.3 06 03 07

Table : Statistics of time series of floods based on yearly flooding area (% of total area of the country)

Normal Flood

The normal floods are converted to either extreme high floods or the extreme low floods.

High Flood events (Inundation > 29% area):

•The return period decreased•The probability increased by about 3 times in recent decades

Low Flood event (Inundation <19% area):

•The return period decreased in recent decades•The probability increase by about 1.4 times.

Moderate Flood events(Inundation =19 - 29% area):

•The return period increased•The probability is reduced to half (1/2) of decreased by about 1.4 times.

13.5

1.72.7

5.4

3.4

1.9

0

4

8

12

16

High Flood Moderate Flood Low Flood

Retu

rn P

erio

d (Y

ears

)

1st set (1954-1981)

2nd set (1981-2008)

7

59

37

19

30

52

0

20

40

60

80

High Flood Moderate Flood Low Flood

Prob

abili

ty (%

)

1st set (1954-1981)

2nd set (1981-2008)

2.3Return Period & Probability:

• Bahadurabad point of Brahmaputra river

• For Q= 76,137 m3/s, Data for 1956 to 2007

Return Period & Probability: Example

Probability of occurrence of characteristic floods at Brahmaputra river is increased from 4% to 28% over the last 50 years.

Bahadurabad

The monsoon rainfall increases very gently as 2.65 mm/year drought events are in decreasing and wet events are found in

increasing trend.The Averaged monthly discharge of Brahmaputra river is

increased about 8% in 50 years.The top 5 extreme floods in terms of inundation area are occurred

in last 20 years in 60 years of history. The number of moderate floods are decreased and they are

converted to either extreme high floods or the extreme low floods. The probability of extreme flood events are found to be

increased up to 3 times in recent decades.

Summary-1:

These temporal changes of hydrologic scenarios in Bangladeshcan be explained as the impact of climate change.

Outline of the Presentation1. Disaster Profile of Bangladesh

2. Impact of CC on the Return period and frequency of floodsHistorical Time-series data of RainfallHistorical Time-series data of river discharge and Historical Time-series data of FloodsChange of Return Period and probability of flood

3. Impact of CC on the intensity of Flood How CC impact magnifies the intensity of flood Analysis of Two Mega Floods (1988 and 1998) Temporary Sea Level Rise in the Bay of Bangle and

Its Impact on Flooding

4. Impact of CC on other Disasters (brief) Cyclone & TornadoesSalinity IntrusionEcosystem of Sundarban

3.1 How the CC will affect the Flood Intensity?

Prediction of change of climate in Bangladesh:

By the year 2100

• Temperature increase = 2.4 0C

• Precipitation increase = 11.8% (Monsoon)

• SLR = 30cm ~ 1 m (9 cm ~ 88 cm by IPCC)

Predicted Changes in Monsoon river flow :

For 2 0C Temperature rise and 10% increase in Precipitation

• Q Ganges will increase 19%

• Q Brahmaputra will increase 13%

• Q Meghna will increase 11%(Source: IWM, 2009)

How the CC will affect the Flood Intensity?

Increased River Discharge (Aprox. 14%)

Increase of Flood Intensity

Flood Intensity Index ＝ duration of flood (days) X the depth of the flood above the danger level（ｍ）

Increase in Inundation depth

Sea Level Rise (Backwater effect to river discharge)

Increase in duration of flood

Impact of Climate Change in Bangladesh

inundated area: 67% in 199861% in 1988

Estimated damage: 2.8 billion USD in 1998 1.2 billion USD in 1988

1998 Flood was the most devastating flood displaced more than 30 million people 20 million homeless.

Analysis of 1988 and 1998 Floods:

Two mega Floods in the history of Bangladesh

0

1

2

3

Kurigra

m

Chilmari

Bahad

uraba

d

Serajga

nj

Aricha

Jamalp

ur

Mymen

singh

Dhaka

Naraya

ngan

j

Taragh

at Wat

er d

epth

abo

ve D

ange

r Lev

el (m

)

2008 (Normal Flood) 1998 1988

3022

66

48

68

31 33

57

7166

16 15

27

44

31

8 10

23

36

65

0

20

40

60

80

100

Kurigra

m

Chilmari

Bahad

uraba

d

Serajga

nj

Aricha

Jamalp

ur

Mymen

singh

Dhaka

Naraya

ngan

j

Tarag

hat

No.

of D

ays

for W

L ab

ove

DL 2008 (Normal Flood) 1998 1988

Fig.: Inundation depth and durations at some locations in Brahmaputra Basin.

Historical Mega Floods (Brahmaputra Basin)

Depth of inundation

Duration of flood

Although the depth of inundation among two mega floods does not differ too much, the duration of flood in 1998 was much higher (up to 4 times) than 1988 flood.

Total Station : 35H.W.L in 1998＝16 st.H.W.L in 19８8＝ 19 st.

3

28 27

6872

25

4

24 23

4147

25

0

20

40

60

80

100

Dinajpur Rajshahi Hardinge Bridge Goalundo Bhagyakul Gorai Rly Bridge

No.

of D

ays

for W

L ab

ove

DL

2008 (Normal Flood) 1998 1988

0

1

2

3

Dinajpur Rajshahi Hardinge Bridge Goalundo Bhagyakul Gorai Rly Bridge

Wat

er d

epth

abo

ve D

ange

r Lev

el (m

)

2008 (Normal Flood) 1998 1988

Depth of inundation and Duration of Historical Mega Floods

Fig.: Inundation depth and durations at some locations in Ganges Basin

Although the depth of inundation among two mega floods does not differ too much, the duration of flood in 1998 was much higher (about double) than 1988 flood.

Depth of inundation

Duration of flood

Flood Intensity IndexFlood Intensity Index ＝ the depth of the flood above the danger level（ｍ） X

duration of flood (days)

0

20

40

60

80

Brahmaputra Ganges Total

Floo

d In

tens

ity In

dex

(m-d

ay)

1988 1998 2007

1.07 m x 60 days

1.09 m x

31 days

Although the depth of inundation among two mega floods does not differ too much, the Flood Intensity Index in 1998 was much higher (about double) than 1988 flood.

Although the depth of inundation among two mega floods does not differ too much, the duration of flood in 1998 was much (up to 4 times) higher than 1988 flood. Why?

Fig. Comparison of Monsoon Rainfall in 1998 with 1988 over three basins in Bangladesh

The Rainfall inside Bangladesh may not be the cause for the prolonged flood in 1998

0

500

1000

1500

Mea

n M

onso

on R

ainf

all (

mm

)

Ganges Basin

Brahmaputra Basin

Meghna Basin

Total monsoon rainfall

1988 1998

•The upstream Rainfall may not be the cause for the prolonged flood in 1998 in Bangladesh

Time lag between Peak Discharges in Ganges and Brahmaputra River Hydrograph

0

2

4

6

0 1 2 3 4 5 6 7 8 9 10 11 12

Month

Dis

char

ge (x

10

4 m3 /s

)

Brahmaputra

Ganges

• Ganges has high discharge in July to September (Peak at end of Aug.).• The Brahmaputra flow has high discharge in June to September (Peak at mid of July.). • The 1.5 months phase difference in the rising limb of their hydrograph • Probably, this is the reason for which most of the annual flood hyetographs show

double peaks: in July and end of Aug.• If the Brahmaputra does not discharge out its peak flow rapidly, its peak will

coincide with the peak of Ganges, it will case a extreme high flood.

1.5 months

Station nameLocation

(Latitude,Longitude)

MSL as perBD tide Table

(m)

MSL during1998 flood (m)

Rise in SeaLevel during

1998 flood (m)Hiron point 21048’N, 89028’E 1.842 1.7 0.142Khepu para 21054’N, 90013’E 2.332 2.06 0.272

Char Changa 22008’N, 91006’E 2.248 2.037 0.211Sandwip 22029’N, 91026’E 3.377 3.243 0.134

Average Rise in Sea Level during 1998 flood (m) = 0.19

2.4 The Sea Level Rise in Bay of Bangle during 1998 Monsoon period

In 1998 July to September, 06 spring tides occur; and during new moon and full moon time declination of moon with the 21-230 Latitude was minimum.On an average, 0.19 m SLR was observed at that

time (for a period of about 2.5 months), which blocked the outflow of the swollen rivers into the Bay of Bengal.The Sea Level Rise due to elevated tides in the Bay

of Bengal was the main cause for the prolonged flood in 1998

Impact of SLR on Flooding Although the depth of inundation among two mega floods does not differ

too much, the duration of flood in 1998 was much (up to 4 times) higher than 1988 flood.

The average intensity of 1998 flood was observed to be two times stronger than that of 1988 flood.

The temporary Sea Level Rise due to elevated tides in the Bay of Bengal was the main cause for the prolonged flood in 1998.

On an average, 0.19 m SLR was observed at that time (for a period of about 2.5 months), which blocked the outflow of the swollen rivers into the Bay of Bengal.

The severity of flood in 1998 due to the impact of temporary SLR of 0.19 m, gives a practical estimation how the SLR due to climate change will affect the flooding scenario of Bangladesh.

Summary-2:

Let’s think about future Impacts!

Nominal flooding + 1m SLR = 24% + 20% =?

1998 flood + 1m SLR = 67% + 20% =?

1m sea level rise (2100) = 20% area flooded (IPCC, 2007)Nominal Flooding = 24% area inundation

0.19m SLR prolonged the flood 2-3 times 1m SLR will prolonged the flood = ??? times

Outline of the Presentation1. Disaster Profile of Bangladesh2. Impact of CC on the Return period and frequency of floods

Historical Time-series data of Rainfall Historical Time-series data of river discharge and Historical Time-series data of Floods

Change of Return Period and probability of flood3. Impact of CC on the intensity of Flood

How CC impact magnifies the intensity of flood Analysis of Two Mega Floods (1988 and 1998) Temporary Sea Level Rise in the Bay of Bangle and Its Impact on

Flooding

4. Impact of CC on other Disasters (brief) Cyclone & Tornadoes Salinity Intrusion Ecosystem of Sundarban

4.1. Climate Change impact on Occurrence of Cyclone

Frequency of major cyclone and affected people (Source: BBS, 2007)

0

2

4

6

8

10

12

1970-1979 1980-1989 1990-1999 2000-2009

Year of Cyclone

Number of occurance

Affected people in million

Coastal Area in Bangladesh

• 710 km long, 32 % of the country, 28% of the people

• Among 19 coastal districts, 12 are directly exposed to the sea.

• Since 1970, the no. of major cyclones striking Bangladesh is 26

• Nos. of occurrences (and the no. of affected people) increased significantly since 1990

Among Top Ten Cyclones in the Worlds history,5 in Bangadesh

• Cyclone Sidr (2007): displace 650,000 people and killed 3,447

• cyclone Bijli and Aila (2009): displace about 200,000 people, water did not discharged out to the sea, some of the area still waterlogged

• Human Casualty decreased due to warning system and improved disaster management

0

100

200

300

Bang

lade

sh, 1

970

Bang

lade

sh, 1

991

Mya

nmar

, 200

8

Chin

a, 1

922

Bang

lade

sh, 1

942

Indi

a, 1

935

Chin

a, 1

912

Indi

a, 1

942

Bang

lade

sh, 1

965

Bang

lade

sh, 1

963

Peop

le K

illed

in th

ousa

nds

Recent Cyclones:

Ganges Dependent Area

Major Salinity Intrusion in the

Southwest Region

4.2. Salinity Intrusion

Increase of Salinity in SW Bangladesh

Year Salinity affected Area (hector)

Salinity Level in Khulna(EC > 2.3 dS/m)

Zone-A Zone-B1973 750,350 3.9 13.952009 950,780 24 47.78

% increase in 4 decades

26.71% 515% 243%

Present Future

Sunderban forest under different salinity zone (ppt); present situation

4.3. Climate Change Impact on Sundarban

Mud

fl ats

(sl o

pe)

High-tide water level

Low-tide water level

Rid

ges

orle

vees

Back-swamps or basins

Mai

nri

ver

chan

nel hoda

nol khagra

golpata

keora/baen goran

Gewa

sundari

hargoza

Destroy

•Soil salinity is increasing and Salinity zone is shifting

•Sundari trees are destroyed and replaced by Keora and Gewa

•Loss of Ecosystem

•Food chain breaks down

Thanks for Your Time and Attention

It is not the strongest of the species that survive,nor the most intelligent, but the onemost responsive to change

Charles Darwin -