Post on 26-May-2018
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 -