HYDROLOGY OF FLOODS IN
SOUTH ASIA
Shafiqul Islam
University of Cincinnati
Cincinnati, Ohio, USA
DRAFT
SaciWATERs Workshop on
FLOODS IN SOUTH ASIA’
BRAC Centre Inn, Dhaka, Bangladesh
28-30 November 2002
1. Hydrology of Floods in South Asia: An Overview
We will provide a review of hydrology of floods in south Asia with a focus on the
Ganges-Brhmaputtra-Meghna basin. Then, bring in recent advances in remote sensing and
computational capabilities to reexamine our traditional understanding of causes and impact of
flood in south Asia from a hydrologic perspective. We will also discuss hydrologic vulnerability
of the region to potential climate change. Finally, we will provide a connection between El-Nino
and Floods in the region and how this information can be used to develop a long-range
forecasting methodology for integrated large scale water resources planning and management.
For this initial draft, our focus is to provide an outline of this chapter and raise some
questions to be addressed within each subtopic. These questions will form the basis of our
discussion during the workshop. With comments and feedback from the workshop, these
questions will be refined, new questions will be defined and expanded in the final write-up of
this chapter.
2. History and Statistics of Floods in South Asia
Flood and water scarcity, may seem paradoxical and incongruous, are real challenges for
the prosperity and sustainability of South Asia. Perhaps nowhere in the world is water so
abundant and yet so scarce than in this region of the world. From food to fish to fiber, livelihood
of primarily rural population of this region critically depends on water. For example, in a typical
year, over half of Bangladesh is flooded during the Monsoon season; yet, during the dry season
water is important not only for food production but also to maintain navigation in the interior
rivers, minimize salinity intrusion in coastal areas, preserve wetlands from continued shrinkage
due to extensive use of groundwater resources for irrigation, and sustain a complex matrix of
terrestrial and aquatic production systems. Here, we begin with a few statistics of floods:
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While the number of geophysical disasters has remained fairly steady, the number of
hydro-meteorological disasters has increased over the last decade. During the past decade
over 90 per cent of those killed by natural disasters lost their lives in hydrometeorological events such as droughts, windstorms and floods. Floods accounted for
over two-thirds of the annual average of 211 million people affected by natural disasters,
worldwide from 1991-2000 (WDR, 2001).
During the 1990s, 140,000 Bangladeshis were killed by cyclones. But the Cyclone
Preparedness Program evacuated and sheltered 2.5 million more people before the
cyclones struck – almost certainly saving their lives (WDR, 2002).
Catchment area of the Ganges-Brahmaputra-Meghna System is 1.55 million sq. km; 90%
of this area is outside of Bangladesh (in India, Tibet/China, Nepal and Bhutan).
“Normal” yearly monsoonal flooding has little adverse impact on rice cultivation; these
lesser floods also add to the soil fertility. These floods begin to have negative effects
when their duration increases beyond “normal.”
The 1987 major floods covered 40% of the land area in Bangladesh, 40 million people
were affected and 1800 people died. The 1988 major floods covered 60% of the land
area, affecting 45 million people and killing 2379.
In 1998, “the flood of the century” covered around 68% of the total area of Bangladesh,
affected 31 million people but killed 918.
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Questions:
Are floods getting worse in South Asia?
What are the metrics to quantify floods?
3. Hydrology of Floods in South Asia
3.1 Ganges, Brahmaputra and Meghna river Basins
The Brahmaputra originates on the Chinese side of the Himalayas and after traversing
about 1800 km through Tibet and India, enters Bangladesh through the northern border. Within
Bangladesh it is called the Jamuna and flows for an additional 275 km, up to its junction with the
Ganges. The Ganges flows for
about 2000 km through India,
enters the Western side of
Bangladesh and flows to the
South-East for another 250
km to confluence with the
Brahmaputra. The Meghna
originates in one of the
rainiest regions of the world,
the Shillong Plateau in Assam.
The headwaters of the
Meghna comprise a number of
streams that meander through
Assam for about 400 km and
then enter Bangladesh from
the North-East in the form of two major tributaries – Surma and the Kushyara – which reunite to
form the main channel and flows to the Meghna (Chowdhury and Sato, 1996).
The characteristics of peak discharges
of Ganges, Brahmaputra and Meghna rivers
are unique. From Table 4 is seen that though
basin area of Ganges is twice that of
Brahmaputra but mean annual peak discharge
of former is considerably lower than latter.
The coefficient of variation (CV) of peak
discharge of Ganges is much higher than of
the other two major rivers. While CV of peak
discharge of Brahmaputra and Meghna
indicates similar precipitation pattern in their
basin areas.
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Hydrographs of the Ganges (lighter
solid line) and Brahmaputra (thicker
solid line) rivers for the typical water
year 1967-68 (m3/sec) are shown here.
Discharge of Brahmaputra river starts
rising in March due to melting of snow
in the Himalayas while Ganges
discharge rises in July with the onset
of monsoon. The flood peaks of the
Brahmaputra occur in July and August,
while the peaks in the Ganges occur in
August and September.
Recently, the frequency
of floods during each
monsoon season at their
terminal gauge/discharge
(G/D) sites in India (at
Farakka for Ganga and
Dhubri for Brahmaputra)
have been studied (Dhar
and Nandargi, 2001).
Average aerial monsoon
rainfall for each
monsoon season was
calculated for 14-year
period and their
percentage departure
along with the number of flood observed each year are given in Table1. In this study, by flood it
is meant the event when flood waters of these rivers overtop their respective danger levels at
Dhubri and Farakka G/D sites. It is seen from Table1 that frequency is more or less of same
order even though rainfall
magnitudes received by each
rainfall distribution are quite
different. It was apparent that the
size of basins does not play any
major role as far as the frequency
of floods in concerned at the
terminal sites.
Dhar and Nandargi
(2000) studied that in the
Brahmaputra basin in India,
variability of annual rainfall over
this region is low, in the order of
10-15%. Before the onset of
monsoon, there is considerable
TABLE 1 Percentage departures of monsoon rainfall and frequency
of floods at the terminal G/D sites of Brahmaputra and Ganges river
systems in India (Dhar and Nandargi, 2001
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thunderstorm activity over the region in the month of May due to the incursion of moisture in the
region from the neighboring Bay of Bengal. The heavy rainfall is mostly caused when the eastern
end of the monsoon trough shifts northwards of the Assam Valley or during the periods when
‘Break’ monsoon situations set in over the country with a northward shift of the monsoon trough
to the foot of the Himalayas. In Figure 2, adapted from Dhar and Nandargi (2000), monthly and
yearly flood frequencies are shown in the Brahmaputra River and its tributaries. It is seen that the
maximum number of floods have occurred in July although the rainfall is at a maximum in the
months of June.
To verify the claims of flood getting worse in GBM basin, a study on the peak discharge
time series and flooded areas was carried out (Mirza et al., 2001). Four statistical tests were
applied on the peak discharge time series and flooded area and the results did not indicate any
conclusive changes in peak discharge or flooded area time
TABLE 2 Results of statistical tests applied to peak discharge data of Ganges, Brahmaputra and Meghna
rivers and the flooded areas in India and Bangladesh (Mirza et al., 2001).
series within Bangladesh. However, at upstream stations in India two rivers showed an increase
in peak discharge which was not registered at downstream stations in Bangladesh which was due
to the regulation of discharge by the Farakka barrage. Factors other that increasing flood event
characteristics (peak discharge and flooded area) which were identified were: (i) improvement in
flood damage assessment techniques; and (ii) increases in human settlement in flood-prone areas.
To develop and analyze alternative regional water planning and management strategies, a
structured planning framework is needed. Such a framework for the formulation, analysis and
evaluation of alternative management strategies would allow accounting for both the short term
and for the cumulative and long-term impacts on the performance of the water resource systems
(WRS) in terms of its contribution to the regional development objectives such as food security
and poverty alleviation.
In developing such a conceptual framework, it is important to pay particular attention
todevelop process based transfer functions that can account for relevant impacts of changes in
availability and utilization of water resources on ecosystems and on social and economic
conditions of different users of the WRS.
For example, hydrologic models that are in use for
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planning purposes in Bangladesh do not simulate some
important characteristics of flood plain processes.
Chowdhury (2002) identified some of these
characteristics as inundation of floodplain by
overflowing river during monsoon and their effects on
groundwater recharge, rise of groundwater level close
to the surface and its impact on the lateral movement of
groundwater, and base flow contribution from
subsurface water during dry season.
Issues of scales, both in time and space, are also
important to develop these process based transfer
functions. For example, water balance studies in GBM
delta are typically based on a decad -- defined by
dividing the month into first ten days, second ten days,
and remaining days of the month – which does not
match with the fortnightly neap-spring tidal cycle.
These tidal cycles play an important role in residual
flows, sediment transport, and saltwater intrusion for the tidal river networks. This mismatch of
temporal scales and their influence on modeling river hydraulics, sediment transport, and
wetland dynamics needs to be explored. In space, differences in the scale and dynamics of rivers
and flood plain processes as well as the mismatch between the measurement scales and modeling
scales needs to be understood.
Questions:
Should we look at hydrology of other basins from South Asia?
How do we characterize the similarities and differences among different
basins and different modeling strategies?
4. Remote Sensing and GIS for Flood Assessment in South Asia
Remote sensing provides an unprecedented spatial and temporal coverage of critical land
surface and atmospheric data that are logistically and economically impossible to obtain through
ground based observation networks. Several studies have investigated remote sensing and
geospatial processing methods for flood monitoring. Satellite-based synthetic aperture radar
(SAR) imagery from RADARSAT has proven especially effective for monitoring the extent of
flooding, due to its ability to image under typical monsoon cloud cover conditions. Most of the
current monitoring and prediction systems are focused on the main river system. Coupling
information from these systems with quantified flood plain dynamics derived from satellite radar
remote sensing could be a key for flood management and planning in the region. The potential
for resource management applications ranges from agriculture and fisheries management to
disaster management. Martin et al., 1998 proposed such a method to develop a detailed flood
depth maps on a daily basis by merging a time series of SAR imagery with a static digital
elevation model (DEM) and water level data from gauges.
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Islam and Sado (2000) studied flooded area and flood hazard assessment in Bangladesh
using remote sensing and geographical information system. Satellite images from the National
Oceanographic and Atmospheric Administration (NOAA) Advanced Very High Resolution
Radiometer (AVHRR) were used to analyze Bangladesh's historical Flood event of 1988.
Recently, Xie et al. (2001) developed a new system and put into operation at the Climate
Prediction Center of NOAA to produce real-time analyses of daily precipitation at 0.100
latitide/longitude grid over the South Asia (700E – 1100E; 500N – 350N). This automated system
defines analysis of daily precipitation by merging satellite as well as four observation based
individual data sets. Such a data set has the potential to fundamentally alter and significantly
improve flood planning and management strategies in South Asia.
Questions:
What is the status of remote sensing and geo-spatial analysis for flood
monitoring and forecasting in the region?
How do we improve flood forecasting methodologies using remotely sensed
data and geo-spatial analysis techniques?
5. Hydrologic Vulnerability of the Region
The United Nation’s Intergovernmental Panel on Climate Change (IPCC) third
assessment report in 2001 has highlighted the severe consequences that are likely to be faced by
the developing countries of the world due to the continued emissions of greenhouse gases. This
region, and Bangladesh in particular, is likely to be one of the most seriously affected, in terms
of total population at risk due to climate change, and most vulnerable country in the world (Huq,
2001). Huq (2001) described
several likely impacts of climate
change on Bangladesh including
(a) sea Level Rise, (b) more intense
cyclones,
(c)
greater
flood
intensity, and (d) health impacts.
Mirza (2002) has studied the effect
of global warming on changes in
probability of floods. For various
warming scenarios, like CSIR09,
HadCM02, GFDL and LLNL ,
changes in the probability of
exceedence of a current flood for
the Ganges, Brahmaputra and
Meghna rivers was calculated.
Results suggest an increase in the
probability of flood for the
warming scenarios considered. (except for Ganges for HadCM02).
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Recent decades have exhibited an increase in extreme rainfall events over northwest India
during the summer monsoon (Singh and Sontakke, 2001). Approximately 70% of the total
annual rainfall over the Indian subcontinent is confined to the southwest monsoon season (JuneSeptember). The number of rainy days during the monsoon along east coastal stations has
declined in the past decade. In Pakistan, seven of 10 stations have shown a tendency toward
increasing rainfall during monsoon season (Chaudhari, 1994).
Questions:
What are the main hydrologic vulnerabilities and consequences of a possible
climate change scenario?
What other studies are available related to hydrologic vulnerabilities of
South Asia?
6. A New Long Range Water Resources Planning Tool
Several recent studies have shown that El-Nino Southern Oscillation (ENSO) index has a
significant influence on various climatic and hydrologic signals across the globe. A recent study
attempted to identify the nature and strength of possible teleconnection between the Ganges river
flow and ENSO and developed a model which can capture, at least in part, the natural variability
of flow and provide a large forecasting lead time (Whitaker et al., 2001). The motivation came
from the fact that in the past hydrologic
forecasts of the basin through rainfallrunoff modeling could provide a lead time
of the order of basin response time, which
is on the order of several days or so. Such a
short forecasting lead time is not adequate
to hedge against extreme events (flood or
drought) in large river basins.
It appears that a significant
relationship exists between the natural
variability of the Ganges annual flow and
ENSO
index.
Through
further
investigation it has been shown that the rate
of change of ENSO index is also
statistically related to the Ganges flow. This model uses current flow data, predicted ENSO data
and its gradient to forecast flow in the Ganges with a forecasting lead time up to one-year
(Whitaker et al., 2001). The model also provides a quantitative measure of forecasting
uncertainty. A key advantage of this model is that it does not require rainfall and stream flow
information from upstream areas and countries. It is encouraging to note that all four of the
validation forecasts during the El Nino and La Nina (1982, 1988, 1991, and 1993) events are
within the ninety-five percent confidence intervals. These results demonstrate the strength of the
proposed model and suggest further exploration of this long-range forecasting methodology for
other major rivers in South Asia. Particularly, utility of such a modeling framework for dry
season flow needs further investigation.
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Questions:
Can we use ENSO related information to develop a long-range water
resources planning tool for major rivers in South Asia?
What are the potentials for such a toll for dry season flow forecasting?
7. Concluding Remarks
It appears that many of the recent studies have focused on flood and hydrologic problems in
Bangladesh. Based on our discussion in the workshop, we hope to expand different subtopics to
cover other regions and river systems in South Asia. Our goal is to examine the similarities and
differences among different river basin of the region to develop a comprehensive overview of
hydrology of floods in South Asia.
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