Asian Development Bank

Project Completion Report (Technical) On

Pilot and Demonstration Activity for Bangladesh: Field-Based Research on the Impacts of Climate Change on Bangladesh

Rivers

Jun 2010 Dhaka, Bangladesh

Part I – Background and Context The Asian Development Bank (ADB) was involved in a Pilot and Demonstration Activities programme for water (the “PDA Programme”) which was part of the regional technical assistance for knowledge and innovation support for ADB’s Water Financing Programme. This project is under the PDA programme, which was approved by the ADB in November 2008 and financed by the Water Financing Partnership Facility. The objective of the PDA programme is to enable testing of new ideas and validation of innovative development approaches that are highly replicable and can strengthen ADB’s core operations, including policy dialogue with government, reform agenda formulation, country programming, and project preparation and implementation. This PDA programme was aimed at assessing the effects of climate change on the Jamuna, Ganges and Padma rivers.

It is well recognised that Bangladesh is one of the most vulnerable countries on earth that would be affected by climate change. Global warming with higher associated rainfall and relative sea level rise will likely cause significant changes in sediment and flood regimes. Consequently, large rivers may be disturbed requiring long periods of adjustment in fluvial processes and morphological forms. It is now time to assess the impact of climate change on the morphology of those rivers as well as the subsequent impacts on floodplain dwellers so that the nation can prepare itself to mitigate the effects. It is expected that this sort of study can be replicated for the other large rivers in Asia.

To manage the possible challenges posed by climate change, this project was aimed at understanding the existing morphological processes and probable adjustments in major rivers. Moreover, the study also attempted to assess the impact of avulsion of the river courses on people in terms of displacement.

With these aims, the ADB started a demonstration project covering the major rivers, the Jamuna, the Ganges and the Padma. Since they carry the major portion of sediments through Bangladesh and deposit to the Bay of Bengal and also cause huge erosion every year, the study area was selected on the adjacent floodplain of these three rivers. The average width of the 220 km long Jamuna is 12 km and its annual average erosion rate is 2,000 ha. On the other hand, the 110 km long Ganges has the average width of 5.3 km and its average annual erosion is 1,000 ha. The erosion rate of the Padma is between that of the Jamuna and the Ganges, which is 1500 ha/y. This river is 100 km long with the average width of 10.3 km.

Part II – Methods Applied and Main Outputs Sea level rise and increase in flood flow in rivers have been assessed from secondary sources for this study. Different scenarios were found in secondary sources for sea level rise and flood flow. A 61 cm sea level rise according to IPCC (2007) was considered in this research. Based on the Winston et al. (2010) projection an average increase in flood discharge in the Jamuna, Ganges and Padma rivers has been assumed in this study to be 20% for 2100. At first, the baseline conditions for river planform, channel dimensions, riverbank erosion processes, and estuary dynamics were established. Different processes acting in the river and estuary systems were identified and an attempt was made to eliminate their influences. The assessment of the impacts of climate change was made on the baseline conditions of the morphological processes, which might be free from large-scale changes induced by exogenous or endogenous factors.

During the analysis different relations were noted between riverbank erosion and maximum annual discharge for different rivers. The relations developed for these three rivers were also used in the assessment of the intensity of riverbank erosion after 100 years due to changes in annual flood discharge. Social and national losses due to riverbank erosion were assessed for the base condition while the effects of climate change on these issues were also addressed.

The study has derived regime type relations for estimating the channel depth and width of the Jamuna, Ganges and Padma rivers in relation with average flood discharge. The estimated results were used as

input data in the analytical model for assessing the impact of climate change and also the process of adjustment of channel dimensions with the changes in flood discharge due to climate change.

An attempt was also made to relate a bank protection structure induced scour with flood discharge in order to assess the effects of climate change on scour depth. The following diagram presents the outline of the methodology used in this study.

The main outputs of formerly mentioned activities are:

An assessment of the probable impacts of climate change on the morphological processes of the rivers system (width, bed level, planform, erosion/accretion processes, and flood regime);

An assessment of the effect on riverbank erosion and existing structures; and

An assessment of the impact of climate change on people in the context of riverbank erosion.

All the activities mentioned in the scope of work were completed with full effort.

Part III – Project Outcomes and Lesson Learned

Outcomes 

1. Human interventions have long term implications. Long-term net accretion rate in the Meghna estuary was about 4 to 5 km2/y. Immediately after the 1950 Assam earthquake the net accretion rate became more than 40 km2/y. During the last 36 years, the rate of net annual accretion has been about 17 km2/y, which is much higher than the long-term accretion rate. The probable reasons are: (i) increased sediment yield from the catchment due to deforestation and intensive agriculture or/or (ii) construction of flood embankment and polders restricting sedimentation within floodplains and tidal plains.

2. Due to sea level rise, the response of the estuary will be instantaneous, i.e. with the rising of sea level the un-poldered tidal plain in the Meghna Estuary will adjust its level with almost the same magnitude. But for the adjustment of the riverbed and river bank including its adjacent floodplain will show some phase lag, which increase with the distance upstream. It has been found that the Bangladesh Rivers are very quickly responding.

3. To determine the role of sedimentation, floodplain or tidal plain inundation or salinity intrusion due to climate change will be assessed differently i.e. the morphological adjustment of the rivers and estuaries will be taken into consideration.

4. With the increase in flood discharge due to climate change, the river will adjust their channel dimensions very quickly. However, flooding in the Jamuna, Ganges and Padma floodplain will not be very significant due to climate change.

5. Flood discharge affects the rate of erosion. It has been found that increasing flood discharge due to climate change would cause increased bank erosion.

6. There will be an effect on the depth of scour holes. Local morphology, especially protrusions may create very deep scour holes in the Jamuna, Padma and Lower Meghna rivers and mitigate the influence of large floods. Increase of flood discharge due to climate change may increase the scour holes in these rivers by 2 to 4 meters and thus raise the cost of riverbank protection works.

7. Climate change has an impact on social life. it is predicted that more than 50% of people would be affected by climate change in the next fifty years.

Lesson Learned 

As sediment input from the upstream catchment is very high, the responses of the river and estuary to climate change will also be very rapid. Thus, any assessment of the impact of climate change on flooding and salinity intrusion without considering the role of river and estuary morphology may lead to wrong conclusions.

The field research will be very much useful for developing a strategy for adaptation to climate change.