Meghna Estuary Study  (1995 - 2000)

 

Study objectives

The development objectives of Meghna Estuary Study (MES) were (1) to increase the physical safety and social security of the some two million inhabitants of the study area; and (2) to promote sustainable development in the coastal areas and on the islands. The immediate objectives were (1) to enhance and strengthen operational knowledge of hydraulic and morphological processes in the Meghna Estuary; (2) to find suitable land reclamation and bank protection methods; (3) to increase the capacity of BWDB to reclaim new land and protect the eroding river banks; and (4) to prepare a plan with priority

projects and programmes for flood protection, agricultural and socio-economic development for early implementation.

The objective of the MES estuarine surveys was to supply a part of the basis for assessment of the physical behaviour of the estuary. The produced data served as a supplement to results and data from other sources, such as ongoing routine monitoring by BWDB, BIWTA, and Department of Meteorology, as well as satellite imagery, and historical data, notably from the Land Reclamation Project and the Cyclone Shelter Preparatory Study.

Physical settingMeghna Estuary is the easternmost sector of the Ganges delta. The Estuary conveys the joint discharge of the Ganges/Padma, Jamuna/Brahmaputra, and Meghna Rivers. Hereby, large volumes of water (some 1,200 km³ per year) and sediment (some 1,100 mio. t per year) pass the area. The catchment area is 1,520,000 km². It covers parts of India and China, all of Nepal and Bhutan, and almost all of Bangladesh.

There is a pronounced seasonal variation of wind, river discharge, and sediment supply from the river system. The highest discharges occur in August-September and the lowest in February. The 10-years peak flow at Chandpur has been estimated at 123,000 m³/s. The estuary forms a complex network of braided tidal channels with strong tidal streams in many places.

The entire Meghna Estuary (and a part of the upstream river system) is tidal-influenced all year. The tidal range increases in the direction from SW (around 4 m range at S Bhola) towards NE (around 7 m range at Sandwip). There is a pronounced seasonal sea level variation. The sea level is highest during the SW monsoon and lowest in the winter. The range of the seasonal variation is about 0.8 m in the southern part of the MES study area and about 2.7 m at Chandpur (at the northern boundary of the area). Extreme set-ups occur during cyclones, where the storm surge can reach 5 - 7 m (on a 20-100 years basis, in the Chittagong-Bhola sector).

In the estuary, fresh water from the rivers meets with saline ocean water from the Bay of Bengal. Due to strong currents and shallow depths, density stratification is not very characteristic. Rather, there are fronts (or transition zones) between the water masses. The location of these transition zones depends on the river discharge and the tide.

The depth of the inner part of the estuary is less than 10 m, except for the thalwegs of the flow channels. Wave heights are generally moderate. In the inner parts of the estuary, and in its extensive shallow areas, the waves are predominantly generated by direct (local) wind action.

Sediments, fine sand and silt, are supplied by the rivers, and are transported within the estuary mainly by the tidal streams. The area is characterized by a highly dynamic morphology, with flow channels shifting their course, and with intermittent erosion and accretion of banks and tidal flats. There is a moderate net accretion, currently estimated by MES at around 10 km² per year (1976-96).

The cause-effect relationships and their interaction can be conceptualized in different ways. One attempt to summarize the most important physical processes is shown below.

The variable forcings can be divided into external and local determinants. They comprise:

Dry season conditions (affecting mainly the salinity):

(i) changed flow caused by natural climate fluctuations; (ii) changed flow caused by upstream irrigation withdrawal or regulation (such as diversion, or large-scale bank protection schemes); and (iii) long-term sea level changes.

Monsoon season conditions (affecting first the sediment budget, and subsequently the salinity and the flood risk):

(iv) changed flow and sediment yield caused by natural climate fluctuations, earthquakes, etc.; and (v) changed flow and sediment yield caused by upstream intervention (such as diversion, large-scale bank protection schemes, or deforestation).

Local intervention in the estuary (affecting first the flow distribution, and in turn the sediment budget, the salinity, and the flood risk):

(vi) changed flow resistance caused by natural morphological development; (vii) changed flow resistance caused by intervention (such as bank protection, cross-dams, etc.); (viii) changed erosive capacity related to a changed flow resistance (causing a re-distribution of the flow); and (ix) changed erosive capacity related directly to intervention (such as bank protection, cross-dams, etc).

As clearly illustrated during the morphological studies carried out by MES, the natural planform development is highly dynamic. The development seems to follow a certain pattern over a period of several years, whereafter the pattern shifts to a new one, and the development continues along a different path.

Regarding time scales for response to external forcing, it is noted that the hydrodynamic effects (including surface water salinity developments) will be rather immediate (occurring within one season or less), while the general morphological effects will develop unevenly and slowly (over several years or even decades).

SurveysDuring Phase 1 of the Project, the estuarine surveys of MES comprised instrumentation of BWDB/SSD's 'Anwesha', field work, laboratory analysis of sediments, and data processing.

The marine surveys comprised 18 cruises, totalling 260 net operation days of 'Anwesha'. The following activities were completed:

Establishment of 35 elevated reference points with accurate positions and elevations, and consistency validation of 15 BIWTA water-level gauge bench marks;

13 station-months of water-level gauging at 7 locations; bathymetric surveys covering 10,376 km², or 10,095 km line; 405 ADCP flow measurements in 19 cross-sections; collection and analysis of 1,200 suspended sediment samples from 827 profiles in

the same 19 cross-sections; 342 temperature/salinity profiles; and collection and analysis of 450 bed samples.

The field work and the sediment analyses were made by BWDB/SSD with participation by SWMC, and with support and backstopping by specialists from the MES project team. The data analysis was made by the MES project team.

The following major accomplishments may be mentioned:

Upgrading of 'Anwesha' with state-of-the-art equipment for accurate, 3-dimensional RTK positioning and high-capacity ADCP current profiling, a spread which is particularly suited for morphological monitoring, and for flow measurements in the tidal-influenced channels of the estuary;

training of professional staff from BWDB/SSD in the related techniques and procedures;

establishment in the field of a grid of geodetic reference points, linked with the Survey of Bangladesh grid, which covers the rest of the country (but not the Meghna Estuary); and

production of a set of data that provides a consistent description of the physical state of the estuary, as well as an important part of the basis for a quantification of the governing hydraulic processes.

Data applicationsData applications within the framework of MES were:

Conceptual design for feasibility analysis of planned intervention (for example seabed elevations and sea levels);

baseline description as a reference for hydraulic, morphological, and general environmental impact assessment (for example flow, sediment transport and salinity); and

basis for set-up and calibration of the numerical hydraulic model, which is in turn applied for hydraulic feasibility and impact studies of potential intervention.

In general, hydraulic data from the estuary can be applied for a variety of purposes, such as:

Monitoring of the morphological development, including an improved description of states and processes, as a basis for adjustment of the general physical planning basis and any coastal zone management efforts;

monitoring of salinity distributions, as an important part of the basis for national management of water resources and water quality;

hydrodynamic monitoring (of water-levels and flow patterns), in connection with the national flood management and flood forecasting;

as a part of the basis for environmental feasibility studies, impact prediction, and impact monitoring in relation to the offshore industry; and

a variety of non-routine purposes, such a specific feasibility, design or impact studies in the downstream river reaches or in the coastal zone (embankments, drainage, irrigation, and other schemes and structures).

Home

 

The project's main objective is to increase the physical safety of the people living in the Meghna estuary.

The Meghna estuary, which covers 6,000km2, is one of the world's largest: the Ganges, Brahmaputra and Meghna rivers all flow through it before discharging their waters into the Bay of Bengal.

The extreme forces of nature - tropical cyclones, storm surges, floods, extremely high river flows - cause severe problems in the area, and have led to the loss of thousands of lives and much property.

The Estuary Development Programme (EDP), financed by the Government of the Netherlands, is a four-year programme, which aims at: mitigating natural disasters; improving natural resource management; and creating opportunities for sustainable development.

The EDP will form the planning framework for the long-term development activities of various coastal zone programs, many of which deal with livelihood development on a local scale.  The DHV-led consortium will be responsible for the masterplan for the development of the Meghna estuary. The EDP requires the continuous updating of hydro-morphological knowledge of the rapidly changing estuary. The consortium will manage extensive surveys of the estuary, and analyze its morphological development using mathematical modeling and remote sensing. The programme will propose cost-effective means of erosion control and enhancement of land accretion.

For specific locations, DHV and its consortium partners will identify, design and implement suitable initiatives - such as cross-dams and erosion control works - in the form of pilot projects.

The strengthening of the Bangladesh Water Development Board, which deals with estuary development, as well as the sharing of knowledge and technology, are also an important part of the project. The consortium includes Royal Haskoning (the Netherlands) and four associated Bangladesh companies.

The project, which is a continuation of the Meghna Estuary Studies (MES), which a DHV-led consortium carried out from 1995 to 2001 under the Flood Action Plan (FAP), commences this month.

Estuarine, Coastal and Shelf Science

Residual Flow in the Meghna Estuary on the Coastline of Bangladesh

The Meghna Estuary is a coastal plain estuary on the coast of Bangladesh. The important driving forces for the flow in the Estuary are the bathymetry, tides and the outflow from the Meghna River. A numerical model covering the northern half of the Bay of Bengal including the Meghna Estuary was set up with the objective to increase our understanding of the hydrographic features and morphological dynamics in the Estuary, especially in the case where man-made physical interventions are constructed. The simulations revealed a

counter-clockwise residual circulation with a northward net flow in the Sandwip Channel of 10 000 m3 s−1 during the dry season and 15 000 m3 s−1 during the wet season. The residual flow is forced by tides together with the bathymetry. During the dry season the flow is approximately equal to the river discharge and during the wet season it is approximately one-third to one-sixth of the river discharge. The residual circulation to some extent traps the river water inside the Meghna Estuary and is one of the reasons for the relatively low salinity in the estuary even during the dry season. It is also believed to be important for the morphological development. Finally, a suggested intervention north of Sandwip shows to stop the residual circulation in the Estuary, for which reason it was advised not to construct such an intervention without further detailed investigations.

Bangladesh is at risk of becoming submerged under the sea, but the government has plans for three natural cross dams that will save 500 kilometers of land. With over 2 billion tons of sediment passing through Bangladesh’s vast network of rivers, the country is claiming the opportunity to compact the sediments into livable land masses.

The cross dams will stretch between two islands, helping the mud to become packed onto each shore. The first dam will be constructed this year at an estimated cost of $5.34 million.

“Effective measures to hold the 2.4 billion tons of sediment passing through the Ganges–Brahmaputra–Meghna rivers system would have given rise to about 200 sq. km of land each year in the Meghna Estuary,” said a report issued by Bangladesh’s Estuary Development Programme under the Ministry of Water Resources at the Bangladesh Water Development Board.

This is not a scheme dreamed up by a greedy developer. Bangladesh needs the space. Its population–150 million people–is the 7th highest in the world and is extremely high for a country that is ranked 94th in country size, one of the smaller countries in the world. 100 square kilometers of land are eroded each year by rivers and with the burdens of an ever-increasing population at the rate of 2.1% each year, the country cannot afford to lose more land.

But 180,000 people are expected to benefit from the cross dams, ensuring there is more land for Bangladeshis to make their home

Bangladesh plans to build a series of dams to reclaim 600 square kilometres (230 square miles) of land from the sea over the next five years, officials said Sunday.

The government has approved the ambitious project under which dams would be built in the Meghna estuary to connect islands and help deposit hundreds of millions of tonnes of sediment, project chief Hafizur Rahman said.

“The project would cost only 1.20 billion taka (18 million dollars). The dams will expedite sedimentations and manage the tidal system. They won’t allow loss of any sediments to the sea,” he told Media.

“The whole process will reclaim at least 600 square kilometres of new land from the sea in just five years.”

The mighty Ganges and the Brahmaputra rivers join in Bangladesh before flowing into the Bay of the Bengal.

Studies have found that the two rivers carry more than one billion tonnes of sediment a year.

Rahman said the dams would be designed so that small islands would become linked with the mainland as shallow areas in the estuary fill up with sediment.

A study by the Dutch-funded Institute of Water Modelling (IWM) has found that the damming process would not affect other parts of the coastline or aggravate erosion of the country’s largest island, Bhola.

“We have done some water models of the project and found some 600 square kilometres of new land could be reclaimed without any side-effects,” IWM principal researcher Jahirul Haq Khan told AFP.

The study has been verified by Dutch experts, he added.

Bangladesh reclaimed 1,000 square kilometres of new land in the Meghna estuary by building two dams in 1957 and 1964. Despite the success, the reclamation process was halted due to lack of donor financing.

The impoverished country has been one of the worse victims of climate change, with the UN’s Intergovernmental Panel on Climate Change (IPCC) predicting that 17 percent of its land would go under a rising sea by 2050.

Bangladesh’s landmass has increased in recent years, thanks to sedimentation in its southern rivers, a study has revealed. The study shows that the country has actually grown in landmass equal to five times the size of Dhaka city.The new land has emerged in the Meghna estuary, where sediments flow down from the Himlayas and collect into charlands. The study found that the 8.5-magnitude 1950 Assam

Earthquake increased the sediment flow and has added a net increase of 1,790 square kilometers to the country’s land mass.

?More charlands have emerged than we have lost due to river erosion over the years,? said Dr. Maminul Haq Sarker, a geo-morphologist who conducted the study at the Center for Environment and Geographical Information System (CEGIS).

The new land, which emerged mostly in Noakhali, was discovered when Sarker and his research team analyzed satellite pictures and other data from 1943 to 2008 tracking sediments coming from the Himalayas and flowing down the Padma (Ganges in India) and Jamuna (Brahmaputra) rivers. The rivers deliver about one billion tons of silt a year from India, Nepal, China , and Bhutan to the Megnha estuary within the Bay of Bengal.

The study found that the 1950 earthquake accelerated the sediment flow by causing huge landslides in the Himalayas, dumping an estimate 45 billion cubic meters of earth into the rivers. Within a few years after the 1950 earthquake, silt and clay began to rapidly accumulate in the estuary. In all, the sediment added 2970 square kilometers in new charland while 1180 square kilometers were erodeda net gain of nearly 1800 square kilometers.

Beside Noakhali, new land has accumulated at the Patuakhali, Shariatpur, Barisal and Chittagong districts.

The findings, formally released yesterday, shines a ray of hope on otherwise dire predictions by groups such as the International Panel on Climate Change (IPCC) that Bangladesh will lose about 17 percent of its land area because melting polar ice daps will increase sea levels.

But Sarker cautioned more research is needed.

?This is an indicative study,? he said. ?We need to continue our research to say something concrete.?

?Now we might think to battle the climate change challenge in different way if we can use the sediment in planned way,? he said. ?We can recover certain amount of our land mass from the aggression of rising sea level.?

Citing a recent study of two American scientists, Saker said that the research suggests that one-third of this sediment is deposited on the floodplain and tidal plain of Bangladesh, thus continuously raising the land. One-third of the sediment is deposited on the estuary thus building new islands. The final third is lost in the deep ocean, he said.

The research also found the main reason behind the erosion of 230 square kilometers at Bhola Island, which many regard as an victim of rising sea levels due to climate change, was instead caused by the shifting flow of the Meghna channel. The shift also eroded a total of 195 square kilometers of land from Sandwip and Hatiya islands

The Bangladesh coast, which is already vulnerable to sea-level-associated disasters like flooding and inundation due to monsoons and the tropical cyclones, is being threatened by the phenomenon of sea-level rise, worsening the situation further in the densely populated deltaic region which is hardly 1 m above mean sea level. The interannual component of sea-level variability is very large on the Bangladesh coast. The present paper deals with the problem of predicting mean tide level 1 month in advance in the estuarine zone of the Bangladesh coast during the adverse weather period from September to December. Regression equations have been presented using the Southern Oscillation Index (SOI) as predictor and mean tide level in the Meghna estuary as predicted. Mean tidal levels during the adverse period were found to be predictable on the basis of SOI 1 month in advance. Consequently, the results may find applications in the disaster preparedness programmes for Bangladesh.

A numerical model with a (1/120)° resolution is used in studying 1) the effects of tide–surge interaction and 2) the effects of cyclone landfall on the Noakhali–Cox's Bazar coast of Bangladesh (i) on the time deviation of the surge arrival from the landfall time of a cyclone and (ii) on the duration of the maximum surge. The present study found that the tide–surge interaction in the Meghna estuary shows, in general, the progressive wave nature of the local tide. If the peak of the maximum surge coincides with the tidal peak near the landfall, the surge propagates toward the north faster than when the surge peak coincides with the tidal trough. Cyclones that make landfall before the arrival of the tidal peak produce higher but shorter duration surges than those that make landfall after the arrival of the tidal peak. If the landfall time of the cyclone is kept fixed, the surge peaks are found to arrive earlier and to be of shorter duration with (i) an increase in the propagation speed of the cyclone and (ii) a decrease in the radius of the maximum cyclonic wind. For the peak of the maximum surge, it may take about 3–4 h to propagate from the southern estuary to the northernmost estuary. The surge, which propagates northward, reaches the northernmost estuary earlier and becomes longer in duration with smaller angles of crossing of the cyclone with the coastline. A slow moving cyclone with a larger radius of cyclonic wind, making landfall in the northernmost area, with a small angle of crossing relative to the Noakhali–Cox's Bazar coastline, and landfall just before the high tide is found to be the most severe cyclone, capable of producing the highest surges in the vast and shallow estuary.A depth integrated two-dimensional numerical modeling was carried out to study the sediment dynamics within the Meghna estuary. The sediment — water dynamics within this estuary are very complex due to its irregular shape, wide seasonal variation, and the changing role of the tide. Both cohesive and noncohesive sediment transport formulations were used to estimate the total transport. An interactive morphological computation was also used to verify the bed level changes over 2 years. Sediment transports of both monsoon and dry seasons (the two most hydrologically pronounced periods in this region) were modeled, and a large seasonal variation in sediment transport pattern was observed. Land reclamation dams were tested by the model and found to be effective in enhancing the accretion in its vicinity.Seasonal variation of residual currents in the Meghna Estuary, located at the northern part of the Bay of Bengal, has been investigated through the use of a 3D numerical model. Residual current in the Meghna Estuary appears to be strongly influenced by tidal currents and Coriolis Effect under average meteorological and hydrological conditions of four different seasons considered,.

Average seasonal variation of wind speed and direction as well as fresh water inflow does not seem to have significant influence on residual current. Only under the influence of average of maximum wind speeds of different seasons, residual currents in the Meghna Estuary show their dependency on wind stress. In general, at the surface layer northward and northwestward flow is created during the pre-monsoon and monsoon periods and southwest and southeastward flow is created during the post-monsoon and winter periods.Negative surges destroy coastal aquaculture installations and hinder rescue-evacuation operations during cyclones and storm surges in the Meghna estuary in Bangladesh. The influence of the characteristics of the cyclones striking the Noakhali-Chittagong-Cox's Bazar coast on the negative surges in the Meghna estuary is examined, This study uses a (1/120)o resolution numerical model, which includes the offshore islands and bathymetric details of the worst-affected northernmost part of the Meghna estuary. The model was verified by the observed data of official agencies like the Bangladesh Department of Hydrography. The negative surges in the Meghna estuary are found to have …Studies are described that use a fine-resolution numerical model and incorporate the islands and detailed bottom topography of the Meghna estuary. They show that depending on the characteristics of the atmospheric cyclone and the astronomical tide, storm surges can be coastal trapped in the Meghna estuary and propagate like edge waves along the coastline causing widespread devastation and enormous loss of life and property. The funneling effect of the narrowing estuary acts strongly on the pressure response and predominantly in the region north of Sandwip Island. The combination of coastal trapping and the funneling effect results in the widespread nature of the surges in the Meghna estuary. The widespread nature of the surges is directly proportional to the wind inflow angle and to the radius of maximum cyclonic wind, but inversely proportional to the angle of crossing of the cyclone as made with the coastline. The cyclone striking the Noakhali-Chittagong coast produces more widespread surges than does a cyclone striking the Chittagong-Cox's Bazar coast. A rapidly moving cyclone drives the surges toward the northern coast. If a cyclone strikes during the ebb tide phase, then nonlinear tide-surge interaction also generates separate surges far to the west in the Khepupara region.

Deciding climate change adaptation and mitigation measures

slide showDr. Maminul Haque Sarker

A very severe earthquake of the magnitude of 8.6 on the Richter scale occurred in August 1950 with its epicenter close to the border of Assam in India and Tibet in China. This earthquake caused huge landslides in the Himalayas, estimated at about 45 billion cubic meter. Within a few years, most of the debris generated from the landslides entered into the Brahmaputra river through numerous tributaries in Assam. The fine fraction of sediment mainly consisting of silt and clay very quickly transported through the Brahmaputra/Jamuna river and deposited in the Meghna estuary area. It moved through the river slowly like a sand wave and caused significant changes in river morphology. Some recent research (by this writer) describes the responses of the rivers through

widening and increasing of erosive capacity during the propagation of the sand wave. That was the main reason why the erosion along the Jamuna and Padma rivers was higher in the 1980s and 1990s than in the previous and later periods.

As the silt and clay fraction of sediment entered within a few years after the 1950 earthquake, it caused very rapid accretion of land in the Meghna estuary area. On the other hand, the leading edge of sand wave entered in the mid 1970s and contributed incremental accretion in the following decades. It took more than 50 years for all the extra sand fraction of sediment generated by the earthquake to enter into the Bay of Bengal from Assam.

Erosion and accretion in the Meghna estuary area A topographic map was created from a survey conducted 70 to 90 years ago, but the landform was revised in the aerial photographs of 1943 and 1944. This map provides the shorelines, locations and sizes of off-shore islands. Time-series satellite images covering the Meghna estuary area are available in CEGIS (Center for Environmental and Geographic Information Services) archives from which images of 1973 and 2008 were used to estimate the erosion and accretion. Maps and satellite images were georeferenced with a common projection system. To avoid errors due to tidal variations during the imaging period, a set of criteria was developed to define the shoreline of the estuary.

The long-term net accretion in the estuary has been estimated by different scientists by comparing different historical maps like James Rennel's map (1776) or Commander Lloyd's chart (1840) with the modern surveyed maps. The results of the estimates for the long-term net accretion in the Meghna estuary area are found to vary from 4 to 10 km2 per year.

Comparing the map of 1943 with satellite images shows that from 1943 to 1973 net accretion was more than 1200 km2. If it is assumed that most of the accretion occurred between 1950 and 1973, the net annual rate of accretion becomes more than 50 km2/year, which is 5 times higher than the long-term average rate. Later, the rate of net accretion slowed down and from 1973 to 2008 it became about 17 km2 per year. The overall land gain after the earthquake of 1943 is about 1800 km2.

In 1957 and 1963 two cross-dams were built in Noakhali, which contributed to the rapid land formation and joining of the islands with the mainland. Nevertheless, the source of the huge sediment that increased net accretion more than 5 times than in other periods the massive landslides caused by the Assam earthquake in 1950. The rapid accretion in the following two decades was mainly due to the rapid transportation of silt and clay to the Meghna estuary.

Noakhali district gains the most As the bay penetrates several kilometers into the mainland of Noakhali in the northeast corner of the estuary, it is likely that the delta building process would be concentrated in Noakhali district. During the 1930s, most of Noakhali district town had been washed away by the Meghna river. After the earthquake the main branch of the Meghna river

flowing though Noakhali rapidly silted up -- Char Alexander and Char Jabbar became part of the mainland causing it to advance about 40 km towards the south. Construction of cross-dams expedited this process. Accretion in Noakhali district was about 1000 km2 which was nearly 5 times of erosion. Thus, over the last 60 years net gain of land has been about 800 km2.

Next to Noakhali district, accretion has been more than 4 times of erosion in Patuakhali district -- and thus net gain of land has been about 325 km2 over the 60 years.

Sandwip, Hatiya and Bhola islands lose the mostAlthough net gain of land after the earthquake was very high, southward movement of the mainland caused huge erosion of the three large islands Sandwip, Hatiya and Bhola. For Sandwip net loss of land has been 160 km2, for Hatiya it has been 35 km2 and for Bhola island it has been 230 km2. However, Bhola district as a whole has gained about 180 km2land.

Adverse effectsRapid advance of the mainland of Noakhali caused drainage congestion in the south of Comilla and north of Noakhali. Huge erosion of old and fertile land in Sandwip, Hatiya and Bhola can also be partly attributed to the rapid advancement of the mainland.

What nextThe long-term sediment yield in the three great rivers -- the Jamuna, the Ganges and the Meghna -- is about one billion tons per year. All extra sediment generated by the Assam earthquake has already reached the Bay. It is likely that the rapid accretion rate will slow down and come closer to the long-term net accretion rate i.e. less than 10 km2 per year. While construction of many flood embankments and polders during the last 40 years has restricted floodplain sedimentation, sea level rise would certainly reduce the rate of net accretion.

Effects of climate change and sea level riseGlobal climate is changing over time, but due to human activities the expeditious rate of climate change and subsequent rise of sea level have become evident now. There are, however, uncertainties about the rapidity of climate change and magnitude of sea level rise in the next 100 years. Whatever might be the rate of sea level rise, there is no doubt that Bangladesh would be the worst victim of climate change. Due to low land elevation, one meter rise in sea level within the next 100 years may cause inundation or enhance drainage congestion in significant parts of Bangladesh.

While assessing the extent of inundation or flooding due to different probable magnitudes of sea level rise and rainfall intensity, it is common practice in Bangladesh to consider that the bed level of the estuary, riverbed and river bank levels, floodplain and tidal plain levels would be constant in the future. Recent research of CEGIS indicates that as the rivers of Bangladesh carry huge amounts of sediment, the levels of riverbeds and banks as well as the bed and unpoldered tidal plains of the Meghna estuary will be raised as well considerably.

Proper sediment management based on better understanding on the sediment distribution processes in the floodplain, tidal plain and Meghna estuary may partly compensate for the inundation of the coastal area, but the difference between these two processes, however, depends on the rate of sea level rise in the coming decades. On the other hand, any adaptation measures without considering appropriate sediment management may further aggravate the situation by causing more drainage congestion and erosion in the coastal areas.

Priorities for adaptation and mitigationThe real situation in terms of extent of inundation and flooding due to climate change would be far more complex and different than shown by different studies. Research should be taken up immediately to assess the morphological responses to climate change in the Meghna estuary as well as other estuaries, rivers, floodplains and tidal plains. Such research would help to generate more reliable extent of inundation and flooding pattern based on which adaptation and mitigation measures could be planned. More research should be directed to (i) finding out the most efficient way of injecting sediment into the existing polders to cope partly with sea level rise where sediment is abundant in the surrounding rivers and (ii) diverting sediment laden flow to the most vulnerable and deeply flooded back swamp areas in southwest and south-central Bangladesh.

A depth integrated two-dimensional numerical modeling was carried out to study the sediment dynamics within the Meghna estuary. The sediment–water dynamics within this estuary are very complex due to its irregular shape, wide seasonal variation, and the changing role of the tide. Both cohesive and noncohesive sediment transport formulations were used to estimate the total transport. An interactive morphological computation was

also used to verify the bed level changes over 2  years. Sediment transports of both monsoon and dry seasons (the two most hydrologically pronounced periods in this region) were modeled, and a large seasonal variation in sediment transport pattern was observed. Land reclamation dams were tested by the model and found to be effective in enhancing the accretion in its vicinity.