Meghna Estuary Study
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.
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).
The marine surveys comprised 18 cruises, totalling 260 net operation days of 'Anwesha'. The following activities were completed:
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:
In general, hydraulic data from the estuary can be applied for a variety of purposes, such as: