Meghna Estuary Study 
(1995 - 2000)

 

Study objectives

Anwesha

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 setting
Meghna 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 km per year) and sediment (some 1,100 mio. t per year) pass the area. The catchment area is 1,520,000 km. 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 m/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).

The survey room of Anwesha

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 km 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.


Cause-effect relationships

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.

The MES Hovercraft

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).

Surveys
During 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 km, 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.

View from a reference station, Tentulia River

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 applications
Data 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.

A tender boat in Tentulia River

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).


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