The Gorai River
The Gorai River is the largest distributary of the Ganges River in Bangladesh. The average flood flow of the river is 4,500 m ³ /s. The annual average sediment transport is about 50 Mtons (million tons) in which about 40% are fine sand and the rest amount consists of silt and clay.

Discharge records from mid 1940s to mid 1970s at the Gorai Railway Bridge gauging station, show that the off-take closed in the dry season only on two occasions in the 1950s. Adams (1919) also describes the cyclic improvement and deterioration of the mouth of this river and other distributaries.

However, due to the reduced dry season flow and the quicker recession of floodwater in the Ganges in the post-Farakka period, dry season flow through the Gorai was affected significantly. Since 1988, the Gorai River began to close completely for some times during the dry season every year. Annual flow volume during the post-Farakka period also shows a clear declining trend (Fig. 3). The wet season flows have also been declining over the same period (Fig. 4).

Figure 3 Changes of annual flow volume of the Gorai River



Figure 4 Daily average discharges of the Gorai River at different periods
Remote Sensing and Hydrologic Data Satellite images are mainly used to identify the morphological changes. On the other hand hydrologic data are used to show the relation between the changes of morphological parameters with the changes of hydraulic regime. The description, sources and processing methods of the data are presented below.

Images Used and Processing Methods
Dry season remotely sensed satellite data were used for a planform analysis. Landsat MSS and TM images of February 1973, February 1984 and January 1997 were organized for analyzing the meandering bend migration, erosion-accretion and sinuosity of the Gorai River.

Processing and analysis of the satellite images were performed using the computer facilities of EGIS. ERDAS Imagine 8.3.1 software was used for analyzing of the images and NT ARC/INFO 7.1.2 was used for vector data analysis. All the digital satellite data were pre-processed to remove data errors and anomalies. The images were then georeferenced and projected to the Bangladesh Transverse Marcator (BTM) Projection system. For each date two scenes of Landsat images were mosaicked to make a seamless image covering the study area.

Hydrologic Data
Time series discharge data (1964-1997) measured by Bangladesh Water Development Board in the Gorai River at Gorai Railway Bridge gauging station were used in the study.

Bankline Delineation
The images were initially classified into Land, Sand and Water. Sand class contains the sandbars during the dry season, which becomes part of the riverbed during wet season. Therefore, banklines were delineated considering both the sand and water class within the rivers. Finally the banklines were edited and refined through onscreen editing keeping the image as backdrop.

Reach Delineation
The Gorai River, whose present length is 199 km, could be divided morphologically into five distinct reaches (Figure 2). Superimposing of the bank lines derived from satellite images of 1973, 1984 and 1997 provide the impression of the characteristics of the reaches of the river. The upper reach, from the offtake to the Gorai Railway Bridge, is the widest and almost straight reach. The next reach, from the Gorai railway bridge to Kamarkhali is characterised by a mixed meander pattern that has remained remarkably stable for several decades (Halcrow, 1993). The following reach is the Kamarkhali bend, which appears to be a permanent feature and makes the flow double back twice within just 20 km (Figure 2), causing considerable energy loss. Although the bends have stayed in the same location since 1860, the loop has grown, cut off and grown again. Because of its complicated shape and behaviour, during the planform study, this reach is excluded. Downstream from Kamarkhali lie two straight stretches, which have also existed for a very long time. The last reach, the so-called "tidal lower course" is, on the other hand, extremely active and sinuous.

Response of the River

River Bed Aggradation
In the post Farakka period, average thalweg depths of the Reach 1 and first 20 km stretches of the Reach 2 have been reduced by approximately 4 m and 2 m respectively (Halcrow, 1993). Further downstream, the reduction of the depth along the river is less significant. With the reduction of the depth, Reach 1 increases its width by approximately 100 m, while the increase of width of Reach 2 is not considerable.

Migration of Meandering Bends
The migration of meandering bend is defined here as the changes of location of the point of highest curvature of a bend from one period to the other. Migration rates (m/year) have been obtained by using the bank lines derived from the images for two different periods (Figure 5, Table 1). When compared with other reaches of the Gorai River, Reach 5 is extremely active. The migration of meandering bends in reaches 1, 2 and 4 are very slow due to the river’s incision into the consolidated high Ganges floodplain. In extreme cases, the rate of bend migration is within few meters per year. Therefore, it is difficult to estimate the migration or bank erosion rate of these reaches with the help of satellite images whose resolutions are 80 to 30 m. The low Ganges floodplain is less consolidated and that is probably the main reason why the Reach 5 of the Gorai is very dynamic. In Figure 5, the migration rates for different periods (1973-84 and 1984-97) and their movement directions are presented. Inspecting the figure and tables, it can be concluded that the migration rate has slowed down and the rate is lower against the terrain slope. Slowing down of migration rate can be attributed mainly to the reduction of annual flow volume (Table 1).

Figure 5 Migration rate and direction of meandering bends of the Gorai