Bank Erosion and Accretion
The definition of the terms bank erosion and accretion as used here is illustrated in Figure 6. Like the bend migration, in Reach 5 the erosion and accretion rate is also quite high (Table 1). The bank lines as derived from satellite images of 80 to 30 m resolution. Therefore, potential of errors in estimating the erosion and accretion is inherent. Areas of erosion and accretion only in Reach 5 of the Gorai River is presented in ha/km/year in the table. Similar to bend migration, in the period 1973 to 84, the river was more active than it was during the period 1984-1997.

Figure 6 Area of erosion and accretion as seen in an over lay of channels of two dates.


Table 1 Changes of meandering bend migration, and erosion and accretion rate in response to the changes of annual flow volume

River	Period	Av. Annual flow volume (Mm 3 )	Meandering bend migration rate (m/yr)	Erosion (ha/km/yr)	Accretion (ha/km/yr)
Gorai Reach 5	1973-84	46,654	110	2.50	3.10
	1984-97	32,708	72	2.00	2.00

Sinuosity
Sinuosity is the ratio between actual length of a reach of the river and the straight length of the reach along the terrain slope. The length of the Kamarkhali bend is oscillating and sinuosity has been changing within a range of 2.75 to 3.5, while the sinuosity of the straight reach downstream of the bend is remained constant at close to 1.16. However, the changes of planform characteristics of these reaches have not been linked with the declining process of the Gorai River. Reach 5 of the river has been elongated by 16 km and the corresponding change of sinuosity is from 1.77 to 2.2 (Table 2). It could be inferred that the river reduces its slope (in response to reduction in water and sediment supply upstream) by becoming more sinuous. It endorses the observation of Adams (1919) in the other distributaries of the Ganges. He noticed that a distributary become more tortuous during their process of declination.

Table 2 Changes of sinuosity in response to the changes of annual flow volume

River	Period	Av. annual flow volume (Mm 3 )	Sinuosity
Gorai Reach 5	Pre-Farakka (1964-73)	47,410	1.77 (1973)
	Post-Farakka (1976-84)	42,430	2.05 (1984)
	Post-Farakka (1984-97)	32,520	2.20 (1997)

Discussions
Applying remote sensing tools to study the morphological characteristics of the river mainly depends on the scale of the river dynamics and the resolution of the images. With images having a resolution of 80m x 80m to 30m x 30m, it is only possible to estimate the changes of morphological parameters of a river, the rate of changes of which are in the scale of tenths to hundredths of meters. The time intervals of the images were selected in such a way that during the interval period the changes are in the range of hundreds of meters. These changes are easily identifiable if bank line derived from one image is superimposed on that of another image. The error that is related to the low resolution is less for the cases of relatively high changes and vice-versa.

Figure 7 Changes of different morphological parameters of the Gorai River in response to the reduction of flow
First, the Reaches 1 and 2 reacted in response to the changes of the hydraulic regime and the planform of the Ganges River near the Gorai offtake. Subsequently, lower reaches reacted on the reduced Gorai flows due to the huge sediment deposits in the first 2 reaches. Due to the consolidated nature of bank materials the morphological parameters of Reach 1, 2 and 4 do not react as quickly as Reach 5. Tables 1 and 2, and Figure 7 show how the meandering migration rate, erosion and accretion rate, and sinuosity of the river reach having loosely packed bank materials reacted with the diminishing of average annual flow volume. It appears that the changes of the meandering migration rate, erosion and accretion rate are proportional and the sinuosity is inversely proportional to the changes of the average annual flow volume (Figure 7). It also suggests that Reach 5 is very dynamic and adjusts its morphological parameter rapidly with the changes of hydraulic regime.

Conclusions
Using remote sensing technology it is possible to identify the trend of morphological changes of the dynamic river reaches only. This trend can be used to predict the morphological changes in response to the declining flow of the river. It is not possible to identify the trends of less dynamic river reaches, because the older images available are of coarser resolution. However, in future this type of study can be carried out even for the less dynamic rivers using the presently available high resolution images.

It is found possible to predict (using Tables 1 and 2, and Figures 5 and 7) the morphological changes of the river reaches for the time scale of decades, because the data presented here are extracted from satellite images at time intervals of 11 and 13 years. It also appeared to be possible to predict (using Figure 5 and Table 1) a time range of occurring loop cuts in the meandering loops.

Acknowledgement
This study was carried out as part of the Environmental Impact Assessment (EIA) study for the Gorai River Restoration Project. The authors like to acknowledge all the team members involved in the EIA study.

References

• Adams, W.C., 1919. History of the rivers in the Gangetic Delta, 1750-1918. Bengal Secretariat Press, 1919, reprinted by East Pakistan IWTA, 1966.
• Delft Hydraulics/DHI, 1996. Morphology of the Gorai Offtake: special report no. 10. River Survey Project (FAP 24), Dhaka, Bangladesh.
• EGIS, 1999. Environmental baseline of the Gorai River Restoration Project. Bangladesh Water Development Board, Dhaka, Bangladesh.
• Goodbred, S.L. and Kuehl, S.A., 1999. Later Quaternary evolution of the Ganges-Brahmaputra River Delta: Significance of High Sediment Discharge and Tectonic Processes on Margin Sequence Development. Submitted to Sedimentary Geology.
• Halcrow, 1993. Morphological studies. Vol 3, Southwest Area Water Resources Management. Project (FAP 4), Dhaka, Bangladesh.
• NEDECO, 1967a. Inventerisation of the waterways. Vol II, part C: surveys of inland waterways and ports (1963-67). EPIWTA, Dhaka.
• NEDECO, 1967b. Hydrological and morphological phenomena. Vol III, part B: surveys of inland waterways and ports (1963-67). EPIWTA, Dhaka.
• Umitsu, M., 1990. Late Quaternary Sedimentary Environments and Land Forms in the Ganges Delta. Sedimentary Geology, 83(1993) pp 177-186.