Morphological response to dredging of the Upper Gorai River

Abstract

The Gorai River is a tributary of the Ganges River and is the only river in the Southwest region of Bangladesh, which provides fresh water from upstream. In the past decades the flows in the Gorai River have been decreasing. From 1988 on, the flows in the lean season were even reduced to zero, due to heavy sedimentation in the off-take. In 1996 the government of Bangladesh initiated the Gorai River Restoration Project (GRRP). The main target of the GRRP is to augment the flows in the Gorai River and to ensure flow in the dry season. Until a permanent solution can be realised, a channel is dredged during the falling stage of the water level to keep the Gorai River flowing. With the Pilot Priority Works, an immediate improvement has been realised and valuable insight into the response of the river to dredging is gained. During the dredging works data has been collected on the Gorai River and the off-take, consisting of measurements of the bed topography, water levels, discharge and sediment transport. In this report the data from April 1999 to April 2001 are used to study the morphological response to dredging. For the analysis, the GIS application ArcView with the Spatial Analyst is used. Besides a qualitative analysis, the effects of dredging are studied based on an analysis of the change in the cross-sections of the river and its effect on the discharge and the sediment transport capacity. Moreover, the sediment balance is extrapolated to include the effect of future maintenance dredging.