Study of aggradation and degradation of the Khowai river

Abstract

The study was conducted to investigate the aggradation and degradation of the Khowai River bed. Assessment of future sedimentation along the Khowai river was also carried out. The reach extends from Ballah to the confluence of Khowai- Dhaleswari River and has an overall length of 87 km. To carry out the investigations 39 sections of the Khowai River form Ballah to the Khowai-Dhaleswari confluence for the year 1988, 17 cross sections form Shaistaganj to Khowai-Dhaleswari confluence for the year 1994 and a thalweg profile for the year 1965 were collected in the digitized form from FAP-6. Data on suspended sediment concentrations at the Shaistaganj station between years 1964 and 1993 were also collected from River Research Institute (RRI), Faridpur. Mean daily water levels for the gauge stations at Ballah, Chunarughat, Shaistaganj and Habiganj form 1964 to 1994 and mean daily discharges at Shaistaganj station for the same years were collected from Surface Water Modelling Centre (SWMC). Bed material gradation curves for the year 1992 at Chunarughat gauge, Shaistaganj gauge, Habiganj gauge and downstream of Habiganj gauge were collected from FAP-6. Suspended sediment particle size distribution curves for the years 1991 and 1992 at Shaistaganj were collected from SWMC. Cross-sectional properties at 17 locations between Shaistaganj and the Khowai-Dhaleswari confluence between years 1988 and 1994 were analyzed and it was observed that the river between the reach is becoming narrower and the average thalweg level has degraded by 1.29 m between Shaistaganj and 6.6 km downstream of Habiganj. The total reach of this degraded thalweg is about 21 km in length. After that up to the confluence of Khowai and Dhaleswari River the average thalweg level has again suffered degradation by 2.03 m on an average during the study period. The cross sectional area at bankfull stage is being increased at the rate of 10.1 m2 per year since 1988. It was also observed that in most of the occasions the cross sectional area above bankfull stage has reduced indicating deposition of sediment on the berms and flood plain on both sides of the river. The overall cross section of the 23.44 km reach below Shaistaganj is being reduced at the rate of 14.84 m2 per year since 1988.

It was also observed from the analysis of the water level records between years 1964 and 1994 that Khowai river bed level is being aggraded at the gauge stations at Ballah, Chunarughat, Shaistaganj and Habiganj by 0.96 m, 0.2 m, over 1 m, and 1.5 m respectively since 1964. Study of the thalweg profiles between years 1965 and 1988 suggested that the Khowai River has aggraded by 1 m between Ballah and Chaunarughat and degraded by 1 m between Chunarughat and Shaistaganj. Mean bed level profiles of 1988 and 1994 suggested average aggradation by 0.4 m between Shaistaganj and 6.6 km downstream of Habiganj. After that up to the end of the river i.e. the confluence of Khowai and Dhaleswari River it suffered degradation by 0.5 m. The mean bed level profile of 1994 showed a scour hole due to the loop cut made by BWDB in the year 1992 at 9 km downstream of Habiganj. Analysis of bed material curves and suspended sediment particle size distribution curves based on Einstein's theory suggested that the inflowing sediment load at Shaistaganj stations consisted of 30% coarser fraction of the suspended load and 70% wash load. Future prediction on the Khowai River sedimentation was made using HEC-6 based on channel geometry of 1988. The study revealed that there would be continued deposition of sediment indicating continued aggradation of the bed level along the study reach of the river.