Assessment of water level and bed level variation in the lower Meghna river

Abstract

This study was conducted to analyze the water level and bed level variation in the Lower Meghna River. Different analysis of cross sectional area, variation of mean bed level, erosion/accretion of the channel and movement of thalweg position were carried out to find the variation of water level and bed level. The assessment was carried out mainly based on the available cross sectional data and water level data from 1985 to 2008 at five different stations along the Lower Meghna River. It may be seen from the analysis of the cross sectional data that there is no consistency in variation of cross-sections over the years. However, the analysis has shown certain trend in the variation of the different hydro-geometric parameters. The cross sectional area varied rapidly at Chandpur station where as the changes in cross sectional area at Sandwip was steady. The change of cross section at Chitolkhali was very dynamic and a rapid increase followed by decrease in cross sectional area occurred at Chitolkhali. The variation of water level and bed level followed a typical trend with few exceptions. In most cases the water level dropped as the bed level is lowered or the water level is raised as a consequence of bed level rise. However the change in water level was not very significant as the change in bed level when the data were compared with two selected years. Analysis showed that there were exceptions in the period of 1985 to 1992 where the water level at Chandpur station is raised by 0.48m with a drop in the bed level by 6.58m. From the analysis of Mean bed level a significant change in the mean bed level of the Lower Meghna River at Chandpur was observed where as the change at Sandwip station was the least. The range of mean bed level rise or fall at Chandpur was from 1.71m to as high as 7.28m over the years. The shifting of thalweg point was also varied at times at different places. The most frequent movement of thalweg was found at Chandpur.