Study on the effect of char movement on river morphology around upper Meghna - Dakatia confluence

Abstract

The present study has been conducted to investigate the planform characteristics and change of river morphology due to Char movement around Upper Meghna-Dakatia confluence lying in Bangladesh. To carry out the study satellite images (LANDSAT MSS, TM and IRS, L1SS) From MIKE 21 C modelling based study it was found that no change in lell bank ncar Chandpur occurred due to bank protection works. At right bank opposite to Chand pur from point 1 (E 548714, N 577998) to point 2 (E 562675, N 567553), few meters of bank line shifting occurred. At the samc time at some location sedimentation occurred, IiIle shi IiiIlg became zero and sand bar formed. From the satellite image of the year 2004, the bank line shifting has been found to be 135 m/year. The model predicted 144,180 and 252 m/year bank line shifting for average (1 in 2-year return period), intermediate (1 in 10-year return period) and extreme (1 in 1OO-yearreturn period) flood events respectively. The study revealed that the actual velocity exceeded the design velocity. As a result the scour depth (-63 m) was found to be higher than that of the design scour depth of the protective works. But when Char-l is extended in northeast direction then the velocity is decreased than the design velocity. Sediment transport of the Lower Meghna at 1.5 km downstream of Chand pur was estimated using half day tid,al cycle measurement data. The value of sediment transport for the month of September was found as 0.93 million tons and corresponds to approximctely 272 million tons/year. Sediment transport was also estimated using MIKE 21C, it was found as 233 million tons/year by Engelund Hansen and 109 million tons/year by Van Rijn formula. Total sediment transpOli load was also calculated by Engelund Hansen and Hossain equation and was found as 423 million tons/year and 893 million tons/year respectively. covering the period ti'om 1973 to 2004, bathymetric data, water level and discharge data has been used. All of the data have been collected from CEG1S and IWM. Several field visits were conducted with a view to obtaining better idea on the complex morphological behavior around Meghna-Dakatia confluence. The length of the study area is considered approximately 82 km from Mawa to Salempur. From the analysis of satillite image of various years, it was appaernt that the river system is not only characterized by a wide river bed of several kilometers in which the various channels developed in combination with large propagating sand bars, but also gradually shifting of the whole lower Meghna both in westward and eastward direction. From analysis it has been found that over 31 years, left bank maximum erosion rate was 323m/year and right bank maximum erosion was 113 m/year. Analysis of time series of satellite imagery shows that from 1973 to 1993 two Chars, named Char-l (between the confluence of Padma and Upper Meghna river) and ChariI' Char was present at upstream of Chandpur. ChariI' Char was almost diminished in 1993. Another Char was formed named Char-2, at downstream of Char -1 during the period of 1993. Area of Ch<ir -1 increased from 1973 to 1998 and Char-2 increased from 1993 to 1998. After high flood of 1998, Char-1 and 2 eroded and divided into small segments. Areas of two Chars werc found to increase again from 2003 to 2004. From satellite image, it was apparent that Char I and Char 2 increased its size and there was a tendency to rejoin together. If this trend continues, these two chars will form into a single char in the near future. The present morphology at the confluence of Padma and Lower Meghna rivers is such that the width of cross-sections increases considerably at upstream of the confluence, leading to a decrease of stream veloeity and consequently the generation of large chars elsewhere. These chars influence the course of the channels. Erosion of the upstream end and sedimentation of downstream end of the chars results in the downstream movement of the chars through the Upper Meghna-Dakatia river, into the direction of the confluence. The study revealed that there were exceptions in pre and post monsoon of 2004, during this period sedimentation occulTed at both end of Char 1 and Char 2.