Review of the bank protection works at Sirajganj

Abstract

Bangladesh is a flat land formed mainly by the sediments deposited by the three mighty , rivers, the Padma, the Meghna and the Jamuna. Rivers flow in alluvial deposits are generally active and associated with the process of erosion and deposition. A study has been conducted to review the bank protection work and associated morphological changes near and around Sirajganj town. The major feature is to assess the bank protection works undertaken so far around Sirajganj. In addition, the nature and causes of erosion and to study the , morphological changes brought about by the river in the vicinity of the town have also been studied~ Study of cross-sections of the Jamuna at Sirajganj indicates that the deep channel of the main flow is moving adjacent to the right bank. The sharp bend on the left bank located at just upstream of Sirajganj is mainly responsible for severe erosion at Sirajganj. From the analysis of bankline movement of the Jamuna near Sirajganj it is observed that the channel is migrating towards west at the rate of 29.19 meter per year. Revetment alone as bank protection measures proved ineffective for a braided and heavy silt carrying river like the Jamuna. The groyne at Sailabari constructed in 1977 was found to be effective upto 1984. The groyne helped formation of shoals and siltation at downstream. During the flood of 1987 & 1988, th,is groyne again proved to be effective and still' working well. The groyne at Ranigram constructed in 1986, and the revetment works, constructed as per design and specifications at this location were found to function satisfactorily. Earlier the revetment was used in isolation and was proved to be unsatisfactory. In the 70's and 80's groynes were used as bank protection structure. It worked well but not upto the mark, as damage was found to occur every year. In 1998 Ranigram groyne was modified to built the Sirajganj hardpoint. This structure is effective for diverting the flood flows. Thus it appears from the study that two or more types of bank protective measures is more suitable than a single type in protecting the erosion. The cross-sections XJ6 (6 km downstream of Sirajganj town) shows the deep channel shifted to the left bank and there was deposition of silt at the right bank. This was possible due to protective works done at Sirajganj. But the deep channel in the. upstream of Sirajganj town flows near the right bank. At a . section located 1555 meter upstream from Sirajganj town, it is observed that the bed had been silted up about 6 meter during the period in between 1988 and 1998. It is also observed that the river bank shifted from west to east during the above period. This happened due to shifting of deep channel towards the leftbank. Thus due to protective works at the right bank Sirajganj town is saved from further erosion.