Technical evaluation of selected river training works on the Brahmaputra-Jamuna river

Abstract

Considerable bank line shifting of Brahmaputra-Jamuna River throughout the past decade entails its erosive trend. The bank line migration has started since 1950. It's westerly migration caused severe erosion at several points in the Bogra and Sirajgonj districts. To save the valuable infrastructures and locality from erosion and to stabilize the riverbank, several river training works at the vulnerable points had been implemented. This study has been carried out to evaluate some of the river training works namely: 1) Sirajgonj Hard point, 2) Meghai Spurs at Kazipur and 3) Kalitola Groyne at Bogra in relation to technical performance and effectiveness of the works. Cross section analysis to conceive the bed scour reveals that there was severe bed scour at Sirajgonj. In addition to the past revetment works a groyne at Ranigram was constructed to save Sirajgonj. However, the recurrent erosion attack in 1990 lead to further river training work and it was planned and designed to develop a hard r;>0int comprising the Ranigram Groyne head as upstream termination and at downstream another termination to avoid out flanking of the River~ Despite the optimum designs and quality construction it could not perform satisfactorily during 1998 flood. From river stage analysis it appears that in the month of July1998 river stage marked a sharp rise. This sharp rise resulted a potential scour of the upstream head of the hard point with a consequent formation of eddy current, which caused scour hole before the upstream termination. As a result slide of the revetment material of the lower part occurred with partial damage of the hard point. In the following year the hard point experienced a severe attack from the Jamuna River.

Reason of recurrent damage of the hard point was mainly focused on the insufficient dimension of the upstream termination and huge scour slightly below the design bed scour level around the upstream termination. Three no R.C.C Spurs in a series have been constructed at Kazipur (Meghail on the Right Bank of the Brahmaputra-Jamuna River. Protrusion of these spurs ranges from 650meter to 1000 meter in the River. After construction bank erosion for a length of 7km approximately has been stopped and Kazipur Thana was saved. Besides this, approximately 350 hectare of land was reclaimed. These groynes/spurs failed partially in 2003. Kalitola Groyne in the Bogra District on the Right Bank of the Brahamputra- Jamuna River was constructed in 1980 and was rehabilitated after 1998 flood. But its performance was not satisfactory as the head of the groyne suffered failure during 1999 monsoon flood. Latter on in 2000 the head was rehabilitated with additional hard material up to the level -30.00m PWD. Though the 2000 monsoon flood made a scour up to -33.00meter PWD, it is still intact and performing it's objective satisfactorily. In addition to the precise study of the river hydraulics, continuous monitoring of the performances of the river training works and its responses to the morphology is required to update the planning of the River training Works.