Evaluation of Chandpur town protection works

Abstract

A study has been conducted to review the bank protective works and associated morphological changes around Chandpur area. The river reach under study is the Padma and Meghna from Mawa to Haimchar. Possible remedial measures are also suggested. From the study it has been observed that the bank line at Chandpur is more or less stable for a period from 1830 to 1929, because the river Meghna was wider and the Padma had separate flow path. Chandpur was hit mainly by the flow of the Meghna and as a result erosion was not severe. Railway department had done protective works by dumping of large volume of rocks along the banks of the Meghna (Natun Bazar) and Dakatia for the protection of railway station for the period of 1966 to 1972. Protective works have been done by the Bangladesh Water Development Board BWDB at Chandpur Natun Bazar, Puran Bazar and Dakatia bank for a length of 2445 metres for the period of 1973 to 2003 at a cost of Tk 8142 lacs. Protective works have also been done at Eklaspur for a length of 3.57 km at a cost of Tk. 3501 lacs. Some minor protective work had been done at Haimchar and Mawa. From the studies it has been observed that failure of protective work occurred due to vertical and lateral migration of slope of the bank. Reason for erosion at the toe of the slope of bank is due to the under design work,ie, hard material used during implementation of revetment work was inadequate .Also from the study it appears that in vertical section at Chandpur Natun Bazar, Puran Bazar and Dakatia bank, e~isting slope of bank on ~xisting protective works is less than the design( design slope one vertical and two horizontal). Proper size and gradation of hard material has not been properly maintained during revetment work. As a result base material below water table under armouring has been lost. Other reasons are due to rapid change of morphological cross sections, change of thalweg line, bar movement, number of channel formation, hitting of flow line, position of bar, change of river pattern, change of river course, change of the 50% conveyance point, variation of discharge etc, which effects depth of scour and location of erosion. In Dakatia bank lateral migration results from bank sliding and bank sloughing for vortex and eddies formed due to mole head at the Natun Bazar Meghna bank. Vortex and eddy current have been diverting the flow path of Dakatia River, which effect excessive degradation resulting failure. From the planform analysis of the SPARRSO maps, it has been revealed that required length of the river is about 80 km from Mawa to Haimchar for effective study. This can provide basis of design for future river training works at Chandpur.

From the analysis of bank line movement, it has been observed that at the upstream of Chandpur the river shifts at an average rate of maximum of 36.55 meter per year to minimum of 3040 meter per year. At few places at the upstream of Chandpur, average rate of erosion was almost nil. At the down stream of Chandpur, the river shifts at an average rate of maximum of 203.36 meters per year to minimum of 3.92 meters per year. At the left bank of the Padma maximum erosion observed at an average rate of 111.28 meter per year and minimum at an average rate of 43.80 meter per year. Maximum erosion observed at krn 25.00 at the left bank of the Padma is about 7.80 krn for the period of 1973 to 1998. From the planform maps and cross sections analysis, it has been observed that, some sections are braided, some are meandering and some are straight. It has also been observed that, creation of char within the river and numbers of channel formation is temporary and periodic in nature. At the left bank of the Padma and Meghna from Mawa to Haimchar, bending nature has not changed, and gradually increases in the northern and eastern direction .Also thalweg lineis found to move along the left bank. The entire length acts as the outer bend of river. It indicates that, if the entire curvature length of this location is considered under protection area, then the river bank of this location will be stable, because meandering pattern is stable in nature. From the analysis it appears that, in absence of Chandpur town protection works the combined flow path of the Padma and upper Meghna will be changed, which will follow the flow path of the Ganges -Jamuna flow path. Reason is that, out of 160,000 cumec discharge, only 20,000 cumec has come from the Meghna, which cannot influence the combined flow path of the Ganges and Jamuna River. Analysis of the SPARRSO maps reveals that the curved length of the river Padma and Lower Meghna from Mawa to Haimchar acts as the meandering pattern. If groyne is constructed for bank protection, then a number of groynes will be required. Insufficient number of groynes will not be effective against erosion within the full meandering length around Chandpur area. In addition to that due to eddy current at the mole head of groyne, depth of scour will be increased. The study observed that revetment work is more effective against erosion. This observation is based on the result obtained from recent construction of revetment of the Dakatia bank of Chand pur Town Protection Works.