Abstract:
Stream being a dynamic system changes its course to attain stability. In this continuous process of attainment of stability planform, morphology and other features of streams change to cope with it. In some reaches the changes are insignificant over a short period of time while in some reaches changes are so dramatic to bring about avulsion. The effects of such changes are eventually noticed in planform and the most common outcome of such adaptation is bank erosion. In this study, an attempt has been made to analyze the bank erosion of a selected reach of the Jamuna River by preparing a morphological model of this river. The selected reach covers about 160km reach of Jamuna River, from downstream of Teesta offtake to Sirajganj.
In this study five vulnerable reaches along five districts (Gaibandha, Bogra, Sirajganj, Jamalpur and Tangail) were identified for incorporating bank protection structures. Revetment was introduced in the model along those vulnerable reaches to evaluate the feasibility of revetments as bank protection measures. Having selected the five reaches to carry out the study the model has been simulated through two flood events; flood events with return period 2.33-year (2005 flood year) and 100-year (1998 flood year). Response of Jamuna River in the vicinity of proposed revetments with respect to these flood events have been evaluated and compared with that from the model of the base condition. Impact assessment of these flood events have been carried out in terms of near bank velocity, bed level changes and bank erosion.
In general near bank velocity reduces after the introduction of revetments along the erosion prone reaches. Oblique flow was noticed in some of the erosion prone reaches rendering the reaches more susceptible to bank erosion. The proposed structure-induced bed scour may vary from 0.8m to 10 m. In general, revetment type of bank protection works executes satisfactory performance to restrain the possible bank erosion along the selected reach of Jamuna River. Based on the results of the two-dimensional morpho-dynamic model MIKE21C, the river bank protection work structures proposed in this study do not cause significant changes in the short term (one flood season) hydro-morphology of the Jamuna River. Besides a correlation between average discharge prevailing during monsoon and shear stress applied by the erosive action of flow, was established. The correlation accounts for hydraulic erosion only, i.e. erosion due to geotechnical failures or other reasons cannot be estimated by this correlation. The correlation was later used to estimate the extent of erosion of the respective reaches for another hydrologic event (2013), which was found to be satisfactory for four out of five reaches.
