Flow and erosion processes at bends and around river training works in a sand bed braided river

Abstract

Riverbank erosion is one of the major problems in Bangladesh. Annually thousands of hectares of land are eroded along the major rivers. Millions of people became homeless and landless, and displaced due to erosion. The erosion also affects livelihood, agriculture and environment. To protect banks from erosion about thirty bank protection structures have been constructed along both banks of the Jamuna River. But almost every year some of the bank protection works are damaged. So, it is essential to investigate the insight causes of failure of the bank protection structures. It is basically a field based research. The present study has been conducted (i) to investigate flow and erosion processes at bends, (ii) to investigate flow and erosion processes around bank protection works and failure mechanism of some selected bank protection structures, (iii) to develop of an analytical erosion prediction model and finally, (iv) to integrate indigenous and technical knowledge for sustainable erosion management. In the present study it is found that the secondary current in a bend of the third order channel within a large scale sand bed braided river is similar to that of the single thread meandering channel. But the secondary cell is divided into multi-cell due to flow over the sandbar. It is also found that the bend channel migrate downstream, lateral direction and upstream direction. The maximum rate of erosion is observed at the upstream and downstream sides of the bend. The rate of erosion is higher at upstream and downstream side of the bend due to the shear velocity is six times higher than critical shear velocity and the thalweg is near the bankline. The flow and erosion processes around Sirajgang hardpoint, Betil and Enayetpur spurs and bandal at Randhunibari have been investigated in the present study. The radical morphological change in a sand-bed braided river is a great challenge for the structural stability of the river training works. Because the flow processes near a river training works is guided by the sandbar which is a prominent characteristic of a dynamic braided river. It is found that the strength of vortices, flow intensity and velocity considerably changes due to drastic morphological change around the Sirajgang hardpoint. From the present study it is clarified that the oblique flow a prime factor for the failure of the river training works. Other factors also accelerate the failure event of the river training works. From the analysis of flow processes and morphology around the Sirajganj hardpoint it has been investigated that the following are the reasons of the failure of the Sirajganj hardpoint such as: generation of oblique flow by sandbar and attacked the hardpoint, washed away of sandbar adjacent to the hardpoint, thalweg shifting at the vicinity of the hardpoint, movement of five meter high dune through the channel passing adjacent to the hardpoint, riprap failure due to movement of dune, development of scour hole because of riprap failure, flow slides from the hardpoint side. The causes of failure of the Betil and Enayetpur spurs have also been investigated from the present study. The causes are as follows: generation of oblique flow as a result of upstream morphology, strong parallel flow upstream of the spurs and development of deep channel due to flow circulation normal to the spurs. It is also found from field measured data analysis that the bandal-like structure is very effective as flow diversion and sediment deposition measure. But selection of site for implementing of such structures is a major factor in a large scale sand bed braided river. An analytical erosion prediction has been developed for the third order channel of a large scale sand bed braided river. The model is developed using the flow parameters, planform parameters, properties of bank materials and indigenous knowledge. The indigenous knowledge regarding effective period of erosion has been integrated into model. The model basically produces an envelope curve. The envelope curve covers eighty seven percent of the observed data and seventy eight percent of EGIS data. The field engineers could estimate the rate of erosion for the third order channel of a sand bed braided river. It is evident from the present study that the riparian people are enriched with experienced based indigenous knowledge on flow and erosion processes around different bank protection works, flow and erosion processes at bends, erosion processes at the straight bankline, historical morphological change, bend development, the trend of the rate of erosion, effective duration of erosion. It is clarified from the present study that there are strong similarities among experience-based indigenous knowledge, field engineer’s knowledge, field measured result and laboratory based scientific knowledge. Interesting outcome of the present study is that the local people can guess the bank protection structure is going to be failed observing bustling out of air bubble, floating bamboo and geo-textile and turbid water. Observing such signs, the local people convey message of probable failure of the bank protection works to the BWDB personnel for taking necessary steps to avoid failure event. Therefore, management program during emergency situation should be formulated integrating the indigenous knowledge.