Sedimentation enhancement and its retention through integration of vegetation with permeable groin-like structure in tidal river

Abstract

Sediment management in the Ganges-Brahmaputra-Meghna (GBM) delta system is considered as one of the challenging issues, even though several studies clarified that unlike many other deltas, it is receiving sufficient sediment to counterbalance the sea level rise. Therefore, it is important to materialize such potential using appropriate techniques through utilizing the natural power of the flow-sediment regime, especially in the tidal rivers. Towards the development of nature-based solution (NbS), the principles of bandal-like structure, a V shape permeable groin-like structure was designed for the bidirectional tidal environments and applied in a tidal reach of the river Pyra. It is found that the structure has a high potential to enhance sedimentation along the bank line, while having difficulty in retaining the trapped sediment within the complex flow-sediment regime of tidal environment. To overcome such a problem, different types of vegetation are integrated with this structure to test the potentiality of vegetation in accelerating the sedimentation and its retention in the study site. Through systematic monitoring of the growth of vegetation and their performance it was found that only the site-specific native vegetation (Hogla and Shoila) grew well, while other planted species did not survive. Moreover, continuous maintenance and proper nursing of plants were required against the hydrodynamic stresses and human and animal interventions till their maturity. Several parameters related to vegetation-flow-sediment data were collected by using ADCP, Eco sounder, total station, and soil sampling to clarify the changes in flow and soil parameters due to vegetation. The results reveal that the cohesiveness of the soil increases by about 72–110 % and flow velocity reduces by 35–40 % in the area with densely grown vegetation having flow depth < 5 m. While velocity reduction is relatively low (10–15 %) in the area with sparsely grown vegetation and having flow depth > 5 m. Such flow reduction resulted in enhanced sedimentation and deceased erosion, whereas the increased soil cohesiveness resulted in retention of already deposited soil and bank and bed stabilization.