Performance evaluation of bandal-like structures for tidal river flow and sediment management

Abstract

Tidal Rivers experience bi-directional sediment transport associated with variable distribution of sediment concentration generated by the periodic flood and ebb tide and the development of meandering planforms in the tidal environment is the consequences through erosion and sedimentation processes. The study site is located at about 60 km upstream from the mouth of Bishkhali/Burishwar estuary in the southern estuarine zone along the right bank of the Payra River on a naturally developed sand bar which is selected for the establishment of Sheikh Hasina Cantonment. Round the year tidal amplitude of that reach varies between 1.5 to 2.0 m and as the land elevation is nearly zero, the maximum area allocated for the cantonment is submerged during daily high tides. Historical evolution of the planform over the period of 40 years exhibits that the outer bank shifting and sediment deposition occur over the floodplain area leading towards the development of natural meandering cutoff, however, during the last two decades the site is nearly stable with intermittent bank erosion. The stability of the river course is the key concern for the safety of cantonment site, but implementation of any permanent bank protection work to safeguard this area requires considerable time and money. As forecasted, the rate of bank erosion will aggravate with time, the authority decided to construct an adapted Bandal-like structures for bi-directional flow. The aim of the protection work is to facilitate sedimentation near the bank side without much intervening the natural flow of the river. The current research aims to evaluate the performance of constructed structures through deploying field measurement using Echotrac CVM single beam echo sounder for bathymetry, River Ray ADCP for flow velocities and discharge and OBS-3A sensor for sediment concentration. Changes in bathymetry, cross sectional profiles, flow velocities, and sediment dynamics at study site are analyzed to evaluate its performance. It is observed that study site is ebb dominated round the year with significant river discharge from upstream especially during pre-monsoon and monsoon period. Micro scale analysis of cross sectional profiles and flow velocities along the constructed structures shows that the implemented structures are capable of reducing flow velocity during flood period that results in sedimentation at upstream region. On the other hand, the structures are also functioning to reduce ebb flow velocity but could not achieve desired hydraulic condition that can result in sedimentation. The above shortcomings have been resolved analytically through fixing two important parameters of the structure: alignment (angle) and blockage height so that Bandal-like structures can be successfully adopted in any tidal environment for stabilizing the channel.