Analysis of the hydro-morphodynamic processes in relation with risk of sedimentation in tidal rivers of southwest Bangladesh

Abstract

In the Ganges-Brahmaputra-Meghna(GBM) delta, the largest on Earth and one of the most dynamic tidedominated deltas, river sedimentation and associated waterlogging within polders is one of the significant water security issues. Despite several efforts, in the form of Tidal River Management (TRM) at specific locations and dredging of rivers in critical areas, the extent of waterlogging continued to rise over the years due to a lack of proper understanding of the hydrodynamic and sedimentation processes inthe Bhabadah area, perennially being considered as a symbol of sufferings. The knowledge gap on the hydrodynamic and sedimentation process in the tidal rivers has been considereda barrier to successful interventions, which has also been duly acknowledged in the Bangladesh Delta Plan 2100.The knowledge gap is principally the result of the non-availability of adequate and systematically measured hydrodynamic and morpho-dynamic data along the southwest coastal rivers. The main objective of this study is to understand the hydrodynamic processes and the sediment transport dynamics as well as their relationship, with potential implications for different types of interventions. An extensive hydrologic and morpho-dynamic measurement (i.e., water level, velocity, discharge, river morphology, and sediment concentration) was done along a 70 km reach from the Hari River in the north to the Sibsa River in the south, at different spatial (at eight locations) and temporal (seasonal, spring-neap and flood-ebb) scales. Within the study area,the river cross-section from Sibsa to Hari Riverdecreased dramatically from 8,996m2 to 252 m2. Depth decreasedfrom 12.2 m to 0.61 m and width from 970 m to 30 m. Because of these rapid changes, a critical hydrodynamic condition has been developed in the Hari-Ghengrail-Sibsa Estuary. At Bhabadah, strong tidal duration asymmetry was observed with 9hours of the ebb tide, whereas the flood tide was 3.5 hours. After examining all asymmetries, namelytidal duration asymmetry, peak current asymmetry, slack water asymmetry, and morphodynamic asymmetry, it wasdetermined that the Hari-Ghengrail-Sibsa River, controlled by tides, is a flooddominated estuary.A progressive increase in tidal amplitude was observed over the last 50 years in all four estuaries in the southwest coastal region.Tidal amplification requires significant mitigating measures to resist the amplified tidal height. Tidal amplification and tidal asymmetry must be appropriately acknowledgedwhile planning any management options in the southwest coastal region of Bangladesh, where considerable polderization has occurred. Following the increasing and decreasing trend in velocity, the Suspended Sediment Concentration (SSC)changed longitudinally in the study area. The SSC increasedfrom downstream to upstream and then reduced further upstream.The maximum annual average sediment concentration of 2988 mg/l was reported at Khornea station,which is 50 km upstream of the Sibsa station and 10 km downstream of the Bhabadah station, and 1321 mg/l at Bhabadah. The average SSC during the monsoon was84 percent higher than in the dry season. Flood-ebb variation was the most alarming as itdirectly estimatesthe amount of sediment retained.The difference between flood and ebb was more prominent during the dry season than during the monsoon, implying increased sedimentation during the dry season. The Turbidity Maximum Zone (TMZ)clearly shows the sediment movement in a complete tidal cycle in the study area. During monsoon, TMZ indicated that large amounts of sediment entered Bhabadah through Khornea station during the first three hours of the flood tide and a large amount of sediment leftduring the last three hours. A distributed sedimentation of 0.58 m was calculated in a 22 km reach downstream of the Bhabadah station.However, the scenarios were different during the dry season. A large amount of sediment entered Bhabadah during flood tides. A smaller amount departed the system during ebb tides, resulting in 1.46 m of sedimentation in an 11 km reachdownstream of Bhabadah. This study reveals that there was practically no upstream flow in the Hari River during the monsoon and dry seasons, as the ebb discharge was less than the flood discharge. The main cause of massive sedimentation in the Bhabadah area is less upstream pressure.Thus, the gate operation at Bhabadah is crucial for managing the discharge and sedimentation in the Hari River.If the gate remains closed, there will be no upstream flow and less tidal flux, reducing the river's flushing capacity. Bangladesh Water Development Board (BWDB) regularly excavates the riverbed from Khornea to Bhabadah station to manage massive sedimentation in the Hari River. Excavation in a few kilometers of length creates a pond-like feature in the riverbed, promoting sedimentation and filling up quickly. TRM is another intervention practiced to manage sedimentation in the Hari River. Not all the implemented TRM have been considered successful. The failure was primarily due to a lack of understanding regarding sedimentation in the beel area.Before starting any TRM operation, it is essential to properly evaluate the possibility of river bed erosion and sedimentation in the beel area by considering river discharge, sediment concentration, and tidal asymmetry in the linked rivers. To better understand the hydrodynamic and sedimentation processes in the study area, more research is required with monthly data from three consecutive years. The other estuary in the southwest coastal region must also be investigated accordingly. The potential for structural interventions such as regulators, closures, and TRM must be carefully assessed in light of the hydro-morphodynamic circumstances.Modeling should be done using the indicator identified in this study to identify suitable interventions for managing sedimentation in the study area.