A Numerical Approach to Predict Water Levels in Ungauged Regions—Case Study of the Meghna River Estuary, Bangladesh

Zakir Syed, Gyewoon Choi, Seongjoon Byeon
2018 Water  
Quantitative flood frequency investigation in a large estuary is somewhat challenging by numerical modelling, because the model optimization depends on the appropriate physical and hydrodynamic properties of the estuarine river. This study attempts to solve the bathymetry configurations of the Meghna River estuary and the assimilation of flow data, which exposed an important role in water level prediction. Upstream flow rates and nonlinear semidiurnal tides have an impact on the instability of
more » ... he flow in this estuarine river. A large amount of flow accumulates in the upstream confluence against or in favor of tides during the rainy season from the adjacent river basins and significantly moves in the Bay of Bengal. The aim of this study is to predict water levels in the un-gauged regions of the Meghna River estuary. A numerical technique was developed using Mike21 flexible mesh, comprising shallow water hydrodynamic components in the estuary. Subsequently, log-normal distribution was employed to analyze the flood magnitudes among the ungauged stations of the estuary. The calibration results comprised with the observed water levels adequately. In conclusion, these water level prediction results can be applied to alleviate the coastal land from extreme flooding and to design hydraulic structures in the narrow streams. machine learning techniques [7, 8] . Chen et al. and Wang et al. provided methodologies for river flow estimation [9, 10] . The prediction methods are important and generally possible to apply with time series data collected for years. However, there are many data sparse regions which need models for data collection. In this study, we tried to develop a model for the prediction of flow characteristics with limited data and the prediction models recently suggested above can be applied for future study with collected data based upon this study. In addition, there are several previous studies for the investigation and prediction of hydrodynamic characteristics in the data sparse regions in Asian countries. Ahmed (2004) indicated that multipurpose river basin development (MRD) is a necessary approach for disaster management, basin-wide development, ecosystem protection and regional institutional framework [11] . Mirza (2011b) states that variations in flood extent and depth are the result of changes in peak discharges in the Ganges, Brahmaputra and the Meghna Rivers [12] . These problems lead to instability in water levels and severe floods and river erosion in this region. Dasgupta et al. (2009b) state that more than 30% of the cultivable land in Bangladesh is in the coastal area and 1m sea level rise could affect around 23% of Bangladesh's total coastal land area [13] . Most of the coastal regions of Barisal, Patuakhali, Sundarbans, Bhola, Hatia and Sandwip wouls be inundated by saline or brackish water, creating serious saline waterlogging problems. Salt-water intrusion along the Meghna River estuary may reach up to the Mymensingh and Sylhet districts upstream. The surface water salinity of the Karnaphuli and Matamuhuri network in the greater Chittagong district may also be affected. The land use practices in watersheds during the last few decades will have a potential impact on this study area: (i) amount of runoff that results from rainfall in a watershed, (ii) water-carrying capacity of a drainage basin, and (iii) change in land elevations with respect to the riverbeds and sea level [14] . In Bangladesh, riverbank erosion and the deposition of sediments are a regular phenomenon which creates enormous problems for the socio-economic and environmental sector [15] . The morphological changes may occur due to the hydrodynamic forces of wave action, tidal effect and river discharge. The banks of the lower Meghna near Chandpur have been eroded continuously and information on the river bank lines for the last 73 years shows progressive recession of the left bank [16] . In a cyclone study it was illustrated that storm surges originate in the central and southern parts of the Bay of Bengal due to the shallow continental shelf and the surge amplifies to the low-lying, poorly-protected coastal areas [17] . In this study, the Mehgna River estuary is adopted as the case study. There are several alternative study areas for this study, however, we selected the Meghna River estuary in consideration of the importance of region, as there are several issues around the region and it is a typical data sparse region with limited data available. Meghna has an estuary which is one of the largest estuaries on the earth in terms of sediment-water discharge located at the central part of the coastline. The area is characterised by high levels of hydro-dynamic activity. Erosion and deposition occur concurrently and the rates of both are high. The area is periodically subject to severe storms and cyclones. The sediment discharge from the Meghna River is the highest and the water discharge is the third highest among all the river confluences in the world. Due to the high sediment discharge, lands are eroded and reformed. The water flow patterns also shift [18] in the Meghna River estuary. The lower Meghna River carries a combined flow of water typically 120,000 m 3 /s, except for extreme flood events. About 85% of the water flow comes from the Ganga, Jamuna and Padma River confluence from the North-West regions, whereas the remaining 15% comes from the upper Meghna River and the North-Eeast regions of Bangaldesh. Along the Meghna River estuary, there are a number of deltaic islands which create the highly dynamic and morphologic features in distinct seasons. The river basin sites are generally submerged due to the continuous rainfall for a short duration (i.e., a week) of time but the water is drained away slowly by semidiurnal tides (max. 1.4 m) occurring twice in a day at the downstream area of Chandpur and the Bay of Bengal [19] . The regions surrounding the Meghna River estuary of Bangladesh are suffering from severe floods with effects on the river flow, tides and monsoon climate. Moreover, Bangladesh has insufficient
doi:10.3390/w10020110 fatcat:fdjtligwo5bq7kkvtjqvzzlf7i