Assessing the impacts of the Padma river stabilization and its embankment failure through a combined 1d and 2d hydrodynamic model simulation

Abstract

Bangladesh is in a unique geographic location with the dominance of floodplains, low lands, proximity to the sea and overwhelming dependence on nature. Flood is a common phenomenon here as it is a riverine country and consists of three large and extremely dynamic rivers – the Ganges, the Brahmaputra and the Meghna. The Government of Bangladesh has a policy regarding better water resource management, disaster risk reduction and land reclamation from the river bed. Such policy includes narrowing down, channelization and stabilization of the Padma River, and construction of infrastructures on its both floodplains. The rehabilitation and construction of embankments have been suggested under a stabilization program. However, such stabilization may result in a change of hydraulic characteristics of the Padma River system and may have both positive and negative impacts. Moreover, the stabilization works are usually carried out with earthen embankments, which are susceptible to failure. Such embankment failure occurs under many conditions including overtopping, seepage, piping, river migration, improper design and construction, and lack of maintenance. Almost every year flooding due to embankment failure causes severe loss to properties and crops. However, very little study has been made for assessing the effects of large-scale river stabilization on the hydraulic behavior of the river and the associated impacts of embankment failure. This study has been conducted to set up a coupled 1D-2D hydrodynamic model using HEC-RAS for the Padma River system and to simulate stabilization and embankment failure impacts. The Padma River, its distributaries and adjoining areas are included in the model setup. The model is calibrated for the 1998 flood and validated for the 2004 flood using the observed water level.Further calibration and validation of the model at the floodplain are done using the observed highest flood level, and through a qualitative comparison of flood inundation with satellite images, respectively. It is found that the model can simulate the observed variation of these floods quite satisfactorily.The simulation results reveal that the peak discharge of the 1998 flood can increase by 20% in the Padma River under a stabilized condition. The peak discharge can increase many folds at the primary distributaries of the Padma River. The peak flood level in the Padma River can increase by 51 cm and maximum average velocity by 25% due to the river stabilization. Between the two embankment failure modes, the overtopping failure can cause slightly more damage than the piping failure mode in terms of the inundated area. The vulnerable areas are easily identified from the model simulated depth of flooding and floodwater arrival time. The outcome of this study will be useful to government agencies, planners and disaster managers in designing river interventions and in flood management and disaster risk reduction.