Morphological changes in channels due to cyclone generated hydrodynamic shock

Abstract

River stability and response to changing environmental conditions are highly dependent on local watershed features and exposed environmental condition. During hydrodynamic shock like severe cyclones, stream power increases above normal levels, resulting in dramatic changes in the riverine landscape. In the natural landscape, cyclone generated storm surges can play a major geomorphic role, especially for sediment transport in channels and deposition on floodplains. The geographical location and climatic condition make Bangladesh one of the most cyclone prone countries in the world. For Bangladesh coast, not much information is available on changes in river morphology due to this phenomenon. To fill this research gap - present study aims to develop a semi-analytical model to assess the morphological changes in channels due to hydrodynamic shocks and apply the model for a cyclonic event in Bangladesh coast. Changes in channel morphology depend on the channel characteristics. In this study channels are classified based on channel conveyance. During cyclone, the morphological changes in channel are mainly controlled by bed shear stress. To develop the semi-analytical model, width depth ratio is used as an indicator of channel morphology. From the Manning’s equation, the relation between channel geometrical shape factor (which is the ratio of channel conveyance and roughness) and channel width depth ratio is developed. Afterward width depth ratio is expressed as a function of channel geometrical shape factor. It is found that conveyance of a channel is directly proportional to the bed shear force. By introducing proportionality constant and assuming that change of channel conveyance due to bed shear force varies linearly, the non-dimensional relation between channel conveyance and bed shear force is established. Later the channel conveyance is replaced by channel geometrical shape factor in the non-dimensional relation. By substituting the width depth ratio with the function of geometrical shape factor in the non-dimensional relation and re-arranging, the semianalytical model is developed. Magnitudes of variables in the semi-analytical model are determined using the Delft 3D model simulations for different cyclone scenarios which are termed as hydrodynamic shocks. To determine the coefficients of the semi-analytical model, these variables are used. Delft Dashboard is applied to incorporate the cyclonic condition in the Delft 3D model simulations. Performance of the semi-analytical model is evaluated by comparing the model results with the Delft 3D model simulation. Performance of the model is found to be within ±0.5% to ±5% of error margin.