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.
