UNCERTAINTY ANALYSIS OF FLASH FLOODS IN NORTH COAST OF EGYPT USING ADJOINT METHODS

ABSTRACT:
The application of adjoint sensitivity analysis to flash flood wave propagation in a river channel has
been studied in the research project reported in this paper . The assessment of flood hazard in a
coastal watershed is applied using the adjoint sensitivity analysis. The developed numerical model
determines the sensitivities of predicted water levels to uncertainties in key controls such as inflow
hydrograph, channel topography, frictional resistance and infiltration rate. Sensitivities are
calculated using the adjoint equations and are specified in terms of water depths being greater than
certain safe threshold depths along the channel. The flood propagation model is based on the St.
Venant equations while the propagation of sensitivity information is based on the corresponding
adjoint equations. This analysis is achieved using a numerical model that integrates The St. Venant
equations forward in time using a staggered finite difference scheme. An enhanced method of
characteristics at the downstream boundary provides open boundary conditions and overcomes the
problem of reflections from the boundaries. Then, the adjoint model is integrated backwards in time
to trace the sensitivity information back through the model domain towards the inflow control
boundary. The adjoint model has been verified by means of an identical twin experiment. As a
study case, the method is applied to the main stream of a certain watershed at the north coast of
Egypt and the location of the specified safe water depths are chosen to be near a culvert beneath the
international coastal highway which extends from Egypt to Libya.