Optimal Flood Control in Alluvial Channel Using Adjoint Sensitivity Analysis

摘要

Flood flows in alluvial channels (rivers) convey suspended sediments from upstream to downstream, and also entrain bed materials along with flood currents. It is obvious that morphological changes in channels due to sediment deposition/erosion will affect water stages, and results in different hydrographs, in comparison with those in a fixed channel bed. During flood seasons, due to speeding flood currents, even drastic bed changes will occur. It is therefore of importance to take into account morphological changes in alluvial channels induced by flood flows for the purpose of flood control operation. This paper presents a systematical numerical investigation on the effect of morphological change in an alluvial channel on optimal flood control. As an example, the numerical optimal flood control is demonstrated by operating a floodgate to withdraw flood waters from a channel. The optimal withdrawal hydrographs for the floodgate operations are obtained by an integrated numerical optimization modeling system which consists of a hydrodynamic model, a sediment transport model, and a bound constrained optimization model. The hydrodynamic module is to solve the one-dimensional nonlinear de Saint-Venant equations. The sediment transport module solves the non-equilibrium transport equations for non-uniform sediments. And the optimization module is to find the optimal solution iteratively by solving the adjoint equations of the Saint-Venant equations obtained by variational approach. In order to study the effectiveness and applicability of the optimal control theory, alluvial channels with single floodgate is considered for different flow conditions and sediment properties. The changes in morphology due to control of flows are studied thoroughly. It is found that the influence of morphological changes on flood control becomes critical under certain flow conditions and the integrated model is applicable to perform optimal flood control under consideration of sediment transport in alluvial channels/rivers.