Morphodynamic equilibrium of width and bed level profile of an estuary in response to adjustable channel widths

Abstract

In this thesis, the morphodynamic equilibrium of the width and bed level profile of estuaries is studied. This is done by using an one-dimensional hydrodynamic and morphodynamic model that includes variable channel widths. The width is determined by a hydraulic geometry relation that relates the width to the peak in total discharge, which is dependent on both the fluvial and tidal influence. A river discharge is forced on the upstream boundary of the channel and a tide is forced upon the downstream boundary of the channel. The channel starts as a straight channel and then evolves towards a steady state with respect to the channel width and bed level. The equilibrium width and bed level profiles largely depend on the coefficients used in the hydraulic geometry relation. These coefficients were underestimated, but the model produced a realistic cross-sectional area tidal prism relation that did not depend on these coefficients. The results generally show that longer estuaries with a larger ratio between the width at the mouth and the width upstream are developed when the tidal amplitude or initial channel depth are larger, or when the river discharge, the initial slope or the drag coefficient are smaller. The width ratio can actually be directly related to the tide dominance ratio, whereas the estuary length mainly depends on the channel depth at the mouth, the estuary bed slope and the tidal amplitude. More convergent estuaries with a shorter e-folding length scale are developed when the initial slope or drag coefficient are larger or the initial channel depth is smaller. Interestingly, the results show that the convergence is greatly dependent on a balance between the river discharge and tidal amplitude. Generally, the estuary is more convergent with larger tidal amplitudes and smaller river discharges. However, if the river discharge is too low or the tidal amplitude too high, the estuary will develop a concave instead of a convex width profile. The equilibrium bed levels were all concave down with an increasing depth towards the mouth. Overall, the model shows great potential for predicting the equilibrium width and bed level profile of estuaries to changing boundary conditions if the hydraulic geometry relation coefficients are optimized.