Numerical Modeling of the Impact of Riparian Soil Water Dynamics on Channel Width Adjustment

摘要

Occurrence of streambank failure is closely related to redistribution of soil water that affects soil shear strength and may lead to seepage-induced erosion. Pore-water pressure in a streambank is affected, among others, by infiltrating rainfall, streambank-material texture, riparian vegetation, and interactions between surface water and groundwater. The hydrologic and biogeochemical components of the riparian ecosystem model REMM were integrated into the channel evolution model CONCEPTS to produce a model capable of simulating bank pore-water pressure distributions. Testing of the model showed that the subsurface flow component of REMM may not adequately simulate pore-water pressure dynamics at depth and near the groundwater table for streambank stability computations. Improved subsurface flow formulations that can be used to evaluate the effects of pore-water pressures on streambank stability at different temporal scales are presented and tested. To resolve the movement of wetting and drying fronts and their impact on bank stability temporally (i.e., time scales ranging from minutes to hours), a model based on the Richards equations describing unsaturated flow is needed. However, if streambank stability assessment uses time intervals of one day or larger, over which the pore-water pressure distribution may have reached a quasi-equilibrium state, the use of simpler models, such as those based on the Boussinesq equation, can satisfactorily predict subsurface flow and pore-water pressure. Further, seepage-induced erosion may be more important to the stability of a streambank than the redistribution of soil water.