FLOW REGIMES IN HOMOGENEOUS AND ISOTROPIC HILLSLOPES

Understanding of the pattern and timing of flow in hillslopes is needed for accurate assessment of mass movement potential. The distribution of moisture in a homogeneous and isotropic hillslope is a transient, variably saturated physical process controlled by infiltration characteristics, hillslope geometry, and the hydrological properties of the hillside materials. The major driving forces for moisture movement are gravity and gradients in soil moisture content. In a saturated hillslope, under the driving force of gravity and a constant pressure boundary at the slope surface, flow is always in the a lateral down slope direction, invariant of transient or steady state conditions. However, under variably saturated conditions, both gravity and moisture gradients drive fluid motion leading to complex flow patterns. In general, the flow field near the ground surface is variably saturated and transient, and the direction of flow can be laterally down slope, laterally upslope, or vertical. Previous work has considered rainfall conditions sufficient to completely control these flow regimes. We use a numerical model, calibrated with results from laboratory physical models, to show that rainfall conditions are not sufficient to determine the flow regime in isotropic and homogenous hillslopes. For example, under decreasing rainfall intensity conditions, down slope and upslope lateral flow can occur concurrently in a hillslope. We hypothesize and demonstrate that the state of wetting or drying in a hillslope defines the temporal and spatial regimes when lateral down slope and/or lateral upslope flow occurs. Our numerical simulations confirm this hypothesis.