Numerical Simulations of Overland Flow in Experimental and Field Scale

Soil erosion from farmlands results in top soil and nutrient loss, and forms gullies on the landscape downgrading the productivity and cultivability. To study the soil and gully erosion process over a landscape, the methods of physical experiments, field observation and numerical simulations are often used. In this paper, a physically based hydrodynamic model, CCHE2D, is applied to simulate the overland flow processes in indoor physical experiments and an agriculture watershed. CCHE2D is a finite element based, depth-integrated two-dimensional hydrodynamic model. It has been modified to simulate the rainfall induced overland flow process in field-sized watersheds. It will be applied to simulate soil and gully erosion processes after the its runoff process simulation capability is validated.

In this paper, several simulations of the overland flow experiments were presented. The simulations involved experimental conditions such as unsteady rainfalls, boundary inflows, tillage types, and soil types. The model results were compared not only with the watershed yielded hydrograph, but also with measured runoff velocities over complex topography. The simulated physical parameters have good agreements to the observed data. The 2D model is then applied to simulate the runoff process of Beasley Lake Watershed. This low relief agriculture watershed is about 9.1km² in size, including an oxbow lake, wetlands, croplands and woods. The natural runoff is interrupted by many culverts. The detailed overland flow patterns and hydrologic processes in BLW and each sub-watersheds can be simulated without predefining a channel network and sub-watersheds. This study validated this physically based model and confirmed its applicability to overland flows on hill slopes with soil erosion and gully development.