Development of A Computer Program to Simulate Erosion by Concentrated Flows (Ephemeral gullies) in Agricultural Fields
Watershed Physical Processes Research Unit
Project Number: 6060-13000-023-09
Start Date: Aug 01, 2010
End Date: May 31, 2012
Existing soil erosion models do not adequately account for the contributions of concentrated flow erosion to sediment delivery from agricultural fields. Recently, RUSLE2 has been modified to improve computational efficiency and enable erosion estimation in a Geographic Information System (GIS) context. The locations of concentrated flow channels that end RUSLE2 hillslopes are first determined within a raster flow net map. Each cell crossed by a channel defines a drainage area outlet, corresponding to the end of an overland flow path. For each channel-containing cell in the network, a shell program analyzes the flow directions map to determine which of the neighboring cells drain to that channel cell. The process is recursive and somewhat complex: if a cell drains into the cell being inspected, focus is shifted to that second cell. This process is repeated in checking uphill cells uphill until no inflow is detected for a cell. The no-inflow cell identifies the beginning of a flow path. A reverse process is then started, following flowpaths downhill, defining the connectivity among the several cells that compose the area draining to the original channel cell. The program provides to RUSEL2 the ordered collection of raster cells comprising a 2-D RUSLE2 profile. The automatic determination of local slope length as the ratio of runoff entering a cell to that leaving the cell has been integrated into the RUSLE2 engine. RUSLE2 internally manages the transfer of runoff and sediment among the profile segments, and efficiently reuses daily estimates of soil residue cover, soil roughness, and canopy cover computations for each unique combination of soil and management within the simulation domain. This new version of RUSLE2 matches the results of the standard equations for uniform profiles but permits the correct representation of complex situations involving topographic flow convergence as well as seasonal and spatial variability in runoff generation related to soil and management combination effects. The result is a spatially explicit representation of sheet and rill erosion. However, the new RUSLE2 still cannot account for erosion of the channels into which hillslope sediment and runoff are delivered. The objective of this project is to overcome that limitation by developing a modern process-based ephemeral gully erosion prediction model.
Phase 1 of the project will develop an erosion model for a single small channel (ephemeral gully) based on the science used by WEPP/CREAMS, but involving a numerical rather than an analytical solution to allow for a complex channel geometry resulting from soil layers of differing erodibility. Erosion and channel geometry will be computed for a storm event, represented by a peak discharge and assumed steady for the duration of each storm event. The channel will have a single slope steepness and constant hydraulic roughness. Flow discharges in the channel will be assumed to increase linearly along the channel and hydraulic properties will be computed using regression curves implemented in the WEPP and CREAMS models. Five sediment size classes will be used to compute sediment transport and erosion (primary clay, primary silt, small aggregate, large aggregate and primary sand). Sediment transport capacity will be determined using Yalin’s formula. Sediment detachment rate will be computed based on the excess shear stress. Channel widening will be computed by extending the approach implemented in the WEPP and CREAMS models. In Phase II, the program will be extended and refined to satisfy the main requirements for integration with the two-dimensional, cell-based RUSLE2 simulation program. The model will be generalized to allow the description of channels that conform to the terrain, with variable channel bed steepness and interconnected channels that form a dendritic network. A new method to determine hydraulic properties must be implemented to allow the specification of spatially variable inputs (runoff and sediment loads) that correspond to RUSLE2 runoff and sheet and rill erosion computations. The method will determine the steady flow discharge along the channels, and compute the water surface profile and related hydraulic properties along the channel, according to the local channel geometry and hydraulic roughness. The method must work satisfactorily even if abrupt changes in channel steepness or inflow discharge are present. The method must also implement a solution for channel confluences (junctions). The new method should also compute backwater effects. A larger number of sediment size classes will be used, to support the new methods recently implemented in RUSLE2.