Erosion Risk Assessment Tool for Construction Sites

ion is the removal of rain from surface runoff processes. In addition to infiltration, it includes surface depressional storage and vegetative interception, of which surface depressional storage is of greater importance. Initial abstraction is the surface depressional storage, vegetative interception and infiltration (Fs for the GAML model) prior to the start of runoff. Most of the total surface depressional storage and vegetative interception occur prior to runoff. Rainfall excess depth is defined as the difference between rainfall and abstraction depths. Excess depths drive the detachment and transport of sediment by surface runoff. Simple vegetative interception models are used in the WATER model where the user specifies whether the relative interception depth as large, medium or small. Adjustments for wind speed are also possible. Two methods for surface depressional storage are available to the user. The simplest approach is to specify whether the relative depth is large, medium, or small. A more rigorous model is available using the results of Onstad’s (1984). Surface depression storage depth is then predicted as function of the roughness and slope of the surface by the following equation. ) 012 . 0 031 . 0 112 . 0 ( L r r d S R R SS − + = (4.3) where SSd is the surface depressional storage (mm), Rr is the roughness of the surface and SL is the slope of the land. The curve number method is a two stage model that directly predicts rainfall excess. If the cumulative precipitation depth is less than initial abstraction, then the excess depth is zero. If the cumulative precipitation depth is greater than initial abstraction, the excess depth is predicted as a a a I P if S I P I P Z > + − − = 2 ) ( (4.4a) where Z is the cumulative excess depth for a cumulative precipitation depth P, Ia is the initial abstraction, and S is a maximum abstraction depth related to a curve number (CN) by