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Hillslope Erosional Processes: Surface Erosion

Surface erosion is the wearing away of the surface soil by water, wind, ice or other geological processes. The potential for surface erosion is associated in part with the amount of bare, compacted soil exposed to rainfall and runoff. An example of overland flow, potentially detaching soil for transport on a compacted slope created by log skidding activity in Idaho, is shown in Figure 2.

Figure 2

The two main types of processes associated with surface erosion are

  1. those that generate erosion such as rill, sheet wash, dry ravel, or freeze-thaw, and
  2. those processes that route eroded soil to drainageways (sediment delivery).

A general approach to predict sediment yields from surface erosion is the PSIAC method (Pacific Southwest Inter-Agency Committee 1968). It involves rating a watershed on nine factors (surface geology, soil, climate, runoff, topography, land use, upland erosion, channel erosion and sediment transport). There are several soil loss models including the Universal Soil Loss Equation (USLE) (Wischmeier and Smith 1978), the Modified Soil Loss Equation as applied in WRENSS (USEPA1980), the Water Erosion Prediction Procedure (WEPP), as well as the methods of Lane and Nearing (1989), Nearing, et al. (1989), and Flanagan and Nearing (1995). Adaptations for forest soils for the WEPP model were developed by Elliot and Hall (1997), and a recent version of the Revised Universal Soil Loss Equation (RUSLE) (Renard et al. 1997) also provides estimates of surface soil loss.

Due to the inherent complexity of sediment routing on hill slopes, sediment delivery is difficult to predict and verify. Direct observation over short lengths of slope is possible if soil has been transported beyond its location of initial detachment. The quantification of a sediment delivery ratio has been extrapolated per unit area of watershed from downstream reservoir surveys (Wischmeier and Smith 1978), however this generally overestimates sediment due to surface erosion as sediment from streambank erosion is not separately expressed. The USLE rainfall erosivity factor used in place of the storm runoff parameter by Williams (1975), is designed to substitute for a sediment delivery ratio and is favored over the drainage area relation. The sediment delivery relation in WRENSS (USEPA 1980) is an approach that evaluates site conditions related to sediment delivery potential. According to Reid and Dunne (1996), Williams and Berndt (1972) provide the best predictions of measured sediment delivery using:


Sdr = 63 Sm 0.40

Sdr is sediment delivery ratio,
Sm is the average slope of the mainstream channel.

The WEPP model adaptation by Toy and Foster (1998) is applied in mined lands erosion studies. Prediction methods associated with road-related sediment using a USLE-based approach are shown by Reid and Dunne (1984), and the FSWEPP-based approach as shown by Elliot and Foltz (2001). Caution should be exercised in the use of this relation if there is slope discontinuity and/or changes in slope shape, surface debris or lack of rills, as Equation II-1 would potentially over-predict delivered sediment from onsite surface erosion in these cases. This relation may be appropriate for hillslopes in very close proximity to the channel slopes used in the prediction.

These surface erosion models rely on the assumptions of Hortonian overland flow, and many were developed for low gradient, agricultural applications. Use of these models often requires local calibration, otherwise their use is limited to the assumptions of the model. These models were also designed for site-specific prediction rather than watershed-wide levels of application. General risk relations are utilized in the RRISSC phase of WARSSS in screening for potential surface erosion/sediment yield contributions. The PLA phase of assessment for specific high risk areas as identified at the screening level will utilize appropriate models as recommended in WARSSS or selected by the assessor.

Vegetative ground cover density is generally the most sensitive variable in these models. Increased risk of erosion and sediment delivery is associated with high soil erodibility; little ground cover; steep, long, continuous slopes; high intensity storms; high drainage density of the slope; and close proximity to streams (e.g., road fills closer than 61 meters or 200 feet). Mitigation for surface erosion generally provides an increase in ground cover or surface protection, and breaks-up continuous slope length (surface drainage diversion, terracing, alternate row crops, lop and scatter logging debris, surface mulch and seeding, etc.). For roads, mitigation involves road and ditch surfacing, vegetating ditches, out-sloping the road surface, cross-drains, vegetating cut-banks and road fills, and proper culvert and bridge designs.

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