This section describes the Global Subcatchment Erosion data. The Universal Soil Loss Equation (USLE) has been adapted for use in SWMM for erosion computations. The adapted USLE estimates average soil loss for a given storm or time period.
Erosion is computed using factors 'K', 'C' and 'P' (soil factor, cropping management factor and control practice factor respectively). A value of the "sediment delivery ratio" may also be included in the erosion computations. Since it is merely another multiplier, if desired it may be incorporated into the 'K', 'C' or 'P' factors.
Erosion is added as an additional water quality constituent, so that one fewer Pollutant can be simulated. Futhermore, if Erosion is simulated, at least one other (arbitrary) pollutant must be simulated. No particular soil characteristics (eg. particle size distribution) are assigned to the erosion parameter, and its name is "EROSION", with units of "mg/l".
Fraction of subcatchment area subject to erosion. Fraction in the range 0.0 - 1.0. This fraction is normally up to the pervious fraction of the subcatchment and could indicate land that is barren or under construction.
Soil factor, K, is a measure of the potential erodibility of a soil, in units of tons per unit of rainfall factor 'R'. This factor can be estimated using an erodibility nomograph (Wischmeir et al., 1971). However, a preferable and often simpler alternative is to use the soil survey interpretation sheet from the Local Agricultural Research Service and Soil Conservation Service.
This factor is similar to the crop management factor except it accounts for the erosion-control effectiveness of super-imposed practices such as contouring, terracing, compacting, sediment basins and control structures. Estimates for this factor for construction sites are given in the following table (Ports, 1973).
|Surface Condition with no Cover||Factor P|
|Compact, smooth, scraped with bulldozer or scraper up and down hill||1.30|
|Same as above, except raked with bulldozer root, raked up and down hill||1.20|
|Compact, smooth, scraped with bulldozer or scraper across the slope||1.20|
|Same as above, except raked with bulldozer root, raked across the slope||0.90|
|Loose, as in a disk plowed layer||1.00|
|Rough irregular surface, equipment tracks in all directions||0.90|
|Loose with rough surface > 12 in. depth||0.80|
|Loose with smooth surface > 12 in. depth||0.90|
Small Sediment Basins:
|Downstream sediment basins:|
with chemical flocculants
without chemical flocculants
Erosion control structures:
This factor is dependent upon the type of ground cover, the general management practice and the condition of the soil over the area of concern. This factor is set to 1.0 for continuous fallow ground which is defined as land that has been tilled and kept free of vegetation and surface crusting. Values for the cropping management factor are given in the following tables (Maryland Dept. of Natural Resources, 1973).
|Type of cover||C Value|
|Sod (laid immediately)||0.01|
Rate of Application
|Hay or straw||0.5||0.35||20|
|Stone or gravel||15.0||0.80||15|
First 90 days
After 90 days
Note: * = as recommended by the manufacturer