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".

Area Subject to Erosion (ERODAR)

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.

Management Factors

Soil Factor 'K' (SOILF)

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.

Control Practice Factor 'P' (CONTPF)

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 CoverFactor P
Compact, smooth, scraped with bulldozer or scraper up and down hill1.30
Same as above, except raked with bulldozer root, raked up and down hill1.20
Compact, smooth, scraped with bulldozer or scraper across the slope1.20
Same as above, except raked with bulldozer root, raked across the slope0.90
Loose, as in a disk plowed layer 1.00
Rough irregular surface, equipment tracks in all directions0.90
Loose with rough surface > 12 in. depth0.80
Loose with smooth surface > 12 in. depth0.90
Structures Factor P

Small Sediment Basins:
0.04 basin/acre
0.06 basin/acre 


0.50
0.30

Downstream sediment basins:
with chemical flocculants
without chemical flocculants

0.10
0.20

Erosion control structures:
normal rate usage
high rate usage


0.50
0.40
Strip building0.75

Crop Management Factor 'C' (CROPMF)

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 coverC Value
None (fallow) 1.00

Temporary seedings:
First 60 days
After 60 days


0.40
0.05

Permanent seedings:
First 60 days
After 60 days


0.40
0.05
Sod (laid immediately) 0.01
Mulch

Rate of Application

(tons/acre)

Max. Allowable

Slope Length

C Value

(Feet)
Hay or straw0.50.3520

1.00.2030

1.50.1040

2.00.05

50

Stone or gravel15.00.8015

60.00.2080

135.00.10175

240.00.05200

Chemical mulches

First 90 days

After 90 days


*

*


0.50

1.00


50

50

Woodchips2.00.8025

4.00.3050

12.00.10100

20.00.06150

25.00.05200


Note: * = as recommended by the manufacturer