2008-03-03_PERMIT FILE - C2006085A ‎(4)‎

• Permit No: C2006085A
• IBM Index Class Name: Permit File
• Doc Date: 3/3/2008
• Doc Name: Drainage and Sediment Control Plan
• Section_Exhibit Name: Exhibit V
• Media Type: D
• Archive: Yes

3.2 RUSLE Annual R Factor
The average annual R method is one option in the SEDCAD4 model used to determine the sediment
storage needs based on the ratio of the Revised LISLE (RUSLE) annual R factor to the calculated R
storm value (calculated by SEDCAD4) and the annual sediment yield to the storm sediment yield.
For sediment storage needs at the Northfield Project site, an annual R factor of 20 (obtained from the
SEDCAD4 Design Manual and User's Guide) was used.
3.3 Erodibility Factor
Susceptibility to erosion is determined by a soils resistance to detachment by rainfall and flowing
water. The soil erodibility factor used for the Northfield Project site, and provided by SEDCAD4 for
clay, clay loam, and silty clay soils, was 0.32.
3.4 Length-Slope (LS) Factor
The effect of topography on erosion is determined through the LS-factor. The LS-factor is calculated
from the representative length L, and the representative slope, S of arun-off area. SEDCAD4 uses
the Modified LISLE (MUSLE) with the addition of the new S-factor developed for the RUSLE.
MUSLE enables the user to calculate the soil loss from a subwatershed based on LISLE parameters,
volume of run-off, and peak flow. Input parameters to determine the LS factor were obtained from
scaled maps of the subwatershed areas. Values measured from the scaled maps are shown in the
SEDCAD4 printouts (Appendix B).
3.5 C Factor
The C factor represents the influence of cover material on or just beneath the surface of the soil. For
Subwatershed C on the Northfield Project site (see Site Drainage Map) where disturbances are
proposed, a C factor of 1.00 for bare soils under construction conditions (as provided by SEDCAD4
tables) was used.
3.6 P Factor
The P factor accounts for specific support practices such as contouring, terracing, and deposition at
the base segment of a concave slope. It also accounts for sediment control barriers such as grass
buffer strips, straw bales, gravel, or filter barriers. Since the final grading plan for the Northfield
Project site has not yet been prepared and the use and location of potential sediment control barriers
is not known, a P factor of 1.00 was used. This P factor assumes that control practices are not used.
4.0 DIVERSION AND COLLECTION DITCHES
Design results for diversion and collection ditches can be found in the SEDCAD4 printouts located in
the Appendix B. In addition, a summary of the ditch designs is provided in Table 2.
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