2010-05-07_PERMIT FILE - C2006085 ‎(7)‎

• Permit No: C2006085
• IBM Index Class Name: Permit File
• Doc Date: 5/7/2010
• Doc Name: Drainage and Sediment Control Plan
• Section_Exhibit Name: Exhibit V
• Media Type: D
• Archive: No

TABLE 1
Northfield Project Curve Numbers
Subwatershed
(refer to Site
Cover Type
Hydrologic
Hydrologic
Curve
. Drainage Ma
Condition
Soil Group
Number
Undisturbed
A
Pinyon, juniper, or both
Fair
D
80
with grassy ndersto
Undisturbed
B
Pinyon, juniper, or both
Fair
D
80
with grassy ndersto
Disturbed
C
Pinyon, juniper, or both
Poor
C
85
with Eassy understo
3.0 SEDIMENTATION CALCULATIONS
The sediment calculations used to model the Northfield Project site were performed utilizing the
SEDCAD4 computer model. The SEDCAD4 model calculates sediment yield using derivatives of the
Universal Soil Loss Equation (USLE). Subwatershed C (see Site Drainage Map) is the only
subwatershed at the Northfield Project site where disturbances are proposed and, therefore, sedimentation
calculations were only performed for this subwatershed. The input parameters used in the sedimentation
analyses are presented in the following sections.
• 3.1 Particle Size Distribution
The particle size distribution of subwatershed soils is entered into the SEDCAD4 computer model along
with particle specific gravity and submerged bulk specific gravity. For the Northfield Project site, soil
types and classifications were determined from NRCS soil maps and tables. The soils are generally silty
sand and lean clay with silt. Although sieve and hydrometer analysis data for site soils were not
available, a typical particle size distribution for a sandy lean clay soil was used for modeling the facilities
subwatershed where disturbances are proposed. Prior to construction of drainage and sediment control
structures, site-.specific soil samples shou be analyzed to verify particle size distribution and erodible
particle sizes. For specific gravity and submerged bulk specific gravity, SEDCAD4 default values of 2.5
and 1.25 were used, respectively.
12 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 USLE ( 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 detaclunent 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.
•