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

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storms used for the Northfield Project drainage. Therefore, the 49 -point distribution was used for all run-
off calculations.
2.5 Subwatershed Area
Each subwatershed, or drainage basin, used in the SEDCAD model has a specific surface area that is
available to collect rainfall and produce stormwater run -off. For the Northfield Project, subwatersheds
were delineated based on available area topography of undisturbed slopes. Subwatershed boundaries are
shown on the attached Site Drainage Map. Prior to construction of drainage and sediment control
structures, subwatershed delineations should be checked following site grading activities.
2.6 Time of Concentration
Time of concentration is used to estimate the timing of peak flows at the Subwatershed outlet. The time
of concentration value is estimated from the longest flow path length (and associated slope) from the top
of the subwatershed to the outlet. Normally, the time of concentration consists of an overland flow
component (calculated by the NRCS upland curve method) and a channel flow component (calculated
using the SEDCAD4 routing calculator). Flow paths for each subwatershed are shown on the Site
Drainage Map and time of concentration values for each subwatershed are provided on the SEDCAD4
printouts in the Appendix B.
2.7 Muskingum Routing
Muskingum routing is used to route flow from an up- gradient to a down - gradient location including
is routing between structures or routing from a subwatershed outlet to its structure, when the outlet of a
subwatershed is not located at the structure. Muskingum parameters are represented by K and X. The K
parameter accounts for the time it takes to proceed from an up- gradient to a down - gradient location. The
X parameter is functionally related to the average stream reach velocity and accounts for the attenuation
of the hydrograph. At the Northfield Project, since designed structures were generally located at the
subwatershed outlets, Muskingum routing was not used.
2.8 Curve Number
Curve number is a function of land use and hydrologic soil group. The curve number will relate the
amount of rainfall to the amount of run -off for a given area. For the Northfield Project, curve numbers
were determined from tables provided in the SEDCAD4 model for Arid and Semiarid Rangelands.
Hydrologic soil group information was obtained from NRCS soil surveys. In undisturbed subwatersheds,
the hydrologic soil condition was considered to be fair and in subwatersheds with proposed grading
disturbances the hydrologic soil condition was considered poor. A summary of curve numbers assigned
to the Northfield Project subwatersheds is provided in Table 1.
2.9 Hydrograph Response Shape
The unit hydrograph methodology is used in SEDCAD4. A double triangle dimensionless unit
hydrograph shape is used to emulate hydrograph response. The hydrograph responses available in the
SEDCAD4 model are slow, medium, and fast. A slow response corresponds to a forested area or an area
with a number of obstructions. A fast response is anticipated for disturbed soils that have been subjected
to compaction by heavy equipment. For undisturbed subwatersheds on the Northfield Project
(subwatersheds A and B as shown on the Site Drainage Map), a medium hydrograph response was used.
A fast hydrograph response was used for Subwatershed C (see Site Drainage Map) where proposed site
• disturbances will occur.