PERMFILE122816

• Permit No: M1983035
• IBM Index Class Name: Permit File
• Doc Name: PROJECT OVERVIEW
• Media Type: D
• Archive: No

L1

APPENDIX A
a
Rational Method:
Assumptions:
1. Antecedent conditions -saturated (all PPT runs off)
2. The rainfall occurs uniformly over the drainage area
3. The peak rate of runoff can be reflected by the rainfall
intensity averaged over a time period equal to the time of
concentration of the drainage area.
4. The time of concentration is the time required for flow to
reach the point in question from the hydraulically most remote
point in the drainage area.
5. The frequency of runoff is the same as the frequency of the
rainfall used in the equation.
6. The bridge abutment of concern (off site) is 50 feet across.
a q = CiA where C is coefficient of runoff for rural
area -rolling pasture with open
sandy loam soil (Columbine gravelly
sandy loam) 0.16
i is intensity of rainfall in inches
per ho,;r for 100 yr, 24 hour event
or G.liS inches per hour
A is the drainage area in acres
a Gravel Pit Area Calculations:
2800' long x 1300' wide = 83.56 Ac
Area presentiy contributing to runoff:
900' wide x 1450' long (parallelogram) = 29.96 Ac
q from gravel pit: q = (0.16)(.175)(8356)
n q 2.34 CFS
u q from existing contributing area: q = (0.16)(0.175)(29.96)
q = 0.84 CFS
Increase due to gravel pit construction:
a 2.34 - 0.84 = 1.50 CFS
Note: This increase of 1.50 CFS e~uld raise the cater level on the
bridge abutment near the site 0.36 inches during a 100 year.
24 hour precipitation event of 4.2 inches.
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