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Kalamazoo County Drain Commissioner

STORMWATER MANAGEMENT RULES

APPENDIX A
RAINFALL FREQUENCY GRAPHS & MODIFIED CHICAGO METHOD

Rainfall Frequency for Michigan

(Figure 1)     2 Year - 24 Hour Rainfall

(Figure 2)     25 Year - 24 Hour Rainfall

Detention Basin Sizing - Modified Chicago Method

SITE LOCATION _____________ DATE _____________
RUNOFF "C" VALUE _____________ DRAINAGE AREA (ac) _____________
MAX. ALLOWABLE OUTFLOW (cfs)
(0.15 cfs/ac = 5yr, 1-hour, "C"=0.1)		 _____________
STORM RECURRENCE INTERVAL _____________


DURATION RAINFALL(in) 100% RUNOFF (cf) RUNOFF"C" RUNOFF(cf) OUTFLOW(cf) STORAGE REQUIRED(cf)
15 min      
30 min      
1 hr      
2 hr      
3 hr      
6 hr      
12 hr      
18 hr      
24 hr      

Rainfall (in) - From Rainfall Frequency Atlas of the Midwest, Huff and Angel (1992)

100% Runoff (cf) - Inches of rainfall divided by 12, multiplied by Drainage Area and then 43,560

Runoff "C" - Proposed "C" value of fully developed site

Runoff (cf) - 100% Runoff multiplied by proposed "C" value

Outflow (cfs) - Multiply Max. Allowable Outflow by Duration in hours and by 3600

Storage Required (cf) - Subtract Outflow from Runoff until peak is defined. Peak value is storage requirement.

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