Drainage Design Manual - the City of San Luis Obispo
Drainage Design Manual - the City of San Luis Obispo
Drainage Design Manual - the City of San Luis Obispo
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Equation 1The Rational Method equation is Q = C * i *C a * A; whereK cRun<strong>of</strong>f CoefficientsQ = Peak rate <strong>of</strong> run<strong>of</strong>f in cubic meters per secondC = Run<strong>of</strong>f coefficient, an empirical coefficient representing a relationshipbetween rainfall and run<strong>of</strong>f (Table 4-1)i = Average intensity <strong>of</strong> rainfall for <strong>the</strong> time <strong>of</strong> concentration (Tc) for aselected design storm (Tables 4-3 through 4-6) in mm/hrC a = Antecedent moisture factor (see Table 4- 2)A = <strong>Drainage</strong> areas in hectares.K c = Unit conversion factor equal to 360 (1 in English Units)The engineer must use judgment in selecting <strong>the</strong> appropriate run<strong>of</strong>f coefficient within <strong>the</strong> range<strong>of</strong> values for <strong>the</strong> landuse. Generally, areas with impermeable soils, moderate to steep slopes, andsparse vegetation should be assigned <strong>the</strong> highest values. Table 4-1 provides run<strong>of</strong>f coefficientsto be used in <strong>the</strong> SLO watershed based on land use, soils, and slope. When using <strong>the</strong> RationalMethod, a composite weighted C value may be calculated if land use within <strong>the</strong> watershed isvaried. Roadway areas must be calculated and used within <strong>the</strong> composite calculation.Equation 2 C 1 (Area C 1 ) + C 2 (Area C 2 )+ .... = Composite C valueTotal Area Total AreaSLO Creek <strong>Drainage</strong> <strong>Design</strong> <strong>Manual</strong> 23 February 2003
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