Drainage Design Manual - the City of San Luis Obispo

Table 4-2. Required Antecedent Moisture FactorsRecurrence Interval (years)Antecedent Moisture Factor(C a )2 to 10 1.025 1.150 1.2100 1.254.1.3 General Procedure for the Rational MethodThe general procedure for determining peak discharge at a given concentration point using theRational Method is as follows:Step 1 - Determine the drainage area (in hectares).Step 2 - Determine the runoff coefficient (c) for the drainage area, using Table 4-1. If thelanduse and soil cover are homogeneous for the entire drainage area, a single runoffcoefficient value can be determine directly from the table. If there are multiplelanduses or soil conditions, a weighted average must be calculated as follows:Step 3 - Determine the hydraulic length or flow path that will be used to determine the time ofconcentration. Also, determine the types of flow (or flow regimes) that occur alongthe flow path.Step 4 - Determine the time of concentration (T c ) for the drainage area for each subarea andflow path. Add all of the travel times to get the time of concentration (T c ) for theentire hydraulic length or flow path.Step 5 - Determine the Rainfall Intensity Factor (i) for the selected design storm by using theRainfall Intensity charts (Tables 4-3 to 4-6). Select the chart with average annualrainfall for the locality closest to project, as shown on Figure 4-A and 4-B.Step 6- - Determine Antecedent Moisture Factor from Table 4-2 based on the design stormbeing considered (i.e. 2- yr, 10-yr, etc)Step 7 - Determine the peak discharge according to Equation 4-1.This procedure can be repeated for each concentration point as one moves down a given drainagesystem. At each point, a new T c will need to be completed by adding the channel time betweenthe previous concentration point and the new point to the previously estimated T c (per Equation4-6). A new run-off coefficient will be computed pursuant to Equation 4-2, and new rainfallintensity; from Tables 4-3 to 4-6.SLO Creek Drainage Design Manual 28 February 2003