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<br />. <br /> <br />. <br /> <br />. <br /> <br />DRAINAGE CRITERIA MANUAL <br /> <br />RUNOFF <br /> <br />3.9 Use of Rational Method For Detention Oesign <br />Section 5.1 of the STORAGE Chapter in Volume 2 of this Manual provides a <br />procedure for using the Rational Method to size on-site detention for <br />watersheds that are to small for analysis using the Colorado Urban Hydrograph <br />Procedure. Because of the inexact nature of this procedure, errors can easily <br />occur in determining the appropriate release rate(s) to maintain "historical" <br />or "undeveloped" flow rates. The Urban Drainage and Flood Control District <br />conducted extensive research into this problem. On the basis of its findings, <br />UDFCO suggests for the area within the Oistrict that designers utilize the <br />allowable maximum unit area release rates given in Table 3-2 whenever the <br />Rational Method is used to size detention ponds. The allowable unit area <br />release rate(s) to be used will depend on the local detention policy (i .e. <br />what recurrence frequency(ies) needs to be controlled). <br />To use Table 3-2, first determine which recurrence frequency(ies) need to <br />be controlled through the use of detention. This can be obtained from the <br />city, county or town in which the land use change is occuring. Knowing the <br />required control frequency(ies) and the SCS Soil Group, read the maximum <br />allowable release rate(s) from Table 3-2. Use the resultant value(s), after <br />multiplying them by the area in acres, in conjunction with the pond sizing <br />procedure described in Secti on 5.1 of the STORAGE Chapter of th i s Manua 1. <br />Although there are other methods of sizing detention ponds using the Rational <br />Method (e.g. triangular hydrograph, trapezoidal hydrograph, etc.), the one <br />described in Section 5.1 of STORAGE with the release rates given in Table 3-2 <br />below appears to provide results that are most consistent with the findings of <br />the Urban Drainage and Flood Control District studies. If the procedure <br />results in an increasing storage volume at the end of 2-hours, use the 2-hour <br />storage volume. <br /> <br />TABLE 3-2 <br />ALLOWABLE RELEASE RATES FOR DETENTION PONDS - cFS/ACRE <br />SOIL GROUP <br />B <br />0.04 <br />0.13 <br />0.23 <br />0.41 <br />0.85 <br /> <br />CONTROL FREQUENCY <br /> <br />2-year <br />5-year <br />10-year <br />25-year <br />100-year <br /> <br />A <br />0.02 <br />0.07 <br />0.13 <br />0.24 <br />0.50 <br /> <br />C & D <br />0.06 <br />0.17 <br />0.30 <br />0.52 <br />1.00 <br /> <br />5-1-84 <br />