(A) Development sites less than or equal to 5 acres in size, with a contributing drainage area less than of equal to 50 acres and no depressional storage.
(1) The required volume of stormwater storage may be calculated using the rational method and based on the runoff from a 100-year return period storm. A computer model that can generate hydrographs based on the NRCS TR-55 time of concentration and curve number calculation methodologies may also be used with the duration storm that produces the highest runoff volume.
(2) The following 11-step procedure, based on the rational method, may be used to determine the required volume of storage. Alternatively, the worksheet labeled as Figure 2 may be completed to determine the required volume of storage.
Step
|
Procedure |
Step
|
Procedure |
1. | Determine total drainage area in acres "A" |
2. | Determine composite runoff coefficient "G" based on existing land use |
3. | Determine time of concentration "t" in minutes based on existing conditions |
4. | Determine rainfall intensity “I” in inches per hour, based on time of concentration and using data given in Table 3 for the 2-year/10-year return periods |
5. | Compute runoff based on existing land use and 2-year/10-year return periods; Qu=CuIuAu |
6. | Determine composite runoff coefficient "Cd" based on developed conditions and a 100-year return period |
7. | Determine 100-year return rainfall intensity "Id" for various storm durations "td" up through the time of concentration for the developed area using Table 3. If the desired time of concentration is not listed in Table 3, it can be calculated using the parameters found in Table 4 with the following equation: Id=c(Tr) a/(t+d)b; where: c, a, d and B are regional coefficients determined by rainfall intensity-duration-frequency curves, Tr is the recurrence interval (in years), and t corresponds to the time of concentration (in hours) |
8. | Determine developed inflow rates "Qd" for various storm durations times "td" up through the time of concentration of the developed area; Qd=Cd1dAd |
9. | Compute a storage rate "S (td)" for various storm durations "td" up through the time of concentration of the developed area; S (td)=Qd-Qu |
10. | Compute required storage volume "Sr" in acre-feet for each storm duration "td;" this assumes a triangular hydrograph of duration (2*td) hours with a peak flow of S (td) at td hours |
11. | Select largest storage volume computed in Step 10 for any storm duration "td" for detention basin design |
(B) Development sites greater than 5 acres in size or contributing drainage area greater than 50 acres or with significant depressional storage. All runoff detention storage calculations for these development sites shall be prepared using a computer model that can generate hydrographs based on the NRCS TR-55 time of concentration and curve number calculation methodologies. The huff third quartile (50%) rainfall distribution and that storm duration up to and including the 24-hour storm duration that gives the highest required storage volume shall be utilized. The allowable release rates shall be based on the storm durations which give the greatest discharge in the pre-developed condition. Examples of computer models that can generate hydrographs include TR-20 (NRCS) and HEC-1 (COE). Other models may be acceptable and should be approved by the city engineer prior to their utilization.
(Ord. 01-2000, passed 7-18-2000)