16.10.360 Determination of storage volume – Rational Method.
A. For areas of 200 acres or less, the Rational Method may be used to determine the required volume of storm water storage. The following 11-step procedure may be used to determine the required volume of storage. Other design methods may also be used, subject to approval of the Board, and as described in JCC 16.10.370.
B. Steps Procedure.
1. Determine total drainage area in acres “A.”
2. Determine composite runoff coefficient “Cu” based on existing land use (undeveloped).
3. Determine time of concentration “tc” in minutes based on existing conditions.
4. Determine rainfall intensity “Iu” in inches per hour, based on time of concentration using Figure 1 or from data given in Table 5A for the 10-year return period.
5. Compute runoff based on existing land use (undeveloped), and 10-year return period: Qu= CuIuA.
6. Determine composite runoff coefficient “Cd” based on developed conditions and a 100-year return period.
7. Determine the 100-year return period rainfall intensity “Id” for various storm durations “td” up through the time of concentration for the developed area using Table 5A.
8. Determine developed inflow rates “Qd” for various storm durations “td,” measured in hours.
Qd = CdId A.
9. Compute a storage rate “Std” for various storm durations “td” up through the time of concentration of the developed area. Std = 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 the peak flow of Std at Td hours. SR = Std (td/12).
11. Select the largest storage volume computed in step 10 for detention basin design.
|
Intensity (Inches/Hour) |
||||||
|---|---|---|---|---|---|---|
|
Duration |
Return Period (Years) |
|||||
|
2 |
5 |
10 |
25 |
50 |
100 |
|
|
5 min. |
5.04 |
6.24 |
7.08 |
8.16 |
9.00 |
9.84 |
|
10 min. |
3.84 |
7.74 |
5.46 |
6.24 |
6.90 |
7.50 |
|
15 min. |
3.20 |
3.96 |
4.52 |
5.16 |
5.72 |
6.20 |
|
20 min. |
2.85 |
3.51 |
4.02 |
4.59 |
5.10 |
5.55 |
|
30 min. |
2.22 |
2.74 |
3.12 |
3.58 |
3.96 |
4.32 |
|
40 min. |
1.85 |
2.28 |
2.61 |
2.99 |
3.30 |
3.60 |
|
50 min. |
1.60 |
1.97 |
2.24 |
2.57 |
2.83 |
3.10 |
|
60 min. |
1.40 |
1.73 |
1.97 |
2.25 |
2.49 |
2.72 |
|
1.5 hrs. |
1.13 |
1.39 |
1.59 |
1.82 |
2.02 |
2.20 |
|
2 hrs. |
0.86 |
1.06 |
1.21 |
1.38 |
1.53 |
1.67 |
|
3 hrs. |
0.61 |
0.76 |
0.87 |
0.99 |
1.10 |
1.20 |
|
4 hrs. |
0.52 |
0.64 |
0.73 |
0.83 |
0.92 |
1.00 |
|
5 hrs. |
0.43 |
0.53 |
0.61 |
0.70 |
0.77 |
0.84 |
|
6 hrs. |
0.37 |
0.46 |
0.52 |
0.60 |
0.66 |
0.72 |
|
7 hrs. |
0.33 |
0.41 |
0.47 |
0.53 |
0.59 |
0.64 |
|
8 hrs. |
0.29 |
0.36 |
0.42 |
0.47 |
0.53 |
0.57 |
|
9 hrs. |
0.27 |
0.33 |
0.38 |
0.43 |
0.48 |
0.52 |
|
10 hrs. |
0.25 |
0.31 |
0.35 |
0.40 |
0.44 |
0.48 |
|
12 hrs. |
0.22 |
0.27 |
0.30 |
0.35 |
0.38 |
0.42 |
|
14 hrs. |
0.19 |
0.24 |
0.27 |
0.31 |
0.34 |
0.37 |
|
16 hrs. |
0.17 |
0.21 |
0.24 |
0.28 |
0.31 |
0.34 |
|
18 hrs. |
0.16 |
0.19 |
0.22 |
0.25 |
0.28 |
0.31 |
|
20 hrs. |
0.14 |
0.18 |
0.20 |
0.23 |
0.26 |
0.28 |
|
24 hrs. |
0.13 |
0.15 |
0.18 |
0.20 |
0.22 |
0.24 |
[Ord. 12-1-97C § 15(E).]