Drainage Design Manual, Hydrology - Flood Control District of ...

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Drainage Design Manual for Maricopa County Hydrology: Application 9.2.2 Procedures for Volume Calculations Volume calculations should be done by applying the following equation: V P = C A 12 where: (3.3) V = calculated volume in, acre-feet. C = runoff coefficient from Table 3.2. P = rainfall depth, in inches. A = drainage area, in acres. In the case of volume calculations for stormwater storage facility design, P equals the 100-year, 2-hour depth, in inches, as discussed in Section 2.2, and is determined from Figure A.56 of Appendix A.1. 9.2.3 Procedures for the Multiple Basin Approach The Rational Method can be used to compute peak discharges at intermediate locations within a drainage area less than 160 acres in size. A typical application of this approach is a local storm drain system where multiple subbasins are necessary to compute a peak discharge at each proposed inlet location. Consider the schematic example watershed shown in Figure 9.2. A peak discharge is needed for all three individual subareas, subareas A and B combined at Concentration Point 1 and subareas A, B and C combined at Concentration Point 2. There are two accepted methods for computing peak discharges for multiple basins using the Rational Method. The first method is the traditional approach that relies upon combining the subbasin areas into a single watershed, computing a new T c , an arithmetically area-weighted value of C for combined sub-basins, and then computing the peak discharge. This approach is referred to as the “Combined Watershed Method.” The second method is the “Triangular Hydrograph Method.” For this method, a triangular hydrograph is created for each sub-basin where the time-to-peak is assumed equal to T c and the hydrograph time base is equal to 2.67T c , as shown on Figure 3.2. Referring to Figure 9.2, the ordinates of hydrographs A and B at CP 1 are added to obtain the total flow hydrograph. That hydrograph is then lagged downstream to CP 2 by the estimated travel time in the roadway, pipe, or channel. The lagged hydrograph is then added to the sub-basin C hydrograph to obtain the peak discharge at CP 2. The triangular hydrograph method is incorporated in the DDMSW computer program, but the combined hydrograph method is not. The combined hydrograph method is intended for use by engineers/hydrologists without access to a computer and DDMSW. Either method may be used but the engineer/hydrologist 9-10 August 15, 2013
Drainage Design Manual for Maricopa County Hydrology: Application should receive prior approval from the jurisdiction before applying the combined watershed method. The procedures for the Combined Hydrograph Method are as follows: 1. Compute the peak discharge for each individual subarea using steps 1 through 5 from Section 9.2.1. 2. Compute the arithmetically area-weighted value of C for subareas A and B. 3. Follow step 4 from Section 9.2.1 to calculate the T c for the combined area of subareas A and B at Concentration Point 1. 4. Compare the T c values from subareas A and B to the T c value for the combined area at Concentration Point 1. Compute the peak discharge at Concentration Point 1 using the i for the longest T c from step 3. If the combined peak discharge is less than the discharges for the individual subareas, use the largest discharge as the peak discharge at Concentration Point 1. The design discharge SHOULD NOT DECREASE going downstream in a conveyance system unless storage facilities are used to attenuate peak flows. 5. Compute the arithmetically area-weighted value of C for subareas A, B and C. 6. Calculate the T c for the combined area at Concentration Point 2 using the following two methods: Method 1 - Follow step 4 from Section 9.2.1 to calculate the T c for the single basin composed of all three subareas. Method 2 - Compute the travel time from Concentration Point 1 to Concentration Point 2 using the Manning equation or other appropriate technique and hydraulic parameters for the conveyance path. Add the computed travel time for the conveyance path to the T c from Concentration Point 1. 7. Compare the T c values from Methods 1 and 2 as well as the T c from subarea C and calculate the peak discharge at Concentration Point 2 as follows: a. If the T c value from Method 1 is the longest, compute the total peak discharge using the Method 1 intensity, the arithmetically area-weighted value of C for all three subareas and the total contributing drainage area at Concentration Point 2. b. If the T c value from Method 2 is the longest, determine i directly from the I-D-F statistics from step 3 of Section 9.2.1. Compute the total peak discharge at Con- August 15, 2013 9-11
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