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

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Drainage Design Manual for Maricopa County Hydrology: Rational Method 3. Compile the site-specific depth-duration-frequency (D–D–F) and intensity-durationfrequency (I–D–F) statistics for the project site using NOAA Atlas 14 (see Section 2.2 and Section 9.1). 4. Calculate the time of concentration. This is to be done as an iterative process. a. Determine the K b parameter from Table 3.1 or Figure 3.1. If the drainage subbasin contains subareas of different K b values compute a K b for each surface roughness class using the total area of the subbasin when applying Table 3.1 or Figure 3.1. Then arithmetically area-weight the values of K b . b. Make an initial estimate of the duration and compute the intensity from the D– D–F data, or derive from the I–D–F curve for the desired frequency. c. Compute an estimated T c using Equation (3.2). If the computed T c is reasonably close to the estimated duration, then proceed to Step 5, otherwise repeat this step with a new estimate of the duration. The minimum T c should not be less than 5-minutes. 5. Determine the peak discharge Q by using the value of i in Equation (3.1). 6. As an alternative to the above procedure, the DDMSW program may be used to calculate peak discharge. 3.6.2 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 3.3. A peak discharge is needed for all three individual subbasins, subbasins A and B combined at Concentration Point 1 and subbasins A, B and C combined at Concentration Point 2. This can be accomplished using two different approaches: the combined watershed approach and the triangular hydrograph approach. 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 should receive prior approval from the jurisdiction before applying the combined watershed method. Steps for applying both approaches follow. 3-8 August 15, 2013
Drainage Design Manual for Maricopa County Hydrology: Rational Method Combined Watershed Approach 1. Compute the peak discharge for each individual subbasin using steps 1 through 5 from Section 3.6.1. 2. Compute the arithmetically area-weighted value of C for subareas A and B. 3. Follow step 4 from Section 3.6.1 to calculate the T c for the combined area of subbasins A and B at Concentration Point 1. 4. Compare the T c values from subbasins 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 combined subbasin T c from step 3. If the combined peak discharge is less than the discharges for the individual subbasins, 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. If there are more than two watersheds being combined, and the combined peak discharge is less than any of the individual subbasin peak discharges, another check needs to be made. A long narrow watershed having a long T c may not be representative of the majority of the combined watershed and could be the reason the combined subbasin peak discharge is too low. A combination of the other subbasins may be more appropriate, using a computed T c for the new combination. 5. Compute the arithmetically area-weighted value of C for combined subbasins A, B and C. 6. Calculate the T c for the combined area of subbasins A, B and C at Concentration Point 2 using the following two methods: Method 1 - Follow step 4 from Section 3.6.1 to calculate the T c for the single basin composed of all three subbasins. 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. August 15, 2013 3-9
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