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

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Drainage Design Manual for Maricopa County Hydrology: Application 9.1 RAINFALL 9.1.1 Procedure for the Development of the Design Rainfall 9.1.1.1 Procedure for the Rational Method 1. Determine the size of the drainage area. 2. Locate the drainage area and determine the point rainfall depth for every duration, and all frequencies of interest from Figure A.1 through Figure A.60 of Appendix A.1. Summarize in a Depth-Duration-Frequency (D-D-F) table. 3. Create an Intensity-Duration-Frequency (I-D-F) table by dividing the individual rainfall depth values from Step 2 by the duration associated with the rainfall depth. The units should be in terms of inches per hour. 4. Plot the results for each frequency on log-log paper and examine the results to be sure they plot as smooth curves. Any anomalies should be checked against Appendix A.1 to be sure the correct depth value was read. Note: Steps 2 through 4 are performed automatically in DDMSW. 9.1.1.2 Procedure for the Unit Hydrograph Method. 1. Determine the size of the drainage area. 2. Determine the point rainfall depth or the areally averaged point rainfall depth, from Figure A.1 through Figure A.60 of Appendix A.1, depending on the desired storm duration and frequency. 3. For a single storm analysis, determine the depth-area reduction factor using or Table 2.1 for a 6-hour local storm and Table 2.2 or Figure 2.2 or a 24-hour general storm. For a multiple storm analysis, determine the drainage areas at key points of interest in the watershed. For each drainage area, determine the depth-area reduction factor using or Table 2.1 for a 6-hour local storm and Table 2.2 or Figure 2.2 for a 24-hour general storm. 4. Multiply the point rainfall depth by the appropriate depth-area reduction factor(s). 5. For a 6-hour local storm, use Figure 2.5 to select the appropriate pattern number(s) (rounded to the nearest 0.1 pattern number). 9-2 August 15, 2013
Drainage Design Manual for Maricopa County Hydrology: Application 6. For a 6-hour local storm, use the dimensionless rainfall distributions of Table 2.4, or Figure 2.4 and Figure 9.7 or to calculate the dimensionless distribution(s) by linear interpolation between the two bounding pattern numbers. For a 24-hour general storm, use the dimensionless rainfall distribution of Table 2.5 or Figure 2.6. Note: Steps 2 through 6 are performed automatically in DDMSW. 9.1.2 User Notes 1. For a multiple storm analysis, areal reduction is accomplished in the HEC-1 program using the JD record option. The use of this record in conjunction with diversion simulations may cause an error at hydrograph combine operations downstream of the diversion. The error is that the model “looses track” of all the upstream tributary area after a diversion. Consequently the peak discharge at hydrograph combines downstream of the diversion are overestimated due to the “loss” of area. This error can be corrected by hard coding the total drainage area on the HC record of the hydrograph combine operation downstream of the diversion. 2. Use of the JD record option prohibits the use of the JR (job ratio) record option. 3. The DDMSW program automatically computes areal reduction factors and the corresponding precipitation mass curves for the 6-hour storm for a multiple storm analysis at predefined intervals. These intervals should be inspected for reasonableness in regard to the study watershed. The JD/PC record sets for storm areas greater than the next largest storm area over the total watershed area can be removed. 4. Precipitation records (PI and PC records) are coded into the HEC-1 program at the time interval specified on the IN record. The DDMSW program automatically populates these records at a time interval of 15 minutes. All other time dependent input data, such as input hydrographs (QI records) will be read into the program at the previously specified time interval unless a new time interval is specified. August 15, 2013 9-3
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