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

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Drainage Design Manual for Maricopa County Hydrology: Application 6. If hand calculating the T c , perform the following: a. Compute incremental rainfall excess for each time step using HEC-1. Rank the rainfall excess values by ordering from largest to smallest. b. The average rainfall excess intensity, i, is estimated by summing the first ten (10) largest rainfall excess values and dividing the result by 10 times the time interval, NMIN, in hours. 7. If a time-area relation is not specified in the HEC-1 model, then the HEC-1 default time-area relation is used which, in general, is not recommended for use in Maricopa County. 9.4.3.2 S-Graph 1. The recommended S-Graphs for Maricopa County (i.e. Phoenix Mountain, Phoenix Valley, Desert/Rangeland, and Agricultural) should only be applied to large natural watersheds. The Phoenix Valley S-Graph is also applicable to large urban watersheds. This is, in part, due to the fact that the original database in Arizona applied the methodology to large watersheds. As a lower limit of application a watershed area of 5 square miles can be considered. 2. K n should be selected from the best available information. General guidance and some regional data is available from the U.S. Army Corps of Engineers (Appendix D.1). A broader range of data for watersheds in Maricopa County is provided in the USBR Flood Hydrology Manual (Cudworth, 1989). The S-Graph study (Sabol, 1987) contains lag and watershed characteristics data that are not generally contained in other publications. These sources should be consulted when selecting K n . 3. The manual discusses two slightly different forms of the lag equation (Equation (5.11)), one by the U.S. Army Corps of Engineers and one by the USBR. The form of the equation that corresponds to the source used in the selection of K n should be used. 4. The length to the basin centroid (L ca ) is measured along L to a point on L that is opposite (perpendicular to the flow path) the basin centroid. L ca is not measured to the centroid unless the centroid happens to lie on the flow path line (L). 5. The transformation of an S-Graph to a unit graph is a function of the selected computational time interval (NMIN). If a new NMIN is desired a new unit graph must be recalculated. 9-46 August 15, 2013
Drainage Design Manual for Maricopa County Hydrology: Application 6. The slope as applied in the calculation of basin lag is not adjusted, regardless of the value. 9.4.4 Unit Hydrograph Example Compute the 6-hour unit hydrograph parameters for each subbasin shown in Figure 9.9 and Table 9.7 using rainfall data and rainfall loss data from the examples in Chapters 2 and 4, respectively and the following data: Subbasin ID Drainage . -- Flow Path . Area, sq. miles Length, Unadjusted miles L ca , miles Slope, ft/mi Unit Hydrograph Method S1 5.438 4.59 2.30 254.8 S-Graph S2 4.401 4.11 --- 227.8 Clark S3 3.302 3.91 --- 222.3 Clark S4 2.366 3.40 --- 197.0 Clark S5 1.079 2.29 --- 157.5 Clark S6 1.055 2.06 --- 144.2 Clark S7 1.181 1.74 --- 215.2 Clark S8 0.958 2.36 --- 201.8 Clark S9 0.827 1.66 --- 537.3 Clark S10 0.444 1.83 --- 438.3 Clark S11 1.820 2.98 --- 126.3 Clark Solution: 1. Select the appropriate unit hydrograph method for each subbasin For subbasin S1, the Phoenix Mountain S-graph is selected because the watershed is natural and has mountainous characteristics. For all other subbasins, the Clark unit hydrograph is selected because they are developed. 2. Develop the unit hydrograph for subbasin S1 a. Compute the basin factor LL ---------- ca S 0.5 August 15, 2013 9-47
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