Data Collection Report - Flood Control District of Maricopa County

III. INITIAL HYDROLOGY REPORTA. IntroductionThe hydrology for this study was developed based on hydrology fromthe Floodplain Delineation of the Tolleson Area (Project FCD 95-26)and modified to reflect changes in land-use and routing which haveoccurred since the original study. The reader is encouraged to read theabove mentioned Hydrology Report to become familiar with thedevelopment of the model.The watershed limits have remained unchanged since the original study.The approximate watershed limits are Interstate 10 on the north, the Salt& Gila Rivers on the south, Interstate 17 on the east and the Agua FriaRiver on the west. The direction of runoff is generally from thenortheast to the southwest. The watershed is characterized by a largeamount of agricultural land with increasing amounts of residential andindustrial development continuing to take place. As a result, overlandflow is the predominant flow condition.B. Hydrology Model UpdateThe U.S. Army Corps of Engineers, HEC-1 Flood Hydrograph Package(HEC-1) computer program was used to develop this model. Guidanceis given in the Drainage Design Manual for Maricopa County, Arizona,Volume I, Hydrology (DDM1) for application of the HEC-1 programwithin Maricopa County. Additionally, the computer program DrainageDesign Menu System (DDMS), developed by the District, was used tomodify land use parameters which have changed due to development.Features within the DDMS used for this study include Computation ofPrecipitation Frequency-Duration Values in the Western United States(PREFRE) and Maricopa County Unit Hydrograph Procedure 2(MCUHP2).Point precipitation rainfall values are taken from NOAA Atlas II,Volume VIII. The PREFRE program included with the DDMS wasused in conjunction with the precipitation isopluvial maps contained inthe DDM1. Four storm events were evaluated for this study. Theyinclude the 10 and 100-year events each having a 6- and 24-hourduration. The rainfall values for each is given as:10-yr, 6-hr 10-yr, 24-hr 100-yr, 6-hr 100-yr, 24-hr2.06" 2.47" 3.23" 3.99"Rainfall losses due to soil types have remained unchanged since theoriginal study. However, because of rapid development, losses due toland use must be revised. For the purpose of this study, the land use wasmodified based on aerial photographs and field reconnaissance. Thecurrent land use is shown on Figure III-1. Any revisions noted for agiven subbasin were coded into the DDMS and new loss parametersgenerated. Printouts of these revised subbasins can be found in theAppendix.C. Modeling Results, Discrepancies, & ConcernsResults from the modeling effort are summarized in the Appendix as aStorm Comparison Table which shows peak flows and the time of peak,at each HEC-1 step, for the four storm events. The sub-basin boundariesand schematic routing diagram are shown on Exhibit 1 and 2 in anenvelope also in the Appendix.Two of the revised sub-basins had significant changes in peak dischargeas a result of the update. Sub-basins VB and JC produced peakdischarges that are approximately one-half the previous values. Thechange is due to a large percentage of the drainage area changing froma “Crops” land use designation with a “wet” moisture condition to“Light Industrial” and “Low Density Residential” with a “Normal”moisture condition. The remaining 26 revised sub-basins had changesin peak discharge within 10% of the original peak discharge. The causeof the significant change in peak discharge resulting from changing“Crops” to other land uses should be investigated. The change couldhave a significant impact on fully developed condition peak dischargeswithin the study area.Proposed South Mountain FreewayAt this time the effects on drainage of the South Mountain Freeway arenot considered in the hydrology model. According to the DrainageDesign Concept Report for the Southwest Loop Highway (September,1988), construction of this Freeway will require several modificationsand improvements to the drainage features in the area. Theseimprovements include channels, storm drains and detention basins – noneof which are included in the model.Stage Storage routing at Holly AcresCurrently, the hydrology model ignores the effects of the Holly AcresLevee. Runoff generated from subbasins AB, AC, and AD is routedDIBBLE & ASSOCIATES 15 DURANGO ADMPDATA COLLECTION REPORT