III. INITIAL HYDROLOGY REPORTA. IntroductionThe hydrology for this study was developed based on hydrology fromthe <strong>Flood</strong>plain Delineation <strong>of</strong> 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 <strong>Report</strong> to become familiar with thedevelopment <strong>of</strong> 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 <strong>of</strong> run<strong>of</strong>f is generally from thenortheast to the southwest. The watershed is characterized by a largeamount <strong>of</strong> agricultural land with increasing amounts <strong>of</strong> residential andindustrial development continuing to take place. As a result, overlandflow is the predominant flow condition.B. Hydrology Model UpdateThe U.S. Army Corps <strong>of</strong> Engineers, HEC-1 <strong>Flood</strong> Hydrograph Package(HEC-1) computer program was used to develop this model. Guidanceis given in the Drainage Design Manual for <strong>Maricopa</strong> <strong>County</strong>, Arizona,Volume I, Hydrology (DDM1) for application <strong>of</strong> the HEC-1 programwithin <strong>Maricopa</strong> <strong>County</strong>. Additionally, the computer program DrainageDesign Menu System (DDMS), developed by the <strong>District</strong>, was used tomodify land use parameters which have changed due to development.Features within the DDMS used for this study include Computation <strong>of</strong>Precipitation Frequency-Duration Values in the Western United States(PREFRE) and <strong>Maricopa</strong> <strong>County</strong> 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 <strong>of</strong> rapid development, losses due toland use must be revised. For the purpose <strong>of</strong> 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 <strong>of</strong> 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 <strong>of</strong> 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 <strong>of</strong> the revised sub-basins had significant changes in peak dischargeas a result <strong>of</strong> the update. Sub-basins VB and JC produced peakdischarges that are approximately one-half the previous values. Thechange is due to a large percentage <strong>of</strong> 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% <strong>of</strong> the original peak discharge. The cause<strong>of</strong> 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 <strong>of</strong> the South Mountain Freeway arenot considered in the hydrology model. According to the DrainageDesign Concept <strong>Report</strong> for the Southwest Loop Highway (September,1988), construction <strong>of</strong> this Freeway will require several modificationsand improvements to the drainage features in the area. Theseimprovements include channels, storm drains and detention basins – none<strong>of</strong> which are included in the model.Stage Storage routing at Holly AcresCurrently, the hydrology model ignores the effects <strong>of</strong> the Holly AcresLevee. Run<strong>of</strong>f generated from subbasins AB, AC, and AD is routedDIBBLE & ASSOCIATES 15 DURANGO ADMPDATA COLLECTION REPORT
directly into the Gila River without any concern for ponding behind theLevee. Additionally, ponded water is likely to reach a point where itwould overtop El Mirage Road, 107 th and, 115 th Avenues and continuetoward the west.RetentionWhile the landuse parameters have been changed to account for rainfalllosses, the question arises as to the issue <strong>of</strong> on-site retention. Hasretention been adequately addressed for these new developments, bothcommercial and residential? While DDMS does adjust parameters forpercent impervious area (RTIMP), no consideration is provided forlosses due to retention.3. The hydrologic routing parameters will be revised along theBuckeye Feeder Canal to more accurately depict the channelized flowconditions within the Canal.4. Large area retention basins constructed with recentdevelopment will be incorporated into the model to the extent that theirexistence and volume can be verified.5. Ponding <strong>of</strong> run<strong>of</strong>f behind the Holly Acres levee will be addedto the model by preparing a stage-storage relationship from the projectmapping.Planned Model RevisionsThis Initial Drainage <strong>Report</strong> is the starting point for a “living” documentthat will evolve over the course <strong>of</strong> the project resulting in a FinalDrainage <strong>Report</strong> at the end <strong>of</strong> the project. The issues just presentedwere reviewed in a meeting with the FCDMC on March 15, 2000. Thefollowing modifications will be made to the model during the nextproject phase:1. The existing conditions land use will be reviewed in moredetail using color aerial photography taken February 15, 2000. Forpurposes <strong>of</strong> this study, the February 15 photography will be the basis <strong>of</strong>“existing conditions.”2. Based on the land use review, sub-basin boundaries may berevised to accurately model differences in land use and to moreaccurately model certain areas within the watershed for master planpurposes.DIBBLE & ASSOCIATES 16 DURANGO ADMPDATA COLLECTION REPORT
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