North Peoria Area Drainage Master Plan - Flood Control District of ...

NORTH PEORIAAREA DRAINAGE MASTER PLANimpact structural, a full structural, and a noaction. The full structural alternative is basedon current federal, state, and local floodplainmanagement regulations that allow encroachmentinto the floodway fringe. The full structuralalternative typically requires, at aminimum, the structural stabilization of washside slopes for the entire reach. The partialstructural alternative also is based on currentfloodplain management regulations, howeverthe partial structural solution is applied atonly specific locations along the watercourse.The low impact structural alternative allowsfor development activity to occur within theerosion hazard zone as long as the activitydoes not significantly alter the natural formand function of the watercourse. The nonstructuralalternative defines a corridor thatallows the watercourse to function naturally.The no action (do nothing) alternative providesflood control management based on currentfederal, state and local floodplainmanagement regulations that allowsencroachment into the floodway fringe. Typically,under current regulations encroachmentsinto the floodway fringe are allowed ona piece-meal fashion without taking into considerationthe effect of the encroachment orcollective encroachments on the entire watercourse.Typical sections of watercourse-basedalternatives are depicted in Figure 16, Figure17, Figure 18, and Figure 19. Stormwater storagealternatives evaluated include the standardpractice of retaining the volume of flowfrom the 100-year, 2-hour event, in-stream, inlinedetention alternative and an in-stream,off-line retention alternative. Perspectives ofthe in-line detention alternative and the offlineretention alternative are presented as Figure20 and Figure 21.14The evaluation/application of an alternativefor a given watercourse is based on physicaland data constraints. Physical constraintsinclude land use, topography, the location,and distribution of rock outcrop, and characteristicsof the floodplain (i.e. floodway limitscoincidental with floodplain limits). Data constraintsare the availability of hydraulic models.Hydraulic evaluations of watercoursebasedalternatives are developed for watercoursesin which detail hydraulic models weredeveloped as part of the study or availablefrom previous studies. The full structuralalternative is applied only to watercourses inthe Twin Buttes area below the CAP canal.The partial structural alternative is evaluatedfor the three unnamed washes that drain theBig Spring Area to the Agua Fria River. Thelow impact structural alternative is evaluatedfor all watercourses in which erosion hazardzones have been delineated as part of thisstudy. The non-structural alternative and theno action alternative are evaluated for allwatercourses in which hydraulic models wereavailable.Both Maricopa County and the City of Peoriarequire retention/detention (stormwater storagefacilities) for all new developments. Thegoal of this requirement is to reduce/minimizethe impacts of the increased runoff dueto development in the watershed. Ideally, thisis accomplished by controlling the post-developmentrunoff such that it is equivalent inmagnitude, duration, and temporal distributionto the pre-development conditions. Toachieve this goal, both Maricopa County andthe City of Peoria use the 100-year, 2-hourstorm as the design event for sizing retention/detention facilities.Traditionally, retention has been accomplishedby storing the 100-year, 2-hour runoff volumein below grade basins. These basins typicallyare drained by percolation into the soil and/ora small outlet structure connected to a watercourseoften via an extensive storm drain system.The effectiveness of this type of facility,both economically and hydraulically, is afunction of the soil and terrain characteristics.Shallow soils with low permeability ratesand/or soils that occur on steep slopes are notconducive to this particular application.Fifty percent of the North Peoria ADMPproject area have soils types that are characterizedby moderate to moderately slow permeability;low to very low available watercapacity and shallow depth to bed rock or