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

NORTH PEORIAAREA DRAINAGE MASTER PLAN• Discussion of how the results of the variousanalyses were combined with the sedimenttransport modeling results, sandand gravel mining impact assessmentanalysis, and were translated into the erosionhazard zone(s).• Recommendation for future updates ofhazard zone boundaries.• Recommendation for long-term monitoring.• Recommendations for how to modify theerosion hazard boundaries and/or underwhat conditions development can occurwithin the boundaries.Earthen Dams (Stock Tanks)There are a number of stock tanks in the NorthPeoria ADMP study area. Stock tanks typicallyconsist of a non-engineered earthen damof varying height placed across a watercourseto impound stormwater runoff. Vegetationtypically lines the impoundment area. Shoulddownstream areas become urbanized theseearthen dams would present a hazard. TheADWR Dam Safety Section, has legal jurisdictionover dams (embankments) which exceedcertain height and storage limits. ADWRdefines a jurisdictional dam as “either 25 feetor more in height or stores more than 50 acrefeet.If it is less than six feet in height regardlessof the storage capacity or does not storemore than 15-acre-feet regardless of height, itis not jurisdictional”. However even though astructure may not be considered jurisdictionalall dams (embankments) in an urban environmentare considered as having high hazardpotential.The structural integrity and safety of existingstock tanks shall be evaluated to access downstreamimpacts to existing or proposed developmentdue to a dam break. Criteria for thedesign and evaluation of dams can be foundin the book entitled “Design of Small Dams”,third edition (1987, Bureau of Reclamation). Aprofessional engineer registered under thelaws of Arizona, and having proficiency in32civil engineering as related to dam technology,shall conduct evaluation and/or designof an earthen structure.Retention/Detention FacilitiesAll detention/retention facilities incorporatedwithin new developments will be designed toretain the peak flow and volume of runofffrom the 100-year, 2-hour duration stormevent. In the special case when a detentiononly facility is allowed, the requirement toretain the 100-year 2-hour runoff volume maybe waived; however, the post-developmentpeak discharge leaving the site can not exceedpredevelopment conditions. In addition to the100-year event, the effects of more frequentevents (2- and 10- year events) of using adetention only facility must be determined.Standard Retention PracticeGuidelines for the standard practice of retainingthe volume of runoff from the 100-year, 2-hour event can be found in “Drainage Regulationsfor Maricopa County, Maricopa CountyDrainage Design Manual Volume II-Hydraulicsand City of Peoria Infrastructure DevelopmentGuidelines.In-stream, In-Line Detention BasinsIn-stream, in-line detention basins are stormwaterpeak reduction facilities, which could beemployed with authorization, from thereviewing agency, instead of the standardpractices of retaining the volume of runofffrom the 100-year, 2-hour event. The detentionfacility is typically located in a watercourseand functions only to reduce postdevelopment peak discharges. The facilitytypically consists of an encroachment into the100-year floodplain (possibly at a roadwaycrossing), an outlet structure sized to conveyrunoff from frequent events such that the naturalform and function of the watercourse isnot disturbed (sediment transport capabilitiesare maintained), and to impound runoff sufficiently,so that there enough storage providedto reduce peak discharges. The followingguidelines/consideration should beaddressed in the design of such a facility: