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

NORTH PEORIAAREA DRAINAGE MASTER PLAN• Design Standard. Engineered flood controlchannels are typically designed to a 100-year standard. Therefore, damage mayoccur to development adjacent to a 100-year channel (or to the channelizationitself) if flow rates greater than the 100-year event occur. If design dischargeschange due to watershed changes or revisionsto hydrologic modeling standards,retrofit solutions are required to maintainthe same standard of protection.• Design Life. Engineered structures have alimited design life, and require regularmaintenance and inspection, and eventualreplacement.only where it can be demonstrated that nolong-term or short-term off-site impacts tochannel stability occur, that downstreamreaches are adequately protected from erosionand flooding, and a long-term maintenanceand inspection program is adopted. Wherestructural flood control measures are necessary,the design and installation of such structuresshould compliment the environmentand be accomplished with the least disturbanceto the natural setting. Design guidelinesand standards for structural flood controlimprovements are provided in the MaricopaCounty Drainage Design Manual Volume II,Hydraulics and the City of Peoria InfrastructureDevelopment Guidelines.• Equilibrium Slope. Because of the increasein discharge, velocity, and depth, the stableslope is generally flatter than the existingchannel slope, which will cause longtermscour and require grade control toprevent undercutting of bank protection.• Habitat. Channelization typically eliminatesmost of the natural floodplain andstream bank habitat, and requires mitigationmeasures.• Sediment Supply. Bank erosion is animportant source of sediment supply forthe streams in the study area. Constructionof bank protection eliminates thissource of sediment, increasing the likelihoodof erosion of adjacent and downstreamreaches.• Downstream Impacts. Excessive instabilityshould be expected at the outlet of a channelizedreach due to the changes in velocity,sediment supply, and discharge.Depending on the channel geometry, theexpected response can range from lateralerosion and scour to sediment depositionand overbank flooding.Channelization, a structural flood controlmeasure, is not recommended as a developmentalternative in the North Peoria ADMPstudy area. Channelization should be allowedRoadway Crossing Drainage StructuresAt-Grade Crossings“At-grade crossings” typically have only minimalor localized impacts on channel stability.More commonly, the streams impact the atgradecrossing, rather than vice versa. Flowover the at-grade crossing can cause erosion ofthe pavement and subgrade, deposition ofsediment in the road section, and disruptionof traffic flow. Channel stability impacts commonlyobserved near “at-grade crossings”include the following including recommendationsfor mitigation:([DPSOHRIDQ´$W*UDGH&URVVLQJµ• A scour hole often forms on the downstreamside of an “at-grade crossing” dueto acceleration of flow over the hydraulicallysmooth pavement surface andincreased turbulence as flow transitions36