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

NORTH PEORIAAREA DRAINAGE MASTER PLANheadwater ponding, discharge of clearwater may result in degradation downstreamuntil the channel slope adjusts tothe new sediment supply. Culverts shallbe designed so that the disruptions to thenatural sediment transport capabilities ofthe wash are minimized.BridgesBridges that span the floodplain typicallyhave no measurable impact on channel stability,as evidenced by the channel conditionsobserved at the Beardsley Canal flumes overCaterpillar Tank Wash and Twin Buttes Washwithin the North Peoria ADMP study area.Bridges with narrow openings are functionallylike a culvert, and have the impacts onchannel stability described above.([DPSOHRI+HDGZDOO&RQVWUXFWHGRI1DWLYH0DWHULDOOversized (relative to natural channel andfloodplain geometry) culverts structures,which increase the width of the natural channelin order to minimize the height or depth ofponding, can also have detrimental impacts toboth upstream and down stream properties.The impacts of oversized culverts on channelstability include the following:• Long-term Aggradation. Increasing thenatural width of a channel to accommodatea culvert structure would change thesediment transport capacity of the channel.During frequent events or eventslesser than the design capacity of the culvertstructure sediment would be depositedin the section of channel that has beenwidened. Accumulation of sedimentwould decease both the capacity of thechannel and the capacity of the structureultimately resulting in flooding impacts toadjacent properties. Culvert span (width)should be as wide as the main channel(top of left bank to top of right bank)where channels are well defined. Culvertsthat do not obstruct the main channel willhave less frequent impacts on channel stabilitythan culverts that block the mainchannel.38([DPSOHRID%ULGJH6WUXFWXUH&ORJJHGZLWK6HGLPHQWBased on theirlikely impactson channelstability, thefollowingguidelines forroadwaycrossingdesign are recommendedfor watercourses in the NorthPeoria ADMP study area:• Bridges are preferable to culverts. Bridgestypically have less impact on channel stabilitythan culverts due to the wider openingand decreased likelihood of headwaterponding.• Bridge span (width) at a minimum shouldbe as wide as the channel or floodway limits,preferably as wide as the floodplain orerosion hazard zone where channels arewell defined. Bridges that do not obstructthe main channel will have less frequentimpacts on channel stability than culvertsthat block the main channel.• Where braided or multiple channels exist,relief structures outside of the main channelshould be provided to maintain overbankflow paths, preserve overbankconveyance, and prevent floodplain sedimentation;instead of widening one of themultiple channels to provide conveyanceof the design event at one location.• Bridge crossings should be regularlymaintained and inspected to identifypotential problems and impacts to channel