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

ack at the natural channel bed. In mostcases, formation of a scour hole doesn’timpact stream reaches located far from the“at-grade crossing”, however the developmentof a scour hole could undermine the“at-grade crossing”, leading to failure ofthe facility. Upstream and downstreamcut off walls shall be designed for at gradecrossings.• An “at-grade crossing” of a watercoursereach which is experiencing degradationwill ultimately function as a grade controlstructure. Until equilibrium is achieved,down stream degradation will continueincreasing the drop immediately downstreamof the “at-grade crossing”. Longterm degradation shall be considered indetermining the depth of cut off walls.If the “at-grade crossing” is constructed at anelevation slightly above the natural channelbed, deposition will occur upstream of thecrossing. Deposition leads to expansion of thefloodplain, and may increase the risk of avulsionsand accelerate formation of the downstreamscour hole. The minimum elevation ofan “at-grade crossing” shall not be higherthan the existing channel invert.stream properties. The impacts of undersizedculverts on channel stability include the followingincluding recommendations for mitigation:• Sediment Deposition. Much of the stream’ssediment load will be deposited in theheadwater pool at the culvert inlet. Thevolume of sediment deposited depends onthe culvert capacity relative to the discharge,the duration of the ponding condition,the geometry of the ponding area,and the size of the sediment in transport.Sediment deposition decreases channel(and culvert) capacity, increases the potentialfor overbank flooding and avulsions,and requires maintenance to restore naturalconditions. Culvert rise (height) at aminimum should be as high as the averagemain channel bank height. In theevent of width and height limitations dueto natural conditions, the structure doesnot convey the design event, increasingthe height dimension or providing reliefculvert structures in the overbank areasshould be considered before increasing thewidth. Culverts that do not obstruct themain channel will have less frequentimpacts on channel stability.“At-grade crossing” is not a recommendedwatercourse/roadway crossing in the City ofPeoria.CulvertsThe design of culvert structures takes intoconsideration public safety, long term functionand maintenance, and impacts to the naturalchannel form and function. Typically, theimpact of culvert crossings on a watercoursesystem is primarily a function of their size inrelationship to design discharge, natural channeland floodplain morphology, cloggingpotential, sediment transport capacity andscour potential.Undersized (relative to natural channel andfloodplain geometry) culverts and culvertsthat create headwater ponding can have detrimentalimpacts to both upstream and down37• Floodplain Encroachment. A culvert is aform of floodplain encroachment, with thesame types of encroachment impactsdescribed in the floodplain encroachmentdiscussion above.• Scour Hole. A scour hole may form at theculvert outlet due to accelerated velocitythrough the culvert, discharge of sediment-deprivedwater, and turbulence atthe culvert/natural channel interface.Design of culvert structures shall includean evaluation of the scour potential at theoutlet of the structure and provisions forchannel protection at the outlet shall beprovided.• Long-term Degradation. Where a significantpercentage of the sediment load isdeposited upstream of a culvert due to