When channels become misaligned with crossing structures they flow through, numerous problems result including excessive scour deposition, reduced capacity, etc. Abrupt change in direction just upstream of the structure is particularly problematic. These management structures move any change in direction upstream to smooth out the flow path and realign the current upstream so it flows straight into the crossing.
Misaligned Channels
As flow approaches the crossing, erosion and deposition often cause the channel to move laterally out of alignment with the crossing structure.
Example Channel Alignment Vane at 2-span Bridge
Example Channel Alignment Vane
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Two-dimensional hydraulic models were developed for several structures to explore the performance of stream control structures.
Most of the debris carried by rivers moves with the main current and, when a pier is encountered, debris accumulates. By dividing the flow, subtly shifting the main current to either side well before it reaches a pier, the debris too is shifted away through the clear spans of the crossing.
Example of Dividing Flow Around Pier
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The common 3-span theme to bridge design has the main current going through the center span and the side spans providing extra capacity for high flows. When channel alignment shifts, one set of piers become in the midst of the main current and are prone to catching debris, scour, and deposition. This vane configuration can be used to shift the main current and keep it centered in alignment with the center span.
Sediment accumulation can reduce flow capacity, particularly when the flow is wider through the crossing than in the stable channel up and down stream.
Three Span Bridges
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Multi Culvert Weir
Sediment accumulation and lost flow capacity are common problems in culverts, particularly if constructed so that base flow through the culvert or culverts is much wider than the width of the naturally stable channel in the vicinity. For twin (or more) box culverts, this practice concentrates the low and intermediate flow through one culvert while utilizing all the culverts at high flow creating. This can flush out previously accumulated sediment within culverts.
Example 1 of Twin Box Culvert Weir
Two box culverts each about the width of the natural stable channel upstream and down with sediment repeatedly accumulating and reducing flow capacity.
Example 2 of Twin Box Culvert Weir
Two box culverts, each about the width of the natural stable channel upstream and down, with sediment repeatedly accumulating and reducing flow capacity.
Downloadable Resources
Wide Culvert Partition
Improving sediment transport through a crossing with multiple culverts is relatively simple with a structure at the entrance to the culverts. However, if the crossing has one very wide span relative to the stream channel, to keep low and intermediate flows concentrated and transporting sediment, partitioning the channel under the crossing may solve sedimentation problems.
Example of a Culvert Partition
Downloadable Resource
