Contraction Scour at Bridges: Analytic Model for Coarse-Bed Channels

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by David C. Froehlich, (M.ASCE),

Document Type: Proceeding Paper

Part of: North American Water and Environment Congress & Destructive Water:

Abstract: Contraction scour at bridges spanning coarse-bed channels is evaluated based on mass conservation of both water and transported sediment between a reference channel section near the crossing and a constricted section created by the bridge. For live-bed conditions, a bed-load transport function obtained specifically for coarse-bed channels yields a nonlinear equation for dimensionless bed shear stress in the constricted section that needs to be solved by iteration. For clear-water conditions, bed shear stress in the constricted section is set equal to a critical value, obtained from the bed-load transport function, at which erosion of bed particles ceases. Flow depth in the scoured channel is easily found from Manning's equation once dimensionless shear stress in the constricted section is known. Measured contraction scour depths at five bridges spanning coarse bed channels in Alaska are more closely predicted by the proposed equation than by equations recommended by the Federal Highway Administration for evaluating scour at bridges.

Subject Headings: Bridge tests | Hydraulic contraction | Hydraulic roughness | Shear stress | Structural models | Model analysis | Hydraulic models | Sediment transport | Water conservation | Bed loads | North America | Alaska | United States

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