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Effects of Flow Duration on Local Scour at Bridge Piers in New York

by Gerard K. Butch, U.S. Geological Survey, Albany, United States,
Richard Lumia, U.S. Geological Survey, Albany, United States,



Document Type: Proceeding Paper

Part of: Hydraulic Engineering

Abstract:

The relation of local scour to the duration of high flows and other hydraulic properties is being studied at 31 bridge sites in New York State. Clear-water scour is common at most of the sites, and local-scour holes that formed during prolonged high flows did not refill during flow recessions. The deepening of about 20 local-scour holes by subsequent high flows indicates that the length of time a flow exceeds a given discharge can affect local scour at a site. Many high flows of short duration did not cause scour. Hourly discharge data collected at nearby U.S. Geological Survey streamflow-gaging stations were used to develop a hydrograph-based factor that incorporates the duration and magnitude of high flows and the shape of the hydrograph (runoff rate per hour). This factor improved statistically derived estimates of local-scour depth. Combining the hydrograph-based factor with a momentum factor that represents flow velocity, water depth, and bed-material size decreased the standard error of estimate from 60 to 55 percent and increased the coefficient of determination from 0.66 to 0.73. The streambed of the Cohocton River at State Route 333 in Steuben County was lowered 40-60 cm near a pier during a high flow that exceeded the mean-annual peak discharge (Q₂) for 71 hours March 31-April 3, 1993. A similar high flow at this site March 30-31, 1993 exceeded the Q₂ for 21 hours but produced no scour. The streambed of the Otselic River at State Route 333 in Cortland County was lowered 170 cm near the pier during a high flow that exceeded the Q₂ for 27 hours April 10-11, 1993; 30 cm of this scour occurred during the flow recession.

Subject Headings: Scour | River flow | Fluid flow | Flow measurement | Flow duration | Piers | Hydrographs | New York | United States

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