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Last modified
1/25/2010 6:45:23 PM
Creation date
10/5/2006 12:21:34 AM
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Floodplain Documents
County
Statewide
Basin
Statewide
Title
Interim Procedures for Evaluating Scour at Bridges
Date
9/1/1988
Prepared By
Federal Highway Administration
Floodplain - Doc Type
Educational/Technical/Reference Information
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<br />I <br />Ic-., <br />., .-\ <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />1'- <br />I <br /> <br />A method to organize such an analysis is to use a three level fluvial syst.. <br />approach. This method provides three levels of detail or stages in an <br />analysis: (1) a qualitative determination based on generll geomorphic and river <br />mechanics relationships; (2) engineering geomorphic Inalysis using established <br />qualitative and quantitative relationships to establish the probable behavior <br />of the stre.. system to various,$cenarios of future conditions and; (3) <br />physical process computer modeling using the available physical process <br />mathematical models such as HEC-6 to ..ke predictions of the quantitative <br />changes in stream bed elevation to be expected as the result of the changes in <br />the stream and watershed. Methods to be used in stages 1 Ind 2 are presented <br />in Highways in the River Environment (6). Additional discussion of this <br />subject is presented in Chapter 4. <br /> <br />D. GENERAL SCOUR AND CONTRACTION SCOUR <br /> <br />General scour at a bridge can be caused by a decrease in flow area Ind an <br />increase in velocity. The decrease in flow area or channel width may be <br />naturally occurring or be caused by the bridge. This fonn of general scour is <br />contraction scour and will be addressed in this publication. General scour <br />can also be caused by short term (daily, weekly, yearly or seasonally) changes <br />in the downstream water surface elevation that controls the backwater and hence <br />the velocity through the bridge opening. Becluse this scour is reversible it <br />is included in general scour rather than in long term scour. General scour can <br />result from the location of the bridge with regard to a bend. If the bridge is <br />located on or close to a bend the concentration of the flow on the outer part <br />of the channel can erode the bed. <br /> <br />General scour can be cyclic. That is, during a runoff event the bed scours <br />during the rise in stage (increasing discharge) and fills on the falling stage <br />(deposition). <br /> <br />General scour from a contraction occurs when the flow area of a stream is <br />decreased from the normal either by a natural constriction or by a bridge. With <br />the decrease in flow area there is an increase in average velocity and bed <br />shear stress. Hence, there is an increase in stream power at the contraction <br />and more bed material is transported through the contracted reach than is <br />transported into the reach. The increase in transport of bed ..terial lowers <br />the bed elevation. As the bed elevation is lowered, the flow area increases <br />and the velocity and shear stress decreases until equilibrium between the bed <br />material that is transported into the reach is equal to that which is <br />transported out of the reach. <br /> <br />The contraction of the flow by the bridge can be caused by a decrease in flow <br />area of the stream channel by the abutments projecting into the channel and/or <br />the piers taking up a large portion of the flow area. Also, the contraction <br />can be caused by the approaches to the bridge cutting off the overland flow <br />that normally goes across the flood plain during high flow. This latter case <br />causes clear-water scour at the bridge section because the overland flow <br />normally does not transport any bed material sedi..nts. This clear water picks <br />up additional sediment from the bed when it returns to the bridge crossing. <br />In addition, if it returns to the stream channel a~ an abutment it increases <br /> <br />5 <br />
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