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<br />IV. BRIDGE DESIGN <br /> <br />A. Improper Bridge Design can Adversely Impact Stream Stability, <br /> <br />B. Scour at Bridges, <br /> <br />1, Three components of scour are: long-term aggradation or <br />degradation, contraction scour, and local scour (see Lesson 8). <br /> <br />2. Proper hydraulic bridge design (e,g" limited encroachment and <br />floodplain constriction) can minimize bridge scour. <br /> <br />C. Abutments, <br /> <br />1, Bridge abutments can be classified as spillthrough or vertical wall, <br />both of which are susceptible to scour. <br /> <br />2, Abutment scour is affected by flow distribution, foundation materials, <br />velocity, skew, and overbank flow diverted to the bridge opE~ning by <br />approach fills. <br /> <br />3, Generally, scour at spillthrough abutments is about 50 percent <br />smaller than at vertical wall abutments. <br /> <br />D. Piers. <br /> <br />1, The number of piers in any stream channel should be limited to a <br />practical minimum, <br /> <br />2. Properly located piers do not contribute significantly to bridge <br />backwater, but can contribute to contraction scour. <br /> <br />3, Piers should be aligned with the flow direction at flood istage to <br />minimize drift accumulation, contraction effects, ice forces, <br />backwater and local scour. <br /> <br />4, If flow direction changes with stage, cylindrical piers or some <br />variation, are probably the best alternative; however, multiple <br />column bents tend to collect more debris than solid piers, <br /> <br />3.24 <br />