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<br />across all or DOS~ of the channel width, This component of <br />scour can result ~rom a contraction of the flow, change in <br />downstream control of the water surface elevation or flow <br />around a bend, The scour is caused by increased velocities <br />and a resulting i~crease in bed shear stresses. <br /> <br />contraction of th,~ flow by bridge approach embankments <br />encroaching onto .:he floodplain and/or into the main <br />channel is the m03t common cause of contraction scour. <br /> <br />3. Local Scour. Local scour involves removal of ma~erial from <br />around piers, abu:ments, spurs, and embankments. It is <br />caused by an acceleration of flow and resulting vortices <br />induced by the flJw obst~uctions. <br />In addition to tha types of scour mentioned above, <br />naturally occurri~g lateral migration of a stream may erode <br />abutments, the ap?roach roadway or change the total scour <br />by changing the flow angle of attack. Factors t~at affect <br />lateral movement also affect the stability of a bridge, <br />These factors are the geomorphology of the stream, location <br />of the crossing on the stream, flood characteris~ics, and <br />the characteristi:::s of the bed and bank materials. (see <br />HEC-20 (6) and HIRE (7)) <br /> <br />The following paragraphs contain additional informa~ion on <br />the types of scour discussed above. <br /> <br />c. AGGRADATION AND DEGRADATION. LONG-TE~1 STRE&V, BED ELEVATION <br />CHANGES <br /> <br />Long-term bed elevation changes may be the natural t~end of the <br />stream or may be the result of some modification to the stream or <br />watershed condition. The stream bed may be aggrading, degrading <br />or in relative equilibrium in the bridge crossing reach. In this <br />section long-term trends are considered. This does not include <br />the cutting and filling of the bed of the stream that might occur <br />during a runoff event (contraction scour), A stream may cut and <br />fill during a runoff event and also have a long-term trend of an <br />increase or decrease ir. bed elevation. The problem for the <br />engineer is to determir.e what the long-term bed elevation changes <br />will be during the life of the structure. What is the current <br />rate of change in the stream bed elevation? Is the stream bed <br />elevation in relative Equilibrium? Is the stream bed degrading? <br />Is it aggrading? What is the future trend in the stream bed <br />elevation? <br /> <br />During the life of the bridge the present trend may change. <br />These long-term change~, are the result of modifications to the <br />stream or watershed. Such changes may be the result of natural <br />processes or man's activities, The engineer must assess the <br /> <br />8 <br />