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D. CONTRACTION SCOUR Contraction scour occurs when the flow area of a stream at flood stage is decreased from the normal, either by a natural constriction or by a bridge, Hith the decrease in flow area there is an increase in average velocity and bed shear stress. Hence, there is an increase in stream power at the contraction and more bed material is transported from the contracted reach than is transported into the reach, T~is increase in transport of bed material lowers the bed elevation. As the bed elevation is lowered, the flow area increases and the velocity and shear stress decrease until relative equilibrium is reached; i.e., the quantity of bed material that is transported into the reach is equal to that transported out of the reach. Contraction scour can also be caused by short-term (daily, weekly, yearly or seascnally) changes in the downstream water surface elevation that controls the backwater and hence the velocity through the bridge opening. Because this scour is reversible it is included in contraction scour rather than in long-term scour. Contraction scour can result from the location of a bridge with regarc to a bend. If a bridge is located on or close to a bend, the ccncentration of the flow on the outer part of the channel can eroce the bed, Contraction scour is typically cyclic, That is, the bed scours during the rising stagE of a runoff event, a~d fills on the falling stage. The contraction of flow due to a bridge can be caused by a decrease i~ flow area of the strea~ channel either naturally or by the abLtments projecting into the channel and/or the piers taking up a large portion of the flow area, Also, the contraction can be caused by the approaches to a bridge cutting off the flood plain flow. This causes clear-',vater scour at the bridge section because the flood plain flow normally does not transport significant concentrations of bed material sediments, This clear water picks up additional sediment from the bed upon reaching the bridge opening. In addition, local scour at abutments may well be srreater due to the clear-water floodplain flow entering the main channel at that point. A guide bank at an abutment decreases the risk from scour at the abutment by its realignment of the stream lines of the flood plain flow to parallel the main channel flow. However, clear-water scour will occur at the upstream Emd of the guide bank. Another method to decrease abutment scour is to install relief bridges. They decrease the scour problem at the bridge cross section by decreasing the quantit~' of clear-water returning to the main channel. Other factors that can cause contraction scour are: (1) a natural stream constriction, (2) long highway approaches over the flood plain to the bridge, (3) ice format:ion or jams, (4) a natural berm forming a:,ong the banks due to sediment deposits, 10