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<br />present state of the stream and watershed and then evaluate <br />potential future changes in the river system, From this <br />assessment the engineer must estimate the long-term stream bed <br />changes. <br /> <br />Factors that affect long-term bed elevation changes are: dams <br />and reservoirs (upstream or downstream of the bridge), changes in <br />watershed land use (urbanization, deforestation, etc.), <br />channelization, cutoffs of meander bends (natural or man made), <br />changes in the downstream channel base level (control), gravel <br />mining from the stream bed, diversion of water into or out of the <br />stream, natural lowering of the total system, movement of a bend, <br />bridge location with respect to stream planform, and stream <br />movement in relation to the crossing. <br /> <br />An assessment of long-term stream bed elevation changes should be <br />made using the principles of river mechanics. Such an assessment <br />requires the consideration of all influences upon the bridge <br />crossing; i,e., runoff from the watershed to a stream <br />(hydrology), the sediment delivery to the channel (erosion), the <br />sediment transport capacity of a stream (hydraulics) and the <br />response of a stream to these factors (geomorphology and river <br />mechanics), Many of the largest impacts are from man's <br />activities. This assessment requires a study of the history of <br />the river and man's activities on it as well as a study of <br />present water and land use and stream control activities, All <br />agencies involved with the river should be contacted to determine <br />possible future changes in the river, <br /> <br />To organize such an assessment, use this three-level fluvial <br />system approach: (1) a qualitative determination based on <br />general geomorphic and river mechanics relationships; (2) an <br />engineering geomorphic analysis using established qualitative and <br />quantitative relationships to estimate the probable behavior of <br />the stream system to various scenarios of future conditions; and <br />(3) physical process computer modeling using mathematical models <br />such as the U. S. Corps of Engineers' HEC-6 and BRISTARS to make <br />predictions of quantitative changes in stream bed elevation due <br />to changes in the stream and watershed. Methods to be used in <br />stages 1 and 2 are presented in Hydraulic Engineering Circular <br />No. 20, "Stream Stability" (6) and "Highways in the River <br />Environment" (7), Additional discussion of this subject is <br />presented in Chapter 4 of this document. <br /> <br />In coastal areas highway crossings (bridge) and/or longitudinal <br />stream encroachments are subject to tidal influences. The impact <br />of the ebb and flow of tides on long-term stream bed elevation <br />changes is relatively indeterminant. <br /> <br />9 <br />