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<br />. <br />% <br />t- <br />eL <br />I&J <br />Q <br />Ct: <br />:) <br />8 <br />(I) <br /> <br />LIVE BED SCOUR <br /> <br />I <br />I <br />( <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />,,: <br /> <br /> <br />MAXIMUM SCOUR DEPTH <br />/" i EQUILIBRIUM SCOUR OEPTA <br /> <br />--'- --.- -- <br /> <br /> <br />---- <br /> <br />CLEAR-WATER SCOUR <br /> <br />TI..., <br /> <br />Figure 2.2. Scour Depth as a Function of Time <br /> <br />G. LATERAL SHIFTIIIG OF A STREAIII <br /> <br />Streams are dynamic. They are always shifting their currents and bank lines. <br />A meandering stream has its .S. shaped plan form continually moving laterally <br />and downstream. A braided stream has its various channels continually <br />changing. Incidentally, the deepest natural scour occurs when two channels of <br />a braided stream come together or when the flow comes together downstream of an <br />island or bar. <br /> <br />A bridge is static. It fixes the stream at one place in time and space. A <br />meandering stream continues to move laterally and downstream eroding the <br />approach embankment and can also affect contraction and local scour because it <br />can change flow directions. A braided stream can shift its channels under a <br />bridge, have two channels come together at a pier or abutment and thus, <br />increase scour. Descriptions of stream morphology are given in Highways in the <br />River Environment (6). <br /> <br />Factors that affect lateral shifting and the stability of a bridge are the <br />geomorphology of the stream, location of the crossing on the stream, bed and <br />bank materials, flood characteristics and the characteristics of the bed <br />material, and washload discharge. <br /> <br />To control lateral shifting requires river training works, bank stabilizing by <br />riprap and guide banks. The design of these works are beyond the scope of this <br />circular. However, design methods are given in FHWA, AASHTO, and U.S. Corps of <br />Engineers publications. Of particular importance are Hydraulic AnalYSIS for <br />the Location and Design of Bridges, Volume VII-Highway Drainage Guidllines, <br />19B2 (10);' Highways in the Rivlr Environment (6); and Stream Stability, <br />Hydraulic Engineering Circular 20 (7) to be issued in 19B9. <br /> <br />10 <br />