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<br />. <br /> <br />51 <br /> <br />structures which are of particular interest in major drainageway design. The <br /> <br />. <br /> <br />term S represents the depth of scour, q is the discharge per unit width, <br /> <br />and hd the depth of flow downstream of the structure. <br /> <br />For the condition in Figure 3-3, a formula defining the scour depth was <br /> <br />. <br /> <br />proposed by Schok1itsch in 1932. It can be written as <br /> <br /> HO.2 q 0.5 <br /> S 4.75 - h (3-10) <br /> D90 0.32 d <br />. <br />where, S = the depth of the scour hole, meters, <br /> hd = the downstream water depth, meters, <br /> <br />q <br /> <br />the water discharge per unit width, cubic meters per second <br />per meter, <br /> <br />. <br /> <br />D90 <br /> <br />= the particle size for which 90 percent of the material is <br />finer, millimeters, <br /> <br />H <br /> <br />= the vertical distance between the energy grade line and the <br />downstream water surface, meters. <br /> <br />. <br /> <br />This formula is in metrjc units. To use the units of S, H, and hd in <br /> <br />. <br /> <br />feet, q in cfs, and DgO in mi 11 i meters, the constant should be 3.75 instead <br /> <br />of 4.75. Also, this equation has not been adequately validated for very large <br /> <br /> <br />particle sizes (D90 ~ l2 inches). For larger sizes of particles, physical <br /> <br /> <br />modeling, or some type of field verification based on performance of similar <br /> <br />. <br /> <br />structures, is advised. <br /> <br />3.8 Total Scour <br /> <br />THE TOTAL SCOUR THAT CAN OCCUR AT A STRUCTURE OR PERTINENT LOCATION IS <br /> <br />EQUAL TO THE SUM OF DEGRADATION, GENERAL SCOUR, AND LOCAL SCOUR. Bank pro- <br /> <br />tection and protection at structures should extend to a depth below the <br /> <br />channel hed equal to the total scour. <br />