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Static head represents the maximum elevation of the water surface above the elevation of the weir <br /> crest as measured upstream of the weir at a point where velocities are low ( 1 to 2 % of the velocity at the weir <br /> crest) . The static head can be measured directly by a stage gauge located at least 40 to 50 ft upstream of the <br /> weir. The water surface elevation can be read directly from the gauge, with the difference between the gauge <br /> elevation and the weir crest elevation indicating the static head . An empirical relationship applicable to <br /> narrow- crested weirs (Walski and Schroeder, 1978) indicates that a design weir loading of 0 . 89 ft /ft-sec <br /> corresponds to a static head 0. 41 ft (4. 9 in . ). Alternatively, the static head can be determined indirectly by <br /> measuring the depth of flow over the weir. The ratio of depth of flow over the weir to static head , estimated <br /> as 0 . 85 for narrow-crested weirs, yields a design flow depth for the IR- 14 facility of 0 .35 ft or 4 . 2 in . If the <br /> head over the weir, as measured by either method, falls below these design values as a result of unsteady <br /> dredge output or intermittent operation , effluent quality should increase. However, if the head exceeds these <br /> values, the ponding depth should be increased by adding flashboards until the mean ponding depth reaches <br /> its 5- ft recommended maximum . To safeguard dike stability, dredging should be temporarily halted rather <br /> than allow the mean ponding depth to exceed the recommended 5 - ft maximum . <br /> At all times , each of the three weir sections must be maintained at the same elevation to prevent flow <br /> concentration and a decrease in effluent quality related to an increase in weir loading . Preventing floating <br /> debris from collecting in front of the weir sections is also important . An accumulation of debris at the weir <br /> will reduce the effective weir crest length and thereby increase the withdrawal depth . This, in turn, may <br /> increase the effluent suspended solids concentration . <br /> To maintain the recommended 4 . 0-ft mean ponding depth throughout the dredging operation , the weir <br /> crest should be raised at approximately the same rate as the rise of the deposition layer. Based on the <br /> projected bulked volume produced by the typical dredging operation within Reach III — 16, 300 to 32,600 <br /> cy per event based on a 5 - to 10-year maintenance interval — the average depth of deposition per event will <br /> range from 1 . 0 ft to 2 . 0 ft. Thus , the typical maintenance operation will result in a final weir crest elevation <br /> ranging from +4 . 6 ft to +5 . 6 ft NGVD at completion of the first dredging operation . <br /> After dredging has been completed, the ponded water that remains within the basin must be slowly <br /> released by gradually removing flashboards — a process known as decanting. Flow over the weir should <br /> drop essentially to zero before the next flashboard is removed . Effluent monitoring must continue during the <br /> 32 <br />