Effects of sedimentation on flow measurements from short crested triangular weirs

A weir is an engineered structure used to measure free surface flows. In streams, canals, and rivers weirs are most often constructed mid-channel and oriented perpendicular to the flow. All weirs create backwater that causes a decrease in flow velocity upstream from the weir. If the flow carries sediment, this velocity reduction results in sedimentation in the channel upstream from the weir. Sedimentation alters approach flow conditions and leads to flow measurement errors, as most weir stage-discharge relations are derived without consideration of sedimentation. The objective of this study was to quantify the effects of sedimentation on the discharge coefficients for the Panama design short crested 120-degree V-notch weirs over a range of flows, channel slopes, and measurement positions along and across the channel. The Panama weir design has twice the crest slope of the ARS short crested weirs to reduce capture of floating woody debris. Physical hydraulic models were built at two different scales in the University of Wyoming Water Resources Laboratory. Laboratory instrumentation included weight-based flow measurements and calibration facilities. Redundant measurements of water levels using ultrasonic sensors allowed statistical evaluation of the accuracy of the results. A 1:2 scale model was built in a large horizontal-bed recirculating flume to test low-flows. A 3:16 scale model was built in a smaller, tilting flume to test higher flows with channel slopes between 0 and 3 percent. In both scale models, plywood and either sand or gravel were used to simulate full sedimentation, which is defined as a planar bed with elevation equal to the weir invert. The fundamental length scale used for Froude dynamical similarity was the thickness of the weir in the flow direction T. Analysis included of the total head H of the approach flow. The 3:16 scale model apparatus used ultrasonic water level sensors mounted at centerline and off-centerline locations on a computer-controlled traverse, which allowed measurement of the water surface profile. Results from the 1:2 scale model showed that sedimentation caused a reduction in discharge coefficients for H/T > 1.36 compared to results of tests without sedimentation. Results from the 3:16 scale model for H/T <3.5 showed that sedimentation reduced discharge coefficients by an average of 3.31 percent compared to tests without sediment. Channel slope was found to have only a slight effect at the steepest slope tested. Measurement location affected the measurement of discharge coefficients near the weir, where drawdown occurred, and sedimentation extended the drawdown region upstream from the weir. The results from these tests were compared to results from Mefford (1979) and the U.S. Bureau of Reclamation Water Measurement Manual. Significant differences were found.