Experimental Analysis of Flow over Rectangular Sharp-Crested Compound Weirs

Compound weirs can be used as flexibly adjustable structures to regulate the flow and the flow distribution over the cross-section. Examples are found in rivers to create additional resistance in one side of the river bifurcations. It is done to distribute the discharge among the branches properly. They are placed at the flood plains and become active only during flooding. Therefore, the expected flow type over the weirs is generally submerged, unlike the modular weir flow which has been studied a lot in the literature. In this study, an experimental campaign was conducted to understand how flow over compound weirs consisting of 12 sections, differs from the flow over uniform weirs. The analyses were conducted under modular and submerged weir conditions to gain a comparative understanding. Configurations of the compound weirs are important as they may lead to horizontal and vertical contraction of flow at various degrees. In this study, nine compound weir configurations were used to include flow variety and a wide range of applications. The experiments were conducted in a 3-meter-wide, 20-m-long rectangular horizontal flume at the Water Lab of Delft University of Technology. Flow depths at the upstream and downstream sides of the weirs were recorded along with the flow rates. Six discharge values were used to include the effect of discharge variations in the results. The measurement results were compared with the standard formulas from the literature to estimate flow rates over uniform rectangular sharp-crested weirs. Experimental data showed strong deviations from the model in submerged cases and moderate to slight deviations in modular cases. The observed deviations showed dependence on weir configurations and discharge. This indicates that discharge coefficients and head losses cannot be treated per weir section, but should be considered in interaction with neighboring sections.