Water Discharge Measurement in Agricultural Catchments Using Critical Depth Flumes Affected by Sediment Deposition

Abstract

Flumes and weirs are hydraulic structures used to measure soil erosion and runoff at the outlet of agricultural catchments. Long-throated flumes use critical flow theory to determine discharge from a single water depth measurement upstream of the flume structure. In erodible catchments, sediments often deposit at the entrance of the flume, just where water level is measured, preventing proper discharge determination. This work proposes the combination of critical depth theory with two models of water flow at free overfalls that may be applied when the exit of the flume presents a drop. The method allows the recording of discharge measurements when the upstream water level sensor fails because of the accumulation of sediments. This paper proposes three discharge predictors: (1) one based on critical flow theory, (2) another based on the momentum balance between the critical section and the free overfall section, and (3) the third based on energy balance between the upstream water section and the free overfall section. Two flume performance indicators and a practical protocol are used to decide, based on the performance indicators, which of the three discharge predictors is most appropriate for every flow situation. The critical flow discharge predictor is recommended when the performance indicators show evidence of no sediment deposition at the upstream section. On the contrary, the momentum balance discharge predictor is recommended if there is evidence of sediments deposition. In case of uncertainty, the recommended discharge predictor is the one based on energy balance between the upstream and the free overfall sections. The proposed equations apply to trapezoidal flumes, but they could be adapted for other geometries.