Settlement and Submergence Adjustments for Parshall Flume

The Parshall flume was developed nearly 70 years ago to measure open-channel discharge accurately for agricultural applications. When the flume is subjected to numerous wet-dry, freeze-thaw, and heating-cooling cycles, the flume becomes susceptible to settlement. As the flume settles, the measurement accuracy becomes suspect. Commercially available Parshall flumes were purchased and assembled in accordance with the manufacturer's instructions and tested in the laboratory. A series of 383 experiments was performed to determine the accuracy of the Parshall flume when subjected to an array of settlement and submergence conditions. The results indicated that although significant difference between apparent and measured discharges exists, the apparent discharge can be corrected. An analysis of the submergence test results determined that for submergence of less than 90%, the flow can be corrected to within ±3%. However, flow conditions with submergence of 90% or greater can only be corrected to within ±5%. A comprehensive procedure is presented for adjusting the apparent discharge for settlement and submergence conditions.