The Parshall flume is a fixed hydraulic structure developed to measure flow in irrigation channels, streams, storm water systems and effluent discharge in wastewater treatment. Named after its creator, Dr. Ralph L. Parshall of the U.S. Soil Conservation Service, the Parshall flume accelerates the flow by contracting both the parallel sidewalls and a drop in the floor at the flume throat. Under free-flow conditions, the depth of water at a specified location upstream of the flume throat can be converted to a rate of flow.
Parshall flumes can be fabricated from a wide variety of materials. Common choices including fiberglass (most popular for its corrosion resistance in wastewater), galvanized steel (economical for irrigation), stainless steel (for harsh, abrasive, or high-temp flows), concrete (for very large sizes), and even lightweight aluminum for portable applications, reflecting the need for durability and cost-effectiveness in diverse flow environments.
Working Principles
- Converging Section (Inlet): Water enters a flat-bottomed, narrowing section, which begins to accelerate the flow.
- Throat: The narrowest part of the flume features a downward drop in the floor, significantly increasing water velocity (from subcritical to supercritical) and creating a critical flow condition.
- Diverging Section (Outlet): The floor rises back up, and the walls expand, slowing the water down (from supercritical back to subcritical) before it returns to the downstream channel.
- Measurement Point: Under “free-flow” conditions (no downstream restriction), the flow rate is determined by measuring the water depth (Ha) at a precise point in the converging section, upstream of the throat.
