How to Select a Weir or Flume for an Open Channel Flow Meter
When selecting the type of weir or flume, consider the flow rate in the channel, the flow regime of the water, and whether free-flow conditions can be achieved. Different weirs or flumes can be selected based on the maximum flow rate.
1. If the maximum flow rate is less than 40 L/s, a V-notch weir is recommended.
2. If it exceeds 40 L/s, a Parshall flume is recommended.
3. If the upstream channel is short and the maximum flow rate exceeds 40 L/s, a rectangular weir is recommended.
When conditions permit, a Parshall flume is the best choice. The water level-flow relationship for a Parshall flume is calibrated in the laboratory and has lower requirements for the upstream approach channel. The water level-flow relationships for V-notch and rectangular weirs are derived from theoretical calculations, which can introduce additional errors if certain operating conditions are overlooked.Use fiberglass to construct the weir or flume. The dimensions of the V-notch and rectangular weir openings must be accurate, and the surface facing the incoming flow must be smooth. For the Parshall flume, the throat dimensions must be accurate, and the inner surface of the flume must be smooth.