Drop-box weir for measuring flow rates under extreme flow conditions

Excerpt

Measuring discharge rates in stream flows laden with sediment using precalibrated flow structures, such as flumes and weirs, is difficult and flow measurements can quickly become inaccurate. This is because large particles ranging from sand-sized particles to rocks easily clog the structures, changing the approach velocities and flow directions during a runoff event. These conditions often occur during extreme runoff events when there is sufficient transport capacity in the flows, causing deposition. Furthermore, assumptions are often made for individual structures regarding the approach conditions in a stream channel, such as a minimum channel slope and direction of flow. The drop-box weir is one of two flow-measuring devices (the other is the critical-depth flume) that can be used to monitor under these conditions and give accurate flow measurements.

The operating principle of the drop-box weir is to maximize the turbulence in stream flows to suspend large sediment particles. This is accomplished by forcing two water streams to flow into each other from opposite sides of a box, after falling into the box. The box is located upstream from a V-measuring section. At low flow rates, the water streams enter the box toward the front near the V section. At increasingly larger …