Sedimentation and Ramp (Palmer Bowlus / RBC) Flumes

Long-throated Ramp Flumes like the Palmer Bowlus and the RBC can cause maintenance challenges when used on flow with high suspended solids contents or low flows. The problem is the way in which the flumes accelerate the flow from a sub-critical (Fr<1) to supercritical state (Fr>1) – whereby the level to flow relationship is generated.

RBC flumes in series - both with upstream sedimentation Both flumes accelerate flow by – in part in the case of the Palmer Bowlus – raising the floor of the flume. This rise in the water level means that at when there is no flow, there is always standing water upstream of the ramp. Under normal mid- to high-range flows, this isn’t a problem and solids will generally push through the flume.

But if the lows are low, the solids particularly dense, or some combination of the two, the sediments may drop out of suspension and collect upstream of the ramp. As the solids accumulate, the process of sedimentation can be self-sustaining – resulting in a change in the flow profile through the flume (decreasing flow accuracy) and increasing site maintenance.

Common materials that we’ve seen collect upstream of Palmer Bowlus and RBC flumes include:

Powder coating shot
Glass
Gravel / sand / stones

Sanitary solids do not usually present a problem unless flows are low and stay low. In which case a different flume should probably have been chose – typically a Trapezoidal flume.

Image: Pudasaini, M., Shrestha, S., Riley, S., Application of Water Erosion Prediction Project (WEPP) to Estimate Soil Erosion From Single Storm Rainfall Events from Construction Sites