River Geomorphology Video created by Little River Research and Design, with funding from the Missouri Department of Natural Resources. http://serc.carleton.edu/details/file…
As the clip opens you see shallow flow with uniform bedmaterial transport throughout. A small low head wier or dam is installed. This produces deep subcritical flow above the dam and critical flow over it. Below the dam we see supercritical flow.
The deeper, low velocity flow above the dam cannot move the coarse bedload (Q = VA, and since A is greatly increased and Q is unchanged above the dam, V is greatly decreased) and we see deposition occur until depth is shallow enough (and A small enough) that the increase in V moved bedload again. Deposition occurs to the top of the dam.
When the dam is installed, we see a classic disruption in sediment transport continuity. Coarse transport essentially ceases through the dam until deposition builds a higher streambed. Sediment is blown out below the dam (often scoured to bedrock in the real world) This is the well known “hungry water” effect seen below dams.
At low-water crossings in the Missouri Ozarks, many of which are essentially low dams, we often see this condition, manifested as a wide, sediment-filled channel with low banks upstream of the bridge. This contrasts with a deep, scoured channel below, sometimes with high, unstable banks.
At the end of the demonstration, the downstream gate is lowered and a hydraulic jump appears which is then drowned as stage increases. The depositional dune and slipface then move past the dam. The gate is then raised somewhat, allowing a jump to reform and sediment is blown out below the dam.