River Geomorphology (40) - Low head dam installation effects on coarse sediment transport...

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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.

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