Understanding Hydraulic Jumps and Weir Flow Dynamics in Rivers

compared to the Great Rivers of the world the water of LEF here in Edinburgh is a tiny stream but we can still see

lots of evidence as to how important it's been to society over the years the Weir behind me here was built to raise

the water level so that it could be used to drive water wheels that ped the industry that was formerly located in

the dean Village here nowadays this particular stretch of water is an important immunity with the pathway

providing a safe route to walk or cycle to work Engineers have had to ensure that

as development proceeds on either side the river banks are protected against floods that occur with increasing

frequency let's look at the way the water negotiat creates obstacles such as the wear

here the very crest of the we is a free overfall there's no resistance to the flow on the downstream side so as soon

as the water has the very minimum of energy it needs to pass the we it plunges over in the torrent that you see

behind me we can see this more clearly in the laboratory here we see water building up

behind a model of a weir the water has been colored for visibility as soon as it has enough energy the

water spills over the wear the depth of water over the wear is called critical depth and this depth is

constant for a given flow rate and Weir Geometry The Water behind the wear is deep and slow flowing a form of flow

known as subcritical flow at the edge of the Weir it spills over and Falls rapidly to the channel

below from the the engineering point of view what is really interesting is the shallow fast flow on the downstream side

of the Weir here we see water with lots of kinetic energy which could do a great

deal of damage Downstream if it were not handled safely this kind of flow the flow we get

Downstream of wears and at various points in Rapids in natural Rivers is called super critical

flow this is often something that Engineers need to control if the depth and flow conditions

Downstream are correct the flow can be forced to return to a slower deeper subcritical regime by means of a

hydraulic jump a hydraulic jump is a standing wave typified by quite violent edies and white

water here we can see the supercritical flow and the hydraulic jump that forms naturally around rocks in the

river this overhead view shows the hydraulic jump running across the center C of the screen with supercritical flow

clearly visible above it we can see the hydraulic jump again in the laboratory

Flume the flow spills over the Weir and becomes super critical but Downstream the flow is restricted under jump forms

forcing the water back to a subcritical regime the view from the side is much more striking the flow over the wear and

the shallow fast supercritical flow contrast with the deeper subcritical flow Downstream of the hydraulic

jump the jump itself takes the form of a jet of water close to the bed with Eddies peeling back off it to form the

white water on the surface in the field the details are not so clear but the basic form of the jump

can still be made out the formation and location of a hydraulic jump depends on the downstream

conditions in the laboratory we use this adjustable slle gate to restrict the flow out of The Flume forcing the flow

to become subcritical again we can see the hydraulic jump gradually creeping up The Flume until it

stabilizes right against the wear finally opening the slle removes the restriction on the flow allowing

supercritical flow to continue Down The Flume and causing the hydraulic jump to be washed out completely