The sluice gate.
The normal function of a sluice gate is to reduce the depth of tranquil flow in a channel. Just as in valves in pipes this means that the sluice gate must cause a loss of energy. The gate is merely a full width gate in a rectangular channel arranged so that it can be opened by raising it from the bed of the channel. Its function is to convert some of the potential head upstream to kinetic energy that can then be dissipated in some way, preferably in the channel immediately down stream of the gate, which will be designed to localise the loss and to resist erosion. There are several methods of dissipating the kinetic energy and these are typically, in a hydraulic jump, in the intense eddying caused by blocks set in the bed and by rapid flow down a sloping apron followed by a hydraulic jump or blocks.
The level downstream of the gate is not determined by the position of the gate but by the position of the next gate downstream and this may mean that the flow from the gate is submerged. This interferes with the normal function of the dissipation devices but somehow or other the surplus energy will be dissipated if only in uncontrolled eddying. This means that the depth profile of a river is determined by the positions of the gates acting together each controlling the upstream depth. Of course changes take place slowly and there is no immediate response to changes of gate positions as would occur in a pipe following valve adjustment.
A section through a gate followed by blocks in the bed is shown in figure 10-21. The gate is shown in the partly open position and it could clearly operate at any position between nearly closed and fully open. Normally the functional range of gate positions is limited by the need to produce rapid flow after the gate.
We cannot predict the relationship between gap under the gate, the steady upstream depth and the flow but rectangular gates with no side contractions will all have the same relationship.
Experimental data on the flow through a sluice gate is available and it is usually stored by the use of a coefficient of discharge. The coefficient is defined through an expression like that for a sharp-edged orifice. It is :-
, where is the width of the gate, is the height under the gate, is the depth before the gate and is a coefficient of discharge. This is clearly a rational expression just like that for the orifice. It is the product of a velocity and an area, the area being that under the gate and the velocity being derived from a measurable height. It is not the outcome of an attempt to predict the flow; it is simply a practical basis for a coefficient. The value of for practical sized gates depends on the ratio and, if the discharge from the gate is submerged, on the ratio . Typical values for , for free discharge, range from about 0.4 for at low values of to about 0.6 for high values such as 10.