Control of rivers.

Most rivers of useful size have been fitted with flow control devices to make them suitable for navigation and/or for extracting power. The basic object in each case is to increase the depth along the river either to give sufficient depth for boats or to impound water and so store potential energy in its new elevation to drive machinery.

 

In order to raise the level of a river, obstructions have to be placed in the bed at suitable intervals. The increase in depth upstream of each obstruction reduces the mean velocity for a given flow and that, in turn, means that the energy loss between the obstructions is much reduced compared with that in natural flow. The slope of the surface is then much less than the average bed slope and as a result the depth increases as the water approaches the obstruction so creating a difference in level at each obstruction. It is this difference in level which permits the operation of water wheels or water turbines to extract  the potential energy of the water before discharging the water into the  next section of the river. If the obstructions are fitted to permit navigation the water must still pass the obstruction and some means has to be found to dissipate the potential energy that has been accumulated if it is not to be turned into a high velocity flow that may damage the bed by scouring.

 

The design of the obstruction must take into account the normal seasonal variation of flow and of flood and drought conditions. During a drought the flow cannot be stopped completely at one obstruction and so deprive river users down stream of what flow may be possible. In time of flood it may be more important to protect centres of population than to reduce all the obstructions to a minimum but generally the design of the obstruction should either permit it effectively to be removed or to be bypassed.

 

Keeping in mind that many flow control devices have designs that may be 200 years old, and the difference in size of rivers, it is not surprising to find a wide variation in design for devices serving essentially the same purpose. Broadly for water-power a dam will be built and a discharge device such as a side channel with a sluice gate used to handle floods. For navigation a dam and a lock are essential although the dam may act as a weir to carry the normal flow when the flow is not carried intermittently by the locks. A side discharge and sluice may also be fitted. All the fittings have to be designed on the essentially simple physics given above. The sluice gate and the drum gate will be considered here as typical examples of such fittings.