Canal & River Locks
Page 7: Use of water
The main problem caused by locks is that, each time a lock goes through one fill-empty cycle, a lockful of water (tens or hundreds of thousands of gallons) is released to the lower pound. In over-simplistic terms: on a canal where only one boat will fit into a lock, a boat travelling from the summit pound to the lowest pound is accompanied on its journey by one 'personal' lockful of water. To prevent the canal from running dry, some method must be used to ensure that the water supply at the canal summit is constantly replenished at the rate that the water is being drained downwards. This is, of course much more of a problem on an artificial canal crossing a watershed than on a river navigation.
When planning a canal, the designer will attempt to build a summit level with a large reservoir, or one supplied by an artificial watercourse from a distant source, or one as long as possible (to act as its own reservoir) or which cuts across as many springs or rivers as possible (or all of these).
Where it is clear that natural supply will not be sufficient to replenish the summit level at the rate that water will be used (or to allow for unexpected periods of drought) the designer may plan for water to be back-pumped back up to the summit from lower down. Such remedies may of course be installed later, when poor planning becomes apparent, or when there is an unforeseeable increase in traffic or dearth of rain. On a smaller scale, some local pumping may be required at particular points (water is continually recycled through some locks on the Kennet and Avon canal).
A way of reducing the water used by a lock is to give it a reservoir whose level is intermediate between the upper and lower pounds. This reservoir can store the water drained from the upper 1/3rd of the lock as a boat descends, and release it to fill the lower 1/3rd next time a boat ascends. This saves the total amount of water lost downhill in each fill-empty cycle. On English canals, these reservoirs are called side ponds, and the gear controlling them is sometimes coloured red. This has given rise to the famous mnemonic "Red before white, you're alright; white before red, you're dead" (referring to the danger of incurring the wrath of the locky, rather than any inherent physical risk in the mechanisms themselves). On some flights of locks with short intermediate pounds, the pounds are extended sideways — in effect to provide a reservoir to ensure that the pound does not run dry (in case, for instance, the lock below leaks more than the lock above). These extended intermediate pounds are sometimes confused with side ponds.
Page 1 - Introduction & use of locks...Go
Page 2 - Basic construction and operation...Go
Page 3 - Details & Terminology...Go
Page 4 - Variations...Go
Page 5 - Illustrations...Go
Page 6 - History & Development...Go
Page 7 - Use of water...(Current page)
Page 8 - Alternatives...Go