HOW A CANAL LOCK WORKS
THIS IS HOW A TRIPLE STAIR CASE LOCK WORKS
Below is the procedure for for a boat which has just arrived at the lock following another boat that has just gone down.
TAKE NOTE OF THE PROCEDURE WHEN ENCOUNTERING THE OTHER THREE OPTIONS (GOING DOWN AFTER A BOAT HAS COME UPSTREAM, GOING UP WHEN A BOAT HAS JUST COME DOWN AND GOING UP WHEN A BOAT HAS JUST COME UP);EACH REQUIRE LOCK LEVELS TO BE ADJUSTED BEFORE ENTRY.
THE BASIC RULE IS THAT:
= WHEN GOING DOWN, THE TOP LOCK IS FULL AND THE REST ARE EMPTY.
= WHEN GOING UP, THE BOTTOM LOCK IS EMPTY AND THE REST ARE FULL.
There are a few exceptions to this, so read the notice board carefully.
Also make sure that each lock is not over empted -
A good and reliable supply of water is essential for a canal to operate. It is important that leakage and waste is kept to a minimum. The greatest demand for water is that used to operate the locks.
For an equal drop or lift height:-
‘COMMON’ LOCKS’ …boats all going in the same direction: the water lost per boat is one lock fill.
…for boats going up and down alternatively. the water lost per boat is half a lock fill.
(This is why boats give way to other boats coming in the opposite direction at a lock. It’s also quicker.)
A DOUBLE STAIRCASE LOCK: boats all going in the same direction: the water lost per boat is one lock fill.
… boats going up and down alternatively: the water lost per boat is one lock fill.
A TRIPLE STAIRCASE LOCK: boats all going in the same direction: the water lost per boat is one lock fill.
… boats going up and down alternatively. the water lost is 1.5 of a lock fill.
A FOUR STEP STAIRCASE: boats all going sequentially in the same direction: the water lost is one lock fill.
… boats going up and down alternatively. the water lost per boat is two locks fill.
All locks in a canal stretch should have the same drop height to avoid water wastage.
The smaller the drop height the less water is used; see pictures above. So in theory a canal should have locks with small drops. This would be expensive to build and take extra time to traverse; so a compromise was reached with most locks having around a 7.5 foot drop.
Locks 1,2 & 3 on the Droitwich Junction Canal have about a 15 foot deep drop. They have side ponds halfway down so that (if used) only a 7.5 ft height of water is passed down to the next lock. This supplies the the rest of the locks on the Junction canal the right amount of water for boats to pass; having themselves only 7.5 ft drops. If the side ponds are not used 7.5 ft lumps of water will be wasted over the spill weirs.
At most staircase locks in the summer there is a queue of boats waiting to go up or down. Here progress can be speeded up by controlling a batch of boats in the longest queue to go through the locks occupying each lock step as soon as it becomes free. This also saves water.
A staircase lock is when locks are joined together by making the top gate of one lock the bottom gate of the next lock. These are used at a steep incline.
They costs less to build and maintain than a series of two gate locks for the same lift height.
Takes much longer to pass through unless there is a team of lock keepers making sure that a batch of boats are following one another through the staircase.
And they use more water.
The largest staircase lock is the Neptune locks on the Caledonian Canal -
On the Droitwich Junction Canal we have a two staircase lock -
For drop ‘H’, volume ‘V’ of water is sent to the next stage or to the sea
For same drop ‘H’, only one third of the volume V/3 is sent to the next stage or the sea
A Typical lock
A lock with a side pond, illustrating that only half the water is going to the next stage or to the sea.
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