In the 12th century King Canute demonstrated to his courtiers and advisors that even the ruler of the land could not exercise command over the tides and by implication the other forces of nature. Water is ubiquitous and without it life is impossible but in the wrong place it can cause harm and or chaos.
So when it is necessary to repair a wall that is normally submerged beneath the murky waters of a canal how might you go about creating a suitable environment for the bricklayers ? One solution would be to simply drain that section of the canal. Well in the case I have in mind the next lock is 2.8 miles (4.5km) further along. A rough estimate of the amount of water that would have to be drained is 158,000 cubic metres, or to use that term beloved of journalists, the equivalent to 63 Olympic size swimming pools. Then there is a matter of all the fish and other life that calls the waters of the ‘cut’ ‘home’. Even the next nearest bridge where there may be ‘stop board’ slots is around half a mile away.
The logical solution is to build a dam so that only a small section needed to be drained. The traditional method would be to use steel sheet piling hammered into the bed of the canal. To install this you need a suitable size crane or excavator equipped with a piling head. After the work is finished the piles have to be extracted and then hope that the clay lining of the canal bed has not been so disturbed as to create a leak into the ground below.
At Lock 27 on the Grand Union Canal there is currently (February 2021) a ‘stoppage’ to allow the replacement of the bottom gates and to undertake repairs to the masonry.
The canal has been dammed using an innovative method consisting of some metal supports and a heavy duty tarpaulin.
A view of the rear side of the structure.
A series of closely spaced metal frames have been staked to the canal bed using standard scaffolding poles. The tarpaulin is anchored to the top of each frame and is presumably weighted at the bottom edge.
At the sides there is a significant wrap around.
This picture shows that the arrangement is working very well and that there is only a small amount of water present. I was also surprised at the small amount of debris, other than an old tyre waiting for an unwary propeller to pass by, most was natural materials rather than objects thrown in.
There are two pumps on the barge, only one of which was running, and judging from the sounds it was making it was not really doing any work.