If the tide was turning by the time the barge was in position to drop the rocks they could become grounded on their own load.
In 1983 a breakwater was built at Rhos-on-Sea to help defend the low lying Penrhyn Avenue from flooding when there was a high tide. [See Rhos section of Beach lowering in front of coastal structures (PDF)]
Rocks were carried in on a hopper barge which split along its length to drop the rocks on to the beach so the diggers could construct the breakwater with them.
If the tide was turning by the time the barge was in position to drop the rocks they could become grounded on their own load.
This required the diggers to clear as much material as possible so it could be floated off on the next high tide.
At which point tugs would try and liberate the barge.
On a good day (from a spectators perspective) a digger would get stuck too and the other diggers would need to pull it out of the sandy gloop.
