The sequence of the new breach on the Llangollen Canal is particularly concerning because the deterioration appears to have progressed from a relatively localised leak to a major structural failure over a period of hours, writes Alan Yates.
THE ENGINEERING QUESTION
Internal erosion of soil is not necessarily a linear process. A small initial flow can progressively remove material, enlarging the passage through which water escapes. As the passage increases in size, the flow and erosion can accelerate rapidly (exponentially!) , eventually undermining the surrounding structure.
This process, commonly known as piping, is a recognised mechanism of earth embankment failure.
The fact that a leak initially appears manageable does not mean that the underlying erosion is progressing slowly or that sufficient time remains to prevent a total collapse.
Questions that need answering
The culvert had apparently been presenting problems for approximately 15 months or more before the final failure.
The important questions are therefore straightforward:
∗ What was the nature of the original culvert obstruction identified in August.
∗ Why was the temporary pumping arrangement required for so long?
∗ What assessment of the culvert's structural condition was undertaken before pressure washing commenced?
∗ At what point was it recognised that the escaping water was undermining the canal bed and towpath?
∗ What measures were considered to isolate the affected section of canal once the culvert failed?
There is an important distinction between stopping visible leakage and stopping the internal erosion responsible for it.
A tarpaulin may temporarily reduce water flow, but it cannot restore soil already washed away beneath a canal bed or towpath.
The fundamental engineering question is whether the initial culvert failure could have been contained before progressive erosion developed into a full breach.
The precise cause and sequence of the structural failures will require investigation. However, the reported events provide another reminder that water escaping through an earth structure can change from a manageable leak to a catastrophic failure with alarming speed i.e hours, not weeks, or months as CRT seems to think.
Photo shows contractor fitting an emergency 'PortaDam' Thursday Evening 12th November, 2026.