Hydraulic erosion incident during discontinuance works triggered by unexpected rainfall events

Basic details

Publish date

27 November 2025

Case ID#

3343

Title

Hydraulic erosion incident during discontinuance works triggered by unexpected rainfall events

Nation

Wales

Regulator reference no.

76

Legal status

Statutory

Reservoir type

Impounding

Reservoir capacity

100,000 - 499,999m3

Year of construction

1850 - 1899

Main construction type

Earth fill embankment

Dam height

15 - 29.99 metres

Dam flood category

A

Hazard class

High-risk reservoir

Reservoir use

  • Water supply

Owner type

Limited company

Incident details

Date & Time of incident

30 September 2024 - 12:00

Date incident closed

25 October 2025

Observations that caused the incident to be declared

  • Water flowing outside of engineered channels

Describe the incident

During planned discontinuance works, an unexpected rainfall event led to elevated water levels and flow through a newly constructed channel. This resulted in erosion at the downstream toe where temporary protective measures had been removed to facilitate construction. The incident was managed promptly through pre-established site controls, including valve operations and temporary pumping. No uncontrolled release of water occurred, and subsequent remedial actions were implemented to reinforce exposed areas.

Supporting photos

Causes and impacts

Natural processes which initiated or contributed to the incident

  • Heavy/persistent rain (no flood)

What were the main contributing factors to the incident occurring?

Dam factors

  • Other dam factors (describe below)

External factors

  • Other external factors (describe below)

Shortcomings

  • Process or procedural shortcoming

What was the root cause of the incident?

Removal of erosion protection during construction and subsequent rainfall.

Impacts on the reservoir

  • External erosion

Supporting photos

Supporting contributions and studies

Describe any human factors which influenced the incident

Quick contractor response, strong collaboration, MIOS deadline pressure, environmental constraints

Describe any instrumentation at the reservoir and how this was used in warning of the incident or providing monitoring during the incident

Instrumentation removed or obsolete; no active monitoring during incident

Was instrumentation effective leading up to and during the incident?

Not Applicable

Describe any assistance by external parties and impacts on the downstream population

SE, QCE, Enforcement Authority notified; no emergency services contacted; no downstream impact

Summary of studies or investigations undertaken

No formal studies; lessons learned exercise conducted

Supporting photos

What are the lessons learnt from the incident

Lesson 1

  • General design and construction
  • Human factors
  • Records and studies
  • Regulatory
  • Structural

Importance of flood contingency planning during construction.

Better engagement with regulators to avoid delays.

Earlier completion of critical works could reduce risk exposure.

Lesson 2

Lesson 3

Lesson 4

Closing comments

Supporting photos