Spillway defect / rutting identified following heavy rainfall

Basic details

Publish date

18 February 2026

Case ID#

3361

Title

Spillway defect / rutting identified following heavy rainfall

Nation

Wales

Regulator reference no.

81

Legal status

Statutory

Reservoir type

Impounding

Reservoir capacity

100,000 - 499,999m3

Year of construction

< 1800

Main construction type

Earth fill embankment

Dam height

5 - 9.99 metres

Dam flood category

A

Hazard class

High-risk reservoir

Reservoir use

  • Recreation or general amenity

Owner type

Public body

Incident details

Date & Time of incident

12 March 2026 - 12:00

Date incident closed

12 May 2026

Observations that caused the incident to be declared

  • Slope or face deformation (slippage, cracking, slumps, mounds, depressions)

Describe the incident

Rutting of the grassed area along the spillway footpath. Voids formed behind the spillway bank walls. Missing blockwork identified on the spillway walls during the S12 inspection.

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

  • Deterioration of materials
  • Instability
  • Spillway or overflow - blockage

External factors

  • None

Shortcomings

  • Maintenance shortcoming
  • Surveillance shortcoming

What was the root cause of the incident?

Very intense rainfall combined with missing/deteriorated blockwork, which allowed water to leave the spillway and erode soil behind the walls. Earlier identification limited escalation, but post‑storm inspections could further mitigate future incidents.

Impacts on the reservoir

  • None - near miss
  • Other (describe below)

Supporting photos

Supporting contributions and studies

Describe any human factors which influenced the incident

Contributing: None identified. 

Mitigating: Early detection during the S12 inspection. Immediate infilling of voids with stone material.

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

No relevant instrumentation was present or used during the incident. (N/A recorded for instrumentation usage)

Was instrumentation effective leading up to and during the incident?

Not Applicable

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

No emergency services were alerted. No downstream population impacts or communications were required.

Summary of studies or investigations undertaken

Defects identified during the S12 inspection. Review of recent intense rainfall events that likely contributed to rutting. Analysis concluded missing blockwork may have allowed water to escape the spillway channel and cause soil washout.

Supporting photos

What are the lessons learnt from the incident

Lesson 1

Greater emphasis required on scheduling inspections after periods of intense rainfall. Maintenance shortcomings identified (missing blockwork). Surveillance shortcomings identified (issue found during routine inspection rather than targeted post‑storm checks).

Lesson 2

Lesson 3

Lesson 4

Closing comments

Supporting photos