Treorchy Phase 2 Flood Alleviation Scheme

Scheme Summary
Strategic Flood Risk Area	Upper Rhondda Fawr
Location	Treorchy
Properties Benefitting	Approximately 234 residential properties and 30 businesses
Type of scheme	Complex Flood Alleviation Scheme
Status	Awaiting funding for detailed design phase
Funding Source	Welsh Government FCERM Capital Grant

 

Scheme Background

Treorchy has been subject to several flooding events in recent years. The most significant incident occurred on the 15-16^th February 2020 following Storm Dennis, which resulted in internal flooding to 44 properties, including 4 commercial properties and flooding to the highway.

Treorchy is noted as an area of high surface water and ordinary watercourse flood risk based on Natural Resources Wales’s Flood Risk Assessment Wales (FRAW) maps and is identified as the 3rd most at risk community for ordinary watercourse and surface water flooding in Wales according to the Communities at Risk Register (CaRR).

The highest risk posed to people and properties in Treorchy is attributed to the network of ordinary watercourse which drain the steep highlands to the north of the town and discharge into the Rhondda Fawr River. These watercourses are heavily culverted beneath Treorchy’s urbanised area, with flooding primarily being sourced from culvert inlets and bank breaches.

Since Storm Dennis, the Council have completed various works to reduce the risk of flooding within Treorchy whilst the wider FAS program is delivered. These works include:

• Carried out survey, jetting and cleansing operations to an estimated 1673 meters of culverted ordinary watercourse network length within Treorchy.

• Undertaken clearance and upgrade works to the culvert inlet structures identified as sources of flooding following Storm Dennis.

• Installed remote telemetry monitoring devices at key culvert structures.

• Completed Treorchy Phase 1A Flood Alleviation Scheme in 2023 which was aimed at facilitating Culvert and Ordinary Watercourse Network improvements and upgrades within Treorchy, specifically the Nant Tyle Du watercourse. 

Proposed Scheme Objectives

• Reduce risk to life by reducing the number of people exposed to risk of flooding of significant depth and velocity.
• Reduce community disruption by reducing the amount of residential and commercial property exposed to flood risk.
• Result in no detriment to downstream receiving River/Watercourse
• Improve community resilience to flood events and climate change, promoting sustainability and wellbeing. Deliver a preferred option that works with natural processes and promotes green infrastructure.
• Enhance local biodiversity and support the resilience of ecosystem services.
• Improve the resilience of flood risk assets against climate change - Promoting accessibility and low maintenance requirements.
• Reduce the impact of climate change – mitigating the extent of the projects carbon footprint.
• Improve community wellbeing through the enhancement of local amenity.

Proposed Scheme Description

The Treorchy Phase 2 Flood Alleviation Scheme aims to develop and deliver a programme of flood alleviation measures in accordance with Welsh Government Flood and Coastal Erosion Risk Management business case guidance and the Future Generations of Wales Act to reduce the risk of surface water and ordinary watercourse flooding within Treorchy.

To date, the RCT appointed consultant, Redstart, has screened and analysed several options, undertaking a technical and economic appraisal.  This process has produced a preferred option for further development.

This preferred option was taken to public consultation which ran from 23^rd September to the 23^{rd of} October and included two in person events to discuss the scheme on the 10^th and 11^th October. Detailed information on the public consultation is available for review online here.

The preferred option includes the following:

• Natural flood management (NFM) techniques which to store and slow down the flow of water from the upper catchment.

• Upper catchment diversion and cemetery drainage improvements with the aims to tackle the surface water issues in the cemetery and divert the water from the upper catchment into a new attenuation basin.

• Construction of grassed channels around the cemetery to redirect the flow into the culverts, alleviating flooding within the cemetery.

• The creation of an attenuation basin to promote wildlife and biodiversity as well as acting as a flood alleviation measure for water storage.

• Urban drainage works within the urban area diverting flows through the town into the river Rhondda.

• Proposed culverts to create greater capacity in the network to mitigate flood risk. These proposed culverts will be larger and used to direct waterflows to the attenuation pond and Nant Orci.

• Existing culverts will remain online during construction and after scheme is completed, we will disuse the culverts which are obsolete and in disrepair.

The feedback gathered from all stakeholders during the consultation has been reviewed by the RCT Flood Risk Management Team and used to produce an Outline Business Case for submission to Welsh Government as part of an application for funding for the next stage which will be the presentation of the Full Business Case to Welsh Government and into the detailed design phase.