Pentre Flood Alleviation Scheme

Scheme Background

Pentre was impacted by flooding on five separate occasions during 2020. The largest of the four 2020 flood events which occurred between the 15 and 16^th  of February 2020 was a result of an extreme rainfall event, designated by the Met Office as ‘Storm Dennis’. The impact of this event at Pentre resulted in internal flooding to 159 residential properties, 10 commercial properties and extensive flooding to the highway.

Pentre 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 highest 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 within Pentre is broadly associated with the four primary ordinary watercourses which drain the highlands in the east and discharge into the Rhondda Fawr River. These watercourses are heavily culverted beneath Pentre’s urbanised area, with flooding primarily being sourced from culvert inlets.

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

• Survey, jetting, and cleansing operations to an estimated 3.2km of ordinary watercourse and 5.5km of surface water drainage network length within Pentre.

• The installation of an overflow arrangement from the highway drainage system in Lewis Street to the Dwr Cymru Welsh Water storm overflow network to increase the capacity of the highway drainage infrastructure.

• Upgrading an ordinary watercourse manhole that overflows into a nearby pumping station at Volunteer Street, maximising the capacity of the existing infrastructure and providing resilience to the pumping station.

• The delivery of a flood routing scheme at Pentre Park, targeting an upgrade to an ordinary watercourse culvert, through the installation of a large catchpit manhole, to reduce the risk of debris entering the culvert.

• The delivery of an overflow structure next to Lewis Street and Pleasant Street, comprising of an over-land flow control route which aims to mitigate flows generated by ordinary watercourse manholes within Lower Pentre. 

• Upgrades to the Pentre Road inlet following damages caused during Storm Dennis include the delivery of new headwall and inlet arrangements, upper debris screen and platform and the installation of an overland flow structure to provide additional capacity and reduce the risk of blockage to the culvert.

• Installation of remote telemetry monitoring devices at key culvert structures.

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 Pentre Flood Alleviation Scheme aims to develop and implement a program of flood alleviation measures in accordance with Welsh Government Flood and Coastal Erosion Risk Management Business Case Guidance. This initiative is aligned with the principles of the Well-being of Future Generations (Wales Act), focusing on reducing the risk of surface water and ordinary watercourse flooding through the implementation of sustainable long-term alleviation measures.

The Council, in collaboration with appointed consultants RPS, has conducted extensive screening and analysis of various options to address the flood risk in Pentre. This process has resulted in the identification of a preferred option for further development. Detailed information on the preferred option is available for review online through a virtual consultation space here.

This preferred option was taken to public consultation which ran from 19^th June 2023 to 17^th July 2023 and included two in person events to discuss the scheme on the 29^th and 30^th June 2023. Detailed information on the public consultation is available for review online here.

The preferred option includes the following:

• Overland Flood Routing in the Upper Catchment through bunds/channels to direct flow to the inlet of a new culverted watercourse network at St Stephens Network inlet location.
• Flow Diversion – The installation of a new culvert is proposed to take all flow from 3 of the previous culverted watercourse network inlets and reduce pressure on the existing network.
• Upgrade of culverts at Pentre Road and St. Stephen’s Avenue storm pipe
• Natural Flood Management (NFM) in the Upper Catchment,
• Sustainable Urban Drainage Systems (SUDs) in the Lower Catchment

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.