Nant y Ffrwd Watercourse Flood Resilience Works
In early 2020, Storm Dennis caused extensive damage to the Nant y Ffrwd watercourse in Mountain Ash, South Wales. The waterway, a tributary of the Afon Cynon, flows through a series of culverted and open channel sections within a residential area.
The open channel sections are bounded by stone masonry river walls and, in some areas, lined with reinforced concrete to prevent erosion of the mobile riverbed material.
The engineered sections of this watercourse suffered scour erosion, leading to undermining and failure of sections of the river walls. This compromised the safety of adjacent residential dwellings and the highway. A sinkhole developed along one culverted section within the driveway of a residential property due to the localised loss of the culvert wall. Flood debris also caused a blockage in the downstream culvert, which subsequently overtopped and flooded nearby dwellings.
EDS was engaged by Rhondda Cynon Taf County Borough Council (RCTCBC) to provide reactive and planned design and construction services, delivering engineering solutions to secure the long-term flood resilience of this watercourse.
Key Challenges:
The works required careful planning and management of public interfaces while operating in multiple locations within a busy residential area on the side of a steep valley. The sequence of works was designed to minimise disruption and included consultation with residents, the development of traffic and pedestrian management measures to assure public safety, maintaining access for school traffic, and accommodating secure construction compounds with adequate space for plant, equipment, materials and welfare.
All works required online management of the existing flows within the watercourse through over-pumping, as spatial constraints prevented offline diversion. The reactive element of the works — clearing the culvert blockage, repairing the culvert wall, and underpinning the highway — had to be completed in the winter period. Works were therefore susceptible to weather delays once the capacity of the installed pumps was exceeded due to the flashy nature of the watercourse. A duty and standby pumping arrangement was implemented to provide redundancy and risk management during concreting activities and for pollution control.
Concrete pours were planned for early mornings to avoid peak school hours and refuse collection times, with a combination of boom and static line pumps used to deliver concrete to the works location. An underwater concrete additive accelerated curing time and reduced washout risk, allowing pumps to be switched off within the agreed working hours to meet noise restrictions and control pollution risks downstream.
All works within the culverted sections to repair the sinkhole, culvert wall and clear debris were completed by confined space trained operatives. Works followed a detailed method statement, risk assessment and rescue plan, with entry controlled by a permit-to-work system.
Solutions:
EDS collaborated with Pebble Engineering Ltd. to provide temporary and permanent works designs for each location. The most challenging section was a 40m stretch of watercourse adjacent to a residential property where the river wall had failed, undermining the property and an adjoining garage.
EDS produced an Options Appraisal Report that set out various short- and long-term solutions. These were evaluated against agreed criteria including cost, constructability and environmental impact. The preferred option involved relining the channel with a reinforced concrete ‘U’-shaped structure built within the confines of the existing channel. The hydraulic performance of the new structure was assessed, with measures incorporated into the detailed design to reduce flow velocities under design flood conditions.
EDS delivered a full turnkey solution, drawing on in-house engineering capability and structural design expertise provided in partnership with Pebble Engineering. Construction began in February 2022 and was completed in May 2022.
The works, part-funded by the Welsh Assembly Government (WAG), were visited by the First Minister of Wales, Mark Drakeford. EDS was praised for providing a resilient, long-term solution to protect this community.
In January 2023, our Principal Engineer, Nathan Walding, visited the site. His inspection confirmed that the works are performing well — a project the EDS team is proud to have delivered.
