Project Outline:

A Section 10 Report issued on 26th March 2018 showed that Pysgodlyn Mawr did not meet the recommended safety standards for a Category B dam. Consequently, Natural Resource Wales (NRW) contracted Walters to carry out the necessary works addressing the inadequacies of the structure so that it meets the requirements of the Reservoir Act 1975.

Pysgodlyn Mawr Reservoir is within Hensol Forest, situated to the east of the village of Welsh St Donats in the Vale of Glamorgan.

Pysgodlyn Mawr is a Category B dam, with an estimated surface area of 20,000m2 and a volume of 17,493m3 when the reservoir is filled. The reservoir is retained by a central concrete gravity section and two adjoining earth filled embankments. The embankments are approximately 66m in combined length, 1m high, 1m wide with 1 in 1 slopes.

The reservoir and surrounding forest were designated a Site of Special Scientific Interest (SSSI) status as of 1972. The project objective was to bring the dam structure up to the required standard for a Category B reservoir as defined in the Floods & Reservoir Safety 4th Edition

Working closely and collaboratively with NRW under a PSC ECI Contract, Walters provided valuable practicable and buildability advice to assist with the design. Utilising our experienced professionals in the field, we were able to provide engineering solutions to technical challengers which proved to contribute to the project’s success. Holding regular client ECI meetings to offer advice and suggestions we, as a collaborative team, were to provide a final Target Cost Setting which was within their budget to move forward to the construction phase of the project.

The project can be summarised in the following elements:

• Concrete dam extension – Raised to the minimum freeboard height
• Earth embankments - Where the embankment was of insufficient height, they were reformed and profiled with clay/soil
• Draw-off facility - an upstream penstock control fitted in a precast headwall.
• Erosion Control - pre-cast concrete, open lattice, interlocking blocks

Project Delivery: -

It was imperative that the public interface was proactively managed and controlled throughout. The site was an extremely popular route for members of the public whilst also being a bridal way for horses - a letter written by a local horse rider thanking the site team for their efforts to reduce noise and plant movement whilst horses passed was received during the construction phase

The project was heavily reliant on the effective use of temporary works to create a dry working area to allow safe construction whilst also ensuring equilibrium of flow downstream. Natural water flows were maintained and passed the construction works and on downstream. Two super-silent suction pumps were used in conjunction to control any water ingress from the dam wall. A specific emergency incident plan was implemented to ensure the safety of the operatives working in the reservoir that involved continual monitoring of weather and water levels.

Added Value and Client Partnerships: -

During construction the reservoir bed level was found to be deeper than shown on the surveys, this led to alterations being required to the temporary works for the second phase of the project. Walters, utilising their extensive knowledge and experience in working in the water environment and utilising existing connections to arrange and install a different temporary work solution with out impacting on programme including the re-issue of the design.

Walters implemented an extensive water management strategy to prevent siltation of the water down stream drawing on our extensive experience and proven success in water management across years of construction projects. This proved especially important at Pysgodlyn as downstream from the works area was a local farm who relied on income from their fishing experiences offered at their pond. Any silt entering the pond could impact on their business and livelihood. Walters methodology and understanding of the flow and filtration methods available enabled the construction to be completed without impacting the downstream pond for which the farmer expressed his gratitude and sent a letter thanking the site team for their efforts throughout the project.  

The works to be undertaken were governed by the reservoirs act and as such required the input from the Qualified Civil Engineer. Following initial meetings Walters developed an in-depth emergency response plan to ensure that at no point during construction there was increased risk of flooding. This required the temporary works and construction sequence to be in a position to handle extreme weather conditions up to 1:1000 year flood event. Walters reviewed historic data and devised an extensively planned construction emergency response plan detailing all the required information to the satisfaction of the QCE enabling the construction phase to commence and worked closely with the QCE and his supervisors during construction to ensure the works met all the requirements of the Reservoirs Act. Following completion the QCE attended site and all works were accepted and signed off without defects, a testament to the attention to detail and hard work of the site delivery team.