When Carmarthenshire County Council (CCC) noticed increased seepage in its Victorian-built dam Trebeddrod, the council took immediate action, asking WSP to carry out a formal investigation. CCC needed to minimise the flood risk of Trebeddrod to the nearby village of Furnace and secure the long-term resilience of the ageing, earth-filled structure.
Completed in 1854, Trebeddrod has evolved into an important habitat for a variety of wildlife. In 2016, major works were carried out to improve the reservoir, which involved creating a new, larger capacity spillway and river training works to deal with extreme weather (Probable Maximum Flood) events. Our challenge would be to pinpoint its exact location of the seepage, to restore the reservoir’s watertightness.
A multi-disciplinary team response
Working with independent reservoir expert Andy Hughes - one of the pre-eminent professionals in this field - we proposed to CCC that it would be expedient, both in terms of time and cost, to use the very latest geophysical technology to further investigate the leaks.
More specifically, we suggested the use of an innovative technology developed by Willowstick for mapping water bodies. This technology works like an MRI scan – it uses electricity to generate a magnetic field, which is analysed to create a 3D model of a water body which can then be checked for anomalies.
In this way, Willowstock enabled us to survey the reservoir non-intrusively and to pinpoint any existing or potential seepage. The technology showed us, very clearly, that the seepage was potentially a significant risk to the stability of the dam, while a second, smaller seepage path could be fixed with a local grout injection.
We brought together a multi-disciplinary team to design and manage the remedial work required, including geotechnical engineers; environmental specialists; pipeline engineers; health and safety specialists; dam engineers; site supervisors; and ground surveyors. The project includes the repairs in the vicinity of the spillway and designing new drawdown facilities.
The multiple benefits of 3D modelling technology
The 3D modelling technology has allowed us to move quickly from design to delivery phase for the project. We have been able to define water flow paths in the reservoir, locate potential seepage, and create designs for improving the structure’s stability. There has been no need to tender for a standard, physical geotechnical investigation, saving the project time and money.
While the project has involved challenges, including logistical restrictions due to Covid and the discovery on site of invasive plant species (Japanese knotweed and Himalayan balsam), Trebeddrod is on its way to becoming a safe, resilient, and future-ready dam and reservoir.
Mike Woolgar, WSP, comments: “Highly innovative technology is enabling us to strengthen a Victorian dam so that it can continue to benefit wildlife, and manage flood risk, in a safe and secure way. Working with Carmarthenshire County Council, we are achieving this result at pace through a collaborative, team-based approach – drawing on experts from across WSP and beyond.”
