2017
Tewkesbury, Gloucestershire
Severn Trent Water
CiM6 (JV of Costain & MHW Global)
Maccaferri Ltd
Flood Protection Upgrade at Mythe Water Treatment Works
Flood protection bund during construction in 2010/11
April 2012 – The Mythe STW
Existing structure before the repair works Flooding of the site during the repai
External side of the structure during construction – Nov 2016
Reno mattress placed over the top of the bund
View of the external side of the completed repair works – Feb 2017
Challenge
In the summer of 2007, extreme flooding from the River Severn and River Avon caused widespread disruption in Gloucestershire, severely impacting the Mythe Water Treatment Works near Tewkesbury. The flooding led to a major water supply failure, leaving over 150,000 homes without access to clean water. This highlighted the urgent need for effective flood defence solutions to protect critical infrastructure.
To mitigate future risk, a flood defence bund was constructed in 2010/11. The structure was designed as a double-sided geogrid-reinforced soil bund, providing an engineered barrier against rising floodwaters. However, in April 2012, Tewkesbury experienced another prolonged flood event lasting over six weeks. While the bund successfully prevented inundation of the treatment works, the extended exposure to floodwater and wind-driven wave action caused significant erosion and damage to the vegetated outer face.
This event revealed key vulnerabilities in the existing flood protection system, particularly the susceptibility of vegetated reinforced soil structures to long-duration hydraulic loading and wave erosion.
Solution
Following the 2012 flooding, a long-term flood defence improvement and erosion protection solution was required. CiM6 commissioned Maccaferri to design a robust upgrade focused on enhancing durability, stability, and resistance to environmental degradation.
The solution involved the installation of polymeric-coated woven gabion walls on both sides of the existing flood bund. These gabion retaining structures were designed to act as gravity walls, providing enhanced structural stability and superior erosion resistance compared to the original vegetated finish. The walls varied in height between 2 m and 3.5 m, including embedment, and extended לאורך 312 m on each side, tying seamlessly into existing concrete flood walls at both ends.
The gabion facing was carefully detailed to maintain a minimum buried depth of approximately 300 mm, ensuring long-term performance and protection against scour. In addition to improving resistance to hydraulic forces and wave action, the gabion system also provided protection against UV degradation of the underlying geogrid reinforcement.
To further strengthen the flood defence system, a 230 mm thick Reno Mattress was installed along the crest of the bund. This addition increased the freeboard above maximum water levels, significantly reducing the risk of overtopping during extreme flood events.
Comprehensive stability analysis was carried out by Maccaferri to verify the performance of the gabion walls under design loading conditions, ensuring compliance with engineering standards and long-term reliability.
The upgraded flood protection system at Mythe Water Treatment Works now delivers a durable, low-maintenance, and highly resilient solution. By combining gabion retaining walls and Reno Mattress technology, the project significantly improves resistance to erosion, wave action, and prolonged flooding, safeguarding critical water infrastructure for the future.
