Historic palace lake dredged for first time in 100 years
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Photo Credit: Blenheim Palace
Civil and environmental engineering company Land & Water is on track to complete dredging work at Blenheim Palace’s Queen Pool in Oxfordshire, UK. 70% of the lake was at a depth of 30cm or less, prior to dredging, and is currently being restored to its original depth of 2m.
Having commenced in May 2022, the work will remove 300,000 m3 of silt, which will then be re-used to create a landform over 16 hectares in the parkland of Blenheim. This land will eventually become grassland. The nine-month dredging campaign has a budget of £5 million and is set for a 2023 completion.
Land & Water won the Blenheim Palace contract in 2019 after competing against three other dredging companies. Charlie Oakes, Land & Water project manager told CEDA Industry News, “The Blenheim project has been an incredible undertaking and we have valued the chance to work on such an iconic landscape.”
The British company had previously worked at Blenheim in June 2020 to renovate the Grand Cascade Apron, a man-made waterfall forming part of a complex dam structure that connects water from the River Glyme to the Blenheim Palace lakes. During this project, the protective walls of the cascade which funnel water into the dam and control its flow were restored.
World Heritage Site
Blenheim Palace is a designated UNESCO World Heritage Site. Additionally, the Queen Pool is a Site of Special Scientific Interest (SSSI) and is also used as a fishery, meaning that the mitigation of any potential environmental impacts of dredging works is of high importance.
Kevin Kirkland, managing director at Land & Water, said: “We will be using HVO (hydrotreated vegetable oil - sometimes known as renewable diesel) fuel throughout the project which is up to 90+% net carbon neutral which will mean we are significantly reducing the environmental impacts of our works.”
Dredging is vital to help ensure the long-term health of the lake, surrounding parkland and waterways and the biodiversity which is dependent on Blenheim’s ecosystem. The removal of silt will change the lake’s ecosystem from a eutrophic to a low-nutrient ecosystem. Eutrophication is a process where a body of water becomes progressively enriched with nutrients such as nitrogen and phosphorus, which result in excessive plant and algae growth. When this occurs in lakes it commonly results in reduced ecological specialisation which can negatively impact the biodiversity living in that location. Dredging helps remove excess nutrients typically found at the bottom of lakes to help restore the environment to its natural low-nutrient state.
In the case of Blenheim, the build-up of excess nutrients is due to accumulations of silt. Typically, bodies of water accumulate around 1-2 cm a year of silt, but in the event of a severe storm, this can reach as high as 20cm. Removing around 1.7m of silt will help restore the lake to its original depth whilst also supporting the lake's ecosystem.
Dredging delayed by two years
The dredging of Queen Pool, which is Blenheim Palace’s largest restoration project to date, was initially planned to start in the spring/summer of 2020. However, the outbreak of the COVID-19 pandemic resulted in the project being deferred to Spring 2021.
In 2020, experts believed that there was a three-to-five-year window to act to prevent the waterway from drying up and becoming a wetland. The Blenheim Palace estate team recognised this urgency to manage the silt levels as crucial, but stated that there was no other option but to delay dredging as contractors would not have been able to carry out the work safely.
Dredging was again deferred in Spring 2021 due to the second wave of COVID-19. During this second delay, the management team used the time to critically assess all aspects of the project to help further improve the delivery and funding of the restoration work. Dredging works eventually commenced in May 2022.
Land & Water has been using a wet dredge technique, where excavators are set on a floating platform that gradually moves across the lake's surface and dredges the area. Hoppers then transport the silt from the platform to land. The manner of execution was designed to protect the lake bed as much as possible as part of Land & Water’s efforts to minimise the project's environmental impact.
The six new ‘Olympic Class’ hoppers were newly constructed this year, and are 20m in length with the capacity to hold up to 80 tons of material. Blenheim Palace launched a competition at the beginning of 2022 to officially name the hoppers with the winning entries being: Clementine, Winston, Swan, Mallard, Fair Rosamund, and Reg who Likes to Dredge.
The Oxfordshire UNESCO World Heritage Site remains open but Queen Pool may not always be accessible. The project site was visited by CEDA in the Summer of 2022.
Land & Water’s Oakes added: “We are thrilled to have reached the 100,000 m3 milestone. We are now a third of the way through our dredging journey at Blenheim Palace, with the final dredge anticipated to finish early next year.”
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