Eric Suesz
In the town of Sidmouth on the South West coast of England, Jacobs, a leading technical consultancy, not only designed a flood alleviation scheme that protects residents and properties but created an innovative, unique, impeccably sustainable, dual-use amenity for residents.
The Sidmouth area has a long history of flooding from sea, river and stormwater, with overland flowpaths draining to a low point directly in the town center. When Devon County Council commissioned Jacobs to produce a stormwater management plan, it showed that over 100 properties were at risk of flooding in a 100-year (1% AEP) storm event.
With its Regency history and architecture, Sidmouth, like many seaside towns in the area, comprises many historic buildings with shallow foundations built on weak soil. Extensive tunneling wasn’t an option and it remains a risky proposition to dig in the town center considering the age of the buildings.
Instead of focusing on the town center, Jacobs sought flood mitigation measures higher up in the catchment. To help benefit the local community as promptly as possible and deliver efficiently, Jacobs suggested a solution that could be accomplished within a tight timeframe.
Jacobs proposed capturing and channelling water above ground from nearby Station Road, utilizing linear drainage channels called crossdrains, with runoff being diverted via a swale to a flood storage area in the adjacent parkland called The Knowle Arena. The council supported this concept but were concerned about the storage area being unavailable to the community for long periods of the year.
Favoring Sustainable Drainage Systems (SuDS), Jacobs focused on creating an environmentally friendly proposal that would be tuned to the needs of nature while also creating something useful for the community. The team was inspired to turn the area into an amphitheater, providing a public space for local communities to visit. This would encourage more people to enjoy this idyllic, natural setting, while simultaneously enhancing the environment for birds, bees and insects by diversifying the flora beyond long grass and weeds.
Initially, Jacobs explored storing the water aboveground inside the amphitheater, but as the team considered other factors, they devised a novel approach to store the water directly beneath the amphitheater.
Putting pen to paper before putting pixels to work
Jacobs Senior Associate Director Paul Hargreaves then put pen to paper to consider initial design details. He already had a lot of interesting research about the historic usage of the parkland from Associate Director Landscape Architect Andy Craven-Webb, who worked alongside Jacobs Landscape Architects Kayleigh Burgess and Jennifer Roberts, helping to research how to fit the project seamlessly into the community.
First and foremost, the spot is a natural amphitheater. It’s been used for events during the Sidmouth Folk Festival since 1955. It was also the location of one of the first zoos in England. Going back still further, Landscape Architect Chris Wilkinson found inspiration via a similar amphitheater created in 1715 by Royal Gardener for Queen Anne, Charles Bridgeman, which still exists as Claremont Landscape Gardens, Esher, West London.
The challenge for the team was to try and find a viable engineering solution, while weaving together each of these possible origin stories and imagine the amphitheater in the present. Considering Jacobs’ SuDS-oriented approach, Hargreaves began by drawing something visually organic, quite literally a spiral-shaped drain. “I was inspired by a vortex, which forms when a plug is pulled in a kitchen sink,” says Hargreaves. “I thought this spiral shape could help tell the story of the water feature linking from above ground to below.”
An impeccably sustainable approach
Jacobs began by turning to Microdrainage to construct a 1D model to represent the flows passing along the swale and into the flood storage area. This was augmented by a 2D model (Network + Flood Flow) of the parkland to help them to fully understand the overland flow routes and demonstrate what would happen in the event of a blockage of the flow control chamber. The team created additional mathematical models to predict sub-surface flow rates, as well as greenfield runoff calculations to estimate the likely overland flows from the parkland itself.
Since this was such a unique undertaking with a strong focus on using recycled, upcycled and sustainable materials wherever possible, in accordance with SuDS best practices, great care was taken to anticipate every detail and solve site-specific challenges.
Slow swales
With so much potential flow during extreme weather events to account for, the team incorporated two check dams into their software models. These important SuDS design details often use rocks (known as “rip rap” in the industry) to help slow the flow at junction points and encourage sediment to settle into the ground before the water reaches the amphitheater. The team also strategically planted vegetation like long grasses on the sides of swales to act as brakes for the water, which is especially useful at bends in the swale.
Percolation destination
Once water reaches the amphitheater, it percolates through a drainage blanket and then two meters of 3,000 geo-cellular crates made from recycled materials that make up only 5% of their volume, leaving 95% available for water storage. “The crates are a highly efficient means of creating very large storage structures,” says Hargreaves. “The alternative would be building a reinforced concrete structure, which obviously would not be very sustainable.” They also placed a second drainage blanket beneath the crates to allow water to percolate at a constant rate across the entire base, as well as allowing groundwater to rise unimpeded into the tank when high.
Permeability
Importantly, while this design encourages water to percolate into the storage tanks, it also includes a self-lifting, spring-loaded, pop-up chamber cover that is beautifully designed and cleverly engineered – the cover can’t fall through or disappear in the night. It lets water in and air out, but it’s also designed to prevent the cover from becoming buoyant. In a worst-case scenario, it also allows water to pass between the surface and sub-surface seamlessly.
Autodesk interoperability
In addition to Innovyze apps, Jacobs worked around multiple shallow utilities by utilizing a combination of Autodesk Civil 3D and InfraWorks software to fit aspects of the project over and around the utilities and trees with Preservation Orders located around this arboretum. They even created a high-quality 3ds Max visualization to bring the scheme to life and get community buy-in.
A satisfying SuDS success story
This former patch of local parkland now contains a stunning outdoor amphitheater with an underground storage system that can hold floodwater for a one in 30-year storm event, and the tiered seating aboveground able to hold up to half a meter of water during a one in 100-year storm event. From a financial perspective, the work was projected to save the town around £10 million in flood damage and recovery costs.
But perhaps most important, this space doesn’t sit unused. The area is flush with visitors, who stop to read the information board and learn about the history of the local landscape, tracing their way around the spiral cobblestone walkway to the center, where they’ll find a poem that’s been laser cut into the stonework surrounding the central flow control chamber – an ode to one of the Monterey Pines that thrives in the parkland.
This incredibly successful project feels like a true benchmark for engineering sustainable water solutions, from the materials chosen to the approaches used, to the array of Autodesk software their engineers employed to create 1D, 2D and even 3D models to help plan, protect and persuade. “I hope this will be an inspiration, not only for drainage engineers,” says Hargreaves, “but also to people who are thinking of how public spaces should be planned and delivered in the future.”
Go deeper into the story
- Read about the project on Jacobs’ website.
- Read the local reaction to the project: “… a fantastic asset for the people of Sidmouth”.
- Watch the 9 videos in Jacobs’ YouTube playlist to discover more about the project. Start with this one about the central chamber. 👇