Two Tree Island is a changing edge where saltmarsh meets sea. It is also a place where the practical problems of coastal protection are hard to ignore. Waves and weather do not politely respect boundaries, and neither do the pressures of erosion.
Through Catchment to Coast we’re protecting places like this in ways that support nature, rather than pushing it out.
Catchment to Coast partners EXO Engineering designed new revetment works that have been installed at Two Tree Island. Their panels are designed to reduce wave energy while creating better conditions for marine life. We spoke to William Coulet, Managing Director at EXO, about exactly what was installed, why the detail matters and what success will look like over time.
A nature-inclusive engineer
William’s route into this work is a blend of environmental science and practical delivery. He studied environmental sciences at the University of East Anglia, and he’s worked on projects focused on dredging and reusing dredged material – the kind of work that quickly teaches you that “waste” and “resource” are often separated by design choices and budget decisions.
“The infrastructure has to do the fundamentals: it must be buildable, durable and maintainable.”
What draws William to nature-inclusive coastal engineering is not the romance of the coast, but the satisfaction of solving real problems with careful design, especially where ecology, engineering, regulation and public expectations all collide.
Breaking the waves
At Two Tree Island, the work includes wave attenuation panels, along the edge of the saltmarsh, designed to break and absorb wave energy. They have a distinctive hexagonal form and could almost be mistaken for a work of contemporary art.
The installation was completed in October 2025, and the project is now in a monitoring phase – moving from baseline observations to post-installation evidence.
Sea walls and hard coastal defences can create a less obvious problem: wave reflection. When waves hit a flat, hard surface, much of that energy can be reflected back into the water. Over time, reflected wave energy can contribute to erosion and put extra pressure on adjacent, softer habitats – including saltmarsh.
Wave attenuation is a way of reducing that reflection. In simple terms, the panels aim to disrupt the energy of incoming waves, breaking and absorbing it rather than allowing it to bounce straight back. It’s not the only factor at play in a coastline as dynamic as this one, but it’s a meaningful design choice, grounded in the physics of how waves interact with structures.
As William puts it, the infrastructure has to do the fundamentals: it must be buildable, durable and maintainable in an environment that is relentlessly hard on materials.
Designing a living edge
Traditional coastal defences often treat the shoreline as something to be held back, and the sea as something empty. Nature-inclusive design starts from a different assumption: marine habitats are shaped by structure, shelter and complexity – and engineered structures can contribute to that, if they are designed properly.
“Designing for marine life can be as specific as designing a bird box. If the openings are wrong, you don’t get the species you hoped for.”
At Two Tree Island, the revetment works include features that are intended to make the “grey” infrastructure more habitable:
- Shelter and refuge: small-scale features can provide protection from strong currents and predators.
- Shade and temperature moderation: shadowing can reduce surface temperature peaks, creating more stable conditions for colonisation.
- Texture and complexity: varied surfaces can give algae, invertebrates and other organisms places to attach and grow.
William offers an analogy that makes the point clearly: designing for marine life can be as specific as designing a bird box. If the openings are wrong, you don’t get the species you hoped for – even if your intentions were good. The detail determines what can actually use the habitat.
EXO Engineering’s role
EXO Engineering are contributing specialist design and manufacturing to Catchment to Coast. Their approach, as described by William, relies on working closely with marine biologists and environmental expertise to make sure the ecological intent of a design is realistic, not decorative.
That matters because nature-inclusive coastal infrastructure sits inside real constraints:
- regulations and permits
- the need to avoid unintended consequences (including invasive species risks)
- budgets and construction practicalities
The result is not a single, perfect solution, but a set of decisions that balance what nature needs with what people will accept, fund and maintain.
Encouraging signs
It’s still early: coastlines are complex systems and the evidence needs time to build. That said, one observation from the site is encouraging: vegetation has grown closer to the seawall than seen previously. The monitoring work will tell us whether that change holds, what is driving it and what it means for the wider habitat.
Success at Two Tree Island needs to mean more than “it looks good”. We’re continuing to look for evidence across two equally important outcomes: protection and ecology.
Success for coastal protection will mean that:
- the intervention performs as intended under wave and weather conditions,
- the structures remain robust and maintainable over time and
- the outcome is acceptable in a visible, community-facing place.
Success for the living edge will mean that there’s:
- an increase in the number of species,
- an increase in biomass and
- a healthy balance – no dominance by a single opportunistic species.
There is also a longer-horizon possibility that we’re interested in: bio-armouring. This is where organisms such as oysters or mussels may, over time, add protective value by stabilising surfaces and absorbing energy. That’s a hypothesis to test, not a claim to make today – but it is part of why long-term monitoring matters.
What happens next
The work at Two Tree Island is part of a wider set of activities in the lower catchment. The value is not only in what is built, but in what is learned – and in sharing that learning so other councils, engineers and researchers can make better decisions, faster.
Coastal protection does not have to mean stripping a place of life. With careful, evidence-led design, infrastructure can be functional, publicly acceptable and ecologically generous too. Two Tree Island is one place where Catchment to Coast is working to prove what that looks like in practice.
