Legakker, narrow strips of peat bordered by water, unique in the Netherlands, is of Dutch cultural and ecological importance. As riparian zones, they support high levels of biodiversity and contribute to national water quality goals set by the EU Water Framework Directive. However, legakkers are eroding due to waves created by boat traffic and wind, the burrowing behavior of the invasive red swamp crayfish (Procambarus clarkii), and due to the lack of vegetated banks. Their characteristically steep banks (1:0–1:1) prevent vegetation from establishing, which would otherwise stabilize the bank through their roots. Due to the lack of this, the crayfish activity and waves, a positive feedback loop in which erosion accelerates, is present. Left without active intervention, these banks will continue to degrade, at both an ecological and cultural cost.
To understand the context of the problem, a literature review, expert interviews, and an analysis of conventional bank protection methods were conducted. This established that bank resilience depends primarily on slope angle, under and above water vegetation cover, and resistance to crayfish burrowing, criteria that existing solutions only partially address. Five design directions were developed and evaluated against twelve drafted design criteria. A two-phase crayfish experiment was carried out to determine what barriers or substrate additions could protect the bank against crayfish burrowing. On top of that, an on-shore embodiment has been prototyped, which has been placed in a water body.
The final design consists of a biodegradable construction of wooden piles, crossbeams, a geotextile liner, BESE element frames, and willow bundles, assembled to form a 1:3 sloped bank that is placed directly in front of a degrading legakker and filled with local dredge. BESE elements form a physical barrier against crayfish burrowing during the first two years, when vegetation is absent, but continue to support the construction for up to 20 years. The sloped geometry and willow bundles dampen incoming wave energy, while the biodegradable geotextile retains the dredge in place as vegetation establishes. After two years, vegetation will start to take over the structure and keep the bank stable. The design was evaluated against the same criteria previously compiled. Further iterations and long-term field monitoring are needed to assess vegetation succession, degradation rates, and crayfish resistance under real conditions.
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