Project background

Thorndell Viaduct, north of Arundel in West Sussex, needed replacing. This required a large work site around the viaduct to construct the new viaduct alongside, which would then be slid into position. A network of ditches were temporarily diverted to enable the work compound to be built over the top.

Previous surveys found Water Voles along both banks scattered across all ditches around the viaduct and the proposed access. ADAS were commissioned to undertake the necessary surveys and mitigation to ensure works could take place without harming wildlife and to apply for any licenses that would be needed.

Actions taken

Exclusion fencing

A water vole conservation license was applied for and granted by Natural England. The mitigation included the erection of water vole exclusion fencing at key points along the ditches on either side of the viaduct. The fencing design had to account for both the size of the site and the hydrology of the site, which had water levels that fluctuated greatly. Due to the shape of the ditch network, rather than encapsulating the entire site, 10m runs of fencing were installed in three locations, with mesh used within the watercourse to allow flow. This innovative method was accepted by Natural England and allowed for cost savings to the client while achieving the best outcome for wildlife. The water level fluctuations and steep banks also require a bespoke trap design, whereby the traps were placed on floating platforms.


Once the viaduct works were complete, the ditches were re-instated and planted with species of value to Water Voles. Some areas previously unsuitable for water voles were re-profiled and planted to become suitable post-works. Selective canopy thinning was also undertaken to improve light penetration to the ground to aid bank planting growth. Annual monitoring will be undertaken to ensure conservation of the population of water voles.

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Viaduct raising