Water vole mitigation for Thorndell Viaduct

We completed necessary surveys and mitigation to ensure viaduct raising could take place without harming wildlife and to apply for any licenses.

Client:

BAM Nuttell

Background

Thorndell Viaduct’s replacement required a large work site around the viaduct to construct the new viaduct alongside. A network of temporarily diverted ditches enabled 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 undertook surveys and mitigation measures to ensure work could take place without harming wildlife and apply for any required licenses.

Actions taken

Exclusion fencing

A water vole conservation license was applied for and granted by Natural England. Mitigation measures included erecting water vole exclusion fencing at key points along the ditches on either side of the viaduct. The fencing had to account for the site’s size and hydrology, which had water levels that fluctuated greatly. Because of 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.

Planting

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.