River Leen: Mitigation for Water Vole and White Clawed Crayfish

Background

To the north of the City of Nottingham subsidence from former deep coal mining lowered the bed of the River Leen. This caused localised flooding of a former mill that had been converted into a residential property. As a result International Mining Consultants Ltd commissioned BSG Ecology to provide ecological advice on how best to re-grade the river whilst protecting and maintaining its ecological value.

BSG Ecology’s role in the project

BSG Ecology undertook a variety of ecological surveys and identified the presence of strong populations of water vole (Arvicola amphibius) and native freshwater crayfish (Austropotamobius pallipes). To address the impacts on water vole and freshwater crayfish and to enhance their riverside habitats in the long term, BSG Ecology designed and help implement a series of mitigation measures.

Water vole

Water VoleMitigation measures for water vole included the excavation of a winding new ditch adjacent to the river to create new habitat for water vole ensuring no net loss of habitat during construction and a net gain post-construction. Water voles were subsequently trapped out from the stretch of river affected by the works and released into the new ditch habitat. To enable the ditch habitat to be used as a receptor site within a short timescale, BSG Ecology worked with the site contractor to devise a method of excavation that would create the necessary ditch profile yet retain the marshy and wet grassland habitat along the banks of the new ditches, as cover and feeding material for water voles. BSG Ecology subsequently undertook the trapping out of water voles and supervised the removal of bankside and in-channel habitat from within the river.

Rather than simply dispose of marginal in-channel vegetation the lowering of the river bed and removal of vegetation was planned such that any vegetation removed from one section downstream was lifted and immediately replaced on a cut shelf in the previous section of re-profiled river channel. This ensured that there was minimal loss of in-channel marginal vegetation (mainly sedge beds) for subsequent recolonization overall and that there was instant mature habitat cover and foraging resources provided in the lowered river channel for water voles to recolonize.

Freshwater crayfish

Crayfish were trapped out of the affected river in advance of the engineering works and released into other parts of the river system. During the trapping out and construction process large stones and smaller rocky material on the bed of the river were retained and returned to the channel once the river bed had been lowered. Larger rocks and stones were placed so as to create riffles and in-channel habitat for crayfish.

Outcome

Following completion of the works, we monitored the local water vole population to enable an assessment of the success of the project to be made. After just one season it was clear that water voles had remained in the new ditch habitats and had re-established themselves within the length of affected river. Freshwater crayfish were also confirmed to be present.

 

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