Silverton Mill

Case Study

Silverton Mill, Devon

• Supplementary site investigation
• Remediation and civil engineering design
• Negotiation and agreement with regulators
• Bat mitigation measures
• Conventional and explosive demolition of above ground structures
• Diversion of the River Culm at Silverton Mills enabling excavation of
• hydrocarbon contaminated soils, demolition of the culvert walls and
• reconstruction of clay lined river banks
• Mill race culvert demolition
• Slab removal and on-site crushing
• Excavation, waste sentencing and off-site disposal of contaminated soils
• Import of fill materials
• River channel reconstruction and landscaping

The former paper mill site had a heavy industrial legacy dating back to the 1800’s, which had resulted in a

harsh industrial landscape and negative visual impact.  In addition to the adverse effect the site had on the local ecology, it was contaminated with significant petroleum hydrocarbons.

The contaminants were present alongside and below the culverted watercourse. Material selection was vital to the longevity of the river design. This included the use of British Waterways approved blue lias clay, a 200mm thick layer formed of recycled no-fine aggregates generated from the demolition process, and carefully selected quarried limestone material ranging in size from cobbles to boulders weighing several tonnes.

Petroleum hydrocarbon impacted soils were excavated from the areas adjacent to the former culvert structures.

Ecological surveys undertaken prior to the commencement of the works, identified maternity roosts for Daubenton’s bats within a small section of culvert and Brown Long-Eared bats within the mill structure itself.  Whilst sensitive species such as reptiles and aquatic animals were not identified in significant numbers, care was required to avoid any potential negative impacts on all wildlife throughout the works.

The design of the newly created river channel required an allowance for natural freedom for the river to find a stable regime. We promoted the construction of a flood defence embankment to protect lower ground and retained listed buildings on the downstream bank on the western edge.

The river base was constructed to natural width with gentle sloping banks to create a significantly widened flood stage channel. The over widening of the channel at flood levels was designed to reduce velocities and hence erosion pressure.

Using a combination of historic analysis and the Environment Agency’s flood data we ensured that adequate water storage was built into the new river and the adjacent floodplain, thereby reducing the risk of flooding for adjacent communities and with additional benefits for local wildlife.

The transformation of the large culverted section of the River Culm into a diverse and attractive river corridor was necessary to improve flood defences; as part of this the range of river and wildlife habitats introduced as part of the project also helped to enrich biodiversity.

Once the remediation works were complete, the former culvert was removed and the new naturalised river channel created. This included a groundwater base flow alleviation system, rock armour flow protection and restoration of ‘natural’ embankments.

We maintained the existing habitats and restored or improved the diversity of habitats to increase the range of flora and fauna indigenous to the area.