Salix were commissioned to restore part of the river Mimram whilst protecting the local population of water voles and increasing the biodiversity
Salix were commissioned to restore part of the River Mimram as part of Affinity Water’s AMP6 NEP framework. A concrete weir had impounded a section of the watercourse, causing sediment deposition which was subsequently smothering the gravels and damaging the local ecosystem. Water vole habitat restoration was a key driver for this project.
The River Mimram in Hertfordshire is one of a number of tributaries of the Upper Lea. Its historic source is to the north of the village of Whitwell.
From there it flows south, passing the villages of Codicote, Welwyn, Digswell, Tewin, and Hertingfordbury, before joining the River Lea in Hertford.
The river still supports a commercial watercress industry that has been running for almost 200 years, and there are several historic mills dotted along the river’s length.
Physical barriers impacting wildlife
The reach at Tewin Water House was a 400m channel that was ponded behind a concrete weir.
The weir was originally built in the late 1700’s when Tewin Water House park and gardens were landscaped by Humphry Repton.
The weir was put in to create a small ‘Broadwater’ along the side of the house and the river was significantly widened.
Overshading from heavy tree cover had limited marginal and submerged macrophyte growth.
The channel was too wide for the flow regime and this, combined with the impounding impact of the weir, meant that the flow velocities very low and the gravel bed was smothered in fine sediment.
The Salix/Arup design focused on removal of the weir and creating a graded bed that would allow all fish the migrate through the reach for the first time since impoundment.
Selective tree thinning allowed more light to access the channel margins and the stream bed to allow marginal and submerged macrophytes to thrive.
Channel narrowing methods were employed in order to concentrate the base flows and allow the river to flush through the fine sediment that have previously smothered the gravel bed.
Salix had to work around existing water vole populations, which had dramatically increased after the tree thinning, as some natural marginal vegetation had become established in the less shaded areas.
Working with consultant Ecologist Julia Massey we developed a technique of a water vole gutter, comprising pre-established coir rolls set out in the channel to achieve the desired channel narrowing, whilst allowing the existing water vole population an open water channel in front of their burrows.
Your staff are always friendly, knowledgeable and helpful and we have always been pleased with the service and the quality of the wetland plants and planting services. Technical knowledge on erosion control is also excellent. Keep up the good work!Living Water Ecosystems Ltd
HydraCX gave an immediate impression the embankment was grassed, making it more visually pleasing to our travelling customers. The rapidity of grass growth was exceptional and it added additional strength to the slope surface.Network Rail
Excellent overall performance – very approachable and offered a good sustainable solutions to rectify a significant erosion scour to the river foreshore without comprising the existing flood defence. Carried out work to agreed timescale and budget. Certainly would value their future input.Environment Agency, Leeds
Atkins has worked with Salix on a number of projects, collaborating on design concept, trialling and implementation. We have found their experience valuable in delivering cost effective and environmentally functional solutions.Atkins
We used HydraCX for the first time on a Network Rail project in Chipping Campden. We wanted to establish vegetation growth as quickly as possible on a slope with poor ground conditions without the need for importing topsoil due to possession time constraints. HydraCX provided an excellent growing medium for rapid germination.Carillion Rail
We have worked with Salix on a number of projects throughout East Anglia over the last 6 years, and we have always found them to be highly professional and competent. We have used their services as suppliers of quality wetland plants, with a traceable provenance, and as contractors to both supply and plant the wetland plants. In addition we have always relied on the technical advice and knowledge of David Holland, in particular, to help answer queries from our clients regarding wetland planting schemes we have been involved in, this is an added value part of their service, for which we are grateful.UPM Tilhill
We invited Salix to re-landscape the watercourse through a design and build project. The works to the watercourse were completed within two weeks of starting on site and, despite a number of prolonged heavy rain events both during and after the works, the pre-established coir rolls and pallets all began putting on new growth within a matter of days providing the watercourse with instant channel vegetation and protection to the toe of the banks. The VMax reinforcement used to stabilise the slopes has also prevented any erosion to the sandy soil beneath and has allowed the rapid establishment of grass which wouldn’t have otherwise been possible.South Ribble Council