A live crib wall of willow were used to protect a high pressure gas pipe that had become exposed due to significant river bed scour and bank erosion on a section of the Afon Elwy near St Asaph.
A high pressure gas pipe had become exposed due to significant river bed scour and bank erosion on a section of the Afon Elwy near St Asaph.
Attempts had been made previously to protect the pipeline with a large gabion structure, but this had actually accelerated the erosion problems directly over the pipe.
Salix were asked by the client to investigate and come up with a design that would prevent the pipeline from further damage and stabilise the river bed and banks longer term.
Cbec eco-engineering undertook a fluvial audit, topographical survey and hydraulic modelling to review the existing geomorphological processes and hydraulic conditions and to assess what interventions could be made.
Existing conditions modelling showed that a large gabion structure upstream of the pipe enforce the flow to make a sudden step to the left, and the shoulder of boulders (which the end gabions sit on) and the end gabions themselves act as a restriction to flow.
The last few gabions have an area of scour in behind them.
Existing conditions modelling showed two mechanisms for erosion on the right bank: at all flows there is an area of recirculation with appreciable reverse velocities on the right bank downstream of the gabions; at high flows there is a fast moving downstream jet of flow from over the top of the gabions and in behind them into an obvious area of erosion.
The gabions jut out into the flow and enforce a left turn, and as the gabions end, the flow separates with a shear layer, expands considerably, and causes the recirculation. The proposed design was to remove the gabion structure and re-grade the right bank.
As the flow in the design area is complex, being composed of a sudden flow direction change and expansion, design conditions modelling was necessary to provide a measure of the success of the design and to guide further design iteration.
The model also demonstrated that the gabion structure was splitting flood flows and creating secondary flow cells. The combination of secondary flows and the deflection of flow of the gabion has led to the bank erosion and bed scour to a depth of over 3m, exposing the pipe.
The large bed scour feature was now self sustaining. A second design iteration model was run to explore the impact of removing the gabions structure and partially filling in the bed scour pool.
Works were undertaken to remove the large 4m high gabion structure. Stone from the gabions was used to partially fill in the large scour pool that had exposed the gas pipe.
Removing this structure gave the channel more room and dissipated shear stress over the pipe. Works were undertaken using Salix’s Menzi Muck spider excavator due to the limited access and steep banks .
Directly over the pipe the area of eroded bank was build back up using an innovative large wood and live crib wall design.
This would ensure that the most robust bioengineering technique would provide structural integrity over the pipeline but also slow the flow and allow native bankside habitat to develop.
Construction of the live crib wall with large rootwards. The tree roots slow the flow locally in flood events and provide shade and shelter for aquatic life
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
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
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
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
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
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 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