Laxey Weir Scour Control Summary
Executive Summary
The report describes a proposal to provide remedial works and scour control measures for Laxey Weir. The narrow, channelised cross section of the river at Laxey Fire Station has caused the adverse hydraulic factors of high stream velocity and energy losses, which have led to the onset of scour problems to the side retaining walls and toe of the weir.
Solutions for remedial works have been considered and discussed in this report.
When considering the options an important aspect of the review was the high velocities predicted by the model during storm events that will carry a heavy bedload with high energy. Potential solutions have been considered but most would be more vulnerable to bedload erosion, particularly gabion baskets and Reno mattresses.
The impact of the bedload being carried by the river can already be seen on the gabions placed to protect the eroded toe of the weir. The wire mesh has failed releasing its contents, which are being washed downstream. This report has concluded gabion style protection is not a preferred option.
Another option considered was the use of low friction materials on the river bed, such as precast or in-situ concrete slabs. There were a number of problems identified with this method:
☐ The hydraulic scour conditions are simply moved further downstream to the downstream edge of the slab.
☐ Fixing precast slabs to the river bed would be a difficult engineering solution in terms of providing anchor bolts as the river bed is not natural bedrock.
☐ Placing of in-situ concrete posed a high risk of contamination that could not be mitigated to a satisfactory level.
☐ Sealing of the joints between the slabs would also be a difficult task to avoid pollution when maintaining river flows and created a long term maintenance programme/cost to replace the sealant at a later date.
☐ The existing scour at the toe of the weir could also undermine the materials under any slabs placed on the river bed. This would cause instability and possible transportation of the whole slab downstream.
The preferred solution proposes the use of locally sourced large size rock to create a stable river bed downstream of the weir. The proposal is to develop an interlocking mass rock armouring that will be stable under controlled hydraulic conditions. In addition to stabilising the bed profile this 'rock armour' will also protect the proposed and existing wall foundations from undermining. The proposed characteristic size of rocks has been based on hydraulic model calculations to provide adequate protection to the bed but also to prevent them being transported along the river. The size or rock also helps to create bed roughness which will help to move the hydraulic jump and local scour back towards the weir.
Quality control of rock selection and rock placement is an important factor for the success of the scheme. Also, construction management and construction sequencing is vital.
It is intended that the use of natural rock will be an acceptable environmental solution, in keeping with the naturally occurring river bed materials and meet the needs of the conservation status of Laxey Weir.
