Project Description
The upper and lower Black Moss reservoirs near Barley provide drinking water for Nelson in Lancashire, and were constructed in the late 19th century. A flood risk assessment identified a spillway upgrade was needed to protect dwellings further down the valley in the event of overtopping.
Concrete was deemed expensive and unsightly, so a grassed spillway was the preferred option to increase the water capacity. This would allow water to safely pass by the dam and be directed away from the local properties into an existing waterway, ultimately draining away safely.
The Challenge
The proposed site for the spillway included a section of slope significantly steeper than the surrounding land. Hydraulic modelling showed flood water passing over this area could reach velocities in the region of 6m/sec. CIRIA Report 116 has shown that well established grass can withstand flows up to 4.5m/s for no more than an hour. In addition, with steeper slopes, there is a risk of landslip where subsurface flows can cause blow out further down the slope. A proven reinforced green solution which could prevent surface erosion and stabilise the subsurface to prevent landslip was required.
The Solution
ABG’s Erosamat Type 3 Turf Reinforcement Mat (TRM) was specified by the project engineer to create a fully vegetated surface that can cope with far higher flows than unreinforced turf.
ABG Erosamat 3/20Z G50 Turf Reinforcement Mat
Erosamat Type 3 erosion control matting is a three-dimensional matrix of thermally bonded polypropylene fibres which create a tough and flexible erosion control mat, with a large open surface area to promote grass growth. Here Erosamat 3/20Z G50 was used which includes geogrid reinforcement within the structure to provide a much higher tensile strength.
The integral geogrid was secured to the surface using ground anchors driven through the subsurface and held and tensioned against washers placed at the surface. This bound the top layer of soil to control the risk of uplift in a flood event. Additional “U” pins were placed between the anchors to ensure good surface contact. Topsoil and seed was brushed into the Erosamat Type 3 geosynthetic matrix to promote the quick establishment of vegetation across the area.
The ABG Service
ABG provided full technical design support for both the use of Erosamat turf reinforcement mat (TRM) and the fixing anchor regime to ensure the success of this project.
