Project Description
The town of Selkirk in the Scottish Borders has a long history of damaging floods from the River Ettrick and more recently from the Long Philip Burn. A severe flash flooding event that inundated over 100 properties, led the Scottish Borders Council (SBC) to identify and prioritise the Selkirk Flood Protection Scheme (FPS) to tackle the issues of flooding in the town. The scheme brief was to construct protection to riverbanks and to raise flood embankments to prevent flooding.
The Challenge
All watercourses (including the Ettrick) which flow into the River Tweed are designated as being a Special Areas of Conservation (SAC) which required the least disruption to the environment during construction. The scheme was one of the first projects submitted under the new Flood Risk Management (Scotland) 2009 Act, with a key requirement that wherever possible solutions should give a natural and vegetated appearance.
The challenge was to maintain a grassed embankment, especially during a flood event. Grass is a good short term erosion protection solution, however it suffers from potential weak areas due to die back or poor growth which can lead to severe erosion.
The Solution
Referring to the guidelines in CIRIA 116 “Design of Reinforced Grass Waterways” it became clear that a robust open Turf Reinforcement Mat (TRM) was suitable. The proven ABG Erosamat 3/20Z500M erosion control matting was selected for the flood embankments which meets the requirements of CIRIA 116. The ABG Erosamat Type 3 is a non-biodegradable solution, with a predicted life span of 120 years. The TRM permanently protects the soil in non-grassed areas and reinforces the roots in grassed areas. The system proved its worth and was tested when the grass was only partially established as both storm events “Desmond” and “Frank” caused a surge event with increased water flows. The system worked well and protected the town from a flood as designed. Where the design allows for an overtopping, the Erosamat was pinned right over the crest. For higher embankments and where access for grass cutting equipment used to allow maintenance of the grass areas was required, the ABG Sudspave 40 permeable paving was used. This was easy to transport, lay and fill and can take light vehicle and footpath traffic.
The ABG Service
ABG provided full specification and design support to aid the approval process.
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