Beach and coastal protection with geosynthetic sand containers
Coastal erosion solution is in the bag
When it came to overcoming the problem of beach erosion, Blyth Valley borough council, working with consulting engineers Royal Haskoning, called upon NAUE Geosynthetics to provide the solution.
The sand dunes at South Beach, Blyth, were at risk from erosion from the sometimes fierce tides on the North Sea. A number of options were considered, and this included soft rock bags. Royal Haskoning contacted NAUE to explore the benefits of this method, and engineer Angus Hamilton at NAUE was charged with looking after the project.
As Angus states, “We felt that this was a particularly interesting project and were keen to design a solution that would be attractive to Royal Haskoning. This involved the use of Secutex® Soft Rock sand container, and Secutex® H geotextile (formerly known as Terrafix®).”
The main option considered for this particular project was that of gabion baskets – wire cages filled with stones. However, being a coastal location meant that corrosion could be a real problem, and the durability of this method was called into question. Re-using the existing tank traps was a further consideration, although this was later dismissed due to insufficient available blocks.
While it had no previous experience of the NAUE solution, Royal Haskoning felt that there were a number of advantages with this method. Project Engineer Ewan Gee takes up the story. “We had the task of trying to prevent dune toe erosion – if the toe of the dune erodes, the bank will then slip down resulting in the entire dune being ‘eaten back’ or a lowering of the crest of the dune.
Why is Secutex® Soft Rock the ideal solution for coastal protection?
“The advantages of the soft rock bags supplied by NAUE included the ease of construction, as the bags arrive on site empty, and then filled from a deposit of wind-blown sand further up the beach using a specially constructed hopper machine.”
“This was a first for us,” Ewan continues. “While we had used soft rock bags previously on river work, we had never specified them for coastal applications. NAUE assured us of the durability of this type of construction, and we wait with interest to see how well it stands up to the local conditions.”
Indeed, NAUE is confident that the use of Secutex® soft rock containers will be as good, if not better, then any of the alternatives on offer. While it may be new to this firm of consulting engineers, Secutex® has a proven track-record all over the world.
While it may be extremely unlikely, should the bag split it will hold an additional advantage to the scheme … it will only spill-out sand that was taken from the beach in the first place. Secutex® is a needle-punched staple fibre non-woven geotextile made from 100% synthetic fibres which gives long-term stability. This material prevents individual and distinct granular layers from contaminating one another, and is recognised for its long-term filter stability and separation performance. Ordinarily, Secutex® is used in sheet form for a range of applications including hydraulic engineering, landfill engineering, and road- and tunnel-construction work.
For the Blyth project, the Secutex® was woven into bags capable of holding 1m3, or 2 tonnes of sand. Its high elongation capacity and robust characteristics make it an extremely durable solution. This project also made use of Secutex® H for additional hydraulics. Made up of two different types of textile – polypropylene and polyester – Secutex® H sheet was wrapped around the Secutex® bags at the toe end of the work. The idea behind this technique was to promote the drainage of sea water as the tide went down, while retaining the sand in the bags and – therefore – preventing failure.
A needle-punched staple fibre nonwoven geotextile, Secutex® H has a three-dimensional fibre structure that creates innumerable labyrinth-like pore openings closely simulating the surrounding beach structure and its hydraulic properties. It offers high water permeability while retaining the sand – essential in this application. Boasting a track-record of more than 30 years of applications in hydraulic engineering, Secutex® H gives a flow rate of 39.9l/sec/m2 … more than capable of the demands likely on the North Sea coastline.