Whittlesey Washes Flood Defence System stores up to 35,000,000m3 of water on an area surrounded by embankments when high tides and high river levels coincide. The stored flood water is then released back into the river as the tide recedes.
Under the 1975 Reservoirs Act, inspections in 2012 identified an essential requirement for remedial works to reduce the risk of a breach in the embankments during an extreme flood event. Flooding in both 2013 and 2014 resulted in noticeable seepage on the South Barrier Bank raising concerns over the integrity of the embankment during a flood event.
The project was implemented by the Environment Agency with support from local Internal Drainage Boards, Natural England and the RSPB. The area is unique with this long embankment and many environmental designations. A solution was sought to safeguard the site for the next 100 years, but, as a Site of Specific Scientific Interest (SSSI) and a Ramsar Site (a wetland of international importance), environmental considerations and biodiversity implications were of paramount importance.
After site investigation and flood modelling it was not considered necessary to increase the overall height of the embankment. However, the weaker areas were sealed and strengthened using additional material placed at the base blended into the surrounding landscape. To meet environmental standards a natural grass finish was required. Grass is a good erosion control surface when maintained; the risk is grass die-back and localised erosion in an overtopping event.
On the occasion of an extreme storm event the embankment will overtop and water will flow down the back face at speeds up to 4m/s. A well-established grass sward can withstand only a short period of high flow. To extend the period and protect against die-back of the grass ABG Erosamat 3/20Z 500M erosion control geosynthetic was chosen as a permanent Turf Reinforcement Mat (TRM) which meets the design requirements of CIRIA Report 116. In areas regularly subjected to light traffic ABG’s grid reinforced Erosamat 3/20Z G50 High Performance Turf Reinforcement Mat (HPTRM) was used. There was a high risk of flooding during the construction of the embankment and it was essential to have a solution which was both quick to install and readily facilitated the establishment of sound vegetation. Erosamat achieved this, being light to handle, and quick and easy to install. Areas of high risk were then hydro seeded to accelerate growth.
ABG Erosamat 3/20Z 500M Turf Reinforcement Mat
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ABG provided full product and design support to aid the approval process.
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Erosamat Type 3 turf reinforcement geosynthetic is robust and light-weight allowing quick and easy installation on this high flood risk site. Site constraints stated that there should be no area of embankment exposed to flooding for more than four hours at any time during the construction. Even with no grass the Erosamat TRM is sufficient to protect the embankment during a flood event.