Conwy Spillway Success

Conwy overtopping winter 2015

Conwy overtopping Winter 2015

In 2009 Team Van Oord installed 9000m2 of Salix’s VMax Shear Stress Turf as part of the Conwy Flood Alleviation Scheme. Seven years and many major overtopping events on and it has stood the test well.

The Conwy Flood Alleviation scheme was a £7 million scheme put into place to protect 96 properties in the area. A flood plain was developed with flood banks and spillways which required a permanent high performing erosion control solution. Natural Resources Wales asked our advice and the conclusion was our VMax Shear Stress Turf Reinforcement Mat.

Conwy spillway after 11 overtopping events in winter, lasting 77 hours

Conwy spillway after 11 overtopping events in winter lasting 77 hours

The VMax Turf Reinforcement Mat (TRM) was used because of its outstanding record in preventing erosion in the most severe tests. The VMax is designed to cope with extended flows and has been used on dam spillways to great effect.

In 2009 the Natural Resources Wales gauging station at Cwmlanerch recorded the highest flood levels on the Conwy since records began. In 2011 it got close to that record again.

This last winter’s extreme wet weather was the ultimate test for the VMax TRM. Torrential rains from storms, such as Jonas, hit the Conwy Valley hard with major spillway overtopping events during late 2015 and early 2016. Apart from some minor repairs required (soil and overseeding) to some small areas where the top vegetation had been stripped (mainly due to cattle/sheep damage at the toe where a hydraulic jump zone is present), the VMax TRM remains undamaged and crucially continues to prevent soil loss below the matting.

With regards to on-going maintenance and performance, Steve Morris the Reservoir Supervising Engineer for Natural Resources Wales states:

“On the Dyffryn Conwy FAS we have no issues with the VMax TRM on the Trefriw barrier bank. We use the Aebi (similar to reform) on the face and the robo-mower on the crest. The only damage is caused by sheep sheltering by the high level outfall. Both spillways are grazed and we manage the thistles when required. Sheep in small numbers seem to be fine but larger animals cause problems.”


Date From To Duration (h)
09/11/15 17:00 20.00 3
30/11/15 14:45 17.00 2
05/12/15 15.00 1.00 10
06/12/15 04.00 11.00 5
10/12/15 05.00 11.00 7
12/12/15 15.00 22.00 7
22/12/15 12.10 17.00 5
25/12/15 18.30 17.30 23
26/12/15 20.00 00.00 4
26/01/16 05.38 13.35 8
26/01/16 15.25 18.15 3