River Trothi Bank Stabilisation
Client: Monmouth Council
Background: The River Trothi is a tributary of the River Monnow with the problem reach located 3 miles NE of Monmouth. A steep section of the river just downstream of a weir had started to erode and undermine several large trees, some of which had collapsed into the channel, creating large scour holes.
At the bank edge the channel was 1m deep and the 1:1 slope was 2-3 metres high. To complicate matters a road is positioned directly above the slope, so the local highways department required emergency stabilisation works.
Design: Initial designs looked at sheet piling and blockstone revetments. However, the design consultants and contractors Salix felt that a softer bioengineering revetment could be suitable. The large trees had reached a point where the effective root reinforcement function was being outweighed by the shear bulk weight of the trees.
As such it was critical to remove the weight of these trees but not remove them completely as the root reinforcement of such large trees was important to the stability of the bank. The trees were cut back to a 1m stump height which also allowed more light onto the riverbank.
Tree cover on such a steep slope was an important stabilising component long term, so locally harvested crack and grey willow (Salix fragilis and Salix cinerea) was used to build up “Willow Spiling” tiers. Each lift was 1m high, the maximum retaining height of such a structure. Every other upright stake was tied to the bank using Duckbill Anchors. Each tier was set back one metre to create a 45º slope. The toe of the bank was in 1 metre of water which in flood becomes turbulent and fast flowing (estimated 2.5 m/s). Under these conditions it was critical that a robust revetment was used below average water level to prevent further scour. A “triangular” pyramid of Rock Rolls was installed with a Coir Roll placed behind the upper rock roll in order to introduce marginal plant species. Species were chosen that are known to grow locally in this marginal zone where grass species struggle to establish due to regular inundations (species include Phalaris arundinacea, Juncus effusus, Lythrum salicaria and Iris pseudacorus).
Results: The “strike rate” of the willow stakes has been impressive (98 out of 99 have started to grow) and the plants in the Coir Rolls had rooted through to coir in less than one month. One day's maintenance every 2 years will keep the willow at its optimum height with a deep fibrous root mass but without excess weight.
Materials used: Willow Spiling, Rock Rolls, Coir Rolls
River Trothi before works
River Trothi immediately after works
The Willow Spiling starting to sprout
July, five months after installation
