River Trothi

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.

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 were important to the stability of the bank. The trees were cut back to a 1m stump height which also allowed for 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 1metre 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).