Bio-engineering uses plants and trees to create engineered structures for slope failures, dropouts and stream restorations. Poplars and willows are generally preferred as they offer many beneficial characteristics that natives do not in the early stages.
Poplars and Willows:
Poplars and willows establish quickly, have good root penetration and a high water uptake. This is particularly important in the first year, as the roots create a cohesive force in the soil and up take large amounts of water to stop further movement.
If a native system is to be established, it is common to include willows and poplars in the planting matrix as they can be removed once the failure has stabilised. Therefore, the benefits of a fast growing system can be captured yet a native system can be the final outcome.
The willows used are a shrub type growing 6-8m in height, which do not sucker or grow from their root systems.
Poplars are preferred for this system which is used on large scale failures, generally covering a hectare or more. The poles are planted as deep as possible, at high density, and drainage is also installed on the site to assist in removing water.
The brush-layer system is used for dropouts or where a reinforced earth wall needs to be made. Willow brush is used to form a matrix of stems across a pre-cut bench. This is then layered up and the dropout can be back-filled behind. Once the brush has been established, it can be cut back with a reach mower.
Live pole-drains are ideal for sites where the wet area of a slip is inaccessible to machinery. A drain can be created and even if there is movement on the site, the willow poles will keep the flow running.