Brush layering: A revegetation technique to revegetate and stabilize streambanks and slopes: Brush layering is a revegetation technique that stabilises and revegetates slopes and streambanks by combining soil with layers of rooted or dormant cuttings.

Description

Instead of living fascines, brush layering involves placing live chopped branches and rooted plants in layers into dug terraces and filling them with compacted soil material.

Brush layers, which are linear constructions, are typically finished with plantation or sowing.

As they are positioned at the bottom of excavated terraces, the cuttings’ tips, containing leaves and buds, escape the slope and can intercept rainwater, limit water flow, and entrap soil sediments (Figure 1). On the other hand, the stem of the cuttings enters the slope and acts as a tensile nail inclusion, reinforcing the soil (Bischetti et al., 2010).

Advantages:

Rey (2009) recommends immediate protection from the first vegetative year for slopes with a steepness of less than 40%.

Suitable for extremely steep slopes.

The deeper the root growth, the stronger the stabilising impact.

Disadvantages:

High demand for materials;

Not appropriate for slopes with limited equipment access (trenches are difficult to implement).

Not recommended for rocky slopes or poor water content.

Modified brush layers

Modified brush layers can also be used on very steep slopes where rolling rocks or derbis might descend, gather speed, and destroy the plant.

These are brush layers supported by a tiny log or board (2 m long and at least 5 m thick) that form a little terrace to chatch potential derbis or small boulders.

They should be spread across the slope to maximise the odds of capturing the potential masses (Figure 2).

Reinforcing steel bar one metre long and 15 mm in diameter can be used as a strong support to keep the modified brush layers in place (Polster, 1999).

Figure 2. Top figure: Modified brush layers in Canada (Rey et al., 2019). Bottom figure: (a) Modified brush layers staggered across a slope, (b) detail of a modified brush layer prior to backfilling, (c) detail of the normal backfill which creates a bit of a bench (Polster, 1999).

Design methods

Brush layering has been widely implemented over the world, following criteria provided by various handbooks based on previous experience (e.g., Schiechtl, 1980; Grey and Sotir, 1996) regarding bench width, inward inclination, and spacing.

Trenches are typically 0.5 to 1.5 meters deep, with spacing distances ranging from 1 to 2 meters depending on the slope aspect (Florineth et al., 2002).

The cuttings should be at least 50 cm long, with a diameter of 2-4 cm and tips that reach 10 to 20 cm from the slope surface (Figure 3).

The average number of cuttings is 20 per linear metre (Rey, 2009). Work is done from the bottom up, so soil excavated from the previous trench can be utilised to fill the next shelf.

This approach can also be used to produce a new slope of fill material. In this case, the live cuttings can be longer (1-5 m), and the slope is created by compacting the soil layer by layer.

Brush layers should be at least 10° sloped, with points protruding from the slope surface (Morgan & Rickson, 1995). Bischetti et al. (2010) proposed a new approach for brush layering design that is based on the computation of the Factor of Safety using equilibrium limit equations and takes into account brush layer design parameters.

Based on this novel model, they found that by employing half of the live materials traditionally used in this procedure, the same stabilisation can be achieved at a significantly lower cost and time.

Installation occurs during the plant’s dormant season.

Materials: Branch cuttings and rooted plants with aventious root growth (such as Salix tetrasperma, Salix purpurea, and Salix incana).

Functional Suitability Criteria