Soil Bio-Engineering for Erosion Control
The Need for Living Slopes
Steep, un-vegetated slopes pose a serious hazard on coastlines and riparian areas - for both human and wildlife habitat.
There are thousands of kilometres of riparian zones and coastal bluffs in the Capital Region and it is our collective duty to ensure they are stable and bio-diverse. If you keep an eye out on walks in these environments, you'll see a wide variety of approaches to slope stabilization. Sometimes, with moderate slopes, it can be as easy as planting trees and shrubs and/or seeding a re-vegetation mix. With severely degraded, extremely steep slopes however, a more comprehensive approach is required.
The 'old-school' engineering approach to this problem involves concrete, steel and rebar - huge retaining walls. However, these inert materials do not respond to nature's ebbs and flows. Let's face it, in living systems, things change. Heavy soil shrinks and swells; sub-surface water movement can cause slumping; and record rain events cause soil erosion. These conventional engineering approaches are not at all responsive these impacts.
Large rock retaining walls are slightly more natural in their appearance, but do not solve the problem of being unable to adapt to shifting soil conditions. They are also extremely expensive to build, involving barging in boulders and placing them with an excavator at low tide. There are indeed times when these 'hardscape' structures are necessary; however, all too often, biological resources are overlooked.
When nature has a say about what to do with bare, steep ground, she almost always chooses biological solutions. Hence, the field of soil bio-engineering.
Soil bioengineering is the use of living plant materials to provide some engineering function. [It] is an effective tool for the treatment of a variety of unstable and/or eroding sites. Soil bioengineering techniques have been used for many centuries." - Wikipedia
Over the last 4 years, I have had the privilege of working on several bio-engineering projects in the Greater Victoria area (see photos on the right). Working with Jeremiah Bain of Trematon Contracting and under the supervision of soil bio-engineering expert David F. Polster, I've learnt a great deal about the logistics and the hands-on practicalities of soil bio-engineering, especially 'wattle fencing'.
Wattle Fencing is one technique used for soil bio-engineering on extremely steep slopes. It is a great alternative to concrete and rock retaining walls that encourages living plants to weave a bank stabilizing root mat. Live cuttings from willow, black poplar and dogwood trees are harvested in the dormant season.
These species coppice and pollard well, meaning for every cut you make when harvesting the building material, more shoots develop. These groves, if well managed, could be harvested indefinitely for wattle fence material, making it a very sustainable, if not a regenerative land management practise.
For vertical posts, 5-8cm wide and 1.25m long cuttings are harvested and sharpened to a point using a hatchet. Starting at the bottom of the slope, holes are pre-drilled across the slope (on contour) every 1m using a steel planting bar. The posts are then pounded in approximately 0.75m-1m, leaving 25-50cm above ground for the fence.
The smaller 2.5cm wide whips are bundled for use as horizontal retaining walls. Starting with the 'butt-ends', the whips are laid horizontally until they are reach the top of the posts.
Soil is then backfilled to create terraces. Once soil temperature rises in early spring, soil-covered buds (on both vertical posts and horizontal whips) begin to grow roots. The resulting root-mat stitches these steep banks together. As a living system, it is much more able to adapt to shifting soil conditions. Any short-term erosion is caught by the wattle fence and offers a new opportunity for buds to root and further stabilize the slope.
Want to See a Wattle Fence in Victoria, BC?
If you'd like to see a two year old wattle fence on public land, head over to Colquitz Creek Park and look for the bus stop on Interurban at Columbine. This was a project I worked on in 2013 with Trematon Contracting and Polster Environmental. It is a great example of responsible riparian slope stabilization.
- See David F. Polster's papers:
- Polster Environmental Services
- Trematon Contracting - [email protected] - local contractor operating in the Capital Region District
- Live Stake Propagation. Permaculture Research Institute.