Using eco-engineering to prevent against hazards in forests on landslide

prone slopes

Alexia StokesINRA-AMAP, TA A51/PS2, Bld de la Lironde, Montpellier cedex 5, 34398 France

Eco-engineering can be defined as the long-term ecological strategy to protect and/or restore a site with regard to natural or man-made hazards

What is eco-engineering?

Usually on a large-scale…

When can eco-engineering be used?

Erosion control

Revegetating pasture lands, Inner Mongolia, China

Storms

Slope restoration after landslide, Sichuan, China

Mass movement

Source: S Mitchell UBC

Source: M Ghestem, INRA

Storm-induced windthrow and landslide, Vancouver, Canada

Not to be (too) confused with ground bioengineering

Railway Lhasa - Beijing

Usually on a small-scale…BUT…

Brushlayering

Vegetation combined with inert structures

Live staking

Source: Sotir, Chao

Sao

A sao was a mixture of stems and branches of willow and crushed rock tied together with twine or bamboo rope. It was recorded in Han Shu (by Ban Gu, 32-92 AD) that the great emperor of the Han Dynasty (Wu Di,) ordered his soldiers and generals to transport Saos to fix the breached levees of the Yellow River and stop the flood water. A section of the Sao-reinforced levee of the Han Dynasty was discovered in 2005 along the Bar River

156-87 BC

Needham Research Institute

Invented by…

Evaluation of site

Consideration of short-term risk to human safety and possibility of recurring hazard (storm, landslide, avalanche etc)

Risk is lowHigh short-term risk to human safety

and infrastructures. Recurring hazard is likely.

Site is large-scale. Expenses incurred

exceed gain

Is site to be restored/protected on a small- or large-scale?Use engineering

methods to protect against immediate, potential dangers

Site is small-scale.Expenses incurred

equal gain

Determine costs over long-term (construction, planting, upkeep, management) and assess gain

Consider ground bio-engineering techniques

Consider eco-

engineering techniques

When can eco-engineering be

used?

What is the best kind of vegetation to improve soil fixation on landslide prone slopes?

1) In plantation forest, does slope stability improve with tree age?

2) On reforested slopes, does natural regeneration lead to instability in the early years?

Broadleaf saplings

Bamboo

Bamboo

9 years 30 years

In Sichuan, China:- We measured vegetation and soil variables for typical Cryptomeria japonica plantations of different ages

In plantation forest, does slope stability improve with tree age?

Can then calculate slope stability safety factor (FOS) using slope stability model (<1.0 is unstable)

9 year old plantation = 2.0 (+27%)20 year old plantation = 1.8 (+15%)30 year old plantation = 1.5 (+17%)

Genet M., Kokutse N., Stokes A., Fourcaud T., Cai X, Ji J, Mickovski S.B., 2008. Root reinforcement in plantations of Cryptomeria japonica D. Don: effect of tree age and stand structure on slope stability. Forest Ecology and Management. In press.

W

Wv

c´R

Three phases of natural regeneration in Sichuan forests

Phyllostachys nidularia (bamboo)

Aralia elata Quercus infectoriaLitsea cubebaCarya cathayensisCynanchum stauntonii Betula laminiferaIdesia polycarpa

Cinnamomum wilsonii Notaphoebe cavaliereiPhoebe nanmuIdesia polycarpa

1st phase 2nd phase 3rd phase

On reforested slopes, does natural regeneration lead to instability in the early years?

Bamboo and 5 year old deciduous mixed forest

Bamboo only 20 year old deciduous mixed forest

An inventory of shallow landslides was made along one valley in the Sichuan Height

WidthAngleDepthExposureVegetation

We then repeated the same procedure as before to determine the Factor of Safety at three different stages of forest succession

16 cm

Landslides often occur in bamboo forest because bamboo is a very shallow rooted pioneer

1st phase of succession = 1.0 ( +0%) – soil only2nd phase of succession = 1.2 (+8%)3rd phase of succession = 1.4 (+32%)

9 year old plantation = 2.0 (+27%)20 year old plantation = 1.8 (+15%)30 year old plantation = 1.5 (+17%)

Genet M, Stokes A, Fourcaud T, Norris J 2008 The influence of plant diversity on slope stability in naturally regenerated sub-tropical montane forests at different phases of succession. Ecological Engineering. Submitted.

Root cohesion mappingThe position and density of trees will have an effect on small local landslips

Upslope

Downslope

Cohesion for one tree calculated from root system architecture

Danjon F., Barker D.H., Drexhage M., Stokes A. 2008. Using 3D plant root architecture in models of shallow slope stability. Annals of Botany, 101, 1281 – 1293.

New Research…

In a perfect (virtual) world….

Root-soil blocks of individual trees

Mesh refinement of root-soil blocks integrating intra tree variation of root properties

D

Z

M. Jaeger

Genet M., Kokutse N., Stokes A., Fourcaud T., Cai X, Ji J, Mickovski S.B., 2008. Root reinforcement in plantations of Cryptomeria japonica D. Don: effect of tree age and stand structure on slope stability. Forest Ecology and Management. In press.

NK Kokutse

Slopes Decision Support System (SDSS) and Species Database

A open source platform to implement Decision Support Systems (DSS) has been developed and published on the web.

Can accept different models

Species database currently being developed to help eco-engineers decide what to plant, how and where on a slope. Will be freely available on the web.

Latin and English name

Region Habitat/Properties Altitude (asl)

Morphology: max. height (H) and max. root depth (R)

Pioneer plant

Efficacy against rockfall

Function Comments

Acer pseudoplatanus L. (Sycamore)

Europe

Moist, cool, mixed broadleaved tree woodland and hedgerows, requires humus-rich soil adequately supplied with water. Tolerant to long periods of gravel aggradation and exposure.

peri-alpine hills to 1700 m

H 25.0mDeep rooted, heart

shaped root system  +++ DR; AB.

Used for vegetated crib walls.

Key to Function classification: AB – anchoring and buttressing of deep taproots; BC – bank and channel reinforcement; DR – deep reinforcement and soil strength enhancement; PH – phreatophytes removing soil moisture, SP – surface protection, shallow reinforcement and erosion control. Key to Efficacy of rockfall: 0 – species is not

suitable for protection from rockfall; + – limited rockfall protection; ++ – good rockfall protection; +++ – excellent rockfall protection

A Series of International Conferences on:

Ground Bio- and Eco-engineeringThe Use of Vegetation to Improve Slope Stability

Thessaloniki, Greece, September 2004

Beijing, China, July 2008

Vancouver, Canada, August 2012

In October 2008, a new International Network of Ground Bio- and Eco-engineers

will be set up