Vegetation is an effective slope stabilisation method and a variety of erosion control products are available for use while it establishes.David Holland examines the options.
Soil tends to be regarded as an infinite resource, but an eroded soil will lose nutrients, structure and therefore the ability to support vegetation. A soil system that may have taken thousands of years to develop can be lost in weeks if exposed during construction work.
Soils should not simply be looked upon as a growing medium, but as a unique habitat of dynamic ecosystems which evolve over time (Holland, 1995).
No specific soil protection policy yet exists in the UK or Europe. The US recently introduced the National Pollutant Discharge Elimination Systems (NPDES) legislation which requires forward planning and early design allowances for soil erosion and stability issues on all land disturbing activities over 0.4ha. More than 97.5% of construction work falls under these regulations.
In the UK, less attention is given to soil erosion and sedimentation at the design stage, even though they can lead to costly remedial works and have implications for environmental quality on site and elsewhere in a catchment. Some form of European soil protection policy, possibly as part of an all-encompassing 'water quality management' directive, is inevitable. Designers and contractors will become directly responsible for strict soil erosion and sediment control practices.
Although it is well known that vegetation plays a critical role in controlling soil erosion and aids slope stability, little consideration goes into how a sustainable vegetative cover can be achieved.
The beneficial effects of herbaceous vegetation and grasses in preventing rainfall erosion are well documented. Plant foliage absorbs rainfall energy and prevents soil particle detachment by raindrop impact. Root systems physically bind or restrain soil particles, while above-ground plant shoots filter sediment out of runoff. Stems and foliage increase surface roughness and reduce runoff velocity.
An optimum slope surface cover should provide a dense and spatially uniform vegetative cover, which is why grasses, legumes and herbaceous species are usually used.
Water is either the primary or a major factor in almost all documented slope failures. Vegetation plays an important role in managing and regulating the amount of water available to a soil and is thus critical to slope stability.
Roots mechanically reinforce a soil as they transfer shear stress to tensile resistance in the roots. Interception of rainfall by plant foliage can limit the build-up of positive pore water pressure within a soil and increase bank stability. Plants also help to maintain soil porosity and permeability, thereby delaying the onset of overland runoff.
This is particularly important on construction sites in the UK, where the temperate climate means soils become saturated for long periods and overland runoff is common. Here, active sheet erosion may not even be visible. In Mediterranean and subtropical regions more intense rainfall produces more dramatic and more obvious soil erosion problems.
The key to successful slope stabilisation and soil erosion control is the rapid establishment of a good and appropriate vegetative cover. But often vegetation is not considered because initial establishment is deemed too difficult.
It is almost always advisable to use some form of temporary erosion control matting during the six-month establishment period. However, selecting the appropriate material is difficult. Over-specification (and overspending) or under-specification (leading to product failure) is common in the UK, reflecting the lack of product selection guidelines and performance information.
Two main types of geotextiles exist:
l short term degradable erosion control materials, usually made from natural fibres, jute, coir, straw and wood fibres l turf reinforcement mats (TRM); permanent threedimensional synthetic mats used to enhance the shear stress resistance of vegetation through root and shoot reinforcement.
Numerous laboratory and field trials have been conducted worldwide to establish the performance of numerous products. There is now good understanding of the geotextile properties that create an effective erosion control barrier and aid revegetation.
An effective erosion control product should closely mimic the function of a vegetative slope cover. Biodegradable erosion control materials will provide ground cover and mimic the rain buffering function of vegetation until the latter establishes.
The percentage of ground cover offered by a product is directly related to the level of erosion control offered. There is also a balance between optimum ground cover and significant light penetration to stimulate seed germination and allow grass shoots to break through the geotextile matrix.
Ground cover of 80%-90% offers the optimum performance level for erosion protection and vegetation establishment (see table).
Traditionally, large amounts of jute and coir nets have been used for slope applications in the UK.
Much more erosion protection can be gained by using higher ground cover products, such as coir and straw blankets - rolled erosion controlled products (RECP) - at minimal additional cost.
Pre-seeded blankets are often used but the designer and end user should ensure the seed is of a certified UK provenance and therefore has no adverse ecological impacts.
A high percentage of ground cover and the use of natural fibres in erosion control products can significantly reduce slope run-off. Natural fibres such as coir, straw and jute retain far higher amounts of water than synthetic materials.
Coir and straw blankets with jute scrims have been shown to be exceptionally efficient at holding and regulating the rate and amount of rain water that enters a slope system (see graph).
By modifying the infiltration rate of a soil, erosion is greatly reduced. Reduced water in the slope will reduce soil pore water pressure and therefore increase soil shear strength. Large volumes of water are held in the blanket's fibre matrix and then slowly released into the slope and lost through evaporation.
Once vegetation establishes and the products become redundant, the vegetation will provide the erosion control function and run-off control.Tensile strength is often given as an important feature of an erosion control material in slope applications.
However, tensile strength has little impact on product performance and is only important if the product is expected to be physically damaged during installation.
Turf reinforcement mats (TRMs) are used on slopes where vegetation alone will not provide sufficient shear strength to provide erosion control and surface stability. By reinforcing plant roots and shoots, TRMs effectively double or even triple the shear stress that vegetation can withstand.
TRMs are generally soil filled and provide effective root reinforcement once mature vegetation has established. However, they have 85-95% open area and so possess very poor short-term erosion control function. Indeed, TRMs have been shown to actually increase soil erosion levels as the dry, fine textured, friable topsoil used to fill the mats is often more erodable than the underlying soil that is being protected.
A new generation of TRM is now available.
Composite TRMs (C-TRM) contain a coir or coir/straw core with all the short-term erosion control properties of a blanket, held within a super high tensile strength 3-D net which gives higher vegetation reinforcing properties. Design information and even design software is available for CTRMs.
Manufacturers need to produce independent research to back up their recommendations for product applications. Some American companies have done this and produce excellent design guidelines and even design software for both slope and drainage channel applications. Greater use of such guidelines is needed to increase confidence in the application of erosion control products in the wider context of water quality management.
David Holland is technical director of Fluvial Environmental Services, a specialist erosion control consultancy.
References Ed Morgan RPC and Rickson RS (1995). Slope stabilisation and erosion control: A bioengineering approach.
Holland D (1995). Soil protection in the UK. Unpublished Masters thesis.University of Gloucestershire.
Urroz G (1995). Effectiveness of selected materials under simulated rain and sunlight. Utah Water Research Laboratory, Utah State University. IECA 1995 conference proceedings.