In nature, a near balanced situation exists between precipitation, runoff and infiltration. An ever-increasing rate of urbanization is upsetting this equilibrium and causing severe soil erosion problems. Over the years, Terraforce and Terrafix blocks have proven their value in providing environmentally sound solutions.
Continue reading for the full presentation given by Mr Rust at the World Summit for Sustainable Development (WSSD) for the Global Alliance for Building Sustainability (GABS), Johannesburg, South Africa in August 2002.
Presentation at the WSSD by Holger Rust
In nature, a near balanced situation exists between precipitation, runoff and infiltration. An ever-increasing rate of urbanization is upsetting this equilibrium and causing severe soil erosion problems. Over the years, Terraforce and Terrafix blocks have proven their value in finding environmentally sound solutions.
Presentation on erosion control – for the Global Alliance for Building Sustainability (GABS), Johannesburg, South Africa, August 2002.
Sustainable development or sustainable building/construction practices should surely be on the mind of any architect, engineer and developer about to tackle a new construction site.
Aiming to create a built environment that “meets the needs of the present without compromising the ability of future generations to meet their own needs” (Our Common Future, WCED, 1987) is possibly one of the most urgent aspects in the global fight to prevent our resources from running dry.
One obvious facet of sustainable development involves protecting our natural (or pristine) environment. This requires that we find a balance between protecting the physical environment and its resources, and using these resources in a way that will allow the earth to continue supporting an acceptable quality of life for human beings, and not, as many wrongly assume, to keep up or “sustain” our current pace of economic productivity to the point of exhaustion (Sustainable Settlement in South Africa, CSIR, 2002).EROSION CONTROL – SUSTAINABLE CONCRETE SOLUTIONS Rampant urbanization is a major problem in many parts of the world. In South Africa an estimated 4/5 of the population live in urban areas. In combination with poor farming practices, this leads to rapid degradation of our rivers and streams. Apart from our precious soil disappearing down to the beach, so too are our undernourished ground water reserves and our biodiversity doing likewise.
The situation demands urgent and decisive steps on various fronts. Since there is little likelihood that population and economic growth will be curbed in the short term, other routes have to be followed to alleviate the pressure.
The “Effects of urbanization on catchment water balance” have been thoroughly researched and documented to the Water Research Commission by Professor D. Stephenson and others of the Water Systems Research Group at Wits University. In a nutshell their findings are summarized below:
“In nature a semi-equilibrium exists between precipitation, runoff and infiltration into the ground. Over years the water table fluctuates about a mean. The construction of impermeable barriers on the surface, such as roads and buildings, reduce the rate of ground water replenishment.
“The water runs off easier and the limited permeable areas restrict infiltration. The groundwater level will therefore drop and the zone above the water table will gradually dry out. Vegetation and the soil characteristics will change. If we are not to affect our environment adversely we should attempt to return some of the storm-water we channel off urban areas back to the ground.”
“This can be accomplished by ensuring adequate permeable surfaces and by directing storm-water into specially selected or constructed seepage areas. We will then not only maintain the regime but also minimize design flow rates downstream.
“Surface water runoff in urban areas has been found to be ± 4 times higher than in rural areas of similar geographical composition. Impermeable buildings, roadways, parking areas and even storm-water drains may cover ± 20% of the total area. Reduced infiltration, increased runoff velocity and erosion are the direct result.”
According to a survey conducted for the Water Research Commission by consultants Economics Project Evaluation, most South Africans undervalue their water supply and a large percentage don’t even know the origin of their Municipal water. Public/private partnerships in the form of maintenance contracts awarded to local communities should help in correcting this deficit.
It is a well-documented fact that surface water runoff is aggravated by alien vegetation. A lack of understanding by large portions of citizens, coupled presently by substantially reduced state funding for nature conservation, do not contribute much toward solving these problems. Again, co-operation with local communities is the best route to follow for developing nations.
The effects of overgrazing, deforestation and veld burning, which result in increased water runoff and soil erosion, are enormous. Ways and means to counter these involve strictly applied and enforced environmentally-compatible farming methods that have been well documented elsewhere.
“The essence of sustainable farming is the attempt to reconcile a short-term survival strategy with a long-term conservation one. The dramatic population growth in sub-Saharan Africa easily tempts us to revert back to survival strategies at the cost of natural resources. Meanwhile, in the developed north, protectionist government policies contribute to agricultural unsustainability” (Dr. Izak Groenewald, Centre for Sustainable Agriculture, University of the Free State).
Most design professionals are discarding some of yesterday’s solutions, such as impervious concrete-lined storm water canals. Instead, permeable catchment storage in the form of shallow channels, roadways or flood plains are recommend at the head of a system, whereas detention pond storage was found to be most economical at the outlet of the catchment. It has also been found that increasing the permeability in most urban situations is more beneficial than reducing roughness in storm-water canals. Research has shown that a well-designed hard-lawn with 25% openings can infiltrate 500L/sec/ha.
Water Research CommissionThe Water Research Commission has suggested various measures, including changes to town planning regulations and practices to achieve the desired effects. Increased state funding for nature conservation projects would also help in raising the levels of understanding amongst citizens.
Rain water harvesting in large format drums, encouraged with public subsidies, could have an enormous regulating effect. These measures will ultimately lead to a regeneration of dried-up springs. A heavy responsibility also rests on the shoulders of engineers, architects and environmental designers to ensure that their projects impact positively on the environment of future generations.
Materials Of the various materials available to fight erosion, dense vegetation cover with deep rooted proven plant species should be considered. Many non-invasive types are known and catalogued, adapted to various climatic and soil conditions. In combination with plastic or natural fibre mats, effectiveness for light erosion control measures can be improved considerably.
- Rock rip-rap is often specified to good effect but has its limitations. Stability is often a problem and drive-over maintenance is impossible.
- Wood and metal-based materials are still popular and can be very effective. Durability is their main stumbling block, meaning that they have to be impregnated or coated. These processes involve highly toxic substances.
- Segmented concrete products are rapidly gaining in popularity over other methods and in combination with vegetation cover are more durable, versatile and cost-effective.
From an environmental point of view this combination stands out head and shoulders above the other methods.
Design considerations (articulated concrete blocks)
Design of erosion control measures should arguably be based on a multi-disciplinary approach that involves engineers, landscape architects, horticulturists and environmental experts. The U.S. based N.C.M.A. (Reference Tec 11-9A Articulated Concrete Blocks for Erosion Control) published a few guidelines:
BASIC GUIDLINES FOR THE USE OF EROSION CONTROL BLOCKS
- Blocks are placed together to form an erosion-resistant overlay with specific hydraulic performance characteristics.
- Includes a geotextile underlay to allow infiltration and exfiltration and to provide particle retention of the soil subgrade.
- Systems are simple to produce, easy to install and environmentally friendly.
- ACBs have excellent resistance to hydraulic shear and overtopping conditions.
- The ability to support the ecosystem’s habitat is a major advantage. Easy and fast to install, easy to inspect.
- Cabled systems can facilitate machine placement. Cables don’t increase hydraulic or structural values however.
- Can be constructed in virtually seamless fields.
- Are not designed to add structural strength to steep slopes.
- A feasibility study conducted for Terraforce by The Council for Scientific and Industrial Research provides further design guidelines.
Minimum Requirements (Hewlett et al. 1987)
Block mass — 15 kg; mass/m² — 135 kg/m² Thickness — 85mm; average block width — 255 mm
Where flow velocities of over 6m/s are expected to occur, these should be reduced with weirs of concrete retaining blocks or gabions. Should this not be possible and where flow depths are greater than 1.5m, sidewalls of concrete retaining blocks or gabions, as well as ground anchors for the bed lining, should be considered.
Erosion control blocks are ideally suited for flow velocities between 6m/s and 3m/s, provided that oscillation is effectively prevented. This is best achieved by providing sufficient lateral restraint between adjacent blocks (more than 75% face contact between blocks) and with mechanical anchors into the subsoil.
Generally no special considerations should be necessary for flow velocities below 3m/s expect standard requirements such as:
• reinforcing for overtopping
• backfill details
• protecting from scour
• providing for lateral inflow
• position of weirs