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.

Yesterdays Solutions	Terrafix solution - keep it permeable!

Urban areas

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.”

Slope reinforcing and stabilisation	Same installation with plants taking root

Permeable surfaces

“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.”

Awareness

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.

Nature Conservation

It is all well documented fact that surface water runoff is aggravated by alien vegetation. A lack ofunderstanding by large portions of citizens, coupled presently by substantially reduced state funding for nature conservation, do not contribute much toward solving these problems. Again the co-operation with local communities is the route to follow for developing nations.

Rural areas

The effects of overgrazing, deforestation and veld burning resulting in increased water runoff and soil erosion are enormous. Ways and means to counter them involve strictly applied and enforced environmentally compatible farming methods that have been well documented elsewhere.

Sand stabilisation	Same installation with plant cover

“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.

Solutions

Most design professionals are presently 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.

Terracrete Hard Lawn	Rural Road protected against erosion

Water Research Commission

The 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 level 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, architect and environmental designs 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 condition. In combination with plastic or natural fibre mats, effectiveness for light erosion control measures can be improved considerably.

Riverbank erosion control	Permeable canal lining

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 the environmental point of view this combination stands out head and shoulders above the other methods.

Design Considerations (Articulated Concrete Blocks)

Arguably design of erosion control measures should be based on a multi-disciplinary approach, involving engineers, landscape architects, horticulturist 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.

Design guidelines

BASIC GUIDLINES FOR THE USE OF EROSION CONTROL BLOCKS

Design process

Minimum Requirements (Hewlett et al. 1987)

Block mass --- 15 kg Mass/m² --- 135 kg/m²
Thickness --- 85mm Ave 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 • drainage • reinforcing for overtopping • backfill details • protecting from scour • providing for lateral inflow • alignment • position of weirs • vegetation • maintenance.