Soil erosion is a natural process, but becomes a problem when human activity causes it to occur much faster than under natural conditions. Annual soil loss in South Africa is estimated at 300 – 400 million tons, nearly three tons for each hectare of land (Enviro Facts, 1999j).

UNDERLYING GEOLOGY

The stable granites of the north has tended to favour developments ranging from single houses to high-rise office parks and residential properties, while the shale groups in the inner city has given it a low lying flat terrain. The quartsite, basic lava and conglomerate underlying parts of Hillbrow all the way to Observatory, Cyrildene and Linksfield, have tended to give these areas a hilly terrain. The dolomite/chert underlying Lenasia, Protea south and surrounding Dobsonville, is well known for its poor qualities to support development (GJMC, 1997).

On the whole, the geological forms across the metropolitan area have influenced the way the area has developed. The more favourable geological areas, like the stable granites and quartsites in the north, have influenced the location of massive developments in these areas, while the shales and dolomites of the south have tended to hinder development and investments.

SOIL TYPES, PROPERTIES AND AGRICULTURAL POTENTIAL

The deep red soils of the ultramafic rocks may be active and/or compressible. The gray, sandy soil of the Halfway House granites are very open textured and collapsible, and excavation instability may occur. The ridges of the Hospital Hill Subgroup causes shallow excavatibility. Compressible soils develop on the West Rand Group, while the Central Rand Group areas all have shallow excavatibility and instability. The dolomites and chert of the Malmani Subgroup forms dolines and sinkholes, and subsidence is a major problem. Alluvium intrusions are associated with a shallow water table, active clays, compressibility and a collapsible fabric.

Due to the geology, soil characteristics and geotechnical constraints, only some areas in the north and patches in the south and west have high agricultural development potential. A substantial volume of food and other resources thus have to be imported into the city to sustain all the people. This increases transport costs and places even more pressure on the city, as a man-made and - sustained environment. The few agricultural holdings also have to be overcultivated in order to supply sufficient products.

LANDFORMS

The landforms largely determine the land use of an area. In Greater Johannesburg a large number of ridges are viewed as status areas for living, while steep and unstable slopes are no-go areas for development. River courses can be ideal for agriculture and irrigation, but clayey soils, especially around the Klip River, are not ideal for cultivation.

SLOPE CATEGORIES

Steep slopes (> 15º) are potentially unstable and pose problems for any development. Development and human activities on the slopes may destabilise them and cause accelerated erosion and even rock falls.

DOLOMITIC LAND

The main environmental problems associated with development on dolomitic land are the generation of ground movement events, such as sinkholes and dolines, as well as groundwater contamination. Sinkholes (resulting from the collapse of cavities) are unpredictable and can cause property damage or loss of life. Most sinkholes are induced by human activities such as dewatering due to mining and the extraction of groundwater. Dolines are areas of shallow depression caused by either the consolidation of low density residual soil material, or through premature termination of the process of sinkhole formation (Buttrick & Stapelberg, 1994). Due to the relatively shallow groundwater levels in some dolomite aquifers, these waterbodies are more prone to leachate from mining, domestic and industrial waste. Where inappropriate development takes place on dolomitic land, residents are prone to the hazards of ground movement events or the pollution of the aquifer.

GEOTECHNICAL PROPERTIES

The anticipated geotechnical problems according to the geotechnical constraints of the land are discussed under the pressure section (Buttrick & Stapelberg, 1994). Geotechnical problems may occur in combination with each other, e.g. where dolomite underlies collapsing soils.

AGRICULTURE

For every ton of maize, wheat, sugar or other agricultural crop produced, South Africa loses an average of 20 tons of soil through erosion (Enviro Facts, 1999j). The loss of protective vegetation through ploughing, overgrazing and fire makes soil vulnerable to being swept away by wind and water. In addition, over-cultivation and compaction cause the soil to lose its structure and cohesion and it becomes more easily eroded. Erosion will remove the topsoil first, and once the nutrient-rich layer is gone, few plants will grow in the soil again. Without soil and plants the land becomes desert-like and unable to support life.

Over-cultivation, over-irrigation can lead to either salinisation or waterlogging. If the irrigated area is subject to heavy soil-water losses through evapotranspiration, the salts in the water remain in the soil, causing salinisation. When the salinity of the soil reaches the limit of the plants, the land must be abandoned. On the other hand, if the infiltration of large volumes of water causes a rising in the water table, the zone of saturation may be brought closer to the surface, causing the soil to be waterlogged. In dry regions, farmers often apply heavy amounts of irrigation to keep salts from accumulating in the soil (Miller, 1993). Crops cannot grow in perpetually saturated soil (Strahler & Strahler, 1992).

Excessive groundwater withdrawal for irrigation may lead to land subsidence and damage to structures.

MINING

In-situ mining impacts include:

• failure of pipes may contaminate air, soil and water systems;
• radon gas released from bore holes and evaporation pond;
• failure of pond liner may contaminate soil and water; and
• long time for ecosystem to recover.

Mine dumps are a major concern in the SMSS especially close to the CBD. The reclamation of the mine dumps causes water and dust pollution. Reclaimed areas can however be used for the location of industries (SMLC, 1997).

URBANISATION, POPULATION GROWTH AND DEVELOPMENT

The hydrological effects of urbanisation are well documented. It can alter the hydrology of a watershed in two ways. First, an increasing percentage of the surface is rendered impervious to infiltration by the construction of roofs, driveways, walks, pavements, and parking lots. It has been estimated that in residential areas, for a lot size of 1400m², the impervious area amounts to about 25%, whereas for a lot of 560m², the area is 80% (Strahler & Strahler, 1992). The impervious surfaces decrease infiltration and increases overland flow, which increases the frequency and height of flood peaks during heavy storms. This means that more topsoil is washed away easier, erosion potential increases on the few patches of open land and there is also less recharge to groundwater bodies. A second change caused by urbanisation is the introduction of storm sewers. Water is now taken directly to stream channels for discharge, with a shortened travel time and increased proportion of runoff. These two changes together conspire to reduce the lag time   between the center of mass precipitation (CMP) and the center of mass runoff (CMR). The chance of flooding is thus increased together with the erosion potential.

The SMLC have reported significant loss of agricultural land and natural systems due to urban expansion. The Klip River wetlands and Klipriviersberg are under severe pressure from urban encroachment and poor management (SMLC: LDO, 1997).

REFERENCES

GJMC 1997: Integrated Metropolitan Development Plan. GJMC: Braamfontein.

Miller, G.T. 1993: Environmental Science – Sustaining the Earth. (4^th ed.) Wadsworth: California.