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Water ~ Urban Run-off

Author: Lt Col Etienne F van Blerk, Staff Officer – Environmental Co-ordination, Department of Defence, RSA

( Article Type: Explanation )

In days when the landscape of the interior was covered in natural vegetation, foliage assisted in the temporary retention of rainwater. While a small quantity of the water would generally evaporate, most of the rest seeped into the soil to replenish aquifers and nourish springs. Recent development has resulted in more homes, shopping centres, streets and highways, sidewalks, driveways and parking lots – all of which shed water more effectively than the proverbial duck’s back. The remaining green spaces in urban areas are subject to pedestrian and other traffic that compresses soil and also reduces the rate at which it absorbs precipitation.

It has been found that there is a significant increase in storm-water run-off, flooding, erosion, pollution, damage to aquatic ecosystems and a drop in water quality, where more than 10% of a developed watershed is covered by impermeable surfaces of asphalt, bitumen, concrete, brick, tile and iron sheeting. Streams and watersheds show severe physical and ecological damage where hard surfaced urban areas exceed 25%. In addition, impervious surfaces, absorb more solar radiation than natural surfaces thereby creating socalled heat islands. By losing its capacity to retain soil moisture, a city – in a sense – loses its capacity to ‘sweat’ and cool itself. Such urban heat islands could be from 0.5°C to 1°C warmer than surrounding rural areas.

Storm-water run-off from these sun-baked hard surfaces in urbanised areas is warmer than run-off from surrounding undeveloped areas, in instances by as much as 6°C. The increased temperature of storm-water run-off together with bacteria, oxygen depletion, pollutants and toxins such as heavy metals, sediment dust, nitrates and phosphates from fertilisers, and carcinogenic polycyclic aromatic hydrocarbons (PAHs) leached from asphalt and coal tar-based sealants, spell sheer death to wetlands and resident aquatic organisms, and peril to water quality and public health. Torrents of unrestrained storm water during rainstorms also cause erosion and structural damage to urban infrastructure.

Innovations are, however, available to control storm-water runoff in urban areas. Porous pavements consisting of open concrete blocks, paving stones and turf-stones that allow plant growth within the paving matrix can absorb water and degrade traces of oil or grease. The use of greenbelts allows run-off to be channelled away from parking lots into porous topsoil supporting vegetation on the verges of these paved areas. Run-off from storm-water inlets can be channelled to flow through a bed of absorbent material before it is discharged into the storm-water system. Wet scrubbing techniques in which water and biodegradable detergent is sprayed onto paved surfaces can be applied to render oil and grease soluble for purposes of surface cleansing. Infrared satellite images mapping paved urban areas can be used to focus costly interventions such as these in the management of urban storm-water run-off.

Measures such as these should be applied as part of a broader strategy to conserve scarce water resources in South Africa and other arid parts of the world. The solution lies in an integrated approach by government and communities in paving the way to sustainable water resources for the future.