Advice and Opinion

Urbanisation and the importance of flood line measures

Flooding in the City of Tshwane.

The question of flood lines is in the spotlight following serious flooding in parts of South Africa declared a national disaster by the National Disaster Management Centre (NDMC).

Heavy rains in February, mainly in the Eastern Cape, KwaZulu-Natal, the Free State, and North West provinces, destroyed property and infrastructure and claimed over 80 lives.

While the national disaster declaration allowed more direct intervention in recovery by national government and organs of state, the NDMC noted that “these threatening conditions call for an all-society and government approach to promote risk reduction”.

Among the standard measures in place to protect communities, businesses, and infrastructure from flooding, is the regulated flood line alongside rivers in urban areas – below which any building or development is not permitted.

According to Xanthe Adams, principal engineer at SRK Consulting, a flood line is an imaginary line on the ground that denotes the edge of the water during a flood. “The floodplain is the area alongside a river that will be expected to be under water during a flood, with the upper edge of this floodplain being designated as the flood line”.

The National Water Act requires these flood lines to be shown on plans for housing and other developments, which show the highest level that a flood could reach every 100 years – the most common stipulated timeframe. All buildings need to be above this flood line to avoid danger of flooding.

Adams highlights that municipalities are responsible for enforcing compliance with flood lines as part of reducing flood risks to communities and infrastructure. Their planning must mitigate and manage the effect of urbanisation, which generally means more rainfall run-off and greater risk of flooding.

Urbanisation is certainly a significant factor increasing the size of floodplains and the velocity of floods,” she said. “We see its effect frequently as we determine flood lines, especially in older urban areas, where we often find that some of the infrastructure is actually already within the flood line.”

Among the efforts by local government to address rising run-off levels is to ensure that developments design water attenuation facilities into new projects. These dams, ponds and wetlands will act to slow down the flow of water and allow more of it to percolate into the ground.

At the most basic level, however, municipalities must ensure that any new developments are outside the flood line and must also upgrade culverts and drainage infrastructure whenever necessary, as part of their emergency planning.  

Other situations where a flood line is required include mining operations which might encroach on rivers or streams,” said Adams. “Holders of water-use licenses might also need to determine flood lines, as their work may potentially disturb the area within the flood line.”

It is likely that a flood line will be required by law if a developer or other entity are planning to build on a property that has a river or stream running across it, she said. It might also be a legal requirement if a river is within a few hundred meters of the property being developed.

Companies who require a full environmental impact assessment (EIA) or even a basic environmental assessment may be required to furnish a flood line as part of the permitting process,” she said. “In fact, planning any activity in or near a river – such as building a bridge or installing a pump – my require a water-use license that includes a flood line.”

She notes that while the recent flooding seemed out of the ordinary, this would still have to be confirmed by detailed studies. It was also difficult to know the extent to which climate change contributed to the magnitude of these rainfall events, as there were many different global climate models – each with a range of predictions.

More important will be the country’s ability to develop flood protection measures alongside flood line studies, as it is likely to be impractical to move houses and infrastructure which today lie within floodplains. Flood management on the catchment scale is another relevant field of enquiry which can significantly reduce flooding so I wouldn’t be surprised to see more regulations focusing on catchment scale management of floods.” 

Flood line studies remain a highly specialised task, and they need to be properly signed off before a local authority will accept the flood line determination. This sign off must be done by a professional engineer registered with the Engineering Council of South Africa.