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Floods: CSIR’s Green Book shows which South African cities are most at risk

Extremely high Flood Hazard Index values are found in areas in Limpopo, KwaZulu-Natal, Eastern Cape and Lesotho. PICTURE: CSIR

Extremely high Flood Hazard Index values are found in areas in Limpopo, KwaZulu-Natal, Eastern Cape and Lesotho. PICTURE: CSIR

Published May 8, 2022

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WEEKS after devastating floods claimed hundreds of lives, displaced thousands and destroyed billions of rands worth of infrastructure, many are still left with more questions than answers. What could we have done better?

While many questions were answered by experts and government officials, these questions and answers have been around for a while.

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The South African Council for Scientific and Industrial Research (CSIR) Green Book is an online planning support tool that provides quantitative scientific evidence on the likely impacts that climate change and urbanisation will have on South Africa’s cities and towns, as well as presenting a number of adaptation actions that can be implemented by local government to support climate resilient development.

The Green Book was co-funded by the CSIR and the International Development Research Centre (IDRC) between 2016 and 2019. The CSIR has partnered with the National Disaster Management Centre (NDMC) and co-developed this product with universities, government departments, NGOs and other peer groups.

THE CSIR estimates that a growing number of people in South African cities and towns will be exposed to the devastating impacts of weather-induced natural hazards such as flooding, heatwaves, droughts, coastal flooding, wildfires and storms which threaten livelihoods, increase vulnerability and undermine hard-earned development gains.

‍The Green Book project was designed to look at the components that increase risk, and provide scientific evidence and information that can be used by local government and other role-players, to better understand local risks and vulnerabilities and to respond to these challenges through climate change adaptation actions.

It seeks to facilitate the integration of climate change adaptation into local government planning instruments and processes, in support of the development of climate resilient cities and settlements.

The Book has extensive free planning tools to assist with sustainable urban development in South Africa with a section dedicated to the potential impacts of flooding and strategies for cities to use to help prevent or reduce these impacts.

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According to the CSIR, floods are the most frequently recorded disasters in southern Africa with South Africa experiencing 77 major floods between 1980 and 2010, claiming the lives of at least 1,068 people. Many severe floods have occurred since 2010 with losses of life, livelihoods and extensive damage to built infrastructure. Recovery has taken years and has required ongoing investment; yet, despite all these endeavours, many communities have not been able to get back to where they were before.

A useful approach to understanding floods is to unpack them using a risk assessment. This approach sees risk as a combination of the factors that create a flood hazard and the consequences of a flood for a society.

Although floods are inevitable, the main ways of dealing with them are to ensure that the catchment storage capacity is maintained or restored and that developments susceptible to flood damage are moved out of areas that are likely to be flooded.

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Protecting natural vegetation, especially along streams and rivers, is still very important because it is resilient and not easily dislodged if it is in good condition and so will reduce the damage caused by the flooding.

For this risk assessment, the CSIR identified municipalities that have a relatively high flood risk so that they can be highlighted for mitigation actions. It assesses the flood risk for settlements based on the characteristics of the climate and the upstream water catchments and the areas of each settlement that are potentially exposed to floods.

Flood risk, like many other risks, can be disaggregated into two main components, a flood hazard and a flood consequence. Flood hazards focuses on the nature of the flood events and includes the likelihood and the severity of flood events.

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Flood consequences focuses on the effects of flood events on people, their livelihoods and infrastructure, and includes the degree of exposure, alternatively the magnitude of the consequences, and the resources available for them to recover from the flood. Flood consequences are also commonly called flood vulnerability.

Several terms and variables are used to describe the consequences of a flood. For this study, CSIR researchers used a combination of exposure, susceptibility and resilience, which are further explained below.

1. Exposure -- Exposure deals with the characteristics of the flood (e.g. depth, duration), the predisposition of people and their assets to flooding, and the value of those assets.

2. Susceptibility -- Susceptibility is about whether or not the community and the authorities are able to take appropriate actions (mitigation or adaptation) and to respond during a flood.

3. Resilience -- Resilience is about the ability of the social system (people) and the governance systems to recover from a flood.

The study looked at historic rainfall patterns in South Africa and found that the eastern coast as well as parts of Mpumalanga and Limpopo experienced the highest rainfall patterns in the country. This means most of the eastern part of the country can expect to have more than 130 mm, and some parts more than 230 mm, of rainfall in a single day about once every 50 years.

This is a considerable volume of water as 130mm equates to 1,300 cubic metres per hectare and if this falls over a 10,000 hectare catchment, it would amount to 130 million cubic metres of water, enough to fill a large dam. To put this in perspective, only 15% of South Africa’s dams have a capacity of 100 million cubic metres or more.

Some of the rainfall is likely to be absorbed by the soil or captured within dams, so the daily rainfall cannot simply be converted to a flood volume. The volume calculated above just provides an indication.

However, if the rainfall intensity is high like the rains which resulted in the KZN floods, then most of that rainfall will become floodwater because the soils and other permeable surfaces simply cannot absorb that amount of water. If the soils were already moist or wet due to recent rainfall, or if the high rainfall continues for many hours or days, then most of the rainfall will become flood water.

The study shares that although there are uncertainties about changes in rainfall under future climates, there is general agreement that rainfall intensities will increase. So, with future climates it is likely that rainfall will increase and that the volumes of storm run-off will increase, leading to the increased occurrence of floods.

This comparison indicates that the extreme daily rainfall will increase in many parts of the country, particularly over the Highveld and northern Drakensberg as well as in a broad belt along the south-eastern and eastern coast. This is consistent with an expectation that increasing temperatures due to climate change will increase the intensity of the convection rainfall systems or thunderstorms, which are characteristic of this part of the country.

The western and south-western regions are likely to experience a decrease, including the winter rainfall region, which may lead to prolonged dry periods and droughts, a story for another day.

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