Flood risk assessment for South African townships
Project facts
- ClientJohannesburg Roads Agency (Pty) Ltd
- LocationJohannesburg, South Africa
- Date2020-2021
- ChallengePredicting the impacts of illegal dumping in Johannesburg’s waterways
- SolutionState-art-modelling, predictive rainfall analysis, and flood risk assessment
The challenge: assessing the impact of illegal dumping
In Johannesburg, the widespread illegal dumping of building materials in rivers is having a noticeable impact on the city’s watercourses – encroaching on floodplains, creating unsafe living conditions, causing damage to existing infrastructure, harming the environment and increasing the risk of flooding.The areas of greatest concern are three townships: Alexandra township along the Jukskei River, Kaalfontein township along the Kaalspruit, and Diepsloot township along a tributary of the Jukskei River.
To assess the flood risk in these areas and better understand the impact of illegal dumping, the Johannesburg Roads Agency (JRA), working on behalf of the City of Johannesburg, appointed Royal HaskoningDHV.
The solution: quantifying economic risk through advanced modelling
To accurately assess this flood risk, we developed six, state-of-the-art, cloud-based flood models; two for each area of interest. In each location, one model represented the situation in 2012, before dumping became a major issue, while the second model reflected the situation in 2019.
We also carried out an in-depth analysis of rainfall in each of the locations to determine critical storm durations and rainfall intensities. This information was then implemented into the hydraulic models.
Armed with this information, we ran a total of 28 different simulations for the three locations, pre- and post-dumping and a variety of likelihoods of occurrence to derive flood lines, water depth maps, water level difference maps, and flood hazard rating maps.
We followed this up with a quantitative flood risk assessment to provide a deeper understanding of the economic flood risk and to create insights how risk levels have changed between 2012 and 2019.
These assessments were performed with our Global Flood Risk tool – a cloud-based platform used to quickly and accurately calculate flood damage and risk.
The result: flood risk insights to guide future investments
As a result of our analysis, we were able to draw several conclusions and provide recommendations to help the City of Johannesburg to improve the model going forward.
The study found that water level increases of up to 1.8 meters could occur as a result of the illegal dumping during a 100-year flood.
We were also able to calculate and visualise an increase in economic flood risk – the expected annual direct flood damage – between 2012 and 2019. Our models showed an increase of 12-15% for Kaalfontein, 33-34% for Diepsloot, and 8-10% for Alexandra which supported the need for improved urban river management.
Recognitions:
This study was presented during the 85th IMESA conference 2022 in South Africa and it received the ‘Best Paper Award’ for non-IMESA members. The research paper “Quantitative flood risk assessments for three townships in Johannesburg using high-resolution modelling” can be found here: https://www.imesa.org.za/wp-content/uploads/paper-2-1.pdf
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