Identifying the causes of flooding on a national route in Ekurhuleni, South Africa
Changing rainfall patterns and intensity are often considered as the main causes of flooding events. Consequently, detailed investigations into other likely causes of flooding are often not carried out. The increased frequency of flooding events and their associated impacts, such as disruption and the loss of both life and property, highlight the need for comprehensive evaluation of the causes of urban flooding. The purpose of the study was to investigate the causes of flooding at the South African National Route-12 (N12), section-19, between the N12/R24 and Gillooly’s Interchanges located in Johannesburg. This followed repeated flooding at the study area most notably on the 2nd of March 2017, 21st of January 2016 and 9th November 2016. The study determined the return period of the rainfall which led to these flood events using a previously developed regional approach of design rainfall estimation. The analysis revealed that only the rainfall event of the 9th of November 2016 exceeded the prescribed design return period of 1:80 years for the drainage structure. Further, hydraulic modelling using the HEC-RAS software package found that the current and older configurations of the drainage structure are inadequate for the recommended 80-year flood, but the upgrades improve capacity by 87.4%. The findings of the study were that the flooding was a result of the inadequate size of the drainage structure. The structure was inadequate for the 2-year recurrence interval flood, far less than the 80-year recurrence interval required for the Class-1 Road. Photographic images showed increased land development (~6%) within the catchment which was estimated to have increased the peak flood by 24%. Lapses in maintenance of the drainage structure were also observed. The intervention to raise the headwall and wingwalls at the inlet to the structure, implemented in 2019, reduces the instances of overtopping of the road but does not mitigate flooding in the surrounding properties and is therefore not holistic. This study provides a methodology to determine the causes of flooding in urban settings and can aid authorities as they seek to understand and address flooding hotspots in urban areas.
A research report submitted in partial fulfilment of the requirements for the degree of Master of Science to the Faculty of Engineering and Built Environment, School of Civil and Environmental Engineering, University of the Witwatersrand, Johannesburg, 2022