Assessment of the lower Isipingo catchment’s ability to mitigate flooding, considering the existing drainage system

Abstract

The Lower Isipingo Catchment is located south of Durban in the KwaZulu-Natal Province, South Africa. It presents a particular challenge to urban flood risk. The Lower Isipingo Catchment comprises the split Isipingo and Umlazi River estuarine systems. The significance of the catchment is that it has become one of Durban's most industrially developed areas. The Isipingo wetland was converted into a flatland to facilitate the economic development of industries in the 1940s. The Isipingo Diversion Works System was implemented in 1960 as a flood mitigation strategy using canals at the tail end of the Isipingo and Umlazi rivers. This strategy was not successful, as extreme flooding is a regular occurrence within the Lower Isipingo Catchment, causing constant socio-economic losses and infrastructure damage. The study evaluated the catchment’s capacity to mitigate flooding, considering the existing drainage system. The study's findings can be used to improve the resilience of the catchment against flooding events. The study objectives were made up of three components. The first was to evaluate the effects of urbanisation on the catchment's drainage system. This was achieved by calculating the peak flow. The Rational Method was used to quantify the volumetric flow rate of surface water draining from the catchment area over 20 years. Peak flows were assessed for pre- and postdevelopment scenarios in 2002 and 2022. The catchment was split into four subcatchments: Isipingo 1, Isipingo 2a, Isipingo 2b, and Isipingo 2c. The findings demonstrated that there has been a rise in economic activity through developments in the Prospecton industrial area and the Isipingo Central Business District (CBD) in the lower catchment. As a result, the upper catchment now includes denser residential zones with fewer green spaces and lower surface infiltration. The unit hydrographs show higher peak flows and reduced lag time under post-development scenarios. Isipingo 1 produced the highest increase in runoff flow at 50%, followed by Isipingo 2a and 2b with 33%, and lastly Isipingo 2c with an increase of only 25%. Development in the catchment has led to more impervious surfaces, which has increased stormwater runoff. This increases the vulnerability to flooding as conventional drainage systems do not consider the effects of urbanisation on runoff volume. The second objective was to determine the extent to which solid waste impacts the drainage system’s functionality. This was achieved by physically inspecting the drainage infrastructure using visual inspections. The study found that the system was not functioning at its designed capacity due to the accumulation of debris and waste within the drainage inlets conveyed by runoff in all four sub-catchments. The drainage structures are blocked with silt and debris and damaged in certain instances, preventing the flow of stormwater within the stormwater networks. It was observed that the culverts in Isipingo 2b are under-maintained to withstand the flows and volumes of stormwater. The result of these factors is that stormwater is unable to enter or leave the stormwater systems efficiently, which can further reduce mitigation capacity and increase the risk of flooding. The third objective was to assess the performance of the existing drainage system with varying rainfall data. This was achieved through the completion of a stormwater hydraulic model for the drainage networks. The Personalized Computer Storm Water Management Model detailed the hydrological characteristics of the catchment and the configuration of the drainage network system. The models are based on a 1-hour storm simulation using the 1 in 5-year design rainfall and the 2019 and 2022 flood rainfall experienced on the catchment. The results specified the flooding networks and the severity of flooding, depending on the digital elevation model data and the distribution and intensity of the rainfall. The model’s results indicated that stormwater infrastructure is sufficient to mitigate stormwater runoff for the 1 in 5-year design rainfall and the April 2019 and 2022 flood rainfall. However, due to hydraulic inconsistency of the stormwater network pipe sizes, slopes, cover, and invert levels, the hydraulic capacity has proven to be insufficient in certain areas. This has resulted in localised flooding in Isipingo CBD located downstream in the sub-catchments of Isipingo 1, 2a and 2b along Phila Ndwandwe Road, Thie Road, Clark Road, Pardy Road, and Lotus Road. There is also visible flooding in Isipingo 2c, the Prospecton industrial area, with the following areas being vulnerable: the N2, Prospecton Road, Winter Road, Avenue East Road, Joyner Road, Ocean Road, Delta Road, Duiker Road, and Inner Circuit Road. It was also noted that relying entirely on model outputs and ignoring real-site circumstances might result in an underestimation of flood hazards associated with high rainfall occurrences. The findings of this study can assist eThekwini Municipality to be more proactive rather than reactive to the frequent flooding in the Lower Isipingo Catchment. Knowing the location of the vulnerable areas within the catchment, including the factors increasing the flood risk, can assist in improved resource allocation and preparedness against frequent floods. The implementation of this study’s recommendations could have positive economic, social, and environmental effects on the Lower Isipingo Catchment. Adopting water-sensitive urban design principles with the use of sustainable urban drainage systems is the new approach to the management of stormwater. Treating stormwater as a resource in the water cycle rather than a nuisance. Sustainable urban drainage systems can be retrofitted into the existing drainage network to increase flood mitigation capacity for frequent heavy rainfalls and reduce stormwater contaminants in receiving waters. An all-inclusive strategy that combines modelling with on-site inspections and maintenance will offer a clearer understanding of the system's capabilities and limits, resulting in improved readiness and reaction strategies in the face of changing weather patterns. A holistic approach can be used through cross-sector collaboration amongst various stakeholders to implement innovative institutional structures, policies, and management methods. This network can implement the following: infrastructure planning and upgrading, public participation, early warning systems, stormwater management, and asset management.