Abstract
A stormwater drainage system is a system of subterranean pipes, channels, and buildings used to transport rainwater to lakes, ponds, streams, and rivers. Worldwide each country sets design guidelines, standards, and policies of stormwater drainage systems from planning, design, implementation and operational monitoring and maintenance as their stormwater management practices. Pollution, flooding, population growth, LULC changes, and aging infrastructure are some of the challenges facing stormwater management. The study aims at investigating the stormwater management practices within the city of Johannesburg and its adjacent catchment, focusing mainly on the Johannesburg Business District (CBD) as a case study. To accomplish the aim three primary objectives were investigated: (i) to determine the quantity of the pre- and post-development stormwater runoff from the Johannesburg CBD area; (ii) to develop an asset register of stormwater management practices encompassing green and grey infrastructure such as rain gardens, rainwater harvesting and bio-swales (iii) to assess and determine the effectiveness and the benefits of sustainable drainage systems (SUDS) in Johannesburg CBD.
Pre-development (1886 to 1900) and post-development (1990 to 2016) quantity of surface runoff were measured using Soil and Water Assessment Tool (SWAT) model and Rational Method (Q = CIA/3.6) using recorded data, data from varies institutions and/or extrapolated data especially for pre-development runoff. The asset register predominantly depended on data received from Johannesburg Roads Agency as they are the custodians of roads and stormwater infrastructure within the city, also GIS was used for the analysis of the land use land cover (LULC) and identification of stormwater infrastructure according to green and grey infrastructure categories. With limited time to design and test an actual infrastructure this study used literature review to assess and determine the effectiveness and the benefits of SUDS.
Wherefrom the study, it was established that the founding of gold in the Witwatersrand near Johannesburg CBD area in 1886 resulted in a drastic change of land use land cover, from a farming community to an industrial mining and financial powerhouse of Southern Africa. Within the catchment area, according to the post-development period, only 35% of the land is used for habitations, while 65% is used for open spaces and other purposes. However, only
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10% of this area was used for urban settlement during the pre-development period, with the remaining 90% being designated as agriculture. In the pre-development condition, there was also evidence of an annual precipitation of 757 millimetres, of which 28% became surface runoff and 45% was lost to evaporation and transpiration. On the other hand, the post-development period saw 772 mm of annual precipitation, of which 29% was lost through evaporation and transpiration and 50% through surface runoff. This study also provides further evidence that the amount of runoff in CoJ has definitely been impacted by LULC variations over time. This resulted in a more urbanised area, consequently resulting in increase in impermeable surface areas that increase surface runoff and damage the environment through flooding. Furthermore, from the assessment of the stormwater drainage infrastructure, it is evident that currently in Johannesburg CBD most of the infrastructure is now grey at 87 percent against only 13 percent green infrastructure and the resident’s properties and the environment is prone to flooding and/or damage.
The evident increase in stormwater runoff requires a proactive, innovative, and sustainable approach by government and developers in managing stormwater. Many stormwater management systems have been researched and tested with their benefits highlighted such as SUDS (bioswales, detention basins etc.) and SWH etc, which promotes green stormwater infrastructure management techniques. And the benefits of a green stormwater infrastructure are the reduction of environmental and property damage and flooding amongst others and has a gentle approach to runoff management. The ancient techniques of stormwater management such as grey infrastructure (impermeable surfaces etc.) are effective in conveying runoff but have their negative impacts such has high cost than green stormwater management infrastructure etc. However, the combination of both green and grey stormwater infrastructure management systems in one stormwater management system has also been proved effective and efficient.
Therefore, the study recommends that the City of Johannesburg should, moving forward, construct sustainable drainage systems as recommended by the new drainage design manual and where upgrades of old drainage infrastructure are required the drainage systems should be retrofitted with SUDS.