Civil Engineering December 2021 | Vol 29 No 11

30 December 2021 Civil Engineering T he Stormwater Design Manual for the City of Johannesburg was pre- pared to meet the stipulation in the City’s Stormwater Bylaws that the calcula- tion of stormwater runoff, and the design of stormwater management facilities, be done in accordance with a manual. While the bylaws, promulgated in 2010, required stringent management of stormwater in the design, construction, and operational phases of most projects that changed the land use on properties within the metropolitan area, their provi- sions were largely ignored because it was not clear how the control of stormwater runoff should be achieved. The operative clause in the Bylaws, rather obscurely buried in the body of the document as Clause 32.(2) stipulates: “The characteristics of all stormwater from a development site with regard to quality, flow rate, velocity and frequency must be the same as the stormwater runoff which would have flowed from the devel- opment site in its natural condition …” The Manual sets out to advise practitioners on how to achieve that target. It recognises that each site and each development is unique with its own set of opportunities and constraints. It is therefore not intended to be a code of practice or to be in any way prescrip- tive, but rather to summarise the state of the art in stormwater management worldwide in a way that can be applied to the Johannesburg metropolitan area and give guidance on the selection of design parameters appropriate to the region. FOUR PART STRUCTURE The Manual was prepared under the auspices of the University of the Witwatersrand by a multidisciplinary team of professionals The first part, in the form of an intro- duction, sets out the principles on which stormwater management is based. This serves as a reference for interpretation, with practitioners being referred to the principles to resolve any uncertainty in application. The second part gives chapter by chapter guidance on the interpretation of the Bylaws. It includes a copy of the Bylaws themselves for ease of reference and gives information on how to achieve the targets set out in each chapter of the Bylaws. In some instances, guidelines are given for the interpretation of specific clauses. The third part comprise seven ap- pendices. The first of these describes the geology and soils of the area which determine the proportion of rainfall that will be converted to runoff. The next two discuss the effectiveness of green infra- structure and how this can be utilised to go beyond simply meeting targets to yield considerable benefits in ecological services and quality of life. The fourth appendix gives advice on calculation methods and modelling, and the fifth is a review of the design rainfall in Johannesburg. This fifth appendix also touches on the likely effects of climate change, showing the shortcoming of the implicit assumption that rainfall in the future will be statistically similar to that of the past. The penultimate appendix discusses and gives guidance on the design of sediment control BMPs. The final appendix gives guidance on the capture and management of data that can be used to improve the accuracy of future calculations. The final part of the document is a list of more than one hundred refer- ences that will be useful to practitioners working to mitigate the negative impact that urban development has on the safety, resource yield, ecology, mor- phology, water quality and aesthetics of our river systems. MULTIDISCIPLINARY PROJECT The Manual represents the end product of a truly multidisciplinary process involving the overlapping skills of civil engineers, landscape architects, architects and lawyers in close collabora- tion with the client’s own team. The writing of the Manual was pre- ceded by extensive study of state of the art stormwater management worldwide. The document contributes significantly to the knowledge of South African practitioners and has helped widen the interest in the field.  SANRAL SA I CE 2021 Nat i onal Awards Category: Techn i cal Excellence FINALIST Technical Excellence category (Sponsored by Maccaferri SA) KEY PLAYERS Client: City of Johannesburg EISD Consultant: cba Specialist Engineers Stormwater Design Manual for the City of Johannesburg The Manual advises practitioners on how the control of stormwater runoff should be achieved