Infiltration experiments in Cape Town

The infiltration experimentation in Cape Town is being undertaken as part of a Civil Engineering PhD study (Mr Craig Tanyanyiwa) entitled ‘An experimental study into the potential use of existing stormwater detention ponds for managed aquifer recharge and reuse in Cape Town, South Africa’. Several other student projects (BScEng and Masters) will complement this study, which comprises three parts; i.e. field investigation with repurposed existing detention ponds; a stormwater harvesting test facility to be set up on UCT campus; and a laboratory-based experiment.

A comprehensive site selection process resulted in the selection of two sites to be used for the field investigation in this study, namely the School Pond and the Green Dolphins Pond (further detail on this aspect of the study is provided in Appendix B). Preliminary designs for retrofitting the ponds to enhance infiltration have been completed that also incorporate some landscaping to extend the amenity of the ponds. This is particularly important at the School Pond which will primarily be used to investigate local resident interactions with the work going on in the pond to explore the possibility of creating local amenity while improving biodiversity. The landscaping designs were informed by a Masters student from University of Copenhagen (Ms Bettina Kappeli) who spent two weeks in Cape Town during March 2020 conducting site visits and meeting with researchers. Prior to, during, and subsequent to her visit, there has been ongoing engagement with local stakeholders (municipal ward councilor, head-mistress of the Leadership College school, and local residents) in an attempt to understand some of the local needs with respect to the detention pond (including the potential for recreational facilities), and to explore the feasibility of including local residents in the construction and monitoring processes. A partnership with the school has been established, to allow for potential educational opportunities and to make use of the school facilities for installing weather station infrastructure, etc. The pilot design of the managed aquifer recharge experiment and the landscaping suggestions for the multifunctional pond area were shared in an online (due to restrictions on in-person meetings as a consequence of Covid-19) meeting with some of the key local stakeholders. The physical establishment of the field experiment has also been affected by Covid-related delays, as has further stakeholder engagement. However, the project manager for the construction phase has now been appointed, and the necessary approvals for the work have been sought from the City of Cape Town – with a view to commencing the work early in 2021 so that the next winter rainfall season can be monitored. Another PhD student (Ms Julia Mclachlan) will be responsible for the engagement processes associated with the multifunctional pond design and the associated landscaping.

In the meantime, planning for the establishment of a permanent test site for monitoring of stormwater runoff quality at the University of Cape Town is at an advanced stage, with the necessary approvals in place and the design completed. The facility will comprise a large (7m x 7m x 2m) tank, divided into three separate cells each filled with soil media to a specified depth to create bioretention cells (that will mimic conditions at the retrofitted detention ponds). The facility will be used to conduct experiments to evaluate the treatment capability of different media, as well as the effectiveness of different indigenous vegetation to, inter alia, mitigate surface clogging – and its placement on UCT campus means that testing can continue even during times of Covid-associated lock-downs / restrictions. Construction of the test facility is expected to commence early 2021.