Photography by:
  • Laven Naidoo

River plastic waste and wetland assessment using airborne radar data

A Council for Scientific and Industrial Research (CSIR)-led campaign is flying high spatial resolution radar and hyperspectral imagery over parts of the Gauteng City-Region (GCR). This is a collaborative project between the CSIR, the University of Pretoria and commercial stakeholders. Multiple collaborative projects have spawned from this campaign and the Gauteng City-Region Observatory team will be involved in two of them:

1) Hennops river plastic island mapping using high resolution multi-frequency (C- and L-band) SAR imagery;

2) Soil moisture estimation and teal carbon estimation using multi-frequency SAR in the Colbyn Wetland Nature Reserve (CWNR) in the City of Tshwane.

Both projects have strong Sustainable Development Goals (SDG) links within the urban GCR boundary. On the one hand, the plastic island topic addresses, in particular, SDG goal 6 on “Clean Water and Sanitation” and goal 12 on “Sustainable Consumption and Production” and can play a big role in plastic waste monitoring and tracing. Most of the literature focuses on marine plastic waste detection mainly using optical Near-Infrared and Shortwave Infrared satellite sensor bands and associated indices (e.g. Plastics Index), however no studies are available within the urban river network where tributaries are small in width and are also obscured by overarching tree lines. The sub-metre spatial resolution of the airborne SAR sensor as well as the ability to penetrate the tree lines makes this study attainable. On the other hand, the soil moisture and teal carbon (including green carbon) estimation in CWNR specifically address SDG goal 6 (target 6.6: Protect and restore water-related ecosystems) and South Africa’s National Wetland Monitoring Programme. Additionally, teal and green carbon estimation contributes to SDG goal 13 which focuses on combatting climate change and mitigating its impacts. The long-term importance of this work is to build a rich layer of detailed estimates at the airborne scale which can be eventually ‘upscaled’ to freely available satellite level (e.g. Sentinel-1 and ALOS PALSAR 2 sensors).

Field work is being conducted for both study areas during the airborne campaign time frame and the focus would be to increase sampling efficiency and sample size. The campaign is expected to acquire three images per month for months January to May 2022 over a variety of land use types (e.g. farming to urban to aquatic wetlands and rivers to mining and defence). The initial project duration is expected to be two years, with an additional three years (starting in 2023) for the two PhD students who will be recruited to achieve some of the goals set out for this project.

Planned outputs of the project include:

  1. At least one collaborative scientific paper – “Estimation of the volume of plastic waste within the 'great poly-island' of the Hennops River, Gauteng, using airborne C- and L-band SAR datasets”
  2. Registration of two PhD students in 2023
  3. Photo Essays or Story Maps on the sampling methodology for the different scientific aims – “Sampling methodology for teal carbon and soil moisture estimation” and “Sampling methodology for estimating poly-island plastic waste volume”
  4. Map of the Month from preliminary results and/or from publications
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