Marine plastic debris has become an established concern as a threat to marine and coastal ecosystems. Despite progress in understanding plastic transport dynamics under deep-water conditions, the characterisation of these processes in the nearshore environment remains incomplete. This poses significant challenges in their parametrisation, essential for the accurate representation of coastal transport dynamics in predictive models.

In this study, experimental measurements of the plastic particles wave-induced transport in intermediate to shallow water depths are presented. The focus is put on the influence of wave steepness as a key parameter affecting the transport of marine plastic debris in the transition from deep water to the shoreline. Its potential as a predictive parameter is investigated through controlled laboratory experiments involving the generation of seven regular breaking wave conditions, characterised by varying offshore steepness, propagating in shallow water depth over a sloped bathymetry.

The results reveal a consistent increase in particle drift speed with increasing offshore wave steepness. While the exact functional nature of the observed positive relationship could not be definitively concluded, the trend appears more likely linear than quadratic, aligning with previous findings for particles deviating from perfect tracers undergoing deep water breaking conditions. Furthermore, wave breaking was observed to play an important role in enhancing particle drift speed. Finally, particle drift speeds were consistently underestimated by the Stokes drift and only partially captured by the wave crest speed estimates, progressively diverging from the former and approaching the latter as offshore steepness increased, though remaining consistently lower than crest speeds. This trend was most recognisable in the breaking zone across all the tested wave conditions.

Overall, the findings suggest offshore wave steepness as a robust predictor for marine plastic debris transport in the nearshore environment, proving its value as a classification parameter for future modelling efforts. By investigating how plastic particles respond to changing wave conditions in the nearshore environment, this study aims to contribute to a better understanding of their dynamics. ... Read more

The Limpopo Lipadi Reserve has the difficult task of restoring its natural ecosystems and protecting them from future challenges. The increasing occurrence of droughts due to climate change and the historical use of this land for cattle farming contribute to concerns about the future availability of water for animals, vegetation and staff, as well as the overall health of the soil. By establishing a water balance and investigating soil health, conclusions could be drawn about the current state of the Reserve’s soil and water resources and recommendations made for future research. The parameters of the water balance were defined by combining the literature and the results of field experiments. A climate change model was applied to the water balance to assess how the Reserve will be affected by changes in precipitation and temperatures. Soil sampling was also undertaken at four characteristic sites in the Reserve to assess the impact of bush clearing on soil health and aquifer recharge through changes in physical, biological and hydraulic properties.
The results of the water balance and the different simulated scenarios show that: 1) the aquifers can currently be accounted as reliable when considered as a total available resource for the entire area of the Reserve; 2) when the bush clearing scenario was simulated, it was found that doubling the amount of clearing has a minor impact and only when 50 % of the reserve is cleared the impact becomes significant; 3) due to climate change and its impact on ecosystems, it was found that there will be an intensification of the hydrological cycle (wetter, hotter summer) with an increased seasonality. However, the results of this scenario indicated that there will be no drastic changes in the main pattern of water dynamics in the next 25 years and therefore no immediate threat to the available groundwater storage.
In carrying out the soil characterisation tests, it was noted that 8 different soil types were being studied, which would certainly include a wider range of values for soil properties. However, looking at the effects of bush clearing and considering the different types of soil, the results showed that there was indeed an outcome in the treated areas. For most of the studied sites, it was consistently found that bulk density had increased in the cleared areas, while porosity levels, soil moisture and organic matter decomposition rate had decreased. It was also discovered that as a side effect of bush clearing, insects such as termites were present, which played a role in some of the soil processes. Furthermore, no clear relationship with clearance status could be observed for hydraulic conductivity. These results were then used in a multi-criteria analysis to assess the health of the soils studied. This assessment showed that, overall and for the specific purpose of the research undertaken, all the soils analysed could be classified as ’healthy’ to sustain the current environmental practices of the Reserve, even after clearing was performed.
Although the results presented in this report take into account the current status of the Reserve, it is noted that there may be differences when different time frames are considered. The results provide valuable insights based on the highlights found and, based on these, recommendations that will impact the future environmental management and land use practices of the Reserve are provided.
Further analysis is recommended to gain a complete understanding of the possible effects of bush clearing on water dynamics and to compare the results presented in this research. It must also be
borne in mind that there may be discrepancies in the results obtained due to lack of equipment and time constraints. ... Read more