Assessing soil erosion associated with main roads in south-eastern South Africa.
Construction of linear infrastructure such as roads is increasing worldwide for the provision of efficient transportation of both humans and commodities. However, roads have been widely recognised as significant causes of increased soil erosion due to their influence on the hydrologic and geomorphic processes through the modification of natural hill-slope profiles, the construction of cut and fill embankments as well as impervious road surfaces that concentrate runoff. Accelerated soil erosion due to roads is of particular concern since the associated environmental impacts have economic ramifications related to water treatment and soil rehabilitation. In the light of the above, a better understanding of road-related soil erosion is required to guide environmentally sustainable future developments and erosion control efforts. The present study assesses soil erosion associated with main tar roads in the south-eastern region of South Africa. The first part of the study provides an overview of the linkages of roads with soil erosion by water, related structural designs that facilitate soil erosion processes as well as available approaches for assessing road-related soil erosion and the available erosion control techniques. Secondly, the study focuses on exploring the characteristics (i.e. gradient, length, and vegetation cover) of degraded and non-degraded roadcuts with a view to understanding why some roadcuts are degraded while others are not. Moreover, the study investigates the relationship between the characteristics of the roadcuts and the dimensions (i.e. width and depth) of the rills. Results show that degraded roadcuts are steeper, longer and have a lower percentage of vegetation cover when compared to non-degraded roadcuts. The results further show that there is a significant relationship between the width and depth of the rills, and the slope gradient and percentage of vegetation cover of the roadcuts. These results prompted the need to evaluate the volume of soil loss, using rill dimensions on roadcuts as well as an assessment of the relationship between the volume of soil loss and the soil properties. Results show that soil loss correlates significantly with all the rill dimensions, and the rill depth is the foremost variable in calculating rill volume than the rill width and length. In addition, the results show that there is a significant relationship between the volume of soil loss and the soil properties of the roadcuts. The study further used remotely sensed data to assess gully erosion related to road drainage release and examined the relationship between physical and climatic factors (i.e. road contributing surface area, vegetation cover, hillslope gradient and rainfall) and the volume of gullies. The results indicate that the road contributing surface area, vegetation cover and hillslope gradient have a significant contribution and influence on the size of the gullies along major armoured roads. Moreover, the results show that remote sensing technologies have the capability to investigate road-related gully erosion where detailed field work remains a challenge due to economic and time constraints. Finally, in order to evaluate the effectiveness of soil erosion control methods along the roads, the study investigates the performance of different soil erosion control methods utilised on the roadcuts. It was observed that most of the slope stabilisation methods are successful in controlling soil erosion while the majority of drainage control methods performed poorly. The results show that good performance is related to vegetation re-establishment, while poor performance may be attributed to improper application, lack of inspection and maintenance. Overall, the study provides an understanding of erosion related to the post construction phase of roads. In this regard, it is expected that the results of this study will contribute to the management of roads from the soil erosion perspective through appropriate interaction with the South African National Roads Authority (SANRAL). It is hoped that this work will lay the foundation for environmentally sustainable road construction, maintenance and the formulation of effective soil erosion control measures in the future.