This chapter describes the processes involved in the transport of material over hillslopes. Hillslopes provide the gradients enabling material to be transported from the slopes themselves towards the valley bottoms, directly by gravity alone, or by water flowing down over the surface. Gravity has the potential to transfer material downslope if the material resistance to counteact it is insufficient. Similarly, water and wind flowing along the surface exert a drag on soil particles and have the potential to entrain material. After the gradient has fallen below a critical threshold to keep the material in tra- port, deposition occurs. Conventionally, slope processes in which gravity alone is the dominant transporting agent are called mass movements (Brunsden 1984). Processes in which other agents dominate are called erosion, e.g. wind or water erosion. Flowing water is an important transporting agent on which the emphasis is placed here. Although mass wasting is a natural process its incidence and impact may be exacerbated by human activities (Crozier 1986; Morgan 2005). A situation may ultimately arise in which human interests become unsafe or unsustainable. This impact is not only directly felt in the areas where material is removed or deposited and indirectly mass wasting may have an effect on soil and water quality in areas located further downstream. In the following sections a brief overview is given of the mechanisms, morphology, causes and consequences of mass movements and erosion.
This book is designed to assist the civil and geotechnical engineer, geomorphologist, forester, landscape architect or ecologist in choosing ecotechnological solutions for slopes that are prone to a variety of mass movements e.g. shallow failure or erosion. Within this book, the ‘engineer’ is used in the global sense to encompass all planners, designers, etc who are involved in the stabilisation of slopes. We review the types of problematic slopes that may occur and describe briefly the nature of mass movements and the causes of these movements. In this book, we focus on the use of vegetation to stabilize soil on slopes prone to mass movements. Before a plant can be chosen for a particular function, its physical and hydrological properties must be determined, thus the root architecture of grasses, shrubs and trees are described and the soil hydrological and mechanical factors which influence vegetation are discussed. Depending on the use of the slope, the engineer may wish to ascertain either the stability of the slope or the mechanical stability of the vegetation or both, therefore slope stability analysis methods are reviewed and the contribution the vegetation has to the stability of the slope are explained. Models to assess the mechanical stability of vegetation are reviewed. This book also introduces new ecotechnological methods for stabilising active rockfalls on steep slopes and slopes that are prone to soil erosion following wild fires, as well as providing user friendly information on traditional ground bio-engineering techniques and tables of plants suitable for different functions. Case studies where ground bio- and eco-engineering measures have been put into practice are also discussed.
