Soil mechanics and geotechnical controls of channel and hillslope processes Special Issue
The dynamics of various landform elements, such as stream banks, mass movements, cohesive deposits and rills, are controlled by principles related to soil mechanics and geotechnics. This particularly is true for cohesive materials where electrochemical forces bond particles, making erosion estimates a function of properties other than particle or aggregate size and weight. Gravitational forces acting on in situ material interact with surface and subsurface hydraulic forces to shape channels and hillslopes. Forces such as shear strength, shear stress, pore-water pressure, and tension in saturated and unsaturated media exert an important influence on rates and types of erosion processes. Recent research provides a new focus on insights into the role of soil mechanics and geotechnics on channel and hillslope processes and forms. This new work led the editors to propose and convene a special session of the American Geophysical Union (AGU) on Soil Mechanics and Geotechnical Controls of Channel and Hillslope Processes. The session was held at the AGU Spring Meeting in Boston during March of 1998. This special issue of Hydrological Processes contains a selection of papers presented at the AGU session as well as some additional related research. The collection of papers included here covers a diverse range of approaches and techniques including laboratory, field and numerical-modelling studies of both channel and upland processes.
The papers in this special issue begin with a study of the controls of gully head retreat. Collision utilizes field evidence and finite-element seepage and effective-stress modelling to propose a cycle of gully expansion controlled by unloading and the consequent development of tension cracks, thereby enhancing pore-water pressures, throughflow and piping leading to collapse of the head wall. Continuing with the theme of subsurface flow, Owoputi and Stolte present a laboratory study on the effects of seepage on soil erodibility and erosion rates of sand and clay till. Erosion rates are shown to increase under the influence of seepage as a result of reduced erodibility, and not due to an increase in total runoff rates. The paper by Hanson and Simon addresses the erodibility of cohesive streambeds in the loess area of the midwestern USA from a hydraulic stress approach by evaluating critical shear stresses in situ with a submerged jet-testing device. Erosion rates are then calculated based on a relationship between excess stress parameters.
The two remaining papers address streambank-stability issues and the role of geotechnical properties of woody, riparian vegetation and bank-toe material. Wood et al. combine hydraulic-stress analysis with measurements of an apparent cohesion between failed cohesive blocks and the underlying substrate to provide an improved mechanism of estimating the entrainment potential of failed, cohesive blocks. Abernathy and Rutherford introduce a unique device to test the tensile strength of riparian tree roots and use the results along with data on root size to develop quantitative estimates of cohesion resulting from root reinforcement. These data are then combined with measures of the distribution of roots in the subsurface and with lateral distance from the tree to provide a model of root reinforcement at their study site in Australia.