The cycle of instability: stress release and fissure flow as controls on gully head retreat

Abstract

Abstract:Gully head and wall retreat has commonly been attributed to fluvial scour and head collapse as a result of soil saturation, sapping or piping. The empirical evidence to substantiate these conceptual models is sparse, however, and often contradictory. This paper explores the hydrological and mechanical controls on gully head and wall stability by modelling the hydrology, stability and elastic deformation of a marl gully complex in Granada Province, southeast Spain. The hydrological and slope-stability simulations show that saturated conditions can be reached only where preferential fissure flow channels water from tension cracks into the base of the gully head, and that vertical or subvertical heads will be stable unless saturation is achieved. Owing to the high unsaturated strengths of marl measured in this research, failure in unsaturated conditions is possible only where the gully head wall is significantly undercut. Head retreat thus requires the formation of either a tension crack or an undercut hollow. Finite-element stress analysis of eroding slopes reveals a build up of shear stress at the gully head base, and a second stress anomaly just upslope of the head wall. Although tension cracks on gully heads have often been attributed to slope unloading, this research provides strong evidence that the so called 'sapping hollow' commonly found in the gully headwall base is also a function of stress release. Although further research is needed, it seems possible that 'pop out' failures in river channels may be caused by the same process. The hydrological analysis shows that, once a tension crack has developed, throughflow velocity in the gully headwall will increase by an order of magnitude, promoting piping and enlargement of this weakened area. It is, therefore, possible to envisage a cycle of gully expansion in which erosion, channel incision or human action unloads the slope below a gully head, leading to stress patterns that account for the tension crack and a stress-release hollow. The tension crack promotes faster throughflow, encouraging hollow enlargement and piping, which undercut the gully head. The tension crack permits the development of positive porewater pressures behind the gully head, leading either to failure or contributing to toppling. Finally the debris may be eroded by fluvial action, unloading a new section of slope and completing the cycle of gully head retreat. Copyright

9 Modelling river-bank-erosion processes and mass failure mechanisms: progress towards fully coupled simulations

“…In other words, only the stresses along the failure surface are accounted for, not the stress distribution within the soil mass. In order to characterise this deformation processes, more complex and sophisticated models used for slope analyses, namely stress-deformation analysis, are required (Griffiths and Lane, 1999;Collison, 2001). Such models have not yet been employed specifically for riverbanks, due to some main reasons: (1) stress-deformation analyses are particularly data-demanding and complex to use; (2) riverbank failures typically occur rapidly, whereas stress-deformation analyses are typically applied to slow landslides, deep-seated deformation, and/or progressive failures on large slopes (e.g., Wiberg et al, 2005;Hu¨rlimann et al, 2006).…”

Bank‐collapse processes in a valley‐bottom gully, western Iowa

“…To do so, we use a simplified model of the gully banks in which a vertical tension crack of unit length strikes roughly parallel to the gully walls (Figure 10). Vertical tension cracks were commonly observed within 1-1·5 m of the gully prior to failure ( Figure 6A) and have been included in previous analyses of slope failures in loess and similarly cohesive sediments (e.g., Lohnes and Handy, 1968;Collison, 2001). …”

“…However, actual heads in tension cracks may have been higher than those measured nearby due to the tendency for such cracks to preferentially channel water (e.g., Collison, 2001).…”

“…Our general goal was to determine through monitoring over a 2-year period whether positive pore pressures associated with subsurface water flow were alone sufficient to instigate bank failures. The role of subsurface water in the failure of gully and river banks has received considerable attention (e.g., Bradford and Piest, 1977;Osman and Thorne, 1988;Higgins et al, 1990;Hagerty, 1991;Darby and Thorne, 1996;Casagli et al, 1999;Simon et al, 2000;Collison, 2001;Simon and Collison, 2001;Amiri-Tokaldany et al, 2003;Dapporto et al, 2001Dapporto et al, , 2003Rinaldi et al, 2004;Darby et al, 2007;Fox et al, 2007;Wilson et al, 2007), and some authors have isolated effects of reduced matric suction in the unsaturated zone (e.g., Fredlund et al, 1978;Casagli et al, 1999;Rinaldi et al, 2004). This study, motivated by observations of bank failures seated below the water table, focuses on only the saturated zone.…”

Tracing shallow lateral preferential pathways of fluid movement using electrical geophysics

“…It involves a dramatic increase in slope relative to the surrounding topography leading into the channel (Collison, 2001). Head cut retreat is the main form of uphill gully extension for continuous gullies but isn't a common focus or analytical feature in assessments of gully erosion.…”