Monitoring and modelling of pore water pressure changes and riverbank stability during flow events

Abstract

Pore water pressures (positive and negative) were monitored for four years (1996-1999) using a series of tensiometer-piezometers at increasing depths in a riverbank of the Sieve River, Tuscany (central Italy), with the overall objective of investigating pore pressure changes in response to flow events and their effects on bank stability. \ud The saturated/unsaturated flow was modelled using a finite element seepage analysis, for the main flow events occurring during the four-year monitoring period. Modelling results were validated by comparing measured with computed pore water pressure values for a series of representative events. Riverbank stability analysis was conducted by applying the limit equilibrium method (Morgenstern-Price), using pore water pressure distributions obtained by the seepage analysis. \ud The simulation of the 14 December 1996 event, during which a bank failure occurred, is reported in detail to illustrate the relations between the water table and river stage during the various phases of the hydrograph and their effects on bank stability. The simulation, according to monitored data, shows that the failure occurred three hours after the peak stage, during the inversion of flow (from the bank towards the river). A relatively limited development of positive pore pressures, reducing the effective stress and annulling the shear strength term due to the matric suction, and the sudden loss of the confining pressure of the river during the initial drawdown were responsible for triggering the mass failure. \ud Results deriving from the seepage and stability analysis of nine selected flow events were then used to investigate the role of the flow event characteristics (in terms of peak stages and hydrograph characteristics) and of changes in bank geometry. Besides the peak river stage, which mainly controls the occurrence of conditions of instability, an important role is played by the hydrograph characteristics, in particular by the presence of one or more minor peaks in the river stage preceding the main on

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

“…contributors to riverbank collapse (e.g., Simon et al, 2000;Rinaldi et al, 2004) -can be ruled out as contributing to bank displacement in this case. This conclusion is reinforced by the observation that the peak frequency of bank displacements occurred 2 months earlier than the mean peak water discharge through the gully (Thomas et al, 2004).…”

“…Bank displacements might also have been caused by additions of water to the unsaturated zone, thereby reducing matric suction and bank strength (e.g., Fredlund et al, 1978;Casagli et al, 1999;Rinaldi et al, 2004). Although gully-wall failures appeared to originate along slip surfaces in the saturated zone, without measurements of matric suction we cannot rule out that it influenced bank movements.…”

“…Nevertheless, we can consider whether increases in hydraulic head within the saturated zone were alone sufficient to promote mass movement by reducing frictional strength (e.g., Simon et al, 2000;Rinaldi et al, 2004). 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).…”

“…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.…”

Numerical simulation of hydrodynamics and bank erosion in a river bend

“…(1) accounting for the effects of positive and negative pore water pressures and confining river pressures [Casagli et al, 1999;Simon et al, 2000;Rinaldi et al, 2004]; and (2) quantification of the effects of riparian vegetation on bank stability [Simon and Collison, 2002;Pollen and Simon, 2005;Pollen, 2006;Van De Wiel and Darby, 2007]. Progress has also been made in understanding and quantifying the effects of seepage erosion on mass failures [Fox et al, 2006Wilson et al, 2007].…”