Quantifying the mechanical and hydrologic effects of riparian vegetation on streambank stability

Abstract

Riparian vegetation strips are widely used by river managers to increase streambank stability, among other purposes. However, though the effects of vegetation on bank stability are widely discussed they are rarely quantified, and generally underemphasize the importance of hydrologic processes, some of which may be detrimental. This paper presents results from an experiment in which the hydrologic and mechanical effects of four riparian tree species and two erosion-control grasses were quantified in relation to bank stability. Geotechnical and pore-water pressure data from streambank plots under three riparian covers (mature trees, clump grasses and bare/cropped turf grass) were used to drive the ARS bank stability model, and the resulting factor of safety (F s ) was broken down into its constituent parts to assess the contribution (beneficial or detrimental) of individual hydrologic and mechanical effects (soil moisture modification, root reinforcement and surcharge). Tree roots were found to increase soil strength by 2-8 kPa depending on species, while grass roots contributed 6-18 kPa. Slope stability analysis based on data collected during bank failures in spring 2000 (following a very dry antecedent period) shows that the mechanical effects of the tree cover increased F s by 32 per cent, while the hydrologic effects increased F s by 71 per cent. For grasses the figures were 70 per cent for mechanical effects and a reduction of F s by 10 per cent for the hydrologic effects. However, analysis based on bank failures in spring 2001 (following a wetter than average antecedent period) showed the mechanical effects of the tree cover to increase F s by 46 per cent, while hydrologic effects added 29 per cent. For grasses the figures were 49 per cent and 15 per cent respectively. During several periods in spring 2001 the hydrologic effects of the tree cover reduced bank stability, though this was always offset by the stabilizing mechanical effects. The results demonstrate the importance of hydrologic processes in controlling streambank stability, and highlight the need to select riparian vegetation based on hydrologic as well as mechanical and ecological criteria. Published in

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

“…Since the 1960s, specific methods of bank stability analysis have been progressively disseminated, with an increasing effort to define closed-form solutions for planar failures representative of characteristic bank geometries (Table 9.1). It is evident that research has progressively sought to account for: (1) a more realistic bank geometry and the influence of tension cracks (Osman and Thorne, 1988); (2) positive pore water pressures and hydrostatic confining pressures (Simon et al, 1991;Darby and Thorne, 1996b); (3) the effects of negative pore water pressures in the unsaturated part of the bank Casagli et al, 1999;Simon et al, 2000); and (4) the influence of riparian vegetation (Abernethy and Rutherfurd, 1998Simon and Collison, 2002;Rutherfurd and Grove, 2004;Pollen et al, 2004;Van de Wiel and Darby, 2004;Pollen and Simon, 2005;Pollen, 2006). Recently, more complex analyses have been utilised for river bank studies (Abernethy and Rutherfurd, 2000;Dapporto et al, 2001Rinaldi et al, 2004) by using various LEM solutions extended to rotational slides (i.e., Bishop, Fellenius, Jambu, Morgestern, GLE) that include features that overcome many of the previous limitations.…”

“…Dates of major reviews of bank erosion in the first Gravel-Bed Rivers I proceedings volume in 1982 (GBR I) and the most recent major review in 1997 (dashed line) are also highlighted. Rutherfurd, 1998Simon and Collison, 2002) and bank hydrology (e.g., Rinaldi and Casagli, 1999;Casagli et al, 1999;Rinaldi et al, 2004) as key controlling influences on bank stability. In contrast, although improvements in the modelling of near-bank flows are starting to be made (e.g., Kean and Smith, 2006a,b), there are still relatively few studies that have been concerned with the process of fluvial erosion (i.e., the removal of bank sediments by the direct action of the flow).…”

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].…”

Uncertainty and sensitivity in a bank stability model: implications for estimating phosphorus loading

“…However, since BSTEM does not allow root depth to exceed bank height, some correlation between these variables was introduced which could artificially increase the calculated importance of root depth. Vegetation is generally considered to be a major control on bank stability (Pollen, 2007;Simon and Collison, 2002;Thorne, 1990), leading to the significant effort of incorporating the RipRoot model into BSTEM (Pollen and Simon, 2005). It is therefore surprising that vegetation parameters do not more significantly influence model output.…”