Landslides in a changing climate

This paper presents recommended methodologies for the quantitative analysis of landslide hazard, vulnerability and risk at different spatial scales (site-specific, local, regional and national), as well as for the verification and validation of the results. The methodologies described focus on the evaluation of the probabilities of occurrence of different landslide types with certain characteristics. Methods used to determine the spatial distribution of landslide intensity, the characterisation of the elements at risk, the assessment of the potential degree of damage and the quantification of the vulnerability of the elements at risk, and those used to perform the quantitative risk analysis are also described. The paper is intended for use by scientists and practising engineers, geologists and other landslide experts.

/pdf/recommendations-for-the-quantitative-analysis-of-landslide-1culjw5db2.pdf

Recommendations for the quantitative analysis of landslide risk

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 (Fs) 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 Fs by 32 per cent, while the hydrologic effects increased Fs by 71 per cent. For grasses the figures were 70 per cent for mechanical effects and a reduction of Fs 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 Fs 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 2002 by John Wiley & Sons, Ltd.

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

Geomorphology, natural hazards, vulnerability and prevention of natural disasters in developing countries

Reciprocal interactions and adjustments between fluvial landforms and vegetation dynamics in river corridors: A review of complementary approaches

Erosion of cohesive channel materials is not fully understood, but is assumed to occur largely as a result of hydraulic shear stress. However, field and laboratory observations of pore-water pressures in cohesive streambed materials reveal the presence of positive and negative pore-water pressure effects that may significantly affect the erosion process, as contributing and resisting forces respectively.



Measurements of pore-water pressures below cohesive streambeds in the loess area of the midwestern USA were conducted in situ and in undisturbed cores with a digital, miniature tensiometer. Results disclosed matric suction values in the range of 15–50 kPa in eastern Nebraska and northern Mississippi. Repetitive tests in soft materials verified a change from positive pore-water pressures in the upper 10–15 cm, to negative pore-water pressures to depths of at least 50 cm. In firm materials, the entire sampled profile was unsaturated.



Laboratory experiments were carried out in which synthetic hydrographs were imposed on undisturbed streambed cores from the same sites. Miniature tensiometers in the cores monitored the resulting pattern of pore-water pressures, and revealed upward directed seepage forces on the recessional limb of the hydrograph. Maximum calculated values of the force ranged from 10 to 275 kN for the materials and heads tested. The maximum value obtained after application and release of a 2·5 m head was 119 kN, with 275 kN after a 5·0 m head. These results were supported independently by subsequent simulations using a finite-element hydrology model coupled with a stress-deformation model.



A numerical scheme was developed to calculate the forces acting on cohesive aggregates in an idealized streambed, and to evaluate the potential for their detachment. The scheme added upward-directed seepage as an additional driving force, and matric suction as an additional resisting force, to the commonly applied factors of particle weight, fluid drag and lift force. Results demonstrate that upward-directed seepage forces of the magnitude measured in the laboratory with 5·0 m stages have the potential to detach particles larger than 10 cm in diameter without requiring fluid drag and lift forces. When added to these hydraulic forces, erosion thresholds are lowered, enabling erosion at lower hydraulic stresses.



A hypothesis for detachment of chips or blocks of cohesive bed material is proposed: (1) large (>5 m) rises in stage increase pore-water pressures or decrease matric suction dramatically in the region just below the bed surface; (2) a relatively rapid decrease in stage causing a loss of water pressure above the bed, combined with low-rates of excess pore-water pressure dissipation just below the bed surface result in steepened hydraulic gradients; and (3) a resulting net upward seepage force is great enough to contribute to detachment of cohesive bed material, or rupture the bed by exceeding the available strength and confining stress. Published in 2001 by John Wiley & Sons, Ltd.

Pore-water pressure effects on the detachment of cohesive streambeds: seepage forces and matric suction

Bank-stability concerns along the Missouri River, eastern Montana are heightened by a proposed change in flow releases from Ft Peck dam to improve habitat conditions for Pallid Sturgeon. The effects of the proposed flow releases on streambank pore-pressures and banktoe erosion needs to be evaluated to properly model bank-stability. The ARS Bank-Stability Model incorporates pore-water pressure distributions, layering, confining pressures, reinforcement effects of riparian vegetation and complex bank geometries to solve for the factor of safety (Fs). To increase the applicability and accuracy of the model for use in predicting critical conditions the hydraulic effects of bank-toe erosion have been added. Upper-bank stability is often a function of the degree of fluvial undercutting that occurs during rises in stage when the bank toe becomes submerged and steepened. This erosion, which is a function of the erodibility of previously failed materials and in situ sediments at the toe has been difficult to measure or estimate in the field. Recent field research on erosion of in situ cohesive streambeds and bank toes with a submerged jet-test device provides a means of calculating bank-toe erosion. Results of almost 200 tests at stream sites across the United States provide the following general relation: k =0 .1τc –0.5 ;w herek = erodibility coefficient in cm 3 /N-s and τc = critical shear stress in Pa. Critical shear stresses are obtained in situ with the jet-test device (92 tests) for bank-toe materials along the Missouri River, Montana to obtain k and to calculate an erosion rate based on an excess shear-stress relation: e = k (τ - τc); where e = erosion rate in m/s and τ = average boundary shear stress in Pa. Inputs for the bank-toe erosion routine are: (1) a rectangular-shaped hydrograph of specified height and duration, (2) bed or water-surface slope, (3) flow depth, (4) bank geometry, and (5) τc for all bank layers and failed debris. Erosion is simulated normal to the submerged bank surface and the resulting bank geometry serves as input into the bank-stability part of the model. According to the proposed flow-release plan, flows of 216 m 3 /s are increased by 38.3 m 3 /s/day for 12 days to 675 m 3 /s, held for 60 days and decreased for 12 days back to 216 m 3 /s according. As expected, results show the important contribution of bank-toe erodibility in controlling mass failure. All sites modeled indicate a destabilizing influence (reduction in Fs) during the lowering of stage after 60 days, much of it due to lateral seepage and losses of matric suction and confining pressure. Banks at river miles 1624, 1676 and 1716 attain Fs < 1.0 indicating imminent failure. These sites contain less resistant sandy-silt material at the bank toe, and experienced simulated undercutting up to 3m. More resistant cohesive, clay bank toes at river miles 1589 and 1762 were undercut only 0.2 m and remained stable.

/pdf/incorporating-bank-toe-erosion-by-hydraulic-shear-into-the-41u3jvqqmb.pdf

Incorporating Bank-Toe Erosion by Hydraulic Shear into the ARS Bank- Stability Model: Missouri River, Eastern Montana

Recent research in geomorphology has considered the significance of progressive pedogenesis and climatic change to slope failure initiation for the Holocene, using physically based models. To date, the significance of vegetation change to slope stability has been largely unexplored through modelling, since available physically based models cannot consider vegetation effects directly. To address the existing deficiency this paper adapts, parameterizes and applies a physically based model of slope hydrology and stability to the combined effect of vegetation change and progessive pedogenesis on slope failure initiation. There is considerable debate in the literature concerning the relative significance of climatic change and vegetation modification to slope failure initiation in the Holocene. This paper uses the model to provide additional evidence for situations in which either climatic or vegetation change is significant to slope failure, depending on the prevailing degree of soil development. The results indicate that young podsols appear to be stable under all the climatic and vegetation conditions considered, but mature podsols may be susceptible to failure. Both climate and vegetation influence slope stability, but their relative significance depends on the stage of soil development. In particular, the stability of young soils is influenced considerably by vegetation, while climate assumes greater significance in mature soils. It is recognized that this conclusion is limited to freely draining podsol profiles, and that more research is needed to consider other soil type and vegetation combinations.

Modelling the role of climate, vegetation and pedogenesis in shallow translational hillslope failure

Beyond Root Reinforcement: The Hydrologic Effects of Riparian Vegetation on Riverbank Stability

Head-cut migration is a common phenomenon in many gullies and channels in loess areas, and occurs by amixture of particle erosion and mass failure. The mechanics of head-cut mass failure are complicated by the factthat, below a relatively shallow wetting front, many gully heads maintain unsaturated pore conditions throughoutrainfall and flow events, considerably increasing soil strength. This additional strength is enough to maintainhead-cut stability in many cases. Finite element hydrology modeling has shown that pore suction can bedestroyed by macropores and tension cracks in the head, and that in many cases strength can only be overcomethrough the presence of such preferential flow paths. Computer simulations show that negative pore waterpressures of 4 KPa can be maintained in a 1 meter high uncracked gully head despite several hours of overlandflow, but that positive pressures of 4KPa develop in head-cuts with cracks behind the head-cut. Combining thehydrologic modeling with slope stability analysis shows that the difference in pore pressure is sufficient toaccount for head-cut instability in many instances, and that the effect of the crack is primarily hydrologicalrather than mechanical. Additional analysis with a finite element stress-strain model has shown the presence oftension cracks and basal hollows to be related to unloading of the slope downstream of a head-cut. The resultssuggest that incorporation of macroporous, as well as matric, hydrologic processes is an essential pre-requisitefor successful simulation of head-cut retreat by mass failure.

Modeling gully head-cut recession processes in loess deposits.

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