Landsliding in response to rainfall involves physical processes that operate on disparate timescales. Relationships between these timescales guide development of a mathematical model that uses reduced forms of Richards equation to evaluate effects of rainfall infiltration on landslide occurrence, timing, depth, and acceleration in diverse situations. The longest pertinent timescale is A/D0, where D0 is the maximum hydraulic diffusivity of the soil and A is the catchment area that potentially affects groundwater pressures at a prospective landslide slip surface location with areal coordinates x, y and depth H. Times greater than A/D0 are necessary for establishment of steady background water pressures that develop at (x, y, H) in response to rainfall averaged over periods that commonly range from days to many decades. These steady groundwater pressures influence the propensity for landsliding at (x, y, H), but they do not trigger slope failure. Failure results from rainfall over a typically shorter timescale H2/D0 associated with transient pore pressure transmission during and following storms. Commonly, this timescale ranges from minutes to months. The shortest timescale affecting landslide responses to rainfall is H/g, where g is the magnitude of gravitational acceleration. Postfailure landslide motion occurs on this timescale, which indicates that the thinnest landslides accelerate most quickly if all other factors are constant. Effects of hydrologic processes on landslide processes across these diverse timescales are encapsulated by a response function, R(t*) = t*/π exp (−1/t*) − erfc (1/t*), which depends only on normalized time, t*. Use of R(t*) in conjunction with topographic data, rainfall intensity and duration information, an infinite-slope failure criterion, and Newton's second law predicts the timing, depth, and acceleration of rainfall-triggered landslides. Data from contrasting landslides that exhibit rapid, shallow motion and slow, deep-seated motion corroborate these predictions.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2000WR900090

Landslide triggering by rain infiltration

The original review of macropores and water flow in soils by Beven and Germann is now 30 years old and has become one of the most highly cited papers in hydrology. This paper attempts to review the progress in observations and theoretical reasoning about preferential soil water flows over the intervening period. It is suggested that the topic has still not received the attention that its importance deserves, in part because of the ready availability of software packages rooted firmly in the Richards domain, albeit that there is convincing evidence that this may be predicated on the wrong experimental method for natural conditions. There is still not an adequate physical theory linking all types of flow, and there are still not adequate observational techniques to support the scale dependent parameterizations that will be required at practical field and hillslope scales of application. Some thoughts on future needs to develop a more comprehensive representation of soil water flows are offered.

/pdf/macropores-and-water-flow-in-soils-revisited-1zhff8ac1e.pdf

Macropores and water flow in soils revisited

Steep, soil-mantled hillslopes evolve through the downslope movement of soil, driven largely by slope-dependent transport processes. Most landscape evolution models represent hillslope transport by linear diffusion, in which rates of sediment transport are proportional to slope, such that equilibrium hillslopes should have constant curvature between divides and channels. On many soil-mantled hillslopes, however, curvature appears to vary systematically, such that slopes are typically convex near the divide and become increasingly planar downslope. This suggests that linear diffusion is not an adequate model to describe the entire morphology of soil-mantled hillslopes. Here we show that the interaction between local disturbances (such as rainsplash and biogenic activity) and frictional and gravitational forces results in a diffusive transport law that depends nonlinearly on hillslope gradient. Our proposed transport law (1) approximates linear diffusion at low gradients and (2) indicates that sediment flux increases rapidly as gradient approaches a critical value. We calibrated and tested this transport law using high-resolution topographic data from the Oregon Coast Range. These data, obtained by airborne laser altimetry, allow us to characterize hillslope morphology at '2 m scale. At five small basins in our study area, hillslope curvature approaches zero with increasing gradient, consistent with our proposed nonlinear diffusive transport law. Hillslope gradients tend to cluster near values for which sediment flux increases rapidly with slope, such that large changes in erosion rate will correspond to small changes in gradient. Therefore average hillslope gradient is unlikely to be a reliable indicator of rates of tectonic forcing or baselevel lowering. Where hillslope erosion is dominated by nonlinear diffusion, rates of tectonic forcing will be more reliably reflected in hillslope curvature near the divide rather than average hillslope gradient.

/pdf/evidence-for-nonlinear-diffusive-sediment-transport-on-532j2v7c1n.pdf

Evidence for nonlinear, diffusive sediment transport on hillslopes and implications for landscape morphology

[1] The relationship between catchment structure and runoff characteristics is poorly understood. In steep headwater catchments with shallow soils the accumulation of hillslope area (upslope accumulated area (UAA)) is a hypothesized first-order control on the distribution of soil water and groundwater. Hillslope-riparian water table connectivity represents the linkage between the dominant catchment landscape elements (hillslopes and riparian zones) and the channel network. Hydrologic connectivity between hillslope-riparian-stream (HRS) landscape elements is heterogeneous in space and often temporally transient. We sought to test the relationship between UAA and the existence and longevity of HRS shallow groundwater connectivity. We quantified water table connectivity based on 84 recording wells distributed across 24 HRS transects within the Tenderfoot Creek Experimental Forest (U.S. Forest Service), northern Rocky Mountains, Montana. Correlations were observed between the longevity of HRS water table connectivity and the size of each transect's UAA (r2 = 0.91). We applied this relationship to the entire stream network to quantify landscape-scale connectivity through time and ascertain its relationship to catchment-scale runoff dynamics. We found that the shape of the estimated annual landscape connectivity duration curve was highly related to the catchment flow duration curve (r2 = 0.95). This research suggests internal catchment landscape structure (topography and topology) as a first-order control on runoff source area and whole catchment response characteristics.

/pdf/hydrologic-connectivity-between-landscapes-and-streams-2eycvo5f64.pdf

Hydrologic connectivity between landscapes and streams: Transferring reach- and plot-scale understanding to the catchment scale

Decades of quantitative measurement indicate that roots can mechanically reinforce shallow soils in forested landscapes. Forests, however, have variations in vegetation species and age which can do...

The variability of root cohesion as an influence on shallow landslide susceptibility in the Oregon Coast Range

Observations from natural rain storms and sprinkling experiments at a steep zero-order catchment in the Oregon Coast Range demonstrate the importance offlow through near-surface bedrock on runoff generation and pore pressure development in shallow colluvial soils. Sprinkling experiments, involving irrigation of the entire 860 m 2 catchment at average intensities of 1.5 and 3.0 mm/h, permitted detailed observation of runoff and the development of subsurface saturation under controlled conditions. A weir installed to collectflow through the colluvium at the base of the catchment recovered runoff equal to one third to one half of the precipitation rate during quasi-steady irrigation. Three key observations demonstrate that a significant proportion of storm runoffflows through near-surface bedrock and illustrate the importance of shallow bedrockflow in pore pressure development in the overlying colluvial soil: (1) greater discharge recovery during both the experiments and natural rainfall at a weir installed approximately 15 m downslope of the weir at the base of the catchment, (2) spatially discontinuous patterns of positive pressure head in the colluvium during steady sprinkling, and (3) local development of upward head gradients associated withflow from weathered rock into the overlying colluvium during high-intensity rainfall. Data from natural storms also show that smaller storms produce no significant runoff or piezometric response and point to a critical intensity-duration rainfall to overcome vadose zone storage. Together these observations highlight the role of interaction betweenflow in colluvium and near- surface bedrock in governing patterns of soil saturation, runoff production, and positive pore pressures.

/pdf/hydrologic-response-of-a-steep-unchanneled-valley-to-natural-4lu8stedhq.pdf

Hydrologic response of a steep, unchanneled valley to natural and applied rainfall

[1] Experimental observations comparing two steep unchanneled valleys in the Oregon Coast Range, one intensively instrumented (CB1) and the other monitored for runoff but which produced a debris flow (CB2), shed light on the mechanisms of shallow flow in bedrock, its interaction with the vadose zone, and its role in generating landslides. Previous work at CB1 led to the proposal that during storms pulses of rainfall transmit pressure waves through the vadose zone and down to the saturated zone to create rapid pore pressure response and runoff [Torres et al., 1998]. Here, we document the associated rapid pore pressure response in the shallow fractured bedrock that underlies these colluvium-mantled sites and examine its influence on the generation of storm flow, seasonal variations in base flow, and slope stability in the overlying colluvial soil. Our observations document rapid piezometric response in the shallow bedrock and a substantial contribution of shallow fracture flow to both storm flow and seasonal variations in base flow. Saturated hydraulic conductivity in the colluvial soil decreases with depth below the ground surface, but the conductivity of the near-surface bedrock displays no depth dependence and varies over five orders of magnitude. Analysis of runoff intensity and duration in a series of storms that did and did not trigger debris flows in the surrounding area shows that the landslide inducing storms had the greatest intensity over durations similar to those predicted by a simple model of piezometric response. During a monitored storm in February 1992, the channel head at the base of the neighboring CB2 site failed as a debris flow. Automated piezometric measurements document that the CB2 debris flow initiated several hours after peak discharge, coincident with localized development of upward spikes of pressure head from near-surface bedrock into the overlying colluvial soil in CB1. Artesian flow observed exfiltrating from bedrock fractures on the failure surfaces at CB2 further implicates bedrock fracture flow in both runoff generation by subsurface storm flow and suggests a connection to landslide initiation. Our observations show that the timing of shallow landslide initiation can be delayed relative to both peak rainfall and peak runoff and support the argument that the influence of fracture flow on shallow landsliding helps explain the wide variability in the occurrence of slope instability in topographically analogous locations.

/pdf/piezometric-response-in-shallow-bedrock-at-cb1-implications-1t1mq6l1tp.pdf

Piezometric response in shallow bedrock at CB1: Implications for runoff generation and landsliding

Logs of standard dimensions from five species of trees were submerged in a stream to evaluate changes in strength and decomposition over a period of 5 years. Changes in structural properties occurred only for wood near the outer surface of the logs. Nearly all bark was removed from the logs within 12 months. Diameter loss for the five species ranged from 10.6 mm (western hemlock Tsuga heterophylla) to 21.8 mm (bigleaf maple Acer macrophyllum) after 5 years. Decreases in the density of surface wood for the five species ranged from 23% (red alder Alnus rubra) to 31% (western hemlock). Modulus of rupture, modulus of elasticity, and wood density did not change for wood more than 12 mm from the log surface for any of the species. Bigleaf maple exhibited the highest resistance to rupture, and western redcedar Thuja plicata exhibited the lowest. Western redcedar was also the most easily flexed. Microbial activity on the surface of the logs was highest at the start of the experiment and decreased rapidly...

Effects of Immersion in Water on Deterioration of Wood from Five Species of Trees Used for Habitat Enhancement Projects

Since the majority of road drainage points in western Washington and Oregon enter small, often ephemeral streams rather than large, fish-bearing waters, impact of road-surface sediment on biota in permanent streams depends, to a large extent, on transport through these small watercourses. A series of experimental additions of road-surface sediment was made to two ephemeral streams to examine the downstream transport of this material as a function of discharge and channel characteristics. These small streams were found to store large amounts of sediment washed from road surface. In no instance did either stream transport more than 45 percent of the added material to their mouths, distances of 95 and 125 m. Larger-sized sediment particles were delivered at a lower rate than finer material. Added sediment <0.063 mm in size was transported efficiently through the systems at all but the lowest flows tested. Material between 0.5 and 0.063 mm and from 2.0 to 0.5 mm in size were retained at progressively higher rates, with sediment in the coarser size category never exceeding a delivery of 10 percent of the added material. There were significant differences in the transport of sediment in the two larger size categories between the two streams. These differences were due to a much greater amount of woody debris in the stream with the lower delivery rates, which acted to trap and hold sediment, as well as a slightly longer and less steep channel.

Transport of road-surface sediment through ephemeral stream channels

Forest practices have progressively changed over the last 30 years in the Pacific Northwest to address water quality concerns. There have been some assessments of these new management practices made at a site scale but very few studies have attempted to evaluate their efficacy at reducing cumulative sediment production at a watershed scale. Such an evaluation is difficult due to the spatial and temporal variability in sediment delivery and transport processes. Due to this inherent variability, detecting a response to management changes requires a long-term data record. We utilized a water quality dataset collected over 30 years at four locations in the Deschutes River watershed (western Washington) to assess trends in turbidity and whether sediment control procedures implemented over this time period had any detectable influence. The sample sites ranged from small headwater streams (2.4 and 3.0 km 2 ) to the mainstem of the Deschutes River (150 km 2 ). Declining trends in turbidity were detected at all the permanently monitored sites. The mainstem Deschutes River site, which integrates sediment processes from the entire study watershed, showed dramatic declines in turbidity even with continued active forest management. For the small basins, logging and road construction occurred in the 1970s and 1980s and turbidity declined thereafter, achieving prelogging levels by 2000. There are no temporal trends in flow that could be responsible for the observed trends in turbidity. Our results suggest that increased attention to reducing sediment production from roads and minimizing the amount of road runoff reaching stream channels has been the primary cause of the declining turbidity levels observed in this study.

John T. Heffner

Papers

Hydrologic response of a steep, unchanneled valley to natural and applied rainfall

Piezometric response in shallow bedrock at CB1: Implications for runoff generation and landsliding

Effects of Immersion in Water on Deterioration of Wood from Five Species of Trees Used for Habitat Enhancement Projects

Transport of road-surface sediment through ephemeral stream channels

Temporal and Spatial Turbidity Patterns Over 30 Years in a Managed Forest of Western Washington