Beyond the angle of repose: A review and synthesis of landslide processes in response to rapid uplift, Eel River, Northern California

In mountainous settings, increases in rock uplift are often followed by a commensurate uptick in denudation as rivers incise and steepen hillslopes, making them increasingly prone to landsliding as slope angles approach a limiting value. For decades, the threshold slope model has been invoked to account for landslide-driven increases in sediment flux that limit topographic relief, but the manner by which slope failures organize themselves spatially and temporally in order for erosion to keep pace with rock uplift has not been well documented. Here, we review past work and present new findings from remote sensing, cosmogenic radionuclides, suspended sediment records, and airborne lidar data, to decipher patterns of landslide activity and geomorphic processes related to rapid uplift along the northward-migrating Mendocino Triple Junction in Northern California. From historical air photos and airborne lidar, we estimated the velocity and sediment flux associated with active, slow-moving landslides (or earthflows) in the mélange- and argillite-dominated Eel River watershed using the downslope displacement of surface markers such as trees and shrubs. Although active landslides that directly convey sediment into the channel network account for only 7% of the landscape surface, their sediment flux amounts to more than 50% of the suspended load recorded at downstream sediment gaging stations. These active slides tend to exhibit seasonal variations in velocity as satellite-based interferometry has demonstrated that rapid acceleration commences within 1 to 2 months of the onset of autumn rainfall events before slower deceleration ensues in the spring and summer months. Curiously, this seasonal velocity pattern does not appear to vary with landslide size, suggesting that complex hydrologic–mechanical feedbacks (rather than 1-D pore pressure diffusion) may govern slide dynamics. A new analysis of 14 yrs of discharge and sediment concentration data for the Eel River indicates that the characteristic mid-winter timing of earthflow acceleration corresponds with increased suspended concentration values, suggesting that the seasonal onset of landslide motion each year may be reflected in the export of sediments to the continental margin. The vast majority of active slides exhibit gullied surfaces and the gully networks, which are also seasonally active, may facilitate sediment export although the proportion of material produced by this pathway is poorly known. Along Kekawaka Creek, a prominent tributary to the Eel River, new analyses of catchment-averaged erosion rates derived from cosmogenic radionuclides reveal rapid erosion (0.76 mm/yr) below a prominent knickpoint and slower erosion (0.29 mm/yr) upstream. Such knickpoints are frequently observed in Eel tributaries and are usually comprised of massive (> 10 m) interlocking resistant boulders that likely persist in the landscape for long periods of time (> 105 yr). Upstream of these knickpoints, active landslides tend to be less frequent and average slope angles are slightly gentler than in downstream areas, which indicates that landslide density and average slope angle appear to increase with erosion rate. Lastly, we synthesize evidence for the role of large, catastrophic landslides in regulating sediment flux and landscape form. The emergence of resistant blocks within the mélange bedrock has promoted large catastrophic slides that have dammed the Eel River and perhaps generated outburst events in the past. The frequency and impact of these landslide dams likely depend on the spatial and size distributions of resistant blocks relative to the width and drainage area of adjacent valley networks. Overall, our findings demonstrate that landslides within the Eel River catchment do not occur randomly, but instead exhibit spatial and temporal patterns related to baselevel lowering, climate forcing, and lithologic variations. Combined with recent landscape evolution models that incorporate landslides, these results provide predictive capability for estimating erosion rates and managing hazards in mountainous regions

Widespread initiation, reactivation, and acceleration of landslides in the northern California Coast Ranges due to extreme rainfall

Episodically to continuously active slow‐moving landslides are driven by precipitation. Climate change, which is altering both the frequency and magnitude of precipitation worldwide, is therefore predicted to have a major impact on landslides. Here we examine the behavior of hundreds of slow‐moving landslides in northern California in response to large changes in annual precipitation that occurred between 2016 and 2018. We quantify the landslide displacement using repeat‐pass radar interferometry and pixel offset tracking techniques on a novel dataset from the airborne NASA/JPL Uninhabited Aerial Vehicle Synthetic Aperture Radar. We found that 312 landslides were moving due to extreme rainfall during 2017, compared to 119 during 2016, which was the final year of a historic multi‐year drought. However, with a return to below‐average rainfall in 2018, only 146 landslides remained in motion. The increased number of landslides during 2017 was primarily accommodated by landslides that were smaller than the landslides that remained active between 2016 and 2018. Furthermore, by examining a subset of 51 landslides, we found that 49 had increased velocities during 2017 when compared to 2016. Our results show that slow‐moving landslides are sensitive to large changes in annual precipitation, particularly the smaller and thinner landslides that likely experience larger basal pore‐water pressure changes. Based on climate model predictions for the next century in California, which include increases in average annual precipitation and increases in the frequency of dry‐to‐wet extremes, we hypothesize that there will be an overall increase in landslide activity

Study of the dynamic behavior of earthflows, with particular reference to the solid-to-fluid transition characterizing stages of rapid movement

Earthflows are landslide developing in clay-rich soil, characterized by elongated tongue-like shapes. Their behavior has been largely debated in literature. The periods of rapid motion have been interpreted in two different ways. Some authors suggest that, in spite of their flow-like morphology, earthflows essentially move like rigid bodies along well-defined slip surfaces. Other authors report that during the surging earthflow material becomes softer and can be considered as a viscous fluid. However, collecting data during the stages of rapid motion is still challenging and only minimal data concerning the real acceleration stage had been presented in literature. The purpose of the current study is to provide an insight into the reactivation stage in order to understand if any fluidization process can occur. Geophysical data have been treated: seismic techniques have been applied to detect shear stiffness variation over time in the periods near the rapid surging and two-pass conventional interferometry has been used to monitor landslide deformation patterns. Seismic data indicate that earthflow soils are affected by stiffness drops during the acceleration stage; the process is slowly reversed in the period following the rapid motion during which the materials become stiffer. This process is probably related to changes in void ratio and soil water content: after the failure the water content decreases, and the porosity of the soils decreases too. Data demonstrate that during the stages of rapid motion earthflows behave like viscous fluids; far from the failure they turn to be stiff and a rigid-like behavior is more likely. Mathematical theories can be used to fully describe the behavior but different types of field data concerning the rapid motion stage are necessary: to obtain a whole dataset we need to know in advance information about earthflow reactivation. Thus, conventional two-pass interferometry has been successfully applied to derive deformation patterns

River channel width controls blocking by slow-moving landslides in California's Franciscan melange

This is the final version. Available from European Geosciences Union via the DOI in this record. To explore the sensitivity of rivers to blocking from landslide debris, we exploit two similar geomorphic settings in California's Franciscan mélange where slow-moving landslides, often referred to as earthflows, impinge on river channels with drainage areas that differ by a factor of 30. Analysis of valley widths and river long profiles over ∼19 km of Alameda Creek (185 km2 drainage area) and Arroyo Hondo (200 km2 drainage area) in central California shows a very consistent picture in which earthflows that intersect these channels force tens of meters of gravel aggradation for kilometers upstream, leading to apparently long-lived sediment storage and channel burial at these sites. In contrast, over a ∼30 km section of the Eel River (5547 km2 drainage area), there are no knickpoints or aggradation upstream of locations where earthflows impinge on its channel. Hydraulic and hydrologic data from United States Geological Survey (USGS) gages on Arroyo Hondo and the Eel River, combined with measured size distributions of boulders input by landslides for both locations, suggest that landslide derived boulders are not mobile at either site during the largest floods (>2-year recurrence) with field-measured flow depths. We therefore argue that boulder transport capacity is an unlikely explanation for the observed difference in sensitivity to landslide inputs. At the same time, we find that earthflow fluxes per unit channel width are nearly identical for Oak Ridge earthflow on Arroyo Hondo, where evidence for blocking is clear, and for the Boulder Creek earthflow on the Eel River, where evidence for blocking is absent. These observations suggest that boulder supply is also an unlikely explanation for the observed morphological differences along the two rivers. Instead, we argue that the dramatically different sensitivity of the two locations to landslide blocking is related to differences in channel width relative to typical seasonal displacements of earthflows. A synthesis of seasonal earthflow displacements in the Franciscan mélange shows that the channel width of the Eel River is ∼5 times larger than the largest annual seasonal displacement. In contrast, during wet winters, earthflows are capable of crossing the entire channel width of Arroyo Hondo and Alameda Creek. In support of this interpretation, satellite imagery shows that immobile earthflow-derived boulders are generally confined to the edges of the channel on the Eel River. By contrast, immobile earthflow-derived boulders jam the entire channel on Arroyo Hondo. Our results imply that lower drainage area reaches of earthflow-dominated catchments may be particularly prone to blocking. By inhibiting the upstream propagation of base-level signals, valley-blocking earthflows may therefore promote the formation of so-called “relict topography”.National Science Foundatio

Investigating slow-moving landslides in the Zhouqu region of China using InSAR time series

In the Zhouqu region (Gansu, China), landslide distribution and activity exploits geological weaknesses in the fault-controlled belt of low-grade metamorphic rocks of the Bailong valley and severely impacts lives and livelihoods in this region. Landslides reactivated by the Wenchuan 2008 earthquake and debris flows triggered by rainfall, such as the 2010 Zhouqu debris flow, have caused more than 1700 casualties and estimated economic losses of some US$0.4 billion. Earthflows presently cover some 79% of the total landslide area and have exerted a strong influence on landscape dynamics and evolution in this region. In this study, we use multi-temporal Advanced Land Observing Satellite and Phased Array type L-band Synthetic Aperture Radar (ALOS PALSAR) data and time series interferometric synthetic aperture radar to investigate slow-moving landslides in a mountainous region with steep topography for the period December 2007–August 2010 using the Small Baseline Subsets (SBAS) technique. This enabled the identification of 11 active earthflows, 19 active landslides with deformation rates exceeding 100 mm/year and 20 new instabilities added into the pre-existing landslide inventory map. The activity of these earthflows and landslides exhibits seasonal variations and accelerated deformation following the Wenchuan earthquake. Time series analysis of the Suoertou earthflow reveals that seasonal velocity changes are characterized by comparatively rapid acceleration and gradual deceleration with distinct kinematic zones with different mean velocities, although velocity changes appear to occur synchronously along the landslide body over seasonal timescales. The observations suggest that the post-seismic effects (acceleration period) on landslide deformation last some 6–7 months

Investigating the Deformation of Slow Moving Landslides in the Northern Appennines of Italy with Differential Interferometry (InSAR)

The present work addresses different aspects of the analysis of landslide motion in the Northern Apennines of Italy by means of spaceborn radar interferometry (InSAR). Datasets from the satellite systems Envisat, COSMO SkyMed and Sentinel 1A were processed with different open source packages in order to assess technical issues related to different InSAR processing techniques and to obtain precise deformation measurements. One particular technical issue was the influence of the external digital elevation model, that is used to calculate and subtract the topographic phase, on final PS-InSAR and SBAS-results (chapter 3). It is common that different digital elevation models are available for a study area and often differences in final PS-InSAR/SBAS results can be observed. Due to the high landslide density in Northern Apennines, slope instabilities often interfere with man made structures. The case of Ripoli and Santa Maria Maddalena, South of Bologna, show in an interesting way this interaction between human activity and landslide deformation (chapter 4). Here, a double road tunnel was excavated under a slope that hosts several old deep seated landslides. Soon after excavation started in 2011 first deformations and damages on buildings were registered. InSAR derived deformation measurements document the landslide acceleration during the construction phase and show a decrease in displacement rates after the construction ceased. During the years 2013 and 2014 Northern Italy was struck by long enduring persistent rainfalls that caused numerous landslide reactivations. InSAR datasets were used to assess kinematics of 25 landslides during the years 2013 and 2015, hosted either by chaotic clay shales or pelitic turbidites (chapter 5). Deformation responses to precipitation were analysed in detail for eight selected cases, four of which are located in turbidites and four that are hosted by chaotic clay shales. Different deformation responses were discussed in relation to precipitation, morphology and landslide material

Multi-scale analysis of active landslides using two-pass differential interferometry

Landslides are common features of the landscape of the north-central Apennine mountain range and cause frequent damage to human facilities and infrastructure. Most of these landslides move periodically with moderate velocities and, only after particular rainfall events, some accelerate abruptly. Synthetic aperture radar interferometry (InSAR) provides a particularly convenient method for studying deforming slopes. We use standard two-pass interferometry, taking advantage of the short revisit time of the Sentinel-1 satellites. In this paper we present the results of the InSAR analysis developed on several study areas in central and Northern Italian Apennines. The aims of the work described within the articles contained in this paper, concern: i) the potential of the standard two-pass interferometric technique for the recognition of active landslides; ii) the exploration of the potential related to the displacement time series resulting from a two-pass multiple time-scale InSAR analysis; iii) the evaluation of the possibility of making comparisons with climate forcing for cognitive and risk assessment purposes. Our analysis successfully identified more than 400 InSAR deformation signals (IDS) in the different study areas corresponding to active slope movements. The comparison between IDSs and thematic maps allowed us to identify the main characteristics of the slopes most prone to landslides. The analysis of displacement time series derived from monthly interferometric stacks or single 6-day interferograms allowed the establishment of landslide activity thresholds. This information, combined with the displacement time series, allowed the relationship between ground deformation and climate forcing to be successfully investigated. The InSAR data also gave access to the possibility of validating geographical warning systems and comparing the activity state of landslides with triggering probability thresholds

Boulders as a Lithologic Control on River and Landscape Response to Tectonic Forcing at the Mendocino Triple Junction

Constraining Earth’s sediment mass balance over geologic time requires a quantitative understanding of how landscapes respond to transient tectonic perturbations. However, the mechanisms by which bedrock lithology governs landscape response remain poorly understood. Rock type influences the size of sediment delivered to river channels, which controls how efficiently rivers respond to tectonic forcing. The Mendocino triple junction region of northern California, USA, is one landscape in which large boulders, delivered by hillslope failures to channels, may alter the pace of landscape response to a pulse of rock uplift. Boulders frequently delivered by earthflows in one lithology, the Franciscan mélange, have been hypothesized to steepen channels and slow river response to rock uplift, helping to preserve high-elevation, low-relief topography. Channels in other units (the Coastal Belt and the Franciscan schist) may experience little or no erosion inhibition due to boulder delivery. Here we investigate spatial patterns in channel steepness, an indicator of erosion resistance, and how it varies between mélange and non-mélange channels. We then ask whether lithologically controlled boulder delivery to rivers is a possible cause of steepness variations. We find that mélange channels are steeper than Coastal Belt channels but not steeper than schist channels. Though channels in all units steepen with increasing proximity to mapped hillslope failures, absolute steepness values near failures are much higher (∼2×) in the mélange and schist than in Coastal Belt units. This could reflect reduced rock erodibility or increased erosion rates in the mélange and schist, or disproportionate steepening due to enhanced boulder delivery by hillslope failures in those units. To investigate the possible influence of lithology-dependent boulder delivery, we map boulders at failure toes in the three units. We find that boulder size, frequency, and concentration are greatest in mélange channels and that Coastal Belt channels have the lowest concentrations. Using our field data to parameterize a mathematical model for channel slope response to boulder delivery, we find that the modeled influence of boulders in the mélange could be strong enough to account for some observed differences in channel steepness between lithologies. At the landscape scale, we lack the data to fully disentangle boulder-induced steepening from that due to spatially varying erosion rates and in situ rock erodibility. However, our boulder mapping and modeling results suggest that lithology-dependent boulder delivery to channels could retard landscape adjustment to tectonic forcing in the mélange and potentially also in the schist. Boulder delivery may modulate landscape response to tectonics and help preserve high-elevation, low-relief topography at the Mendocino triple junction and elsewhere

High-Resolution Soil-Moisture Maps Over Landslide Regions in Northern California Grassland Derived From SAR Backscattering Coefficients

Slow-moving landslides are destabilized by accumulated precipitation and consequent soil moisture. Yet, the continuous high-resolution soil-moisture measurements needed to aid the understanding of landslide processes are generally absent in steep terrain. Here, we produce soil-moisture time-series maps for a seasonally active grassland landslide in the northern California coast ranges, USA, using backscattering coefficients from NASA's uninhabited aerial vehicle synthetic aperture radar at 6-m resolution. A physically based radar scattering model is used to retrieve the near-surface (5-cm depth) soil moisture for the landslide. Both forward modeling (backscattering estimation) and the retrieval (soil-moisture validation) show good agreement. The root-mean-square errors (RMSE) for vertical transmit vertical receive (VV) and horizontal transmit horizontal receive (HH) polarizations in forward model comparison are 1.93 dB and 1.88 dB, respectively. The soil-moisture retrieval shows unbiased RMSE of 0.054 m³/m³. Our successful retrieval benefits from the surface and double-bounce scattering, which is common in grasslands. The retrieved maps show saturated wetness conditions within the active landslide boundaries. We also performed sensitivity tests for incidence angle and found that the retrieval is weakly dependent on the angle, especially while using copolarized HH and VV together. Using the two copolarized inputs, the retrieval is also not sensitive to the change of orientation angles of grass cylinders. The physical model inversion presented here can be generally applied for soil-moisture retrieval in areas with the same vegetation cover types in California