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Abstract. Estimation of erosion rate is an important component of landscapeevolution studies, particularly in settings where transience or spatialvariability in uplift or erosion generates diverse landform morphologies.While bedrock rivers are often used to constrain the timing and magnitude of changes in baselevel lowering, hilltop curvature (or convexity), CHT, provides an additional opportunity to map variations in erosion rate given that average slope angle becomes insensitive to erosion rate owing to threshold slope processes. CHT measurement techniques applied in prior studies (e.g., polynomial functions), however, tend to be computationallyexpensive when they rely on high-resolution topographic data such as lidar,limiting the spatial extent of hillslope geomorphic studies to small studyregions. Alternative techniques such as spectral tools like continuouswavelet transforms present an opportunity to rapidly document trends inhilltop convexity across expansive areas. Here, we demonstrate howcontinuous wavelet transforms (CWTs) can be used to calculate the Laplacianof elevation, which we utilize to estimate erosion rate in three catchmentsof the Oregon Coast Range that exhibit varying slope angle, slope length,and hilltop convexity, implying differential erosion. We observe thatCHT values calculated with the CWT are similar to those obtained from2D polynomial functions. Consistent with recent studies, we find thaterosion rates estimated with CHT from both CWTs and 2D polynomialfunctions are consistent with erosion rates constrained with cosmogenicradionuclides from stream sediments. Importantly, our CWT approachcalculates curvature at least 103 times more quickly than 2Dpolynomials. This efficiency advantage of the CWT increases with domainsize. As such, continuous wavelet transforms provide a compelling approachto rapidly quantify regional variations in erosion rate as well aslithology, structure, and hillslope sediment transport processes, which areencoded in hillslope morphology. Finally, we test the accuracy of CWT and 2Dpolynomial techniques by constructing a series of synthetic hillslopesgenerated by a theoretical nonlinear transport model that exhibit a range oferosion rates and topographic noise characteristics. Notably, we find thatneither CWTs nor 2D polynomials reproduce the theoretically prescribedCHT value for hillslopes experiencing moderate to fast erosion rates,even when no topographic noise is added. Rather, CHT is systematicallyunderestimated, producing a power law relationship between erosion rate andCHT that can be attributed to the increasing prominence of planarhillslopes that narrow the zone of hilltop convexity as erosion rateincreases. As such, we recommend careful consideration of measurement lengthscale when applying CHT to estimate erosion rate in moderate tofast-eroding landscapes, where curvature measurement techniques may be prone to systematic underestimation. 

Debris flows pose persistent hazards and shape high‐relief landscapes in diverse physiographic settings, but predicting the spatiotemporal occurrence of debris flows in postglacial topography remains challenging. To evaluate the debris flow process in high‐relief postglacial terrain, we conducted a geomorphic investigation to characterize geologic, glacial, volcanic, and land use contributions to landslide initiation across Southeast Alaska. To evaluate controls on landslide (esp. debris flow) occurrence in Sitka, we used field observation, geomorphic mapping, landslide characteristics as documented in the Tongass National Forest inventory, and a novel application of the shallow landslide model SHALSTAB to postglacial terrain. A complex geomorphic history of glaciation and volcanic activity provides a template for spatially heterogeneous landslide occurrence. Landslide density across the region is highly variable, but debris flow density is high on south‐ or southeast‐facing hillslopes where volcanic tephra soils are present and/or where timber harvest has occurred since 1900. High landslide density along the western coast of Baranof and Kruzof islands coincides with deposition of glacial sediment and thick tephra and exposure to extreme rainfall from atmospheric rivers on south‐facing aspects but the relative contributions of these controls are unclear. Timber harvest has also been identified as an important control on landslide occurrence in the region. Focusing on a subset of geo‐referenced landslides near Sitka, we used the SHALSTAB shallow landslide initiation model, which has been frequently applied in non‐glacial terrain, to identify areas of high landslide potential in steep, convergent terrain. In a validation against mapped landslide polygons, the model significantly outperformed random guessing, with area under the curve (AUC) = 0.709 on a performance classification curve of true positives vs. false positives. This successful application of SHALSTAB demonstrates practical utility for hazards analysis in postglacial landscapes to mitigate risk to people and infrastructure.

 

Climate change is causing increasingly widespread, frequent, and intense wildfires across the western United States. Many geomorphic effects of wildfire are relatively well studied, yet sediment transport models remain unable to account for the rapid transport of sediment released from behind incinerated vegetation, which can fuel catastrophic debris flows. This oversight reflects the fundamental inability of local, continuum-based models to capture the long-distance particle motions characteristic of steeplands. Probabilistic, particle-based nonlocal models may address this deficiency, but empirical data are needed to constrain their representation of particle motion in real landscapes. Here we present data from field experiments validating a generalized Lomax model for particle travel distance distributions. The model parameters provide a physically intuitive mathematical framework for describing the transition from light- to heavy-tailed distributions along a continuum of behavior as particle size increases and slopes get steeper and/or smoother. We show that burned slopes are measurably smoother than vegetated slopes, leading to 1) lower rates of experimental particle disentrainment and 2) runaway motion that produces the heavy-tailed travel distances often associated with nonlocal transport. Our results reveal that surface roughness is a key control on steepland sediment transport, particularly after wildfire when smoother surfaces may result in the preferential delivery of coarse material to channel networks that initiate debris flows. By providing a first-order framework relating the statistics of particle motion to measurable surface characteristics, the Lomax model both advances the development of nonlocal sediment transport theory and reveals insights on hillslope transport mechanics.

 

Abstract We deployed a network of 68 three-component geophones on the slow-moving Two Towers earthflow in northern California. We compute horizontal-to-vertical spectral ratios (HVSRs) from the ambient seismic field. The HVSRs have two prominent peaks, one near 1.23 Hz and another between 4 and 8 Hz at most stations. The 1.23 Hz resonance is a property of the background noise field and may be due to a velocity contrast at a few hundred meters depth. We interpret the higher frequency peaks as being related to slide deposits and invert the spectral ratios for shallow velocity structure using in situ thickness measurements as a priori constraints on the inversion. The thickness of the shallowest, low-velocity layer is systematically larger than landslide thicknesses inferred from inclinometer data acquired since 2013. Given constraints from field observations and boreholes, the inversion may reflect the thickness of deposits of an older slide that is larger in spatial extent and depth than the currently active slide. Because the HVSR peaks measured at Two Towers are caused by shallow slide deposits and represent frequencies that will experience amplification during earthquakes, the depth of the actively sliding mass may be less relevant for assessing potential slide volume and associated hazard than the thicknesses determined by our inversions. More generally, our results underscore the utility of combining both geotechnical measurements and subsurface imaging for landslide characterization and hazard assessment. 

The response of eroding topography to changes in base level is driven by patchy and intermittent mass flux, both on hillslopes and in channels. However, most models of soil‐mantled landscape evolution use continuously differentiable equations that average over these patchy transport processes. Because of the limited time and space resolution of field observations, the relationship between zeroth‐order landscape evolution (i.e., over long space and time scales) and first‐order fluctuations due to patchy, intermittent transport (e.g., tree throw and landsliding) remains unclear. Here, we use five physical experiments of an eroding experimental landscape to examine how the signature of first‐order transport, as described by autocorrelation functions of local elevation time series, varies as a function of the vigor of hillslope transport relative to channel incision. Our results show that experiments with higher hillslope transport efficacy have higher autocorrelation coefficients, suggesting that differences in zeroth‐order transport coefficients may be driven by differences in patchy, first‐order transport processes. These higher autocorrelation coefficients also imply that in landscapes where hillslope transport dominates, landscape dynamism is reduced and patterns of elevation change are more persistent over time. These findings suggest that the balance between channelized and hillslope transport processes is fundamental to landscape response to perturbation and may control landscape susceptibility to unsteady processes like large‐scale reorganization.

 

Abstract. To explore the sensitivity of rivers to blocking from landslidedebris, we exploit two similar geomorphic settings in California'sFranciscan mélange where slow-moving landslides, often referred to asearthflows, impinge on river channels with drainage areas that differ by afactor of 30. Analysis of valley widths and river long profiles over∼19 km of Alameda Creek (185 km2 drainage area) andArroyo Hondo (200 km2 drainage area) in central California shows avery consistent picture in which earthflows that intersect these channelsforce tens of meters of gravel aggradation for kilometers upstream, leadingto 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 upstreamof locations where earthflows impinge on its channel. Hydraulic andhydrologic data from United States Geological Survey (USGS) gages on Arroyo Hondo and the Eel River, combinedwith measured size distributions of boulders input by landslides for bothlocations, suggest that landslide derived boulders are not mobile at eithersite during the largest floods (>2-year recurrence) with field-measured flow depths. We therefore argue that boulder transport capacity isan unlikely explanation for the observed difference in sensitivity tolandslide inputs. At the same time, we find that earthflow fluxes per unitchannel width are nearly identical for Oak Ridge earthflow on Arroyo Hondo,where evidence for blocking is clear, and for the Boulder Creek earthflow onthe Eel River, where evidence for blocking is absent. These observationssuggest that boulder supply is also an unlikely explanation for the observedmorphological differences along the two rivers. Instead, we argue that thedramatically different sensitivity of the two locations to landslideblocking is related to differences in channel width relative to typicalseasonal displacements of earthflows. A synthesis of seasonal earthflowdisplacements in the Franciscan mélange shows that the channel width ofthe Eel River is ∼5 times larger than the largest annualseasonal displacement. In contrast, during wet winters, earthflows arecapable of crossing the entire channel width of Arroyo Hondo and AlamedaCreek. In support of this interpretation, satellite imagery shows thatimmobile earthflow-derived boulders are generally confined to the edges ofthe channel on the Eel River. By contrast, immobile earthflow-derivedboulders 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 upstreampropagation of base-level signals, valley-blocking earthflows may thereforepromote the formation of so-called “relict topography”.