Luke A. McGuire

Calibration and testing of upland hillslope evolution models in a dated landscape: Banco Bonito, New Mexico

[1] In this study we tested upland hillslope evolution models and constrained the rates of regolith production, colluvial transport, and eolian deposition over geologic time scales in a dated volcanic landscape in northern New Mexico using field measurements of regolith thickness; geochemical analyses of regolith, bedrock, and regional dust; numerical modeling of regolith production and transport; and quantitative analyses of airborne light detection and ranging (lidar) digital elevation models (DEMs). Within this volcanic landscape, many topographically closed basins exist as a result of compressional folding and explosion pitting during eruption. The landscape has evolved from an initial state of no regolith cover at 40 ± 5 ka to its modern state, which has highly weathered regolith ranging from 0 to 3+ m, with local thickness values controlled primarily by topographic position. Our models constrain the maximum rate of regolith production in the study area to be in the range of 0.02 to 0.12 m kyr ! 1 and the rate of colluvial transport per unit slope gradient to be in the range of 0.2 to 2.7 m 2 kyr ! 1 , with higher values in areas with more aboveground biomass. We conclude that a depth! dependent colluvial transport model better predicts the observed spatial distribution of regolith thickness compared to a model that has no depth dependence. This study adds to the database of estimates for rates of regolith production and transport in the western United States and shows how dated landscapes can be used to improve our understanding of the coevolution of landscapes and regolith cover.

Model simulations of flood and debris flow timing in steep catchments after wildfire

Debris flows are a typical hazard on steep slopes after wildfire, but unlike debris flows that mobilize from landslides, most postwildfire debris flows are generated from water runoff. The majority of existing debris flow modeling has focused on landslide-triggered debris flows. In this study we explore the potential for using process-based rainfall-runoff models to simulate the timing of water flow and runoff-generated debris flows in recently burned areas. Two different spatially distributed hydrologic models with differing levels of complexity were used: the full shallow water equations and the kinematic wave approximation. Model parameter values were calibrated in two different watersheds, spanning two orders of magnitude in drainage area. These watersheds were affected by the 2009 Station Fire in the San Gabriel Mountains, CA, USA. Input data for the numerical models were constrained by time series of soil moisture, flow stage, and rainfall collected at field sites, as well as high-resolution lidar-derived digital elevation models. The calibrated parameters were used to model a third watershed in the burn area, and the results show a good match with observed timing of flow peaks. The calibrated roughness parameter (Mannings n) was generally higher when using the kinematic wave approximation relative to the shallow water equations, and decreased with increasing spatial scale. The calibrated effective watershed hydraulic conductivity was low for both models, even for storms occurring several months after the fire, suggesting that wildfire-induced changes to soil-water infiltration were retained throughout that time. Overall, the two model simulations were quite similar suggesting that a kinematic wave model, which is simpler and more computationally efficient, is a suitable approach for predicting flood and debris flow timing in steep, burned watersheds.

Development of topographic asymmetry: Insights from dated cinder cones in the western United States

Topographic asymmetry, that is, differences in the morphology of landscapes as a function of slope aspect, can be used to infer ecohydrogeomorphic feedback relationships. In this study, we document the dependence of topographic gradients and drainage densities on slope aspect and time/age in four Quaternary cinder cone fields in Arizona, Oregon, and California. Cinder cones are particularly useful as natural experiments in geomorphic evolution because they begin their evolution at a known time in the past (many have been radiometrically dated) and because they often have simple, well-constrained initial morphologies. North-facing portions of cinder cones have steeper topographic gradients and higher mean vegetation cover (i.e., Normalized Difference Vegetation Index, or NDVI, values) under current climatic conditions compared with corresponding south-facing portions of cones within each volcanic field. Drainage density is also higher on north-facing portions of cones in three of the four volcanic fields. These differences in topography were not present initially but developed progressively over time, indicating that the asymmetry is a result of post-eruption geomorphic processes. To test alternative hypotheses for the slope-aspect control of topography, we developed a numerical model for cinder cone evolution and a methodology for estimating local paleovegetation cover as a function of elevation, slope aspect, and time within the Quaternary. The numerical model results demonstrate that rates of colluvial transport were higher on south-facing hillslopes in at least three of the four cinder cones fields. Our paleovegetation analysis suggests that in the two Arizona volcanic fields we studied, higher rates of colluvial transport on south-facing hillslopes were the result of greater time-averaged vegetation cover and hence higher rates of sediment transport by floral bioturbation. Our results illustrate the profound impact that relatively small variations in solar insolation can have on landscapes via feedbacks among hydrology, vegetation cover, and sediment transport.

The influence of vegetation on debris-flow initiation during extreme rainfall in the northern Colorado Front Range

We explored regional influences on debris-flow initiation throughout the Colorado Front Range (Colorado, USA) by exploiting a unique data set of more than 1100 debris flows that initiated during a 5 day rainstorm in 2013. Using geospatial data, we examined the influence of rain, hillslope angle, hillslope aspect, and vegetation density on debris-flow initiation. In particular, we used a greenness index to differentiate areas of high tree density from grass and bare soil. The data demonstrated an overwhelming propensity for debris-flow initiation on south-facing hillslopes. However, when the debris-flow density was analyzed with respect to total rainfall and greenness, we found that most debris flows occurred in areas of high rainfall and low tree density, regardless of hillslope aspect. These results indicate that present-day tree density exerts a stronger influence on debris-flow initiation locations than aspect-driven variations in soil and bedrock properties that developed over longer time scales.

Elucidating the role of vegetation in the initiation of rainfall-induced shallow landslides: Insights from an extreme rainfall event in the Colorado Front Range

More than 1100 debris flows were mobilized from shallow landslides during a rainstorm from September 9-13, 2013 in the Colorado Front Range, with the vast majority initiating on sparsely vegetated, south-facing terrain. To investigate the physical processes responsible for the observed aspect control, we made measurements of soil properties on a densely forested north-facing hillslope and a grassland-dominated south-facing hillslope in the Colorado Front Range and performed numerical modeling of transient changes in soil pore water pressure throughout the rainstorm. Using the numerical model, we quantitatively assessed interactions among vegetation, rainfall interception, subsurface hydrology, and slope stability. Results suggest that apparent cohesion supplied by roots was responsible for the observed connection between debris flow initiation and slope aspect. Results suggest that future climate-driven modifications to forest structure could substantially influence landslide hazards throughout the Front Range and similar water-limited environments where vegetation communities may be more susceptible to small variations in climate.

Amplification of postwildfire peak flow by debris

In burned steeplands, the peak depth and discharge of postwildfire runoff can substantially increase from the addition of debris. Yet methods to estimate the increase over water flow are lacking. We quantified the potential amplification of peak stage and discharge using video observations of postwildfire runoff, compiled data on postwildfire peak flow (Qp), and a physically based model. Comparison of flood and debris flow data with similar distributions in drainage area (A) and rainfall intensity (I) showed that the median runoff coefficient (C = Qp/AI) of debris flows is 50 times greater than that of floods. The striking increase in Qp can be explained using a fully predictive model that describes the additional flow resistance caused by the emergence of coarse-grained surge fronts. The model provides estimates of the amplification of peak depth, discharge, and shear stress needed for assessing postwildfire hazards and constraining models of bedrock incision.

Effects of tsunami wave erosion on natural landscapes: Examples from the 2011 Tohoku-oki tsunami

The 2011 Tohoku-oki Tsunami affected approximately 600 km of the northeastern coast of the Japanese Honshu Island, leaving traces of destruction on man-made buildings and depositing mud- to boulder-sized sediment. Our field observations at Aneyoshi along the Sanriku “ria” coast, where a maximum run-up height of 39.2 m was recorded, add to the limited number of studies of tsunami wave effects on natural landscapes. We found evidence for (1) tsunami wave erosion that exposed bare rock by stripping basal hillslopes of regolith and vegetation, including trees, (2) transport and deposition of coarse gravel, and (3) scour-hole generation around a large boulder and a large sea wall fragment. Computer simulations indicate that the highest first wave reaching the Aneyoshi coast may have been about 20 m high, that the combined duration of the first three waves was tens of minutes to 1 h, and that the maximum wave velocity on land reached over 10 m/s and probably exceeded 20 m/s in the lower, wide reach of the Aneyoshi valley. We hypothesize that hillsides along the Sanriku Coast have been stripped by erosion of numerous ancient tsunami events recurring at century or even decadal scales, since at least the mid-Holocene. The cumulative effects of tsunami erosion on the hillslopes and their long-term evolution are important potential topics for future studies.

Debris flow initiation by runoff in a recently burned basin: Is grain-by-grain sediment bulking or en-masse failure to blame?†

Postwildfire debris flows are frequently triggered by runoff following high-intensity rainfall, but the physical mechanisms by which water-dominated flows transition to debris flows are poorly understood relative to debris flow initiation from shallow landslides. In this study, we combined a numerical model with high-resolution hydrologic and geomorphic data sets to test two different hypotheses for debris flow initiation during a rainfall event that produced numerous debris flows within a recently burned drainage basin. Based on simulations, large volumes of sediment eroded from the hillslopes were redeposited within the channel network throughout the storm, leading to the initiation of numerous debris flows as a result of the mass failure of sediment dams that built up within the channel. More generally, results provide a quantitative framework for assessing the potential of runoff-generated debris flows based on sediment supply and hydrologic conditions.

Constraining the relative importance of raindrop- and flow-driven sediment transport mechanisms in postwildfire environments and implications for recovery time scales

National Geographic Society [Proposal002929-2009-0046-1]; National Science Foundation [02-39749, 09-34131]; U.S. Geological Survey (USGS) Landslide Hazards Program

Which way do you lean? Using slope aspect variations to understand Critical Zone processes and feedbacks: Which way do you lean?

Soil-mantled pole-facing hillslopes on Earth tend to be steeper, wetter, and have more vegetation cover compared with adjacent equator-facing hillslopes. These and other slope aspect controls are often the consequence of feedbacks among hydrologic, ecologic, pedogenic, and geomorphic processes triggered by spatial variations in mean annual insolation. In this paper we review the state of knowledge on slope aspect controls of Critical Zone (CZ) processes using the latitudinal and elevational dependence of topographic asymmetry as a motivating observation. At relatively low latitudes and elevations, pole-facing hillslopes tend to be steeper. At higher latitudes and elevations this pattern reverses. We reproduce this pattern using an empirical model based on parsimonious functions of latitude, an aridity index, mean-annual temperature, and slope gradient. Using this empirical model and the literature as guides, we present a conceptual model for the slope-aspect-driven CZ feedbacks that generate asymmetry in water-limited and temperature-limited end-member cases. In this conceptual model the dominant factor driving slope aspect differences at relatively low latitudes and elevations is the difference in mean-annual soil moisture. The dominant factor at higher latitudes and elevations is temperature limitation on vegetation growth. In water-limited cases, we propose that higher mean-annual soil moisture on polefacing hillslopes drives higher soil production rates, higher water storage potential, more vegetation cover, faster dust deposition, and lower erosional efficiency in a positive feedback. At higher latitudes and elevations, pole-facing hillslopes tend to have less vegetation cover, greater erosional efficiency, and gentler slopes, thus reversing the pattern of asymmetry found at lower latitudes and elevations. Our conceptual model emphasizes the linkages among shortand long-timescale processes and across CZ sub-disciplines; it also points to opportunities to further understand how CZ processes interact. We also demonstrate the importance of paleoclimatic conditions and non-climatic factors in influencing slope aspect variations. Copyright