Debris flows are fast, water-charged slurries of sediment and rock, and among the most destructive processes in steep terrain, yet many of their basic mechanics long resisted direct measurement. Our foundational work established how debris flows initiate from rainfall runoff, in contrast to the prevailing view that they begin as the liquefaction of shallow landslides; how they entrain sediment from the bed to grow as they travel; and how large the forces are that they exert on the channel below.
Through sustained field monitoring (notably at the Chalk Cliffs natural debris-flow laboratory in Colorado) and a set of in-situ sensors we designed and built ourselves, we captured some of the first direct measurements of debris-flow dynamics, basal forces, and erosion in nature, quantities long thought too difficult to record. (McCoy et al., 2010; McCoy et al., 2012; McCoy et al., 2013; Kean et al., 2015; Coe et al., 2010)
These results remain widely cited foundations of debris-flow science, and they underpin much of the applied, post-fire hazard work our group pursues today. A continuing thread extends the same mechanistic lens to steepland sediment transport more broadly: the thresholds, rates, and channel forms that govern how mass moves through mountain landscapes. (Feehan et al., 2023; McCoy, 2015; Ring, 2024)
References
2024
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Hydraulic Geometry of Debris-Flow Channel Networks With Implications for Discharge Estimation
Caleb M. Ring
University of Nevada, Reno, 2024
Publication Title: ProQuest Dissertations and Theses
In steep landscapes, debris flows scour much of the upper channel network and significantly alter longitudinal profile form at drainage areas less than 1 to 5 km². The degree to which debris-flow processes alter other aspects of the upper channel network is largely unexplored. Here we compile a large data set of bedrock channel cross-sections following debris-flow scour to constrain the downstream hydraulic geometry in the upper channel network. We find that bedrock channel networks scoured by debris flows display systematic power-law relationships between discharge Q and channel width w ~ Q0.26-0.35 and mean depth d ~ Q0.40-0.46 and hence display hydraulic geometry akin to that found further downstream in bedrock and alluvial rivers. Observed w/d ratios are narrowly distributed (6.0+/- 2.5) despite w and d not having identical scaling with Q. However, the scaling observed when substituting upstream drainage area A for Q depends on initiation mechanism and network structure. For branching networks scoured by debris flows initiated from rainfall runoff, we find w ~ A0.31-0.33 as found in many bedrock rivers, whereas for debris flows initiated from failure of shallow landslides we find w ~ A0.08-0.16. In all cases, debris-flow channel widths are 2-10 times wider at A=105 km2 than that found in fluvial channels, highlighting that when present, debris flows are systematically larger at a given drainage area than rivers. If channel networks are nonbranching, we find almost no dependence of w on drainage area, w ~ A0.01-0.05. We use these relationships in combination with simple empirical velocity relationships and a constant w/d ratio to develop debris-flow discharge estimation techniques that require only measurements of scoured channel width or drainage area. We find good agreement when tested against debris-flow monitoring sites around the world. Hydraulic geometry relationships specific to bedrock channel networks scoured by debris flows should aid in developing more accurate models of landscape evolution that incorporate debris-flow processes, as well as provide first-order guidance on expected discharges emanating from these networks.
2023
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Quantifying Variability of Incipient-Motion Thresholds in Gravel-Bedded Rivers Using a Grain-Scale Force-Balance Model
Scott A. Feehan, Scott W. McCoy, Joel S. Scheingross, and 1 more author
Journal of Geophysical Research: Earth Surface, 2023
_eprint: https://onlinelibrary.wiley.com/doi/pdf/10.1029/2023JF007162
Predicting thresholds of sediment motion is critical for a range of applications involving sediment transport. However, thresholds for sediment motion can vary over an order of magnitude for a single characteristic flow and bed configuration. Lacking simple ways to incorporate this variability, many assume thresholds are constant for rough, turbulent flow. Here, we quantify variability of incipient-motion thresholds based on a commonly used grain-scale force-balance model, with model parameter distributions determined from published experiments. We show that variability in the threshold of motion within the 2D force-balance model occurs predominantly due to variability in the lift coefficient and grain protrusion, and secondarily due to drag coefficient variability. For a known grain size, the mean threshold of motion, and variability about the mean, can be predicted from a family of power laws. These power laws can be altered with site-specific parameter distributions, allowing for site-specific application to well-studied reaches and other planets. Using compiled flume and field data we show that constraining force-balance parameter distributions with independent data results in narrower distributions of the predicted threshold of motion, consistent with constrained flume experiments. This analysis highlights that while the threshold of sediment motion is variable, the magnitude of variability is predictable within the force-balance model based on site-specific physical constraints of local flow and bed conditions.
2015
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Estimating rates of debris flow entrainment from ground vibrations
J. W. Kean, J. A. Coe, V. Coviello, and 3 more authors
Geophysical Research Letters, 2015
Debris flows generate seismic waves as they travel downslope and can become more dangerous as they entrain sediment along their path. We present field observations that show a systematic relation between the magnitude of seismic waves and the amount of erodible sediment beneath the flow. Specifically, we observe that a debris flow traveling along a channel filled initially with sediment 0.34 m thick generates about 2 orders of magnitude less spectral power than a similar-sized flow over the same channel without sediment fill. We adapt a model from fluvial seismology to explain this observation and then invert it to estimate the level of bed sediment (and rate of entrainment) beneath a passing series of surges. Our estimates compare favorably with previous direct measurements of entrainment rates at the site, suggesting the approach may be a new indirect way to obtain rare field constraints needed to test models of debris flow entrainment.
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RESEARCH FOCUS: Infrequent, large-magnitude debris flows are important agents of landscape change
Scott W McCoy
Geology, May 2015
2013
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Field measurement of basal forces generated by erosive debris flows
S. W. McCoy, G. E. Tucker, J. W. Kean, and 1 more author
Journal of Geophysical Research: Earth Surface, Jun 2013
It has been proposed that debris flows cut bedrock valleys in steeplands worldwide, but field measurements needed to constrain mechanistic models of this process remain sparse due to the difficulty of instrumenting natural flows. Here we present and analyze measurements made using an automated sensor network, erosion bolts, and a 15.24 cm by 15.24 cm force plate installed in the bedrock channel floor of a steep catchment. These measurements allow us to quantify the distribution of basal forces from natural debris‐flow events that incised bedrock. Over the 4 year monitoring period, 11 debris‐flow events scoured the bedrock channel floor. No clear water flows were observed. Measurements of erosion bolts at the beginning and end of the study indicated that the bedrock channel floor was lowered by 36 to 64 mm. The basal force during these erosive debris‐flow events had a large‐magnitude (up to 21 kN, which was approximately 50 times larger than the concurrent time‐averaged mean force), high‐frequency (greater than 1 Hz) fluctuating component. We interpret these fluctuations as flow particles impacting the bed. The resulting variability in force magnitude increased linearly with the time‐averaged mean basal force. Probability density functions of basal normal forces were consistent with a generalized Pareto distribution, rather than the exponential distribution that is commonly found in experimental and simulated monodispersed granular flows and which has a lower probability of large forces. When the bed sediment thickness covering the force plate was greater than ∼ 20 times the median bed sediment grain size, no significant fluctuations about the time‐averaged mean force were measured, indicating that a thin layer of sediment (∼ 5 cm in the monitored cases) can effectively shield the subjacent bed from erosive impacts. Coarse‐grained granular surges and water‐rich, intersurge flow had very similar basal force distributions despite differences in appearance and bulk‐flow density. These results demonstrate that debris flows can have strong control on rates of steepland evolution and contribute to a foundation needed for modeling debris‐flow incision stochastically. , Key Points We measured 36-64 mm of bedrock incision by debris flows over four years Basal force variability was large (\textgreater+/-10x mean) and increased with mean force Granular surge fronts and watery tails had similar basal force distributions
2012
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Sediment entrainment by debris flows: In situ measurements from the headwaters of a steep catchment
S. W. McCoy, J. W. Kean, J. A. Coe, and 3 more authors
Journal of Geophysical Research: Earth Surface, Sep 2012
Debris flows can dramatically increase their volume, and hence their destructive potential, by entraining sediment. Yet quantitative constraints on rates and mechanics of sediment entrainment by debris flows are limited. Using an in situ sensor network in the headwaters of a natural catchment we measured flow and bed properties during six erosive debris‐flow events. Despite similar flow properties and thicknesses of bed sediment entrained across all events, time‐averaged entrainment rates were significantly faster for bed sediment that was saturated prior to flow arrival compared with rates for sediment that was dry. Bed sediment was entrained from the sediment‐surface downward in a progressive fashion and occurred during passage of dense granular fronts as well as water‐rich, inter‐surge flow. En masse failure of bed sediment along the sediment‐bedrock interface was never observed. Large‐magnitude, high‐frequency fluctuations in total normal basal stress were dissipated within the upper 5 cm of bed sediment. Within this near surface layer, concomitant fluctuations in Coulomb frictional resistance are expected, irrespective of the influence of pore fluid pressure or fluctuations in shear stress. If the near‐surface sediment was wet as it was overridden by a flow, additional large‐magnitude, high‐frequency pore pressure fluctuations were measured in the near‐surface bed sediment. These pore pressure fluctuations propagated to depth at subsonic rates and in a diffusive manner. The depth to which large excess pore pressures propagated was typically less than 10 cm, but scaled as ( D / f i ) 0.5 , in which D is the hydraulic diffusivity and f i is the frequency of a particular pore pressure fluctuation. Shallow penetration depths of granular‐normal‐stress fluctuations and excess pore pressures demonstrate that only near‐surface bed sediment experiences the full dynamic range of effective‐stress fluctuations, and as a result, can be more easily entrained than deeper sediment. These data provide robust tests for mechanical models of entrainment and demonstrate that a debris flow over wet bed sediment will be larger than the same flow over dry bed sediment. , Key Points Entrainment rates were 2‐10x faster for wet channel sediment than dry sediment Flow induced near‐surface pore pressures facilitated progressive entrainment Entrainment occurred during dense granular surges and water‐rich debris floods
2010
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Evolution of a natural debris flow: In situ measurements of flow dynamics, video imagery, and terrestrial laser scanning
Scott W. McCoy, Jason W. Kean, Jeffrey A. Coe, and 3 more authors
Geology, 2010
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Chalk Creek Valley: Colorado’s natural debris-flow laboratory
Jeffrey A. Coe, Jason W. Kean, Scott W. McCoy, and 2 more authors
In Through the Generations: Geologic and Anthropogenic Field Excursions in the Rocky Mountains from Modern to Ancient, 2010
Recent debris flow studies in Colorado indicate that the state is most susceptible to debris flows that initiate from surface-water runoff that erodes and entrains hillslope and channel sediment. These runoff-initiated debris flows grow in size by entraining sediment along travel paths, thereby increasing their destructive potential. Yet, the mechanics of initiation, erosion, and entrainment processes for runoffinitiated debris flows are poorly understood. The steep, bedrock-dominated flanks of the formerly glaciated Chalk Creek Valley near Nathrop, Colorado, generate an average of two runoff-initiated debris flows per year, making the valley an ideal natural laboratory for debris-flow research. This two-day field trip to the Chalk Creek Valley will examine debris-flow initiation areas, transport zones, deposits, and the impact of large pulses of debris-flow sediment on the morphology of Chalk Creek. On the first day, participants will hike into a particularly active basin at Chalk Cliffs where debris flows are being monitored by the U.S. Geological Survey, the University of Colorado, and East Carolina University. The second day will focus on debris-flow deposits in Chalk Creek and on recent debris flows in and near the community of Alpine in the central part of the valley.