Mechanisms for avulsion on alluvial fans: insights from high-frequency topographic data

Leenman, Anya; Eaton, Brett C.

doi:10.31223/x5z01g

Cited by 3 publications

(6 citation statements)

References 51 publications

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“…Consequently, they postulated that overall channel stability reflects the stability of a small population of immobile or partially mobile large grains. In a previous study analyzing Run 1 in more detail, we also observed that in-channel deposition around accumulations of the largest grains disrupted autogenic flow pattern cycling (Leenman & Eaton, 2021). The non-linear relation between morphologic change and flow in our data may therefore indicate that channel dimensions are regulated by the (im)mobility of the coarsest grains on the fan.…”

Section: Vertical (Morphologic) Changesupporting

confidence: 63%

“…We collected data using an adaptation of Structure-from-Motion photogrammetry. The data collection system and its spatial accuracy are described in detail in Leenman and Eaton (2021) and Leenman (2021); here we give a brief summary. We mounted nine digital single-lens reflex cameras above the stream table to 'view' the experiment from different angles (Figure 1).…”

Section: Model Set-upmentioning

confidence: 99%

“…In this paper, we present and analyze all data from 12 hours of experimental running time and onward. Following Leenman and Eaton (2021), we exclude data from earlier in the experiments, as fan morphology and dynamics appeared to be scale dependent prior to this cutoff.…”

Section: Data Processing and Analysismentioning

confidence: 99%

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Floods on alluvial fans: implications for reworking rates, morphology and fan hazards

Leenman¹,

Eaton²,

MacKenzie³

2021

Preprint

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Flood events are the agents of change on alluvial fans. However, most alluvial fan experiments have used constant flows to model fans and the channels upon them. Here, we present results from a series of alluvial fan experiments with different patterns of flow variation (i.e. different hydrograph shapes). We conducted experiments with 1) constant flow, 2) alternating high and low flows, 3) a moderate flood peak that decayed slowly, alternating with a constant low flow, and 4) a high flood peak that decayed rapidly, alternating with a constant low flow. Importantly, all experiments had the same mean flow and sediment supply, but the different hydrographs generated fans with different slopes. In addition, higher peak flows led to increased lateral migration rates and increased erosion and deposition. These results challenge the notion that a single representative flow can be used to approximate the geomorphic effects of a range of flows in a natural stream. Moreover, the data suggest that hydrograph shape can govern the geomorphic impact of a flood event. Our findings indicate how altered basin hydrology (for instance, through changes to land cover) could influence geomorphic change and natural hazards on alluvial fans.

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Section: Vertical (Morphologic) Changesupporting

confidence: 63%

Section: Model Set-upmentioning

confidence: 99%

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Floods on alluvial fans: implications for reworking rates, morphology and fan hazards

Leenman¹,

Eaton²,

MacKenzie³

2021

Preprint

Self Cite

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“…It could be that incorporating avulsion dynamics even in detachment-limited settings could reveal an important component that drives models toward more realistic outcomes. Given that the frequency of avulsions depends on the micro-topography of the system, sediment characteristics, and the magnitude, burstiness, and sequencing of floods (Field, 2001;Stock et al, 2008;de Haas et al, 2016;Leenman and Eaton, 2020), such modeling efforts may reveal new mechanisms through which climate, lithology, and tectonics influence the rate of landscape response.…”

Section: Discussionmentioning

confidence: 99%

The rate and extent of wind-gap migration regulated by tributary confluences and avulsions

Shelef

Goren

2021

Earth Surf. Dynam.

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Abstract. The location of drainage divides sets the distribution of discharge, erosion, and sediment flux between neighboring basins and may shift through time in response to changing tectonic and climatic conditions. Major divides commonly coincide with ridgelines, where the drainage area is small and increases gradually downstream. In such settings, divide migration is attributed to slope imbalance across the divide that induces erosion rate gradients. However, in some tectonically affected regions, low-relief divides, which are also called wind gaps, abound in elongated valleys whose drainage area distribution is set by the topology of large, potentially avulsing side tributaries. In this geometry, distinct dynamics and rates of along-valley wind-gap migration are expected, but this process remains largely unexplored. Inspired by field observations, we investigate along-valley wind-gap migration by simulating the evolution of synthetic and natural landscapes, and we show that confluences with large side tributaries influence migration rate and extent. Such confluences facilitate stable wind-gap locations that deviate from intuitive expectations based on symmetry considerations. Avulsions of side tributaries can perturb stable wind-gap positions, and avulsion frequency governs the velocity of wind-gap migration. Overall, our results suggest that tributaries and their avulsions may play a critical role in setting the rate and extent of wind-gap migration along valleys and thus the timescale of landscape adjustment to tectonic and climatic changes across some of the tectonically most affected regions of Earth, where wind gaps are common.

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“…Avulsions are formative processes on debris‐flow fans, and shift the active channel, area of deposition, and hazard through space and time (de Haas, Densmore, et al, 2018). Until recently, much of our understanding of avulsion processes came from observations of fluvial systems (e.g., Bryant et al, 1995; Hamilton et al, 2013; Leenman & Eaton, 2020; Reitz et al, 2010; Reitz & Jerolmack, 2012; Schumm et al, 1987). Recent experimental and field studies implicate factors that affect debris‐flow avulsions, including flow composition, longitudinal position on the fan, path obstructions, degree of lateral confinement, channel depth, lobe thickness, fan slope, event volume, deposition history, and frequency–magnitude distributions (de Haas et al, 2016, 2019; de Haas, Densmore, et al, 2018; Pederson et al, 2015; Schaefer et al, 2021).…”

Section: Introductionmentioning

confidence: 99%

Exploring new methods to analyse spatial impact distributions on debris‐flow fans using data from south‐western British Columbia

Zubrycky

Mitchell

McDougall

et al. 2021

Earth Surf Processes Landf

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Predicting the spatial impact of debris flows on fans is challenging due to complex runout behaviour. Debris flow mobility is highly variable and flows can sporadically avulse the channel. For hazard and risk assessments, practitioners typically base the probability of spatial impact or avulsion on their experience and expert judgement.To support decision-making with empirical observations, we studied spatial impact distributions on 30 active debris-flow fans in south-western British Columbia, Canada. We mapped 146 debris-flow impact areas over an average observation period of 74 years using orthorectified airphotos, satellite imagery, topographic base maps, LiDAR data, orthophotos, and field observations. We devised a graphical method to convert our geospatial mapping into spatial impact heat maps normalized by fan boundaries, enabling comparison of runout distributions across different fans. About 90% of the mapped debris flows reached beyond the mid-points of fans, while less than 10% avulsed more than half-way across the fan relative to the previous flow path. Most avulsions initiated at distances of 20% to 40% of the maximum fan length from the fan apex and upstream of the fan intersection point. Large volume events tend to be more mobile in the down-fan direction, but the relation between volume and cross-fan runout (e.g., avulsions) is more complex. Differences in spatial impact distributions can be explained, in part, by the degree of fan incision and whether a fan is truncated at its toe by a river or lake. There were no significant differences in spatial impact distributions based on the geology of the source area, sediment supply condition, or hydrogeomorphic process classification.

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Mechanisms for avulsion on alluvial fans: insights from high-frequency topographic data

Cited by 3 publications

References 51 publications

Floods on alluvial fans: implications for reworking rates, morphology and fan hazards

Floods on alluvial fans: implications for reworking rates, morphology and fan hazards

The rate and extent of wind-gap migration regulated by tributary confluences and avulsions

Exploring new methods to analyse spatial impact distributions on debris‐flow fans using data from south‐western British Columbia

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