Dryland Alluvial Fans

Harvey, Adrian

doi:10.1002/9780470710777.ch14

Cited by 52 publications

(40 citation statements)

References 154 publications

(258 reference statements)

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“…They form by deposition in repeated debris flows, and are thus an archive of past flow magnitude, timing, composition and depositional pattern (Schumm et al. ; Harvey, ; Dühnforth et al. ).…”

Section: Introductionmentioning

confidence: 99%

Effects of debris‐flow magnitude–frequency distribution on avulsions and fan development

Haas

Kruijt

Densmore

2018

Earth Surf Processes Landf

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Shifts in the active channel on a debris-flow fan, termed avulsions, pose a large threat because new channels can bypass mitigation measures and cause damage to settlements and infrastructure. Recent, but limited, field evidence suggests that avulsion processes and tendency may depend on the flow-size distribution, which is difficult to constrain in the field. Here, we investigate how the flow magnitude-frequency distribution and the associated flow-magnitude sequences affect avulsion on debris-flow fans. We created three experimental fans with contrasting flow-size distributions: (1) a uniform distribution; (2) a steep double-Pareto distribution with many flows around the mean and a limited number of large flows; and (3) a shallow double-Pareto distribution with fewer flows around the mean and more abundant large flows. The fan formed by uniform flows developed through regular sequences of stepwise channelization, backstepping of deposition toward the fan apex, and avulsion over multiple flows. In contrast, the wide range of sizes in the double-Pareto distributions led to distinct avulsion mechanisms and fan evolution. Here, large flows could overtop channels, creating levee breaches that could initiate avulsion immediately or in subsequent events. Moreover, sequences of small-to moderate-sized flows could deposit channel plugs, triggering avulsion in the next large flow. This mechanism was most common on the fan formed by a steep double-Pareto distribution but was rare on the fan formed by a shallow double-Pareto distribution, where large flows were more frequent. We infer that some flow-size distributions are more likely to cause avulsions -especially those that produce abundant sequences of small flows followed by a large flow. Critically, avulsions in our experiments could occur by either large single events or over multiple flows. This observation has important implications for hazard assessment on debris-flow fans, suggesting that attention should be paid to flow history as well as flow size.

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Section: Introductionmentioning

confidence: 99%

Effects of debris‐flow magnitude–frequency distribution on avulsions and fan development

Haas

Kruijt

Densmore

2018

Earth Surf Processes Landf

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“…Lane and Richards, 1997;Harvey, 2001). The strength of this linkage is driven by landslide, gully and alluvial fan stability as well as the potential for slope wash contributions (Harvey, 2011). Likewise channel-floodplain linkages are thought to be driven by the magnitude and inundation frequency of overbank flow events (Walling and He, 1997;Walling and Owens, 2003).…”

Section: Introductionmentioning

confidence: 99%

Channel–floodplain connectivity during an extreme flood event: implications for sediment erosion, deposition, and delivery

Croke

Fryirs

Thompson

2013

Earth Surf Processes Landf

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The term connectivity has emerged as a powerful concept in hydrology and geomorphology and is emerging as an innovative component of catchment erosion modeling studies. However, considerable confusion remains regarding its definition and quantification, especially as it relates to fluvial systems. This confusion is exacerbated by a lack of detailed case studies and by the tendency to treat water and sediment separately. Extreme flood events provide a useful framework to assess variability in connectivity, particularly the connection between channels and floodplains. The catastrophic flood of January 2011 in the Lockyer valley, southeast Queensland, Australia provides an opportunity to examine this dimension in some detail and to determine how these dynamics operate under high flow regimes. High resolution aerial photographs and multi-temporal LiDAR digital elevation models (DEMs), coupled with hydrological modeling, are used to assess both the nature of hydrologic and sedimentological connectivity and their dominant controls. Longitudinal variations in flood inundation extent led to the identification of nine reaches which displayed varying channel-floodplain connectivity. The major control on connectivity was significant non-linear changes in channel capacity due to the presence of notable macrochannels which contained a > 3000 average recurrence interval (ARI) event at mid-catchment locations. The spatial pattern of hydrological connectivity was not straight-forward in spite of bankfull discharges for selected reaches exceeding 5600 m 3 s -1. Data indicate that the main channel boundary was the dominant source of sediment while the floodplains, where inundated, were the dominant sinks. Spatial variability in channel-floodplain hydrological connectivity leads to dis-connectivity in the downstream transfer of sediments between reaches and affected sediment storage on adjacent floodplains. Consideration of such variability for even the most extreme flood events, highlights the need to carefully consider non-linear changes in key variables such as channel capacity and flood conveyance in the development of a quantitative 'connectivity index'.

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“…Debris flows are common geomorphic processes in mountainous regions worldwide (e.g., Iverson, ; Takahashi, ). Deposition of sediment by repeated debris flows results in the formation of debris‐flow fans where channels emerge from confined catchments into areas of local accommodation (Beaty, ; Blair & McPherson, ; De Haas et al, ; De Haas, Kleinhans, et al, ; Harvey, ; Hooke, ; Ventra & Nichols, ). Debris‐flow fans owe their characteristic semiconical form to shifts of the active channel and locus of deposition in space and time, termed avulsions (e.g., De Haas et al, ; De Haas, Densmore, et al, ; Schumm et al, ; Schürch et al, ; Whipple & Dunne, ).…”

Section: Introductionmentioning

confidence: 99%

Fan‐Surface Evidence for Debris‐Flow Avulsion Controls and Probabilities, Saline Valley, California

et al. 2019

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Debris‐flow fans form by shifts of the active channel, termed avulsions. Field and experimental evidence suggest that debris‐flow avulsions may be induced by depositional lobes that locally plug a channel or superelevation of the channel bed above the surrounding fan surface, by analogy to fluvial fans. To understand debris‐flow avulsion processes, we differentiate between these controls by quantifying the spatial distribution of debris‐flow lobe and channel dimensions, along with channel‐bed superelevation, on nine debris‐flow fans in Saline Valley, California, USA. Channel beds are generally superelevated by 2–5 channel depths above the fan surface, and locally by more than 7 channel depths, thereby substantially exceeding superelevation on fluvial fans. Depositional‐lobe thickness and channel depth decrease with distance from the fan apex, although both are highly variable across the fans. Median channel depths roughly correspond to the 50th–75th percentiles of lobe thicknesses, while minimum channel depths roughly correspond to the 10th–25th percentiles. In contrast, the thicknesses of lobes that have triggered avulsions roughly equal local channel depths and are on average twice as thick as the local median lobe thickness. The spatial correspondence between avulsion locations and thick lobe deposits, and the lack of correlation with channel‐bed superelevation, leads us to infer that avulsions on these fans are mostly caused by thick lobes forming channel plugs. Although results may vary with climatic and tectonic setting, our findings indicate that avulsion hazard assessment on populated fans should include mapping and monitoring of channel depths relative to typical deposit thicknesses on a given fan.

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Dryland Alluvial Fans

Cited by 52 publications

References 154 publications

Effects of debris‐flow magnitude–frequency distribution on avulsions and fan development

Effects of debris‐flow magnitude–frequency distribution on avulsions and fan development

Channel–floodplain connectivity during an extreme flood event: implications for sediment erosion, deposition, and delivery

Fan‐Surface Evidence for Debris‐Flow Avulsion Controls and Probabilities, Saline Valley, California

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