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Fundamental Flood Hazard Issues in the Alluvial Fan Environment
Published in Saeid Eslamian, Faezeh Eslamian, Flood Handbook, 2022
Debris and mud flows may be the most hazardous of the common alluvial fan flooding processes and may result in the most severe types of flood damages. However, the available methodologies for assessing mud and debris flow hazards are oriented more toward identifying whether such hazards exist, rather than quantifying the risk in terms of depths, velocities, and probability of occurrence. It is likely that many past floodplain delineations on active alluvial fans that are subject to mud and debris flows fail to adequately account for debris flow hazards and thus, significantly underestimate the true flood hazard.
Geology of Urban Watersheds
Published in Daniel T. Rogers, Urban Watersheds, 2020
Alluvial deposits are typically formed by fast-flowing stream water. They range in size from fine silt to boulders, and also include debris flows and rock slides from adjacent mountain slopes. One example of an alluvial deposit is an alluvial fan. An alluvial fan is a flat to gently sloping mass of sediment and loose rock material shaped like a fan. These deposits are created at the base of a mountain slope where fast moving steams coming out of the mountains meet the relatively flat surfaces of a basin floor or broad valley. At this junction, the stream gradient is significantly decreased, which removes much of the energy available for sediment transport by the flowing water (Bloom 2004). The result is the deposition of sediment in a fan shape as depicted in Figure 2.37. Alluvial fans are common in the western United States.
Fluvial depositional landforms
Published in Richard J. Chorley, Stanley A. Schumm, David E. Sugden, Geomorphology, 2019
Richard J. Chorley, Stanley A. Schumm, David E. Sugden
Alluvial fans occur in a variety of environments, particularly in arid and semi-arid or seasonally dry regions, where there is a large sediment supply to a point where accumulation can occur. Such locations are particularly common along faulted or tectonic mountain fronts. Depending on the magnitude and persistence of the fluvial processes, the radii of alluvial fans can vary from a few hundred metres to tens of kilometres. Smaller fans commonly have gradients of 3°-6° or less, steepening to some 10° near the apex where they may grade into marginal screes lying at angles approaching 30°. The gross form of a fan depends on the magnitude of the sediment supply from the contributing drainage basin and the climatic controls over the local hydrological characteristics.
Mid-Phanerozoic microbialite forms and associated facies in the northern Perth Basin, Western Australia, and their relationship to the end-Permian mass extinction
Published in Australian Journal of Earth Sciences, 2022
L. J. Olden, M. Barham, J. Cunneen, H. K. H. Olierook, E. Suosaari, G. C. Smith
Large alluvial fans form where streams, sourced from the hinterland, encounter a sudden decrease in slope, typically at the base of the upland region (Miall & Postma, 1997). Sediments are deposited rapidly as a consequence of loss of energy and ‘fan’ out in a radial pattern. Typical characteristics of an alluvial fan include poorly sorted coarse-grained sediments that are locally sourced (Blair & McPherson, 1994). Fans typically exhibit lateral fining textures. Main feeder channel sediments exhibit winnowing textures and imbrication of clasts. Reported occurrences of alluvial stromatolites commonly colonise the primary channel of the fan (Elmore, 1983). The PBS exhibits negligible winnowing textures and has clear normal grading textures where present. Thus, a pure alluvial depositional environment is inconsistent with observations for the PBS. However, hyper-concentrated alluvial sedimentary processes may have played a part in the episodic deposition of the coarse-grained siliciclastic facies.
Pleistocene marine terraces of the Wellington south coast – their distribution across multiple active faults at the southern Hikurangi subduction margin, Aotearoa New Zealand
Published in New Zealand Journal of Geology and Geophysics, 2022
Dee Ninis, Timothy Little, Nicola Litchfield, Ningsheng Wang, Katrina Jacobs, C. Mark Henderson
The erosional shore platforms are unconformably overlain by both marine and non-marine deposits. Marine deposits typically consist of well-sorted and well-rounded sands, pebbles and gravels that directly overly the shore platform, similar to the modern-day beach deposits on this coastline. Younger, non-marine gravelly deposits are interpreted to have been deposited as alluvial fans, whereas massive, homogeneous units of silt-size grains, are inferred to be aeolian deposits such as loess. Subaerial erosion and mass wasting of the cliffs behind the terraces has shed aprons of angular and irregular-sized clasts of younger colluvium atop the aforementioned deposits. These partially bury the older, and otherwise flat, terrace treads.
Lithofacies characteristics and sedimentary model of a gravelly braided river-dominated fan: a case study of modern Poplar River alluvial fan (northwest Junggar Basin, China)
Published in Australian Journal of Earth Sciences, 2021
D.-W. Liu, Y.-L. Ji, C.-L. Gao, J. Zhong, Y. Qi
Controls on the development of alluvial fans include autocyclic (e.g. hydrodynamic conditions) and allocyclic (e.g. structure, climate and base-level change) factors (DeCelles et al., 1991; Fernandez et al., 1993; Fidolini et al., 2013; Jonathan et al., 2008). Specifically, several sedimentary models of alluvial fans have been built based on some single factors, such as a dependence on source-river types (De Haas et al., 2015; Harvey, 2002; Martinez et al., 2010), climate background differences (Waters et al., 2010) and tectonic activity (Blair, 2000; Hu et al., 2012). More recently, many scholars have focused on the study of river-dominated alluvial fans or megafans, because this fan type has a large spatial extent (generally larger than 100 km2) and contains more complex depositional characteristics than traditional debris and fluvial deposits. Currently, the mainstream classification scheme of river-dominated alluvial fans refers to source-river types, and the alluvial fans are divided into fluvial-flow-dominated alluvial fans, and debris-flow-dominated alluvial fans (Blair, 1999b; Calvo & Ramos, 2015; Carnicelli et al., 2015; Kim & Lowe, 2004; Stanistreet & McCarthy, 1993). Moreover, the fluvial-dominated alluvial fans can be further divided into meandering river-dominated alluvial fans, straight river-dominated alluvial fans, and braided river-dominated alluvial fans (Allen, 1981; De Haas et al., 2014. Ridgway & DeCelles, 1993; Stanistreet & McCarthy, 1993). Blair and McPherson (2009) proposed sheet-flow alluvial fans based on tractive flow genesis enriching the fluvial alluvial fan models of Stanistreet and McCarthy (1993).