Abstract
AbstractThe wavelength of stationary water‐surface waves and their associated antidune bedforms are related to the mean velocity and depth of formative flow. In past published sand‐bed flume experiments, it was found that lens structures were preserved during antidune growth and change, and the dimension of the lenses was empirically related to antidune wavelength, and thus could be used to estimate flow velocity and depth. This study is the first to compare observations of formative flow conditions and resulting sedimentary structures in a natural setting, testing the previously published relationship at a field‐scale. Trains of stationary and upstream migrating water‐surface waves were prevalent during the flash flood in October 2012 in the Belham Valley, Montserrat, West Indies. Wave positions and wavelengths were assessed at 900 sec intervals through the daylight hours of the event within a monitored reach. The wave data indicate flow depths up to 1·3 m and velocity up to 3·6 m sec−1. Sedimentary structures formed by antidune growth and change were preserved in the event deposit. These structures include lenses of clast‐supported gravel and massive sand, with varying internal architecture. The lenses and associated low‐angle strata are comparable to sand‐bed structures formed from stationary and upstream migrating waves in flume experiments, confirming the diagnostic value of these structures. Using mean lens length in the event deposit underestimated peak flow conditions during the flood and implied that the lenses were preserved during waning flow.
Highlights
Sedimentary structures associated with supercritical flow conditions are very poorly documented in comparison with structures produced in subcritical flow conditions and, are still rarely recognized in the rock record. (Maejima et al, 2009)
Sedimentary structures resulting from formation, migration and truncation of antidunes have been documented in sand-bed flumes with and without bed aggradation (e.g. Alexander et al, 2001; Yokokawa et al, 2010), but there are no published studies in natural settings that compare the formative flow conditions and the resulting sedimentary structures
This study provides a test for the usefulness of antidune lens size as an alternative indicator of depositional flow conditions
Summary
Sedimentary structures associated with supercritical flow conditions are very poorly documented in comparison with structures produced in subcritical flow conditions and, are still rarely recognized in the rock record. (Maejima et al, 2009). Alexander et al (2001) suggested that the dimensions of lenses formed in flumes under stationary water-surface waves are related to antidune wavelength and could be used to estimate flow velocity and International Association of Sedimentologists. While this demonstrated the utility of the relationship to palaeoflow reconstruction, there were no direct measurements of the flow conditions which generated the documented deposit
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