AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Experimental Study on Bedforms Created by Density Currents
(1) Earth and Atmospheric Sciences, St. Cloud State University, St Cloud, MN.
(2) ExxonMobil Upstream Research Company, Houston, TX.
(3) Saint Anthony Falls Laboratory, University of Minnesota, Minneapolis, MN.
We report results from more than 350 experimental runs performed with the purpose of investigating bedform patterns created by saline density currents. Combinations among several values of the water discharge, fractional density excess, sediment sizes composing the bed, and bed slopes, allowed for the analysis of the sedimentologic response of the bed to the flow across a full spectrum of the densimetric Froude number (i.e. supercritical, critical, and subcritical flows). Among the major findings we highlight our experimental observation that for such gravity flows, ripples and dunes can form within the supercritical flow regime. This suggests first that bedform regimes are in general much richer than thought before and secondly, that bedforms developed in deep water systems are somewhat different from those developed by subaerial flows (rivers). Therefore, inversion in deep-water environments based on shallow water bedform diagrams and regimes -as currently practiced- is likely to be incorrect. Other important experimental observations include: (1) density current bedform transition from flat beds to different bedform successions follows analogous sequences than subaerial bedforms, when using dimensionless shear stress and sediment size criteria for classification; (2) while ripples were observed to develop through a wide range of supercritical and subcritical flows, large dunes were commonly observed in supercritical flows; (3) upper flat bed regimes where observed to appear at Froude numbers of about 1.6 for fine sediments, and of about 3 for coarser sediments; (4) flat beds or small ripples were the characteristic bedform for flows around and at the critical condition; and (5) an interesting -and somewhat surprising- result included the observation of a different (and perhaps new) kind of bedform that originated as a typical dune- for intermediate and coarser sediment sizes, and typically for values of the Froude number of about 1.2-1.5, evolved with a height comparable to flow thickness, thus interacting with the flow interface, to become a symmetrical, short-wavelength, downstream migrating bedform that resembles a typical antidune. Finally, we investigated first order bedform scaling, and propose an alternative scaling for the case of deep-water supercritical dunes, which appear to follow a similar scaling law than subaerial subcritical dunes, but with an internal characteristic length (the Richardson layer) rather than the current thickness.