Bed form genesis from bed defects under unidirectional, oscillatory, and combined flows

Abstract

New experimental data on bed form initiation under unidirectional, oscillatory, and combined flows are presented to gain quantitative insight into bed form genesis from artificially generated defects on a flat sediment-laden bed. Planform changes revealed in time-lapse photography allowed study of the evolution of the downstream and upstream edges of defects from their initial geometric center. Based on this temporal data set and flow velocity profiles, it was observed that combined flow bed forms share the same bed form initiation processes as unidirectional and oscillatory flows, which are reflected directly in the generation of similar geometric patterns regardless of the hydrodynamic conditions. The development of these bed defects is strongly coupled to the direction and magnitude of the shear stress applied to the bed throughout the wave cycle. Under current-dominated combined flow conditions, no defect propagation occurred in the upstream direction, despite the presence of flow reversal. In addition, spectral analysis of the evolution of the downstream and upstream edges of the defects demonstrated that (i) periodicity in the defect growth pattern scales with the wave period for pure oscillatory flows; (ii) wave-dominated combined flows possess a period-driven peak with respect to defect growth in experiments with relatively short wave periods, but this peak is absent in experiments possessing relatively long wave periods (T >15 s); and (iii) current-dominated combined flows and pure unidirectional flows do not display a period-driven peak in defect growth. These results suggest that the occurrence of long-period combined flow bed forms may be underrepresented in the sedimentary record.

Direct Numerical Simulation of Transverse Ripples: 1. Pattern Initiation and Bedform Interactions

“…Although the mechanism leading to the formation of these chevron trains is still not fully understood, in section 5 we will explore the tendency for an incipient cross‐stream bedform to form a train of self‐similar structures. The mechanism of bed formation at the very early stages is difficult to analyze, especially in experiments, due to the very rapid growth of these bedforms and their quick evolution to fully formed ripples (Perillo, Prokocki, et al, ). These chevron features have very small amplitudes of approximately one sediment grain size.…”

“…It is this strong coupling between bed and flow that is essential for furthering our understanding of the various mechanisms of bed formation. Experimental studies (Baas, ; Coleman & Melville, ; Coleman et al, ; Fedele & García, ; Kennedy, ; Nelson & Smith, ; Perillo, Best, et al, ; Perillo, Prokocki, et al, ; Venditti et al, ) have illuminated aspects of short and long time evolution of the bed morphology. However, it has been difficult to establish the connection between individual turbulent flow structures and their effect on the formation and evolution of the ripples.…”

“…Indeed, having a sand pileup of only a few grains in height under a turbulent flow is difficult to set up. Consequently, experiments of this type are usually handled by an initial negative defect, that is, an initial cavity in the bed (Perillo, Prokocki, et al, ). Figure shows the evolution of such an isolated sediment pileup (Gaussian bump) in the frame of reference of the pileup.…”

Predicting bedforms and primary current stratification in cohesive mixtures of mud and sand

“…In sedimentary geology, as well as in hydraulic engineering, the characterization of these relationships for non-cohesive silt, sand and gravel is based on descriptive and empirical methods, because the physical processes responsible for the initiation, growth and stability of bedforms are still not fully known. Yet numerous scientific papers have investigated bed defects and wavelets that constitute the first expression of bedform development on a flat sediment bed, and their relation to coherent structures in the near-bed flow (Kennedy 1964(Kennedy , 1969Allen 1968Allen , 1979Southard & Dingler 1971;Williams & Kemp 1971;Kaneko & Honji 1979;Richards 1980;Kobayashi & Madsen 1985;Best 1992Best , 1993Best , 1996Rubin 1992;Baas 1994;Nelson et al 1995;Coleman & Melville 1996;Coleman et al 2003;Colombini 2004;Carling et al 2005;Venditti et al 2005Venditti et al , 2006Wierschem et al 2008;Chou & Fringer 2010;Fourrière et al 2010;Coleman & Nikora 2011;Bose & Dey 2012;Charru et al 2013;Perillo et al 2014a).…”

The morphology of fluvial‐tidal dunes: Lower Columbia River, Oregon/Washington, USA

“…O 'Donoghue, 2006;Du Buat, 1786;Harms et al, 1975;Kennedy, 1969;Kleinhans, 2001;Perillo et al, 2014aPerillo et al, , 2014bPerillo et al, , 2014cReesink & Bridge, 2009;Southard, 1991;Venditti et al, 2005aVenditti et al, , 2005b. One of the most common subaqueous bedforms are dunes (Best, 2005;Venditti, 2013), whose strata represent a fundamental building block of the rock record (Ashley, 1990;Bridge, 2003;Martinius & Van den Berg, 2011;Myrow et al, 2002;Myrow & Southard, 1991;Reynaud & Dalrymple, 2012).…”