Observations of wave-generated vortex rippleson the North Carolina continental shelfFabrice ArdhuinCentre Militaire d’Oce´anographie, Service Hydrographique et Oce´anographique de la Marine, Brest, FranceT. G. DrakeDepartment of Marine, Earth and Atmospheric Sciences, North Carolina State University, Raleigh, North Carolina, USAT. H. C. HerbersOceanography Department, Naval Postgraduate School, Monterey, California, USAReceived 18 May 2001; revised 15 November 2001; accepted 4 December 2001; published 10 October 2002.[1] Sand ripples with wavelengths between 0.5 and 3 m were observed on the bottomacross the U.S. east coast continental shelf off North Carolina during three side-scan sonarsurveys in September and December 1999. Ripples were present in about 75% of thesurvey images, in particular, in regions with coarser sediments. Analysis of surficialsediment samples shows that median grain diameters range from 0.1 to 4.7 mm with largevariations on the inner shelf over distances <1 km. The observed ripple properties areconsistent with wave-generated vortex ripples. Analysis of concurrent wave observationsindicates that the ripple crests were aligned perpendicular to the average direction of near-bottom wave-induced motions during preceding events that were s ufficiently energetic tomobilize surficial sediments. Furthermore, the ripple wavelengths proportionality to near-bottom wave orbital excursions is consistent with wave-formed vortex ripples. Thesefindings support the hypothesis that the observed strong attenuation of waves across theshelf resulted from form drag over large vortex ripples.INDEX TERMS: 3022 Marine Geologyand Geophysics: Marine sediments—processes and transport; 3045 Marine Geology and Geophysics: Seafloormorphology and bottom photography; 4211 Oceanography: General: Benthic boundary layers; 4560Oceanography: Physical: Surface waves and tides (1255); KEYWORDS: ripples, waves, shelf, SHOWEX,observations, North CarolinaCitation: Ardhuin, F., T. G. Drake, and T. H. C. Herbers, Observations of wave-generated vortex ripples on the North Carolina continentalshelf, J. Geophys. Res., 107(C10), 3143, doi:10.1029/2001JC000986, 2002.1. Introduction[2] Bed forms on the sandy sea floor are an importantsource of hydrodynamic roughness for waves and currentsin the surf zone and on the inner shelf [Zhukovets, 1963;Scott and Csanady, 1976]. Bed forms have a strong impacton the transport of sediments, either as a r esult of theirmigration or because of their influence on the flow thatshapes them. Their presence, formation, and evolution havebeen observed extensively in nearshore environments [e.g.,Hunt, 1882; Forel, 1883; Dingler, 1974; Vincent andOsborne, 1993; Gallagher et al., 1998; Traykovski et al.,1999]. In the absence of mean currents, waves can generateripples that are symmetric in cross section. The formation ofsuch wave ripples was first investi gated in the laboratory byDarwin [1883] using a rotating bath. He noted the importantrole of the vortices generated in the lee of the ripples, furtherobserved by Ayrton [1910], eroding the ripple troughs andbuilding up the crests. Such bed forms, termed ‘‘vortexripples’’ by Bagnold [1946], exert a much larger drag on theflow than friction on sand grains. Vortex ripples occasion-ally have been called ‘‘orbital ripples’’ because their wave-length is related to the near-bed orbital diameter of the wavemotion, or ‘‘megaripples’’ when they exceed some largewavelength, although they should not be confused withnearshore short-crested megaripples generated by differentprocesses [Gallagher et al., 1998].[3] Based on dimensional analysis and numerical mor-phodynamic modeling, Andersen [1999] and Andersen andFredsøe [1999] found that the wave flow over ripples isessentially governed by two nondimensional parameters,l/d and h/l, where l and h are the ripple wavelength andheight and d is the diameter of the bottom orbital excursionof water parcels. Sediment motions are governed by twoadditional parameters, the ratio ws/umaxof the settlingvelocity of sand grains wsand the maximum near-bedorbital velocity umaxand the maximum ratio of frictionand buoyant forces acting on a sand grain, represented bythe Shields number ymax(often denoted q0max)ymax¼f0wu2max2 s 1ðÞgD; ð1ÞJOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. C10, 3143, doi:10.1029/2001JC000986, 2002Copyright 2002 by the American Geophysical Union.0148-0227/02/2001JC000986$09.007 - 1where f0wis a skin friction factor, s is the specific density ofthe sand grains, D is the grain diameter, and g is theacceleration of gravity [Shields, 1936]. When ymaxis largerthan a critical value yc, the flow is able to move sand grains.If the bed is initially flat, ‘‘rolling-grain ripples’’ will formas a result of an instability. Each grain creates a region ofweaker flow (‘‘shadow’’) in its lee, and grains tend to groupand form ripples with larger shadows [Blondeaux, 1990;Vittori and Blondeaux, 1990; Andersen, 1999]. Theserolling-grain ripples eventually evolve into vortex ripples[Sherer et al., 1999; Faraci and Foti, 2001]. The height h ofvortex ripples is generally closely related to l. The ripplesare steepest for 1 < ymax/yc< 4, when the vortices createdin the lee of the crests maintain h/l values between 0.1 and0.2 [Nielsen, 1981]. For very large values of ymaxof theorder of 10yc[Li and Amos, 1999], a layer of sediment (the‘‘sheet flow’’) moves with the water column, ripples arewashed out, and the dissipation of wave energy in thebottom boundary layer is relatively weaker.[4] Numerical model simulations of the morphodynamicsof one-dimensional ripples under sinusoidal waves confirmthat the vortex forming in the lee of the crest with a size of theorder of the orbital diameter d exerts a strong shear on the lee-side slope of the ripples that tends to build up the creststogether with the flow on the upstream slope of the ripple[Andersen, 1999]. If two ripples are initially closer than aminimal stable wavelength lm, the vortex in the lee of theupstream ripple will erode the downstream ripple and oneripple may disappear creating a default in the regular spacingof the bed that will allow l to grow. Conversely, ripples thatinitially are much further apart than lmwill promote thegeneration of ripples in the troughs, dividing l by a factor of2. Values of lmwere found to be related to the orbitaldiameter with lm=0.4d when ws/umax< 0.07, and lm=0.63d