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Small-Scale Sediment Transport Patterns and Bedform Morphodynamics: New Insights from High-Resolution Multibeam Bathymetry

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Small-Scale Sediment Transport Patterns and Bedform Morphodynamics: New Insights from High-Resolution Multibeam Bathymetry Geo-Mar Lett DOI 10.1007/s00367-011-0227-1 ORIGINAL Small-scale sediment transport patterns and bedform morphodynamics: new insights from high-resolution multibeam bathymetry Patrick L. Barnard & Li H. Erikson & Rikk G. Kvitek Received: 8 July 2010 /Accepted: 18 January 2011 # Springer-Verlag (outside the USA) 2011 Abstract New multibeam echosounder and processing between surveys indicates the impact of different flow technologies yield sub-meter-scale bathymetric resolution, regimes on the entire bedform field. This paper presents revealing striking details of bedform morphology that are unique fine-scale imagery of compound and superimposed shaped by complex boundary-layer flow dynamics at a bedforms, which is used to (1) assess the physical forcing range of spatial and temporal scales. An inertially aided and evolution of a bedform field in San Francisco Bay, and post processed kinematic (IAPPK) technique generates a (2) in conjunction with numerical modeling, gain a better smoothed best estimate trajectory (SBET) solution to tie the fundamental understanding of boundary-layer flow dynam- vessel motion-related effects of each sounding directly to ics that result in the observed superimposed bedform the ellipsoid, significantly reducing artifacts commonly orientation. found in multibeam data, increasing point density, and sharpening seafloor features. The new technique was applied to a large bedform field in 20–30 m water depths Introduction in central San Francisco Bay, California (USA), revealing bedforms that suggest boundary-layer flow deflection by The shape, size, migration, and orientation of bedforms can the crests where 12-m-wavelength, 0.2-m-amplitude bed- reveal important information about local and regional two- forms are superimposed on 60-m-wavelength, 1-m- dimensional flow structure, sediment transport processes, and amplitude bedforms, with crests that often were strongly forcing/boundary conditions, including the lateral distribution oblique (approaching 90°) to the larger features on the lee of net bedload transport direction and the associated residual side, and near-parallel on the stoss side. During one survey flow velocity distribution (e.g., Langhorne 1982;Beldersonet in April 2008, superimposed bedform crests were contin- al. 1982;Kuboetal.2004). For example, Knaapen et al. uous between the crests of the larger features, indicating (2005) showed that net sediment transport rates as derived that flow detachment in the lee of the larger bedforms is not from sand wave migration are well correlated with model always a dominant process. Assessment of bedform crest predictions of residual sediment transport. Some excellent peakedness, asymmetry, and small-scale bedform evolution reviews of bedform features and distribution have been published (e.g., Ashley 1990; Dalrymple and Rhodes 1995). P. L. Barnard (*) : L. H. Erikson For superimposed bedforms, spatial variations in bedform Pacific Coastal and Marine Science Center, morphology are thought to be due to local flow separation United States Geological Survey, associated with larger bedform morphology, unsteady flows, 400 Natural Bridges Drive, or possibly secondary flows (cf. Allen and Collinson 1974; Santa Cruz, CA 95060, USA e-mail: [email protected] Allen 1978). Understanding bedform morphology and controlling processes can provide insight into (1) the R. G. Kvitek fundamental understanding of bedform dynamics in complex Seafloor Mapping Lab, Institute for Earth Systems Science & hydrodynamic and morphological settings, (2) the influence Policy, California State University, Monterey Bay, 100 Campus Center, of boundary condition variations (e.g., sediment supply, sea Seaside, CA 93955-8001, USA level rise, tidal forcing) on bedform evolution, and (3) Geo-Mar Lett anthropogenic impacts on bed evolution and sediment be even higher. Malikides et al. (1989) deployed near-bed transport, which are critical to addressing estuarine sediment current meters on the crest and trough of large (9 m management issues such as channel dredging, aggregate amplitude) subtidal bedforms, and documented current mining, and damming. deflection and acceleration over bedform crests associated Allen and Friend (1976) observed bimodal (i.e., super- with the pattern of small superimposed bedform crests at an imposed) bedforms over three consecutive sets of spring– acute angle to the larger ones. Wienberg et al. (2004) and neap cycles along the British North Sea coast. In their Wienberg and Hebbeln (2005) quantified the impact of investigation, over 85% of the superimposed features formed dredge disposal on a bedform field, where, despite within one-quarter to one-third of a wavelength upstream significant morphological changes and complete burial in from the crest of the major bedform. They found the places, there was not a persistent long-term effect, as superimposed features to be more three-dimensional, mostly bedforms regenerated within a few months and continued lunate, and abundant only during the spring tidal cycle. The migrating. large bedforms were two-dimensional, and more temporally Within the context presented above, large- and small- and spatially consistent in size and shape. Terwindt (1971) scale bedform occurrence, shape, and evolution over a observed mega-current ripples superimposed at up to 45° seasonal and decadal timescale were investigated in central angles to the crests of the larger sand waves, with the San Francisco Bay, using recent advances in multibeam superimposed height increasing from trough to crest, and echosounder and processing technology from surveys in suggested that the obliquity was likely evidence that the 2008, and incorporating comparison with earlier, lower- direction of tidal currents in the bottom boundary layer resolution data extracted from a 1997 survey by Dartnell varied between trough and crest, a result of topographic and Gardner (1999). The fine details of the observations steering. Dalrymple and Rhodes (1995) note that divergences from the recent surveys were only possible due to the of up to 90° have been observed, but that 30–60° are more advance in technology for mapping the seafloor (cf. common. Sweet and Kocurek (1990)demonstratedintheir Ernstsen et al. 2006b), and the associated improvements study of eolian dunes that the oblique orientation of in post-processing. This paper aims to describe a new superimposed bedforms is due to deflection of the near-bed technique for resolving the seabed at an extremely fine flow by the three-dimensional shape and oblique orientation scale, and explores a direct application of this fine of the larger dunes, and that when lee-side slopes are <20° resolution for gaining a better understanding of bedform flow separation does not occur, but rather stays attached to morphodynamics. the bed and is deflected into an along-crest orientation. Ernstsen et al. (2005) observed superimposed bedforms oriented on the lee sides of larger, saddle-shaped bedforms Physical setting that were oblique to the tidal current directions and perpendicular to larger bedform crests, and attributed their San Francisco Bay, California (Fig. 1), experiences a modest formation to topographic steering as well. Terwindt (1971) diurnal tidal range (i.e., difference between mean higher high also observed superimposed bedforms on either side of the water and mean lower low water) of 1.78 m (NOAA 2010), crest oriented toward the crest, the lee-side features referred but due to the size of the estuary over ~7.5 trillion liters of to as backflow ripples by Allen (1980)thatwereformed water is forced through the 1.5-km-wide Golden Gate each during high flow conditions. Ernstsen et al. (2006a)observed day, generating tidal currents that can exceed 2.5 m/s in the similar features on the lower lee side of large asymmetric inlet throat (Barnard et al. 2007). These powerful currents bedforms. and an ample supply of coarse sediment result in large Rubin and McCulloch (1980) showed that superimposed bedforms that can be up to 317 m in wavelength in the ebb bedform heights increase with bed shear velocity. Parsons jet (Barnard et al. 2006), with many other large bedforms et al. (2005) performed the first integrated study of detailed locally persistent where currents are focused by erosion- three-dimensional flow fields and bedform morphology in a resistant bedrock outcrops. By contrast, shorter superim- fluvial setting, showing that lateral flows and secondary posed bedforms have to date not been recorded in this area, circulation can have significant impacts on morphology. In except along spatially restricted, discrete transects using their study, superimposed features were absent in the lee- analog side scan in the 1970s by Rubin and McCulloch side scour region, and tended to increase in height and (1980). Relatively low-resolution (~4 m) surveys performed wavelength toward the crests of major bedforms. Topo- in 1997 by Dartnell and Gardner (1999)wereunableto graphic forcing of flow by the bedforms resulted in detectanysuchfeatures.BarnardandKvitek(2010) differences in flow direction of >15° between flow over investigated the regional-scale bathymetric change between the crest and in lee-side areas. Laboratory measurements by the surveys conducted in 1997 and 2008, but did not analyze Maddux et al. (2003) suggest that the flow deflection could any fine-scale seabed patterns. Geo-Mar Lett o 45 N Depth (m) 0 o 40 N Fig. 6a 110 Angel Study Island o area 35 N Pacific Ocean o o 120 W 115 W Focus area, N Marin Fig. 6b Figs. 2-5 Fig. 6c Fig. 6d Kilometers San Francisco 031.5 Fig. 1 Location of the study area with depth measurements from the April 2008 survey in shaded relief (sun azimuth=240° and angle=25°). Also shown are the locations of five sites selected for more detailed analyses (boxes) The study was carried out in 2008 in a 1 by 1.5 km bedform However, they likely play only a minor role in sediment field in central San Francisco Bay, south of Angel Island. A transport in the deeper sectors, including our study area. 750 by 450 m focus area within this bedform field (Fig. 1), with water depths ranging from ~20 to 30 m, was selected for more detailed analysis of bed characteristics using Materials and methods multibeam bathymetry.
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