Giant sand waves at the mouth of San Francisco Bay

Abstract

A field of giant sand waves, among the largest in the world, recently was mapped in high resolution for the first time during a multibeam survey in 2004 and 2005 through the strait of the Golden Gate at the mouth of San Francisco Bay in California (Figure la). This massive bed form field covers an area of approximately four square kilometers in water depths ranging from 30 to 106 meters, featuring more than 40 distinct sand waves with crests aligned approximately perpendicular to the dominant tidally generated cross‐shore currents, with wavelengths and heights that measure up to 220 meters and 10 meters, respectively. Sand wave crests can be traced continuously for up to two kilometers across the mouth of this energetic tidal inlet, where depth‐averaged tidal currents through the strait below the Golden Gate Bridge exceed 2.5 meters per second during peak ebb flows. Repeated surveys demonstrated that the sand waves are active and dynamic features that move in response to tidally generated currents. The complex temporal and spatial variations in wave and tidal current interactions in this region result in an astoundingly diverse array of bed form morphologies, scales, and orientations. Bed forms of approximately half the scale of those reported in this article previously were mapped inside San Francisco Bay during a multibeam survey in 1997 [Chin et al., 1997].

Guidance on Setup, Calibration, and Validation of Hydrodynamic, Wave, and Sediment Models for Shelf Seas and Estuaries

“…ese data can be used to determine net sediment budgets and transport pathways and assist model calibration (e.g., [103,104]). Repeat subtidal bathymetric surveys can provide valuable information on bedform mobility from which net sediment responses can be determined (e.g., [105]).…”

Découverte de mégadunes dans l’estuaire moyen du fleuve Saint-Laurent, Québec, Canada

“…Les mégadunes de surface (champs « A » et « B ») seraient également contemporaines de ce régime hydraulique. Par analogie avec les mégadunes de la baie de San Francisco, formées par l'empilement d'eau au passage d'un secteur ouvert à plus étroit (BARNARD et al, 2006), les mégadunes enfouies inférieures du champ « B », localisées en amont de la tête du chenal Laurentien, pourraient aussi correspondre à une accumulation d'eau lors de la marée montante après une diminution abrupte des profondeurs d'eau (-225 m à -50 m) et donc de la section du fleuve. Ces conditions hydrodynamiques n'auraient alors persisté que pour un temps, puisque les dunes enfouies supérieures et les dunes en surface ne montrent pas de migration vers l'amont.…”

“…De telles dunes ont été rapportées dans des environnements modernes, tels que la baie de San Francisco (BARNARD et al, 2006) et le détroit de Long Island (FENSTER et al, 2006). Ces dunes semblent associées à de fortes marées montantes (détroit de Long Island) ou à un rétrécissement important à l'entrée de la baie de San Francisco.…”

Garden of the Gods at Colorado Springs: Paleozoic and Mesozoic sedimentation and tectonics

“…A possibility that has not yet been considered, but that might be explored, is deposition of the Lyons by subaqueous sandwaves. Large subaqueous sandwaves are rather common features on the continental shelf (e.g., see Barnard et al, 2006). A model like this may help us understand (1) why the Lyons does not appear to be particularly well-sorted in the cross-bedded facies as we might expect from eolian sands, (2) why so many aqueous facies are present within the Lyons, (3) why the Lyons transitions into aqueous facies to the east in the subsurface, and (4) why channels of crudely sorted arkoses occasionally occur in the main cross bedded body of the Lyons.…”