Field Observations of Small Scale Sedimentation Processes

Hanes, Daniel M.; Thosteson, Eric D.; Chang, Yeon-Sihk; Conner, C.C.; Vincent, Christopher E.

doi:10.1061/9780784404119.176

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“…Field observations have yielded a lot of information on suspension processes above rippled and plane beds. The influences of wave conditions, wave groups, sediment size, sediment gradation, etc., have been studied for example by Vincent and Green [1990], Hanes [1991], Kroon [1994], Hoekstra et al [1994], Houwman and Ruessink [1996], Lee and Hanes [1996], Van Rijn [1998], Hanes et al [1998], and Jimenez et al [1998]. However, in the field it remains very difficult to perform detailed measurements within the dense mobile sediment layer close to the seabed.…”

Section: Introductionmentioning

confidence: 99%

Sheet flow dynamics under monochromatic nonbreaking waves

2002

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[1] For the first time, detailed measurements of sediment concentrations and grain velocities inside the sheet flow layer under prototype surface gravity waves have been carried out in combination with measurements of suspension processes above the sheet flow layer. Experiments were performed in a large-scale wave flume using natural sand. Sand transport under high waves in shallow water is mainly contained within the so-called ''sheet flow layer,'' a thin layer (10-60 grain diameters) in which the volume concentration of sand decreases by an order of magnitude from a value near 0.6 at the stationary bed. The thickness of the layer varies over a wave cycle and the maximum thickness increases with increasing peak Shields stress. The concentrations within the sheet flow layer vary approximately synchronously with the orbital velocity measured by an Acoustic Doppler Velocimeter (ADV) located 0.1 m above the bed, with typical phase lags of 0-p/5. In contrast, the suspended sediment concentrations a few centimeters and higher above the bed exhibit larger phase lags. Grain velocities were successfully measured in the middle and upper portions of the sheet flow layer around the time of their maximums. These velocities increased weakly with elevation from approximately 50% to 70% of the velocity outside the wave boundary layer. The observations are compared to previous experimental work and are found to be mainly consistent with observations in steady unidirectional flows and in oscillating water tunnels (OWTs), although differences in the suspended sediment concentration and the total sediment transport rate are apparent. Observations are also compared to two very different models: a 1DV suspension model for oscillatory flow with enhanced boundary roughness and a two-phase collisional grain flow model for steady unidirectional flow. While the suspension model describes the velocity profile fairly well and the collisional model describes the concentration profile well, neither model accurately predicts both the velocity and the concentration and therefore the sediment flux over the full vertical extent of the sheet flow.INDEX TERMS: 4546 Oceanography: Physical: Nearshore processes; 4558 Oceanography: Physical: Sediment transport; 3022 Marine Geology and Geophysics: Marine sediments-processes and transport; 3020 Marine Geology and Geophysics: Littoral processes; KEYWORDS: sediment transport; sheet flow; bed load; suspension; large wave flume; sand transport models Citation: Dohmen-Janssen, C. M., and D. M. Hanes, Sheet flow dynamics under monochromatic nonbreaking waves,

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