Sheet flow dynamics under monochromatic nonbreaking waves

Abstract

[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,

A semianalytical model for sheet flow layer thickness with application to the swash zone

“…The effects of boundary layer streaming [Dohmen-Janssen and Hanes, 2002;Nielsen, 2006;Yu et al, 2010] and of groundwater in/exfiltration [Turner and Masselink, 1998], which affect net sediment transport, are not included in the model. For example, groundwater infiltration induces a net downward force on sediment grains (counteracting sediment mobilization) but also decreases the boundary thickness, increasing the shear stress (enhancing sediment mobilization) [Butt et al, 2001].…”

Experimental Study of Turbulent Oscillatory Boundary Layers in a New Oscillatory Water Tunnel

“…To generate prototype flow conditions of such high Reynolds numbers, the laboratory wave flume has to be very large. For example, the large wave flume in Hanover (Dohmen-Janssen and Hanes, 2002) is 280m long, 7m deep and 5m wide. Therefore, it will be very cumbersome to set up experiments and deploy instruments for velocity measurements such as PIV.…”

Laboratory Experiments for Wave-Driven Sand Transport Prediction

“…The experiments have yielded measures of net sand transport rates for a wide range of flow and sand conditions and have provided insights that have informed the development of Figure 7. Measured sand transport rates from progressive surface waves [Dohmen-Janssen and Hanes, 2002;Schretlen et al 2011] and oscillatory flow tunnel experiments with "similar" flow and sand conditions (0.21 ≤ d 50 ≤ 0.27mm; 0.57 < R < 0.72; 5s < T < 10s). practical predictors.…”

“…Dohmen-Janssen and Hanes, 2002;Schretlen et al, 2011]. Also shown are measured net transport rates from large-scale oscillatory flow tunnel experiments with near-bed velocity-skewed flows and sand size similar to those of the progressive wave cases.…”