JOURNAL OF WATERWAY, PORT, COASTAL,AND OCEAN ENGINEERING / MAY/JUNE 2001 / 141 SAND TRANSPORT IN OSCILLATORY SHEET FLOW WITH MEAN CURRENT By S. R. McLean, 1 J. S. Ribberink, 2 C. M. Dohmen-Janssen, 3 and W. N. Hassan 4 ABSTRACT: The sheet-flow layer, under a combination of waves and mean current, is typically a few milli- meters thick and characterized by very high sediment concentrations. Small conductivity probes, capable of measuring high sediment volume concentrations, were employed to estimate sediment fluxes within the sheet- flow layer. Two probes, spaced 20 mm apart in the streamwise direction, were deployed upward through the sand bed in a water tunnel. Using correlation techniques, the sensors provided concentration information and estimates of particle velocity. These measurements yielded direct measurements of sediment flux. Two different sand sizes (120 and 320 ) were investigated with mean velocities between 0.3 m/s and 0.5 m/s and oscillatory amplitudes between 0.9 m/s and 1.5 m/s that were created using monochromatic, sinusoidal waves of 7.2-s period. Mean sediment-velocity estimates, averaged over a wave period, were smaller than predictions of water velocity from a combined wave/current boundary layer model. On the other hand the oscillatory component of the velocity was approximately in-line with predictions from the model. Flux estimates clearly show that the bulk of the sediment transport takes place within the sheet-flow layer. INTRODUCTION Sand transport by waves and currents in the near-shore en- vironment occurs under the influence of several physical re- gimes that are related to the small-scale morphology of the sea bed. This paper addresses the sheet-flow regime that cor- responds to high near-bed velocities washing ripples out and causing the sea bed to become flat. Sheet-flow conditions gen- erally occur on the shallow part of the shoreface under storm conditions. Large transport rates take place in a thin layer (thickness of order 10 mm) with high sediment concentrations (greater than 10% by volume). The present state of knowledge of the sheet-flow process is based on experimental research in oscillating water tunnels. In these laboratory facilities, even though no vertical oscillatory motions are present, the oscillatory horizontal flow due to nat- ural waves can be simulated in the high-velocity regime and basic research on wave-boundary layers and sheet-flow layers can be carried out. The understanding and modeling of hori- zontal oscillatory sheet flow, and its interaction with suspen- sion, is a necessary step toward the modeling of sheet flow/ suspension processes over the full water depth for real-surface waves, where vertical orbital velocities and wave-induced net currents exist. During recent years insight has been obtained into the ver- tical structure and time-dependent behavior of oscillatory sheet flow layers from laboratory experiments of Horikawa, Wata- nabe, and Katori (1982), Staub, Jonsson, and Svendsen (1984), Sawamoto and Yamashita (1986), Asano (1995), Dick and Sleath (1991, 1992), Ribberink and Al-Salem (1994, 1995), Janssen and Ribberink (1996), Janssen, Hassan, van der Wal, and Ribberink (1997), Zala-Flores and Sleath (1998) and Doh- men-Janssen (1999). Various measuring techniques and ma- terials were employed in a range of oscillatory flow conditions; 1 Prof., Mech. and Envir. Engrg., Dept., Univ. of California, Santa Bar- bara, CA. 2 Assoc. Prof., Facu. of Tech. and Mgmt., Univ. of Twente, Enschede, The Netherlands. 3 Postdoctoral Student, Facu. of Tech. and Mgmt., Univ. of Twente, Enschede, The Netherlands. 4 PhD Student, Facu. of Tech. and Mgmt., Univ. of Twente, Enschede, The Netherlands. Note. Discussion open until November 1, 2001. To extend the closing date one month, a written request must be filed with the ASCE Manager of Journals. The manuscript for this paper was submitted for review and possible publication on December 23, 1998; revised October 2, 2000. This paper is part of the Journal of Waterway, Port, Coastal, and Ocean Engineering, Vol. 127, No. 3, May/June, 2001. ASCE, ISSN 0733- 950X/01/0003-0141–0151/$8.00 + $.50 per page. Paper No. 19911. time-dependent, as well as time- or wave-averaged results, were obtained. Modeling of sheet-flow dynamics is compli- cated by the interaction of processes such as sediment entrain- ment and settling, unsteady flow effects, and/or phase effects (Davies, Ribberink, Temperville, and Zyserman 1997). Rib- berink (1998) showed that net (i.e., time-averaged, sand trans- port) rates can be described successfully with the formula used by Meyer-Peter and Mueller (1948) for a wide range of oscil- latory and steady sheet-flow and flat-bed conditions. However, the formula assumed the sand transport is quasi-steady through a wave cycle and could not be used for relatively fine sands (D < 0.2 mm) and small wave periods (T < 3 s). Dohmen- Janssen, van der Hout, and Ribberink (1998) explained that phase effects, such as delayed entrainment and settling of sand grains, are responsible. These effects occur for large values of the parameter p = /w s that relates particle settling times to wave period, where  =2/ T,  is the sheet-flow layer thick- ness, and w s is the settling velocity of a single grain in still water. Based on measurements of Dick and Sleath (1991, 1992) and new tunnel measurements, Zala-Flores and Sleath (1998) showed that different sheet-flow regimes exist for low and high values of the parameter S = U osc /g, which is a ratio of oscillatory acceleration to reduced gravity, where U osc is the oscillating velocity amplitude, g is the acceleration of gravity and  =( s - )/ is the relative density, where  s and  are the sediment and fluid densities, respectively. Very lightweight materials, such as PVC and nylon, seem to have a different dynamic behavior than sand. For improved mod- eling of oscillatory sheet flow, additional insight is necessary into the internal flow structure of the sheet-flow layer. The available data sets are too limited to obtain this insight and new experiments are needed. As a follow-up to experimental research in the large oscil- latory water tunnel (LOWT) of WL | Delft Hydraulics (Janssen and Ribberink 1996; Janssen et al. 1997; Dohmen-Janssen et al. 1998), a set of new sheet-flow experiments were carried out with the primary objective to obtain simultaneous mea- surements of time-dependent (grain) velocities and sand con- centrations inside the sheet-flow layer. The following consid- erations were used for the experimental set-up: 1. Since phase effects are of special interest, measurements should be made outside the influence of the side-wall boundary layer (wall effects). 2. For application in natural settings, the measurements should preferably be made with sand in a relevant range of diameters, avoiding light-weight materials (polyvinyl chloride, nylon).