Introduction

Weathered oil in the surf zone after an oil spill may mix with suspended sediments to form sand and oil agglomerates (SOAs) (Operational Science Advisory Team [OSAT], 2013). Sand and oil agglomerates may form in mats on the scale of tens of meters (m) and may break apart into pieces between 1 and 10 centimeters (cm) in diameter. These more mobile pieces are susceptible to alongshore and cross-shore transport, and lead to beach re-oiling on the time scale of months to years following a spill (OSAT-2, 2011; OSAT-3, 2013).

Understanding the transport of SOAs is critical to cleanup planning and response. In a modeling study to predict Deepwater Horizon SOA movement probabilities in the northern Gulf of Mexico (NGOM) for a variety of size classes, wind conditions, and wave conditions, it was found that SOAs are less mobile than sand, are not transported during low-energy wave conditions, and not likely to move alongshore in the surf zone, except during storm events (Plant and others, 2013; Dalyander and others, 2014). Instead, these heavier-than-water masses fall out of the water column onto the sea floor where they may be repeatedly buried and exhumed by wave action and currents. This model used the Shields parameter (Shields, 1936), along with modified versions of the Shields parameter developed in riverine systems for mixed-grain-sized environments (Fenton and Abbott, 1977; Andrews, 1983; Wiberg and Smith, 1987; Wilcock, 1998; Bottacin-Busolin and others, 2008) to predict SOA incipient motion. Field experiments with artificial SOAs (aSOAs) later determined that incipient motion predictions made by using a modified (lowered) critical Shields threshold to account for the exposure of larger objects in a mixed-grain bed were consistent with observed incipient motion of aSOAs in the surf zone during the low-energy wave conditions of the field experiment (Dalyander, 2015). Although shear-stress-based motion is typically used to predict the mobilization of sediments, it may not include the dominant mechanisms driving the motions of larger centimeter-scale objects such as SOAs. Other studies have found that pressure-gradient-driven motion (Madsen, 1974; Sleath, 1999), or a combined pressure-gradient and shear-stress parameterization (Foster and others, 2006; Frank and others, 2015) may more fully capture incipient motion processes and thus may be more accurate in describing the motion of SOAs.

The purpose of the current work was to expand the available data on SOA motion; test shear- stress-based incipient motion parameterizations in a controlled laboratory setting, and directly observe SOA exhumation and burial processes. The aSOAs created for the 2015 U.S. Geological Survey (USGS) field experiment (Dalyander and others, 2015) were deployed in two sets of experiments in a small-oscillatory flow tunnel. In the first set of experiments, individual aSOAs were placed on an immobile rough bottom and flow conditions were varied to determine the threshold of incipient motion. In the second set of experiments, the immobile rough bottom was replaced with coarse grain sand. The aSOAs were placed on the surface of the sand bed that then evolved under the varying flow conditions, forming sand ripples and other microtopographic features. More detailed descriptions of the experimental setup are included in the Methods section of this report, and techniques used to process the raw data are included in the Data Processing section. Video and velocity data obtained during these experiments are linked in the Data Catalog section and are provided in an associated USGS data release (Jenkins and others, 2017). Analysis of aSOA incipient motion is included in the Results section, along with qualitative descriptions of video data from the moving-bed experiments.