Coastal Dynamics 2017 Paper No. 201 1462 RIP CURRENTS AT WAVE-AVERAGED TIME SCALES VIA RADAR REMOTE SENSING Annika O’Dea 1 and Merrick C. Haller 2 Abstract Rip currents, seaward-directed currents flowing through the surf zone and beyond, are an important component of nearshore circulation systems. In this study, X-band radar images of rip currents from Duck, NC, along with accompanying wind and wave measurements, are used to assess the forcing mechanisms controlling the offshore obliquity of rip currents relative to the shoreline. The relevant parameters in the alongshore momentum balance are assessed at the locations of three current sensors in the domain, and the associated radar images are used to determine how these different forcing mechanisms influence the obliquity of the rip current in and out of the surf zone. Results suggest that the direction of rip obliquity outside of the surf zone is primarily driven by the alongshore wind stress, although additional analyses are necessary to address simplifications made in the analysis. Preliminary observations of large rip currents from Newport, OR, are presented along with a discussion of future research areas focusing on the parameters influencing the obliquity and cross-shore extent of these large-scale rip systems. Key words: remote sensing, microwave radar, rip currents, nearshore hydrodynamics, nearshore momentum balances 1. Introduction Rip currents are narrow offshore-oriented currents, generated in the surf zone, that flow through the breaker line and extend offshore. They are frequent features on many beaches all over the world, and play an important role in the exchange of waters, nutrients, sediment, and organisms between the surf zone and inner shelf. Rip currents also pose a serious danger to ocean swimmers, and are the leading cause of lifeguard rescues on public beaches (Fletemeyer and Leatherman, 2010). Because of their importance in nearshore zones and their status as a major public safety hazard, significant efforts have been made to improve our understanding of the factors influencing the formation, strength, and morphology of rip currents as well as our ability to accurately model and forecast rips. A number of field studies have been conducting using nearshore pressure and current meter arrays to collect information on rip currents (e.g. Sonu, 1972; Guza and Thornton, 1989; Aagaard et al., 1997; Brander 1999; MacMahan et al., 2005). However, the transient nature of many rips and the need for dense spatial and temporal measurements make capturing and evaluating rip currents using these methods a difficult and expensive task. Lagrangian techniques such as drifters have provided a more synoptic picture of rip current circulation (Schmidt et al., 2003; Johnson and Pattiaratchi, 2004; MacMahan et al., 2010), but are still limited in their capacity to provide long-term observations over large areas. The application of remote sensing techniques to nearshore areas has expanded the type of data we can collect relating to rip currents as well as the temporal and spatial scales over which these data can be collected. Optical assessments of rip current location and persistence over long time scales (i.e. several years) have been conducted using video techniques (Holman et al., 2006). However, this technique is based on the identification of rip-associated morphological features (primarily rip channels in alongshore bars), and therefore is limited in the amount of information it can provide about the flow field itself. A few other studies have employed Doppler sonar (Smith and Largier, 1995; Vagle et al., 2001), airborne infrared imagery (Marmorino et al., 2013), and synthetic aperture radar (SAR) (da Silva et al., 2006; da Silva, 2008) to identify rip currents or rip-associated hydrodynamic features, but to date these methods have only been applied to a limited number of rip current events. Preliminary observations from shore-based microwave radar suggest that these instruments could 1 School of Civil and Construction Engineering, Oregon State University, USA. odeaa@oregonstate.edu 2 School of Civil and Construction Engineering, Oregon State University, USA. merrick.haller@oregonstate.edu