Mapping and Mitigating the International Rip Current Health Hazard

Rip currents are concentrated seaward flows of water originating in the surf zones of beaches. Rips cause hundreds of international drownings each year. Calculating exact numbers is barred by logistical difficulties in obtaining accurate incident reports, but annual rip current fatalities are estimated at 100, 53 and 21 in the United States (US), Costa Rica, and Australia respectively. Notably, Australia's lifeguards rescue 17,600 swimmers from rips each year. This project addresses the geophysical, social, and systematic causes of fatalities in hopes of decreasing the global number of rip-related deaths. We demonstrate a novel method for mapping bathymetry in the surf zone (20m deep or less), specifically within rip channels (topographic low spots in the nearshore that result from feedback amongst waves, substrate, and antecedent bathymetry). We calculate bathymetry using 8-band multispectral imagery from the Digital Globe WorldView2 (WV2) satellite and field measurements of depth, generating maps of the changing nearshore at two embayed, rip-prone beaches: Playa Cocles, Costa Rica, and Bondi Beach, Australia. WV2 has a 1.1 day pass-over rate with 1.84m ground pixel resolution of 8 bands, including `yellow' (585-625 nm) and `coastal blue' (400-450 nm). Methods are tested for consistency amongst dates and locations. Previous research shows drownings result from a combination of the physical environment with personal and group behaviors; for this reason we build on rip-detection by evaluating tourists' and locals' knowledge and understanding of their beach's rip behavior. By combining the geomorphologic maps developed from WV2 with interview data, we evaluate how the physical environment dictates the exposure of certain swimmers. Controls include rip channel location, beach access points, and environmental factors favored by swimmers. The project serves as an evaluation of the landscape's creation of a physical feature that becomes a hazard when vulnerable humans swim into it. Results are meant to inform policy makers so that they may implement the methods: developing frequent nearshore maps to observe rip channel behavior, designing beach access controls informed by morphology, posting informative signs in ideal locations, and funding lifeguard programs.