Additional information on rip currents from the National Weather Service

Wendy Carey, University of Delaware Sea Grant Program
Spencer Rogers, North Carolina Sea Grant Extension Program

Through partnerships, agencies such as the NWS and Sea Grant have established rip current warning programs and public information campaigns in many coastal states. Lessons learned through the success of these projects, especially those in Florida and North Carolina, have been used as springboards for Sea Grant outreach efforts in many locations. For example, coordinated efforts between local beach patrols and Delaware Sea Grant have culminated in rigorous training programs for rescue personnel and production of printed material and interpretive signs on rip currents. In New Jersey, coastal engineers are working with NWS personnel to develop nearshore wave and circulation models that may improve rip current prediction capabilities.

Cooperation between regional programs will enhance national public education regarding rip current safety. Sea Grant specialists, the NWS, and beach patrols are developing programs that quickly broadcast warnings and incorporate rescue data into analyses. Additionally, technology transfer and application of engineering research findings on surf zone physics and rip current evolution will serve to enhance outreach efforts to increase awareness and safety for citizens visiting and using coastal waters.

Rip currents are narrow channels of water flowing out to sea past the surf zone. They pose significant dangers to beach goers, as they can pull even strong swimmers into deep water. Rip currents are usually narrow (50-100 feet in the alongshore direction), may extend 1,000 feet offshore, and generally span the entire water column. Recent research has demonstrated that offshore of the surf zone, they tend to be confined near the surface. Rip current velocities may reach or exceed 3-5 mph, and attempting to swim directly back to shore against the rip current can result in exhaustion and possibly drowning.

Rip currents develop as waves "pile up" water along the shore. The water begins flowing laterally along the beach (longshore current), and a low spot in the nearshore topography or a break in an offshore sand bar allows the water to move seaward as a rip current. Large amounts of water trapped behind a sand bar, and/or a convergence of the laterally flowing currents product the strongest rip currents. Rip currents may also develop near a fixed structure such as a groin, jetty or pier.

RIP CURRENT RESEARCH

Sea Grant rip current research at the University of Florida has focused on the persistence of rip currents over time, and the relationship between rip current strength, weather and surf parameters, and near shore bathymetry (Engle et al., 2002). MacMahan et al. (2001) conducted research which correlates rip current development to wave and current data obtained from instrument buoys deployed in near shore areas. Rip current pulsations were related to temporal variability and motion of sea-swell wave groups (MacMahan et al., 2001).

With California Sea Grant funding, oceanographers at Scripps Institution of Oceanography are using satellite-tracked drifters to track water movements within the surf zone in La Jolla. Near shore circulation patterns, wave patterns, current velocities, and eddies are observed and mapped through deployment of the surf drifters. The drifters have demonstrated that rip currents accelerate as they flow seaward, with the highest velocities occurring at the edge of the surf zone (Sea Grant in Brief, 2002).

Research on rip currents has been conducted by coastal engineers at University of Delaware's Center for Applied Coastal Research (CACR) for several decades (Dalrymple, 1978; Haller et al., 1997). Currently, Sea Grant funded rip current research has focused on wave basin and computer models. In the CACR wave basin, data are gathered on rip current channel widths, distances from the shore, and wave heights. These measurements have then been integrated into CACR's SHORECIRC computer model to improve its ability to predict when and where rip currents might occur and what their effects might be (Haas and Svendsen, 2000). Svendsen and Haas are continuing to test the SHORECIRC model to further study the hydrodynamics of rip currents, including those induced by coastal structures such as submerged breakwaters (Haas and Svendsen, 2002).

RIP CURRENT EDUCATION AND OUTREACH
Public education and outreach efforts regarding the dangers of rip currents have been widespread throughout shoreline states in the U.S., not only along the ocean coasts, but also along the Great Lakes shores. Sea Grant rip current outreach programs in Wisconsin, Florida, and North Carolina have been used as springboards for education and awareness projects in other states such as Delaware and New Jersey. Coordination and cooperation between Sea Grant and agencies such as the National Weather Service, U.S. Lifesaving Association, local lifeguards, emergency response personnel, and coastal communities have resulted in effective educational campaigns on many beach safety issues. North Carolina has developed posters, brochures, videos, and educational signs to improve rip current awareness and public safety (North Carolina Sea Grant, 2000; Mosher, 2002). Similarly, Delaware has worked with several coastal towns to place interpretive signs about rip current safety on boardwalks, beaches, and lifeguard stands.

An ongoing public education and outreach Sea Grant project in Delaware involves a coordinated program with the United Open Water Rescue program. A coalition of community and state park lifeguards, the U.S. Lifesaving Association, Delaware State Police Aviation Unit, the U.S. Coast Guard, and other rescue groups have joined forces to improve open water safety programs along Delaware's coast. One aspect of the Delaware program consists of ongoing efforts to film rip currents and mock rip-current rescues from the air with the cooperation of Dewey Beach Patrol lifeguards and the Delaware State Police helicopter. Experiments have been conducted using red food coloring as a dye to highlight the location of the rip current. Ultimately, the videotape and data gathered will be used in the future as a teaching tool for rescue personnel and as part of an educational public service announcement.

DEVELOPMENT OF RIP CURRENT PREDICTIVE INDICES

The National Weather Service (NWS) has been issuing rip current predictions, statements and advisories in various southern coastal states for several years. As describe by Engle et al. (2002), "the determination of a predictive index for rip currents is vitally important for the protection of human life. Such an index allows governmental agencies to issue rip current warnings directly to the public and allows lifesaving corps to set-up preventative measures according to the magnitude of the rip current threat." In Florida during the early 1990's, the NWS developed forecast models for prediction of rip currents which were based on collected rip current rescue data as well as physical parameters such as wind speed and direction, wave/swell height and period, and stage of the tide (Lushine, 1991; Lascody, 1998). Engle et al. (2002) report on modifications of the empirical forecast techniques originally developed by Lushine (Lushine Rip Current Scale or LURCS) and adapted to east coast of Florida by Lascody (ECFL LUCRS). Their analysis of rescue data and wind/wave/tide parameters demonstrated a correlation between rip current-related rescues and wave/wind measurements along a shoreline with periodically spaced rip channels. It was determined that the frequency of rip current rescues increased during conditions of: 1) shore-normal wave incidence, 2) mid-low tidal stages, 3) deep water wave heights of 0.5 to 1.0 meters, and 4) wave periods from 8 to 10 seconds (Engle et al., 2002).

In North Carolina, the NWS recently developed a Recreational Beach Forecast which includes rip current guidelines and prediction information (Pfaff, 2002). Their rip current forecast system is not only broadcast over local television and radio systems, but is also available online. National Weather Service success stories from Florida and North Carolina have encouraged offices in the northern states to consider doing the same beginning this season. The NWS office in Mt. Holly, New Jersey has initiated plans to coordinate with Sea Grant personnel and others to develop a rip current forecast product for use in New Jersey and Delaware coastal regions (Eberwine, personal communication). Additional cooperation with coastal engineers at Stevens Institute of Technology may further enhance forecast capabilities by upgrading their wave model into a nowcast/forecast model, and through use of a high-resolution nearshore wave model that may be able to eventually drive a model like SHORECIRC in a predictive mode (Herrington, personal communication).

LITERATURE CITED
Dalrymple, R.A. 1978. Rip Currents and Their Causes. Proc. 16th Intl. Conf. Coastal Engineering, ASCE, pp 1414-1427.
Eberwine, Jim. 2003. Personal communication. National Weather Service Forecast Office, Mt. Holly, NJ.
Engle, Jason, J. MacMahan, R. J. Thieke, D. M. Hanes, R. G. Dean. 2002. Formulation of a Rip Current Predictive Index Using Rescue Data. Proc. National Conf. on Beach Preservation Technology, FSBPA, January 23-25, 2002, Biloxi, MS.
Haas, K. A. and I. A. Svendsen. 2000. Three-dimensional modelling of rip current systems. Rep. No. CACR-00-06, 250 pp.
Haas, K. A. and I. A. Svendsen. 2002. Laboratory measurements of the vertical structure of rip currents. Jour. Geophys. Res. (in press)
Haller, Merrick C., R. A. Dalrymple, and I. A. Svendsen. 1997. Rip Channels and Nearshore Circulation. Proc. Conf. Coastal Dynamics, ASCE, pp 594-602.
Herrington, Tom. 2002. Personal communication. Stevens Institute of Technology, Hoboken, NJ.
Lascody, R. L., 1998. East Central Florida Rip Current Program. National Weather Digest, Volume 22, No. 2, p. 25-30.
Lushine, J. B., 1991. A study of rip current drownings and related weather factors. National Weather Digest, Vol. 16. pp 13-19.
MacMahan, J., Thieke, R. J., Dean. R. G., Hanes, D. M., and R. A. Holman. 2001. Analysis of rip channel stability. Journal of Marine Geology.
Mosher, Katie. 2002. Rip Currents .... Do You Know What To Do? Coastwatch Magazine, Early Summer 2002, North Carolina Sea Grant, Raleigh, NC, pp 7-11.
North Carolina Sea Grant. 2000. Be Prepared: Know the Signs of Dangerous Rip Currents. UNC-SG-00-01, Raleigh, NC.
Pfaff, Steven. 2002. Marine Program Guide, Recreational Beach Forecast. National Weather Service, WFO, Wilmington, NC.
Sea Grant in Brief. 2002 Teasing Apart the Physics of Rip Currents with a Satellite-Tracked Surf Drifter. July-August 2002 Newsletter of the California Sea Grant College Program.