DETERMINING RIP CURRENT CIRCULATION PATTERN AND VELOCITIES USING GPS EQUIPPED DROGUES

Rip currents pose a significant hazard to swimmers on Galveston Island, Texas. Groin structures built to protect the Galveston Seawall from erosion consequently are responsible for the generation of quasi-permanent rips. A field study was conducted to better understand and quantify the current speeds and circulation patterns associated with rip currents at this location. Eight GPS equipped drogues were deployed over a range of wave and tidal conditions over 2 weeks from May 3rd to 14th, 2012. The drifter data reveals a fairly consistent circulation cell with peak offshore velocities ranging between 0.10m/s up to 1m/s although long wave pulsing caused the rip speeds to reach 1.5m/s. The areal extent of the rip cell circulation was dependent on the strength of the rip current and wind and wave conditions of the specific day. Green flag days resulted in larger circulation patterns often resulting in occasional drogue releases from the circulation cell and moving alongshore hundreds of metres. Yellow flag days decreased the size of the circulation cell to the order of tens of metres and distances of alongshore drift. Onshore winds aided in the creation of the rip cell circulation directing drogues back to shore. However, with an offshore wind drogues moving off shore in the rip were swiftly driven further out to sea resulting in 3 drogues drifting up to 2 km offshore, providing more evidence of the hazard associated with nearshore circulation processes. Throughout the experiment, the rip channel accreted sediment, contributing to the decrease of rip current velocities and limiting the influence of bathymetric gradients on the magnitude of the flow. Being able to quantify the rip current hazard in Galveston, Texas by correlating rip current speeds and flag colours provides the Galveston Island Beach Patrol with information that can be used to better educate the public and beach staff. Ultimately the increased knowledge or rip current processes around groin structures can lead to the reduction in loss of life.