MONITORING BEACH CHANGES USING GPS SURVEYING TECHNIQUES

A need exists for frequent and prompt updating of shoreline positions, rates of shoreline movement, and volumetric nearshore changes. To eff ectively monitor and predict these beach changes, accurate measurement s of beach morphology incorporating both shore-parallel and shore-norm al transects are required. Although it is possible to monitor beach dy namics using land-based surveying methods, it is generally not practic al to collect data of sufficient density and revolution to satisfy a t hree-dimensional beach-change model of long segments of the coast. The challenge to coastal scientists is to devise new beach monitoring met hods that address these needs and are rapid, reliable, relatively inex pensive, and maintain or improve measurement accuracy. The adaptation of Global Positioning System (GPS) surveying techniques to beach monit oring activities is a promising response to this challenge. An experim ent that employed both GPS and conventional beach surveying was conduc ted, and a new beach monitoring method employing kinematic GPS surveys was devised. This new method involves the collection of precise shore -parallel and shore-normal GPS positions from a moving vehicle so that an accurate two-dimensional beach surface can be generated. Results s how that the GPS measurements agree with conventional shore-normal sur veys at the 1 cm level, and repeated GPS measurements employing the mo ving vehicle demonstrate a precision of better than 1 cm. In addition, the nearly continuous sampling and increased resolution provided by t he GPS surveying technique reveals alongshore changes in beach morphol ogy that are undetected by conventional shore-normal profiles. The app lication of GPS surveying techniques combined with the refinement of a ppropriate methods for data collection and analysis provides a better understanding of beach changes, sediment transport, and storm impacts.