SWASH: A New Method for Quantifying Coastal Change Surveying Wide Area Shorelines

This measure of the shifting land/water interface is also an important source of information for scientific investigations of coastal change, for determinations of the sediment budget, and for conducting numerical simulations of shoreline change.

Despite the importance of this measure of coastal change, the methods available for collecting shoreline position data are very limited. The most commonly applied method-shoreline interpretation from aerial photography-is expensive, labor-intensive, and involves a considerable amount of subjectivity in identifying the shoreline. There is a significant need for a method that can provide an unambiguous and repeatable measure of shoreline position, can cover large sections of coast within a single low tide period, is inexpensive to operate, and can be used for both long-term monitoring and rapid-response surveys of storm impacts. In response to this need, the USGS developed SWASH, a vehicle-based system for measuring shoreline position, which utilizes recent advances in the Global Positioning System (GPS). SWASH stands for “Surveying Wide Area Shorelines.

Methodology – The SWASH system is mounted on a six-wheel, amphibious, all-terrain vehicle. As the vehicle transits the coast, an array of GPS sensors are used to make high-accuracy measurements of horizontal position, vertical position, and beach slope. Following the field survey, position and slope data are combined to compute shoreline position, defined as the horizontal location of a target elevation contour's intersection with the beach. In keeping with historical sources on shoreline information, the Mean High Water (MHW) contour is usually chosen as the definition of the shoreline.

In contrast to shorelines derived by most previous methods, SWASH shorelines have well-defined error bars, important for determining the statistical significance of shoreline change. Error bars are calculated on a point-by-point basis as a function of beach slope and the deviation between the elevation driven and MHW. SWASH can survey more than 70 km of shoreline within a single low tide period and provide near real-time information on shoreline changes during storms. SWASH is also very inexpensive to operate relative to previous methods for obtaining shoreline position.

Current Survey Program and Example Results – The SWASH system is currently being applied to study storm-induced and fair weather shoreline change on beaches in North Carolina and Massachusetts. The most extensive set of measurements are within the Cape Cod National Seashore, where both the short-term impact of storms and the longer seasonal cycle of change has been measured in a continuing survey program initiated in April 1998. The shoreline erosion response was extraordinarily non-uniform, with zones of significant erosion (more than 20 m of shoreline recession) alternating with zones of virtual stability (less than 2 m of change). In the period of decreasing waves following the storm, the pattern of change almost entirely reversed, with the erosional zones showing strong accretion and the stable zones still exhibiting no significant change. Similar results have been obtained for other storms on Cape Cod, as well as along the Outer Banks of North Carolina. Although the processes responsible for these erosional “hotspots” are unknown, their identification has important implications for management of both cultural and environmental resources along the coast. Research is ongoing to better characterize the locations and persistence of erosional hotspots and to understand their cause or causes.

For study-area maps and figures related to the SWASH system, please visit http://woodshole.er.usgs.gov/operations/swash/