|
Ground Surveys Beach Movement Beach Profiles |
 The Southwest
Washington Coastal Erosion Study is utilizing two LIDAR data sets,
one collected in August 1997 and one in April 1998. Integrating
LIDAR data with beach monitoring data from the study helped fill
gaps in the existing data set. The regional scale of the LIDAR
data provides a more detailed coverage of the study area than is
possible to obtain in a reasonable amount of time through ground
surveys. At left is an image generated by the Washington
Department of Ecology showing a beach surface derived from LIDAR.
Warm colors (red, orange, yellow) represent higher elevations,
while cool colors (green, blue) represent lower elevations. Also
depicted on the graphic are the 1.0-, 2.0- and 3.0-meter contours
as derived from the April 1998 LIDAR data set. By comparing
subsequent LIDAR and/or ground surveys, changes in contour
positions can be quantified, leading to an improved understanding
of seasonal to interannual beach migration.
|
|
The Southwest Washington Coastal Erosion Study initiated a beach morphology monitoring program in summer 1997. Global Positioning System (GPS) equipment is used to conduct two types of ground surveys, cross-shore beach profiles and three-dimensional beach topographic surfaces. Beach profiles are collected using GPS mounted to a backpack. These surveys are conducted at 47 sites in the littoral cell by walking from the landward side of the primary dune, over the dune crest, across the beach and out to wading depth at low tide. Although analysis of beach profiles can reveal both cross-shore beach elevation variability and volume change over an individual profile, little information about the longshore component of morphologic change can be obtained. |
|
In lieu of multiple closely spaced cross-shore transects, three-dimensional surface maps are being generated by mapping the beach surface with a GPS antenna mounted to a six-wheel-drive amphibious all-terrain vehicle called the CLAMMER (CoastaL All-terrain Morphology Monitoring and Erosion Research vehicle). CLAMMER data are collected at 16 sites throughout the littoral cell between the base of the primary dune and the swash zone. Each site extends approximately 4 kilometers, providing over 64 kilometers of coverage in the littoral cell. As opposed to beach profiles that extend across the beach, the CLAMMER is driven along the beach. Individual measurements are densely spaced in the alongshore direction, approximately one every five to ten meters, to resolve relatively small-scale features such as beach cusps. The data are gathered over long-enough distances (approximately 4 kilometers) to resolve larger-scale and potentially migrating features, such as mega-cusps, rip-current embayments, sand waves, and regional gradients. In general, the LIDAR and CLAMMER data sets are directly comparable. |
Return to Top
|
Shoreline position (based on a specified elevation contour) can change quite drastically over a period of several months in response to a number of natural influences, including tides, wave height, wave direction, and water level. In the Pacific Northwest, seasonal variability in wave height and direction generally results in northerly offshore sediment transport in the winter (beach erosion) and southerly onshore sediment transport in the summer (beach accretion). The figure shown at left displays the 2.0-meter contour location for six surveys conducted between summer 1997 and summer 1999, as well as the April 1998 LIDAR data. The green line represents the 2.0-meter contour derived from the spring 1998 LIDAR data. The other lines are derived from CLAMMER surveys conducted as part of the study's beach morphology monitoring program. |
Return to Top
|
The image at left compares a profile collected with GPS equipment mounted on a backpack in February 1999 and the LIDAR survey from April 1998. The LIDAR data are represented by the red line, and the ground data, or Southwest Washington Coastal Erosion Study (SWCES) data is represented by the dark blue line. By extracting profiles from a LIDAR data set at some specified interval, important beach parameters can be extracted from the data, including beach slopes, dune toe and dune crest elevations, and the volume of sediment above some datum. The regional coverage of the LIDAR data can assist in the derivation of these parameters throughout the entire littoral cell in a more efficient manner than is possible by conducting numerous ground surveys. |
Return to Top