HR: 0800h
AN: H41C-0324    [Abstracts]
TI: Quantifying Shoreline Change Using Mean High Water and High Water Line Shorelines: Should Proxy-Datum Offsets be Incorporated?
AU: * Moore, L J
EM: laura.moore@oberlin.edu
AF: Oberlin College Department of Geology, 52 West Lorain Street, Oberlin, OH 44074 United States
AU: Ruggiero, P
EM: pruggiero@usgs.gov
AF: Coastal and Marine Geology Program, U.S. Geological Survey, 345 Middlefield Road, MS-999, Menlo Park,, CA 94025 United States
AU: List, J
EM: jlist@usgs.gov
AF: Coastal and Marine Geology Program, U.S. Geological Survey, 384 Woods Hole Road, Woods Hole, MA 02543 United States
AB: Studies of large-scale geomorphic change in coastal environments often rely on shoreline change analyses to determine rates and identify alongshore patterns of change. Although the high water line has traditionally been the shoreline indicator of choice in such endeavors, the use of datum-based shorelines derived from lidar is becoming more common. For this reason, it is important to understand the effects of combining traditional shoreline indicators and datum-based shorelines in shoreline change analyses. We compare a high water (proxy-based) shoreline, interpreted from aerial photography, with a mean high water (datum-based) shoreline derived from a lidar survey. The aerial photography and lidar surveys were collected simultaneously along 45 km of Assateague Island, which has a relatively steep reflective beach (1V:14H), and experiences a moderately energetic wave climate (annual average significant wave height = 1.2 m). Results show an average horizontal offset of approximately 20 m between the two types of shoreline indicators, with the mean high water shoreline seaward of the high water line. Vertical offsets are also substantial and are correlated with foreshore beach slope and corresponding variations in wave run-up. Incorporating the average horizontal offset into both an end-point and a linear regression shoreline change analysis produces average shoreline change rate shifts of -0.5 m/yr and -0.1 m/yr, respectively. The rate shift increases with increasing horizontal offset and decreasing measurement intervals. However, results suggest that in most cases the error due to a rate shift will be small relative to shoreline change rates. Investigation of pre-survey hydrodynamic conditions using a total water level model suggests that the high water line was generated by a combination of large waves and a high tide several days prior to the aerial survey. This result illustrates the complexity of the high water line as a shoreline indicator and calls into question traditional definitions of this feature, which consider the high water line a wetted bound or "marks left by the previous high tide." Especially for this reason, datum-based shorelines provide a more reliable measure of coastal change than traditional high water line shorelines.
DE: 1894 Instruments and techniques
DE: 3020 Littoral processes
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting