HR: 13:40h
AN: G43D-01 INVITED    [Abstracts]
TI: Determination of Event-Dependent Depth of Closure for the South Florida Atlantic Coast Using Airborne Laser Bathymetry
AU: * Robertson, W
EM: qrobertson@coastalplanning.net
AF: Coastal Planning and Engineering, 2481 NW Boca Raton Blvd., Boca Raton, FL 33431, United States
AU: Zhang, K
EM: zhangk@fiu.edu
AF: International Hurricane Research Center, 11200 SW 8th St. FIU UP Campus MARC 360, Miami, FL 33199, United States
AU: Finkl, C
EM: cfinkl@coastalplanning.net
AF: Coastal Planning and Engineering, 2481 NW Boca Raton Blvd., Boca Raton, FL 33431, United States
AU: Whitman, D
EM: whitmand@fiu.edu
AF: Florida International University, Department of Earth Sciences 11200 SW 8th St., Miami, FL 33199, United States
AB: Depth of closure (DOC) is an important concept in coastal engineering that defines the seaward limit of significant net sediment transport along a wave-dominated sandy beach profile. Few surveys measured the DOC over large areas because traditional methods for measuring DOC are time consuming and cost prohibitive. With a dramatic increase in airborne laser bathymetric data in recent years, it has become possible to measure DOC over many kilometers. Reported here is a new method that identifies the DOC using airborne laser bathymetric (ALB) data. The horizontal location of the DOC was determined by differencing 2004 pre- and post-hurricane airborne laser data sets along Palm Beach, Broward, and Miami-Dade counties. Noise in the ALB data were approximately +/- 0.3 m, thus bathymetric variations greater than +/- 0.3 m were considered significant change. The seaward depth where change was less than +/- 0.3 m was interpreted as the DOC. The measured DOC was compared horizontally and vertically to DOC positions that were calculated based on wave data and to geomorphic units at 1046 locations spaced 100 m along the coastline. Calculated DOC values were on average within 2.8 m vertically to the measured DOC in the northern end of the study area. In the southern segment of the study area, however, the calculated DOC was on average deeper than the measured DOC. Small horizontal differences (90 m, on average) between geomorphic boundaries (rock outcrop, hardgrounds) and measured DOC suggest geologic control south of Hillsboro Inlet. Diabathic channel fields match the measured DOC to the north, with a vertical difference of 0.3 m and a horizontal difference of 161 m, on average. Because diabathic channels are hydrodynamically formed (hydromorphodynamic forms), the northern study area appears to be hydrodynamically controlled. Given the ALB data represent a before and after surface for the 2004 hurricane season, the DOC extracted from ALB data in this study is event-dependent, which is different from a normal DOC based on a typical annual wave condition. Because this study measured an event-dependent DOC where waves generated by two hurricanes were larger than large waves in a typical year, the measured DOC represents a deeper DOC than the normal DOC. It is essential to understand temporal changes of the DOC in order to effectively apply the DOC to various applications. Determination of the DOC from ALB data is an accurate, cost-effective and technologically advanced means of determining DOC over large areas that can range from quiescent phases to large storm events.
DE: 3020 Littoral processes
DE: 3045 Seafloor morphology, geology, and geophysics
DE: 4546 Nearshore processes
SC: Geodesy [G]
MN: 2007 Fall Meeting