Precise Airborne LiDAR Surveying For Coastal Research And Ge(5)

ALSM elevation biases. Shorelines were delineated from 1m ×1m digital elevation models (DEM). Long and short ALSMranges (e.g. clouds, birds, and multi-paths) were edited andALSM elevation biases were removed. The edited and bias-corrected ALSM data were then imported into ARC/INFO andinterpolated using the TOPOGRID module, which is based onthe ANUDEM interpolation method of Hutchinson (1989). TheDEM’s were converted from HAE to orthometric height usingthe G99SSS gravimetric geoid model (Smith and Roman, 2000)and adjusted vertically so that the zero-elevation conformed tomean sea level (MSL) at the nearest tide station.3.2 Rollover Pass

Rollover Pass is a small artificial inlet on the southeast Texascoast that connects East Bay of the Galveston Bay system withthe Gulf of Mexico. The channel was dredged across a narrowportion of Bolivar Peninsula in 1954/55 and has stabilized at awidth of 61m. Bolivar Peninsula is an area of naturally higherosion rates, however the shape of the shoreline shows that theartificial inlet has altered rates of shoreline movement bychanging the littoral drift rate in the area.

From 1996 to 1999, Tropical Storms Josephine and Francescaused a total of 27m of scarp retreat 3.2km to the west ofRollover Pass. The process of shoreline retreat in the RolloverPass area involves episodic and dramatic scarp retreat duringstorms followed by post-storm recovery and widening of thebeach in front of the scarp. Eventually, the long-term erosionprocess resumes and the beach begins to narrow, allowing asubsequent storm to erode the scarp again.

We collected ALSM data along Bolivar Peninsula beforeTropical Storm Frances on August 6, 1998, and after the stormon September 17, 1998 using an Optech 1020 ALSM system.All the HAE were transformed into orthometric heights usingthe National Geodetic Survey G96SSS geoid model. All theALSM data were adjusted by –0.35m vertically so that the zero-elevation would conform to the local mean sea level asmeasured at the Port Bolivar tide gauge.

Figure 10. ALSM shaded relief images of Rollover Pass. Upperpanel is the pre-Tropical Storm Frances shoreline with the 1mcontour in white. The lower panel is the post-Frances shoreline.

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We computed pre- and post-Frances 2m × 2m DEMs from thevertically adjusted data sets. Figure 10 shows the coastaltopography at Rollover Pass before and after Frances. The 1melevation is the white contour line on both shaded relief images.We digitized the 1m contour lines along the beach for a distance of10km on either side of Rollover Pass. Figure 11 shows theshoreline change as represented by the movement of the 1mcontour from August 6 to September 17, 1998. The shoreline datashow a complex pattern of erosion. This pattern reflects theinteraction of factors including offshore topography and waverefraction, piers and other man-made shoreline structures, and pre-storm beach morphology in determining the response of the beachto the storm. Except for a small area within 300m west of RolloverPass where as much as 30m of retreat occurred, it appears that thepass had no unusual effect on beach erosion during this storm.

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Figure 11. Change in 1m contour at Rollover Pass during

due to Tropical storm Frances.

Figure 12. Geotube installed in front of the beach scarp at BolivarPeninsula during July 2001