Precise Airborne LiDAR Surveying For Coastal Research And Ge(6)

International Archives of Photogrammetry and Remote Sensing, Volume XXXIV-3/W4 Annapolis, MD, 22-24 Oct. 2001

Figure 13. ALSM shaded relief image of Rollover Pass on 17 July 20001 showing geotubes installed behind the beach and in front of theeroding beach scarp. The 1m elevation contour is shown in white. A shore-normal beach profile (GLO-21) is to the left of Rollover Pass.In 1999, communities on Galveston Island and Bolivar Peninsulabegan installing geotextile tubes (geotubes) along the mosterosion-prone stretches of shoreline. The geotubes are sand-filledsleeves of geotextile fabric with an approximately 4m oval crosssection (see figure 12). The ALTM 1225 system was used to mapthe Galveston and Bolivar shorelines, including the geotubes, on17 and 18 July, 2001 (see figure 13).

Kinematic GPS and a total station were used to measure a set ofshore-normal profiles after the ALSM surveys were flown. Theprofiles extended across the geotubes, the beach, and for 100-200m offshore. Figure 14 compares the topography measured byALSM with the total station profile at location GLO-21 (seefigure 13). The ALSM elevations agree well with the groundcontrol except were dense vegetation behind the geotubes masksthe true ground surface. Thick deposits of sargassum on the back-beach also cause the ALSM elevations to be slightly higher thanthe true ground surface. These new data will be used to study theresponse of the beach and geotubes to coastal processes.

HAE(m)Shore-normal Distance (m)

an estimated $900 million loss. Honduras is still rebuilding thehousing and infrastructure destroyed by Hurricane Mitch. Tominimize future flood disasters, the Honduran government needsmaps that accurately delineate probable areas of inundation byflooding.

From February to March 2000, the BEG, the U.S. GeologicalSurvey (USGS) and Optech collaborated to map the channelgeometry of the floodplains within 15 Honduran municipalitiesusing ALSM. Between January 7-21, 2001, the USGS and BEGcollaborated again to measure the geometry and location of 21bridges in these 15 municipalities using a total station and GPSequipment. The USGS will use the bridge geometry and ALSMdata to generate new, accurate 50-year flood inundation maps foreach Honduran municipality.

The construction of the Honduran inundation maps involved threegeneral steps. We estimated the 50-year stream discharges for therivers in each municipality using a statistical analysis ofprecipitation and a rainfall-runoff model. We then computedwater-surface elevations using channel geometry informationfrom ALSM-derived DEM’s and the HEC-RAS hydraulicsimulation model (U.S. Corps of Engineers, 1998). HEC-GeoRAS, an ArcView extension, was used to define the streamthalweg, banks, overbank centerlines, and extract channel cross-sections from the DEM’s (U.S. Corps of Engineers, 2000). Oftena shaded relief image of the DEM was used as background tohelp locate these various lines. Manning roughness coefficients,n, were estimated by the hydrologists from field observations orby reviewing a shaded relief image of the DEM. The shadedrelief image gave a good view of the density of vegetation in thestream channel – the higher densities were given higher n values.Finally, the simulated water levels from the hydraulic mode wereplotted as depth and area of inundation over the DEM.4.2 Tegucigalpa

We installed the ALTM 1225 system in a Beech King Air A-90aircraft in the U.S. and ferried the aircraft to Toncontin Airport inTegucigalpa, Honduras. Tegucigalpa was mapped during 1-2March, 2000. We operated the instrument at a laser repetition rateof 25kHz, a laser scanning rate of 28Hz, and a laser scan angle of±20° off nadir. We flew the aircraft at an average airspeed of 140knots (72 m/s). This resulted in a spacing of about 2.6m betweenlaser scan lines. The aircraft altitude varied between 800m to1200m above ground level (AGL). To generate an approximately1m × 1m ground point spacing, we mapped the city with a grid oforthogonal flight lines with approximately 30 percent side-lapbetween adjacent swaths (see figure 15).

Figure 14. A beach profile across a geotube measured with totalstation on 19 July, 2001 is compared to ALSM data collected on17 July, 2001.

4 FLOOD HAZARD MAPPING

4.1 Hurricane Mitch

From October 27 to November 1, 1998, Central America wasdevastated by Mitch, a category 5 hurricane on the Saffir-Simpson scale with winds up to 155 mph. Mitch is responsiblefor over nine thousand deaths, making it one of the deadliestAtlantic tropical cyclones in history and comparable to the greatGalveston storm of 1900. In Honduras, the human toll is anestimated 5,000 deaths. Whole villages were washed away and anestimated 70-to-80 percent of the transportation infrastructurewas destroyed. At least 70 percent of the crops were destroyed;

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