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Methods
Beach
profiles
Beach
profiles are topographic surveys oriented perpendicular to the shoreline
and extend from behind the dune to wading depth. Sand volume, beach shape
and elevation, and precise vegetation and shoreline positions can be determined
from beach profiles. Frequent beach profiles will help decipher the temporal
and spatial variation occurring in the area and will alert us if undesired
conditions are developing. The Bureau of Economic Geology (Bureau) is
using some previously established profile sites and some newly established
sites in the geotube project areas (Figs.
1, 2, and 3). There are 16 beach profiles being measured for this study. Eight
of these profiles are within the geotube projects and 8 others are on
adjacent beaches.
The starting
points of each profile were surveyed using geodetic-quality Global Positioning
System (GPS) techniques. All survey data are provided in the UTM zone
15 coordinate system using the NAD 83 datum. Vertical measurements are
expressed as heights above the reference ellipsoid (HAE). Using the Geoid99
model, HAE heights are converted to orthometric heights relative to NAVD
88 which approximates mean sea level.
Geotube,
Shoreline, and vegetation line surveys
This
study is surveying the shoreline and vegetation lines and mapping the
conditions of the geotubes. The surveys cover the project areas and at
least 600 m (2,000 ft) to each side. For the purposes of these surveys,
the shoreline is defined as a geomorphic feature such as the berm crest
or as the limit of the last high tide (the wet/dry line). The surveys
are conducted using a kinematic differential GPS system mounted on a vehicle
or a backpack. Although accurate elevations are not provided by these
surveys, they compliment the beach profiles because they provide alongshore
data on the vegetation line and the width of the beach.
Airborne
LIDAR survey
In conjunction
with the State Aircraft Pooling Board, the Bureau is conducting an airborne
LIDAR survey along the geotube projects and adjacent shorelines. This
survey will provide an essentially continuous topographic model of at
least the first row of houses, geotubes, and subaerial portions of the
beach. Average data point spacing of 1 m or less will be acquired with
a vertical accuracy of 15 cm or better. A 1 meter digital elevation model
will be derived from the LIDAR data points.
The LIDAR
survey will accurately map the locations and dimensions of the geotube
projects. These data will allow the efficient and quantitative assessment
of the state of the geotubes and beaches. The new survey conducted for
this project will be compared with a survey conducted by the Bureau during
May 2000.
Compilation
of process measurements
Hourly
wave and wind information are compiled from the National Data Buoy Center's
buoy offshore Galveston Entrance. Hourly readings from the open coast
tide gauge on the Pleasure Pier in front of the Galveston Seawall are
also being compiled. Water level and water level standard deviation measured
by the Pleasure Pier gauge will be combined in a parameter that indicates
the reach of wave activity during storms.
Development
of criteria for assessing geotube performance and effects
Several
parameters will be devised that describe the effect the geotubes have
on the fronting and adjacent beaches and dunes. Parameters that will be
investigated include alongshore offset in the shoreline and vegetation
line, variations in beach width, variations in beach height, and variations
in beach shape (slope).
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