CSG
 

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).

 

     
BEG