for a complete bathymetric survey. The setup parameters for each panel are
saved in three different "settings.set" files and are usually named "Setting_
downdrift.set," "Settings_middle.set," and "Settings_updrift.set." The updrift
and downdrift configuration files contain approximately 10 transects in each.
Each transect requires approximately 5 min to complete; therefore, a total
bathymetric survey consisting of 60 transects requires approximately 5 hr to
complete. The operator only needs to be involved in this process at the
beginning and end of each of the three segments of the survey.
The manufacturer's performance specifications indicate a vertical sensitivity
of +0.2 mm and a linearity of 0.1 percent of full scale. Although these are
excellent performance characteristics, only the resolution of the potentiometer is
considered, rather than the entire system. Nonetheless, during preliminary
performance tests conducted with a rigid piece of metal positioned along one of
the profile transects, the profile indicator measured the elevation of the rigid
metal surface within +1 mm of the actual elevation after 10 repetitions.
Therefore, the accuracy and repeatability of the sensor has proven to be excellent
and meets the requirements of the facility.
Analysis of beach profiles
The raw output files containing the measured beach profile data are in ASCII
file format and consist of three columns of data, the x, y, and z values. The
profile depth is measured every 0.005 m in the cross-shore direction. The x, y,
and z positions are recorded with an accuracy of 0.001, 0.01, and 0.0001 m,
respectively. Consecutive beach profiles are listed one below the other. The data
are clean and need not be filtered or altered in any way.
This large amount of high-resolution bathymetric data can be analyzed using
various qualitative and quantitative techniques. The standard analysis for each
test segment consists of two types of qualitative analysis. First, a 2-D contour
plot is generated using surface mapping software to visualize the uniformity of
the bathymetry. These contour plots provide considerable qualitative information
and are used, in part, to determine whether or not the magnitude of the erosion at
the updrift end of the beach is sufficient to dictate dredging of the sediment traps.
Secondly, a contour plot of the difference between the posttest and pretest
bathymetry gives insight in regard to the degree of erosion and accretion in
various regions of the beach.
Two types of quantitative analysis are also conducted after each test segment.
First, each of the measured beach profiles from the posttest survey is
superimposed onto the corresponding beach profiles from the pretest survey.
These plots are used to quantify various changes in the beach profile, such as the
location of the still water shoreline and the maximum depth of erosion or
accretion. Secondly, the net volumetric change along each beach profile is
calculated assuming that each profile is representative of a 0.5 m wide slice of
beach which runs along the beach profile (i.e., 0.25 m on each side of the profile
line). The net volumetric change within the spatial domain of each slice is
plotted as a function of longshore position. This type of analysis is very useful in
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Chapter 8
Sensor Calibration, Measurement, and Data Analysis
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