ERDC/CHL CETN-IV-24
March 2000
pass. The directional wave follower contains a flux-gate compass to determine the direction of
the wave by measuring the orientation of the buoy to true north. Therefore when displacements
are measured with the accelerometers, a direction component to the wave motion can be
calculated. Non-directional wave followers measure heave, pitch, and roll with accelerometers.
Some companies have incorporated velocity and pressure measurements into instrument
packages to obtain wave direction and amplitude data. These instruments can capture wave
motion in and around structures, whereas the wave follower/buoy gauge is more applicable
the water column. All of these instruments have specific areas where they perform better. The
user will have to consult the manufacturer's literature for the best model for the application
according to rapid advancements in the instrument industry. Figure 1 also shows where the wave
data was collected for this study.
5. Weather stations. Meteorological data, such as wind speed and direction, precipitation and
evaporation, temperature, relative humidity, and barometric pressure, are recorded by weather
stations. The weather station should be located at some central location in the study area and,
following American Meteorology Society standards, mounted approximately 10 m over land.
No standards have been established for mounting weather stations over open water; however,
depending on the meteorological parameters being monitored and their purpose, the weather
station should be mounted approximately 2 to 3 m above the highest wave height during a storm.
There are numerous types of weather equipment that can meet the requirements for the project
depending on the accuracy of the instrument. Self-contained weather stations use a battery-
powered microcomputer with a real time clock, a serial data interface, and programmable
analog-to-digital converter. The battery is constantly charged by a solar panel-charging system
located near the system. Various programming options are available to set the sampling interval
and averaging period. The system can be programmed to sample input signals each second over
a set period of time to determine the mean, maximum, and minimum values of the various
parameters.
6. Current meters. Acoustic techniques are used to obtain current velocity and direction
ADCP and Acoustic-Doppler Profiler (ADP) transmit sound bursts into the water column, which
are scattered back to the instrument by particulate matter suspended in the flowing water. The
ADCP and ADP sensors listen for the return signal and assign depth and velocity to the received
signal based on return time and the change in the frequency caused by the moving particles,
respectively. This change in frequency is referred to as the Doppler shift. The ADCP is also
capable of measuring vessel direction, current direction, water temperature, and bottom depth.
Communications with the instrument for setup and data recording are performed with a portable
computer using manufacturer-supplied software, hardware, and communication cables. The
instrument can be mounted over the side of a boat with the acoustic transducers submerged. The
data are collected while the vessel is underway. Both ADCP and ADP can be mounted on a
stable platform and placed on the bottom of the river or ocean. Self-contained recording point
current meters are used to obtain long-term fixed-depth data. This type of current meter is
tethered to a stationary line or structure. The measurement method can range from
electromagnetic to acoustical. An internal microprocessor computes the velocity vectors, which
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