R.J. Sobey, S.A. Hughes r Coastal Engineering 36 (1999) 1736
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current meter in an assemblage such that both instruments have the same horizontal
position and adjacent but wholly submerged vertical positions. In shallow waters, this
assemblage would commonly be deployed near the bed. In deeper waters, deployment
may be much higher in the water column, because of the considerable attenuation in
signal magnitudes with depth. Because these gauges are not surface-piercing, they
escape the most extreme environmental conditions and have the potential to optimize
data recovery.
The PUV gauges provide simultaneous traces of submerged pressure pZ t; xa , z P . and
submerged horizontal velocity components Z u,. or ua Z t; xa , z UV .. Both instruments are
at the same fixed horizontal position Z x, y. or xa but at different Zthough fixed and
adjacent. vertical elevations z P and z UV , respectively, as sketched in Fig. 1. For
notation convenience, the horizontal tensor notation will be adopted; the position
corresponding to the x and y directions, respectively, of a cartesian coordinate system.
The z-axis is directed vertically upwards from the horizontal plane of the local mean
water level ZMWL., in opposition to the gravity vector TM
g.
Short extracts from two measured PUV traces are shown in Fig. 2. Trace Za. was
measured in 46 m of water at Platform Edith, about 9 km offshore from Huntington
Beach, southern California. To a MWL datum, both the pressure gauge and the
directional current meter are at y7.4 m. The sampling rate is 1 Hz. Trace Zb. was
measured in 17.6 m of water off the Columbia River on the Pacific coast of the United
States. To a MWL datum, both the pressure gauge and the directional current meter are
at y16.8 m. The sampling rate is 4 Hz. There is a substantial ambient current in record
Zb..
Although combined into a single instrument, the precision and response of the
pressure and directional current sensors are not identical. It is immediately clear from
Fig. 2 that there is significant instrument noise in the UV traces; this is especially
Fig. 1. Definition sketch and PUV gauge location.
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