Page 172 - Fundamentals of Ocean Renewable Energy Generating Electricity From The Sea

In Situ and Remote Methods for Resource Characterization Chapter | 7 161

well, tracks the movement of the tide over time, and the level can be recorded
on a rotating drum of paper. Stilling wells have probably made the greatest
contribution to our records of sea level, and have led to the evidence for sea-
level rise over the last two centuries [6].
Pressure Measuring Systems
Rather than tracking the sea surface with a float, the pressure at a fixed
point below the sea surface can be measured and converted into a level using
the hydrostatic relationship. This can be achieved by a basic pressure sensor
installed below lowest astronomical tide (LAT), provided the relationship of
the pressure sensor to some local datum can be measured with accuracy. For
more precise measurements, two common types of pressure systems suitable for
permanent installation are half-tide and full-tide bubblers. In a full-tide bubbler
system, a low flow of dry air is fed down an air tube to the top of the pressure
point located at some distance below LAT. When the air pressure in the air
line is equal to the pressure exerted by the column of water above it, then the
excess air is released as bubbles through a nozzle. This means that the pressure
in the air line is proportional to the weight of the water column. In a half-tide
bubbler system, the measuring point is mounted at the mid-tide height, and so
the pressure point is immersed for half of the tidal cycle. During times when the
measuring point is exposed, it can be accurately levelled and incorporated into
geodetic networks.

Radar Sensor
Finally, because we know the speed of sound in air (based on local measure-
ments of air temperature, pressure, and humidity), a transmitter that is mounted
at some height above the water surface can transmit a pulse of sound and mea-
sure its double path length from transmitter to reflection at the water surface and
back to the instrument. This method is best used in conjunction with a stilling
well to eliminate reflections directed away from the receiver (e.g. due to an ir-
regular or sloping sea surface). Alternatively, the transducer can be placed on the
seabed looking upwards, using the speed of sound in seawater (again, calculated
from local measurements of the sea water properties) as the reference point.

7.1.2 Mechanical and Electromagnetic Current Meters
Traditionally, oceanographers and coastal engineers have used current meters
to make measurements of water currents. These are generally of the horizontal
axis design, and convert the number of rotations of a calibrated propeller (per
unit time) into a current speed (Fig. 7.2). Current meters record measurements
at a single point in the water column and, although they have a directional vane
to calculate the direction of flow, do not measure the vertical component of
velocity, which is generally an order of magnitude lower than the horizontal
component.