478 CHAPTER 8. LABORATORY MEASUREMENT AND ANALYSIS
Directional Wave Gauges
A single wave gauge located at a point will provide only a record of sea
surface elevation fluctuations at that point. This measurement may be
adequate in two-dimensional wave tanks; however, in multi-directional wave
basins, a single gauge provides no directional information about the wave
field.
Estimates of multi-directional wave conditions at a point can be obtained by collecting sea surface elevation measurements simultaneously
from an array of wave gauges. The pattern, spacing, and number of gauges
in the array depends on the intended analysis method. Arrays vary from
polygon arrangements to linear arrays; and in general, resolution of the
directional spectra increases with the number of gauges in the array.
A variation of the multi-gauge array consisting of surface-piercing wire
gauges is an array comprised of a capacitance wave gauge and a miniature electromagnetic current meter that obtains two orthogonal horizontal
components of the wave orbital velocities.
8.2.3 Flow Velocity Measurement
One of the true benefits of small-scale laboratory models is the capability to
reproduce complex flow situations at reduced scale. However, much of this
benefit would be lost if we could not quantify the flow to some degree with
accurate flow velocity measurements obtained at time intervals suitable to
capture the temporal detail at the model scale.
Fluid flows in coastal engineering laboratory studies can be either unidirectional, oscillatory, or a combination of both. Although the term “unidirectional” implies flow in only one direction, the term is commonly applied
to any steady or unsteady flow4 that does not undergo flow reversals. Examples of unidirectional flow in coastal models include ebb or flood tidal
currents, longshore currents, and river flow into a harbor.
Orbital wave velocities beneath progressive or standing waves constitute the primary source of oscillatory flows in coastal models. Naturally,
measuring oscillatory velocities with periodic flow reversals is more difficult
than measuring unidirectional flow velocities.
Another interesting flow phenomenon that can be studied in the laboratory is turbulence, which occurs in both unidirectional and oscillatory
flows. Turbulence can occur near the boundaries or in the main body of
the flow, depending on the circumstances. The requirement to measure the
rapidly fluctuating turbulent velocities at a point has led to development of
Here we are referring to the predominant flow and ignoring whatever turbulent fluctuations might be present in the flow field.
Directional Wave Gauges
A single wave gauge located at a point will provide only a record of sea
surface elevation fluctuations at that point. This measurement may be
adequate in two-dimensional wave tanks; however, in multi-directional wave
basins, a single gauge provides no directional information about the wave
field.
Estimates of multi-directional wave conditions at a point can be obtained by collecting sea surface elevation measurements simultaneously
from an array of wave gauges. The pattern, spacing, and number of gauges
in the array depends on the intended analysis method. Arrays vary from
polygon arrangements to linear arrays; and in general, resolution of the
directional spectra increases with the number of gauges in the array.
A variation of the multi-gauge array consisting of surface-piercing wire
gauges is an array comprised of a capacitance wave gauge and a miniature electromagnetic current meter that obtains two orthogonal horizontal
components of the wave orbital velocities.
8.2.3 Flow Velocity Measurement
One of the true benefits of small-scale laboratory models is the capability to
reproduce complex flow situations at reduced scale. However, much of this
benefit would be lost if we could not quantify the flow to some degree with
accurate flow velocity measurements obtained at time intervals suitable to
capture the temporal detail at the model scale.
Fluid flows in coastal engineering laboratory studies can be either unidirectional, oscillatory, or a combination of both. Although the term “unidirectional” implies flow in only one direction, the term is commonly applied
to any steady or unsteady flow4 that does not undergo flow reversals. Examples of unidirectional flow in coastal models include ebb or flood tidal
currents, longshore currents, and river flow into a harbor.
Orbital wave velocities beneath progressive or standing waves constitute the primary source of oscillatory flows in coastal models. Naturally,
measuring oscillatory velocities with periodic flow reversals is more difficult
than measuring unidirectional flow velocities.
Another interesting flow phenomenon that can be studied in the laboratory is turbulence, which occurs in both unidirectional and oscillatory
flows. Turbulence can occur near the boundaries or in the main body of
the flow, depending on the circumstances. The requirement to measure the
rapidly fluctuating turbulent velocities at a point has led to development of
Here we are referring to the predominant flow and ignoring whatever turbulent fluctuations might be present in the flow field.