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Processes and Measurement Requirements
Coastal measurement systems designed for sensing the properties
of waves and currents are usually categorized into one of two temporal
domains: high-frequency and low-frequency. High-frequency wave
and current motions are defined here as those with periods of five min-
utes or less and include both the gravity: and the infragravity2 por-
tions of the surface wave spectrum (see Figure 3-1~. Low-frequency
motions are defined here as those having periods greater than five
minutes and range from tsunami to tidal or longer oscillation periods.
The current velocity measurement techniques can be categorized as
Eulerian or Lagrangian, and in situ or remotely sensed. Eulerian
measurements are those collected by moored current meters mea-
suring velocity at a fixed location. Lagrangian measurements are
obtained by a tracer (drifter floats, dye, or tagged particles) follow-
ing the current stream over a period of time. In situ measurements
are obtained by placing instruments in the ocean. Remote-sensing
measurements are those collected by "noncontact" methods from
satellites, aircraft, ships, or ground stations, using electromagnetic
Gravity waves are those waves with lengths between their crests of a
meter or so to a few hundred meters typical of wind-generated waves.
2Infragravity waves have lengths of hundreds of meters to tens of kilometers
and are usually driven by gravity waves.
26
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28
radiation. Modern acoustic-Doppler current meters and similar sen-
sors that sense current velocity using acoustic waves scattered from
particles drifting with the current should be considered a combina-
tion of ~ situ and remote-sensing techniques.
After the hydrodynamic forces generated by high- and low-
frequency waves are deterrn~ned, the engineer and researcher must
measure and understand the responses of the sediment particles to
these wave and current forces. This chapter addresses the various
measurement techniques used for determ~n~g sediment movement
on scales from particle-size up to regions and locations from near
(within a few centimeters) the seabed to the upper water column.
Finally, the engineer must be able to measure the effect of the
hydrodynamic forces of the sea on structures in, or at, the wave zone.
How structures, in turn, affect both hydrodynamics and sedimentary
response also must be gauged if the engineer is to establish adequate
design guidelines. This chapter also clears with these fluid structure
interactions.
HIGH-PREQUENCY WATER MOTIONS
Processes
High-frequency motions in the coastal environment can be cate-
gorized as being wave-induced flows (including gravity and infragrav-
ity waves), turbulent flows, and averaged currents (that is, averaging
the current in time for more than five minutes). Because Lagrangian
measurement techniques generally do not lend themselves as well to
high-frequency field measurements, only Eulerian systems are con-
sidered here.
In contrast to waves, currents are physically difficult to measure
with a single instrument except in extremely simple cases. Spatial
variation in waves primarily depends on the bathymetry, whereas
current variation is influenced by the shape of both the bottom and
the shoreline. Also, strong vertical variation in the current velocities
can exist due in part to inhomogeneity in the water density caused
by salinity and temperature variations, as well as changes caused
by the viscous boundary layer effects on turbulence and velocity.
Currents modify wave velocities in a complicated fashion when both
are varying in time, particularly where the shape of the bottom is
also changing as a result of this interaction.
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29
TABLE 3-1 Summary of Data Needs for Coastal Engineering
1. SHORE STABILIZATION
Sediment Characteristics
Grain size distribution
Concentration of suspended
fraction
Near-bed transport rates
Beach Characteristics
Beach profiles
Local areas of deposition or
erosion
Stabilizing structures
Hydrodynamic Characteristics
Incident wave heights and
steepness
Wave direction
Velocities of wa~re-dri~ren
currents
Velocities of other currents
Bottom shear stress
Turbulence characterization
2. BACKSHORE PROTECTION
Sediment Characteristics
Grain size distribution
Beach Characteristics
Beach profiler
Longshore sediment transport
Crose-shore sediment
transport
Hydrodynarruc Characteristics
Wave direction
Wave height and steepness
Wave runup
Storm surge
Tsunarrii runup
S. INLET STABILIZATION
.
Sediment Characteristics
Grain size distribution
Packing density
Inlet Characteristics
Bathymetry
Net sediment flux
Patterns of erosion and
deposition
Protective structures
Hydrodynamic Characteristics
Wave height and steepness
Wave direction
Current velocities
Bottom shear stress
(wave/current interaction)
4. HARBOR PROTECTION
Sediment Characteristics
Grain size distribution
Harbor Characteristics
Bathymetry
Shoreline changes
Protective structures
Patterns of erosion and
deposition
Hydrodynamic Characteristics
Wave height and steepness
Wave direction
Current velocities
Bottom shear stress
Measurement Requirements
The proper design of a measurement system requires (~) knowI-
edge of the expected environmental conditions at the measurement
site, (2) appropriate design of the supporting moorings of platforms,
and (3) the selection of suitable measurement devices. Each of these
components, ant] their interaction, contributes significantly to the
quality of the data, and a total system must be considered collec-
tively. The requirements for precision and resolution of the data
must also be considered.
The set of measurement requirements listed in Table 3-1 was
established by assessing the engineering needs of each of the four
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30
application areas or features discussed in Chapter 2 (see Figure
2-1~. The optimum spatial and temporal resolutions are specified
for each measurement requirement (see Tables 3-2, 3-3, and 3-4)
and existing sensing systems are identified for evaluation of their
measurement capability. If the measurement capability of an existing
sensing system failed to meet the necessary spatial and temporal
resolution requirements, a need was then identified for instrument or
system development.
Measurement Capabilities
Velocity Measurements In Situ
A wide variety of physical measurement techniques incorporate
current measurements taken at a point. A primary difficulty for
nearshore velocity measurements is that oscillatory wave-induced
velocities are generally present throughout the water column, super-
imposed on steady currents. In the wave, simple impeller (speed)
and vane (direction) current meters were found to rectify the oscilIa-
tory velocity imposed by the waves, thereby biasing the results (e.g.,
McCullough, 1978~. Vanes generally do not respond to wave motion
rapidly enough to resolve direction accurately, giving Aliases" or
noisy directional information. This condition has led to the develop
ment of a number of current meters that respond to wave motion and
that have been adapted especially for shallow-water measurements.
These instruments resolve the wave motion into velocity compo-
nents at right angles to each other. Instruments include biaxial
rotor vector-measuring current meter (VACM) (Figure 3-2), electro-
magnetic current meters (Figure 3-3), forward-scatter or backscatter
acoustic-Doppler current profiler (ADCP) (Figure 3-4), and laser ve-
locimeter. The EuTerian current meters have a relatively high capital
cost per unit, ranging from over $4,000 for a meter to as much as
$65,000 for a current profiler system, depending on the data record-
ing and internal processing capability. Some of the difficulties and
problems encountered with Eulerian measurement systems are bi-
ological fouling, corrosion, orientation uncertainties, disturbance of
the flow fields by the instrument, bubbles, cavitation, extreme storm
waves, and disturbance of sensors by fishermen.
Velocity Measurements Remote Sensing
Remote-sensing techniques avoid many of the disadvantages of
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31
FIGURE 3-2 Davis-Weller Vector Measuring Current Meter, Model 630.
SOURCE: EG&G Environmental Equipment Division, Burlington, Mass.
FIGURE 3-3 Electromagnetic Current Meter, Model 511. SOURCE: Marsh-
McBirney, Inc., Gaithersburg, Md.
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32
~ `11 ~ ~
/
/
/
/
FIGURE 3-4 75-KHz Self-Contained Acoustic-Doppler Current Profiler.
SOURCE: RD Instruments, San Diego, Calif.
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33
in situ measurements and have the advantage of generally providing
rapid coverage of a wide area. The disadvantages in most techniques
include sensing only of the surface currents (providing no information
about the subsurface velocity profiles) and integrating over large
areas so that the local gradients are filtered out.
One method for the remote sensing of currents relies on the
scattering of radar signals from the sea surface. Current speed is d~
term~ned from the Doppler shift (frequency change) ~ the scattered
signal after a theoretical shift due to wave speed has been removed.
The speed of the current in the direction of the radar beam is deter-
mined. Directional components are measured by turning the radar or
using multiple radars. Synthetic Aperture Radar (SAR) uses a sin-
gle radar mounted on a moving platform (plane or satellite), which
allows good definition of currents over large areas. Other remote-
sensing techniques are being developed, including optical systems,
which offer additional capabilities for the future. Because of the
large surface area observed by most of these systems, they are not
generally applicable to the nearshore region, where currents change
rapidly over short distances.
Wave Measurements—In Situ
A wide variety of wave-measuring devices for waves based on var-
ious physical principles have evolved, including direct, indirect, and
remote techniques. Each technique has inherent advantages depend-
ing on the application. Desirable characteristics of any wave sensor
should include good accuracy, linearity, ruggedness, dependability,
and low cost. The utility of any wave sensor is highly dependent on
the type of installation for which it can be adapted, such as bottom
or pier mountings.
The most common direct measurement of the sea surface is
by wave staffs (wires that penetrate the sea surface) that measure
resistance, capacitance, or inductance. The disadvantages of wave
staffs are that they usually require mounting on a rigid structure
such as a tower or pier piling. In measuring plunging breakers there
is a question as to exactly what is measured, because of multiple
interfaces in the plunge portion of the wave. The influence of foam
is unknown for direct wave sensors.
Other direct measurement techniques include acoustical devices
directed up from beneath such as inverted fathometers and lasers
and microwaves (infrared) looking down from above; both of these
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34
techniques are based on measuring the tone for a pulse to travel from
the sensor, be reflected off the surface, and return. Acoustical and
electromagnetic methods have the advantage that they do not disturb
the surface. The disadvantage of the inverted fathometer is that the
speed of sound is a function of temperature and salinity, which can
vary temporally and spatially over the vertical. Steep waves may not
provide a good acoustic return rn~rroring exact surface configuration
because of side-Iobe reflection problems. The inverted fathometer
does not work in the highly turbulent region near the surf zone due
to scattering of the sound by turbulence and bubbles. Because of
beam spreading, the sampled surface area may be wide and as a
result, the signal return provides an area average depth; this may
be an advantage or disadvantage depending on the mtencled use of
the device. The inverted fathometer does offer the advantage that
it can be mounted on the bottom, generally a much easier type of
mounting than that required for a laser beam or microwave, which
must be mounted from a structure looking down onto the surface.
Pressure sensors that provide indirect measurements of the wave
surface generally have the advantage of ease of installation, durabil-
ity, and low vulnerability to environmental forces. For this reason
and because pressure sensor technology is highly advanced, they are
a popular means of measuring the waves. The water column above
the pressure sensor acts as a hydraulic filter, partially filtering out
the high-frequency components of the wave spectrum. The wave
spectrum is increasingly filtered as the depth of the sensor increases.
A practical deep-water limit is approximately 20 m for placement
of the pressure sensors to determine sea and swell waves. Some un-
certainty exists concerning the performance of the sensors when the
waves are quite steep (Forristall, 1982~.
Nearshore direction characteristics are typically either measured
locally in shallow water with "slope arrays" or measured in deep wa-
ter with buoys and then transformed to the nearshore location. The
important problem in predicting littoral sediment transport is that
an accuracy of 1-3° is required to obtain good estimates. This means
that the deep-water waves must be resolved to within 2-5° to obtain
the desired nearshore directional accuracy after transformation.
Wave directional buoys are used primarily in deep water to mea-
sure the heave, pitch, and roll with accelerometers and an inclinome-
ter. A second type of "orbit following" directional buoy, developed
by ENDECO (Brainard and Gardner, 1982) infers the surface slope
due to the forcing by the horizontal motion of water particles. The
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TABLE 3-2 Assessment of Needs for Measurements of High-Frequency Motions
Measurement Accuracy Measurement
Objective Requirements Technique
Capability Need
Currents
Longshore +2 cm/e Electromagnetic I 2, 6, 7
cross-shore 5 Hz Acoustic
1-2O Mechanical ;
Turbulence +1 cm/e Acoustic III 2, 4, 6, 7
high- Hot film
frequency
Water level
Tides +5 cm Pressure, I 7
Mechanical
Storm surge is cm Pressure I 7
Wave setup +5 cm Pressure III 7
Wave runup +5 cm Photo/Video I 4, 6, 7
Electrical
Wa~recharac- 0-15 m +5% Many I 6, 7
teristice
Wave direction O Slope array III 1, 2, 6, 7
deep water >lOm 2-50 Remote sensing
shallow water 1-3
Breaker Photographic II 2, 6, 7
characteristics
Meteorological
Wind velocity 5% Many I 7, 8
Wind direction 1056 Many I 7, 8
LEGENDS:
Need:
1 Major development
2 Improve information detail
3 Improve physics
4 Improve efficiency
5 Improve tuning
6 Special data needed
7 Verification needed
8. None
Capability:
I Good
II Adequate
III Possible but
not satisfactory
IV None
buoys have their own individual dynamic response functions, and
the mooring also can influence the dynarn~c response of the buoys.
Therefore, the dynamic response of the combined system of the buoy
and mooring system must be known so that proper calculation of
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TABLE 3-3 Assessment of Needs for Measurements of Low-Frequency Motions
Measurement Accuracy Measurement
Objective Requirements Techniques
Capability Need
Currents
Offshore 3 cm/e EMC\I, ATTCM,
(10-20 m depth) VACM, AD CM III 3, 4, 5, 6
Nearshore 3 cm/s (same as above) II 3, 4, 5
Inlets 3 cm/e (same as above) III 3, 4, 5, 6
Radiation stress 10% Slope array II 5
Water level
Offshore 10 cm relative Pressure sensor, II 5, 6, 7, 8
- Remote altimeter
Nearshore 10 cm absolute Pressure sensor, I 5, 6, 7, 8
Tide gauge
Back~hore 20 cm absolute Pressure sensor, I 5, 6, 7, 8
Float gauge
Runup 0.3 m Contact sensors, II 3, 4, 5
Photogrammetry
Wave setup 10 cm Slope array III 5, 6
Direction spec. 10%, 25° Buoys, SAR, PUV, II 3, 4, 6
Slope array
Meteorological
Wind velocity 5%, 10° Many I 6, 7, 8
Barometric 1 mb Many I 6, 7
pressure
Morphological
Bathymetry 5% or 0.5 m Fathometer, I 9
(large-ecale) Precise leveling
Bathymetry 0.3m Fathometer, III 2, 8
(small-ecale) Precise
leveling,
Side-scan
surface displacement can be made. Buoys have not been used as reli-
~bly in shallow water as in deep water because of the demand placed
on shallow-water surface moorings (large displacements, steep waves,
etch. Although buoys provide some directional wave information in
deep water, these buoys do not give the high-resolution direction
information often required for coastal engineering (see Table 3-2 to
3-43. More accurate, inexpensive inertial compasses based on optical
interferometry should be available soon. These solid-state devices
may give improved accuracy and reliability to the directional buoy
measurements but will probably not improve the directional resolu-
tion.
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55
can significantly change the computed longshore sediment flux rate.
Consequently, improved wave directional resolution ~ required in
measurement systems for sedimentation assessment and forecasting.
This measurement requirement Is also important for other aspects of
coastal engineering, as discussed earlier in this chapter.
Spatial-Scale Observations
Several nearshore engineering functions, such as beach protection
and beach nourishment planning and engineering, require data from
simultaneous observations over large sections of the nearshore or
coastal zone with methods that may be less accurate or precise than
the site-specific requirements discussed in the preceding sections.
Remote-sensing techniques have been developed, over the past
decade, that may provide significant potential for application in the
surf zone. These techniques rely on satellite, airborne, and ground-
based sensors that can rapidly scan or observe a variety of param-
eters. Examples showing some limited application of these remote-
sensing methods Include deterrn~nation of nearshore bathymetry from
airborne systems and the use of satellite imagery detecting nearshore
turbidity to map coastal circulation patterns. Generally, remote-
sensing technology has been developed for use in other fields; how-
ever, many of the specific methods have potential for coastal engineer-
ing applications. Some examples are briefly summarized in Table 3-5,
which also includes some of the in-water acoustic and photographic
methods discussed in the foregoing sections. Large-scale spatial av-
eraging may limit the usefulness of some of these technologies in the
surf zone.
Since no agency or institution exists that coordinates or system-
atically funds instrument development, the needs of the coastal engi-
neer~g community have been addressed historically on an individual
basis as dictated by funding for field research. Most of the major co-
operative field studies e.g., NSTS, C2S2, Duck '85, SUPERDUCK,
etc. (Kraus, 1987) have been responsible for coordinating moderate
development efforts carried out by individual participants. This ad
hoc approach has been effective in the sense that instrument develop-
ment is closely tied to scientific needs. It may not, however, provide
adequate or long-term funding and facilities for developing some of
the major instrument systems listed in the foregoing discussions.
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TABLE S-5 Remote-Sensing Instruments, Techniques, and Potential
Application in Surf Zone
Instruments
Potential Application
Deployment: SATELLITE/AIRCRAFT
Altimeter
Scatterometer
BAR (Synthetic Aperture Radar)
Radiometer (microwave)
Color scanner
Deployment: AIRCRAFT
SCR (Surface Contour Radar)
ROWS (Radar Ocean-Ware Spec-
trometer
AOL (Airborne Oceanographic Lidar)
Geostrophic currents, wave and
storm surge characteristics
Sea state
Sea state, wave spectrum,
wave direction
Suspended sediment distribution
Sea surface
Directional wave spectrum
Surface warren
and bottom topography
SLAR (Side-Looking Airborne Radar) Mapping
ALM (Airborne Electromagnetic Coastal bathymetry
Method)
Deployment: GROUND-BASED
CODAR tOSCR)
SLR (Side-Looking Radar)
Cameras: time lapse
and time exposure;
(photographic
and video imaging)
Coastal circulation
Wave runup, breaker
conditions, low-
frequency wave
characteristics,
bar morphology
Techniques
Deployment: IN WATER
Acoustic back~catter
Laser Doppler
Acoustic Doppler
Acoustic tomography
Suspended sediment
concentration, flux,
bed-load transport
characteristics,
local scour and deposition,
turbulent
and suspended sediment
field.
F[UID/STRUCTURE INTERACTIONS
Structure Apes and Measurement Requirements
Coastal structures can be divided into the following categories:
Breakwaters
Elevated platforms, on piles or cylinders
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Seawalb and bulkheads
Jetties and groins
Dredged entrance channels into harbors
· Navigation aids
.
Artificial Elands.
Design of each of these types of structures requires knowledge
of environmental forces sufficient to establish criteria for location,
materials that should be used, and dimensions. These criteria may
come from physical models, or, if the physics and mathematics are
understood well enough, from numerical models.
Breakwaters
The purpose of a breakwater may be different in one location
than in another location because of different reasons for reducing
wave energy. Some of these purposes are to:
tion,
provide quieter water in an entrance channel for safe naviga-
. provide a calm harbor for loading and unloading cargo or
passengers from ships,
protect the shore from wave damage during storms, and
. reduce beach erosion or accumulate sand.
Breakwaters can be properly designed only with site-specific
data about the wave spectrum—both for significant and maximum
waves and wave grouping, information on runup and overtopping,
shock pressures exerted by breaking waves, the effect of the breakwa-
ter on currents, the intensity of air bubbles during storms, and the
engineering characteristics of bottom sediments at the seabed and
for some depth below.
Breakwaters are of various types. The most common is the
rubble mound, faced with large stone or concrete units of di~er-
ent shapes. Caisson breakwaters, made of rows of hollow boxes
(of concrete, usually) reflect wave energy, since long-period narrow-
spectrum waves can cause significant scour. Design of these break-
waters requires specific knowledge of the erodibility characteristics
of the local seabed.
Composite breakwaters, partly caissons and partly rubble, have
been built- in many configurations, most notably in Japan. These
structures are characterized by a vertical wall near the top so that
impact forces are usually a critical design consideration.
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Another variety of breakwater is that composed of a line or lines
of sheeting, sheetpiling, or connected cylinders, forniing a vertical
wall. This design also requires detailed information about the foun-
dation materials below the seabed and also about scour resistance in
front of the structure.
Any of the foregoing types of breakwaters may be built with
different top elevations, depending on the need to prevent, or to
allow, overtopping during storms that are so violent that vessels
would not be entering. Still lower top elevations may be built as
submerged breakwaters designed to reduce the height c)f waver halt.
not to absorb or reflect all wave energy.
cat · · ~ . ~ _ _
A special class of structures is floating breakwaters. Their effec-
tivenes~ depends on their width as compared with the wave length of
incoming waves. Consequently, they are mainly suitable for relatively
short wave, such as those usually occur in marinas and other small-
boat harbors. They offer advantages, compared to bottom-mounted
breakwaters, in sheltered deep water or In regions of unusually high
tide ranges. A literature search by the Corps of Engineers Water-
ways Experiment Station (WES) shows a wide variety of designs for
floating breakwaters, including arrays of tires, boxes, baffles, and
diaphragms (COE, 1982~.
A working group of PlANC (Permanent International Associa-
tion of Navigation Congresses) has been established to analyze 163
breakwaters from many parts of the world in efforts to find common
factors among breakwaters that have contributed to their success or
failure, consider model tests, evaluate safety factors, and propose
ways to respond to engineering and construction problems. This
assessment is still underway, but it is clear from such reviews, as
well as from experience in the analysis of several failures, that more
information is needed about the progress of shock pressure waves
through the armor and core materials.
Elevated Platforms
Elevated platforms supported by piles or cylinders embedded in
the sea bottom have been extensively used and researched. Wave
forces on the cylinders have been measured, and the body of data
is sufficient for design, except for the magnitude of shock pressures
for breaking waves. Better knowledge is needed about the effect of
groups of piles on the scouring of the seabed. Examples of this lack
of understanding are demonstrated by the eject of pilings used in
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so
construction of several research piers over the past decade (for ex-
ample, at the Corps of Engineers facility, Duck, North Carolina, and
Scripps Institution of Oceanography) that have caused unexpected
deepening near the pier. In addition, predictions of scour depth an a
function of wave cInnate and the engineering properties of the beach
soils and subsoils are not possible with any great accuracy at this
tune.
The measurement of the height of storm wave crests ~ a critical
requirement for safe design. Many tests and observations of behav-
ior of of! industry platforms have shown the need to maintain the
elevation of the underside of the deck above the crests of the highest
waves, to avoid] excessive impacts.
Seawalb
The design of seawalIs and bulkheads along the shore requires
the same kmds of measurements of wind and wave forces as break-
waters, with added emphasis on wave overtopping and the effects of
impounded rainwater on the landward side.
Jetties ~d Groins
Jetties and groins are physically similar to breakwaters but are
built perpendicular to the shoreline. In the United States, jetties
are structures protecting and stabilizing the entrances to harbors,
although the term Jetty is used more broadly elsewhere. Groins,
by interfering with the transport of sand along the shore, are used
to attempt to stabilize the shoreline position. Design problems for
these structures are s~rnilar to the design problems for breakwaters.
Entrance Channels
Dredged entrance channels into harbors may be considered] coast-
al structures with negative elevation. In designing entrance channeh,
information is required about the source and movement of sediments
that tend to fill up these channels and impede shipping. In addition,
entrance channels may cause adverse environmental effects in the
form of downdrift erosion when sediment supplies are cut off. The
relations between current velocity, wave climate, grain size, distribu-
tion of sediments, and turbulence are not clearly understood.
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Navigation Aids
Navigation aids to guide ships and boats are a broad class of
coastal structures. They may be fixed or floating and consist of buoys,
lights, ranges, markers, and communications stations (acoustical,
visual, radar, or radioJ. Measurements of environmental forces such
as those developed by wind, current, and ice are important to the
effective design of navigation aids and their supporting structures.
Artificial Islands
The creation of land in the ocean for terminals, airports, and
other activities has been advanced, particularly by the Japanese,
during the past two decades. The fill is typically contained within
a conventional breakwater structure so it presents no unique design
problems.
Measurement Capabilities
To measure the dynamics of wave/structure interactions, a vari-
ety of instrumentation may be required, including the following:
Fast response pressure sensors
Strain gauges
Tensiometers
Accelerometers
Anemometers
Water-level gauges
Current meters
Bathymetric and topographic measurement systems
Optical motion indicators, remote sensing.
Measurement requirements, capabilities, and needs have been
summarized in Tables 3-6 and 3-7 for application to structures of
two general types: fixed position (rubble mounds, solid blocks, or
vertical-walled caissons) and floating breakwaters. These structures
may be located on shorelines, harbor entrances, or channels. In
situ instruments, imbedded in rubble-mound structures, can present
data-Iogging problems because of the susceptibility of cables to dam-
age.
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TABLE 3-6 Assesement of Needs for Measurements of Fluid/Structure
Interaction (F~ced-Position Structures)
Accuracy Measure- Opera-
Measurement Require- Measurement ment Capa- tional
Objective meets Technique bility Need
Profile movement: 10 cm Sunrey III
external 10 cm Survey + strain
gauge II 5, 6
internal 5 cm Inertial IV 1
Water level
Wave Pressure +
charactenstice 10 cm wave riders I 3, 6
Wore direction B° Slope arrays
+ photos III 2
Wave runup Photos II 5
Wave reflection Photos II 2, 6
Wave transmission Photos II 6
Wave overtopping Photos,
Acoustic II ~
Pore pressures Pressure gauge III 1
Currents
Toe of structure 10 cm/e Acoustic II 4
Inside structure 10 cm/s Acoustic II 5, 6
Forces
Mooring ~t10% Strain gauges I 5, 6
Torsional +10% Strain gauges I 5, 6
Seismic +10% Strain gauges I 5, 6
LEGENDS:
Capability:
I Very good
II Adequate
III Not adequate
IV None
Need:
1 Major development
2 Improve information detail
3 Improve reliability
4 Improve durability
5 Improve installation and use
6 More sensors or lower data cost
Measurement Requirements
Erosion at the Base of Rubble-Mound Structures
If the foundation soils are eroded by storm waves, causing the
sediments to move seaward, the undermining will cause raveling of
the slope and loss of material. To counteract this effect, the base
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TABLE 3-7 Assessment of Needs for Measurements of Fluid/Structure
Interaction (Floating Breakwatere)
Measurement
Requirement
Measurement Measurement
Capability Neede
Wave data
Currents,
instantaneous
Time series, in stonne
Stochastic
Heights
Directions
Pore pressures,
core and
foundation in storms
Velocity in interstices
Rate of overtopping
Transmission through
reflection,
in 3 dimensions
Runup
Air content
of breaking waves
Ice forces
Storm currents, toe
Seismic forces
Structure data
Cross section,
as built
Movement of units
Internal strain
Floating structures
Mooring forces
Torsional stresses
Ice forces
Sensors, Adequate;
at toe, none
Adequate
Adequate
Adequate;
at toe, none
Inadequate
Sensors, Adequate
None
None
Adequate
Partial
None
None
None
Sensors, Adequate;
Installation, none
Adequate
Poor, especially
for rubble
Poor
Adequate, outside
Adequate
Adequate
Adequate
At toe; installation
Improved reliability
Improved reliability
At toe; installation
Better resolution
Durable installation
Installation, none
Development and
installation
Development
Better spatial coverage
Development
Development
Development
Development
Installation
Improvement
Development
Development
Improvement
Improvement
Improvement
.
areas are often reinforced with a berm or placed in a previously ex-
cavated trench to prevent scour. However, the effectiveness of these
eros~on-prevention techniques so far has not been accurately mea-
sured. Physical models do not adequately reflect the scale relation-
ship between foundation sediments and rubble or armor materiab.
Prototype measurements of waves and currents, scour, and transport
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are required; numerical and physical modeb need to be developed on
the basis of these measurements.
Shock Pressure Implosion
When a wave breaks directly against a rubble slope, or against
a concrete wall embedded within it, the shock pressure rapidly pen-
etrates the interstices. These shock pressures are suspected of being
involved in rearrangement of the structure and breaking of concrete
units. Measurements are needed of the pressure gradients that exist
at the peak of such ~xnplosions, and of the movement of individual
elements of concrete, stone, gravel, and sand as they are jostled by
the impacts.
Storm Surge
During the height of a storm, mean sea level may rise consid-
erably as a result of the combined effects of wind setup, reduction
in barometric pressure, and tides or Ketches. This brings the zone
of magnum wave action nearer to the top of the structure, where
the structure is often the most vulnerable. There are few reliable
measurements available at this time of the structural ~rnpact or the
structural response, so that models are of questionable value.
Loss of Fine Particles
Uncler the cyclic action of waves, fine particles in the breakwater
may work their way through the gaps in the armor (these channels
are called piping), or subarmor, and escape. Progressive piping may
then cause cavities within the breakwater and loss of support for
materiab above, leading to eventual reduction of the crown elevation
and allowing detrimental overtopping. At present, there is no tech-
nique for detecting and locating internal cavities and piping; such
information could affect design and material selection criteria.
Overtopping
If sufficient "green waters crosses the top of the rubble mound,
the crests can attack the structure and flood the interstices at the
roadway or crown. Both tests and prototype experience show that
flows that overtop are more likely to cause damage. In fact, breakwa-
ter ~roundheads," or ends, are known to be most vulnerable and are
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often designed with heavier units or flatter slopes, or both. Repeated
overtopping is thought by some to be a dominant factor in struc-
tural failure. Better measurements of wave/structure interaction are
needed.
Armor Decomposition
Wave attack, sun and salt, heat and cold (and perhaps ice), and
the unp act of smaller stones and movement of units themselves ad
cause gradual deterioration of stone or concrete. The rounded corners
of old armor units attest to this effect. As the units become more
spherical they become less stable and shift position. Efforts have been
made to build scale models with units of softer material, to reproduce
this effect, but these efforts have not been wholly satisfactory. A long-
term monitoring program of armor decomposition would provide
needed information.
Multidirectional Waves
When waves from different directions approach a breakwater,
convergence of crests can produce a local breaking wave far larger
and steeper than the average conditions. There is a need for more
directional wave measurements in the vicinity of breakwaters.
Breakwater Stability
Irnp act of concentrated wave energy against any structure at
the top of a rubble mound can precipitate a loss of slope stability.
A rubble-mound slope has been characterized as an incline close to
the failure point. For reasons of economy, slopes are often made
as steep as is believed to be practical; there is a Tower factor of
safety common to the design criteria for rubble-mound structures
than for steel and stone structures. There is a need for more mea-
surements of rubble-mound breakwater on prototype and mode!
rubble-mounded structures in order to improve understanding of
stability coefficients.
Summed
Instruments are available to measure many parameters related to
interaction between water and structures. Some parameters cannot
be measured, largely because the sensors and data links have not
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been successfully deployed in the harsh environment of a stormy
sea. Measurements that are needed but cannot yet be measured
adequately on fixed structures are
.
tures,
the types and directions of breaking waves attacking struc-
the nature of movement of different structural components
during storm conditions,
pore pressure within the interstices and in the foundation,
wave transmission and reflection,
torsional stresses in floating structures,
scour depths as they progress during storms,
the air content of water at contact points,
the velocity of wave crests,
the amount and nature of overtopping, and
the amount of wave runup.
The priority research areas are those that will lead to more
rational design of coastal structures. This research focuses on mea-
surement of the pressures and forces, not only external but also
internal, from storm waves at the instant of breaking on structures,
so that the behavior observed can be understood and predicted. To
undertake much of this research it will be necessary to develop in-
stallation methods for mounting instrumentation on or in structures
and to recover the data.
Representative terms from entire chapter:
wave direction
