Abstract- Obtaining ocean wave data has traditionally
been an expensive undertaking. Deployments are often open-ended,
because extreme conditions are usually the most important to
measure. Advances in electronic miniaturization have produced
wave instrumentation with impressive memory and telemetry capabilities
that are very small and inexpensive. Descriptions of miniature
wave gages and logistics for this new approach are provided.
Benefits and costs of past and potential applications are analyzed
to determine the value of this new wave measurement methodology.
Obtaining ocean wave data has traditionally
been an expensive undertaking. Deployments are often open-ended,
because extreme conditions are usually the most important to
measure. While a long term, continuous wave data set is almost
always useful and often necessary, capture of a relatively few
number of specific events will frequently satisfy the needs of
a project. But such events are rare and may not occur in the
first year, or even first few years, of a wave study. In addition,
robust validation of wave models requires measurements of a wide
range of conditions (e.g., various directions and periods as
well as heights) that may take years to accumulate. With potential
costs running several hundred thousand dollars, many, if not
most, coastal engineering projects are designed without the benefit
of site-specific measurements.
There are several reasons for such expenses in today's wave measurement
programs, but the major factor is the circular nature of the
design and logistics constraints. Gages are often deployed and
operated for a year or longer only so they will be in place and
operating when and if the event of interest occurs. Large, robust
gages (e.g. buoys) are required to have sufficient electrical
power (batteries and sometimes solar panels) for long deployments.
Large instrument systems require large vessels, crews, and equipment,
and relatively calm conditions to safely deploy, so instruments
must be deployed far in advance of the conditions of most interest
for engineering applications.
Advances in electronic miniaturization have produced wave instrumentation
with impressive memory and telemetry capabilities that are very
small and inexpensive. For example, all of the electronics for
a non-directional wave buoy can be less than the size of a soda
can. Battery technology has not kept pace. A battery pack with
year-long operation capability still weighs hundreds of kg.
Thus, small instruments with small battery packs are constrained
to short-term deployments. Rapid, event-triggered deployment
methods, including air-deployment, deployment from small vessels
of opportunity (e.g., jet skis), cast-out and reel-back from
platforms, or even an expendable release are required to realize
the potential cost savings of miniature wave gages. If capture
of extreme event data will satisfy a project's requirements,
system and logistic costs can be decreased by an order of magnitude.
II. PRIOR ART
A. Stationary Wave Gages
Wave gages measure water surface displacement over time. Fixed-mount
gages measure the surface directly (e.g., staff gages) or indirectly
(e.g., pressure or velocity gages) from a rigidly-mounted sensor.
Fig. 1 is typical of a state-of-the-art fixed-mount wave gage.
It is the DWG-1 developed at the US Army Engineer Coastal and
Hydraulics Laboratory (CHL). It consists of 3 pressure sensors
in an array and a central electronics and battery module. The
batteries are sufficient to enable the gage to sample and store
hourly wave and water level observations for over one year. It
is mounted in a steel hexapod, trawler-resistant frame that is
pinned to the sea floor by divers using steel pipe pilings. The
frame and instrument weigh about 500 kg. Installation from a
vessel requires a dive team plus vessel with crew and takes about
a day, once the vessel is moored on station. Mild sea states
(less than 1-m waves) are required not only for placing the frame
over the side and on positioning it correctly on the bottom but
for the safety of the dive team.
Fig. 1. DWG-1 gage deployment.
Fig. 3. Waverider (TM) buoy at
Diablo Canyon, CA.
Floating gages measure displacements
indirectly from the accelerations (and tilt, if directional)
of a surface-following or particle-following) buoy that is moored
to a stationary anchor. Perhaps the most well-known floating
gage is incorporated into the familiar "weather buoy"
operated by the National Data Buoy Center. Mid ocean buoys are
6 to 10m in diam. Fig. 2 shows one of the smallest operated by
NDBC: the 3-m-diam. discuss buoy. The 3-m buoy measures meteorological
parameters as well as directional wave information for up to
18 months using a combination of solar and battery power. It
weighs about 1000 kg, not counting the anchor and mooring assembly.
NDBC buoys are usually installed with US Coast Guard buoy tenders
or cutters. Actual deployment can take less than an hour, once
the cutter is on station, but steaming to the deployment site
can take days.
Fig. 2. NDBC 3-m discuss buoy.
One of the more commonly used wave gages
is the Waverider (TM), manufactured by Datawell. The 1-m-diam.
Spherical buoy weighs about 300 kg and can operate for over a
year on its internal batteries. The Scripps Institution of Oceanography
(SIO) operates a network of 20 or more Waveriders, and has recently
developed a streamlined deployment method using a trailerable
10-m boat and a crew of two. Actual release of the system may
only take minutes, but again, steaming time to the site takes
hours. Deployments can take place in mild to moderately rough
conditions, but not in waves over about 2m.
B. High Wave Deployment of Stationary
The US Army Engineer Filed Wave Gaging Program (FWGP) and the
SIO pioneered the use of helicopters for placing stationary wave
gages. Both Waveriders and pressure arrays have been placed using
the US Army Chinook CH-47 helicopter. When placing a Waverider,
the buoy and its anchor are slung under the helicopter while
the mooring is coiled inside. Once on station, the buoy is released
and allowed to drift downwind while the mooring is paid out the
stern cargo ramp. The anchor is released last. When placing pressure
arrays, the array is slung from the helicopters central cargo
hook (Fig. 4). Typically, an armored communication cable is laid
from the gage to the beach (1-2 km) on the same flight. The
cable is on a reel inside the helicopter and leads out the stern
cargo ramp. The heavy-lift Chinook helicopter with its rear-opening
cargo entrance is considered essential for these operations.
The principal incentive for developing these methods was to enable
stationary gage deployment under higher wave conditions than
could be safely accomplished from vessels. In some regions,
such as the Pacific Northwest, waves can exceed safe operating
thresholds for months at a time. When continuous records are
desired, waiting for calm conditions is not always an option.
Deployments in this category took place in spite of high waves,
not because of them. However, there are times when measurements
under extreme conditions are the study's only objective.
Fig. 4. Chinook with DWG-1.
C. Roving Gages
CHL conducted a study in which current measurements were required
during high wave conditions in the immediate vicinity of the
jetties at the Siuslaw River Entrance. It was too dangerous
to position vessels that close to the rocks, so a US Coast Guard-supplied
helicopter was used to lower an InterOcean S-4 meter at numerous
positions around the structure . The helicopter lowered the
instrument on a side-mounted winch, and remained on station in
a hover for several minutes while the current measurements were
Another CHL study required wave, water level, and current profiles
at multiple stations on top of the Columbia River Bar during
a typical winter storm. This is considered one of the most treacherous
bodies of water in the world, and extreme conditions can persist
for months at a time. Simply passing through the channel in a
vessel is risky in the winter; an over-side operation in the
surf zone was not an option. An instrument package could be deployed
in the summer, but significant scour and deposition rates make
recovery of a fixed instrument the following year doubtful.
The technique used at Siuslaw was considered,
but was impractical. The instrument package was significantly
heavier, and required over 1200 kg of lead weights to insure
stability during the measurements. This could be accommodated
by a heavy-lift helicopter, such as the Chinook. However, wave
measurements require the gage to be stationary for longer periods
- typically 18 minutes. Stationary hovering for that long a
period is extremely challenging for the pilot, and is nearly
impossible over open water, without fixed visual targets in the
near field of view. This demanded the helicopter to place, release,
and recover the instrument repeatedly. The method is illustrated
in Fig, 5 and described in Reference .
Fig. 5. Release and recovery of instruments with Chinook
While these two methods could be called
event-deployed, the planning, preparation, and staging time,
not to mention the cost, is significant and they are not likely
to be widely utilized. In addition, most events of interest provide
little advance warning - perhaps a few days at best, often just
hours. A practical event-deployed gage must inherently be a rapidly-deployed
III. MINIATURE GAGES
An event-deployed measurement plan is possible with a new-generation
of miniature wave gages developed by Neptune Sciences, Inc (NSI).
The largest model is the Wave Sentry Buoy (Fig. 6). It is made
from aluminum, PVC, and urethane foam and weighs 19 kg; the foam
floatation collar is 75 cm in diam. It measures buoy motions
with solid state sensors and an on-board micro processor calculates
directional wave spectra. Typical wave parameters, such as significant
wave height, peak wave period, and dominant wave direction, are
automatically calculated from the spectra. Power comes from 27
alkaline D-cell batteries. Operating life is a function of the
programmable sampling scheme; it will last about 1 month taking
hourly, 17-min samples.
Fig. 6. NSI Wave Sentry Buoy
Fig. 7. Two NSI Mini Sentry Buoys
The next smaller model, the Mini Sentry Buoy, is a hand-held
floating wave gage (Fig. 7). It is 9 cm in diam. by 58 cm tall,
not counting the 40 cm antenna, and weighs 3.6 kg. The Mini Sentry
measures non-directional wave spectra. The battery pack accounts
for most of its mass, and will last about one week taking hourly
The smallest model from NSI is the Micro Sentry buoy. It measures
6 by 47 cm and weighs a little over ½ kg (see Fig. 8).
It has essentially the same electronics as the Mini Sentry but
only holds enough batteries for about 1 day of operation. Like
all of the Sentry buoys, the sampling scheme is programmable,
so increasing the sampling interval to say, two hours would double
the operating period. This is truly a hand-held, "hand-tossable"
instrument. It is based on the original model designed for the
US Navy that is air-deployed and free-falls to the surface, so
it is extremely rugged.
Fig. 8. NSI Micro Sentry Buoy
Measured data are both stored internally on solid state memory
and transmitted in near real-time via spread spectrum UHF telemetry
to a plane overhead, to a nearby vessel, to shore, or to an ARGOS
satellite. Horizontal telemetry range is approximately 10 km,
depending upon receiving antenna height. The receiving station
consists of a UHF transceiver, a radio modem, and a PC. Even
if the data are not required as soon as they are measured, telemetry
offers valuable redundancy for capturing the measurements.
The buoys can be either free floating or moored. An onboard GPS
receiver tracks the buoy and its position is included in the
telemetry stream. Not only does this allow recovery if visual
tracking is lost, but turns the buoy into a trackable drifter
that provides surface current measurements as well as waves.
A Wave Sentry or Mini Sentry (and its mooring if desired) can
be deployed in a few minutes from a small craft by two people
(Fig. 9). The Micro Sentry can be tossed out from an airplane,
a small boat, or even a personal watercraft by one person, without
even reducing speed.
The low cost (see next section), ruggedness, and ease of deployment
of these gages open an entirely new way to capture design wave
information In one scenario one or more buoys would be stored
in secure locations near sites where wave data are desired (harbors,
airports, field offices, on board a dredge, etc.). When weather
forecasts predict an event of interest, local trained personnel
Fig. 9. Two Wave Sentry Buoys ready for Installation.
deploy the buoy(s). In many cases, these
would be volunteers with an intimate knowledge of the region
and its weather and a personal or organizational stake in the
wave information - harbor pilots, marine police, US Coast Guard
search and rescue patrols, and lifeguards are all candidates
for potential deployment teams. Access to real-time output by
these organizations will more than compensate for the minimal
investment in time. After adequate data are obtained, the buoy
is recovered and readied for the next event or immediately redeployed
at another site.
IV. COST COMPARISON
The principal costs of acquiring wave data are the hardware (gage
and associated mooring or mount and shoreside electronics) operations
(installation, repairs, and recovery), and data management (analysis,
QC/QA, reporting, distribution, and archiving). Often, it is
assumed the instrumentation is the major cost, but for the typical
stationary gage, each of these three categories account for approximately
equal shares of the total project cost.
Costs are compared for the three models of Sentry gages, a Waverider,
a DWG-1, and an NDBC 3-m discuss buoy. Acquisition costs for
the first three gages are firm, but operational costs are projections
based upon typical labor and travel costs, since there is little
field experience available. For the latter three gages, cost
estimates are based upon the principal author's twelve years
of experience managing the FWGP between 1986 and 1998. During
those years, the FWGP operated the world's largest network of
stationary ocean wave gages, including nearshore coastal stations
and deep water buoys. While it is understood that a streamlined
private company may realize a more efficient operation than the
government, it would not have the benefit of the economy of scale
attained from operating a network approaching 100 gages for multiple
years. Occasionally a "problem" site would cost considerably
more than average to maintain in some years. However, the only
way to realize significant savings was by neglecting maintenance.
The overall data recovery rate for the FWGP was 90 %. Thus, these
are considered conservative estimates applicable to a project
that maintains the gage to assure capture of events of interest.
Fig. 10 (top) compares some basic operating characteristics of
the 6 gages. "Duration" is the operating time for
the gage using its internal batteries. As a fixed-mount gage,
the DWG-1 is usually installed with a power and signal cable
connecting it to a shore station. It could theoretically operate
indefinitely on shore power. In practice, other considerations
usually required approximately annual service trips. "Range"
is the maximum distance it can send data in real-time. For the
DWG, this is the maximum a signal can be sent on the cable without
using in-line amplifiers. For the Sentry and Waverider, it is
the line-of-sight radio range without using extraordinary receiving
antenna height. Since the NDBC buoy uses the GOES satellite network
for telemetry, its range is global. Thus, from a site selection
standpoint, only the NDBC buoy offers access to real-time data
across oceanic scales. If access to the data can wait until the
gage is discovered, the Sentry buoys can also be deployed without
concern for distance to a receiver. While the DWG can also store
data internally, its pressure sensors limit the deployment depth
for measuring wind waves to about 20m, so it is effectively a
Fig. 10 (middle) shows the acquisition and annual operating costs
for each gage. Acquisition costs include telemetry and shoreside
receiving electronics. If operated in internal storage mode only,
about $2,500 could be deducted from the Sentry gages and about
$10,000, depending upon cable length, from the DWG. A single
Micro Sentry costs $1500. NDBC does not sell its gages to clients;
it charges a fee for operation including, presumably, an amortized
hardware cost. Because the principal mission of NDBC is to collect
and disseminate weather (including wave) observations, outside
clients usually benefit from a significant government subsidy.
Operating costs for the Sentry buoys are linearly dependent upon
the number of deployments, so a range between the minimum and
the maximum expected operating costs is presented. The minimum
cost represents one deployment and recovery. The maximum expected
cost represents a 50 % duty cycle. That would be two one-day
deployments a week for the Micro, two one-week deployments per
month for the Mini, and six one-month deployments for the Wave
Sentry Buoy. If a continuous record is required, a long-term
gage is preferable.
Fig. 10. Gage Performance (top), Acquisition and Operating Costs
(middle) and Wave Record Costs (bottom).
Fig. 10 (bottom) recognizes that the
desired product is the data, not the hardware. It plots the number
of wave records measured in a year by each gage, assuming 90%
successful data recovery. Finally, the unit cost of each wave
record recovered is plotted. For the Sentry gages, the minima
and maxima are shown, as described above; minimum costs per record
comes form maximizing usage. Note that per-record cost for the
Sentry gages is comparable to the long-term gages if it is fully
utilized. Maximum cost per record means using the gage for only
one deployment. Obviously, buying a Micro Sentry and throwing
it away after one day results in the highest cost per wave record.
But if those 24 measurements fulfill the study requirements,
it is still the lowest cost option.
Low cost, miniature instrumentation is not a low-cost replacement
for traditional gages in all situations. For example, they will
not serve NDBC's primary mission to monitor weather conditions.
It is just as important for a mariner to know with certainty
when the waves are less than his safe operational threshold as
when they are above it. Neither are they suited to obtaining
climactic statistics directly from measurements. They are not
suited for sediment transport studies or structural fatigue studies,
where the integrated effect over time of all loading is required.
They also are more susceptible to two common hazards faced by
gages in unprotected locations - accidental collisions with vessels
and deliberate theft or vandalism. However, there are situations
where they can replace conventional gages, and others new applications
where wave measurements have not been considered.
Damage Threshold Measurements: In structural stability/integrity
studies, the threshold of conditions that cause damage or failure
is critical information. For example, the (INSERT HARBOR NAME
HERE) breakwater failed unexpectedly under conditions that were
estimated to be much lower than the damage threshold predicted
by the physical model used for design. However, no gage was in
place at the project site to provide a detailed measurement of
the incident conditions. Knowledge of the incident wave spectrum
that caused the damage would not only help design an adequate
repair, but allow better understanding and improvement of a tool
used to design hundreds of coastal structures.
Episodic Events: The US Army Engineer District, New Orleans,
is using four Mini Sentry gages to capture hurricane wave conditions
on Lake Pontchartrain to study potential for overtopping of the
levees surrounding New Orleans . Many years could pass before
the gages are needed; when they are required, warning time could
be a few days or less. The low cost and rapid deployment capability
allow them to obtain the desired data at multiple sites for much
less than the cost of a single long-term gage.
Emergency Response: The optimal tool for responding to an oil
spill depends on the wave conditions. Rapid deployment of a gage
directly into a spill would prevent managers from mobilizing
resources (e.g., booms) when conditions preclude their use. Search
and rescue missions are most effective when the right vessel
for the conditions is dispatched. Air deployment of a gage could
prevent the rescue crew from becoming victims themselves. Hurricane
evacuation routes on bridges or causeways may become impassable
when wave conditions become excessive. Real-time monitoring could
allow civil defense authorities to reroute traffic before lives
Wave Model Validation: Calibration and verification of numerical
and physical wave models is expensive but necessary. Usually,
one gage is deployed for a year, and the model validated for
whatever conditions occur during that time. Questions can remain
about unusual conditions that weren't measured - not only high
energy events, but unusually long periods or crossing wave trains.
If the model domain includes widely varying boundaries, selecting
the one site for calibration is a compromise. If a large number
of sites could be measured during these unusual events, confidence
in the model would be greatly enhanced. Once a deployment crew
is mobilized, additional gages could be deployed at negligible
additional operational cost
Small Projects: Budgets for smaller structures or harbors rarely
have several hundred thousand dollars for obtaining site specific
data. If data collection cost several tens of thousands or less,
managers of smaller projects could afford to design a safer,
more efficient project. Examples include low-cost shore protection
projects, small boat harbors, lakes and reservoirs.
Operations: Most construction contracts include provisions for
weather delays. Claims and disputes over the wave conditions
can become a major burden to both parties. A contractor could
document the exact conditions at any time using a $1500 instrument,
a fishing pole and line to deploy it, and 15 minutes of non-skilled
New miniaturized wave gages provide a low-cost alternative to
traditional, long-term stationary gages. Battery capacity, and
thus operational life, is much reduced compared to larger gages,
and they face greater risk from accidental and deliberate encounters
with mariners. However, in those applications where it is critical
to capture specific events, savings of more than an order of
magnitude can be realized. The lowest overall cost is realized
by minimizing the number of deployments. On the other hand, if
a miniature gage is utilized to its maximum practical capacity,
the cost per wave record collected costs can is actually less
than for traditional long-term gages.
 C. Pollock, D. McGehee, R. Neihaus, S. Chesser, C. Livingston,
"Effectiveness of spur jetties at Siuslaw River, Oregon:
Report 1, prototype monitoring study," Technical Report
CERC 95-14, US Army Engineers Research and Development Center,
Vicksburg, MS, November, 1995.
 D. McGehee, and C. Mayers "Deploying
& recovering ocean instruments with a helicopter," CETN
VI-34, US Army Engineer, Vicksburg, MS, June 1998.
 H. Winer and D. McGehee, "A
high-reliability system for capturing hurricane wave data,"
Solutions To Coastal Disasters '02, pp 100 - 107, ASCE, San Diego,
CA, Feb 2002.