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Proceedings ArticleDOI

The capabilities of Doppler current profilers for directional wave measurements in coastal and nearshore waters

09 Nov 2004-Vol. 3, pp 1418-1427
TL;DR: In this paper, the capabilities and limitations of two different Doppler current profilers for directional wave measurements in shallow coastal waters of 0-25 m water depth were compared with bottom mounted PUV (pressure-velocity) sensors sampling at wave frequencies and wave buoys.
Abstract: The adaptation of Doppler current profilers to measure directional wave spectra has provided a new instrumentation approach to coastal and nearshore oceanographic studies Past studies have shown favorable comparisons between Doppler current profiler wave instruments with bottom mounted PUV (pressure-velocity) sensors sampling at wave frequencies and wave buoys In this paper, we examine the capabilities and limitations of two different Doppler current profilers for directional wave measurements in shallow coastal waters of 0-25 m water depth Data collection programs using Doppler current profilers for wave measurements have been conducted for one month long periods in the early spring of 2002, 2003 and 2004 on Roberts Bank in the Fraser River foreslope region of the Strait of Georgia, British Columbia, Canada In 2004, an RD Instrument ADCP along with the newly-released 1000 kHz Nortek AWAC current profiler and wave instrument were co-located in 7 m water depth at a different site on the edge of Roberts Bank Inter-comparisons between these bottom mounted instruments are used to examine the capabilities of the directional wave spectral parameters, in terms of: resolvable frequencies for directional and nondirectional wave spectra; wave directional resolution and reliability, and limitations arising from the use of linear wave theory For a preliminary assessment of the capability of Doppler wave spectra in deeper waters of 20-25 m depths, in particular for very long wave periods, some experiences derived from a long-term measurement program being conducted off the west coast of Africa are presented
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Presen t ed a t Ocea n s 04 MTS/IE E E / Tech n o-Ocea n s 04, Kobe J a pan, Novem ber 2004. 1
The Capabilities of Doppler Current Profilers for Directional
Wave Measurements in Coastal and Nearshore Waters
Rick Birch, David B. Fissel, Keath Borg, Vincent Lee and David English
ASL Environmental Sciences Inc.
1986 Mills Road
Sidney, British Columbi
a, V8L 5Y3, Canada
rbirch@aslenv.com
Abstract
-
The adaptation of Doppler current profilers to
measure directional wave spectra has provided a new
instrumentation approach to coastal and nearshore
oceanographic studies. Past studies have shown
favorabl
e comparisons between Doppler current
profiler wave instruments with bottom mounted PUV
(pressure
-velocity) sensors sampling at wave
frequencies and wave buoys. In this paper, we
examine the capabilities and limitations of two different
Doppler current profilers for directional wave
measurements in shallow coastal waters of 0-25 m
water depth. Data collection programs using Doppler
current profilers for wave measurements have been
conducted for one month long periods in the early
spring of 2002, 2003 and 2004 on Roberts Bank in the
Fraser River foreslope region of the Strait of Georgia,
British Columbia, Canada. In 2004, an RD Instrument
ADCP along with the newly-released 1000 kHz Nortek
AWAC current profiler and wave instrument were
co
-located in 7 m water depth at a different site on the
edge of Roberts Bank. Inter-comparisons between
these bottom mounted instruments are used to
examine the capabilities of the directional wave
spectral parameters, in terms of: resolvable
frequencies for directional and
non
-directional wave
spectra; wave directional resolution and reliability, and
limitations arising from the use of linear wave theory.
For a preliminary assessment of the capability of
Doppler wave spectra in deeper waters of 20-25 m
depths, in particular for very long wave periods, some
experiences derived from a long-term measurement
program being conducted off the west coast of Africa
are presented.
I.
INTRODUCTION
Measurements of directional waves in coastal zones are
required in support of many applications including input to
engineering design and operational planning for ports and
marine terminals, sediment transport and beach erosion for
scientific and engineering purposes and dispersal of
pollutants from outfalls or accidental spills.
Tradition
ally, directional wave measurements are made
with: (a) instruments which are complex to install such as
capacitance wire gauges or distributed arrays of bottom
pressure sensors, (b) directional wave buoys which tend to
be expensive, are prone to damage/vandalism, and have
mooring issues in shallow coastal waters or (c) bottom
mounted pressure-velocity (PUV) sensors which are less
expensive but are limited in effective working depths to less
than 10-15 m due to the high degree of attenuation in the
high frequency portion of the wave signal of pressure and
velocity.
Acoustic Doppler Current Profiler (ADCP) instruments,
which were first developed for remote measurements of
current profiles in the 1980
s, have been adapted to the
measurement of directional waves over the past several
years [1,2,3] with initial tests conducted in comparatively
shallow waters of 7
8 m depth. In these shallow waters,
pressure sensors were also used along with the
measurement of orbital wave velocities. Intercomparison
studies have
been extended to deeper waters using wave
buoys [4,5], both directional and non-directional types.
More recently, the capabilities of the Doppler Current
Profiler wave instruments have been augmented through
the use of acoustic surface tracking methods with upward
looking sonars [6], which is not attenuated with depth as is
the case of the wave orbital velocities and especially,
bottom pressure.
Our purpose is to provide insights into the capabilities
of Doppler Current Profilers for directional wave
meas
urements based on:
(a)
an inter-comparison of the RDI 1200 kHz Doppler
Current Profiler, using its Wave Array algorithm,
along with surface tracking with a newly introduced
Nortek 1000 kHz AWAC Doppler Profiler having an
enhanced surface tracking capability, co-located in 7
m water depth, and
(b)
some preliminary experiences of using a RDI 600
kHz ADCP for directional wave measurements in
20
-25 m water depths in West Africa.
II. INTERCOMPARISON OF NORTEK AWAC AND RDI
ADCP
A.
Location and Mooring
Measurements of waves, currents and sediments
have been conducted by ASL Environmental Sciences Inc.
on the Roberts Bank foreshore portion of the Fraser River
delta in the Strait of Georgia, British Columbia, Canada.
As part of studies of sediment dynamics and transport on
Roberts Bank, in 2002, PUV data collected with a 1000
kHz Nortek Aquadopp Doppler velocity instrument was
compared to PUV data using a conventional
electromagnetic current sensor in 10 m water depth [7].
In 2003, a 1200 kHz RDI Doppler Current Profiler using
the wave array configuration, as well as surface tracking
and pressure sensors for non-directional wave spectral
measurements, was deployed in 8 m water depth [7]. In
2004, an intercomparison study using two different
ADCPs was conducted as part of engineering and
environmental studies related to the planned expansion of
the Roberts Bank container terminal operated by
the
Vancouver Port Authority (VPA). Discussion of these
2004 intercomparison results will be the focus of this
section.

Presen t ed a t Ocea n s 04 MTS/IE E E / Tech n o-Ocea n s 04, Kobe J a pan, Novem ber 2004. 2
Fig. 1.
Map of the southern Strait of Georgia and the Fraser River
delta. The Westshore/Deltaport container terminal is at the end
of the T
-
shaped causeway.
ASL is providing long-term representative wave and
current data for the terminal expansion study. The con
tainer
terminal is located on the Fraser River delta at the end of a
causeway that extends across Roberts Bank (Fig. 1, 2).
Deltaic deposits crowd the receiving waters of the Strait of
Georgia, with water depths increasing to 200 m 10 km
offshore. The Strait is aligned approximately
northwest/southeast, which is also the two main directions
for winter storm winds. The largest fetch is to the
northwest, 80 km.
The mooring was located on the edge of the Bank in
7.2 m water depth (relative to low tide). A
diver
-serviceable bottom frame was built to house both
instruments (Fig. 3). This minimized the disturbance to
the bottom since only the instrument cube is recovered for
servicing, while the heavy base remains on bottom.
Fig. 2. Deltaport/Westshore Terminals. The location of the
mooring (
) in 7.2 m water depth at the edge of Roberts Bank is
shown. From CHS chart # 3492.
Fig.3. The diver serviceable bottom frame used to moor both the
Nortek AWAC, and the RDI ADCP.
The instruments are always deployed in exactly the
same location, which is important as the wave regime
varies over short distance scales at this site on the edge of
the Bank. Servicing is done at 3-month intervals with
divers and a small boat. The divers level the cube to
within a few degrees of vertical each time. Concrete pier
blocks were added to the base to increase the weight. It
is important when measuring waves in shallow water that
the instrument is stable and does not respond to wave
orbital velocities.
B.
Instrumentation
Both a Nortek AWAC and an RDI ADCP were
deployed to measure currents as well as directional waves
(Table 1). The Nortek AWAC (Acoustic Wave and
Current Meter) is a 1 MHz 3
-
beam Doppler instrument that
also has a vertical beam for direct measurement of th
e
wave heights. An RDI 1200 kHz WH ADCP instrument
was also deployed for the 13 April
3 June 2004 period.
The RDI ADCP has the standard 4
-
beam configuration.
The Nortek AWAC uses Acoustic Surface Tracking
(AST) for direct measurement of the wave heights. A
receive window is defined, based on the pressure, and
partitioned into multiple cells (2.5 cm size). Then a short
acoustic pulse is transmitted and a match filter applied
over the series of cells to locate the surface. Quadratic
interpolation is used to determine the surface location.
Vertical resolution is stated to be about 1 cm or better.
Sampling rates of up to 4 Hz allows high frequency wave
resolution out to 2 Hz. The frequency limitation is
determined by a combination of the Nyquist freque
ncy
(0.5 times the sensor sampling rate) as well as being
limited by the acoustic
footprint , i.e. when half the
wavelength is on the order of the diameter of the footprint
[4]. For the 7-10 m water depth (low-high tide), the
frequency resolution due to the 22-30 cm footprint, is
1.9
-1.6 Hz, for a sampling rate of 4 Hz. Note that if the
AST data are unreliable, wave heights can also be
computed from the pressure, or velocity spectra.
Three other beams, at 25 degrees from vertical, are
used to measure cur
rent profile and wave orbital velocities
based on the Doppler effect.
The depth of the cell for wave
orbital velocity measurements is adaptively positioned for
AWA C &
ADCP

Presen t ed a t Ocea n s 04 MTS/IE E E / Tech n o-Ocea n s 04, Kobe J a pan, Novem ber 2004. 3
each wave measurement burst.
The
Waves feature of the RDI ADCPs determines
wave height by one or more combinations of three
measurement methods: velocities, surface tracking and
bottom pressures. Measurements of wave orbital
velocities, as well as pressures, are used to derive
independen
t wave spectra, with correction for
depth
-dependent attenuation using linear wave theory.
The surface tracking measurements are also used to
determine a third version of the wave spectra. The primary
wave spectra for the RDI ADCP is normally derived from
doppler orbital velocities which are measured much closer
to the surface, and therefore less affected by attenuation
at high frequency than is the case for the bottom pressure
sensor. For this study we computed wave spectra from
both the velocities and surface tracking, to determine the
non
-
directional wave parameters H
s
and T
p
.
Note that the precision of the surface tracking for the
RDI ADCP is approximately the bin size divided by 3.5, or
in the case of the intercomparison study, 0.1 m, vs. the
Nortek AW
AC surface tracking precision that is about 0.01
m. The lesser precision of the RDI ADCP surface
tracking, by comparison to the Nortek AWAC, is due to the
use of longer pulse widths on each of its four beams,
consistent with the need to measure the frequency shifts
to determine Doppler velocities. In contrast, the AWAC
uses a dedicated vertical beam which allows a very short
acoustic pulse width to be employed.
Fig. 4. Sampling parameters for the Nortek 1 MHz AWAC (upper
two panels), and for
the RDI 1200 kHz ADCP (lower panel). The
AWAC AST samples at twice the velocity rate, or at 4 Hz.
The RDI Sentinel Workhorse ADCP derives directional
wave spectra from the orbital velocities.
RDI uses a wave
array
algorithm [1] based on twelve (three or more from
each beam) independent measurements of velocity that
increases the resolution of the directional wave spectrum
estimates. By comparison, the AWAC uses three
measurements of velocity (one from each beam). Also,
the RDI ADCP uses the Broadband acoustic pulse signal
processing technique [5], which effectively increases the
precision of the measured velocities.
TABLE 1: MANUFACTURER S SPECS FOR THE AWAC AND
ADCP INSTRUMENTS USED IN THE INTERCOMPARISON
N
ortek AWAC
RDI ADCP
Transducers
Frequency
1 MHz
1200 kHz
Orientation
3-
beam convex,
25
from vertical;
one vertical (AST)
4-
beam convex,
20
from vertical
Beam Width
1.7
(3 dB)
1.4
(3 dB)
Wave Data
Data types
AST vertical beam
surface track; one
or
bital velocity cell
along each beam;
pressure
4-
beam surface
track; 3
-
5 orbital
velocity cells along
each beam;
pressure
512, 1024 or 2048
variable
Cell size
0.4
4.0 m
0.01
-
2 m
Max sampling rate
2 Hz (4 Hz AST)
2 Hz
Internal sampli
ng rate
4 Hz
2 Hz
Velocity Measurements
Range
10 m/s horizontal
20 m/s horizontal
Accuracy
1% of measured
value or
0.5 cm/s
0.25% of
measured value
or
0.25 cm/s
Doppler uncertainty Waves: 3.5 cm/s
at 1 Hz for 1 m
cells
One ping al
ong
beam precision of
5.4 cm/s w/ 0.35
m bin
Sensors
Flux
-
gate Compass
Acc/res 2
/0.1
for
tilts <20
Acc/res 2
/0.01
Piezoresistive
Pressure
Range 0-
50 m;
Acc/res 5cm /
0.25cm
Range 0
-
20 m;
Acc/res 5cm /
0.05cm

Presen t ed a t Ocea n s 04 MTS/IE E E / Tech n o-Ocea n s 04, Kobe J a pan, Novem ber 2004. 4
C.
Sampling Parameters
The AWAC recorded 17-minute wave bursts and the
RDI ADCP 20 minutes. Only 17 minutes of the ADCP data
were used
for spectral analyses. The AWAC bursts began
on the hour and the ADCP on the half hour. Overlap in
sampling was avoided to eliminate any possibility of
acoustic interference or
cross talk if the two instruments
were operated simultaneously. The non-directional wave
spectra parameter data from each instrument were
interpolated to a common hourly increment on 15 minutes
after the hour, mid-way between the measurement time of
the Nortek AWAC (on the hour) and RDI ADCP (on the
half
-hour). All wave direction data are reported in degrees
magnetic.
The set-up parameters used for the AWAC and ADCP
are shown in Fig. 4.
D.
Wave Results
Spectral Parameters (H
s
, T
p
)
For both the AWAC and ADCP, the wave spectra are
computed and the wave parameters are derived a
s:
significant wave height (H
s
) equal to four times the square
root of the integrated spectra. The peak period (T
p
) is the
centre frequency of the spectral band with the most energy.
Because the AWAC measurements are also available as a
time series of wave height, the additional parameters H
max
,
H
1/10
, H
1/3
and T
mean
are readily determined using the
zero
-
upcrossing
method in the time domain. The RDI
ADCP can also provide a time series representation of
surface tracking, although the precision is reduced a
s
discussed above.
Waves during the 13 April
3 June 2004 data collection
period were generally 0.2-0.4 m in height, increasing to
almost 2 m during a northwesterly storm on 27
-
28 April (Fig.
5). A cutoff of H
s
=0.3 m has been used to reduce the
scatter at small wave heights. Wave periods are small,
generally 6 seconds or less. The wave directions (from)
were largely confined to northwest/southeast by the
orientation of the Strait and by the two dominant wind
directions.
Fig 5. Comparison of the wave parameter time series from the
AWAC and RDI ADCP (Red=AWAC [surface track]; Blue=RDI
ADCP [velocity-derived]); H
s
, T
p
, Peak Dir (for H
s
>0.3m). The
black line in the upper panel represents the difference in H
s
of the
RDI ADCP less the AWAC (see special
scale on the right).
Comparison of Wave Heights
Fig. 6a,b compare the significant wave heights from the
two instruments, based on the moderate to larger waves,
i.e. H
s
0.3 m, amounting to a total of approximately 200
one
-hourly samples. The regression is very good with R
2
values of 0.93 and 0.91. In the case of the AWAC
surface
-
tracking versus RDI ADCP velocity
-
derived values,
the AWAC tended to produce H
s
values slightly larger (~
0.05 m) than the RDI ADCP.
Fig. 6a Scatter plot of H
s
; (H
s
>0.3m); A
WAC [surface track] versus
ADCP [velocity
-derived]. Y=0.99X+0.05 R
2
=0.93
Fig. 6b. AWAC H
s
[surface track] versus ADCP H
s
[surface track]
for H
s
>0.3 m. Y=0.99X+0.02 R
2
=0.91.
For this comparison of H
s
, RDI ADCP values were
computed to an upper frequency limit of 0.5 Hz while the
upper frequency limit, used for the AWAC computation,
was 1 Hz (very little energy was present in the autospectra
from 1 to 2 Hz). A similar regression analysis for the RDI
ADCP surface tracking
-
(over frequencies of up to ne
arly 1
Hz) vs. velocity-derived H
s
results in a mean difference of
0.04 m. In Fig. 6b, we compare the H
s
values computed
from surface tracking on both the AWAC and the RDI
ADCP (now with an upper frequency limit of nearly 1 Hz).
In this case, the difference between the AWAC and RDI
ADCP H
s
values is reduced to only 0.02 m. While this
difference in H
s
is very small, we note that a higher
positive noise floor in the AWAC wave spectra (Fig. 9 and
10), relative to that of the RDI ADCP, has approximately
the
correct magnitude to account for this small difference.
Comparison of Wave Periods
A similar regression was performed for the peak
periods from the two instruments (Fig. 7a,b), again in
cases where H
s
0.3 m. As one would expect, there is
more scatter (R
2
= 0.65 - 0.7) among the peak periods
from the AWAC and ADCP due to the nature of this
particular spectral parameter, especially at longer periods
where the effective spectral resolution is reduced.
0 0.2 0.4 0.6 0.8 1 1.2
0
0.2
0.4
0.6
0.8
1
ADCP - Hs (m) [vel.]
AWAC - Hs (m)
AWAC vs ADCP - Hs (Hs
0.3m)
y = 0.99x + 0.05; R
2
= 0.93
0 0.2 0.4 0.6 0.8 1 1.2
0
0.2
0.4
0.6
0.8
1
ADCP - Hs (m) [surf]
AWAC - Hs (m)
AWAC vs ADCP - Hs (Hs
0.3m)
y = 0.99x + 0.02; R
2
= 0.91
04/17 04/21 04/25 04/29 05/03 05/07 05/11 05/15 05/19 05/23 05/27 05/31
0
0.5
1
1.5
ADCP Hs(m)
-2
-1.25
-0.5
0
0.25
ADCP-AWAC (m)
04/17 04/21 04/25 04/29 05/03 05/07 05/11 05/15 05/19 05/23 05/27 05/31
0
5
10
Tp(s)
AWAC
ADCP-vel
04/17 04/21 04/25 04/29 05/03 05/07 05/11 05/15 05/19 05/23 05/27 05/31
0
45
90
135
180
225
270
315
360
PeakDir(deg)

Presen t ed a t Ocea n s 04 MTS/IE E E / Tech n o-Ocea n s 04, Kobe J a pan, Novem ber 2004. 5
Fig. 7a. AWAC T
p
[surface track] versus ADCP T
p
[velocity] for
H
s
>0.3m. R
2
=0. 70
Fig. 7b. AWAC T
p
[surface track] versus ADCP T
p
[surface track]
for H
s
>0.3m. R
2
=0.65
Overall, the scatter plot approximately tracks a 1:1
relationship; however, the regression fit deviates from this
due to 5 pairs of values where the AWAC T
p
lies in the
2.5
-3.5 s range, while the ADCP values are approximately
5-7 seconds period range. The wave auto-spectra for
these 5 sets of values were examined (from the RDI
ADCP
software) in detail, as were 4 cases of large
deviations in T
p
, selected from similar comparisons (not
shown) of T
p
values derived from the RDI ADCP surface
tracking
- and velocity-derived wave spectra. For all of
these auto-spectra, two distinct spectral peaks occurred,
one at a higher frequency and the other at a lower
frequency of 4.5 to 6.5 second periods. In the case of
the RDI ADCP derived values, the peak period was
observed to change abruptly from the low to the high
frequency peak, or vice versa, over time scales as short
as one hour. In the case of the Nortek AWAC, the
selected peak period was always the high frequency or
low peak period value. Although the wave spectra were
similar, the preferential tendency of the AWAC to exhibit
comparative
ly larger spectral densities at higher
frequencies may reflect a larger degree of smoothing
which is apparently applied in the Nortek software over
that used in the RDI software (see Fig. 8, 9 and 10 below),
which is discussed further below. In any case,
an
examination of the full wave auto
-
spectra provides a much
more complete assessment of the frequencies (or periods)
at which appreciable energy is present, whereas the
single value of T
p
can be misleading.
E.
Wave Results
Non
-
Directional Wave Spectra
Wave height spectra from the AWAC and RDI ADCP
are compared for small (H
s
=0.3 m), moderate (H
s
=0.8 m),
and large (H
s
=1.1 m) waves. In all cases the AWAC
spectra are based on the surface track, whereas the
ADCP plots show the spectra from the pressure (re
d),
orbital velocities (green), and the surface track (blue).
The RDI ADCP computed H
s
, T
p
parameters, for both
velocity
- and surface-track derivations, are shown in the
figure captions. The spectral resolution for the RDI
ADCP is 0.016 (1/64) Hz while the Nortek AWAC has a
spectral resolution of 0.01 Hz.
The spectrum from the pressure sensor does not
include energy above 0.29 Hz (3.4 sec period) and, if used,
would underestimate significant wave heights. The
ADCP surface track spectra (blue) extend to 0.94 Hz (1
sec). An orbital velocity spectra cut off at 0.5 Hz (2 sec)
was used as a best upper frequency limit for the majority
of bursts. (Note: Agreement between the surface
tracking and velocity spectra for the RDI ADCP was found
out to higher frequencies approaching 1 Hz for the
moderate and larger wave spectra, but for the small wave
case, going beyond the 0.5 Hz cut
-
off would result in noise
dominating signal in the auto
-
spectral values.)
The AWAC spectra based on the surface track was
cut
-off at 1 Hz, although the 4 Hz sampling rate of the
AWAC AST actually provides spectral information out to 2
Hz, since there is little wave energy beyond 1 Hz.
Small Waves
For small waves (H
s
~0.3 m, T
p
~2.6 sec) the energy in
the AWAC wave spectra (Fig. 8b) is centered at 0.38 Hz
(T
p
=2.62 sec) and the integration resulted in a significant
wave height of 0.31 m.
The surface track from the RDI ADCP (Fig. 8a) is most
similar to the AWAC spectra. The H
s
and T
p
parameters
computed from both the velocity- and surface-
tr
ack
derived ADCP spectra, in this one case, agree well with
those from the AWAC, even though the ADCP
velocity
-derived spectra are cut off at 0.5 Hz. As is the
case in comparisons of spectral parameters though-
out
this study, the AWAC and RDI ADCP measure
ments were
separated in time by 30 minutes, so individual
comparisons can reflect time varying changes in this
highly fetch
-
and duration
-
limited wave generation regime.
Fig. 8a. ADCP Energy Spectra for small waves; April 24 12:29
Hs=0.28m, T
p
=2.56s [velocity], H
s
=0.34 m, T
p
=2.67 s [surface
track]
0 1 2 3 4 5 6 7
0
1
2
3
4
5
6
7
ADCP - Tp (s) [vel]
AWAC - Tp (s)
AWAC vs ADCP - Tp (Hs
0.3m)
y = 0.74x + 0.80; R
2
= 0.70
0 1 2 3 4 5 6 7
0
1
2
3
4
5
6
7
ADCP - Tp (s) [surf]
AWAC - Tp (s)
AWAC vs ADCP - Tp (Hs
0.3m)
y = 0.72x + 0.89; R
2
= 0.65

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Journal ArticleDOI
TL;DR: In this paper, the authors used subsurface pressure and lidar data to study the non-linear and non-hydrostatic character of surf zone waves and found that the nonlinear effects remain strong over the entire surf zone; that is, fluid accelerations are important and the hypothesis of a hydrostatic pressure field leads to large deviations of the real surface elevation.
Abstract: In the surf zone, non-hydrostatic processes are either neglected or estimated using linear wave theory. The recent development of technologies capable of directly measuring the free surface elevation, such as 2-D lidar scanners, allow for a thorough assessment of the validity of such hypotheses. In this study, we use subsurface pressure and lidar data to study the non-linear and non-hydrostatic character of surf zone waves. Non-hydrostatic effects are found important everywhere in the surf zone (from the outer to the inner surf zones). Surface elevation variance, skewness, and asymmetry estimated from the hydrostatic reconstruction are found to significantly underestimate the values obtained from the lidar data. At the wave-by-wave scale, this is explained by the underestimation of the wave crest maximal elevations, even in the inner surf zone, where the wave profile around the broken wave face is smoothed. The classic transfer function based on linear wave theory brings only marginal improvements in this regard, compared to the hydrostatic reconstruction. A recently developed non-linear weakly dispersive reconstruction is found to consistently outperform the hydrostatic or classic transfer function reconstructions over the entire surf zone, with relative errors on the surface elevation variance and skewness around 5% on average. In both the outer and inner surf zones, this method correctly reproduces the steep front of breaking and broken waves and their individual wave height to within 10%. The performance of this irrotational method supports the hypothesis that the flow under broken waves is dominated by irrotational motions. Plain Language Summary In the surf zone, waves undergo rapid changes in shape, passing from steep and skewed waves right before breaking to sawtooth-shaped asymmetric bores. Capturing and understanding these changes is crucial for coastal researchers and engineers since the breaking wave-induced hydrodynamics shape beaches at various temporal and spatial scales. In this study, we use lidar scanners and pressure sensors to study the non-hydrostatic and non-linear character of surf zone waves. We show that non-hydrostatic effects remain strong over the entire surf zone; that is, fluid accelerations are important and the hypothesis of a hydrostatic pressure field leads to large deviations of the real surface elevation. More specifically, wave crests elevation are underestimated, and the sharp-crested shape of broken waves is rounded off. A recently developed non-linear weakly dispersive method to reconstruct the free surface from subsurface pressure is found to consistently outperform the hydrostatic or classic transfer function reconstructions over the entire surf zone, with relative errors on the surface elevation variance (related to the wave energy) and skewness (related to wave shape) around 5% on average. The performance of this irrotational method supports the hypothesis that the flow under broken waves is dominated by irrotational motions.

17 citations


Cites background from "The capabilities of Doppler current..."

  • ...The presence of air bubbles associated with wave breaking processes prevents sound waves to reach the surface, hence making this method inappropriate for surf zone applications (Birch et al., 2004)....

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  • ...These can be deployed at the bottom of the water column (Pedersen et al., 2002; Birch et al., 2004; Mouragues et al., 2019) or above the surface (Turner et al., 2008), although the latter study focuses on the swash zone....

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Journal ArticleDOI
TL;DR: In this article, the performance of an improved 5-beam ADCP, with a vertical beam, when deployed to measure non-directional waves in waters of 40m depth is compared with four co-located directional wave buoys.

11 citations

References
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Proceedings ArticleDOI
11 Mar 1999
TL;DR: In this article, it is shown that it is possible in shallow water to estimate both wave height and direction from a conventional bottom-mounted, upward-looking acoustic Doppler current profiler.
Abstract: Routine monitoring of waves and currents in the nearshore region is of great interest both scientifically and to the general public because of their role in coastline erosion and their impact on recreational activities Historically, the technology for measuring these quantities has been distinct, requiring separate instrumentation for each In this contribution the authors show that it is possible in shallow water to estimate both wave height and direction from a conventional bottom-mounted, upward-looking acoustic Doppler current profiler Height and direction spectra compare well with a co-located array of pressure gages

67 citations


"The capabilities of Doppler current..." refers methods in this paper

  • ...RDI uses a wave array algorithm [1] based on twelve (three or more from each beam) independent measurements of velocity that increases the resolution of the directional wave spectrum estimates....

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  • ...Acoustic Doppler Current Profiler (ADCP) instruments, which were first developed for remote measurements of current profiles in the 1980 s, have been adapted to the measurement of directional waves over the past several years [1,2,3] with initial tests conducted in comparatively shallow waters of 7 8 m depth....

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Proceedings ArticleDOI
11 Sep 2000
TL;DR: In this article, a bottom-mounted upward looking ADCP was used to determine wave height and direction in coastal-depth waters, and the ADCP yielded three independent estimates of the non-directional wave height spectrum and hence provided an internal consistency check on the performance of the instrument.
Abstract: The authors have shown that a bottom-mounted, upward looking ADCP provides a robust means of determining wave height and direction in coastal-depth waters. When equipped with a pressure sensor, the ADCP yields three independent estimates of the non-directional wave height spectrum, and hence provides an internal consistency check on the performance of the instrument. Directional spectra obtained from the ADCP tend to be sharper than those from point measurements, such as PUV triplets or directional wave buoys and, because of the greater number of degrees of freedom in the measurement, the ADCP can resolve complex multi-directional wave distributions.

45 citations

Proceedings ArticleDOI
29 Mar 2002
TL;DR: In this paper, the authors compare PUV wave observations from two collocated velocity/pressure sensors (Aquadopp Current Profiler and Vector Velocimeter) to evaluate their performance.
Abstract: We compare PUV wave observations from two collocated velocity/pressure sensors (Aquadopp Current Profiler and Vector Velocimeter). Our objective was to use the differences in the two sensors to evaluate their performance. We evaluate limitations and uncertainties in the wave measurements, focusing particularly on the high frequency cutoff and uncertainties in direction and spreading. We model direction and spreading uncertainties with a simple Monte Carlo simulation, which compares well with our wave data. We conclude that either instrument is able to observe wave spectra and wave height with an uncertainty of a few percent and with wave direction uncertainties of a few degrees.

20 citations

Proceedings ArticleDOI
29 Oct 2002
TL;DR: In this paper, a vertical, acoustic beam that detects the surface is introduced, which allows for directly measuring waves as opposed to interfering wave estimates from wave energy spectra, which improves the accuracy at higher frequencies.
Abstract: Nortek has improved upon its AWAC, a current and wave measurement sensor package, by introducing a vertical, acoustic beam that detects the surface. This added functionality allows for directly measuring waves as opposed to interfering wave estimates from wave energy spectra. Traditionally, wave measurements from bottom-mounted instruments, such as the combine pressure-velocity (PUV) approach, are limited in their frequency response. This is due to attenuation of the surface signal with increasing depth. Recent advances employ the alternative solution of measuring orbital velocities close to the surface and incorporating the Maximum Likelihood Method (MLM) estimate technique (Krogstad et al., 1988). This improves the accuracy at higher frequencies. However, for deployment depths of 10 metres or deeper, these methods cannot resolve waves periods that are 3 seconds or shorter. Moreover, these bottom-mounted systems do not measure the real surface time series, which makes it difficult to calculate extreme value statistics. The following paper provides an overview of the process of (1) developing the surface track algorithms, (2) comparing with a Datawell wave buoy off the coast of Carqueirance, France (3) and testing limiting conditions such as breaking waves and greater depths (35 metres).

20 citations

Proceedings ArticleDOI
15 Jun 2004
TL;DR: In this article, a vertical, acoustic beam that detects the surface is used to measure the orbital velocities close to the surface and incorporating the Maximum Likelihood Method (MLM) estimate technique (Krogstad et al., 1988).
Abstract: Nortek has improved upon its AWAC, a current and wave measurement sensor package, by introducing a vertical, acoustic beam that detects the surface. This added functionality allows for directly measuring waves as opposed to inferring wave estimates from wave energy spectra. Traditionally, wave measurements from bottom-mounted instruments, such as the combined pressure-velocity (PUV) approach, are limited in their frequency response. This is due to attenuation of the surface signal with increasing depth. Recent advances employ the alternative solution of measuring orbital velocities close to the surface and incorporating the Maximum Likelihood Method (MLM) estimate technique (Krogstad et al., 1988). This improves the accuracy at higher frequencies. However, for deployment depths of 10 meters or deeper, these methods cannot resolve waves periods that are 3 seconds or shorter. Moreover, these bottom-mounted systems do not measure the real surface time series, which makes it difficult to calculate extreme value statistics. The following paper provides an overview of (1) the process of developing the surface track algorithms, (2) comparing with Datawell wave buoys off the coasts of Carqueiranne, France and Gabbard, UK (3) and finally we show how the same technology has been transferred from a 1 MHz to a 600 kHz AWAC to achieve similar accuracy and resolution at depths of 60 meters.

12 citations