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Proceedings ArticleDOI

A configurable multi-sensor tripod for the study of near-bottom ocean processes

01 Jan 2003-Vol. 4, pp 2254-2260

AbstractA cost effective and highly configurable scientific platform, dubbed Norton, has been developed for the scientific study of near-bottom ocean processes. The basic platform is a collapsible lightweight aluminum tripod that can be transported in the back of a pick-up truck, and is easily accommodated on most vessels. Ballast is supplied in the form of large ship anodes bolted to the base of each leg. The desired sensors are attached to the tripod legs and bracing. A typical deployment might utilize the following sensors: Acoustic Doppler Current Profiler for current profiles and directional wave data. Marsh McBirney electromagnetic flow sensor for measurements close to the bottom, supporting both wave orbital burst sampling and average current measurements. Optical Backscatter Sensors (OBS) for sediment concentration. Sector-scanning sonar to supply bed-form images; an Imagenex variable frequency scanning head has been integrated into a self-contained housing containing a control module, data logger and power supply. Other sensors can be added as desired. Norton is usually deployed by lowering with a line from the ship. An acoustic release is used to let go of the tripod once it is on bottom. A customized 'tilt-pinger' provides confirmation to the crew that the tripod is upright. Recovery can be via a pop-up buoy and/or a ground-line.

Topics: Tripod (photography) (56%), Acoustic release (53%), Data logger (51%)

Summary (2 min read)

Introduction

  • When a similar requirement measurements on the Fraser River d ASL was contacted to build a suitab.
  • While its origins are obscure, there naming bottom tripod/frames after Honeymooners TV series.
  • For current profiles and etic flow sensor for supporting both wave rrent measurements, also known as A ollowing sensors.

C. Deployment / Recovery

  • To date Norton has been deployed twice off the Fraser River delta, once in 10 m water depth, and most recently on a tidal bank that dries at low tide.
  • On both occasions, the Canadian Coast Guard Hovercraft Siyay was used for deployment and recovery.
  • At the 10 m depth site, the tripod was deployed with a ground-line and an acoustic release-activated pop-up buoy at the end of the ground-line.
  • The field team then disembarked to measure the tripod heading, collect sediment samples, take photographs, and measure the bedforms.
  • When the tripod was recovered a month later, the tide was higher but the top of the tripod was above water and easily recovered.

A. Environment, Background Oceanography

  • The Strait of Georgia separates Vancouver Island from the BC mainland.
  • The Fraser River enters the ocean at Vancouver and many of its suburbs are located on the subaerial delta.
  • The main channel is dredged and the Steveston Jetty was built along the northern portion of the channel extending 5 km into the Strait.
  • Significant waves heights up to about 3.4 m height, and about 7.5 second period, have been recorded in the deeper water off the delta (Fisheries and Oceans Canada station #108; http://www.meds-sdmm.dfompo.gc.ca/meds/Databases/Wave/).
  • There is evidence for sand from the South Arm becoming incorporated in a clockwise gyre across the north half of Roberts Bank [5].

B. Winter 2002 Deployment on Roberts Bank

  • Previous multibeam surveys show outcropping beds aligned roughly north-south in the 5-15 m depth range, indicative of net erosion [8].
  • Further offshore (20-30 m depths) were subaqeous dunes aligned northeast-southwest, perpendicular to the predominant direction of tidal flow.
  • The currents were tidally dominated with flood tides stronger than ebb, up to 1 m/s versus 0.4 m s-1 respectively.
  • Several storm events were recorded with significant wave heights up to 1 m, and 5 second peak periods.
  • The scanning sonar (set to 30 m range and default gain and frequency settings) showed high suspended sediment concentrations just trailing the storms and the emergence of long wavelength (c. 10 m), low-relief, irregular patches [8].

C. Preliminary Results from the Winter 2003 Deployment on Roberts Bank

  • The Imagenex scanning sonar settings were optimized for this deployment using tank tests.
  • The first images from the scanning sonar (after the tide had risen enough to submerge the instrument) show ripple-like features, particularly in the northwest quadrant with wavelengths of the same magnitude as seen visually (Fig. 6).
  • During this period of low tide, the authors expect that suspended sediment levels are low, due to the ‘slack’ current.
  • Gwyn Lintern (U. Victoria Post Doctoral Fellow) - assistance during deployment and recovery.

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A Configurable Multi-Sensor Tripod
For the Study of Near-Bottom Ocean Processes
R. Birch
1
, P. R. Hill
2
, M. Clarke
1
, D. Lemon
1
, & D. Fissel
1
1
ASL Environmental Sciences,
1986 Mills Road,
North Saanich, BC, Canada, V8L 5Y3
asl@aslenv.com
Abstract- A cost effective and highly configurable scientific
platform, dubbed Norton, has been developed for the scientific
study of near-bottom ocean processes. The basic platform is a
collapsible lightweight aluminum tripod that can be
transported in the back of a pick-up truck, and is easily
accommodated on most vessels. Ballast is supplied in the
form of large ship anodes bolted to the base of each leg. The
desired sensors are attached to the tripod legs and bracing. A
typical deployment might utilize the following sensors:
· Acoustic Doppler Current Profiler for current profiles and
directional wave data.
· Marsh McBirney electromagnetic flow sensor for
measurements close to the bottom, supporting both wave
orbital burst sampling and average current measurements.
· Optical Backscatter sensors (OBS) for sediment
concentration.
· Sector-scanning sonar to supply bed-form images; an
Imagenex variable frequency scanning head has been
integrated into a self-contained housing containing a control
module, data logger and power supply.
Other sensors can be added as desired. Norton is usually
deployed by lowering with a line from the ship. An acoustic
release is used to let go of the tripod once it is on bottom. A
customized ‘tilt-pinger’ provides confirmation to the crew
that the tripod is upright. Recovery can be via a pop-up buoy
and/or a ground-line.
I. INTRODUCTION
As part of the Georgia Basin Geohazards Initiative, the
Geological Survey of Canada is studying erosion and
sediment transport on the Fraser River delta. The
Geological Survey of Canada has had considerable success
on Canada’s east coast using a platform termed Ralph [1],
[2]. When a similar requirement for sediment transport
measurements on the Fraser River delta became necessary,
ASL was contacted to build a suitable instrument platform.
While its origins are obscure, there seems to be a trend in
naming bottom tripod/frames after characters from the old
Honeymooners TV series. The New Zealand National
Institute of Water & Atmospheric Research called theirs
Alice, after the lead female character, and as mentioned, the
Geological Survey of Canada on the east coast named
theirs Ralph, after the lead male role. The Ralph platform is
quite big and burly, much like the actor Jackie Gleason
who played that role. When the Canadian west coast
version was built, it was more lightweight and mobile, and
we named it Norton after Ralph’s scrawny and somewhat
idiosyncratic sidekick. Characteristically, Norton’s
aluminum legs buckled slightly during the latest recovery,
after encountering a much larger mass (the recovery
vessel).
2
N
atural Resources Canada,
Geological Survey of Canada,
P.O. Box 6000, Sidney, BC, Canada, V8L 4B2
phill@NRCan.gc.ca
GSC Publication No. 2003086
II. NORTON TRIPOD CONFIGURATION
A. The Tripod
The Norton platform is an aluminum tripod with a base
dimension of about 3 m and an overall height of 2.2 m.
Aluminum was chosen for its light weight and resistance to
corrosion. Large ship anodes are added to the base of the
legs for stability as well as corrosion protection. A mid-
section brace adds strength as well as a means of attaching
instruments. The legs are hinged and with the mid-section
removed they collapse, making the platform easily
transportable (it fits in the back of a pickup truck).
Fig.1. Norton being recovered from its inaugural
deployment off the Fraser River delta.
B. Sensor Suite
1234
presented at Oceans 2003, San Diego

Norton is easily configurable with whatever sensors are
required. Instruments such as OBS sensors and Doppler
current profilers are generally attached to the tripod legs
whereas the scanning sonar and electromagnetic flow
sensors are normally mounted in the open area directly
under the tripod.
At present the instruments are stand-alone with each having
its own power supply and data logger. Time
synchronization is used to later merge the data streams.
This approach provides a highly configurable suite of
various sensors that can be optimized for a particular
application, and it also ensures simple field assembly and
operation.
Instruments that have been used on Norton to date include:
Coastal Leasing MiniSpec current/wave gauge
. This
instrument is equipped with a Marsh McBirney flow
sensor for measuring near-bottom currents, and in
burst mode measures the wave orbital velocities. It
also has a Paroscientific pressure sensor for wave
height measurements. When combined with the
orbital velocity data, wave directions can also be
determined (PUV method).
Imagenex 881A Scanning Sonar
. The multi-frequency
(175-1000 kHz) scanning sonar head was integrated to
a custom-built data logger/power supply. A large
alkaline battery pack and up to 1 GB of data storage,
allow the instrument to be deployed for 8-10 months
using an hourly scan rate.
OBS Sensors
. Norton is usually outfitted with two
D&A Optical Back Scatter (OBS) sensors, at different
heights above bottom. The OBS sensors are stand-
alone using an Applied Microsystems CTD platform
for power and data logging. Sediment samples are
usually collected from the site to allow post-survey
conversion of the OBS sensors output to suspended
sediment concentration units.
Acoustic Doppler Profiler
. Doppler current profilers
can be added to provide current profile data, as well as
(directional) wave data.
C. Deployment / Recovery
To date Norton has been deployed twice off the Fraser
River delta, once in 10 m water depth, and most recently on
a tidal bank that dries at low tide. On both occasions, the
Canadian Coast Guard Hovercraft Siyay was used for
deployment and recovery. At the 10 m depth site, the
tripod was deployed with a ground-line and an acoustic
release-activated pop-up buoy at the end of the ground-line.
During recovery the pop-up buoy failed to surface and the
ground line was snagged with a grapple. In the
oceanographic community there is often some doubt as to
whether a bottom frame is upright after deployment. ASL
has developed a tilt-pinger that pings only if it is within 20°
of vertical, providing assurance that the tripod was
deployed upright.
On the tidal bank, the deployment was relatively easy. The
hovercraft settled onto the sand bank at low tide and the
crane was used to swing the tripod over the side. The field
team then disembarked to measure the tripod heading,
collect sediment samples, take photographs, and measure
the bedforms. When the tripod was recovered a month
later, the tide was higher but the top of the tripod was
above water and easily recovered.
III. TWO DEPLOYMENTS ON ROBERTS BANK, THE
FRASER RIVER DELTA
A. Environment, Background Oceanography
The Strait of Georgia separates Vancouver Island from the
BC mainland. The Fraser River enters the ocean at
Vancouver and many of its suburbs are located on the
subaerial delta. The main channel is dredged and the
Steveston Jetty was built along the northern portion of the
channel extending 5 km into the Strait. This and other river
management projects have deprived some areas of the delta
front of much of the sediment formerly received during the
peak spring discharge.
Tides within the Strait are mixed, mainly semi-diurnal,
with ranges up to about 3-5 m. Tidal current speeds can
exceed 1 m s
-1
. The Strait has limited fetch for wave
generation with the largest waves affecting the delta
coming from the west-northwest (approximately 80 km
fetch in this direction). Wave heights are generally less
than 0.5-1 m [3]. Significant waves heights up to about
3.4 m height, and about 7.5 second period, have been
recorded in the deeper water off the delta (Fisheries and
Oceans Canada station #108; http://www.meds-sdmm.dfo-
mpo.gc.ca/meds/Databases/Wave/).
Fig. 2. 2001 Landsat image of the Fraser River delta
showing the 2002 site (
=Norton) in 10 m water depth,
and the 2003 locations in 8 m (
=ADCP) and 0 m
(
=Norton) depths. Roberts Bank is outlined in yellow.
1235
presented at Oceans 2003, San Diego

Roberts Bank is located just south of where the main
(South Arm) Fraser River channel enters the Strait of
Georgia. The substrate of Roberts Bank is primarily sand
with a mean grain size of about 0.125 mm [4]. The sand
becomes finer-grained towards the shore.
It is generally thought that much of the slope of Roberts
Bank is being eroded, with the possible exception of the
northern portion of the Bank. There is evidence for sand
from the South Arm becoming incorporated in a clockwise
gyre across the north half of Roberts Bank [5]. Numerical
model results indicate the formation of a clockwise eddy on
the Bank during weak flood tides, apparently caused by the
Steveston Jetty [6]. The Steveston Jetty provides partial
protection from winter storm waves from the west-
northwest. The dominant tidal currents and breaking waves
can generate a net longshore current over Roberts Bank [7].
B. Winter 2002 Deployment on Roberts Bank
The Norton tripod was deployed in 10 m water depth off
the edge of Roberts Bank (
in Fig. 2 above). Previous
multibeam surveys show outcropping beds aligned roughly
north-south in the 5-15 m depth range, indicative of net
erosion [8]. Further offshore (20-30 m depths) were
subaqeous dunes aligned northeast-southwest,
perpendicular to the predominant direction of tidal flow.
The currents were tidally dominated with flood tides
(northerly) stronger than ebb, up to 1 m/s versus 0.4 m s
-1
respectively. The peak tidal flows re-suspended bottom
sediment on both the flood and ebb.
Several storm events were recorded with significant wave
heights up to 1 m, and 5 second peak periods. The
scanning sonar (set to 30 m range and default gain and
frequency settings) showed high suspended sediment
concentrations just trailing the storms and the emergence of
long wavelength (c. 10 m), low-relief, irregular patches [8].
C. Preliminary Results from the Winter 2003 Deployment
on Roberts Bank
The 2003 deployment of Norton was at a location on
Roberts Bank where bedforms were expected to consist of
small ripple features about 2 cm high with a 5-10 cm
wavelength. The Imagenex scanning sonar settings were
optimized for this deployment using tank tests.
A series of three ripples were created in the laboratory
using fine-grained sand to simulate the expected bedforms
on the Bank. Frequency and gain settings were then varied
until optimal sonar images were obtained (Fig. 3). Echo
returns at approximately 1.6 m range are from the tank
wall. The final settings chosen were: 1 MHz frequency, 5
m range, and 18 dB gain.
Fig. 3. Imagenex sonar image (left) of test ripples (about 1-
2 cm height, about 10 cm length) in tank. Red range rings
are 1 m increments. Three sand ripples are shown starting
at 1 m range (first red ring).
The Norton tripod was deployed on Roberts Bank (
in
Fig. 2) at low tide (Fig. 4 below).
Fig. 4. Deployment of Norton on Roberts Bank March 23,
2003 using the Canadian Coast Guard Hovercraft Siyay. A
radar reflector and flashing light were mounted on a pole
above the tripod.
A 1200 kHz RD Instruments WH ADCP™ with Waves
firmware was deployed in 9 m water depth just offshore of
the edge of the Bank (
in Fig. 2). The ADCP was to
measure the ‘offshore’ waves so that we could determine
the wave attenuation over the Bank. During the
deployment, winds were blowing from the northwest and
waves about 1 m high could clearly be seen breaking on the
edge of the bank, reducing in height as they moved onto the
Bank.
Using the settings previously determined in the laboratory,
the Imagenex scanning sonar head was suspended from the
tripod at a height of 45 cm above bottom.
1236
presented at Oceans 2003, San Diego

The scanning sonar often recorded radial patterns, with a
generally low reflectivity background (Fig. 7 below).
These images occurred frequently and nearly always during
a lower high tide (consecutive high tides are different
heights due to the diurnal inequality), and not when
successive low tides were of similar height (more purely
semi-diurnal tide). OBS levels (0.3 m off bottom) were
low at these times (about 10 FTU, up to 20-30 max).
Currents were generally weak with speeds of about
At the time of deployment (Fig. 5 below) the bedforms
consisted of sand ripples about 1-2 cm high, and about 5-10
cm wavelength, similar to the ones fabricated in the test
tank. The ripples were not continuous, nor linear, and far
more complicated than the ones created for the tank test.
20 cm s
-1
.
Fig. 5. Bedforms at time of deployment: ripples about 1 cm
high, with about 7 cm wavelength. Overall trend in ripples
was approximately northeast-southwest.
The first images from the scanning sonar (after the tide had
risen enough to submerge the instrument) show ripple-like
features, particularly in the northwest quadrant with
wavelengths of the same magnitude as seen visually (Fig.
6). The ripple features are oriented northeast-southwest.
The reversed-grey color option best shows the ripple
features.
Fig. 7. Radial-pattern sonar image (image 040909).
Another interesting occurrence was high suspended
sediment concentrations at diurnal time scales, centered on
the lower low tides. Sonar images such as the one below
were recorded just prior to, and immediately following, the
emergence of the sonar head from the water as the tide was
falling.
The suspended sediment signal from the OBS sensor 30 cm
above bottom clearly shows the diurnal nature of the signal
(Fig. 9 below). The ‘dropouts’ in the middle of the peaks
are when the sensor was in air at low tide. Waves from the
west added to the high OBS readings on the 21
st
.
Fig. 6. First image (image 03231900) from the scanning
sonar after the tide had submerged the sonar head. Tripod
legs and instruments show up darkest, with ripple-like
features evident, particularly in the northwest quadrant.
Parallel lines are used to highlight ripple features having a
NE-SW orientation.
Ripple-like features were occasionally evident on sonar
images at other times, but their characteristics (wavelength,
orientation, etc) did not vary significantly from those
observed initially. Several ‘storms’ occurred during the
deployment, but significant wave heights in the deepwater
increased to only about 1 m, and the sonar images
following the storms were not noticeably different than
before.
Fig. 8. High suspended sediment concentration during
moderate westerly winds and near low tide (image
04051200).
1237
presented at Oceans 2003, San Diego

Fig. 9. Uncorrected OBS readings from the sensor 30 cm
off bottom, showing high suspended sediment
concentrations at lower low water.
These OBS diurnal peaks occurred only during the lowest
low tides, the lower low tides that occurred when the
diurnal inequality was most pronounced. Lower low tides
are accompanied by strong ebb and flood currents, up to 50
cm s
-1
ebb and 125 cm s
-1
flood, as recorded by the ADCP
located in 8-10 m water depth off the edge of the Bank
(Fig. 10). Note that the flood tide associated with the
preceding higher low tide is negligible.
Data from the Norton sensors help explain the processes
causing these high suspended sediment values (Fig. 11).
Starting at high tide, suspended sediment values are low
and currents weak (near slack). As the tide ebbs and the
water level drops, the current speed increases to 40-50 cm
s
-1
, and suspended sediment levels increase. As low tide
nears, the water level falls below the level of the sensors
which are then exposed to the air for up to 7 hours. During
this period of low tide, we expect that suspended sediment
levels are low, due to the ‘slack’ current. As the tide turns,
the flood waters submerge the sensors. The flood tide is
already running fairly quickly, and OBS levels are high.
As slack high tide is approached, the current weakens and
OBS levels fall.
The high suspended sediment levels occur during the
strong ebb/flood currents associated with the lowest of the
low tides. Sediment re-suspension occurs when the current
speed (as measured by the sensor 80 cm above bottom)
reaches about 30-40 cm s
-1
. This is consistent with a 25 cm
s
-1
threshold current for re-suspension of fine sand
(0.125 mm), based on the Hjulstrom Curves.
Fig. 10. ADCP current profile data for April 22, 2003 (north component of velocity is in mm s
-1
). The speed direction time
series for a mid-depth is shown in the lower portion, along with the pressure time series.
1238
presented at Oceans 2003, San Diego

Citations
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Patent
20 Dec 2012
Abstract: Apparatus and methods useful in surveying to provide information rich models. In particular, information not readily or possibly provided by conventional survey techniques can be provided. In some versions targets provide reference for baseline positioning or improving position information otherwise acquired. Scanning may be carried out in multiple locations and merged to form a single image. Machine mounted and hand mounted scanning apparatus is disclosed.

58 citations


Journal ArticleDOI
Abstract: Waves and currents were monitored over a range of incident forcing between March 23 and April 24, 2003, on Roberts Bank, the sandy intertidal portion of the Fraser River Delta. A comparison of the dimensionless, current-induced skin friction with the critical skin friction for the initiation of sediment motion suggests that the currents are only capable of entraining sediment briefly with the ebbing tide or when enhanced by the wind. Since these wind-generated currents are associated with storm waves, which typically exceed the critical skin friction, they have a disproportionately large impact on the direction of the sediment transport. An energetics-based model, driven by locally measured near-bottom currents, is used to characterize the rate and direction of bedload and suspended load transport. The largest transport rates were predicted in response to storm waves and were initially directed onshore with weak oscillatory transport and alongshore by wind-generated currents that turned offshore ...

13 citations


Cites background or methods from "A configurable multi-sensor tripod ..."

  • ...Wave, current, and suspended sediment concentration sensors were mounted on a Norton tripod platform (Birch et al., 2003), hereafter referred to as the bank station....

    [...]

  • ...Despite the low frequency of onshore winds, uv analysis in this same time period suggests that 80% of offshore wave energy approached from the SSW and NW (Birch et al., 2003), with time-averaged significant wave heights of 0.17 m and 0.24 m, respectively....

    [...]

  • ...Based on uv analysis, Birch et al. (2003) found that waves at the peak frequency approached the bank station from the WSW, even during the April 7 storm when winds were offshore....

    [...]


Proceedings ArticleDOI
09 Nov 2004
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

10 citations


Proceedings ArticleDOI
14 Jul 2006
Abstract: A fully three-dimensional, nested grid, integrated circulation-wave-sedimentgeomorphology numerical model, COCIRM-SED, was developed to study sediment dynamics on Roberts Bank, Fraser River foreslope, Strait of Georgia, Canada. Roberts Bank is an extensive intertidal zone, located just south of where the main (South Arm) Fraser River channel enters the Strait of Georgia. The substrate of Roberts Bank is primarily silty sand, and a total of four grain size classes were considered in the model. The study area of interest is modeled using a fine grid size of 100 m by 100 m, and is nested within the much larger domain of the Strait of Georgia with a coarser grid size of 500 m by 500 m. These two model domains are solved together at every time step using a fully dynamic and two-way connection scheme. Following initial testing with idealized analytical cases, the COCIRM-SED modeled flows and suspended sediment concentration were calibrated and verified using the data obtained with a configurable multi-sensor tripod system on the Bank and at the edge of the Bank, respectively. These field data, including month-long ADCP current profiles, OBS sediment concentrations, and directional wave measurements, are used to examine the model performance. The preliminary model results provide improved understandings of the sediment dynamics on Roberts Bank.

4 citations


ReportDOI
01 Jan 2007
Abstract: Between 1 March and 26 March 2002, a wave and current measurement station was deployed on the upper delta slope of Roberts Bank in approximately 12 m of water. The field data were analyzed and used in a parabolic wave model to determine wave characteristics for fair-weather, moderate, and storm conditions. The model shows that storm waves act nonuniformly along the beach depending on the local morphology and tidal elevation. Waves propagating during high tide are characterized by smooth dissipation and progressive refraction, whereas waves propagating during low tide show intense refraction and breaking in front of or over a low-tide bar. Sediment transport divergence in the nearshore is strongly controlled by the temporal relationship between storm waves and tidal height. Resume : Entre le 1er et le 26 mars 2002, une station instrumentee a ete deployee sur la partie superieure de la pente deltaique du banc Roberts, dans approximativement 12 m d’eau, afin de mesurer la houle et les courants. Les donnees de terrain ont ete ensuite analysees et utilisees dans un modele parabolique de houle afin de determiner les caracteristiques des houles pendant les conditions de beau temps, moderees et de tempete. Le modele montre que les houles de tempete agissent non uniformement le long de la plage selon la morphologie locale et le marnage. La propagation de la houle a maree haute est caracterisee par une legere dissipation et une refraction progressive tandis que les houles se propageant a maree basse montrent une intense refraction et deferlent sur le front ou au-dessus d’une barre de maree basse. La divergence du transport sedimentaire dans la zone littorale est fortement controlee par la relation temporelle entre les houles de tempete et le niveau de la maree. Current Research 2007-A11 1 S. Meule et al.

2 citations


Cites methods from "A configurable multi-sensor tripod ..."

  • ...Field measurements and data processing A Norton instrumented tripod (Birch et al., 2003) was deployed at Roberts Bank in approximately 12 m water depth from 1 to 26 March 2002....

    [...]

  • ...Field measurements and data processing A Norton instrumented tripod (Birch et al., 2003) was deployed at Roberts Bank in approximately 12 m water depth from 1 to 26 March 2002....

    [...]


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Abstract: THOMSON, R.E. 1981. Oceanography of the British Columbia coast. Can. Spec. Publ. Fish. Aquat. Sci. 56: 291p. This book deals with the physical aspects of the sea as exemplified by the Pacific Ocean and the contiguous waters of the British Columbia coast. Although principally devoted to waves, currents, and tides, the book spans a broad spectrum of topics ranging from meteorology and marine biology to past and present marine geology. It attempts to elucidate the nature of oceanic motions and to relate them to everyday experience for the general interest of the casual reader and for the practical benefit of the professional mariner, scientist, or engineer.

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Journal ArticleDOI
Abstract: Large wave ripples and upper-plane bed sheet flow were observed under combined waves and currents during storms at 39 m water depth on Scotian Shelf. These data show that the critical wave mobility number and Shields parameter for sheet flow are not constant, but rather their values decrease with an increase in grain size. The critical sheet-flow Shields parameter under combined flows is found to be about 50% smaller than that for pure waves. Though the bed shear stress based on bedload roughness or wave parameterization using the one-tenth largest waves both offer adequate resolution to this difference, a definitive explanation does not exist at present. The large wave ripples (LWR) observed under combined flows are generally symmetrical, rounded and 3-dimensional, with small current ripples superimposed. The wavelengths of these LWR are approximately half of the wave orbital diameter and their average steepness is about 0.1. Analyses of storm processes on continental shelves show that as a storm builds up, ripples generally change directly into upper-plane beds. LWR generally form following the peaks of storms due to sediment fall out from suspension and moulding of the seabed by the long wave oscillation. If the spin up of a storm is gradual and strongly wave dominant, however, LWR can also develop from small ripples before sheet-flow conditions are reached. We interpret the three-dimensional, mound-like, low-relief LWR as combined-flow hummocky megaripples. This suggests that hummocky megaripples and hummocky cross-stratification (HCS) on continental shelves are formed under wave-dominant combined flows, and that they can form in medium sand as well as in silt and fine sand. The application of the combined-flow boundary layer model of Grant and Madsen (1986) and a modified Rouse suspension equation shows that the incorrect prediction of the presence of ripples in place of upper-plane bed causes over-estimation of shear velocities and suspension concentration and a 300% over-prediction of the suspended sediment transport rate. By contrast, failing to predict correctly the formation of LWR causes an under-estimation of shear velocities and sand resuspension, and will result in under-prediction of the suspended sediment transport rate by nearly one order of magnitude.

71 citations


Journal Article
Abstract: @. ABSTRACT I BARRIE, J.V. and CURRIE, R.G., 2000. Human impact on the sedimentary regime of the Fraser River Delta, Canada. Journal of Coastal Research, 16(3), 747-755. West Palm Beach (Florida), ISSN 0749-0208. The Fraser River Delta is a Holocene, river dominated feature in a macrotidal environment, built into the deep (>300 m) waters of the Strait of Georgia on Canada's Pacific coast. The delta has been modified to provide port facilities and a navigable channel for the city of Vancouver. Prior to the confinement of the Fraser River to its present channels, during the early part of this century, the distributary river channels regularly switched and migrated across the entire delta front. The annual river load (approximately 17.3 X 106 tonnes) is 65% silt and clay, and 35% sand. Today, most of the sand is removed from the system by dredging and the mud is transported in a plume past the intertidal estuary and northwards into the basin by the dominant flood tidal flow. Three causeways that cross the intertidal zone to the delta foreslope act as barriers to the dominant northward sediment transport causing estuarine and localized seabed erosion. The presence of the causeways results in tidal flow separation with clockwise back eddies forming behind the structures focusing the tidal energy to the intertidal zone. On the delta foreslope, off the southern causeways, an eroded submarine distributary channel subaqueous fan complex has been exposed by enhanced tidal flows that scour the seabed and form northward migrating subaqueous dunes increasing the delta slope and, consequently, the risk of slope failure.

27 citations


Proceedings ArticleDOI
D. Heffler1
23 Sep 1996
Abstract: A seafloor instrument, RALPH, has been developed and used at the Geological Survey of Canada (Atlantic). RALPH was designed to study sediment dynamics and the seafloor processes involved. Over the past 17 years, it has grown from a small tripod with a crude tape recorder to a system comprising 5 computers, 800 Mbytes of disk storage and many sensors, RALPH has been deployed in the intertidal zone and on the continental shelf. It is capable of continuous monitoring for several months. It logs time series of waves, tides, and near bed flows. It takes time lapse photographs of the seabed to monitor bedform migration and also images the seafloor acoustically. Suspended sediment is monitored acoustically and optically.

8 citations


"A configurable multi-sensor tripod ..." refers methods in this paper

  • ...The Geological Survey of Canada has had considerable success on Canada’s east coast using a platform termed Ralph [ 1 ], [2]....

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