Wave and current measurements in the St.Lawrence Estuary near Gros-Cacouna, Quebec : Implication on sediment transport (original) (raw)
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Continental Shelf Research, 2014
Multibeam sonar mapping and geophysical and geological groundtruth surveys were coupled with tidal current and sediment transport model calculations to investigate the sediment transport and formation processes of the complex seabed features off the Cape Split headland in the upper Bay of Fundy. The Cape Split banner bank, composed of coarse to very coarse sand, is a southwest-northeast oriented, large teardrop shaped sand body with superimposed sand waves that show wavelengths from 15 to 525 m and heights from 0.5 to 19 m. Isolated and chains of barchan dunes occur on top of a shadow bank to the southeast of the banner bank. The barchan dunes are composed of well-sorted medium sand and are oriented northwest-southeast. Their mean height and width are 1.5 and 55 m, respectively. A gravel bank, with an elongated elliptical shape and west-east orientation, lies in the Minas Passage erosional trough east of the headland to form the counterpart to the sandy Cape Split banner bank. The southern face is featureless but the northern face is covered by gravel megaripples. Tidal model predictions and sediment transport calculations show that the formation of the banner bank and the gravel bank are due to the development of the transient counterclockwise and clockwise tidal eddies respectively to the west and east of the headland. The formation of barchan dunes is controlled by the nearly unidirectional flow regime in outer Scots Bay. Sand waves on the flanks of the Cape Split banner bank show opposite asymmetry and the barchan dunes are asymmetric to the northeast. The tidal current and sediment transport predictions corroborate bedform asymmetry to show that sand wave migration and net sediment transport is to southwest on the northern flank of the banner bank but to northeast on the southern bank. Long-term migration of the Scots Bay barchan dunes is to the northeast. Spring-condition tidal currents can cause frequent mobilization and high-stage transport over the banner bank and barchan dunes. Strong currents in Minas Passage can cause infrequent lowstage transport over the megarippled northern face but are not high enough to mobilize the coarser gravels on the southern face of the gravel bank.
Two benthic landers were deployed behind the St. Georges Bank, a linear sand bank located in proximity of the northern bank of the mouth of the Gironde Estuary, for several tidal cycles, to measure near bed processes. One of the main outcomes of the deployments is the role of the bank in modulating wave energy. The wave height measured behind the bank oscillates in response of the tidal variations (i.e. higher waves coincide with high water). Therefore, the bank acts as a natural offshore breakwater, protecting the beaches behind it. Man-made activities such as dredging could have an impact on the shoreline, therefore should be avoided unless a proper impact assessment is undertaken.
Estuarine, Coastal and Shelf Science, 2011
Field measurements were conducted in Mont-Saint-Michel Bay, a megatidal embayment (spring tidal range of 15 m), in order to monitor, over the course of a tidal cycle, sediment transport variability due to waves and tides on the upper part of a tidal flat characterised by shallow water depths. Sensors used to measure currents, water depth and turbidity were installed just above the bed (0.04 m). Two experiments were conducted under contrasting hydrodynamic conditions. The results highlight wave activity over the tidal flat even though observed wind waves were largely dissipated due to the very shallow water depths. Very high suspended sediment concentrations (up to 6 kg/m 3 ) were recorded in the presence of wave activity at the beginning of the local flood, when significant sediment transport occurred, up to 7 times as much as under conditions of no wave activity. This influence may be attributed to the direct action of waves on bed sediments, to wave-induced liquefaction, and to the erosive action of waves on tidal channel banks. The sediment composition, comprising a clay fraction of 2e5%, may also enhance sediment transport by reducing critical shear stress through the sand lubrication effect. The results also show that antecedent meteorological conditions play an important role in suspended sediment transport on the tidal flat. Total sediment flux directions show a net transport towards the inner part of the bay that contributes to deposition over the adjacent salt marshes, and this tendency also prevails during strong wave conditions. Such sediment transport is characterised by significant variability over the course of the tidal cycle. During fair and moderate weather conditions, 83% and 71% of the total flux was observed, respectively, over only 11% and 28% of the duration of the local tidal cycle and with water depths between 0.04 and 0.3 m. These results suggest that in order to improve our understanding of sediment budgets in this type of coastal environment, it is essential to record data just at the beginning and at the end of tidal submergence close to the bed.
Effects of Tide on Waves in the Outer Seine Estuary and the Harbor of Le Havre
Coastal Engineering Proceedings, 2012
The present study examines the influences of time-varying tide-induced water depths and currents on waves in the outer Seine estuary (southern central English Channel) and their penetration in the harbor of Le Havre and its new infrastructures Port 2000. The investigation is based on a numerical procedure which links regional phase-averaged wave modules with a local phase-resolving wave module within Port 2000 harbor. Required spatio-temporal evolutions of tidal free-surface elevation and current are computed by circulation modules. Numerical results of wave height are compared with field data collected at three wave buoys in the access harbor channel and its inner basin. Predictions exhibit a local increase (up to 30 %) of wave height induced by current refraction at slack tide in the access harbor channel. Respective mappings of the wave height modified by the tide, the water levels alone and the currents alone confirm this finding. The effect of currents on waves are pronounced a...
Effects of Tidal Power Generation on Hydrodynamics and Sediment Processes in the Upper Bay of Fundy
The upper Bay of Fundy in Nova Scotia is known for its large tidal range, high current velocities and extensive intertidal flats with fine sediments. This region is actively being studied as a candidate for in-stream tidal power generation using an array of large turbines on the seabed. We present observations of current velocities in Minas Passage and Minas Basin that, in combination with total suspended matter estimates from the satellite-based MERIS sensor, were used to characterize the system and calibrate a hydrodynamic and sediment transport model. This has elucidated strong trends in surficial suspended sediment concentrations in the central part of Minas Basin on the seasonal timescale, ranging from less than 10 g/m 3 in the summer to 10-30 g/m 3 in the winter. Model results suggest that lower sediment concentrations in the water column may occur because critical bed shear stress values are higher, due to biological activity in the sediments in summer. This result emphasizes the sensitivity of the system to flow and sediment properties that undergo seasonal changes. Using increased flow drag to simulate in-stream turbines in Minas Passage, the model was applied to turbine array scenarios to determine possible farfield effects of tidal energy dissipation in Minas Passage on sediment processes throughout Minas Basin.
An intercomparison of electromagnetic and acoustic current meters in a shallow estuary
1999
The authors describe and compare the performance of a well known electromagnetic current meter and two types of acoustic Doppler current meters. Two Aanderraa Instruments DCS3500s were separated by 84 cm and deployed on a mooring between two Interocean S4s and allowed to collect data for 145 hours in a shallow (10 m depth) site in Eastern Long Island Sound. Contemporaneously, two 300 kHz R.D. Instruments Workhorses were deployed on the bottom 30 m to the north and south of the mooring. The results of the intercomparison experiment showed that: (1) there were intermittent, unreasonably large amplitude spikes in the velocity records obtained by the DCS3500, and (2) that once the data spikes were removed and the vertical sheer of the tidal flow has been accounted for, the mean difference between the DCS3500 and S4 observations was within the range expected
Bathymetric influences on tidal currents at the entrance to a highly stratified, shallow estuary
Continental Shelf Research, 2013
Bathymetric effects on tidal currents are investigated at Main Pass, which is the primary inlet of Mobile Bay, Alabama. A 12-h ship-mounted ADCP survey, which covered nearly one-half of the diurnal tide during flood conditions, included 24 repetitions. The resulting velocity data demonstrate significant tidal variability in the horizontal and vertical current structure between the ship channel and the shoals. The diurnal tidal flows, the dominant tidal forcing, are 721 (4.8 h) ahead of the water level throughout shallower areas of Main Pass, indicating near-standing wave conditions. Moving across the mouth, a phase lag (5.371 or 20 min) develops with the deep channel tidal currents lagging the shoal region. The vertical tidal structure is also modified across the mouth where near-bottom flows change their direction first in the ship channel, while near-surface flows change their direction first over the western shoal. This may be related to the seaward pressure gradient associated with the relatively large (~1715 m 3 /s) freshwater discharge or the discharge interaction with a nearby opening, Pass-aux-Herons. Current magnitudes over the shoals and in the ship channel vary by as much as 1 m/s. Flows at the east side of Main Pass, close to Mobile Point, behave oppositely to those in the rest of the transect during the survey. This inconsistent flow pattern is caused by an anticyclonic eddy that is triggered by flow separation at Mobile Point.
Tide and wave dynamics on a sand bank from the deep shelf of the Western Channel approaches
Marine Geology, 1999
The sedimentary response of deep shelf sediments to the interplay of tides and waves, based on the example of the Celtic Sea sand banks is described. Grab samples and 1500 km of multibeam and side scan data have been collected from the 300 km 2 densely surveyed Kaiser bank area. The bank is 140-170 m deep, 30 m high and 60 km long, oriented perpendicular to the shelf edge and nearly parallel to the major axis of the tidal ellipse. Surficial sediments of the bank consist of medium to gravely biolithoclastic sands. They are swept by tidal currents that reach 0.9 mrs 1 m above the seabed, and under the occasional influence of waves. The mobile sands are commonly bedformed and rest on a highly backscattering lag. The flanks below y140 m mainly exhibit transverse tidal dunes and sand ribbons, whereas the top of the bank mostly displays discontinuous sand patches and wave ripples. The sand patches are interpreted as the remnants of tidal bedforms reworked by waves. Calculations of the threshold of motion of four modal grain sizes under various conditions at the seabed show that tidal bedforms are active during spring tides sometimes associated with waves, whereas only the largest annual waves may explain the observed wave ripples. As in active tidal banks, there is a reversal of tidal bedload transport from one bank side to the other. The sedimentation rate is very low on the Kaiser bank. In addition, there is a loss of sediment at both bank extremities and the tidal bedload cross-bank transfer is very small, probably controlled by shelf residual currents and long-term drift of sediment resuspended by waves. However, the present-day action of waves at the bank top is less intense