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The vertical structure of downslope velocity within sub-aqueous gravity-driven flows of (smoother) glass beads and (rougher) natural sand is investigated for both fixed roughness and erodible beds using high-resolution, MHz-frequency acoustics. The observed velocity profiles within the O(1) cm thick, O(10) cm/s flows exhibit a negative shear layer extending downward from the sediment–water interface to a velocity maximum at ∼ 9 grain diameters depth within the layer, below which the velocities decrease to near-zero values at the pre-flow bed location for fixed roughness beds and to non-zero values for mobile beds. The attenuation of sound transmitted through the moving layer is used to constrain the depth-averaged solids concentration to a value of ∼ 0.52. The observed negative shear at the interface indicates that, unlike the sub-aerial case, interfacial friction is dynamically important in gravity-driven sub-aqueous granular flows. It is shown that the observed vertical structure of velocity within the layer can be well represented by continuum viscous flow models. Solids concentration and effective viscosity are estimated from the best-fit model parameters using the Zarraga–Hill–Leighton (2000) empirical relation for suspensions of negatively buoyant particles, yielding vertically averaged values ∼ 0.57. While the sub-millimeter vertical resolution of the measurements is too coarse to provide precise estimates of the friction velocity at the interface, the model-data comparisons nevertheless indicate that the vertical structure of the downslope flow consists of a weakly stratified dense layer and a thin, dilute transition layer between the dense flow and the overlying water. [ABSTRACT FROM AUTHOR]

Wake measurements are critical for quantifying the hydrodynamic effects of tidal energy extraction and for designing arrays of tidal turbines. In this investigation field measurements from the wake of PLAT-I, a commercial-scale floating array of four tidal turbines, are presented. Instrumented surface-following drifters are used to measure velocity and turbulence in the wake of PLAT-I while deployed in Grand Passage, an energetic tidal channel in the Bay of Fundy, NS, Canada. Field measurements were obtained when the turbines where fully operational under optimal conditions during spring tides. The collected data are used to construct tridimensional maps of acoustic backscatter, mean velocity, vertical velocity variance and turbulent kinetic energy dissipation rate downstream of the platform at different stages of the tide. The operational turbine array wake was captured for ebb and flood tides. For both ebb and flood, reduced velocities are observed in the upper water column at the depths spanned by the turbine rotors. Vertical profiles of velocity mix quickly during flood tide but velocities remain slower than the undisturbed flow outside of the wake up to 20 effective diameters downstream of the platform. In contrast, during ebb, the velocity deficit persists farther downstream, and the reduced velocity increases slightly. Elevated turbulence levels peak at 2 effective diameters downstream of the turbines and recover to undisturbed flow levels at approximately 10 effective diameters downstream of the platform for both ebb and flood tides. Total energy dissipated through turbulence within the wake is estimated to be less than 10% of the energy extracted for electricity production. • Field measurements of the wake from an operational commercial scale floating tidal turbine array. • The wake is confined to the upper water column during both ebb and flood tides. • Wake velocity deficit recovers faster during flood tide than during ebb tide • Wake recovery occurs beyond 20 effective diameters downstream of the turbines. • Wake elevated turbulence intensities recover quickly for both ebb and flood tides [ABSTRACT FROM AUTHOR]

In this investigation, instrumented mobile platforms are used to spatially map the turbulent flows in Grand Passage, one of the Bay of Fundy's more energetic tidal channels in Nova Scotia, Canada. The aim is to characterize the flow around the PLAT-I floating tidal energy platform developed by Sustainable Marine Energy Canada (SMEC). GPS-tracked surface drifters equipped with fast-sampling acoustic Doppler current profilers (ADCPs) provide turbulence-resolving vertical profiles of velocity and turbulent kinetic energy dissipation rate along drifter trajectories, while vessel-mounted ADCP transects complement the mean flow velocity measurements. These data are used to construct tridimensional quasi-synoptic maps of mean velocities and turbulence parameters for several stages of the tide around PLAT-I's location including peak ebb and flood currents. The data set includes measurements under natural flow conditions, and while the turbines installed on PLAT-I were both not operational and operational. The measurement techniques and resulting maps successfully capture the spatial and temporal structure of the flow for unsteady conditions for various tidal conditions. The combined wake of the four 6.3 m operational PLAT-I turbines was measured for a single tidal stage. Closer to the turbines, the vertical extent of the wake is about 6 m (approximately one turbine diameter), increasing with distance downstream, while the maximum observed velocity deficit is 26% relative to not operating conditions, decreasing with distance downstream. [ABSTRACT FROM AUTHOR]

Acoustic reflection coefficients are reported for water-saturated granular media at frequencies from 1.2 to 2.0MHz using a narrow-beam broadband transducer in a monostatic geometry at near-normal incidence. Natural sand and glass beads with median grain diameters ranging from 0.22 to 0.40mm were used. For each granular medium, bed elevation and root-mean-square roughness were measured using side-on photographs of the sediment-water interface. The probability density distributions of the bed elevations are Gaussian. The roughness parameter is close to 1, indicating that the reflected pressure field is mainly due to coherent scattering. The probability distribution of the observed reflection coefficients is nearly Gaussian, consistent with the predictions from a coherent single-scattering model. The horizontal decorrelation length of the observed reflection coefficients is˜4 mm, with no consistent dependence on either frequency or grain size, and approximately equal to 20% of the transducer diameter. This behaviour, which is reproduced by the single-scattering model, is due to speckle. The size/frequency-dependence of the reflection coefficients are well described by Eckart’s [(1953). J. Acoust. Soc. Am. 25(3), 566–570] prediction for a rough surface with Gaussian-distributed surface elevations. Comparisons are made to previously reported reflection coefficient measurements. [ABSTRACT FROM AUTHOR]

Measurements of the phase velocity of compressional sound waves in water-saturated granular materials are reported for the 1.0–2.0 MHz frequency range. The sound speed estimates are based on travel times through granular layer thicknesses ranging from 8 to 17 mm. Three types of granular media were used: 336 μm median diameter glass beads and two natural sands with median diameters of 219 and 406 μm. These grain sizes and frequency range correspond to 0.5 < k a < 1.2 , where k is the wavenumber and a the grain radius. To remove trapped air, the samples were boiled under pressure before transfer to the measurement tank. The results are compared to previously reported experimental results and to the Schwartz and Plona [J. Appl. Phys. 55(11), 3971–3977 (1984)] multiple scattering prediction, confirming negative dispersion for ka > 0.5. Scaling the data by a factor depending on porosity and grain density reduces the spread among the available phase speed estimates by nearly a factor of 2, from 12.5% to 6.9%. [ABSTRACT FROM AUTHOR]

Attenuation and group speed measurements are reported for water-saturated granular materials (natural sand and glass beads) at frequencies of 1.0 to 1.8 MHz. Median grain diameters were 219 to 497 μm, corresponding to k d ≳ 1 , i.e., the scattering regime. The measurements were made for different thicknesses of sediment resting on a reflective surface using a monostatic geometry. The attenuation estimates compare well with previously reported experimental results and to the predictions of multiple scattering theory, confirming in particular the tendency toward f4 dependence for k d ≳ 1. Group speed estimates exhibit the negative dispersion predicted by theory and are comparable in magnitude to previously reported measurements made using transmission geometries. It is found that the available data exhibit a O(10)% spread among the sound speed measurements at a given kd value, and that this spread is reduced to 2.2% when the data are scaled by a factor dependent on porosity and grain density, and that essentially all of the reduction can be attributed to differences in porosity. [ABSTRACT FROM AUTHOR]

This study characterizes the turbulence in a 3.6 m s −1 tidal channel in the Bay of Fundy, Nova Scotia that has been identified for development as a tidal power resource. A horizontally aimed fast-sampling single beam acoustic Doppler profiler was deployed on a subsurface buoy, and the flow-aligned profile is used to test the cross-spectral (Garbini 1982a,b) and the spatial structure function methods for estimating dissipation rates of turbulent kinetic energy. Second and third order structure functions computed over separation distances in the mean flow direction vary with spatial scale as predicted by the Kolmogorov hypotheses. Cross-spectra from adjacent spatial bins have a noise floor an order of magnitude lower than the corresponding auto-spectra and phase lags over increasing spatial ranges demonstrate the advection of coherent structures by the mean flow. The spectral and structure function estimates of dissipation rate vary linearly with U 3 and agree to within a factor of 2.5 on ebb and 6 on flood, the spectral estimates being larger. Dissipation rates are an order of magnitude higher on ebb than flood tide, likely due to asymmetry in the local topography. [ABSTRACT FROM AUTHOR]

The short-pulse acoustic backscatter amplitude calibration technique of Dragonette, Numrich, and Frank [(1981), J. Acoust. Soc. Am. 69, 1186–1189] is evaluated for MHz frequency systems, as are used to measure environmental suspended sediment concentration. The technique involves measuring backscatter from a short transmit pulse reflected by a solid spherical target, and treating the first received reflection as equivalent to that from a perfectly rigid sphere. In this implementation, the target sphere is 16mm diameter tungsten carbide. The target is mounted on a stiff support rod rather than being suspended on a filament, enabling precise positioning of the sphere relative to the transducer. That the support rod does not significantly influence the calibration is demonstrated by reconstructing the backscatter time series, including the sequence of pulses from surface (Rayleigh) waves, using elastic scattering theory. Calibration results are verified in a separate experiment in which the backscatter from dilute aqueous suspensions of 400–600 lm diameter polystyrene beads in water is measured within the 1–2MHz frequency band. After correcting for the additional attenuation within the scatterers, the resulting values of the backscatter form factor are within 20% of the theoretical predictions. [ABSTRACT FROM AUTHOR]

Spectral and structure function methods are implemented to compute the dissipation rate e from broadband, diverging-beam acoustic Doppler current profiler (ADCP) data collected at four sites in a high-flow tidal channel. This paper shows that middepth estimates of ϵ obtained from spectral and second-order structure function (SF2) methods are both lognormally distributed with comparable means and variances. Speed bin-averaged e values agree to within 16%, depending on the site and tidal phase (ebh/flood). The close agreement between the two independent methods provides further support for the argument put forward by McMillan et al.: that is, that the factor-of-2 difference between shear probe and (spectral) ADCP estimates of e was likely caused by spatial differences in turbulence levels. The agreement between the spectral and both second- and third-order structure function methods also supports the use of C2 = 2.0 for the SF2 universal constant. Notably, however, the SF3 method was less robust for these data. Two additional aspects of the SF2 approach are examined in some detail: 1) the differences from upstream- and downstream-facing beams are shown to arise from the Reynolds stress and 2) the inability of the ADCP to resolve small-scale motions does not affect the estimates of ϵ but yields apparent Doppler noise levels that--counterintuitively--decrease with increasing flow speed and increasing dissipation rate. A modified SF2 method that accounts for the variance associated with the unresolved scales removes the flow speed dependence and yields noise level estimates that agree with the spectral values. [ABSTRACT FROM AUTHOR]

Natural tracer pebbles and cobbles were deployed on a steep (slope: 0.1) mixed sand and gravel mega-tidal beach located at the head of the Bay of Fundy. Two experiments were carried out to investigate the response of natural pebble- and cobble-sized material, both to energetic wave conditions during individual storm events, and to tidal-only forcing conditions between storms. Net displacement of different tracer sizes was the focus of the first experiment, while the effect of different shapes was tested in the second. The pebbles and cobbles were painted in fluorescent colors, labeled, and their positions measured at low tide over periods of 10–12 days. The tracers were emplaced at mid-tide level on the beach face at low water, and later recovered and re-deployed on successive low tides. Net travel distance and direction were measured upon recovery. Hydrodynamic forcing conditions and local morphology at mid-tide level on the beach face were monitored continuously during the experiments. In the first experiment, recovery rates were 100% during calm and moderate wave conditions ( H s ≤ 0.5 m), but decreased significantly in more energetic wave forcing ( H s up to 1 m). Recovery rates were lower for smaller tracers. During a three day wind-wave event, net displacements of up to 48 m were measured. Comparable net displacements were observed for all tracers. Maximum displacements were 39 m onshore, 42 m offshore, and 46 m alongshore. In calm conditions, maximum displacements were less than 1 m, indicating that the bottom stresses due to tidal currents alone are too low to cause significant tracer displacement on time scales of weeks, despite the high tidal range. In the second experiment, recovery rates were lower, due to more energetic wave forcing: significant wave heights reached H s >0.5 m on 80% of the particle tracking days and H s >1 m on 30% of the days. Flat pebbles and cobbles reached the longest transport distances, with a maximum net displacement of 98 m. The flat tracers were observed to be flipped by the shore break and to “ride” the swash bore, and were in this manner transported long distances, shoreward up to 66 m. In both experiments, the directions of alongshore displacements were consistent with deviations of the angle of wave incidence from the shorenormal. Tracer burial and re-exposure was observed – via rotary imaging sonar – with the evolution and migration of m-wavelength ripples. The study allowed a direct correlation of pebble and cobble tracer displacements to hydrodynamic forcing during individual storm events, with consideration of tracer particle size, and shape. [ABSTRACT FROM AUTHOR]

The ability to estimate the rate of dissipation ( ε) of turbulent kinetic energy at middepth in a high-speed tidal channel using broadband acoustic Doppler current profilers (ADCPs) is assessed by making comparisons to direct measurements of ε obtained using shear probes mounted on a streamlined underwater buoy. The investigation was carried out in Grand Passage, Nova Scotia, Canada, where the depth-averaged flow speed reached 2 m s−1 and the Reynolds number was 8 × 107. The speed bin-averaged dissipation rates estimated from the ADCP data agree with the shear probe data to within a factor of 2. Both the ADCP and the shear probe measurements indicate a linear dependence of ε on the cube of the flow speed during flood and much lower dissipation rates during ebb. The ebb-flood asymmetry and the small-scale intermittency in ε are also apparent in the lognormal distributions of the shear probe data. Possible sources of bias and error in the ε estimates are investigated, and the most likely causes of the discrepancy between the ADCP and shear probe estimates are the cross-channel separation of the instruments and the high degree of spatial variability that exists in the channel. [ABSTRACT FROM AUTHOR]

A methodology is developed and tested for simultaneously extracting time-resolved one-dimensional profiles of the mass-concentration and velocity of two different particle types in a mixed suspension, using a multi-frequency pulse-to-pulse coherent Doppler instrument. The technique involves inversion of a model for frequency-dependent acoustic backscatter amplitude and phase. Results are presented from a laboratory settling column experiment, measuring a mixture of polystyrene beads (slowly-settling, strongly-scattering) and glass beads (quickly-settling, weakly-scattering) in a vertical pipe section. [ABSTRACT FROM AUTHOR]

Abstract: The vertical structure of flow and stress within turbulent oscillatory boundary layers above evolving sand ripples is investigated in experiments carried out using a prototype wide-band coherent Doppler profiler in an oscillating boundary facility with beds of 0.216mm median diameter sand, 10s oscillation period, and 0.9m excursion (Shields parameter=0.11). The bed evolved from an initial nominally flat state through 5cm wavelength, nearly two-dimensional ripples to 12cm wavelength, three-dimensional ripples. Average ripple height increased from 0.2cm to 1.4cm, corresponding to roughness Reynolds numbers between 68 and 7700. Average ripple steepness increased from 0.03 to 0.15. Bed elevation spectra exhibit the shift towards lower spatial frequencies observed by and, at higher spatial frequencies, a saturation range analogous to that in surface gravity wave spectra. Horizontal velocity profiles exhibit the phase lead and overshoot expected for oscillatory boundary layer flow. Bottom stress estimates are obtained from the acceleration defect, the Reynolds stress and the law-of-the-wall. The defect stress estimates are bounded above and below by the Reynolds stress and the law-of-the-wall estimates, respectively. The values of the bottom friction coefficient and hydraulic roughness from the defect stress estimates are consistent with results from previous work on equilibrium orbital-scale ripples, as summarized by , indicating that ripple evolution was quasi-steady. [Copyright &y& Elsevier]

Sound scattering by solid particles suspended in a turbulent jet is investigated. Measurements of the scattered amplitude were made in a bistatic geometry at frequencies between 1.5 and 4.0 MHz, and at scattering angles from 95° to 165° relative to the forward direction. Two types of particle were used: nearly spherical lead-glass beads and aspherical natural sand grains. For each particle type, experiments were carried out using ∼200 and ∼500 μm median diameter grain sizes, corresponding to 0.7≤=ka≤=4. The sphericity of the sand grains, defined as the ratio of projected perimeter size to projected area size, was 1.08. The lead-glass bead results are consistent with an elastic sphere model. A rigid movable sphere model provides the best fit to the sand data, and the best-fit diameter is within 4% of the equivalent volume size. However, the scattering pattern for sand is systematically smoother than predicted: that is, the undulations in the angular scattering pattern predicted by spherical scatterer theory are present, but muted. This observed departure from spherical scatterer theory is attributed to disruption of the interference among creeping waves by the irregular surfaces of natural sand grains. [ABSTRACT FROM AUTHOR]

Theoretical solutions for the wave bottom boundary layer (WBL) over a sloping bed are compared with field measurements in the nearshore zone. The WBL theory is constructed using both viscoelastic-diffusion and conventional eddy viscosity turbulent closure models. The velocity solutions are then matched with those of the interior flow, given by Chu and Mei potential theory for surface gravity waves over a sloping bottom. The field measurements were obtained with a coherent Doppler profiler over a 2° bed slope. Results are presented for both flat and rippled bed conditions, the latter being characterized by low steepness, linear transition ripples. Close to the bed, the observed velocity profiles change rapidly in amplitude and phase relative to potential flow theory, indicating the presence of a wave boundary layer with a thickness of 3-6 cm. The observed velocity and shear stress profiles are in good agreement with the theory. The sloping bottom has significant effects on the vertical velocity, but not on the horizontal velocity and shear stress. Bottom roughness and friction velocity are estimated from optimizing the model-data comparisons. The friction velocities and wave friction factors are found to be consistent with values obtained from the momentum integral method and from the nearbed turbulence intensity, and with Tolman's semiempirical formulation. [ABSTRACT FROM AUTHOR]

Wave friction factors are estimated from vertical profiles of near-bed turbulence and horizontal velocity spanning the wave bottom boundary layer. Measured values are partitioned by bed state, which ranged from irregular ripples to flat bed, and are examined as a function of two traditionally selected parameters—physical bed roughness and outer flow Reynolds number. The measurements are from two field experiments in very different nearshore environments: a relatively protected unbarred pocket beach and a linear barred beach exposed to the open shelf (Duck). Measured wave friction factors are remarkably similar for the two beaches and are highest for low-energy rippled beds and lowest for the high-energy flat bed conditions. The reduction in the friction factor for high-energy conditions corresponds to a decrease in the physical roughness of the bed as wave energy increases. As a function of relative roughness, measured friction factors are generally consistent with previous laboratory results and theoretical results for the high-energy cases, but the predicted values for the low-energy rippled beds show some significant differences. A new expression is derived for the bed roughness and is found to have a stronger dependence on ripple steepness than previously suggested laboratory-based relationships. Estimated friction factors exhibit a power-law dependence on Reynolds number and occupy a narrow band within the rough turbulent and transition regions. [ABSTRACT FROM AUTHOR]

A pulse-to-pulse coherent acoustic Doppler profiler has been developed for high-resolution particle velocimetry in the ocean, in particular for remote measurements of suspended sediment flux and turbulence in the nearshore and continental shelf bottom boundary layer. Acoustic backscatter estimates of suspended particle concentration and velocity are determined simultaneously from the phase and amplitude of the backscattered signal over an O(1 m)-long profile with subcentimeter resolution, at an ensemble-averaged rate of O(25 Hz). To characterize the performance of the profiler as a remote turbulent flux sensor, laboratory experiments were carried out in a particle-laden high--Reynolds number round jet. The results include comparisons between the acoustic Doppler and standard laboratory sensors, and to previous experimental and theoretical results for turbulent jets. The observed mean radial particle flux profile is found to be consistent with the computed mean flux profile for a turbulent jet. The measured entrainment rate is within 10% of the accepted value for turbulent round jets. Energy spectra of the turbulent motions demonstrate the 25/3 slope characteristic of the inertial subrange. The kinetic energy spectral densities obtained with the Doppler profiler match observations with a Sontek acoustic Doppler velocimeter and are in qualitative agreement with hotfilm measurements within the jet. Space--time domain images of particle flux exhibit well-defined coherent structures. Cospectral analysis demonstrates that these larger structures dominate the particle flux. [ABSTRACT FROM AUTHOR]

Field observations are presented of the morphological evolution of beach cusps on a 1:10 slope, megatidal (8–12 m range), mixed sand-gravel beach at the head of the Bay of Fundy, Nova Scotia. Cusps had mean wavelengths of 3–6 m and displayed pronounced horn/bay sediment size segregation, with sand-sized material in the bays and gravel-sized material in the horns. Cusp occurrence was limited to the upper third of the beach face. Shoreline position during the tidal cycle was estimated at three minute intervals from time-averaged video imagery. Three cusp events are examined in detail, two exhibiting pronounced topographic relief, and the third demonstrating sensitivity of the rate of cusp evolution to the beach surface grain size distribution. Forcing conditions were weak, with significant offshore wave heights of 10–20 cm and peak periods of 4–7 s. Relict cusp morphology was inundated with the rising tide and destroyed or reworked during high tide. New cusps formed during the falling tide through a combination of accretion at the horns and erosion in the embayments. Timescales of growth and decay were short, ranging from 10 to 30 min. The location and dynamics of cusp horns appeared to depend on the high water line and its location relative to any pre-existing cusp morphology. The apparent sensitivity of cusp formation timescales to the local grain size distribution suggests that size segregation is intrinsic to the process of mixed sand-gravel cusp evolution. • Timescales of initial beach cusp emergence were of the order of 10 min. • The very broad grain-size distribution affects cusp evolution timescale. • Positions of emergent cusps were influenced by pre-existing morphology. • Cusp evolution was modulated by the 10-m range tide. [ABSTRACT FROM AUTHOR]

Acoustic backscatter observations at 200 kilohertz were made of the buoyant plume from a submarine spring at a depth of 47 meters in Cambridge Fiord, Baffin Island. Vertical velocities of up to 37 centimeters per second are inferred from the ascent rates of discrete scattering structures in the plume. [ABSTRACT FROM AUTHOR]

This paper considers the problem of accurately measuring the sediment transport over bedforms where flow evolves continuously both in time and space. For this purpose, we have developed a pulse-to-pulse coherent Doppler sonar system designed in bistatic geometry with two fan-beam transmitters symmetrically positioned on each side of a multielement receive array. The system resolves 2D velocity components within a ±20° (~0.5m by ~0.5m) swath. The software-defined radio data acquisition and control system limited us at present to eight independent receiver channels, and consequently the azimuthal resolution of the systemis 48. As a preliminary test of the sonar system, the system operation was simulated using a model developed to predict coherent sonar performance. The uncertainties with respect to the prescribed values and mean measurements in the model results were confined to 0.35 and 0.23 cm s-1, respectively, in the presence of strong shear (~150 s-1) and 50 cm s-1 horizontal flow. An important thing is that the model allowed us to test and develop the signal processing algorithms necessary to invert the multibeam sonar data. Using sand of 0.4-mm median diameter, the laboratory trials were carried out in active sediment transport conditions over dunes with 2-m wavelength and ~0.90m s-1 unidirectional flow velocities. The results presented here focus mainly on 2D velocity field and indicate an average 4% deviation from the wake law and 8% from independent observations made with the wide-band multifrequency coherent Doppler profiler (MFDop) instrument under similar flow conditions. [ABSTRACT FROM AUTHOR]

Highlights • BBL environment in the axis of Barkley Canyon (−985 m) is stable on time scales >1d and <3y. • Epibenthic megafaunal community is composed of species adapted to low oxygen levels. • Two major species of the community, C. tanneri and A. fimbria , are of fishery interest. • Seasonal variations are observed in the epibenthic megafauna of the axis of the canyon. • Inter-annual trends in abundance of major species is linked with "the blob" event. Abstract Understanding the impact of the environment on temporal trends in the composition and abundance of deep-sea species is essential for forecasting evolution of the community in the context of climate change. The recent development of deep-sea observatories enables multidisciplinary studies of long duration and high temporal resolution. We used a platform at the Ocean Networks Canada NEPTUNE Observatory located in the axis of Barkley Canyon between June 2012 and January 2015 to: (1) characterize the megabenthic community and the environmental conditions in the canyon; (2) investigate temporal patterns in the faunal community; and (3) determine the influence of environmental conditions on the observed patterns. The megafaunal epibenthic community, which included commercially important species, was composed of species known to be able to adapt to low oxygen conditions. A strong seasonal pattern characterized wind speed and direction, wave height, temperature, and chlorophyll concentration at the sea surface. The megafaunal epibenthic community exhibited seasonal patterns which were influenced by in-situ benthic boundary layer currents and temperature, weather conditions and chlorophyll concentration at the sea surface. At inter-annual scales, the surface temperature anomaly event observed in 2013 in the region could potentially impact the community structure with significant changes in species' densities, highlighting the need for long-term monitoring in the region. [ABSTRACT FROM AUTHOR]

Results are presented from the acoustic remote sensing component of Dalhousie University’s interdisciplinary Coastal Ocean Observatory in Lunenburg Bay, Nova Scotia, operational since June 2002 (www.cmep.ca/bay). Remote acoustic measurements of water column velocity and backscatter are made from bottom-mounted instruments hard-wired to surface buoys. The data are telemetered in near real-time to a shore station via wireless Ethernet, and to Dalhousie over the Internet. The fixed-point measurements from the bottom pods are augmented by a vessel-mounted profiling system, and by opportunistic deployments of internally-recording instruments. Highlights from the first three years of the observing system operation will be presented, including sediment suspension from the seabed and bubble penetration from the surface during Hurricane Juan. [ABSTRACT FROM AUTHOR]

Coherent Doppler sonar is a useful tool for noninvasive measurement of ocean currents, sediment transport, and turbulence in coastal environments. Various methods have been proposed to separately address two of its inherent limitations: velocity ambiguity and measurement noise. However, in energetic turbulent flows, both factors may be present simultaneously. The presence of measurement noise complicates velocity ambiguity resolution, and conversely velocity ambiguity presents a challenge for existing noise suppression methods. A velocity estimator based on maximum a posteriori (MAP) estimation has been developed to resolve velocity ambiguity and suppress measurement noise simultaneously rather than separately. The estimator optimally combines measurements from multiple acoustic carrier frequencies and multiple transducers. Data fusion is achieved using a probabilistic approach, whereby measurements are combined numerically to derive a velocity likelihood function. The MAP velocity estimator is evaluated using a high-fidelity coherent Doppler sonar simulation of oscillating flow and data from a towing tank grid turbulence experiment where both velocity ambiguity and backscatter decorrelation were present. Time series and spectra from MAP velocity estimation are compared to those obtained with conventional Doppler signal processing. In addition to robustly resolving velocity ambiguity, the MAP velocity estimator is shown to reduce high-frequency noise in turbulence spectra. [ABSTRACT FROM AUTHOR]

A new formula is derived for the asymptotic form of the magnitude of an autocorrelation coefficient for coherent Doppler sonar. The autocorrelation magnitude is shown to be a biased estimator in the limit of infinite ensemble length. Numerical simulation of a Gaussian random process is used to verify the asymptotic formula and to show that a bias persists for finite pulse-pair averages. Validity of the asymptotic formula is also confirmed using a high-fidelity coherent Doppler sonar simulation, and from sonar measurements in a towing tank. It is shown that the distribution of observed autocorrelation coefficients is well predicted by a Gaussian random process once the autocorrelation bias has been removed. [ABSTRACT FROM AUTHOR]

An extreme weather event (Hurricane Juan) made landfall in Nova Scotia, Canada, in September 2003. The storm produced an ∼70-cm storm surge and ∼40 cm s-1 coastal currents in Lunenburg Bay, registered by a coastal observing system. A fine-resolution (60 m) coastal circulation model is used to examine the response of Lunenburg Bay to Hurricane Juan. The model is forced by local wind stress at the sea surface, and tides and remotely generated waves specified at model open boundaries. The model performance is assessed in terms of γ2, the variance of the model errors normalized by the observed variance. The model reproduces very well the observed surface elevations with γ2 values of less than 0.05, and reasonably well the observed currents with γ2 values between 0.2 and 1.1 in the bay during Hurricane Juan. The model–data comparisons demonstrate that the coastal circulation in the bay is significantly affected by local wind associated with the storm. The model results are also used to demonstrate the importance of nonlinear dynamics in the barotropic response of the bay to the storm. [ABSTRACT FROM AUTHOR]