Daniel G. MacDonald

Turbulent energy production and entrainment at a highly stratified estuarine front

[1] Rates of turbulent kinetic energy (TKE) production and buoyancy flux in the region immediately seaward (� 1 km) of a highly stratified estuarine front at the mouth of the Fraser River (British Columbia, Canada) are calculated using a control volume approach. The calculations are based on field data obtained from shipboard instrumentation, specifically velocity data from a ship mounted acoustic Doppler current profiler (ADCP), and salinity data from a towed conductivity-temperature-depth (CTD) unit. The results allow for the calculation of vertical velocities in the water column, and the total vertical transport of salt and momentum. The vertical turbulent transport quantities (u 0 w 0 , S 0 w 0 ) can then be estimated as the difference between the total transport and the advective transport. Estimated production is on the order of 10 � 3 m 2 s � 3 , yielding a value of e(nN 2 ) � 1 on the order of 10 4 . This rate of TKE production is at the upper limit of reported values for ocean and coastal environments. Flux Richardson numbers in this highly energetic system generally range from 0.15 to 0.2, with most mixing occurring at gradient Richardson numbers slightly less than 1 = 4 . These values compare favorably with other values in the literature that are associated with turbulence observations from regimes characterized by scales several orders of magnitude smaller than are present in the Fraser River. INDEX TERMS: 4546 Oceanography: Physical: Nearshore processes; 4568 Oceanography: Physical: Turbulence, diffusion, and mixing processes; 4235 Oceanography: General: Estuarine processes;

Lateral spreading of a near‐field river plume: Observations and numerical simulations

[1] Data collected from the near-field region of the Merrimack River (Massachusetts) plume were analyzed to examine plume spreading. Estimates of the plume spreading rate were derived from a direct assessment of the spreading of clustered surface drifters and from density and velocity data along a cross-plume arc. Additionally, plume spreading rates were also derived from highly resolved numerical model output (ROMS). These three distinct observational and numerical approaches have reasonable agreement and are compared favorably with observations using a control volume approach in a previous study. It is demonstrated that these three methods are valid to estimate the plume spreading rate. The observations and numerical simulations also allowed estimation of terms in the lateral momentum balance along streamline normals, perpendicular to surface layer streamlines. One principal finding of this study is that the dominant terms in the lateral momentum balance are the centrifugal force term associated with streamline curvature, the buoyancy term, the Coriolis term, and the interfacial stress term. Furthermore, the lateral momentum balance shows that the lateral spreading process is significantly affected by interfacial stress within the first 1 to 2 km but, thereafter, is locally inviscid. However, turbulent mixing continues to play an important role in modifying the spreading rate by adjusting the internal wave speed.

Hydraulic control of a highly stratified estuarine front

Abstract Observations at the mouth of the Fraser River (British Columbia, Canada) indicate an abrupt frontal transition between unstratified river outflow and a highly stratified river plume with differences in salinity greater than 25 psu across a few meters in the vertical direction and several hundred meters in the horizontal direction. The front roughly follows a natural break in the bathymetry, crossing the channel at an angle of approximately 45°, and is essentially stationary for a period of approximately 3.5 h centered on the low tide following the larger of two daily ebbs. The location of the front is coincident with observations of significantly supercritical internal Froude numbers at the front, based on velocities in the along-flow direction. This observation contradicts the one-dimensional theory, which indicates that the Froude number should be 1. However, because the front is oriented obliquely to the outflow, a coordinate system can be selected that is normal to the front and for which a c...

The role of wind in the near field and midfield of a river plume

The role of wind in the near-field and midfield regions of the Merrimack River plume is quantified using observations from surface drifters released near the river mouth during ebb tide in 2009, 2010, and 2011 under a range of wind and river discharge conditions. Comparison of momentum balance terms and analysis of plume trajectories suggests that the plume is sensitive to wind direction for wind speeds >4 m s−1 in all regions of the plume, including the near field, where wind influence has typically been considered second order. Intermediate ranges (4–12 km) were more strongly influenced by the wind than the near field (0–4 km). However, the influence of the instantaneous wind diminished farther from the mouth, presumably, due to the growing importance of longer time scale process (i.e., Ekman transport). The plume was less sensitive to cross-shore winds than alongshore winds, particularly near the river mouth, where momentum dominates.

Evidence of Cobble Habitat Preference in Age-0 Winter Flounder, Pseudopleuronectes americanus

Juvenile winter flounder utilize a wide variety of habitats, including those that are structurally complex. Although a common nearshore habitat in the Northwest Atlantic, selection of cobble habitat by juvenile winter flounder has not been the focus of previous study. In this laboratory experiment, we investigated habitat selection and preferences of age-0 winter flounder in sand and cobble of varying complexity. Fish were observed every 0.5 h for 6 h in tanks with four cobble treatments; bare sand, sand with low complexity cobble, sand with intermediate complexity cobble, and sand with high complexity cobble. Habitat preference was inferred when the proportion of observations in treatments were greater than what would be expected by chance based on availability. Results suggest that age-0 winter flounder strongly prefer cobble to sand, particularly cobble of an intermediate complexity. This study serves as a first step in the investigation of the importance of cobble habitat to juvenile winter flounder, and highlights the need for a more integrative approach when monitoring this species.

Mixing processes and hydraulic control in a highly stratified estuary

Thesis (Ph. D .)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 2003.

Relationship between body size and habitat complexity preference in age-0 and -1 year winter flounder Pseudopleuronectes americanus

The interaction between body size, habitat complexity and interstice width on habitat preference of age-0 and -1 year Pseudopleuronectes americanus was examined using continuous remote video observation. The habitat choices of juvenile P. americanus were recorded over a 6 h period in tanks with four treatments: bare sand, sand with low complexity cobble, sand with intermediate complexity cobble and sand with high complexity cobble. Both age-0 and -1 year fish preferred cobble to bare sand. Within cobble treatments, age-0 year fish preferred intermediate complexity cobble, with a 1.59 ratio of interstitial space to body width. The largest age-1 year fish (123-130 mm standard length, L(S) ) preferred low complexity cobble. While a significant preference was not detected, medium age-1 year fish (83-88 mm L(S) ) tended to select low complexity cobble, whereas small age-1 year fish (73-82 mm L(S) ) tended to select low and intermediate cobble, with an interstitial space to body width ratio of 1.05. For medium and large age-1 year fish, there was an increased selection of low complexity cobble, corresponding to larger interstitial space to body size ratios. This study indicates that juvenile P. americanus prefer complex habitat to unstructured habitat and that this preference is mediated by a relationship between fish body size and the size of structure interstices. These results contribute to the growing body of knowledge of complex habitat selection and drivers of habitat choice in flatfishes.

Enhanced Surface Cooling as an Alternative for Thermal Discharges

Excess heat is an unavoidable by-product of electricity generation from fossil and nuclear fuels. In most cases, excess heat is transferred to a cooling water stream and discharged to a local receiving water body, or processed through on-site cooling towers. In many cases existing discharges are potentially responsible for significant ecological impacts, and regulatory authorities are mandating the construction of cooling towers, often at significant expense. Most existing cooling water discharges are designed to reduce excess temperatures through rapid dilution. Enhanced surface cooling is an alternative approach which involves the development of a thin surface plume, while limiting mixing of the discharge with ambient waters. This process encourages rapid transfer of heat to the atmosphere while limiting impacts to sensitive benthic environments and most of the volume of the receiving water body. This discharge approach may be particularly effective for receiving water bodies which have limited natural ...

Natural and Anthropogenic Influences on the Mount Hope Bay Ecosystem: Concluding Remarks

Abstract The papers in this volume have provided a detailed focus on various aspects of the Mount Hope Bay ecosystem, from the local heat budget to Pseudopleuronectus americanus Waldbaum (winter flounder) stock declines. In this conclusion, we attempt to place these individual studies within the broader context of research performed in Mount Hope Bay over the last several decades.

The Effect of Discharge, Tides, and Wind on Lift-Off Turbulence

Data from three deployments of a 1200 kHz moored Acoustic Doppler Current Profiler (ADCP) were used to study the factors affecting turbulent kinetic energy (TKE) production in the lift-off zone of a mid-sized river plume (Merrimack River, Newburyport, MA) during the spring freshets of 2007, 2010, and 2011. TKE production was estimated from the ADCP data, during periods of minimal wave activity, using the variance method, with significant variability in plume thickness and TKE production observed between ebbs. Correlations with this observed variability and the primary environmental variables, such as river flow, wind speed/direction, and tidal range, were noted. On the basis of these observations, we quantify the contribution of these forcing mechanisms to the observed TKE production using an empirical approach based on the marginal value of the discharge Froude number (which is scaled from the environmental variables) above a critical value of one. The resulting regression provides a means for estimating TKE production in the lift-off zone as a function of only the environmental variables, and produces results consistent with previous observations from other turbulence measurement techniques in the Merrimack plume. The regression also provides an indication of the relative importance of the various forcing mechanisms, and suggests that onshore (east) winds and river discharge are the most important factors in controlling TKE production in the Merrimack plume, with tidal range of lesser significance.