Connecticut River Plume Research Articles

Mixing of the Connecticut River Plume During Ambient Flood Tides: Spatial Heterogeneity and Contributions of Bottom‐Generated and Interfacial Mixing

AbstractThe Connecticut River plume is influenced by energetic ambient tides in the Long Island Sound receiving waters. The objectives of this modeling study are (a) characterizing the spatial heterogeneity of turbulent buoyancy fluxes, (b) partitioning turbulent buoyancy fluxes into bottom‐generated and interfacial shear contributions, and (c) quantifying contributions to plume‐integrated mixing within the tidal plume. The plume formed during ambient flood tides under low river discharge, spring tides, and no winds is analyzed. Turbulent buoyancy fluxes (B) and depth‐integrated B through the plume (Bd) are characterized by pronounced spatial heterogeneity. Strong mixing (Bd ∼ 10−5‐10−4 m3/s3) occurs near the mouth, in the nearfield plume turning region, over shoals, and nearshore shallow areas. Low to moderate mixing (Bd ∼ 10−8‐10−6 m3/s3) occupies half the plume. Buoyancy fluxes are first partitioned based on the depth of the shear stress minimum between plume‐generated and bottom‐generated shear maxima. Four other tested partitioning methods are based on open channel flow and stratified shear flow parameterizations. Interfacial and bottom‐generated shear contribute to different areas of intense and moderate mixing. All methods indicate a significant plume mixing role for bottom‐generated mixing, but interfacial mixing is a bigger contributor. Plume‐integrated total and interfacial mixing peak at max ambient flood and the timing of peak bottom‐generated mixing varies among partitioning methods. Two‐thirds of the mixing occurs in concentrated intense mixing areas. A parameter space with the ambient tidal Froude number and plume thickness to depth ratio as axes indicates many tidally modulated plumes are moderately to dominantly influenced by bottom‐generated tidal mixing.

Comparison of structure and turbulent mixing between lateral and leading-edge river plume fronts: Microstructure observations from a T-REMUS AUV

The energetic frontal region of the tidally pulsed Connecticut River plume was sampled with the T-REMUS Autonomous Underwater Vehicle (AUV) which navigated ten cross-front transects at various depths. Synchronous and high resolution hydrographic, microstructure, velocity and backscatter observations allowed the creation of detailed views of frontal structure and turbulent mixing in the reference frame of the propagating front. The front was defined by a sharp horizontal density gradient of ∼18.5 kg m−4 across only 0.7 m in the horizontal, and strong downwelling velocities of 0.17 m/s. Three successive zones were consistently identified. The first O (10 m) wide downwelling zone in which buoyant plume water was forced downward, thereby forming a highly energetic frontal head that extended down to 6 m depth and had TKE dissipation rates (ε) of order 10−3 m2/s3. Beyond the downwelling zone to O (100 m), a transition zone was observed, characterized by substantial entrainment of ambient water. This zone had high amplitude and high frequency density anomaly (σθ) variability consistent with elevated but decreasing turbulent kinetic energy dissipation rates (ε) which ranged between 10−4 and 10−5 m2/s3. Beyond O (100 m) from the front, stable stratification, high Ri values, and low Reb values, suppressed the growth of instabilities and diminished turbulent mixing. In this stable interior region the plume base shoaled to 2 m depth and ε decreased further, from 10−5 to 10−7 m2/s3. In ambient Long Island Sound waters, ε ranged between 10−6 and 10−7 m2/s3. The cross-front distributions of σθ and ε did not change substantially as the front transformed from a lateral boundary, where plume flow was primarily in the along-front direction, to one that was at the leading edge of the plume, where plume flow was mostly in the cross-front direction. In both cases, a significant convergence was present at the front due to strong ambient tidal velocities. The observed ε values and those from four previous studies of large and mid-sized plumes were nondimensionalized using the velocity and length scales associated with the downwelling region at the front. The effective normalization suggests that local dynamics primarily control ε in the near-frontal region.

Freshwater Composition and Connectivity of the Connecticut River Plume During Ambient Flood Tides

The Connecticut River plume interacts with the strong tidal currents of the ambient receiving waters in eastern Long Island Sound. The plume formed during ambient flood tides is studied as an example of tidal river plumes entering into energetic ambient tidal environments in estuaries or continental shelves. Conservative passive freshwater tracers within a high-resolution nested hydrodynamic model are applied to determine how source waters from different parts of the tidal cycle contribute to plume composition and interact with bounding plume fronts. The connection to source waters can be cut off only under low-discharge conditions, when tides reverse surface flow through the mouth after max ambient flood. Upstream plume extent is limited because ambient tidal currents arrest the opposing plume propagation, as the tidal internal Froude number exceeds one. The downstream extent of the tidal plume always is within 20 km from the mouth, which is less than twice the ambient tidal excursion. Freshwaters in the river during the preceding ambient ebb are the oldest found in the new flood plume. Connectivity with source waters and plume fronts exhibits a strong upstream-to-downstream asymmetry. The arrested upstream front has high connectivity, as all freshwaters exiting the mouth immediately interact with this boundary. The downstream plume front has the lowest overall connectivity, as interaction is limited to the oldest waters since younger interior waters do not overtake this front. The offshore front and inshore boundary exhibit a downstream progression from younger to older waters and decreasing overall connectivity with source waters. Plume-averaged freshwater tracer concentrations and variances both exhibit an initial growth period followed by a longer decay period for the remainder of the tidal period. The plume-averaged tracer variance is increased by mouth inputs, decreased by entrainment, and destroyed by internal mixing. Peak entrainment velocities for younger waters are higher than values for older waters, indicating stronger entrainment closer to the mouth. Entrainment and mixing time scales (1–4 h at max ambient flood) are both shorter than half a tidal period, indicating entrainment and mixing are vigorous enough to rapidly diminish tracer variance within the plume.

Sill effects on physical dynamics in eastern Long Island Sound

This study investigates how Mattituck Sill influences circulation patterns and physical dynamics in eastern Long Island Sound, a major estuary on the U.S. east coast. Observations show there is pronounced across-estuary transport in the area and suggest there may be subtidal anticyclonic flow around the sill. Model runs, with and without sill bathymetry, exhibit this across-estuary transport and anticyclonic circulation. Comparison between these runs indicates that the sill intensifies the anticyclonic circulation. This study finds the sill does not exert internal hydraulic control during neap, mean, or spring tidal conditions. Nevertheless, along-estuary exchange is reduced over the sill and across-estuary fluxes are increased. The Connecticut River plume enters close to the estuary mouth. The sill deflects more of the plume waters towards the mouth, causing less freshwater to take the long looping route through the estuary. The subtidal circulation balance around the sill indicates a barotropic balance between the tidal advection of tidal vorticity and friction. The subtidal vorticity balance indicates the net effect of tidal advection of relative vorticity is balanced with frictional curl associated with lateral speed gradients and vorticity dissipation. Previously developed scalings based on the circulation balance (Nature 290:549–555, 1981), frictional vorticity generation mechanisms (Deep-Sea Res 28:195–212, 1981), and tidal diffusion of potential vorticity (J Phys Oceanogr 29:821–827, 1999) are applicable to Mattituck Sill and predict circulation with a similar magnitudes to model results.

Small-scale variability in suspended matter associated with the Connecticut River plume front

[1] We report high spatial resolution observations of optical proxies for suspended particles and dissolved matter measured at the boundary of the Connecticut River plume (CRP) in Long Island Sound (LIS) in April 2000 when river discharge was near the annual maximum. The magnitude of beam attenuation, cp, backscatter, bb, and absorption at short wavelengths indicated higher concentration of suspended particles and colored dissolved matter within the LIS relative to the adjacent CRP. The fractional backscatter from particles indicated relatively higher organic matter fraction within the LIS. An absorption feature centered at 429 nm (aPB), indicated the presence of pigmented heterotrophic bacteria unique to the LIS and the strongest signals were observed at locations closest to shore. The spectral slope of beam attenuation associated with particulate matter (γ) indicated the presence of relatively larger particle assemblages within the LIS. Strong linear relationships between γ and salinity were observed within the energetic CRP boundary region, within 30 m of the front location. Regression residuals indicated a shift to smaller particles and were greatest at the front and decreased with distance toward the plume interior with a length scale similar to previous reports of the kinetic energy dissipation rate. At the same time, the magnitude of cp and bb remained uniform. These results are consistent with the disruption of low fractal dimension particle assemblages due to enhanced turbulence and mixing. The residuals in γ were weakly correlated with salinity and aPB suggesting that aggregate disruption was primarily associated with entrained Long Island Sound water.

Turbulence in coastal fronts near the mouths of Block Island and Long Island Sounds

Measurements of turbulence were performed in four frontal locations near the mouths of Block Island Sound (BIS) and Long Island Sound (LIS). These measurements extend from the offshore front associated with BIS and Mid-Atlantic Bight Shelf water, to the onshore fronts near the Montauk Point (MK) headland, and the Connecticut River plume front. The latter feature is closely associated with the major fresh water input to LIS. Turbulent kinetic energy (TKE) dissipation rate, ε, was obtained using shear probes mounted on an autonomous underwater vehicle. Offshore, the BIS estuarine outflow front showed, during spring season and ebb tide, maximum TKE dissipation rate, ε, estimates of order 10 − 5 W/kg, with background values of order 10 − 6 to 10 − 9 W/kg. Edwards et al. [Edwards, C.A., Fake, T.A., and Bogden, P.S., 2004a. Spring–summer frontogenesis at the mouth of Block Island Sound: 1. A numerical investigation into tidal and buoyancy-forced motion. Journal of Geophysical Research 109 (C12021), doi:10.1029/2003JC002132.] model this front as the boundary of a tidally driven, baroclinically adjusted BIS flow around the MK headland eddy. At the entrance to BIS, near MK, two additional fronts are observed, one of which was over sand waves. For the headland site front east of MK, without sand waves, during ebb tide, ε estimates of 10 − 5 to 10 − 6 W/kg were observed. The model shows that this front is at the northern end of an anti-cyclonic headland eddy, and within a region of strong tidal mixing. For the headland site front further northeast over sand waves, maximum ε estimates were of order 10 − 4 W/kg within a background of order 10 − 7 –10 − 6 W/kg. From the model, this front is at the northeastern edge of the anti-cyclonic headland eddy and within the tidal mixing zone. For the Connecticut River plume front, a surface trapped plume, during ebb tide, maximum ε estimates of 10 − 5 W/kg were obtained, within a background of 10 − 6 to 10 − 8 W/kg. Of all four fronts, the river plume front has the largest finescale mean-square shear, S 2 ~ 0.15 s − 2 . All of the frontal locations had local values of the buoyancy Reynolds number indicating strong isotropic turbulence at the dissipation scales. Local values of the Froude number indicated shear instability in all of the fronts.

Optical determinations of suspended sediment dynamics in western Long Island Sound and the Connecticut River plume

The magnitude of beam attenuation attributed to suspended sediments, cp, and the slope of the log‐transformed attenuation spectrum, γ, were used to investigate the properties and dynamic nature of matter suspended in the waters of western Long Island Sound (LIS) and the adjacent Connecticut River plume (CRP). Within the LIS, cp and γ indicate a robust relationship between sediment concentration and particle size distribution (PSD). As concentration increased, the PSD shifted to larger particles. The highest concentrations and particle sizes were found in a nepheloid layer adjacent to the sound floor. Within the adjacent CRP, sediments were observed to shift towards smaller particles at the lateral plume boundary, where current shear stress may have disrupted large particle aggregates, relative to sediments in the more quiescent plume interior. A strong linear correlation between γ and salinity was also found, indicating that mixing between the two water masses may also have altered the PSD of the plume sediments. A suspended sediment attenuation model based on Mie theory, a power law form of the PSD, and a single sediment source indicates that the observed changes in LIS cp and γ are consistent with sediment removal as particles settle with size‐dependent rates. In contrast, within the CRP, the model supports the hypothesis that turbulence‐induced aggregate disruption at the lateral plume boundary is responsible for the observed variability in cp and γ. However, mixing between the LIS and CRP particle populations would violate the single source requirement of the model and necessitate a more complicated set of particle dynamics in order to correctly interpret the observed variability in particulate attenuation.

Spatial Processing of Range-Bin ADCP Data to Resolve Small-Scale Frontal Features

Abstract A new approach is proposed for the processing of acoustic Doppler current profiler (ADCP) data that has the potential to better resolve the structure of small-scale dynamical features such as strongly convergent current fronts. The essence of this approach is to assign each range bin of radial velocity and backscatter data an x, y, and z position, and then to spatially average the bin data relative to the feature of interest. The approach is illustrated by an analysis of surface-towed ADCP data collected across the Connecticut River plume front. Results using an across-front averaging length of 5 m show that the frontal morphology and circulation (gravity-current head structure, shallow inflow layer, strong sinking motion) are consistently resolved, while 1-m averaging appears additionally to resolve flow instabilities occurring along the frontal interface. Other possible applications are briefly discussed.

Convergence and Downwelling at a River Plume Front

Abstract The small-scale structure of the circulation and hydrography at the frontal boundary of the Connecticut River plume in Long Island Sound has been resolved using a novel combination of instruments: a towed acoustic Doppler current profiler (ADCP) and a rigid array of current meters and conductivity–temperature sensors. Observations were made during the latter half of the eastward ebb tide, when the river plume was well established and the front was moving to the west at approximately 0.3 m s−1. Two across-front transects revealed a horizontal convergence rate in the across-front velocity components at 0.6 m of 0.05–0.1 s−1. This was associated with a salt-induced horizontal density gradient of 10−2 kg/m4. Observations obtained during a period in which the towed ADCP was caught in the zone of maximum surface convergence showed significant downwelling with a near-surface maximum of 0.2 m s−1. Vertical velocities of this magnitude are consistent with observed magnitudes of the convergence rate at 0.6...

Observations of near‐surface currents and hydrography in the Connecticut River plume with the surface current and density array

It has been well established that the Connecticut River forms a large plume in the vicinity of its mouth during periods of high river discharge. The plume is bounded on the offshore side by a distinct front where the water color and turbidity changes rapidly and foam and detritus accumulate. Though there have been many observations of similar fronts throughout the world, the details of the density and velocity structure in the vicinity of a plume front and the propagation velocity of the feature have not been adequately resolved. To better observe the properties of these phenomena, we have constructed a ship‐mounted array of instruments to measure the near‐surface currents and density (SCUD) distribution in plumes and fronts. SCUD provides current velocity components at four levels and conductivity and temperature at five levels in the upper 3 m of the water column. By using the global positioning system in differential mode to obtain position, the system has a horizontal resolution of 5 m at a ship speed of 1–2 ms−1. Estimates of the ground referenced water velocity components have an uncertainty of approximately 0.1 ms−1. To demonstrate the performance of the SCUD array, we report the preliminary results of an observation program to examine the variability of the front of the Connecticut River plume. In the spring of 1994, several 200 m across‐front SCUD transects separated by 200 m were obtained during the mid ebb in Long Island Sound. During this period the front advanced to the west with a velocity of the order of 0.1 ms−1 in opposition to an eastward tidal current of 1.0 ms−1. A practical salinity increase of 20 in less than 10 m was observed at the front on crossing into the plume. In concert, the front normal velocity component decreased by 0.5 (±0.1) ms−1 over 10 m, and the vertical shear in the front parallel component decreased dramatically. The performance and potential of the instrumentation and the implication of the observations for the development theories of river plume fronts are discussed.

A steady state model for buoyant surface plume hydrodynamics in coastal waters

Shallow, surface plumes of buoyant water are frequently discharged into coastal waters by rivers, bays, power plants, and other sources. The most readily observed portion of these plumes is the near field where the initial expansion of plume water occurs, and where there is large contrast between plume and ambient water. In contrast to existing turbulent jet models of such plumes, the present model divides the plume near field into two domains, a frontal domain where turbulent exchange between plume and ambient water is intense, and the remainder of the flow where the dynamics of inviscid, nonlinear gravitational spreading of a shallow buoyant layer dominates. Using this approach, the steady state flow produced by the supercritical outflow of buoyant water from a channel at a coast into ambient water with a uniform alongshore current is modelled. Earth rotation is neglected. Frontal jump conditions developed in an earlier paper are applied to match the flow at the frontal boundary with the remainder of the plume. Numerical solutions for the entire flow are found by the method of characteristics, but much of the flow can be determined by simpler analytic computation. The computed flow fields show that a frontal boundary forms on the upstream or offshore side of the plume where the oncoming ambient current contains the gravitational spreading of the buoyant water. No front forms on the corresponding downstream, inshore side. In the body of the plume the flow simultaneously expands and turns downstream toward alignment with the ambient current. Relatively little change in plume water speed occurs, but the pressure field required for turning the flow downstream results in deepening of the plume interface offshore near the front. This, in turn, concentrates plume volume flux along its offshore side near the front. The maximum angle through which the flow may be turned initially is less than 66°. Outlet channel angles greater than this produce plumes which are separated from the shoreline by ambient water. The model results explain many of the observed features of the Connecticut River plume in Long Island Sound. DOI: 10.1111/j.2153-3490.1982.tb01818.x

Observations of the motion field of the Connecticut River plume

Observations of the motion field associated with the plume formed by the outflow of the Connecticut River into the coastal seawater of Long Island Sound are presented. Approximately 35 drogues and drifters were tracked for each of three experiments using an airborne camera. The trajectories and the Eulerian velocity field deduced from them are presented. The offshore boundary of the plume was formed by a front where there was a strong discontinuity at the surface in both the velocity and density fields. In addition to a vigorous outflow of plume surface water away from the river mouth and parallel to the plume axis, the observations showed a pronounced surface flow toward the front and normal to the axis which was consistent with frontal convergence. The speed of plume water was found to be highly supercritical. The motion of nearby ambient seawater appeared to be little affected by the plume.

Frontal structure of a river plume

Field observations are reported of the frontal structure found along the boundary of the Connecticut River plume in Long Island Sound. The hydrography and the horizontal current field were determined in the vicinity of the front. The front separates ambient saltwater on the surface from the light brackish water of the plume. Beneath the surface an inclined frontal layer is found where static stability is a maximum. Isopycnals forming the frontal layer slope upward from a typical depth of 1 m and intersect the sea surface on a horizontal scale of 50 m. A sharp transition in surface color appears in the frontal zone. The primary motion relative to the front is manifested by a vigorous surface convergence from both sides. Sinking motion at the front is locally intense. Saltwater approaching the front is pressed downward beneath the pool of brackish water. The motion parallel to the front is much weaker and independent of density structure. The structure of a section of the front is destroyed by mixing over a period of several hours.