Alexander R. Horner-Devine

River Influences on Shelf Ecosystems: Introduction and synthesis

[1] River Influences on Shelf Ecosystems (RISE) is the first comprehensive interdisciplinary study of the rates and dynamics governing the mixing of river and coastal waters in an eastern boundary current system, as well as the effects of the resultant plume on phytoplankton standing stocks, growth and grazing rates, and community structure. The RISE Special Volume presents results deduced from four field studies and two different numerical model applications, including an ecosystem model, on the buoyant plume originating from the Columbia River. This introductory paper provides background information on variability during RISE field efforts as well as a synthesis of results, with particular attention to the questions and hypotheses that motivated this research. RISE studies have shown that the maximum mixing of Columbia River and ocean water occurs primarily near plume liftoff inside the estuary and in the near field of the plume. Most plume nitrate originates from upwelled shelf water, and plume phytoplankton species are typically the same as those found in the adjacent coastal ocean. River-supplied nitrate can help maintain the ecosystem during periods of delayed upwelling. The plume inhibits iron limitation, but nitrate limitation is observed in aging plumes. The plume also has significant effects on rates of primary productivity and growth (higher in new plume water) and microzooplankton grazing (lower in the plume near field and north of the river mouth); macrozooplankton concentration (enhanced at plume fronts); offshelf chlorophyll export; as well as the development of a chlorophyll ‘‘shadow zone’’ off northern Oregon.

Infrared-Based Measurements of Velocity, Turbulent Kinetic Energy, and Dissipation at the Water Surface in a Tidal River

Thermal infrared (IR)-based particle image ve locimetry (PIV) is used to measure the evolution of velocity, turbulent kinetic energy (TKE), and the TKE dissipation rate at the water surface in the tidally influenced Snohomish River. Patterns of temperature variability in the IR imagery arise from disruption of the cool-skin layer and are used to estimate the 2-D velocity field. Comparisons of IR-based PIV mean velocity made with a colocated acoustic velocimeter demonstrate high cor relation (r² >; 0.9). Over a tidal period, surface TKE computed from the IR velocity varies from 10^-4 to 3 × 10^-3 J · kg^-1, with an average difference from the in situ measurements of 8%. IR-derived TKE dissipation rates vary from approximately 3 × 10^-6 to 2 × 10^-4 W · kg^-1 at peak ebb, agreeing on average to within 7% of the in situ velocimeter results. IR-based PIV provides detailed measurements of previously inaccessible surface velocities and turbulence statistics.

Laboratory Investigation of the Impact of Lateral Spreading on Buoyancy Flux in a River Plume

AbstractThe relationship between lateral spreading and mixing in stratified gravity currents is investigated by comparing laterally confined and unconfined currents in a series of laboratory experiments. The vertical turbulent buoyancy flux is determined using a control volume approach with velocity and density fields derived from combined particle image velocimetry (PIV) and planar-laser-induced fluorescence (PLIF). Lateral spreading is determined in the unconfined experiments based on plan-view imaging using the optical thickness method (OTM). The authors find that lateral spreading dramatically modifies the plume structure; the spreading plume layer consists of approximately linear density and velocity profiles that extend to the surface, whereas the channelized plume profiles are uniform near the surface. Lateral spreading decreases the average plume density relative to laterally confined currents with similar inflow conditions. However, the local turbulent buoyancy flux in the spreading experiments i...

An Autonomous Open-Ocean Stereoscopic PIV Profiler

Abstract Over the past decade, a novel free-fall imaging profiler has been under development at the Scripps Institution of Oceanography to observe and quantify biological and physical structure in the upper 100 m of the ocean. The profiler provided the first detailed view of microscale phytoplankton distributions using in situ planar laser-induced fluorescence. The present study examines a recent incarnation of the profiler that features microscale turbulent flow measurement capabilities using stereoscopic particle image velocimetry (PIV). As the profiler descends through the water column, a vertical sheet of laser light illuminates natural particles below the profiler. Two sensitive charge-coupled device (CCD) cameras image a 25 cm × 25 cm × 0.6 cm region at a nominal frame rate of 8 Hz. The stereoscopic camera configuration allows all three components of velocity to be measured in the vertical plane with an average spatial resolution of approximately 3 mm. The performance of the PIV system is evaluated ...

Wave breaking turbulence at the offshore front of the Columbia River Plume

Observations at the Columbia River plume show that wave breaking is an important source of turbulence at the offshore front, which may contribute to plume mixing. The lateral gradient of current associated with the plume front is sufficient to block (and break) shorter waves. The intense whitecapping that then occurs at the front is a significant source of turbulence, which diffuses downward from the surface according to a scaling determined by the wave height and the gradient of wave energy flux. This process is distinct from the shear-driven mixing that occurs at the interface of river water and ocean water. Observations with and without short waves are examined, especially in two cases in which the background conditions (i.e., tidal flows and river discharge) are otherwise identical.

Frontogenesis and Frontal Progression of a Trapping-Generated Estuarine Convergence Front and Its Influence on Mixing and Stratification

Estuarine fronts are well known to influence transport of waterborne constituents such as phytoplankton and sediment, yet due to their ephemeral nature, capturing the physical driving mechanisms and their influence on stratification and mixing is difficult. We investigate a repetitive estuarine frontal feature in the Snohomish River Estuary that results from complex bathymetric shoal/channel interactions. In particular, we highlight a trapping mechanism by which mid-density water trapped over intertidal mudflats converges with dense water in the main channel forming a sharp front. The frontal density interface is maintained via convergent transverse circulation driven by the competition of lateral baroclinic and centrifugal forcing. The frontal presence and propagation give rise to spatial and temporal variations in stratification and vertical mixing. Importantly, this front leads to enhanced stratification and suppressed vertical mixing at the end of the large flood tide, in contrast to what is found in many estuarine systems. The observed mechanism fits within the broader context of frontogenesis mechanisms in which varying bathymetry drives lateral convergence and baroclinic forcing. We expect similar trapping-generated fronts may occur in a wide variety of estuaries with shoal/channel morphology and/or braided channels and will similarly influence stratification, mixing, and transport.

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.

Structure of turbulence and sediment stratification in wave‐supported mud layers

We present results from laboratory experiments in a wave flume with and without a sediment bed to investigate the turbulent structure and sediment dynamics of wave-supported mud layers. The presence of sediment on the bed significantly alters the structure of the wave boundary layer relative to that observed in the absence of sediment, increasing the TKE by more than a factor of 3 at low wave orbital velocities and suppressing it at the highest velocities. The transition between the low and high-velocity regimes occurs when ReΔ ≃ 450, where ReΔ is the Stokes Reynolds number. In the low-velocity regime (ReΔ 450) the ripples are significantly smaller, the near-bed sediment concentrations are significantly higher and density stratification due to sediment becomes important. In this regime the TKE and Reynolds stress are lower in the sediment bed runs than in comparable runs with no sediment. The regime transition at ReΔ = 450 appears to result from washout of the ripples and increased concentrations of fine sand suspended in the boundary layer, which increases the settling flux and the stratification near the bed. The increased stratification damps turbulence, especially near the top of the high-concentration layer, reducing the layer thickness. We anticipate that these effects will influence the transport capacity of wave-supported gravity currents on the continental shelf.

The impact of storms and stratification on sediment transport in the Rhine region of freshwater influence

We present measurements of along and across-shore sediment transport in a region of the Dutch coast 10 km north of the Rhine River mouth. This section of the coast is characterized by strong vertical density stratification because it is within the midfield region of the Rhine region of freshwater influence, where processes typical of the far-field, such as tidal straining, are modified by the passage of distinct freshwater lenses at the surface. The experiment captured two storms, and a wide range of wind, wave, tidal and stratification conditions. We focus primarily on the mechanisms leading to cross-shore sediment flux at a mooring location in 12 m of water, which are responsible for the exchange of sediment between the nearshore and the inner shelf. Net transport during storms was directed offshore and influenced by cross-shelf winds, while net transport during spring tides was determined by the mean state of stratification. Tidal straining dominated during neap tides; however, cross-shore transport was negligible due to small sediment concentrations. The passage of freshwater lenses manifested as strong pulses of offshore transport primarily during spring tides. We observe that both barotropic and baroclinic processes are relevant for cross-shore transport at depth and, since transport rates due to these competing processes were similar, the net transport direction will be determined by the frequency and sequencing of these modes of transport. Based on our observations, we find that wind and wave-driven transport during storms tends move fine sediment offshore, while calmer, more stratified conditions move it back onshore.

Cross‐shore transport of nearshore sediment by river plume frontal pumping

We present a new mechanism for cross-shore transport of fine sediment from the nearshore to the inner shelf resulting from the onshore propagation of river plume fronts. Onshore frontal propagation is observed in moorings and radar images, which show that fronts penetrate onshore through the nearshore and surf zone, almost to the waterline. During frontal passage a two-layer counterrotating velocity field characteristic of tidal straining is immediately set up, generating a net offshore flow beneath the plume. The seaward flow at depth carries with it high suspended sediment concentrations, which appear to have been generated by wave resuspension in the nearshore region. These observations describe a mechanism by which vertical density stratification can drive exchange of material between the nearshore region and the inner shelf. To our knowledge these are the first observations of this frontal pumping mechanism, which is expected to play an important role in sediment transport near river mouths.