[1] Time-dependent buoyant plumes form at the outflow of tidally dominated estuaries. When estuary discharge velocity exceeds plume internal wave speed c, a sharp front forms at the plume's leading edge that expands from the time-dependent source. Using observations of the Columbia River tidal plume from multiple tidal cycles we characterize time-evolving plume structure and quantify front speed U f , plume internal wave speed c, front curvature, and ultimate extent. We identify three distinct stages of propagation: (1) Initially, the plume is strongly influenced by shallow bathymetry near the river mouth.(2) As the front advances offshore the plume detaches from the bottom and expands as a freely propagating gravity current with relatively constant U f , c and frontal Froude number F = U f /c. Ambient currents explain intracycle variability in U f and winds alter front shape. Variability in ambient stratification associated with previous cycles' plume remnants leads to complex fronts and internal waves. (3) Finally, the plume decelerates, adjusts toward geostrophy, and may radiate additional internal waves. Using a simple kinematic model, we suggest that constant frontal propagation speed, U f = 0.9 ± 0.1 m/s, during stage 2 is primarily controlled by linearly increasing volume flux from the Columbia River mouth. As this discharge rate subsides, the plume expands as a fixed volume with decreasing front speed (stage 3). The plume's final extent is controlled by the Rossby radius, which scales with a length based on the total volume discharged. This provides an integral description of plume front evolution based on the time-dependent estuary discharge.

After a decade of rising costs and technical challenges, project financial data indicates that offshore wind may finally be on a downward cost trajectory while the industry logged its best deployment year ever in 2015. Historically, rising offshore wind costs have been attributed to a myriad of hindrances, including increasing siting challenges (e.g., deeper water, greater distances from shore) and a wide range of installation and operational difficulties that have frustrated developers and offset gains made in technology, learning, and experience. The resilience of the European offshore wind industry to overcome these daunting cost challenges can be attributed to stable European policy commitments, the introduction of new offshore-class turbine and substructure technologies, and the creation of an offshore wind industry supply chain.

[1] The initial composition of a river plume depends on the cumulative turbulent entrainment within the estuary and how this dilutes the supplied freshwater. Here we examine the relative roles of turbulence and freshwater input using observations from the Columbia River estuary and plume during two periods with contrasting river flow. Within the estuary, intense turbulence observed on flood and ebb stages is controlled by the bottom stress and scales with tidally dominated near-bottom velocity as u tidal 3 . Shear associated with the estuarine circulation is found to have a much weaker influence on turbulence dissipation rates. On the basis of these observations, we suggest that properties of the Columbia River tidal plume should be controlled by the ratio of horizontal advection to turbulent mixing within the estuary. This ratio depends on the magnitude of freshwater river input (characterized by its volumetric flow rate Q f ) as compared to turbulent fluxes due to tidal mixing. This is summarized in terms of the estuary Richardson number Ri E , a nondimensional ratio between Q f and u tidal 3 . From 17 tidally resolving offshore surveys during spring/neap tides and low/high river flows, we find that the plume's median salinity, thickness, and turbulent mixing are each predicted through Ri E . It is hoped that these simple formulations will provide guidance in assessing critical properties of river plumes and their influence on coastal circulation.

Wind turbine power output is known to be a strong function of wind speed, but is also affected by turbulence and shear. In this work, new aerostructural simulations of a generic 1.5 MW turbine are used to rank atmospheric influences on power output. Most significant is the hub height wind speed, followed by hub height turbulence intensity and then wind speed shear across the rotor disk. These simulation data are used to train regression trees that predict the turbine response for any combination of wind speed, turbulence intensity, and wind shear that might be expected at a turbine site. For a randomly selected atmospheric condition, the accuracy of the regression tree power predictions is three times higher than that from the traditional power curve methodology. The regression tree method can also be applied to turbine test data and used to predict turbine performance at a new site. No new data are required in comparison to the data that are usually collected for a wind resource assessment. Implementing the method requires turbine manufacturers to create a turbine regression tree model from test site data. Such an approach could significantly reduce bias in power predictions that arise because of the different turbulence and shear at the new site, compared to the test site.

[1] Turbulence controls the composition of river plumes through mixing and alters the plume's trajectory by diffusing its momentum. While believed to play a crucial role in decelerating river-source waters, the turbulence stress in a near-field river plume has not previously been observationally quantified. In this study, finely resolved density, velocity, and turbulence observations are combined with a control-volume technique to describe the momentum balance in the Columbia River's near-field plume during 10 tidal cycles that encompass both large and small river flow. Turbulence stress varies by 2-3 orders of magnitude, both within a given ebb and between ebbs with different tidal or river forcing; its magnitude scales with the strength of the instantaneous ebb outflow, i.e., high stresses occur during peak flow of strong ebbs. During these periods, the momentum equation is represented by a balance between stress divergence and plume deceleration. As the flow relaxes, the stress divergence weakens and other terms (pressure gradient and Coriolis) may become appreciable and influence plume deceleration. While the momentum balance could not be closed during these weaker flow periods, during strong tidal pulses the time scale for decay based on observed stress is significantly less than a tidal half-period, indicating that stress divergence plays a fundamental role in the initial deceleration of the plume.