Kevin A. Haas

With the increasing size of modern vessels, destructive long-period ship-induced waves are being reported more frequently, causing damage to adjacent waterways, promoting erosion, and posing risks to human safety and natural ecosystems. Particularly in shallow areas near embankments, unexpectedly large waves have been observed. Recent insights into the long-period components of ship wakes suggest that ship-generated edge waves, trapped along waterway margins, are a key mechanism that must be considered to fully understand ship-wave impacts. Edge waves can be viewed as long-period waves that reflect off the shoreline and subsequently re-reflect from an offshore depth transition. This repeated reflection can generate long sequences of waves traveling parallel to the coast. This work investigates whether the above outlined reflection mechanism applies to ship-induced edge waves. Results from a parametric study with the depth-averaged numerical tools FUNWAVE and REEF3D::SFLOW indicate that reflection of the inverse wave at the super-sub-critical depth-Froude boundary (caustic) governs the near-bank trailing wave system. These reflections of the ship-generated drawdown in shallow water determine both the wave heights and wave lengths along channel margins. The main findings show that edge waves' lengths depend solely on ship speed and local bathymetry and are independent of the vessel geometry itself. Moreover, under idealized conditions, near-bank wave heights may increase by up to a factor of four relative to the offshore wave height due to repeated reflections in shallow-water regions, underscoring the need to account for edge waves when predicting loads on waterways.

Cargo ship wake can be a strong and frequent source of energy along shorelines in the vicinity of commercial shipping channels. Large vessels in narrow channels produce low-frequency wake that consists of a positive front wave at the bow of the vessel, a depression (drawdown) spanning the length of the vessel, and an upwards surge produced by the stern wave at the stern. This low-frequency wake is followed by high-frequency wedge-shaped Kelvin wake, as well as continued low-frequency oscillations that may persist in the channel more than 30 minutes after a vessel passage. Low-frequency cargo ship wake with wave heights up to 2.8 meters was identified as the dominant source of erosion at Bird/Long Islands in the Savannah River, Georgia, USA based on the analysis of one month of Aquadopp measurements that captured over 300 large vessel wake events. The islands divide the river into the Main Channel, which contains the shipping channel, and the South Channel, a shallow, sheltered secondary channel trafficked only by small craft. Cargo ship wake was the primary energy source in both channels. The dominance of cargo ship wake energy in the sheltered South Channel demonstrated that the effects of wake extend beyond the shipping channel shorelines. This finding motivated three follow-up studies to quantify the extent and significance of cargo ship wake in the far-field, secondary channels connected directly or indirectly to the shipping channel.

Low-frequency (LF) cargo ship wake is a major source of hydrodynamic energy in shipping channels, where it has increasingly been linked to shoreline erosion. The LF wake consists of a drawdown called the “Bernoulli depression,” a return surge, and finally a series of trailing waves that may persist longer than 30 minutes after the vessel passage. While the Bernoulli depression and surge are well-explained by conservation principles, we presently lack a robust explanation of the trailing waves. They are a ubiquitous feature observed at many locations (e.g. Garrel, Lopez, and Collins, 2008), and have frequently been attributed to cross-channel seiching; however, field measurements from the Savannah River, GA, USA suggested that some of the waves may have alongshore progressive characteristics, which contradicts this assumption.

The ocean contains a variety of renewable energy resources, little of which has been exploited. Here, we review both tidal range and tidal stream energy, with a focus on the resource, feedbacks, and environmental interactions. The review covers a wide range of timescales of relevance to tidal energy, from fortnightly (spring-neap) and semi-diurnal variability, down to array, and device-scale turbulence. When simulating the regional tidal energy resource, and to assess environmental impacts, it is necessary to account for feedbacks between the tidal array and the resource itself. We critically review various methods for simulating energy extraction, from insights gained through theoretical studies of “tidal fences” in idealized channels, to realistic three-dimensional model studies with complex geometry and arrays of turbines represented by momentum sinks and additional turbulence due to the presence of rotors and support structures. We discuss how variability can be reduced by developing multiple (aggregated) sites with a consideration of the enhanced phase diversity offered by exploiting less energetic tidal currents. This leads to future research questions that have not yet been explored in depth at first-generation tidal sites in relatively sheltered channels (e.g., the interaction of waves with currents). Such enhanced understanding of real sea conditions, including the effects of wind and waves, leads to our other identified primary future research direction—reduced uncertainties in turbulence predictions, including the development of realistic models that simulate the interaction between ambient turbulence and the turbulence resulting from multiple wakes, and changes to system-wide hydrodynamics, water quality, and sedimentation.

Numerical models have been widely used for the resource characterization and assessment of tidal instream energy. The accurate assessment of tidal stream energy resources at a feasibility or project-design scale requires detailed hydrodynamic model simulations or high-quality field measurements. This study applied a three-dimensional finite-volume community ocean model (FVCOM) to simulate the tidal hydrodynamics in the Passamaquoddy–Cobscook Bay archipelago, with a focus on the Western Passage, to assist tidal energy resource assessment. IEC Technical specifications were considered in the model configurations and simulations. The model was calibrated and validated with field measurements. Energy fluxes and power densities along selected cross sections were calculated to evaluate the feasibility of the tidal energy development at several hotspots that feature strong currents. When taking both the high current speed and water depth into account, the model results showed that the Western Passage has great potential for the deployment of tidal energy farms. The maximum extractable power in the Western Passage was estimated using the Garrett and Cummins method. Different criteria and methods recommended by the IEC for resource characterization were evaluated and discussed using a sensitivity analysis of energy extraction for a hypothetical tidal turbine farm in the Western Passage.

Classification systems for wave energy resources and wave energy converter (WEC) technologies could provide similar benefits to those for the wind energy industry: resource classification facilitating reconnaissance studies and project planning at both regional and national scales; and WEC classification streamlining and reducing costs of WEC device design and manufacturing. In the present study, a classification system for U.S. wave resources is used to investigate the feasibility of WEC classification. Wave spectra inputs from three wave energy resource classes delineated in this system are used to derive distributions of optimized WEC design scaling factors, as well as WEC design responses. Preliminary results indicate that a single standard WEC design class could serve within a given resource class, and corresponding regional wave climate, due to distinct wave energy distributions and concentrations of energy within partitioned period bands for each resource class. The WEC response to extreme loads was found to vary considerably within the most energetic of the resource classes examined, suggesting the need for these standard design classes to meet structural design requirements based on the upper limits of load response within a given resource class. However, the observed load metric variation is lower than the inter-region resource variations.