Low Frequency Vessel-Generated Waves in a Tidal River

Large shipping vessels generate highly energetic waves that can contribute significant energy to the hydrodynamics of a tidal river. A hydrodynamic characterization in the Savannah River, Georgia, indicates that the wake of commercial shipping vessels up to 366 m in length is the predominant source of energy at the shoreline and is a strong contributor to the erosion observed at the site. During large vessel wake events at the study site, the majority of the energy flux is contained in its low frequency components of the wake. Furthermore, the first low-frequency wave to arrive on shore, the primary wave, typically contains the strongest peaks in energy flux. This 2-3 minute wave is generated by the pressure field produced by the vessel and within a constrained channel can be theoretically explained with the Bernoulli equation. In observations of vessel wake on a shelf adjacent to the shipping channel at the study site, the primary wave is followed by waves of a similar frequency with decaying amplitudes that may persist for up to an hour after the vessel passage; it has been hypothesized that these persistent waves are the result of cross-channel seiching excited by the vessel passage and the primary wave. To further investigate these low-frequency waves, wake events are modeled in FUNWAVE, a 2D Boussinesq model; vessel passages similar to those observed at the study site are simulated within a simplified domain featuring a shipping channel and a shelf. Preliminary results suggest that, in addition to the effects of seiching, the observed low-frequency waves may be the result of a wave train trapped on the shallow shelf and propagating down the river along the shoreline. Through the analysis of field measurements and FUNWAVE modeling, the characteristics of the persistent low-frequency waves are described, and the mechanisms underlying their generation are explored.