Effects of Primary Ship Waves in a Far-Field Waterway Network

Abstract

Ships navigating confined channels produce a low-frequency (about 0.005–0.010 Hz) primary wave that can cause erosion and damage coastal structures, among other effects. Here, far-field wake signals originating from the primary wave are examined using pressure and velocity observations collected along the Savannah River shipping channel in Georgia, and throughout a connected far-field network of smaller waterways. First, analysis of far-field hydrodynamic power at primary wave frequencies reveals that episodic packets of elevated power are linked to ship passages and therefore identified as far-field wake. Comparison of the time-averaged power of far-field wake and tidal processes indicates similar contributions, but the peaks of far-field wake power exceed the peaks of tidal power by 1–2 orders of magnitude. For each of three “source” locations and each ship passage, long-wave celerity is used to predict the propagation of a low-frequency wake signal out of the shipping channel and into the far-field. Agreement between propagation predictions and the arrival of waves at the field instrumentation reveals that low-frequency wake enters the far-field at all source locations and remains detectable for at least 15 km of propagation, the spatial extent of this study. Wave heights of isolated far-field wake scale linearly with near-field primary wave heights and range from 2 to 18 cm in the observations. Wave heights of potentially overlapped far-field wake signals reach 28 cm. As wake propagates into the far-field, it is attenuated by waterway junctions and frictional losses, the effects of which are enhanced at higher tidal stages. By documenting the extensive propagation and potentially significant power of far-field wake stemming from the primary wave, this study demonstrates the need for further research into the factors governing far-field wake generation, magnitude, and impacts.