Ship Wakes and Their Potential Impacts on Salt Marshes in Jamaica Bay, New York

Abstract

Aerial photographs and field studies have revealed a rapid deterioration of salt marshes in Jamaica Bay, New York. Past studies have linked marsh deterioration to sediment supply, water quality, storms, and sea level rise. Yet ship wakes and their potential impacts on marsh edge erosion are not understood. Here, we study ship wake transformation in Jamaica Bay and their potential impacts on salt marsh erosion. We apply short-time, Fourier transform (spectrogram) on existing water level measurements collected during 2015 and 2016. Our analysis reveals the existence of typical wake components. Among the observed wake components is a long wave component which propagates over shallow areas where short wind waves do not reach. We further implement a phase-resolving wave model to study wake transformation in the vicinity of salt marsh islands Little Egg and Big Egg and the consequent morphological changes. The selected marshes are located near a deep shipping channel and a ferry station, making them exposed to wakes of vessels with different size and sailing speed. A series of numerical experiments show that ship wakes can result in erosion spots near the border of deep shipping channels and their banks, i.e., edges of mudflats and marsh substrates. We show that the cumulative erosion increases rapidly with the number of vessels that pass through the study area. For instance, the magnitude of final bed erosion after the passage of 10 vessels is two to three times larger than that after the passage of five vessels.

Spectrogram analysis of surface elevation signals due to accelerating ships

“…8(a) by deriving a modified dispersion curve as follows. Instead of solving (11) or (13) for k, which is not possible outside of the caustic, we set k such that min…”

“…Waves generated by the ship travel with phase velocity c p and group velocity c g . The right angle triangle between U and c p is a consequence of (11).…”

“…The tool we employ to achieve this goal is the spectrogram, which uses short-time Fourier transforms to decompose each wave signal from the fixed sensor into a time-frequency heat-map. In recent times, some success has been achieved in using spectrograms to identify different features of ship wakes (e.g., their transverse and divergent waves) observed in real shipping channels and in experimental towing tanks [2,3,[5][6][7][8][9][10][11][12][13]. The theory for this line of enquiry has been almost entirely for steadily moving ships [1,3,5,8], with very brief studies of accelerating ships moving in one direction [1,5].…”

Spectrogram analysis of surface elevation signals due to accelerating ships

“…respectively. That is, we move as close as possible to the solutions of the second equation in (11). In terms of the method of stationary phase, instead of forcing the derivatives ∂g 1,2 /∂τ to vanish, we minimise them.…”

“…The tool we employ to achieve this goal is the spectrogram, which uses short-time Fourier transforms to decompose each wave signal from the fixed sensor into a time-frequency heat-map. In recent times, some success has been achieved in using spectrograms to identify different features of ship wakes (eg., their transverse and divergent waves) observed in real shipping channels and in experimental towing tanks [2,3,[5][6][7][8][9][10][11][12]. The theory for this line of enquiry has been almost entirely for steadily moving ships [1,3,5,8], with very brief studies of accelerating ships moving in one direction [1,5].…”