In recent years, severe scour at the Kingston ferry terminal in Washington state has caused concern for terminal safety, as underwater erosion has caused an undersea cliff face to migrate shoreward toward the onshore ferry trestle structure. To investigate the role of ferry-generated turbulence in causing the erosion, this project characterized the ferry vessel wake and wash structure and developed a model to better predict seabed stress at ferry terminals caused by propeller wash.
The researchers used state-of-the-art oceanographic instruments to take high resolution measurements of velocity and turbulent bottom stress at the seabed and along the length of each of the two slips at the Kingston ferry terminal during vessel arrivals and departures. They also developed an empirical model for estimating bed stress on the basis of observed turbulent stresses and applied it to a 9-year-long record of vessel activity and tidal stage data.
The researchers found that vessel arrivals and departures were associated with 10 to 30 times larger velocities and 10 to 100 times larger stresses than during quiet periods. The ferry wake and propeller wash structure was affected by the difference in bathymetry (depth of the seabed) between Slip 1 and Slip 2, particularly the steeper slope at Slip 1. Vessel arrivals were generally associated with lower bed stresses than vessel departures. Different variables were found to influence the magnitude of bed stresses during ferry arrivals and departures. During departures, higher bed stresses occurred at lower depths because of tidal variability, whereas during arrivals, higher bed stresses were primarily associated with larger vessels.
The model showed that over periods of longer than one to two weeks, the most important factor related to cumulative seabed stress is the frequency of vessel activity at a given slip. The stress on the seabed during departures can be influenced by changing the seabed elevation, and this investigation showed that filling the scour hole at Slip 1 would likely cause an increase in seabed stress.
The developed model predicts expected bed stress but not scour. It will be portable to other Washington State Ferries terminals, and calibrated for other vessels, it could also provide information about seabed stress potential in other ferry and vessel systems.
Authors:
Samuel E. Kastner
Alex R. Horner-Devine
Jim M. Thomson
UW Department of Civil and Environmental Engineering
Chris Stearns
Washington State Ferries Terminal Engineering
Sponsor: WSDOT
WSDOT Technical Monitor: Chris Stearns
WSDOT Project Manager: Jon Peterson