Thom, R.M., A.B. Borde, P.J. Farley, M.C. Horn and A. Ogston. Batelle Marine Sciences Laboratory. 1996. Passenger-only ferry propeller wash study: threshold velocity determinations and field study, Vashon Terminal. Report to WSDOT. PNWD-2376/UC- 000. 15+pp.

REGION: PNW SPECIES: Eelgrass

STUDY OBJECTIVES: Impact assessment. Determine critical current velocities that damage eelgrass and compare experimental flume data with a field verification study at the Vashon Passenger-only Ferry Terminal documenting current velocities, suspended sediment concentrations and PAR.

METHODS: Empirical: controlled flume experiment at Battelle Laboratory to assess currents that damage eelgrass leaves and rhizomes. The successive treatments were not independent rendering cumulative eelgrass with each treatment. Empirical: on-site field investigation of actual bottom current speeds were measured at various prop speeds and at varous distances from the ferry, covering the predicted region of bottom impact over various speeds. Instruments used included velocimeter, backscatter sensor and PAR data logger, digital compass and tilt sensor.

VELOCITY IMPACTS: Bottom currents were increased from 2 to 30 cm per second by prop speeds of 550 rpm and 750 rpm at 32 m from the boat. While bottom currents increased at 750 rpm and slightly for 1000 rpm at 57 m from the boat. At the nearest station, only prop speed of 550 rpm increased bottom currents. Prop wash reached the bottom nearer the ferry at slower prop speeds. Conversly, at higher prop speeds, the wash contacted bottom further behind the boat. The prop's spiraling effect and bottom impact resulted in high varability in currents along the horizontal axis. This turbulence is likely to stir up bottom sediments and disrupt eelgrass and other benthic organisms. Temporal variability was great between prop speeds and prop-induced current speeds across varying distances from the boat.

LIGHT IMPACTS: PAR decreased with increasing prop speed: 30% at 550 rpm, 50% at 750 rpm and 70% at 1000 rpm. The greatest impacts were at the sites closest to the boat. The reduction in transparency was due to increased suspended matter and bubbles which increased with increased prop wash. The reflective nature of the prop bubbles resulted in some higher PAR values with at 550 rpm than at 0 rpm. It is also possible that suspended matter settles out of the water column prior to the dissipation of the bubble plume. The findings suggest that the shading from the ship's hull may be more important than prop wash in reducing light.

SUSPENDED IMATTER IMPACTS: At 41 m from the boat, a slight increase in bottom current speeds resulted in a slight increase in suspended matter. The pass-over runs did not affect suspended matter. The depth of the water (6.7m) prevented the wash plume from affecting the bottom sediments.

THRESHOLD VELOCITIES (flume studies):Current speeds on the order of 50-80 cm per second potentially erode eelgrass patches with speeds over 180 cm per second severely damaging patch edge. Cumulatively, the erosive events remove sediments from the root rhizome system and expose below ground plant parts to degradative processes. The eelgrass mat did not completely erode at the greatest velocities tested, suggesting a significant capacity to remain in place despite erosive prop wash forces. Repeated erosion and changes to sediments around the plants can result in plant death or meadow migration. Eelgrass depends upon biogeochemical processes in the sediments to maintain its growth. Sediments also protect the plants from drying and animal foraging.




Conclusions: Current speeds over 180 cm per second could severely damage the edge of an eelgrass patch. However, eelgrass patches in Puget Sound can survive tidal currents velocies as great as 200 cm per second.
The displacement of sediments brought on by increased currents threatens the integrity of the plant. Effects varied with both distance and prop speed with lower prop speeds increasing bottom currents close to the vessel and higher prop speeds increasing currents away from the vessel. The net effect of high turbulence may be important in loosening sediment particles and eroding eelgrass. A mean of the 30 greatest velocities measured may be more indicative of the erosive stresses of ecological significance. These maximum and mean velocities should be factors when designing ferry terminal setback. At 57 m from the boat, it is likely that the prop wash has little effect on the existing eelgrass. The strongest correlation was between propeller speed and PAR. This indicates that increased prop speed increases suspended matter and bubbles that lower bottom light levels. Increased prop speed increases the light reduction impact which is manifested at increasing distances from the boat.