Transport and Dispersion within the Upper 10 m layer of Port Valdez, Alaska driven by Winds, Estuarine Circulation and Mesoscale Eddies

A recent study of circulation in Port Valdez, Alaska was sponsored in 2016 and 2017 by the PWS Regional Citizens Advisory Council to address the fate of oil potentially spilled at the Alyeska tanker loading facility. The research revealed seasonal patterns in circulation and dispersion within the upper 10 m layer caused by varied effects of wind stress, freshwater input, tides and large-scale turbulence in the form of mesoscale eddies. During the summer, maximum eastward (up-fjord) transport occurred in the afternoons due to sea-breezes, but at night westward transport occurred from a combination of down-fjord winds and estuarine flow concentrated along the northern shoreline. At these times drifters at 1 m reached maximum speeds of 40 to 50 cm s^-1 after 10 to 17 hrs, with and average distance from the center of mass (DCM) of 0.6 to 1.3 km. The surface drifters also attained high speeds (50 cm s^-1) but maximum DCMs were much lower (0.2 to 0.6 km). At 10 m currents reached speeds of only 10 to 15 cm s^-1 but drifters remained very close together and followed widening cyclonic orbits in the center of the fjord. After about 30 hr, however, the DCM along the northern shoreline increased to 0.5 km due to a relaxation of up-fjord winds combined with an ebb tide and estuarine outflow. In the fall, freshwater input increased significantly and winds initially blew westward for an extended period during a storm. At this time both the surface and 1 m drifters moved rapidly down-fjord at speeds ranging from 40 to 70 cm s^-1 with DCMs of 1.2 to 2 km. The 10 m drifters, by contrast, showed highly varied flow patterns and although all drifters initially followed cylonic eddies, drifters released in the central basin gradually moved inward along the southern shoreline whereas those released in the inner basin moved outwards along the northern side of the fjord. After 30 to 40 hr, however, both drifter sets exhibited high amounts of dispersion with DCMs ranging from 1.7 to 5.5 km. After the storm, wind speeds were markedly reduced but returned to a quasi diurnal sea-breeze pattern. Due to the strong estuarine flow, however, both the surface and 1 m drifters still moved outward. The surface drifters attained maximum speeds of 80 to 100 cm s^-1 but at 1 m the speeds reached only 17 to 27 cm s^-1. Dispersion for both groups was also very limited with DCMs of only 0.1 to 0.7 km. At 10 m the drifters again exhibited varied flow directions but the net motion was westward with very limited dispersion (0.4 to 0.7 km). The speeds also showed cyclical bursts going from < 1 to 10 cm s^-1 that were in phase with the tides. The varied seasonal patterns of dispersion over depth in Port Valdez showed that oil dispersed in the water column could reach very different regions of the fjord if dispersants were applied following a spill.