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Where is the oil likely to spread?

Dr. Thomas Church

Richard Garvine, Maxwell P. and Mildred H. Harrington Professor of Marine Studies, UD College of Earth, Ocean, and Environment. Phone: (302) 831-2169   E-mail: rgarvine@udel.edu

Dr. Richard Garvine at the University of Delaware College of Earth, Ocean, and Environment provides this analysis of the Delaware River and Bay's circulation:

"From our group’s study of the circulation and mixing of water in the Delaware Estuary (the Delaware Bay and the tidal portion of the Delaware River, which extends to the falls at Trenton, New Jersey) over the past two decades, we have assembled a coherent view of how estuarine circulation works in general.

The dominant source of current in the estuary is the tidal period (12.4 hr) rise and fall of the water level impressed on the estuary’s mouth by the tidal variations on the adjacent continental shelf. Typical tidal currents in the estuary are about 0.8 meters/second or about 1.5 mph. The mechanism creating the next rank of tidal current is the horizontal difference in water density between the fresh water in the Delaware River and the heavier water seaward in the estuary where shelf water has intruded at depth and mixed upward with seaward flowing fresh water. This force creates a strong conveyor belt-like circulation pattern in the vertical with the lighter water passing through seaward in the upper half of the water depth and landward in the bottom half. A similar circulation is established in a room with a wood stove in operation. Hotter (less dense) air rises and travels upward along the ceiling while (more dense) air sinks and travels along the floor inward toward the stove. Typical current speeds for density-driven circulation in the Delaware estuary are about 0.05 to 0.10 m/s or about 0.1 to 0.2 mph.

Wind produces current, also. Wind from the northwest induces a seaward flow for the upper 2/3 of the water depth and a landward flow in the lower 1/3 with typical speeds the same as the density driven flow. The directions reverse for wind from the southeast. But there is a special twist to the wind-forced current. Strong winds from the southwest impact the estuary little, as they are crosswinds there, while on the shelf nearby they produce coastal upwelling and falling sea level. This is imposed on the estuary mouth causing falling water levels there, too. In the reverse with winds from the northeast (as in a ‘northeaster’), sea-level rise results, often with coastal flooding. Currents in the estuary then reflect this water-level change by flowing landward in northeasters and seaward in southwesterly winds. The speeds from this action are a fraction of the locally driven winds discussed above.

In the estuary, the strong tidal currents themselves with daily and twice-daily periods produce a steady or very slowly varying current called the tidal residual. This flow is weaker at about 0.05 mph, but makes up in persistence by its long term of activity in transporting materials, including petroleum. Last in intensity is the current driven by the water added to the estuary by its tributaries, the largest being the Delaware River. This flow drifts seaward at all depths much like water running downhill in a rain gutter. Typical speeds are 0.005 mph or about 0.25 miles/day.

Averaging over the tides, we find under most conditions a net seaward flow, especially in the upper half of the depth. Most of the oil from a spill will be in this part of the water column, and we can expect typical current speeds of 0.1-0.2 mph or about 2-5 miles per day.

 Once at the mouth at Cape Henlopen, the flow continues into the waters of the continental shelf, but because of the action of the Earth’s rotation (Coriolis force) it turns right and continues to follow the coast of Delmarva sometimes as far south as the mouth of Chesapeake Bay.  We refer to this feature as the Delaware Coastal Current. If it originally was transporting oil at the mouth of Delaware Bay, it will likely distribute that oil while gliding along the coast. If southwesterly winds blow, however, sea level falls at the coast in connection with offshore movement of the surface water. This transports oil and water offshore with rapid mixing with coastal seawater.

Our group has applied a mathematical numerical model of the circulation and mixing of the Delaware Estuary and Delaware Coastal Current that has given good agreement when tested against field observations. We are planning to use this model to study mixing of fresh water from the Delaware estuary with the seawater on the continental shelf.  Then we will apply the model to the detailed transport of oil seeking to simulate a spill in the estuary. Results from this modeling effort are expected by 2006."

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