UDMessenger

Volume 13, Number 1, 2004


Connections to the Colleges

Estuary research gets to the bottom of things

Children always have loved to play at the river's edge, building castles in the sand and making mud pies. And, if you ever waded in a river, can you forget that feeling of mud oozing between your toes?

For Christopher K. Sommerfield, assistant professor of oceanography in the College of Marine Studies, that mud has taken on a greater significance. He studies it to understand how human activities and natural processes have affected the Delaware River Estuary.

Sommerfield says he became "hooked" on coastal rivers and estuaries when he was a graduate student at the State University of New York at Stony Brook. However, he admits that he always has been fascinated by the ocean, even as a child wondering what lay beneath the surface.

The importance of the Delaware River Estuary cannot be understated, Sommerfield says. The mixture of salt water from the ocean with fresh water from rivers has created critical habitat for a wide variety of plant and animal life, including the world's largest population of horseshoe crabs. Stretching 134 miles from Trenton, N.J., to the mouth of the Delaware Bay, the estuary also supports numerous recreational activities and commercial businesses, including the largest freshwater port on the Eastern seaboard. About 70 percent of the oil that arrives on the East Coast is transported through the Delaware Estuary.

According to Sommerfield, the sediments in the estuary play a crucial role in maintaining the delicate balance of its ecosystem. Currently, an estimated 1.4 million metric tons of sediment (about 100,000 dump-truckloads of mud) washes into the estuary from rivers each year. But, where does all this sediment go?

To help answer this question, Sommerfield is working to develop a "sediment budget," which accounts for the amount of sediment that is added to and removed from Delaware Estuary waters. The sediment budget can be used to understand how different processes affect the movement of sediment from its sources in the Delaware River watershed to its resting places, or sinks, in the estuary seafloor and fringing tidal marshes. Such budgets are commonly used to manage sediments and shorelines in coastal waters worldwide.

"Tackling the sediment budget for the Delaware River Estuary is extremely difficult," Sommerfield says. "The sediment sources and sinks are variable in space and time, so it is a challenge to track the comings and goings of the river mud."

Sommerfield has compared historical records of river flow and sediment discharge from the late 1800s, the mid-1950s and the late 1980s with changes in the depth of the estuary, as indicated by bathymetric maps, for the same time periods. This comparison indicates that, although shoreline developments have tended to decrease the width of the upper estuary, the lower estuary has widened as a result of sea-level rise and shore erosion. On the whole, the estuary has deepened through time.

"This analysis also indicated that tidal currents have been eroding the seafloor in some areas, providing a source of muddy sediment that had previously been unaccounted for," Sommerfield says. "Although this process is natural, engineering practices such as the construction of bulkheads and dredging have modified the native shape of the estuary and thus might exacerbate this effect."

He notes that even though a very thin layer of the seafloor is being eroded each year on the order of four-hundredths of an inch or less, it creates a large source of sediment because such a large area is affected. "In addition to the sediments that are washed into the estuary from rivers, there is this previously underappreciated source from the seafloor," he says.

To help identify pathways and rates of sediment movement, Sommerfield has deployed various oceanographic instruments in the river. "These instruments allow us to make continuous measurements of sediment transport for a specific period of time and can provide glimpses of how the estuary responds to events such as coastal storms and floods," he says.

Most recently, Sommerfield has used a technique called multibeam sonar to map the river bottom. In this technique, a group of sound beams is emitted from a transducer, or an electronic device, mounted on the hull of Cape Henlopen, the College's 120-foot research vessel. The transducer records the two-way travel time of each outgoing sound pulse or "ping" and converts it to water depth. The width of the multibeam swath is generally seven to eight times the water depth, which allows large areas of seafloor to be imaged rapidly.

In addition to providing a continuous picture of the bottom, this technique is capable of resolving very small features, on the order of 4 inches, that convey information on sediment transport. "These small details shed light on how the bottom behaves from place to place," Sommerfield says. "From theory and experience, we know that particular bottom types form only under specific conditions of current flow and sediment size."

Another advantage of the technique is that it provides an accurate baseline of seafloor elevation. Repeat surveys of a seafloor area can be conducted through time to determine whether the bottom has been eroding or accumulating sediment and in what amounts.

With the Delaware River Estuary project, Sommerfield has been able to satisfy his interest in how natural and human factors influence sedimentation in coastal environments. "My goal is to understand how human activities affect the Delaware River Estuary," he says. "This will ultimately help us make more informed decisions about how to manage the estuary without adversely affecting it for future generations."

--Kari K. Gulbrandsen, EG '91M