The Carlisle Mosquito Online

Friday, July 30, 2010

Submerged landscapes – marine mapping off the New England coast

“Massachusetts has been one of the leaders in doing a very systematic mapping of their offshore sea floor,” said U.S. Geographical Survey (USGS) oceanographer Dave Twichell. “It’s the only state doing this right now.” It is important to map the sea floor, he explained, so that decisions on where to place Liquefied Natural Gas (LNG) terminals and wind turbines can be made intelligently. The data is also useful for cable and pipeline placement and for fisheries management.

Twichell is based in Woods Hole, Mass. but his most recent work has taken him to the Gulf of Mexico to study the oil spill. He gave a presentation called “Submerged Landscapes” at the Gleason Public Library on July 26, as part of the “Go Green at Your Library” summer reading program.

Woods Hole research is extensive, interdisciplinary

The marine geology team in Woods Hole is one of three such teams in the U.S., he explained. The other teams are located in St. Petersburg, FL and Menlo Park, CA. Each team has roughly 100 people with a variety of backgrounds. Twichell said, “Our work is collaborative.” He explained that his team was made up of geographers, geologists, geophysicists, chemists, mechanical and electrical engineers, oceanographers, photographers, mathematicians, computer scientists and accountants, stressing the need for people with many different skills.

Twichell said, “Last year there were 36 field programs. Some were a day long, some lasted over five weeks.” The average is about two weeks long. “That’s a lot of time that people spend at sea.” He said they work as far north as the Arctic Ocean and as far south as the Caribbean, using a range of boats including ice-breakers in the Arctic, submersibles so the sea floor can be observed and kayaks for local data collection.

The major emphasis of the field programs are in the areas of climate change, human impact on the sea floor, permafrost melt, beach erosion, coastal and marine sediment contaminants, fish habitats, invasive species, landslides causing tsunamis and determining international boundaries.

Mapping the ocean bottom

Twichell explained the methods used to map the sea floor. “Sound travels underwater remarkably well.” Swath bathymetry, sidescan sonar and seismic reflection are different acoustic tools used to build a profile of the sea floor. Bathymetric sonar collects soundings of water depth in a wide swath beneath the ship. Sidescan sonar is essentially a low-angle black and white photo of the sea floor. The sidescan sonar “illuminates” the sea floor with a pulse of sound rather than light. This method defines the shape of the sea floor but not depth. Seismic reflection provides images of the subsurface, creating a profile of the sub-bottom sediments. The acoustic data from these methods provides seafloor topography, the type and distribution of the seafloor sediments and the thickness of the sediment deposits.

Underwater landslides

“There are other ways to form tsunamis than just earthquakes,” said Twichell. “The U.S. Nuclear Regulatory Commission has been accepting applications for five new nuclear power plants in coastal areas. They don’t want a tsunami to come and flood a nuclear reactor.” So the commission has asked the USGS team to look at the underwater landslide problem to decide if it is a hazard they need to be concerned about. Twichell said in 1929, a 21-foot tsunami hit Newfoundland killing 27 people. This was caused by an earthquake off the Grand Banks which caused an underwater landslide which generated the tsunami. “So they can happen. They are rare.”

Twichell showed an underwater landslide in an area south of New England. He said the landslide was four to five times the area of Carlisle, some 200 feet thick. “That’s a lot of dirt being moved.” He didn’t know how large a tsunami would be generated by such a landslide, but he hopes to find out. He also questioned how old the landslide was. He said he could figure that out by taking core samples.

Coastal Zone mapping

Twichell said with the new tools, a much better map of the coastal zone is being formed. This is important because the new maps show larger gravel and rocky areas which is good lobster habitat. “Massachusetts is trying to understand what they have out there so they can manage it effectively.” Better mapping also helps in laying new cables and pipelines.

Coastal erosion

“20,000 years ago, the sea level was 120 kilometers lower than it is now. A lot of the North American continent was covered in glaciers,” said Twichell. As the glaciers melted, the water rose, moving the coastline over 80 miles. He showed a chart of the rising sea. It was not linear. In the last 6,000 years, the sea has risen very gradually, but over the 20,000 years, some periods of time have seen a rapid rise in sea level. Twichell said the hitch is that the future could look a lot like the distant past. He noted that sea level has gone up ten inches in the last 100 years at Woods Hole, roughly an eighth of an inch per year.

Oil spill in the Gulf

Twichell showed aerial views of the oil in the Gulf of Mexico and discussed the plan to build berms around some coastal marshes. “First, you need enough sand to build it. You need enough time to build it. You need good weather to build it.” Finally, you need to understand sand movement in that area.

“Our mapping showed where the sand is,” said Twichell. The USGS also determined how thick the sand is. This information caused the project to shift where they dredged for sand. “The project has started.”

“Whether this is a wise decision or not is debatable,” said Twichell. “It’s certainly not something they can get done in time.” He asked the crowd, “So, the information we collected is being used, but is it being used wisely?”

This lecture was presented by the Susan Zielinski Natural Science Fund and the Friends of the Gleason Public Library. As a government employee, Twichell could not be paid for speaking; however the funds will be used to purchase new library books that he will help choose. ∆


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