The shrinking Beaufort Sea coastline
A USGS team is assessing changes along the North Slope coastline using historical and contemporary maps and aerial surveys
A recent paper published by geologists from the U.S. Geological Survey reported accelerating rates of Beaufort Sea coastal erosion along a 37-mile section of shoreline bluffs, north of Teshekpuk Lake, at the northeastern edge of the National Petroleum Reserve-Alaska. The geologists said that erosion rates have increased from 6.8 meters per year pre-1979 to 13.6 meters per year in the period 2002 to 2007, with as much as 25 meters of erosion occurring in some places in 2007.
“Concurrent Arctic changes potentially responsible for this shift in the rate and pattern of land loss include declining sea ice extent, increasing summertime sea surface temperature, rising sea level, and increases in storm power and corresponding wave action,” the geologists said. “Taken together these factors may be leading to a new regime of ocean-land interactions that are repositioning and reshaping the Arctic coastline.”
Major concernBut how do the rates of erosion discovered in the relatively short stretch of coastline investigated in this particular study compare with rates elsewhere along the Beaufort Sea coast? Shoreline erosion has become a major concern for coastal communities in northern Alaska and could perhaps impact the oilfield facilities of the central North Slope at some point in the future.
A USGS team is engaged in a study of shoreline change along the entire U.S. Beaufort Sea coast, as part of a multiyear assessment of land erosion and accretion around the entire U.S. coastline, USGS geologist Bruce Richmond told Petroleum News Feb. 25.
“Coastal land loss is an issue all around the country and it looks like rates are accelerating,” Richmond said.
The coastline project, involving cooperation in Alaska between various government agencies including USGS, the U.S. Bureau of Land Management, the U.S. Fish and Wildlife Service and the Alaska Department of Natural Resources, is collecting data from a variety of sources and assembling that data in a consistent format. The idea is to develop an historic archive and then update the data every five years or so, without having to keep re-investigating earlier data, Richmond said.
“The USGS is trying to come up with some consistent-methodology datasets so that we can look at one part of the United States in the same way as we look at somewhere else with respect to coastal erosion,” said USGS geologist Ann Gibbs.
And the USGS team started studying the Beaufort Sea coast around 2006.
“We actually collected oblique aerial photography and video … along the coast from a small floatplane,” Gibbs said.
Since then the team has been using more of an office-based approach, by tracking down and compiling historical data, from sources such as National Oceanic and Atmospheric Administration maps dating back as far as the 1940s, and then digitizing the data in an appropriate format.
“BP and ConocoPhillips have (also) provided us some of their imagery to use for our analysis,” Gibbs said.
Part completedThe team is using the shoreline data to make transects every 50 meters along the coast, west from the Canadian border, including the Beaufort Sea barrier islands but excluding major sea inlets, Richmond said.
“We’re probably about one-third of the way finished with the analysis of the historical information,” Gibbs said.
And in the summer of 2009 the team plans to conduct an airborne Light Detection and Ranging, or LIDAR, survey along the coast to document the location of the modern shoreline and obtain accurate land elevation data. The LIDAR data will enable the team to compile an up-to-date time history of erosion along the cost by comparing the position of the current shoreline with the historic data.
A LIDAR survey uses a laser system mounted in an aircraft to make precision measurements of landforms. It has 15 to 20 centimeter vertical accuracy, Richmond said.
Although the analysis of historic data is still a work in progress and remains subject to technical review, preliminary results suggest that erosion in the Prudhoe Bay area has been accelerating but that it is occurring at a much slower rate than the erosion found north of Teshekpuk Lake. In the period between 1981 and 2000, even those places with the highest erosion rates around Prudhoe Bay appear to have been losing less than 2 meters of land per year to the encroaching sea, Gibbs said.
Reasons for variationHowever, the variation in erosion rates along the coast raises some interesting questions about why rates are higher in some places and lower in others. That is a question that the USGS geologists have yet to address, although Richmond speculated that the grain size of the land material may prove to be an important factor — the bluffs north of Teshekpuk Lake tend to be composed of finer grained material than the more sand-rich material found at Prudhoe Bay.
After completing its analysis of the shoreline changes, the USGS team hopes to use its video of the coastline to classify different shoreline types and then link those types to the erosion rates, Gibbs said.
“After we come up with the (erosion) rates there’s a whole other level of the research project to try to reason it all out,” she said.