|
HELPING NPS UNDERSTAND LAKE MICHIGAN LANDSLIDE
|
|
Bruce Jaffe recently made the news in northern Michigan, where he is
|
|
studying a biglandslide that removed the beach along 500 m of shoreline in
|
|
Sleeping Bear Dunes National Lakeshore in February 1995. Approximately
|
|
100,000 cubic meters of sand above the shoreline of Lake Michigan and
|
|
1,000,000 cubic meters of sand below the shoreline slid into deep water
|
|
offshore, removing the beach and changing a gently sloping lake bottom into
|
|
a sharp 20-m dropoff near the shore. The National Park Service, concerned
|
|
about public safety and long-term effects on the coastal environment, asked
|
|
the USGS to study the slide. The request presents a unique opportunity to
|
|
Jaffe and his team, who conducted yearly topographic and bathymetric surveys
|
|
of the area from 1989 to 1993 as part of a shoreline-change study in the
|
|
Great Lakes Wetlands Project. With a detailed picture of pre-slide
|
|
conditions, they are well equipped to advance our understanding of coastal
|
|
and submarine landslides in lacustrine environments. The USGS team includes
|
|
Rob Kayen, Ken Israel, Tom Reiss, Homa Lee, Hank Chezar, and Guy Cochrane
|
|
from WUSGS and Randy Jibson from the Central Region Hazards Team. They are
|
|
receiving help from Max Holden of the Park Service and Jaques Locat of Laval
|
|
University, Quebec, Canada. The team has already begun conducting repeat
|
|
surveys of the area, using GPS and high-precision surveying equipment to
|
|
obtain elevations and positions accurate to within several centimeters. This
|
|
September they plan to (1) map the slide using 100-kHz sidescan sonar, (2)
|
|
conduct a bathymetric survey to quantify the volume of sediment moved during
|
|
the slide, (3) map sub-bottom lake geology using high-resolution seismic
|
|
reflection, and (4) deploy an underwater video camera to determine failure
|
|
type and slide-plane character. In FY97 they will install instruments to
|
|
measure geotechnical properties that could trigger sliding, and they will
|
|
perform a slope stability analysis to help the National Park Service predict
|
|
when the area may slide again. Two previous slides in this century--in 1913
|
|
and 1971- -indicate that landsliding will recur at the site. The team's work
|
|
was recently featured in front-page articles in two northern Michigan
|
|
newspapers, the Traverse City Record-Eagle (daily) and the Leelanau
|
|
Enterprise (weekly), and in a 10-minute National Public Radio broadcast
|
|
aired on local stations.
|
