Can variable erosion rates be determined for shorelines composed of unconsolidated glacial sediment

1
Can variable erosion rates be determined for
shorelines composed of unconsolidated glacial
sediment?
Or When Will that Rich Guys House Fall Off the
Cliff?

• Heather Rogers
• University of Washington
• 2007

2
Outline

• Motivation
• Field Area
• Collecting Data
• Geomorphic Evidence
• Cosmogenic Nuclide Dating
• Proposed Treatment of the Data

If there are any terms youve never heard before,
raise your hand!
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Perkins Lane area of Seattle following the
1996-97 El Nino winter
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Miles of shoreline versus the miles of unstable
shoreline by county for Washington State.
Perkins Lane area of Seattle following the
1996-97 El Nino winter
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• Population is expected to increase 50 for Island
  and 25 for King counties by 2030 (WA State DOT)
• Little systematic research has been carried out
  regarding the quantitative assessment of
  shoreline erosion rates around the Puget Sound
  region (Shipman, 2004).

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• Population is expected to increase 50 for Island
  and 25 for King counties by 2030 (WA State DOT)
• Little systematic research has been carried out
  regarding the quantitative assessment of
  shoreline erosion rates around the Puget Sound
  region (Shipman, 2004).

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To quantify long term shoreline retreat rates, I
propose to use both of the following

• Geomorphic Evidence
• Surface Exposure Dating

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Whidbey Island, WA

• Field Area

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Why Whidbey Island, WA?

• Presence of ground water sapping channels graded
  to the marine high stand immediately following
  deglaciation.

LIDAR image of eastern Possession Point Whidbey
Is, WA.
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Explaination of Ground Water Sapping

• Groundwater sapping is the geomorphic process in
  which groundwater exits a bank or hillslope
  laterally as seeps and springs and erodes soil
  from the slope. This often causes the slope to be
  undermined and undergo mass wasting, hence the
  word sapping. Erosion by sapping tends to produce
  steep-sided U-shaped valleys of fairly uniform
  width with box-like, "theater-shaped" headwalls.
  This contrasts with the more common branching or
  dentritic pattern of V-shaped valleys produced by
  overland flows that become wider with distance
  from their source.

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Reconstruction of the Original Shoreline

• Southern Whidbey Island deglaciated 15,500 yrs
  BP
• Channels incised easily into bare glacial
  sediment
• Holocene sediment filled in those channels that
  became inactive

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Shoreline Emergence /Submergence History

• 15,500 years BP
• Whidbey Island deglaciated (Swanson and Caffee
  2001). Sea level 140 feet higher than modern
  (Dethier et al. 1995).
• 15,500 to 13,000 years BP Sea level reached the
  marine low-stand (Dethier et al. 1995).

• 13,000 to 6,000 years BP Eustatic sea level
  rise exceeds isostatic uplift.
• 6000 years BP
• Eustatic sea level is near modern sea level.

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Shoreline Emergence /Submergence History

• 15,500 years BP
• Whidbey Island deglaciated (Swanson and Caffee
  2001). Sea level 140 feet higher than modern
  (Dethier et al. 1995).
• 15,500 to 13,000 years BP Sea level reached the
  marine low-stand (Dethier et al. 1995).

• 13,000 to 6,000 years BP Eustatic sea level
  rise exceeds isostatic uplift.
• 6000 years BP
• Eustatic sea level is near modern sea level.

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Shoreline Emergence /Submergence History

• 15,500 years BP
• Whidbey Island deglaciated (Swanson and Caffee
  2001). Sea level 140 feet higher than modern
  (Dethier et al. 1995).
• 15,500 to 13,000 years BP Sea level reached the
  marine low-stand (Dethier et al. 1995).

• 13,000 to 6,000 years BP Eustatic sea level
  rise exceeds isostatic uplift.
• 6000 years BP
• Eustatic sea level is near modern sea level.

15
Shoreline Emergence /Submergence History

• 15,500 years BP
• Whidbey Island deglaciated (Swanson and Caffee
  2001). Sea level 140 feet higher than modern
  (Dethier et al. 1995).
• 15,500 to 13,000 years BP Sea level reached the
  marine low-stand (Dethier et al. 1995).

• 13,000 to 6,000 years BP Eustatic sea level
  rise exceeds isostatic uplift.
• 6000 years BP
• Eustatic sea level is near modern sea level.

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Reconstruction of the Original Shoreline

• LIDAR
• Using LIDAR (Light Detection and Ranging), we can
  define longitudinal profiles of these relict,
  post-glacial channels that are graded to the last
  glacial marine high-stand.

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LIDAR Data Collection Technique
LIDAR data is collected by measuring the travel
time of laser pulses from an aircraft to the
surface of the earth and back. Other data
collected during the flight includes the GPS
located (Global Positioning System) position of
the aircraft and the INS (Inertial Navigation
System) measured direction that the pulse was
fired. This information can then be converted
into map coordinates and elevation data.
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Reconstruction of the Original Shoreline

• Using LIDAR accompanied by ground truthing in the
  field, one can define the base of these roughly
  linear stream channels.

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• Surface Exposure Dating using Cosmogenic Nuclides
• Cosmogenic nuclides are rare isotopes created
  when high-energy cosmic rays interact with the
  nucleus of an in situ atom. These nuclides then
  accumulate in rock, and the concentration can be
  measured to determine the age of exposure.

Courtesy University of Glasgow CfG-CNL
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• The veneer of Vashon Till draped over the
  drumlinoid topography of Whidbey Island contains
  large till boulders, which are exposed to the
  cosmic flux as the finer glacial material is
  eroded away.

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Has shoreline retreat been constant through time?
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Surface Exposure Dating Using Cosmogenic Nuclides

• Potential Complications
• Are the boulders still in original position
  following deposition?
• Prior exposure while the boulders are still
  contained in the bluff
• Occasional water/organic debris cover

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Surface Exposure Dating Using Cosmogenic Nuclides

• Potential Complications
• Are the boulders still in original position
  following deposition?
• Prior exposure while the boulders are still
  contained in the bluff
• Occasional water/organic debris cover

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Surface Exposure Dating Using Cosmogenic Nuclides

• Potential Complications
• Are the boulders still in original position
  following deposition?
• Prior exposure while the boulders are still
  contained in the bluff
• Occasional water/organic debris cover

25
Recap of Proposed Research

• Geomorphic Data Relict Stream Channels Graded
  to the Marine High Stand
• Cosmogenic Nuclide Data Surface Exposure Dating
  of Lag Boulders

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Questions/Suggestions?

• Special thanks to
• National Science Foundation
• Harvey Greenburg
• T.W. Swanson
• J.O. Stone
• Joy Laydbak