Predictive Coastal Erosion Modeling

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Predictive Coastal Erosion Modeling for
Teshekpuk Lake Special Area Tom Ravens (Univ. of
Alaska, Anchorage) with Joel Schmutz, Ben Jones,
Chris Arp (USGS, Anchorage)
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Smith
Harrison Bay
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International Migratory Bird and Subsistence
Value
WHY?
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Thermo-erosional niche at Dalton Wellhead
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Conceptual Model of Coastal Erosion
1. Thermo-erosional niche
2. Niche growth
active layer
active layer
bluff
bluff
Thermo-erosional niche
Niche growth
3. Block collapse/ slump
active layer
active layer
bluff
bluff
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Mathematical models of thermo-erosional niche
growth(Kobayashi 1985)
heat used to melt ice
warm water
cooler water
q
Rate of niche growth/erosion (m/yr) dxm/dt C
H3/4(Ta-Tm)
where C is a constant (including latent heat of
fusion), H is wave height,
Ta is ambient water temperature, and
Tm is the melting temperature.
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Model of rate of retreat of frozen bluff
(Kobayashi et al. 1999)
lc
Rate of retreat (m/year) lc hc (Ta-Tm)
/Lc(Hc-Bc) where hc convective heat transfer
coef. C2 H Or, . rate of retreat C3 H
(Ta-Tm)
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Conventional coastal engineering
approach(Without accounting for permafrost
melting)

• Cross-shore (offshore) sediment transport will
  result if the beach profile is not in equilibrium
  with the wave climate and the sediment grain size
• Transport will be related to the wave energy
  flux, F
• F E Cg 1/8 ? g H2 (gh)1/2 H2.5

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Our approach

• Assume an erosion rate (ER) expression
• ER C Hn (Ta-Tm)
• Use measured erosion rates (from 1955-1979 and
  2002-2007), and data on H and Ta (from those two
  periods) to calibrate model (i.e., determine
  best fit C and n)
• Use the model and projected H and Ta (for 2045)
  to estimate future erosion rates

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SWAN wave modeling

• Model accounts for wind generation, refraction,
  breaking, and energy exchange between frequencies
• Model was operated in quasi-steady mode, using
  wind data from Barrow
• Extent of open water in four open water months
  was determined from sea ice data

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Example Beaufort Sea Ice Data
Study site
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Bathymetry for wave model
depth m
Effective ice edge October 2002-2007
Effective ice edge October 1955-1979
latitude
study site
Barrow
longitude
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SWAN-calculated wave height(12 m/s NE wind, Oct
2002-2007)
wave height m
study site
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Regional temperature increase in West Beaufort
Sea 1970-2007
4
2
temp C
0
-2
1960
1980
2000
From Steele et al. 2008
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Nearshore/offshore long-term temperature
differential
From Steele et al. 2008
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Monthly average Prudhoe Bay temperature
(1991-1994)
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Assumed temperatures used in erosion rate model
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Increasing wind speed in 2002-2007
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Study site North of Lake Teshekpuk (70 53
N, 153 52 W) Erosion rate 5.3 0.5
m/yr (1955-1979) 7.4 0.6 m/yr
(1979-2002) 13.0 1.0 m/yr
(2002-2007)

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Model Calibration and Results

• Erosion rate 11.3 H 2.6 (Ta-Tm) m/yr

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Projection of cumulative erosion
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Variation of erosion rate with month
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Conclusions

• The erosion rate is likely to increase
  significantly in the coming years
• The erosion rate on the Beaufort Sea coast by
  Drew Point may increase to about 50 m/yr by 2045
• The erosion rate went as H2.6 but the dependence
  on H might be reduced if water level was
  explicitly accounted for by the model

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Future work

• Extend erosion projections throughout study site
  (Smith Bay to Harrison Bay).
• Develop a more mechanistic or physics-based
  accounting of the erosion process.
• Account for niche growth, erosion of fallen
  block, dispersal of sediments
• Include additional environmental variables (e.g.,
  bluff height, soil ice content, soil grain size,
  beach elevation/slope, sea level rise)
• Storm-surge modeling

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Future work (continued)

• Develop a better understanding of wave generation
  and propagation in ice floes.
• More precise accounting of historic temperature
  using original data
• More precise projection of future ice and
  temperature conditions (if possible)

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Evolution of a North Slope barrier island
(Narwhal Island, North Arctic Alaska)
1955-2007 Thomas M. Ravens and William
Lee University of Alaska, Anchorage
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Westward movement 5 m/yr, 1955-1990 24
m/yr, 1990-2007
1955
1990
2007
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Questions?
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