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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)
Smith




        Harrison
        Bay
   International Migratory Bird
WHY? and Subsistence Value
Thermo-erosional niche at Dalton Wellhead
   Conceptual Model of Coastal Erosion
1. Thermo-erosional                        2. Niche growth
   niche
                      active layer                                 active layer

                        bluff                                      bluff


                        Thermo-erosional
                        niche                                      Niche growth


3. Block collapse/                         4. Erosion of fallen
   slump                                      block and sediment
                                              dispersion
                           active layer                                 active layer

                            bluff                                       bluff
Mathematical models of thermo-erosional niche growth
                (Kobayashi 1985)




     heat used
     to melt                      cooler       “warm”
                                  water    q    water
     ice




  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.
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)
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 ρ g H2 (gh)1/2 ~ H2.5
                   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
        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
Example Beaufort
Sea Ice Data




         Study site
                    Bathymetry for wave model
                                                      depth
                                          Effective    [m]
                         Effective
                         ice edge         ice edge
                         October          October
                         1955-1979        2002-2007
latitude




                                     study site
           Barrow
                        longitude
  SWAN-calculated wave height     wave
                                  height
(12 m/s NE wind, Oct 2002-2007)    [m]




                 study site
Regional temperature increase in West Beaufort Sea 1970-2007




                                                   4

                                                   2
                                                        temp
                                                   0
                                                         [C]
                                                   -2



                         1960     1980      2000




                                            From Steele et al. 2008
Nearshore/offshore
long-term
temperature differential




                           From Steele et al. 2008
Monthly average Prudhoe Bay
 temperature (1991-1994)
                 6

                 5

                 4
temperaute (C)




                 3

                 2

                 1

                 0
                      July   Aug   Sept   Oct
                 -1
       Assumed temperatures used in
            erosion rate model
                        6
                        5
                        4
Water temperature (C)




                        3
                        2                            1955-1979
                        1                            2002-2007

                        0
                             Jul   Aug   Sep   Oct
                        -1
                        -2
                        -3
                          Increasing wind speed in 2002-2007


                    0.6


                    0.5
fraction of month




                    0.4


                    0.3
                                                                   1955-79
                                                                   2002-2007
                    0.2


                    0.1


                     0

                          2       7         12           17   22
                                      wind speed (m/s)
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)
             Model Calibration and Results
• Erosion rate = 11.3 H 2.6 (Ta-Tm)                                                                  [m/yr]
                      60


                      50              Erosion rate = 2.52 x 10-24 e0.0284 t
erosion rate (m/yr)




                      40


                      30

                                                                                                        Model
                      20                                                                              calculation


                      10

                                                                          Measured data
                      0
                       1960   1970   1980        1990         2000            2010   2020   2030   2040       2050
                                                                time (year)
Projection of cumulative erosion
                          1400



                          1200



                          1000
cummulative erosion (m)




                           800



                           600



                           400



                           200



                             0
                             2005   2010   2015   2020   2025      2030       2035   2040   2045   2050   2055
                                                                time (year)
       Variation of erosion rate with month

                        0.5

                       0.45

                        0.4

                       0.35
erosion rate (m/mo.)




                        0.3

                       0.25                           1955-79
                                                      2002-7
                        0.2

                       0.15

                        0.1

                       0.05

                         0
                              Jul   Aug   Sep   Oct
                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
                     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
       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)
   Evolution of a North Slope barrier island
(Narwhal Island, North Arctic Alaska) 1955-2007
         Thomas M. Ravens and William Lee
           University of Alaska, Anchorage
2007   1990   1955   Westward movement
                     ~ 5 m/yr, 1955-1990
                     ~ 24 m/yr, 1990-2007
Questions?

				
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posted:9/1/2012
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