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“Ecosystem Restoration Environmental and Social Constraints

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“Ecosystem Restoration Environmental and Social Constraints Powered By Docstoc
					  “ECOGEOMORPHOLOGY OF THE MISSISSIPPI
RIVER DELTA: IMPLICATIONS (THRESHOLDS) FOR
   RESTORATION OF COASTAL LANDSCAPES
                               Robert R. Twilley
                Department of Oceanography and Coastal Science
                          Louisiana State University

                                                       “In a hundred years,
                                                       Louisiana as a whole had
                                                       decreased by a million
                                                       acres.“ [1.2 million since
                                                       1900, USGS]
                                                       Plaquemines Parish is
                                                       coming to pieces like old
                                                       rotted cloth. A hundred
                                                       years hence, there will in
                                                       all likelihood be no
                                                       Plaquemines Parish, no
                                                       Terrebonne Parish.”
Taken from ‘The Control of Nature’ by John McPhee, 1989
• Robert R. Twilley, Charles Sasser, Guerry Holm,
Azure Bevington: Louisiana State University.
• Chris Paola, University of Minnesota
• Gary Parker, Wonsuck Kim, University of Illinois
• David Mohrig, University of Texas-Austin




• Breton Sound Ecosystem Study, LSU
                      ECOSYSTEM THRESHOLDS

                                               Trajectories




DELTA LANDSCAPE
                                     Reference Restoration


                   THRESHOLDS
                  Degradation       REHAB     Rehabilitation


                       Degraded
                                  THRESHOLDS



                    ECOSYSTEM ATTRIBUTES
                                             1. Natural Systems Vulnerability:
                                                Susceptibility to
                                                biogeophysical impacts of sea-
             PRIMARY PRODUCTION                 level rise and river/sediment
Tidal/River
Flooding
                                                discharge on wetlands (climate
       Nutrient
                                                and land use changes) relative
                   SEA    Increase Sea Level
       Input
                   LEVEL                        to their adaptation
                                                (thresholds);

                          SUBSIDENCE



                        61 – 190 cm
                        change RSL
                          by 2100
  (3) What
Processes are
  at Work ?

Managing River and
 Coastal Processes
    linked with
Engineering Design
 to Develop a Self-
 Maintaining Delta
     Landscape
     (footprint)
There are critical thresholds of accretion relative to rise in water
levels that determine the stability of coastal wetlands.

                                                                 Subsidence, 4 mm/yr




                         Relative Rise in Water Level, mm/yr




                                                                                                             Threshold Accretion Rate, mm/yr
                                                                                              Delta HGM
             Sea Level
             Rise
                                                               Degradation                     Restoration
             SUB-
             SIDENCE



                                                                                       Adaptation Envelope


 61-190 cm
change RSL                                                                      Subsidence, 0 mm/yr
  by 2100

                                                                        Sea Level Rise, mm/yr
I believe the Bush administration should continue to withhold
money for coastal restoration in Louisiana. …. Most of the
Mississippi Delta, some 10,000 square miles, lies less than three
feet above sea level. Beset by land subsidence and rising sea
levels, much of this vast area will inexorably sink beneath the
waters by the end of this century.

BRUCE BABBITT
Washington Post, Friday, May 18, 2007
           ECOGEOMORPHOLOGY
              of Deltaic Coast

              Wax Lake Delta – An
              analogue to study the
             processes of prograding
                       delta;
Salinity   In contrast to coastal basins
            in the delta that represent
           degrading stages (ecosystem
              state change) of delta
LAND
SUMMARY: The Delta Mass Balance

The Dynamic Delta Top = competition between sea-level rise +
subsidence and deposition of sediment and organic matter:
                                    Wonsuck Kim, Gary Parker, David
                                    Mohrig, Robert Twilley, submitted
Atop the area of the delta                        1990
top        .
where H is eustatic sea
level,
σ spatially averaged
subsidence rate,
Qs total volumetric
sediment supply,
fr the fraction retained in the
delta top, and
rorg the rate of storage of
organic matter in the
sediment column,
SUMMARY: The Delta Mass Balance

The Dynamic Delta Top = competition between sea-level rise +
subsidence and deposition of sediment and organic matter:
                                    Wonsuck Kim, Gary Parker, David
                                    Mohrig, Robert Twilley, submitted
Atop the area of the delta                        2000
top        .
where H is eustatic sea
level,
σ spatially averaged
subsidence rate,
Qs total volumetric
sediment supply,
fr the fraction retained in the
delta top, and
rorg the rate of storage of
organic matter in the
sediment column,
SUMMARY: The Delta Mass Balance

The Dynamic Delta Top = competition between sea-level rise +
subsidence and deposition of sediment and organic matter:
                                    Wonsuck Kim, Gary Parker, David
                                    Mohrig, Robert Twilley, submitted
Atop the area of the delta                        2002
top        .
where H is eustatic sea
level,
σ spatially averaged
subsidence rate,
Qs total volumetric
sediment supply,
fr the fraction retained in the
delta top, and
rorg the rate of storage of
organic matter in the
sediment column,
SUMMARY: The Delta Mass Balance

The Dynamic Delta Top = competition between sea-level rise +
subsidence and deposition of sediment and organic matter:
                                    Wonsuck Kim, Gary Parker, David
                                    Mohrig, Robert Twilley, submitted
Atop the area of the delta                         2005
top        .
where H is eustatic sea
level,
σ spatially averaged
subsidence rate,
Qs total volumetric
sediment supply,
fr the fraction retained in the
delta top, and
rorg the rate of storage of
organic matter in the
sediment column,
MIGRATION OF THE MODEL FROM WAX LAKE TO THE
    MISSISSIPPI RIVER BELOW NEW ORLEANS:
Two diversions: Barataria Bay and Breton Sound (Envisioning the Coast)
     Developing a Self-Maintaining Coast is Possible
  BASE CASE: SEA-LEVEL RISE = 2 mm/yr, SUBSIDENCE = 5 mm/yr = 918 km2
VARIANT CASE: SEA-LEVEL RISE = 4 mm/yr, SUBSIDENCE = 10 mm/yr = 701 km2
       Solid line: variant case
       Dotted line: base case
    Thresholds Have to Account for Cause/Effect
 Relationships – Challenges to Restoration Approach
“In particular, hurricanes appear to be the overwhelming
pathway of new inorganic sediments for coastal wetlands
in western Louisiana, because the few riverine sources
bring relatively trivial amounts inorganic sediments into
the marsh.” (1)
“Restoration programs that restrict the underlying cause-
and-effect relationships and remedies to focus on inorganic
sediment source and delivery may be fatally flawed.” (2)
(1)R. E. Turner, J. J. Baustian, E. M. Swenson, J. S. Spicer, Science 314, 449 (2006).
(2)R. E. Turner, E. M. Swenson, C. S. Milan, J. M. Lee, Limnology and Oceanography 52,
1231 (2007).
                               Old River Control

                      2008 Flood Year

            Bonnet Carre

                                                     Caernarvon



                           Davis Pond


   Wax Lake


                                  Birds Foot Delta
Experimental Coastal Basins
Rank   Year
                Atch R.                                                          2008 Flood Year
                Peak Q (cms)
                                           • 74-yr record (1935-2008)
1        1973           22,118

2        1975           20,362             • 2008: 10-yr flood
3        1983           19,258

4        1945           18,720             • Since delta emergence - 1973:
5        1997           18,040                 13 of 15 record floods have
6        1950           17,842                 occurred




                                  Atchafalaya R. Discharge (cfs * 1000)
                                 Atchafalaya Riverdischarge (cfs *1000)
7        2008           17,728                                            1000            1970-2007 maximum             2008

8        1979           17,134
                                                                           800
9        1991           16,256
                                                                           600
10       1974           15,944

11       1990           14,981                                             400

12       2005           14,925
                                                                           200
13       1993           14,726

14       1984           14,641                                               0
                                                                                 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

15       1999           14,528
      Bonnet Carre
                                                     Caernarvon



                     Davis Pond

   Wax Lake


                                  Birds Foot Delta
Experimental Coastal Basins
     Hurricane       Year        Hurricanes Gustav & Ike
      Andrew         1992
                                                   400

      Georges        1998




                                 Water level (m)
                                                   300




                                Water level (m)
        Lili         2002
                                                   200

      Katrina        2005
                                                   100

       Rita          2005
                                                    0

      Gustav         2008
                                                    25


        Ike          2008




                                 Salinity (g/kg)
                                                    20




                                Salinity (psu)
                                                    15

• Return periods (1901-2005):                       10

      All hurricanes = 10 yr                        5

       cat 3 = 26 yr                               0

• 17 years                                           29 Aug   02 Sep   06 Sep   10 Sep   14 Sep

• 7 destructive storms
          Guerry Holm, Charles Sasser, Azure Bevington, Robert Twilley

2008 Elevation Surveys

• 7 transects (2,000 m)
   – geomorphic elevation
     gradient
   – near channel , natural levee,
     flank, interior
• 87 plots 1 m-2
• Class I laser level
• Surveys:
   – Feb: pre-flood
   – Jul-Aug: post-flood
   – Sep: post-storms
Mass Sediment Deposition
(bulk density = 0.7 g/cm 3)
Wax Lake Sediment Deposition
   (Current Delta Area = 112 km2)

                            This study
                         = 5.22 x106 MT
                  (82% flood event; 18% hurricane event)


                      Tornqvist et al 2007
                   (geometric / mass balance)
                         = 5.2 x106 MT
                       (4.3-5.8 x106 MT)
 Future: Trajectory
Soil-Plant Development
  Mineral vs Organic
   matter transition:
  When does organic
  matter production
  become dominant
    control on soil
      elevation?
            Conclusions & Perspectives:
Thresholds of Deltaic Coasts – Restoration Practices
  Ecosystem Adaptation is function of Thresholds
 In Deltaic Coasts – Pulses Significant to Thresholds
  Restoration Practice – Manipulating Thresholds
    River Management Reduces Deltaic Coast
   Adaptation to Climate Change (sea level rise)
 Experimental Coastal Basins of Mississippi River
   Delta and Restoration Practices offer Unique
  Opportunities in Modeling and Observations of
       Thresholds and Delta State Change
a) Two of the assumptions of the model presented are that land
   area estimates do not include the following: a) the contribution
   of soil formation from plant organic matter as part of the
   vertical elevation of land;
b) the redistribution of new sediment that is delivered along the
   coastal shoreline that may be deposited in wetlands during
   storm events. The role of soil formation in land development
   during century long simulations may be 50% by volume of the
   total soil horizon (33, 34).
a) The river delta is not a static board sinking into the sea or
   drowning under sea level rise, but instead is a dynamic surface
   that has been able to maintain its level slightly above sea level
   for millions of years, by maintaining a balance between
   sediment deposition and subsidence.

a) That sediment has over the same thousands of years been
   supplied primarily from the Mississippi River itself.
Plots with Accretion




Plots with Erosion
     Comparison to other Louisiana Wetlands
                      Time                 Elevation
Wetland Type
                     period                (cm yr-1)
Prograding delta
  Wax Lake            2008                      6.6
  Atchafalaya       1998-2007                   1.7
Degrading delta
  Fresh#            Since 1963                 0.56
  Brackish#         Since 1963                 0.52
  Salt#             Since 1963                 0.64




                                 #   Holm, Steyer, Piazza in prep, 45 cores
                                 137Cs  dating
                                                                Patterns in Tidal Frame:
                                                       Accretion/erosion along an Elevation Gradient


                                                               Flood                                                                                                        Hurricanes




                                                                                      Post-Storm elevation (cm MSL)
                                                                                                                      Post-storm accretion/erosion (cm)
 Post-flood accretion/erosion (cm)
Post-Storm elevation (cm MSL)




                                     25                                                                                                                   25

                                     20                                                                                                                   20

                                     15                                                                                                                   15

                                     10                                                                                                                   10

                                      5                                                                                                                    5

                                      0                                                                                                                    0

                                      -5                                                                                                                   -5

                                     -10                                                                                                                  -10
                                           -80   -60     -40    -20    0    20   40                                                                             -80   -60   -40   -20     0     20   40
                                                  Pre-flood elevation (cm MSL)                                                                                        Pre-storm elevation (cm MSL)