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Sustainability of Living Shorelines in a World of Rising Sea Level and Increasing Wave Energy
RAE December 2016
Carolyn Currin, Jenny Davis, Amit MalhotraNOAA Beaufort Lab
Carolyn.currin@noaa.gov
How salt marshes respond to SLR
Modified from: Cahoon, DR., J.W. Day, Jr., and D. J. Reed. 1999.
• Marshes grow sediment volume over time
• Suspended sediment supply crucial parameter
• Sediment accretion facilitates the carbon sequestration ability of marshes
Keep UP Move LANDWARDPontee. 2013. Ocean & Coastal Management
• Landward transgression of salt marsh determined by topography and absence of development
• May preserve marsh habitat acreage even with accelerated SLR
Morris et al. 2002, Cahoon 2012, Kirwan et al. 2016
• 50% of wave energy reduced within 5 m (15’) of marsh edge; >90% over 25 m of marsh (S. alterniflora)
• Wave energy reduction increases with plant biomass
• Belowground biomass binds sediments
• Wave energy reduction decreases as inundation depth exceeds canopy height
• Linear Relationship between wave energy or wave power and marsh erosion over large scales, other factors important locally and regionally
Salt marshes effectively attenuate wave energy and reduce erosion
(Knutson, Marani, Moller, Fagherazzi, Leonard, Leonardi, Tonelli and others )
Natural Fringing Marsh Width vs. Wave Energy
Width of natural fringing marshesis markedly decreased at RWE> 300
Wave energy calculated at 50 m intervals with a Wave Energy Model (WEMo)• Fetch, wind speed and direction,
nearshore bathymetry• Fringing marsh width determined across
wave energy ranges
What are the wave energy settings appropriate for Living Shoreline approaches?
7 mile fetch
WEMo Malhotra and Fonseca 2007
1 Representative Wave Energy (WEMo)2 Estimates Boat Wave Energy3 Proximity of existing Marsh
Factors Driving Final Score:
Developing Guidance for Living Shoreline Implementation
0
50
100
150
200
250
300
350
400
450
1 2 3Ave
rage
Mar
sh W
idth
(m
)
Representative Wave Energy …
No impacts
Boat traffic
Boat Wakes add to site Wind Wave Energyand reduce marsh width
NOAA/ NERRS Research & Monitoring
Natural reference marsh SETSET
4 paired Natural and Sill marsh sites• Sills built between 2002 - 2004• Sill heights < MHW, length 20 – 125 m
SETS established at Lower and Upper edge of S. alterniflora• Measure net change in marsh surface elevation
Annual measures of marsh vegetation in permanent plots
20 m5 10 15-1 0
NC Sentinel Site
North Carolina
Virginia
SET reading
O m Veg plot
SET
1/04 1/05 1/06 1/07 1/08 1/09 1/10 1/11 1/12 1/13 1/14 1/15 -40
-20
0
20
40
60
80
-40
-20
0
20
40
60
80
100
PI
NCMM
PKS
HI
Sill Lower
Sill Upper
Natural Lower
Date
1/04 1/05 1/06 1/07 1/08 1/09 1/10 1/11 1/12 1/13 1/14 1/15 -140
-120
-100
-80
-60
-40
-20
0
20
Natural Upper
Sur
face
ele
vatio
n ch
ange
(m
m)
-140
-120
-100
-80
-60
-40
-20
0
20
PI
NCMM
PKS
HI
Net Marsh Surface Elevation Change
Natural and Sill Fringing Salt Marshes
Surf
ace
elev
atio
n c
han
ge(
mm
)
Currin et al. In Press, CRC
Elevation ChangeMarsh type (mm/yr)Nat Upper 0.1Nat Lower -6.0 *
Sill Upper 3.4*Sill Lower 3.2*
Nat Lower
• Sill marshes significantly greater surface elevation increase than Natural marshes
• Natural marshes losing elevation at lower edgeBoat wake and wave energy contribute at 2 sites
Sill Upper
Surf
ace
elev
atio
n c
han
ge (
mm
)
20 m510
15-1 0
PLOT -1 m PLOT 0 m PLOT 5 m
Loss of vegetation at lower edgeMaintained interior vegetation
Increase in vegetation at lower edgeMaintained interior vegetation
Spartina alterniflora Stem Density2006 - 2016
Natural Fringing Marsh
Sill Marsh
Marsh Vegetation MonitoringNOAA NCCOS and NC NERRS
aa
b
b
Sill sediment accretion resulted increased Spartina biomass at lower edge,
Sill marshes have less low marsh habitat
Low marsh edge High marsh
2011Spartina biomass vs Plot Distance from Shore
Distance from Shore
Pontee. 2013. Ocean & Coastal Management
Lo
“Coastal Squeeze”
Habitat Tradeoffs May Ensue
Stone Sills
• Fish habitat• Oyster settlement• Increase sediment accretion
• Reduce/eliminate shallow subtidal
• Reflect wave energy• Non-native hard substrate;
Invasives
Low Marsh
• Absorb wave energy• Faunal utilization• Denitrification• Sediment trapping• C sequestration
• Less SLR resiliency• Lower plant diversity
High Marsh
• Greater SLR resiliency• Greater plant biodiversity
• Less faunal utilization• Reduced denitrification• Reduced Sediment trapping• Lower C sequestration
BUT ECOSYSTEM FX GREATER THAN BULKHEADS
-
+
Leslie Irwin, NOAA communications
Sustainability of Living Shorelines in a World of Rising Sea Level and Increasing Wave EnergyCarolyn Currin, NOAABio: Research scientist working on natural and restored salt marshes. Work focuses on marsh response to SLR, and ecosystem structure and function.Description: Living Shorelines offer an alternative to shoreline hardening to protect property from erosion. However, given the vulnerability of salt marshes to SLR and wave energy, what are the limits on this approach? A literature review of marsh response to SLR and wave energy suggests guidelines that can be followed, identifies the crucial parameters to measure, and identifies the practical limits to the Living Shoreline approach. Sediment supply, topography and wave energy are the most important drivers of fringing marsh resilience. We will summarize current literature on the relationship between marsh response to SLR, and discuss model results which point to a 20 mg/ l suspended sediment concentration as a benchmark for marsh resilience to SLR. An analysis of the relationship between wave energy and marsh shoreline erosion rates in NC is consistent with reports from other systems, and the relationship between fringing marsh width and wave energy from the NC study suggest Relative Wave Energy limits for Living Shoreline installations. Results from a decade-long assessment of marsh surface elevation and vegetation change in natural and stabilized (hybrid Living Shorelines) marshes in North Carolina will also be presented. This study shows a significant increase in marsh surface elevation change in hybrid Living Shoreline sites with a stone sill, compared to natural fringing marshes, which are losing marsh surface elevation. Oyster reefs provided moderate protection from loss of marsh elevation. Marsh vegetation responded to changes in surface elevation, and over the short-term, were resilient to losses in surface elevation. However, model results suggest limits to the sustainability of shoreline marshes. We combine results from an analysis of recent literature and the North Carolina case study to provide guidance on the physical settings in which fringing marsh and hybrid living shorelines can be considered. We utilize models of marsh response to SLR and marsh transgression to predict the long-term sustainability of fringing salt marshes in a variety of geomorphic settings.
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