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

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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

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80

-40

-20

0

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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

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-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

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+

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.