How, and How Effectively, can Vegetation Dissipate Storm Surge, Wave, and Flooding?

8/10/2019 How, and How Effectively, can Vegetation Dissipate Storm Surge, Wave, and Flooding?

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The Big Pictureafter Sandy in NYC

Tremendous interest in the role ofcoastal wetlands in reducingstorm surge and inundation

But, how well does coastal wetlands reduce storm surge?


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What role does Piermont Marsh play in reducing flooding? What are the relative roles of various species?


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Can Ecosystem Service (Healthy Marshes and Mangroves)be used to dissipate storm surge and coastal flooding?

Yes. It depends on many things:

Vegetation type, height, density, and distributionLocal bathymetry and topography

Local climate and weather wind and pressure deficitLocal hydrodynamic condition waves and currentsLocal salinity and temperature and water quality

Sediment supply Is rate of SLR ~ rate of sediment accretion? Available space for vegetation migration

Will seawalls and other structures block vegetation migration?


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Vegetation-Resolving Hydrodynamic Modeling

Improves modeling of the influence of vegetation onstorm surge, without using the empirical Manningscoefficient

Quantifies the influence of vegetation on storm surgeInvestigates vegetations performance during real storms

Some example publications:

1. Sheng, Y.P., A. Lapetina, and G. Ma (2012). The reduction of storm surge by vegetationcanopies: Three-dimensional simulations, Geophysical Research Letters , 2012.

2. Lapetina, A. and Y.P. Sheng (2014). Three-Dimensional Modeling of Storm Surge andInundation including the Effects of Coastal Vegetation, Estuaries and Coasts , 2014.

3. Lapetina, A. and Y.P. Sheng. The Influence of Vegetation on Storm Surge and CoastalInundation in the Northern Gulf of Mexico, Submitted, JGR-Oceans .


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How does vegetation affect surge & wave?

Vegetation plays a major role in coastal circulation and waveprocessesSource of drag in the water column from profile drag andskin friction, reducing mean flow and dissipate wave energy

and wave heightProfile drag and skin friction drag depend on flow speed as well as Frontal Area and Wetted Area of vegetationSource of turbulence from wake production and shear,affecting turbulent kinetic energy Sink of turbulence through dissipation


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Incorporation of vegetation processes into a 3D modelCH3D-SSMS (Sheng et al.2010, Sheng and Liu 2011,Sheng et al. 2012), alreadyincludes bottom stress, windeffects, wave effects,radiation stresses, and vortexforces

Permits the inclusion of vegetation effects such asprofile drag, skin frictiondrag, and vertical Reynoldsstresses

Includes wave effects of vegetation through two-waycoupling to SWAN (Booij etal. 1999; Suzuki et al. 2012)

How does this 3Dmodel compare tosimpler 2D model tosimulate the effects of vegetation? Ref. #2

How can the influenceof vegetation bequantified? Ref. #1 What is the effect of vegetation during a realhurricane? Ref. #3


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Y(cm)

TKE Data (cm/s)

0 0.5 1 1.5 2 2.50102030

0

0.1

0.2

X (m)

Y(cm)

TKE Model Results (cm/s)

0 0.5 1 1.5 2 2.50

102030

0

0.1

0.2

Vegetation Model Validation (Ref. #1)

Neumeier (2007) experimentReal Spartina anglica

32 cm deep water19 cm tall canopy 1200 stems/m 2

0 0.5 1 1.5 2 2.50

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Y(cm)

Velocity Data (cm/s)

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Y(cm)

Velocity Model Results (cm/s)

0 0.5 1 1.5 2 2.50

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r2=0.98, rms error=0.59 cm/s

r2= 0.95, rms error=0.015 cm/s


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How well does coastal vegetation reduce storm surge?Likely varies with:

-Canopy height (cm)-Canopy width (km)-Canopy density (stems/m 2)

Consider a storm: Cat. 2, 6.71 m/s (15 mph) forward speedConsider the following canopy conditions:

Height (50, 75, 100, 125 cm)

Width (0.5, 1, 1.5 km)Density (100, 200, 300 stems/m 2)

Run the vegetation-resolving modeling system and compare the resulting TIV (TInundation Volume) and Vegetation Dissipation Potential (VDP) for various

vegetation conditions in a square basin with the following transect


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Vegetation Dissipation Potential (VDP)

= 1

0= 1

0


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Do storm characteristics affect dissipation rates? Yes, likely depends upon

-Storm forward speed -Storm intensity


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Vegetation reduced flooding during H. Ike

VegetationFree

VegetatioResolving

Domain wideRMSE

60.6 36.0Domain Wide

Average% Error

11.9% 7.6%

ChambersCountyRMSE

108.4 61.2

ChambersCounty

Average% Error

17.9% 9.6%


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Vegetation Free Mangroves Case 1:300 stems/m2, 150 cm

Vegetation reduces flooding in Miami-Dade County(maximum inundation map during Hurricane Andrew)


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Case Name Vegetation Type Vegetation Height(cm)

Vegetation Density( / )

Case 1 Mangrove 150 300




Increasing density and height further reduces flooding

Case 1- Case 2 Case 1 Case 3 Case 1 Case 4


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What Happened to Piermont during Sandy (2012)


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Assuming 25 m/sec wind from the East


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How can we use the vegetation resolvingmodeling to predict the effectiveness of

Piermont Marsh in protecting people fromfuture flooding risk?

Conduct a forensic study of H. Sandy to assess therole of Piermont Marsh;

Conduct modeling in conjunction with the USGSSWATH field program in Piermont Marsh;

Calculate the flooding risk under current climateand future climate, using ensembles of storms

Documents

How, and How Effectively, can Vegetation Dissipate Storm Surge, Wave, and Flooding?