Response of an ecomorphodynamic model of tidal marshes to oscillatory sea

level rise rates

-Matthew Kirwan & Brad Murray-Duke University

Assumed equilibrium(Long term accretion = SLR)

Unchanging accretion rate, depth, channel density

How long between equilibrium states?

How far out of equilibrium?

How dynamic?

Model Approach

Cellular bed surfaceflooded and drained bytidal flow containing finesediment

Basic transport processes influenced by vegetation builds 3D marsh topography

(kilometers and decadal-millennial time scales)

Sedimentary Processes

Deposition rate = (k1*SSC + k2*Biomass)*Depth, k1=.0135,

k2=.000015

Erosion rate

Slope driven transport (diffusion, slumping)

critical

criticalbottomm

m= 1.4-3 kg/m2 sec

(Fagherazzi and Furbish, 2001)

= (k * slope) (Murray and Paola, 2002)

where k is inversely related to biomass

Suspended sediment concentration(Morris et al., 2002)

Vegetation

TreatmentCalculate biomass in eachcell with productivityfunction (Morris, 2002)

Depth below high tide (m)

Pro

du

ctio

n(g

/m3/

yr)

0.5 1 1.5 2 2.5 3

Increased inundation Increased biomass Increased deposition rates

Shallower platformLess extensive channel network

Steep, abrupt channel edges

2m4m water depth3m

No vegetation

1 mm/yr

Vegetation

1 mm/yr

Platform depths increaseVelocity, erosion rates increase

Channels deepen, expand slightly

10 mm/yr

2m4m water depth3m

Accelerated Sea Level Rise: Vegetated

Accelerated Sea Level Rise: Unvegetated1 mm/yr 10 mm/yr

Channel density infinityRemoval of vegetation productivity feedback

Lack of plants to constrain creek bank slump

(eq. depths subtidal)

Oscillating instead of abrupt changes in rate of sea level rise(moving forcing term)

Sea level rate approximated by sin function

Experiments with varying period and amplitude of oscillation

Track accretion, vegetation, and channel changes

•SLRR exceed AR: water depth•Accretion α depth: AR•Platform always adjusting depth to continuously changing SLR rate, causing accretion lag and phase shift

SLRR > AR

(depth inc)sea level

accretionSLRR < AR

(depth dec)

More out of phase with smaller period

Physical reason for lag

Forcing term always moving, so never get to equilibrium

tooshallow

too deep

Amplitude of SL Rateoscillation has no effect on lag

• Need bigger depth change to accommodate bigger sea level rate change,

• But deepening occurs faster

• Amplitude of accretion smaller than sea level rise rate

expanding tidal prism Oak Island. Near Cape Fear, NC

Channel network and biomass change

Channel Network and Biomass Change

Biomass proportional to depth (never exceed optimum depth)Channel network expansive when platform depths are shallow

Effects of biomass win over effects of expanding tidal prism!

Channel

Biomass

Depth

Sea Level Rate and Accretion Rate970 – 1970 AD

• Inferring sea level rise rates from accretion rates complicated• Today’s Sea level rates > Accretion rates

not indicative of a marsh unable to “keep up”

Acknowledgements

Jim Morris, University South Carolina

Lincoln Pratson, Duke University

Andrew W. Mellon Foundation

•SLR rate: 1 mm/yr to 2.5 mm/yr in late 19th century•Coincides with increased global temperatures

•Are marsh accretion rates accurate indicators of sea level history?•How would lag effect timing of acceleration?

Donnelly et al., 2004

Case Study One

Case Study Two

Many authors note long-term AR < SLRR, and infer marsh will be lost.

Would expect SLRR > ARin a normal, healthy marshwhenever SLRR has increased

210Pb (1850-present)