1. -Matthew Kirwan & Brad Murray-
Response of an ecomorphodynamic model of tidal marshes to oscillatory sea level rise rates
-Matthew Kirwan & Brad Murray-
Duke University

2. How long between equilibrium states? How far out of equilibrium?
Assumed equilibrium
(Long term accretion = SLR)
Unchanging accretion rate, depth, channel density
How long between equilibrium states?
How far out of equilibrium?
How dynamic?
Revise for roberts bank….

3. decadal-millennial time scales)
(kilometers and
decadal-millennial time scales)
Model Approach
Cellular bed surface
flooded and drained by
tidal flow containing fine
sediment
Basic transport processes influenced by vegetation builds 3D marsh topography

4. Sedimentary Processes
Deposition rate = (k1*SSC + k2*Biomass)*Depth, k1=.0135, k2=
Erosion rate
Slope driven transport (diffusion, slumping)
(Morris et al., 2002)
Suspended sediment concentration
m= kg/m2 sec
(Fagherazzi and Furbish, 2001)
= (k * slope) (Murray and Paola, 2002)
where k is inversely related to biomass

5. Increased inundation Increased biomass Increased deposition rates
Vegetation
Depth below high tide (m)
Production
(g/m3/yr)
Treatment
Calculate biomass in each
cell with productivity
function (Morris, 2002)
Increased inundation Increased biomass
Increased deposition rates

6. No vegetation Vegetation
1 mm/yr
NEED DISCUSSION OF EQUILIBRIUM 2m 4m
water depth 3m
Shallower platform
Less extensive channel network
Steep, abrupt channel edges

7. Accelerated Sea Level Rise: Vegetated
1 mm/yr
10 mm/yr
NEED DISCUSSION OF EQUILIBRIUM 2m 4m
water depth 3m
Platform depths increase
Velocity, erosion rates increase
Channels deepen, expand slightly

8. Accelerated Sea Level Rise: Unvegetated Channel density  infinity
1 mm/yr
10 mm/yr
(eq. depths subtidal)
Channel density  infinity
Removal of vegetation productivity feedback
Lack of plants to constrain creek bank slump

9. 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

10. SLRR exceed AR: water depth Accretion α depth: AR
SLRR < AR
accretion
(depth dec)
SLRR > AR
sea level
(depth inc)
SLRR exceed AR: water depth
Accretion α depth: AR
Platform always adjusting depth to continuously changing SLR rate, causing accretion lag and phase shift

11. too
shallow
too deep
More out of phase with smaller period
Physical reason for lag
Forcing term always moving, so never get to equilibrium

12. oscillation has no effect on lag
Amplitude of SL Rate
oscillation 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

13. Channel network and biomass change
expanding tidal prism
Oak Island. Near Cape Fear, NC

14. Effects of biomass win over effects of expanding tidal prism!
Channel Network and Biomass Change
Channel
Biomass
Depth
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!

15. Sea Level Rate and Accretion Rate 970 – 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”

16. Acknowledgements Jim Morris, University South Carolina
Lincoln Pratson, Duke University
Andrew W. Mellon Foundation

17. SLR rate: 1 mm/yr to 2.5 mm/yr in late 19th century
Case Study One
Donnelly et al., 2004
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?

18. Case Study Two Many authors note long-term AR < SLRR,
and infer marsh will be lost.
210Pb (1850-present)
Would expect SLRR > AR
in a normal, healthy marsh
whenever SLRR has increased