Symbols used

1
Feedbacks Between Flow, Sedimentation, and
Standing Biomass on Salt-Marsh Platforms
Simon M. Mudd1, David J. Furbish1, and James T.
Morris2 1Center for Earth Surface Processes
Research and Department of Geological Sciences,
Florida State University, Tallahassee
FL 2Department of Biological Sciences and Belle
W. Baruch Institute, University of South
Carolina, Columbia, SC
Plant community and platform evolution
Abstract
Salt marsh macrophyte population dynamics
Tidally induced flood-and-ebb flows over
salt-marsh platforms are modeled using a depth
integrated model in which plant drag and water
surface slope drive fluid motion. Plant drag is
related to the plant population characteristics
on the salt- marsh, which vary through space and
time. Sedimentation on the platform is due to
particle settling. Feedbacks exist between flow,
suspended-sediment transport, sedimentation, and
the population dynamics of the macrophytes (i.e.
Spartina alterniflora) living on the marsh.
Simulations suggest that, on short timescales
(annual to decadal) the elevation of the marsh
surface is the dominant factor in determining
average depth of flow on the salt marsh in
comparison to flow impedance effects associated
with variations in drag due to the plant stems.
Therefore the spatial pattern of plant density on
the platform surface is more sensitive to the
platform elevation (including its slope) than to
the details of how the flow dynamics vary with
biomass. Small spatial variations in flow due to
spatial variations in biomass do affect the
residence time of water on the salt marsh,
however, and at longer timescales the spatial
variation in sedimentation due to these subtle
flow effects becomes important.
Above ground biomass varies through the seasons,
reaching a maximum in the summer and a minimum in
the winter. The stem density is at a minimum near
the time of maximum biomass due to self thinning
in the macrophyte canopy.
The distribution of salt marsh macrophytes
changes through time as deposition and flow
resistance alter the average depth of flow over
the platform
Figure 7 Biomass as a function of time and
distance inland. Seasonal variations in biomass
dominate, but as time progresses in this example,
sedimentation and flow feedbacks increase the
amount of biomass on the marsh.
Figure 2 Stem density and Biomass through time
for Spartina alterniflora at Goat Island, South
Carolina
The stem density and stem diameter, and projected
stem area per unit volume of salt marsh
macrophites can all be described as functions of
the plant biomass. Additionally, the peak biomass
is a function of the average depth of water above
the root mass.
Flow on salt-marsh platforms
Drag exerted by salt marsh macrophytes resists
tidally induced flows over coastal wetlands. The
drag force can be expressed as (Nepf, 1999)
figure 1 Drag coefficient as a function of ad
(digitized from fig. 6 in Nepf, 1999 over range
of ad found in field). These data are based on
laboratory experiments at Red 2400
Figure 9 sedimentation rate through time.
Sedimentation rate is highest near the boundary
(a tidal creek).
Figure 9 Average depth through time. As sediment
is deposited on the marsh platform, the average
depths change, which affect biomass. There is no
sea level rise in this model run
Figure 3 Projected stem area per unit volume
(calculated) as a function of biomass for
Spartina alterniflora at Goat Island, South
Carolina. Figure represents 18 years of monthly
data.
Figure 4 Biomass as a function of water depth
(from Morris, 2002, in Press)
Plant and flow interactions
In flows over salt marshes, velocities are
typically 0.001-0.5 m/s (Red 1-4000), and
inertial forces are much smaller than gravity and
drag forces. The continuity and momentum
equations are
Conclusions
The changing seasons can affect flow velocities
and water depths across a marsh surface due to
changes in the macrophyte populations
The drag coefficient can be calculated as a
function of the biomass. Both the drag
coefficient and the projected stem area per unit
volume vary through time and space.
Flow hydrodynamics, macrophyte population
dynamics, and sedimentation interact on
salt-marsh platforms. The drag forces due to flow
through plant stems can cause minor changes in
water surface elevation. This will lead to minor
changes in biomass on the marsh, but the small
changes in water surface elevation and the
corresponding changes in flow velocities caused
by plant resistance can change sedimentation
rates on the marsh surface. Marsh surface
elevation changes cause changes in average water
depth on the marsh platform, which in turn affect
the biomass production. The data used to
calculate drag coefficients was experimentally
determined (Nepf 1999) at Reynolds numbers higher
than those typically found in the field. At lower
Reynolds numbers (Re 1-1000) the drag
coefficient on isolated cylinders is inversely
proportional to the Reynolds number (see
Schlichting, 1968). More data is needed for flows
at moderate Reynolds number for arrays of
cylinders. Data from a single cylinder would
indicate that there are possibly stronger
feedbacks between flow and stem density.
Because biomass and platform elevation are
sensitive to sedimentation rates, more field and
experimental data on particle and floc settling
rates, plant sediment trapping efficiency, and
concentration boundary conditions are needed.
Sedimentation on salt marsh platforms

• Field studies on salt marshes generally indicate
  the following features of sediment transport in
  this setting
• There is little or no resuspension of sediment
  under normal (non-storm) flow conditions.
• The sedimentation rate is highest near the tidal
  creeks, and diminishes inland.
• Ebb flows containing waters retuning from the
  marsh surface contain some sediments.
• From these observations a simplified
  sedimentation model is developed with the
  assumptions that there is no erosion on the
  marsh, sedimentation occurs as settling of
  suspended sediment, and there is some mixing of
  suspended sediment during marsh flows such that
  the vertical concentration profile is uniform.
  This is expressed as

figure 5 Calculated drag coefficient and
projected stem area as a function of time for a
site at Goat Island, South Carolina
figure 6 Computed water surface elevation and
velocities for a marsh with a slope of 0.001.
Seasonal changes in biomass can affect water
surface depths and velocities. Tidal forcing is
sinusoidal in this example.
Symbols used
References

• Cd plant drag coefficient
• a projected plant area per unit volume
• d plant diameter
• g gravitational acceleration
• ad dispersivity
• ws settling velocity

• U depth averaged velocity
• D diffusion coefficient
• ? water surface elevation
• marsh surface elevation
• h (z-h) flow depth
• C depth averaged concentration

Morris, J.T. P.V. Sundareshwar, C.T. Nietch, B.
Kjerfve, and D.R. Cahoon, Responses of coastal
wetlands to rising sea level, Ecology, in
press. Nepf, H.M., Drag, turbulence, and
diffusion in flow through emergent vegetation,
Water Resources Research, 35, 479-489,
1999. Schlichting, H. Boundary-Layer Theory,
McGraw-Hill, New York, 1968.
Dispersivity is a function of the stem density
and the settling velocities are calculated using
the Stokes equation.