Chapter 8 Fluid Dynamics in Seagrass Ecology
207
Fig. 7. Wave attenuation (open circles) as a function of water depth/tidal fluctuation (black boxes). Wave attenuation was based on the
significant wave height in a Ruppia maritima bed in comparison to an adjacent unvegetated area at Bishop’s Head Point, Chesapeake
Bay, USA. Note that these data were collected in June when the plants were reproductive. Wave attenuation was highest at low tide
when the canopy occupied the entire water column. Negative wave attenuation represents periods in which wave height was larger in
the vegetated site than the unvegetated site. Source of data: E.W. Koch.
applied to Z. marina with some success, although
there were a number of inconsistencies with field observations as would be expected (Abdelrham, 2003;
Peterson et al., 2004). This type of approach provides some indication of the general pattern of flow
within an eelgrass canopy, but its utility will likely
be limited by species-specific differences in canopy
vegetative profiles and the lack of detailed studies of
canopy flow in these systems. Future development
will need to apply the mixing layer analogy. Realistically, the ability to model canopy flow phenomena
is a goal that speaks to the need for detailed canopy
flow profiles in the laboratory and the field.
B. Wave Attenuation by Seagrass Canopies:
A Concept in Development
“Seagrasses are able to modify current flow and sediment composition, yet little information exists describing their effect on waves.” This statement with
which Fonseca and Cahalan (1992) started their paper more than 10 years ago is still true today. Many
papers begin by describing the importance of seagrasses, including their capacity to attenuate waves,
but the studies which led to this generalization are
few. A flume study measured wave attenuations between 20 and 76% over 1 m length when the plants
were occupying the entire water depth (Fonseca and
Cahalan, 1992), whereas field studies measured values between 1.6 and 80% (Koch, 1996; Prager and
Halley, 1999). Our general understanding is that
wave attenuation is highest when seagrasses occupy
a large portion (>50%) of the water column (Ward
et al., 1984; Fonseca and Cahalan, 1992; Fig. 7),
but reduction in wave energy (15 s waves) has also
been observed in a 5 m deep Amphibolis antarctica
bed (Verduin and Backhaus, 2000) and reduction of
orbital velocities (3–4 s waves) at a 15 m deep Posidonia oceanica bed (Granata et al., 2001) where the
plants occupied only a small portion of the water
column.
C. Monamis
Canopy flow is complex because it is a function of
the drag or resistance exerted by the vegetation on
the fluid, which is likely to vary spatially due to
stem spacing and vertically due to the vegetation
profile (Okubo et al., 2002). One important consequence of this realization is the propagation of wavelike oscillations or monamis (mo = aquatic plant;
nami = wave; Ackerman and Okubo, 1993) caused
207
Fig. 7. Wave attenuation (open circles) as a function of water depth/tidal fluctuation (black boxes). Wave attenuation was based on the
significant wave height in a Ruppia maritima bed in comparison to an adjacent unvegetated area at Bishop’s Head Point, Chesapeake
Bay, USA. Note that these data were collected in June when the plants were reproductive. Wave attenuation was highest at low tide
when the canopy occupied the entire water column. Negative wave attenuation represents periods in which wave height was larger in
the vegetated site than the unvegetated site. Source of data: E.W. Koch.
applied to Z. marina with some success, although
there were a number of inconsistencies with field observations as would be expected (Abdelrham, 2003;
Peterson et al., 2004). This type of approach provides some indication of the general pattern of flow
within an eelgrass canopy, but its utility will likely
be limited by species-specific differences in canopy
vegetative profiles and the lack of detailed studies of
canopy flow in these systems. Future development
will need to apply the mixing layer analogy. Realistically, the ability to model canopy flow phenomena
is a goal that speaks to the need for detailed canopy
flow profiles in the laboratory and the field.
B. Wave Attenuation by Seagrass Canopies:
A Concept in Development
“Seagrasses are able to modify current flow and sediment composition, yet little information exists describing their effect on waves.” This statement with
which Fonseca and Cahalan (1992) started their paper more than 10 years ago is still true today. Many
papers begin by describing the importance of seagrasses, including their capacity to attenuate waves,
but the studies which led to this generalization are
few. A flume study measured wave attenuations between 20 and 76% over 1 m length when the plants
were occupying the entire water depth (Fonseca and
Cahalan, 1992), whereas field studies measured values between 1.6 and 80% (Koch, 1996; Prager and
Halley, 1999). Our general understanding is that
wave attenuation is highest when seagrasses occupy
a large portion (>50%) of the water column (Ward
et al., 1984; Fonseca and Cahalan, 1992; Fig. 7),
but reduction in wave energy (15 s waves) has also
been observed in a 5 m deep Amphibolis antarctica
bed (Verduin and Backhaus, 2000) and reduction of
orbital velocities (3–4 s waves) at a 15 m deep Posidonia oceanica bed (Granata et al., 2001) where the
plants occupied only a small portion of the water
column.
C. Monamis
Canopy flow is complex because it is a function of
the drag or resistance exerted by the vegetation on
the fluid, which is likely to vary spatially due to
stem spacing and vertically due to the vegetation
profile (Okubo et al., 2002). One important consequence of this realization is the propagation of wavelike oscillations or monamis (mo = aquatic plant;
nami = wave; Ackerman and Okubo, 1993) caused