428
14 Tides and Waves on Vegetated Coasts
seaward motion. The intensity of the transport depends also on the gradient of
suspended sediment concentration (SSe). Measurements of sse indicate that
the concentration decays exponentially with distance from the shoreline. For
example, in the Bay of Fundy, the decrease of sse takes the form (Amos, 1995):
SSC(x) = SSC(O)e- kx ,
(14.5)
in which the decay coefficient varies with the season between 0.023 and 0.091
m- I (mean value is 0.049 m- I ).
A model for sediment accretion/erosion on the tidal flats of The Wash, England, and Minas Basin, Canada, was developed by Amos (1995). Computer
simulation included the time of inundation, velocity of current, instantaneous
bed shear stress, cumulative deposited and eroded mass, and suspended sediment concentration. The tide and current speed were determined for eighty
M2 tidal cycles. The starting sse was set in turn to 10, 100, 1000 and 10,000
mg/l, and was assumed to be constant across the flats. The simulation for The
Wash indicated that peak erosion appeared to exceed peak deposition during
spring tides and the converse was evident during neap tides. The net predicted
result was one of long-term erosion of the flats, and an overall increased in sse,
modulated by the spring-neap cycle.
The presence of plant cover along tidal flats may lead to modification in water
circulation and sedimentation rates. For example, Spartina alternifiora marshes
act at the surface through their stems and leaves, and at the subsurface through
their roots and rhozomas. These marshes are typical features of the southern
Brazilian estuaries and lagoons. Netto and Lana (1997), show that Spartina
alternifiora is a major source of sediment heterogeneity between vegetated and
unvegetated areas.
Most organic material accumulating on tidal flats is processed in the sediments rather than in the water column. The traditional concept of a closed
carbon cycle consists of the microphytobenthos living on the flat as the producer, and the benthic fauna and micro-organisms living in the sediments as the
consumer of organic matter and the suppliers of nutrients to the phytobenthos
(Posma, 1988). However, recently Alongi (1998) pointed out the importance
of the role of bacteria and protozoa in utilizing and mineralizing sediment organic matter. The bacterial biomass, growth, and productivity in sediments are
greater than was previously believed, and the oxidation reactions and pathways
of microbiological decomposition in sediment are very complex. The inquisitive
reader should consult Alongi (1998) for an in-depth discussion.
14.4 Waves at Coral Reefs
14.4.1 A Brief Overview
Coral reefs are truly magical places. Although remote from the natural environment of humans, coral reefs are increasingly becoming an area of tourist
14 Tides and Waves on Vegetated Coasts
seaward motion. The intensity of the transport depends also on the gradient of
suspended sediment concentration (SSe). Measurements of sse indicate that
the concentration decays exponentially with distance from the shoreline. For
example, in the Bay of Fundy, the decrease of sse takes the form (Amos, 1995):
SSC(x) = SSC(O)e- kx ,
(14.5)
in which the decay coefficient varies with the season between 0.023 and 0.091
m- I (mean value is 0.049 m- I ).
A model for sediment accretion/erosion on the tidal flats of The Wash, England, and Minas Basin, Canada, was developed by Amos (1995). Computer
simulation included the time of inundation, velocity of current, instantaneous
bed shear stress, cumulative deposited and eroded mass, and suspended sediment concentration. The tide and current speed were determined for eighty
M2 tidal cycles. The starting sse was set in turn to 10, 100, 1000 and 10,000
mg/l, and was assumed to be constant across the flats. The simulation for The
Wash indicated that peak erosion appeared to exceed peak deposition during
spring tides and the converse was evident during neap tides. The net predicted
result was one of long-term erosion of the flats, and an overall increased in sse,
modulated by the spring-neap cycle.
The presence of plant cover along tidal flats may lead to modification in water
circulation and sedimentation rates. For example, Spartina alternifiora marshes
act at the surface through their stems and leaves, and at the subsurface through
their roots and rhozomas. These marshes are typical features of the southern
Brazilian estuaries and lagoons. Netto and Lana (1997), show that Spartina
alternifiora is a major source of sediment heterogeneity between vegetated and
unvegetated areas.
Most organic material accumulating on tidal flats is processed in the sediments rather than in the water column. The traditional concept of a closed
carbon cycle consists of the microphytobenthos living on the flat as the producer, and the benthic fauna and micro-organisms living in the sediments as the
consumer of organic matter and the suppliers of nutrients to the phytobenthos
(Posma, 1988). However, recently Alongi (1998) pointed out the importance
of the role of bacteria and protozoa in utilizing and mineralizing sediment organic matter. The bacterial biomass, growth, and productivity in sediments are
greater than was previously believed, and the oxidation reactions and pathways
of microbiological decomposition in sediment are very complex. The inquisitive
reader should consult Alongi (1998) for an in-depth discussion.
14.4 Waves at Coral Reefs
14.4.1 A Brief Overview
Coral reefs are truly magical places. Although remote from the natural environment of humans, coral reefs are increasingly becoming an area of tourist