434
14 Tides and Waves on Vegetated Coasts
more important process than tidally driven circulation in reef flushing.
The exposure time of resident biota to water-borne materials and accumulation of sediments or adsorbed substances in lagoon depend on the so-called
lagoon residence time, T. There are a few definitions of residence time, depending on the method used for the calculation of T. Residence time, T, can be
calculated in the simplest way using the tidal prism approach (Prager, 1991;
see also Chap. 8):
lagoon volume
T = daily volume input'
(14.10)
or:
lagoon depth x tidal period
T=
tidal range
(14.11)
In particular situations, the residence time varies in the large extent. For
shallow, narrow lagoons time, T, may be relatively short, less than one day,
while in more isolated and larger systems it may be of the order of one to four
months. For example, in Enewetak Atoll lagoon, the average residence time
of water in the lagoon is 28 days, and contribution from the wind-ward reef
comprises 85% of the total inflow, when time, T, is calculated according to
Eq. (14.10) by dividing the lagoon volume by the rate of water input (Atkinson
et al., 1981). On the other hand, Kraines et al. (1998) after measurements and
numerical modelling of the lagoon at Bora Bay, Miyako Island, showed that
in this case the residence time is in the range of 1.5 hour to 3.7 hours. This
means that lagoon is flushed several times each tidal cycle. The residence time,
T, calculated using the tidal prism method for this lagoon was almost 10 times
longer.
14.4.4 Impact of Waves on Physical Degradation of Reefs
Cyclone waves affect the density, structure and local distribution of coral assemblages by acting as agents of mortality and colony transport. In general, the
probability that an organism will be destroyed (by breakage or dislodgement)
is a function of organism size, its structural strength, wave-induced velocity
and acceleration, and the probability of encountering waves with given parameters. We will illustrate two different mechanisms of coral damage, using as
the example hard corals Acropora formosa and Porites. In the first case, we
will examine a relationship between coral branch proportions and water flow.
The second case is dealing with a probability of coral survival in a given wave
environment.
Coral Branch Proportion. Consider a single vertical coral beam of the
cylindrical shape with dimensions as in Fig. 14.lOa. The axis z is vertical
and oriented upwards from substratum, while x is the horizontal axis. We
14 Tides and Waves on Vegetated Coasts
more important process than tidally driven circulation in reef flushing.
The exposure time of resident biota to water-borne materials and accumulation of sediments or adsorbed substances in lagoon depend on the so-called
lagoon residence time, T. There are a few definitions of residence time, depending on the method used for the calculation of T. Residence time, T, can be
calculated in the simplest way using the tidal prism approach (Prager, 1991;
see also Chap. 8):
lagoon volume
T = daily volume input'
(14.10)
or:
lagoon depth x tidal period
T=
tidal range
(14.11)
In particular situations, the residence time varies in the large extent. For
shallow, narrow lagoons time, T, may be relatively short, less than one day,
while in more isolated and larger systems it may be of the order of one to four
months. For example, in Enewetak Atoll lagoon, the average residence time
of water in the lagoon is 28 days, and contribution from the wind-ward reef
comprises 85% of the total inflow, when time, T, is calculated according to
Eq. (14.10) by dividing the lagoon volume by the rate of water input (Atkinson
et al., 1981). On the other hand, Kraines et al. (1998) after measurements and
numerical modelling of the lagoon at Bora Bay, Miyako Island, showed that
in this case the residence time is in the range of 1.5 hour to 3.7 hours. This
means that lagoon is flushed several times each tidal cycle. The residence time,
T, calculated using the tidal prism method for this lagoon was almost 10 times
longer.
14.4.4 Impact of Waves on Physical Degradation of Reefs
Cyclone waves affect the density, structure and local distribution of coral assemblages by acting as agents of mortality and colony transport. In general, the
probability that an organism will be destroyed (by breakage or dislodgement)
is a function of organism size, its structural strength, wave-induced velocity
and acceleration, and the probability of encountering waves with given parameters. We will illustrate two different mechanisms of coral damage, using as
the example hard corals Acropora formosa and Porites. In the first case, we
will examine a relationship between coral branch proportions and water flow.
The second case is dealing with a probability of coral survival in a given wave
environment.
Coral Branch Proportion. Consider a single vertical coral beam of the
cylindrical shape with dimensions as in Fig. 14.lOa. The axis z is vertical
and oriented upwards from substratum, while x is the horizontal axis. We