Environmental Engineering Reference
In-Depth Information
expected to magnify the impact of pollutants discharged
into them.
Estuaries are particularly susceptible to problems
associated with cultural eutrophication because the
estuarine circulation pattern tends to recycle discharged
nutrients and hence magnify their impact on production
and biomass. Estuaries are naturally eutrophic systems
because of the high efficiency with which the estuarine
circulation pattern recycles nutrients, and, because of
their high productivity, estuaries such as those found
along the coastline of the United States account for
some important coastal fisheries and serve as breeding
and/or nursery grounds for many marine organisms that
are usually associated with the open ocean, such as
sharks and whales.
An estuarine
tidal barrier
, which typically consists of
a fully or partially submerged weir at the mouth of an
estuary, is sometimes used to inhibit natural tidal intru-
sion, mixing, and flushing processes that would other-
wise operate in the absence of the barrier. These
structures can enhance the aesthetic, recreational, and
environmental qualities of estuaries. However, if not
properly designed, these barriers can also cause a dete-
rioration of estuarine water quality.
V
Q
S
S
T
=
1
−
(9.101)
f
f
0
Alternative formulations have also been used to calcu-
late the flushing time (Ji, 2008). The lower limit of flush-
ing time can be calculated using the
tidal prism formula
given by
V
V
T
T
=
(9.102)
f
tp
where
V
tp
is the volume of the tidal prism (l
3
), and
T
is
the tidal period (T), which is equal to 12.42 hours for
the M
2
tide. In cases where the surface area of the
estuary does not differ significantly between high and
low tide, the volume of the tidal prism can be estimated
by
V
=
∆
H A
×
(9.103)
tp
e
where Δ
H
is the tidal range, and
A
e
is the plan area of
the estuary. The tidal prism formula, Equation (9.102),
is derived by assuming that the seawater brought into
the estuary during a flood tide is completely mixed with
the freshwater from the river, and that the mixture is
completely flushed out of the estuary during the ebb
tide. Regardless of the method used to calculate the
flushing time, long flushing times are usually associated
with poor water-quality conditions, and factors influenc-
ing flushing times are tidal ranges, freshwater inflows,
and wind. Typical flushing times range from days in
small estuaries to months in large estuaries. Estuaries
with short flushing times (less than 1-2 weeks) are
unlikely to have algal blooms since algae would likely
be flushed out of the system before they can grow
significantly.
In river estuaries, the
replacement time
is sometimes
used instead of the flushing time and is defined as the
time required for all the water contained in the estuary
to be replaced by tidal flows, and the replacement time,
t
R
, can be estimated using the relation
9.3.5.1 Flushing Time.
An important parameter used
to characterize flow in estuaries is the
flushing time
,
which can be defined as the average time needed to
remove a particle from a point in the estuary into the
sea. The flushing time,
T
f
(T), is commonly calculated
using the relation
V
Q
f
T
=
(9.98)
f
f
where
V
f
is the volume of freshwater in the estuary (l
3
),
and
Q
f
is the rate at which freshwater flows into the
estuary (l3T−1).
3
T
−1
). The freshwater volume of an estuary,
V
f
,
is calculated using the following relation
S
−
S
S
S
∫
0
(9.99)
V
f
=
dV V
=
1
−
S
V
0
0
where
S
0
is the
sa
linity of pure seawater,
S
is the salinity
in the estuary,
S
is the mean salinity in the estuary, and
V
is the estuary volume. The mean salinity in the estuary
is calculated using the relation
L
E
2
t
= 0.4
(9.104)
R
L
1
where
L
is the length of the estuary (l), and
E
l
is the
longitudinal dispersion coefficient (l2T−1).
2
T
−1
). The magni-
tudes of longitudinal dispersion coefficients in several
river estuaries in the United States and the United
Kingdom are given in Table 9.11.
∫
S
=
SdV
(9.100)
V
V
Combining Equations (9.98) and (9.99) gives the
flushing time as
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