Environmental Engineering Reference
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these changes indefinitely. Of particular concern are
flood control dams that produce controlled discharges
to an estuary rather than relatively short but massive
discharge during high-flow periods. Dams operated to
impound water for water supplies during low-flow
periods may drastically alter the pattern of freshwater
flow to an estuary, and although the annual discharge
may remain the same, seasonal changes may have sig-
nificant impact on the estuary salinity distribution and
its biota.
Well mixed
Stratified
Partially
mixed
Stratification Classification of Estuaries.
Stratiication
is most often used for classifying estuaries influenced by
tides and freshwater inflows. Three stratification classes
of estuaries are: (1) highly stratified estuaries, (2) par-
tially mixed estuaries, and (3) vertically homogeneous
estuaries. The primary parameter used to classify the
stratification potential of estuaries is the
Richardson
number
, Ri, which measures the ratio of buoyancy to
shear forces and for estuaries can be put in the form
Salinity
Figure 9.16.
Salinity gradient in an estuary.
Source
: National
Oceanography Centre, Southhampton, UK (2005).
1.5757 10
×
5
6.6423 10
×
7
c
=
exp
−
139.34
+
−
o
=
∆ρ
ρ
gQ
WU
3
T
T
2
f
Ri
a
a
(9.82)
(9.81)
10
8.6219 10
11
4
T
a
1.2438 10
×
×
t
+
−
3
T
a
where Δ
ρ
is the difference between freshwater and sea-
water density, typically 25 kg/m
3
;
ρ
is the reference
density, typically 1000 kg/m
3
;
g
is gravity (m/s
2
),
Q
f
is the
freshwater inflow (m
3
/s);
W
is the width of the estuary
(m); and
U
t
is the mean tidal velocity (m/s). If Ri is large
(>0.8), the estuary is expected to be strongly stratified
and dominated by density currents, and if Ri is small
(<0.08), the estuary is expected to be well mixed. Transi-
tion from a well-mixed to a strongly mixed estuary
occurs in the range 0.08 < Ri < 0.8. The stratification
classifications of several estuaries in the United States
are shown in Table 9.10.
It is apparent from Equation (9.80) that as the salinity
increases, the saturation concentration of DO decreases.
Salinity distribution has a dominant effect on mixing
in estuaries. Highly stratified (stable) estuaries offer sig-
nificant resistance to vertical mixing. Typical (vertical)
salinity profiles in estuaries are illustrated in Figure 9.16.
In stratified estuaries, there is a significant vertical varia-
tion in salinity, with a relatively uniform top layer over
a relatively uniform bottom layer, with a sharp salinity
gradient in between. In well-mixed estuaries, there is a
relatively small vertical variation in salinity, and par-
tially mixed estuaries have vertical salinity distributions
somewhere in between those of stratified and well-
mixed estuaries.
The two most important sources of freshwater to
an estuary are inflow from streams and rainfall, with
stream flow typically representing the greater contribu-
tion. The location of the salinity gradient in a river-
controlled estuary is to a large extent a function of
stream flow, and the location of the isoconcentration
lines may change considerably depending on whether
stream low is high or low. This in turn may affect the
biology of the estuary, resulting in population shifts
as biological species adjust to changes in salinity.
Most estuarine species are adapted to survive tempo-
rary changes in salinity either by migration or some
other mechanism (e.g., mussels can close their shells).
However, many estuarine organisms cannot withstand
9.3.4 Dissolved Oxygen: The Estuary
Streeter-Phelps Model
The Streeter-Phelps model can be adapted for applica-
tion to river estuaries, in which case the governing equa-
tions for DO and BOD are given by
V
dD
dx
K
d D
dx
2
(9.83)
=
+
k L k D
−
L
d
a
2
V
dL
dx
K
d L
dx
2
(9.84)
=
−
k L
L
r
2
where
V
is the longitudinal mean velocity (lT
−1
),
D
is
the oxygen deficit (l),
x
is the along-stream coordinate
(l),
K
l
is the longitudinal dispersion coefficient (l
2
T
−1
),
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