Environmental Engineering Reference
In-Depth Information
concentration on the beaches. How does this compare
with the coliform concentration if decay is neglected?
Solution
From the data given, x = 6 km = 6000 m, u a = 22 cm/s =
0.22 m/s, c e = 40,000 CFU per 100 ml, and the near- and
far-field dilutions (neglecting decay) are given by
net ow
river
tidal river
river estuary
c
c
c
c
e
o
=
20
and
=
110
o
b
ocean
bay estuary
where c e is the coliform concentration in the effluent, c o
is the coliform concentration after initial dilution, and
c b is the coliform concentration on the beaches. Neglect-
ing decay, c b is given by
Figure 9.15. Typical estuary system.
endeavors. An example of a comprehensive field valida-
tion study of a far-field mixing model of the Boston
outfall can be found in Roberts et al. (2011).
c
c
c
c
1
20
1
110
=
o
b
c
=
c
40 000
,
= 18 / 100
mL
b
e
e
o
If decay is considered, T 90 = 6 hours, and the decay coef-
ficient, k , is given by Equation (9.73) as
9.3 ESTUARIES
An estuary can be broadly defined as the water body
that connects a river to the ocean. By this definition,
estuaries are complex transitional water bodies in which
the inflow is nontidal fresh water and the outflow is tidal
saltwater. A schematic diagram of a typical estuary
system is shown in Figure 9.15. At the upstream end, a
nontidal freshwater river transitions into a tidal river in
which tidal motions affect the flow to some extent, but
the water is still relatively fresh. The tidal river then
transitions into the upper part of the estuary, which still
appears (physically) like a river but has full tidal rever-
sals and the water is brackish. The lower part of the
estuary typically widens into a bay, which is tidal and
has a much higher salt concentration than the riverine
portion of the estuary. The key difference between a
river estuary and a bay estuary is that the flow in a river
estuary can be approximated as one dimensional,
whereas the flow is either two- or three-dimensional in
a bay, depending on the depth in the bay.
Many large population and industrial centers have
developed adjacent to estuaries because of the easy
access to both the ocean and inland river systems for
water transportation. The wastes from these large
population/industrial centers have often been dis-
charged into estuarine waters, with little or no aware-
ness of the biological importance of the receiving
estuary or of the tendency of pollutants to be retained
within the estuary. Brief descriptions of some of the
major estuaries in the United States and their associated
water-quality threats are given in Table 9.9. It is appar-
ent from this table that sewage discharges and runoff
ln10
2.30
6
k
=
=
=
0.383
h
1
=
0.000106
s
1
T
90
and
c
c
1
110
1
110
b
kx u
/
(0.000106)(6000) 0.22
/
=
e
=
e
=
0.000505
a
o
Therefore, the far-field dilution is given by
c
c
1
0.000505
o
=
=
1980
b
and the fecal coliform concentration on the beaches, c b ,
is given by
c
c
c
c
1
20
1
1980
=
=
o
b
c
=
c
40 000
,
1 / 100
mL
b
e
e
o
Therefore, consideration of coliform decay yields an
order-of-magnitude reduction in the expected level of
fecal coliforms on the beaches.
Models of far-field mixing are more complex than
models of near-field mixing. Comprehensive models of
far-field mixing have been proposed by Chin and
Roberts (1985a,b) and Chin et al. (1997). There has been
limited field validation of far-field models of outfall dilu-
tion, presumably due to the cost and complexity of such
 
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