Geoscience Reference
In-Depth Information
In small-roughness, uniform channels, the reaeration rate can be described by the
following formula derived from the small-eddy model for oxygen interfacial transfer
(Moog, 1995; Moog and Jirka, 1999):
0.161Sc 1 / 2
1
/
4
K L
=
(εν)
(12.58)
ε
where Sc is the Schmidt number; and
is the turbulent dissipation rate per mass near
the water surface, which may be estimated as
u 3
h .
In channels with large-scale form roughness, the reaeration rate is enhanced by
the large bed variations that produce depth-scale form drag. Moog and Jirka (1999)
proposed a modification for Eq. (12.58) to take this enhancement into account.
For standing water bodies, such as lakes, lagoons, and bays, reaeration is affected
by wind. Banks and Herrera (1977) suggested the following relationship:
ε =
/
0.728 U 1 / 2
w
0.0372 U w
K L
=
0.317 U w
+
(12.59)
day 1 , and U w is the wind speed (m
s 1 ) at 10 m above the
where K L has units of m
·
·
water surface.
The saturation DO concentration, C sDO , is a function of temperature and salinity.
In fresh and saline waters, C sDO is approximated by (Benson and Krause, 1984; see
Huber, 1993)
exp c 0
C sa c 5
T 2 (12.60)
c 1
T +
c 2
T 2 +
c 3
T 3 +
c 4
T 4 +
c 6
T +
c 7
C sDO
=
+
+
m 3 ; T is in K; C sa is the salinity in ppt, which is related to
chlorinity or chloride concentration C chl by C sa
where C sDO is in g
·
=
1.80655 C chl , with both in ppt; and
10 5 , c 2
10 7 ,
the coefficients: c 0
=−
139.34411, c 1
=
1.575701
×
=−
6.642308
×
10 10 , c 4
10 11 , c 5
c 3
=
1.243800
×
=−
8.621949
×
=−
0.017674, c 6
=
10.754, and
c 7
=
2140.7.
Photosynthesis and respiration of phytoplankton
The presence of aquatic plants, such as phytoplankton, weeds, and algae, can
significantly affect the DO concentration in a water body through photosynthesis.
These plants containing chlorophyll can utilize the radiant energy from the sun, con-
vert water and carbon dioxide into glucose, and release oxygen. The photosynthesis
reaction can be written as
6H 2 O photosynthesis
6CO 2
+
−−−−−−−−→
C 6 H 12 O 6
+
6O 2
(12.61)
Because the photosynthetic process is dependent on solar radiant energy, the
production of oxygen proceeds only during daylight hours. Concurrently with this
production, however, the algae require oxygen for respiration, which can be con-
sidered to proceed continuously. These two processes result in a diurnal variation
 
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