Environmental Engineering Reference
In-Depth Information
the velocity,
V
, in meters per second and depth,
D
, in meters were given as functions of discharge,
Q
, in
cubic meters per second as follows:
Figure 9.3 shows the values of
K
a
estimated for this river for a range of flows. If, for a discharge of
2.8 m
3
/s, the measured value of
K
a
was 1.45 d
-1
, the O'Connor and Dobbins (1958), Owens et al. (1964),
Bennett and Rathbun (1972), Tsivoglou and Neal (1976), and Shindala and Traux (1980) equations all
would offer a good match to the measured value. However, if the design flow for waste-load allocation
was 0.5 m
3
/s, the “good match” equations would estimate
K
a
values ranging from 0.6 to 3.2 d
-1
. Thus, the
waste-load allocation could be drastically different using different “good match” equations.
V
0.05977
Q
0.5
and
D
0.6024
Q
0.4
Fig. 9.3
Reaeration-rate coefficient values as a function of discharge computed using different estimation equations
for the hypothetical river of St. John et al. (1984)
3) A set of
K
a
measurements is made for a group of rivers representative of a region or state, and these
measurements are used to derive
K
a
estimation equations specific to that region or state, e.g., Cleveland
(1989) for Texas and Hren (1984) for Ohio.
Generally, equations for estimation of
K
a
on the basis of stream hydraulic conditions are needed because
either
K
a
measurements are not made or the low-flow conditions for which measurements are available
may be substantially different from the low-flow conditions that must be considered for waste-load allocation.
These equations provide a rational basis for estimation of
K
a
for unmeasured low flows. Furthermore,
water-quality management often is done on the basis of continuous simulation of long periods—months
or years—[e.g., Demuynck et al. (1997); Melching et al. (2004)]. In such cases, equations are needed to
relate
K
a
to changing streamflow conditions throughout the year.
Reaeration varies with surface exposure and volume of water and the rate at which water is mixed by
vertical and horizontal currents that distribute absorbed oxygen and bring fresh volumes of unsaturated
water into contact with the atmosphere (Fair et al., 1971, p. 651). Considering the foregoing physical
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