Environmental Engineering Reference
In-Depth Information
flow conditions along the stream by com-
puting a series of steady state water surface pro
The program simulates changes in
ow
rate, velocity, cross-sectional area, and water depth serve as a basis for determining
the heat and mass
les. The calculated stream
ow.
Mass balance determines the concentrations of conservative minerals, coliform
bacteria, and non-conservative constituents at each computational element.
In addition to material
fluxes into and out of each computational element due to
fluxes, major processes included in the mass balance are
the transformation of nutrients, algal production, benthic and carbonaceous
demand, atmospheric reaeration, and the effect of these processes on the dissolved
oxygen balance. QUAL2K uses chlorophyll a as the indicator of planktonic algae
biomass. The nitrogen cycle is divided into four components: organic nitrogen,
ammonia nitrogen, nitrite nitrogen, and nitrate nitrogen. In a similar manner, the
phosphorus cycle is modeled by using two components. The primary internal sink
of dissolved oxygen in the model is biochemical oxygen demand (BOD). The major
sources of dissolved oxygen are algal photosynthesis and atmospheric reaeration
(Khodadadi Darban
2010
).
The hydrodynamic model was developed on the foundation of the continuity
equation, the momentum equation, and the mass-balance equation for salt. The
water quality model is based on the laterally integrated equation describing the
mass-balance of a dissolved or suspended substance in the water column
(Eq. (
6.1
)).
o
ð
CB
Þ
o
t
þ
o
ð
CBu
Þ
o
t
þ
o
ð
CBw
Þ
o
t
K
x
B
o
C
o
x
K
z
B
o
C
o
z
¼
o
o
x
þ
o
o
z
þ
BS
i
þ
BS
e
ð
6
:
1
Þ
where; t = time [T]; x = distance seaward along river axis [L]; z = distance upward
in the vertical direction [L]; B = river width [L]; C = laterally averaged concen-
tration [M/L3]; u and w = laterally averaged velocities in the x and z directions,
respectively [L/T]; K
x
cients in the x and z
directions, respectively [L2/T]; Si
i
= time rate of internal increase (or decrease) by
biochemical reaction processes [M/L3T]; S
e
= time rate of external addition (or
withdrawal) across the boundaries [M/L3T].
As indicated on Fig.
6.1
, the water quality model consists of eight interlinked
components including organic nitrogen (ON), ammonium nitrogen (NH
4
-
N),
nitrite
and K
z
= turbulent diffusion coef
nitrate nitrogen (NO
2
+
NO
3
-
N), organic phosphorus (OP), inorganic phos-
phorus (PO
4
-
P), chlorophyll
-
(chl), carbonaceous biochemical oxygen demand
(CBOD), and the dissolved oxygen (DO) (Khodadadi Darban
2010
).
Each of the water quality components can be represented by the same equation as
Eq. (
6.1
), but with its own representations of external (Se) and internal (Si) source
and sink terms. Each rectangular box in Fig.
6.1
represents a component being
simulated by the model. The arrows between components represent the biochemical
transformation of one substance to the other. An arrow with one end unattached to a
component (rectangular box) represents an internal source (or sink) due to the
biochemical reaction or an external source (or sink) (Khodadadi Darban
2010
).
'
a
'
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