SWQM - A simple river water quality model for assessment of urban ...

12 nd International Conference on Urban Drainage, Porto Alegre/Brazil, 10-15 September 2011
SCON has been introduced as an (optional) user-definable conservative substance, i. e. a
fraction not being subject to any degradation processes. For example, it could be used to
include phosphorous in a simplistic way when necessary. SALK and SIC are used in order to
calculate pH, which is required for determination of the ammonia (NH3-N) part of SNH, using
the well-known relation by Emerson et al. (1975). The difference of these two fractions (SIC
– SALK) represents the CO2 contents of the liquid phase. SWQM therefore allows also to
model CO2 and its exchange with the atmosphere (Alex, 2009a).
Table 2 provides a summary of the biochemical transformations considered within SWQM
using the Petersen-Gujer notation as established in wastewater treatment modeling and also
used in the RWQM descriptions. As SWQM also considers pH and carbonate, it allows the
determination of NH3–N (toxic to aquatic life) rather than just using NH3+NH4 as a lumped
constituent.
Table 2. SWQM - the Simple Water Quality Model
Among the processes modelled by SWQM, reaeration is crucial one for the DO balance.
Although a wide variety of relationships describing the reaeration rate (most of them using
average values of water depth and velocity) has been described in the literature (see, for
example, Chapra (1997) and Bowie et al. (1985) for reviews, the modified approach by Wolf
(ATV-DVWK, 2002) has been applied here. For wide ranges of velocities and water depths,
this approach renders reaeration rates similar to other approaches. For very low water levels,
however, most equations in the literature, due to their exponential nature, result in very
(unrealistically) high reaeration coefficients. A correction term introduced by Wolf – hence
the term “modified approach by Wolf” - ensures that also for low water depths, seemingly
realistic reaeration coefficients k2 will be determined:
k
2
40 0
( 3 + ) v
k
=
h
St
1.
7
0
. 7
0.
5
+
h
⎧
with h0
= ⎨
⎩
h
− 0.
588
( 10 −13.
5*
h)
for h ≥ 0.
5
else
(kSt: roughness coefficient [m -1/3 /s], v: flow velocity [m/s], h: water depth [m])
As opposed to the many other model implementations of river water quality models where
often average values for water depth and velocity are used in the water quality module, both
implementations of the SWQM reported in this paper (see next section) use the actual values
4 SWQM - A simple river water quality model
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