Chapter 3 : Reservoir models - KU Leuven

In this figure 3.3 the constant throughflow is presented on the horizontal axis
(in [m 3 /h/ha] for the upper horizontal axis and in [mm/h] for the lower horizontal axis),
while the maximum storage in the system is presented on the vertical axis (in [m 3 /ha]
at the left side and in [mm] at the right sight). The lines from the left top to the right
bottom are the isolines for the overflow frequency (from 2 to 25 p.a.).
The major disadvantage of this ‘dot’ method is the simplification of both rainfall and
model parameters. The method does not take into account the antecedent rainfall nor
the variability of the rainfall within a storm. By linking every storm to only one storm
duration (i.e. the duration of the total storm event), short overflow events for sewer
systems with a smaller critical storm duration may be missed. All this will lead to
an underestimation of the overflow frequency. There is also no criterion used to
distinguish dependent overflow events. Furthermore, the parameters of the combined
sewer system (storage and throughflow) are used as static parameters and are most
often determined very roughly. It can be concluded that the dot graph of Kuipers can
provide a rough estimation of the overflow frequency, but there remains a very high
uncertainty on the results. The advantage of this method is its simplicity and in earlier
days it was the only relevant possibility for the prediction of the overflow frequency.
It is obvious that by using computer technology, better methods can be developed,
while retaining the simplicity of the methodology.
In the early eighties at the Hydraulics Laboratory of the University of Leuven, a single
reservoir modelling system was developed [Berlamont & Smits, 1984a,b,c], which was
later extended to the multiple reservoir modelling system Glas [Glas, 1986; Berlamont
et al., 1987]. Glas is a linear reservoir modelling system, which means that the
throughflow of the reservoir is defined as a linear function of the storage in the
reservoir. Several reservoirs for subcatchments can be connected in series or parallel.
With such a reservoir model the real variability of the rainfall can be included. Also
the antecedent rainfall is included, because the time variation of the storage in the sewer
system is taken into account. The reservoir modelling system Glas used hourly rainfall
data as input for the period 1967-1986 measured at Uccle. This might be too smoothed
as rainfall input, because the rainfall variation within one hour can be high. Because
of the fact that the real succession of the rainfall is taken into account for a long period
(20 years) and a statistical analysis on the results is performed afterwards,
the assessment of the overflow parameters will be much better than using the Kuipers
graph. Using this kind of model, parameters other than the overflow frequency can be
easily determined (statistically), as there are overflow volumes, discharges and
durations. The remaining disadvantage of the Glas modelling system is the assumption
of the linearity between storage and throughflow, which is for many applications not
justified. The major bottleneck in the use of reservoir models is the calibration of the
sewer system parameters. The calibration is a very important stage, which is often
disregarded.
Chapter 3 : Reservoir models 3.3