Chapter 3 : Reservoir models - KU Leuven

In the reservoir modelling system Remuli up to 10 reservoirs can be connected in series
or parallel, each with one throughflow and one overflow. Although in the current
version the number of reservoirs is limited to 10, this can be increased without any
problem. An example is shown in figure 3.27. Each of these reservoirs contains the
four modules which are shown in figure 3.26 (eventually the dynamic storage can be
0 and only three modules are then present).
Figure 3.27 : Schematic view of the simplified model for the sewer system of Dessel;
each reservoir contains (part of) the modules in figure 3.26.
3.3.2 Piecewise linear relationships
As the instantaneous relationship between storage and throughflow is important, models
should be built that can take into account a possible non-linear behaviour of the
system. However, most existing modelling systems assume a fixed type relationship
between storage and throughflow (most often a linear relationship or a constant
throughflow). When the parameters of such models are well calibrated, they can
provide a rough estimation of the effects, but the results will not always be accurate.
Therefore, it is better to use modelling systems that do not prescribe the model
structure, but allow a wide range of possibilities. This can be achieved using a
piecewise linear (also named multi-linear) relationship between storage and
throughflow. With this model structure every possible relationship can easily be
approximated by a combination of several successive linear parts. The only restriction
is that the throughflow must monotonously increase with increasing storage.
An example for the ‘static’ modelling approach is shown in figure 3.28.
3.30
The influence of rainfall and model simplification on combined sewer system design