Chapter 3 : Reservoir models - KU Leuven

Although this is an unrealistic (artificial) situation, this state is most often a very good
approximation of the situation at the end of an overflow event (figure 3.7). At least the
maximum static storage in a sewer system can be assessed in this way, even if there is
a flow in the system which creates an extra storage (this is most often the case). In the
same way the static storage at any time can be assessed as the storage during the
emptying of the system when no rainfall input is present. This means that the
instantaneous static storage, for instance for the case of figure 3.6, must be very close
to the storage during the falling branch of the storm (slightly smaller (see figure 3.15)).
In figure 3.7 it can be seen that the storage at the end of the overflow event is a good
indication of the maximum static storage in the system. If only the static storage is
incorporated, the storage at the beginning of the smallest overflow event can also be
a good estimation for the static storage (values near to 0 on the horizontal axis in
figure 3.7). Although the maximum static storage can be easily obtained at the end of
the overflow event, it is important to use the instantaneous relationship between storage
and throughflow for the rising branch of the storm up to the moment the overflow starts
spilling. This is necessary to obtain a good estimation of the start of the overflow.
The primary objective should be to detect if an overflow occurs or not and when it
starts. The emptying of the system after the overflow event only has a second order
effect, because the storage in the system can have an influence on the following
overflow event. This means that in the first instance the dynamic storage can be
neglected. As for sewer system calculations in Flanders the composite storms are used,
it is logical to use the composite storm that will just lead to an overflow event to
determine this storage/throughflow-relationship in a first approach. This first
approach has been applied to a number of different sewer systems and provides already
a lot of information on the sewer systems behaviour. In figure 3.9 the ‘static’
storage/throughflow-relationship is shown for a small sewer system with a gravitary
throughflow.
3.10
The influence of rainfall and model simplification on combined sewer system design