Chapter 3 : Reservoir models - KU Leuven

Figure 3.14 : Longitudinal profile of a sewer system on which the static
and dynamic storage is indicated using a strict definition of
storage under and above the crest level.
As already mentioned, the maximum dynamic storage is correlated with the maximum
overflow discharge (figure 3.7). Therefore, it could be assumed that there is
a relationship between the instantaneous dynamic storage and the global outflow from
the system (overflow + throughflow). It is more common to link the dynamic storage
to the inflow (e.g. Muskingum method [Cunge, 1969]) than to link it with the overflow
discharge, although the optimum relationship will be a combination of both. If the
inflow hydrograph is first smoothed in the same way as during a hydrodynamic routing,
the instantaneous values of this smoothed hydrograph will be near to the sum of the
instantaneous values of the overflow and the throughflow. This means that the dynamic
storage can be assessed as a function of the smoothed inflow hydrograph. In this work
this smoothing is obtained by an averaging of the runoff over the concentration time as
will be further addressed in paragraph 3.3.1. In figure 3.15 an example is given where
the dynamic storage is included as a linear function of the smoothed inflow. The total
storage can then be obtained by the summation of the static and the dynamic
storage. The correlation between the relationship for the total storage obtained with
a hydrodynamic simulation and those obtained using the linear relationships between
the dynamic storage and inflow on the one hand and between the static storage and the
throughflow on the other hand is very good for this sewer system (figure 3.15).
The incorporation of the dynamic storage can certainly improve the estimation of the
overflows, especially with respect to the overflow duration and the (instantaneous)
overflow discharges, although with only a static storage a good result is already
obtained in many cases. The incorporation of the dynamic storage not only improves
the shape of the overflow events (i.e. the variation of the overflow discharge in time),
it also allows a better approximation of the emptying of the system. The antecedent
rainfall conditions for the next storm will be better incorporated in this way.
Chapter 3 : Reservoir models 3.15