Chapter 3 : Reservoir models - KU Leuven

Other more detailed runoff models could be used, but the calibration of the parameters
requires large amounts of high quality data. Although a detailed runoff model for the
pervious areas might be necessary for the modelling of runoff to brooks and rivers
(e.g. [Clarke, 1994; DHI, 1993]), the influence of pervious areas and certainly
the influence of the detail of the infiltration description will be small for urban
drainage. If it is necessary to incorporate the infiltration, in most cases the rough
description by Horton (equation 3.5) will suffice.
Next to the losses, also the smoothing by the overland flow should be considered.
Many complex models can be used for this, as was described in paragraph 1.3.3.
However, if this is already taken into account in the single storm simulations that are
used for the calibration of the routing model, the smoothing is already incorporated in
the routing part of the simplified model.
3.2.5 Complex systems
In the previous paragraphs, the sewer system characterisation was applied to simple
sewer systems with one overflow. In reality, sewer systems are more complex, but it
is not always necessary to incorporate this complexity into the model. Before a model
is built, the modeller has to decide which output he wants to obtain. Then, the model
has to be built in such a way that the required output is achieved with the desired
accuracy.
The first step in the transformation of a combined sewer system into a reservoir model
is to separate the hydraulically independent subcatchments. This can be done
e.g. at pumping stations, internal weirs, places where the critical water depth always
occurs, ... The number of places where these conditions occur may be limited.
However, other places can be used to disconnect different subcatchments as long as no
significant backwater effects occur at these places. Whether a certain place is suitable
for disconnecting subcatchments, can be seen in the storage/throughflow-relationships.
If the storage volume in the subcatchment cannot be linked unambiguously to the
inflow and outflow, the place is not suitable for disconnection of subcatchments. In all
other cases the disconnection can be applied at that place.
As a reservoir model is a conceptual model, the outflows of the reservoir do not
necessarily have to agree with the flow in a single pipe or at a single overflow.
The throughflow can be the sum of different pipes which connect two
subcatchments and the overflow can be the sum of two overflows which spill into
the same brook or river. This gives more possibilities to separate subcatchments,
but also to simplify the models.
Chapter 3 : Reservoir models 3.25