In spite of its considerable parallels with the analysis of pressurised networks, urban drainage systems are much more complex. This is, for other reasons, due to the uncertainty involved in determining the "loads" that these have to take, depending not only on the return period adopted but also on the always complex rainfall-runoff ratio. Apart from this complexity there are also the difficulties inherent to free surface hydraulics. All of this is why up to now the analysis of these systems has always been more simplified than it should be.
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