Urban stormwater control is an issue present since the oldest civilizations.  At the beginning of the 20th 
century, urban sewer systems begin to be configured as nowadays. In combined networks, it is intended to 
send so much flow as possible to waste water treatment plants (WWTP); during a rainfall event, when WWTP 
and interception sewers capacity are surpassed, excess flow is directly spilled into the receiving water body, 
generating a Combined Sewer Overflow (CSO). Some decades ago, if these discharges complied with some 
restrictions towards their dilution, they could be perfectly assumed by the receiving water body. Nevertheless, 
around 1960, urban overflows are identified to be one of the main causes of the receiving water bodies 
degradation. Since then, this problem is seriously studied and some CSO control and treatment methods taken 
into account so that, environmental criteria are introduced in urban drainage system design guidelines.  
Detention tanks are efficient elements to reduce negative effects of runoff spills produced during rainfall 
events to the receiving water bodies. Their use is nowadays quite generalised; nevertheless, there are still 
some divergences in optimum storage volume dimensioning criteria, some of them even obsolete. In fact, the 
problem has some local aspects with a great influence on the development of these methodologies, 
aggravating the possibility of a universal formulation.  All probabilistic methods for detention tank sizing begin 
with a proper stochastic rainfall characterization. This is also important for event based approaches, 
continuous simulation analysis and for simple derived methodologies. Consequently, even if methodologies can 
follow some common general standards, applied developments and, obviously, the results obtained cannot 
become widespread.
This analysis is conditioned by the requirements established by the Framework Water Directive 2000/60/CE 
and all its derived legislation. A protection or quality objective in the receiving water body must be fixed 
because the storage volume of the tank depends on it. The environmental impact of spills from the stormtank 
on receiving water bodies can be studied from two different perspectives, establishing an Emission Standard 
(ES) or an Environmental Quality Standard (EQS).
The first level of protection objectives corresponds to emission standards. They allow the planner to analyse 
and impose some restrictions on spills, evaluating their frequency, volume, pollutant load, etc. There are 
some practical ways to define an emission standard. The most extended ones deal with spills frequency and 
reduction of runoff volume or pollutant load spilled to the receiving water body. An analysis based on emission 
standards produces normally easy methodologies, and this represents an advantage. But the weak point is that 
they do not consider the receiving water body as an element of the model, so that, real impacts produced by 
the spills are not quite discriminated. This is why the second level of protection objectives (EQS) is necessary. 
Environmental Quality Standards are objectives defined directly on the receiving water body, instead of 
regarding the spills. 
This PhD deals with a probabilistic model to obtain some indicators to allow the detention tank efficiency 
evaluation from ES and EQS points of view. Results obtained by the way of this probabilistic method are 
applied and validated by complete and continuous simulation in a catchment in Valencia.
The probabilistic method development must begin with a proper rainfall characterization.  Exponential 
functions have been usually used for this but the scale variables of the event rainfall pattern (event volume 
and peak intensity) do not fit properly to this model in the application developed in Valencia. Alternatives 
suggested and fitted focus on the Pareto distribution. Nevertheless, the variable describing event durations 
fits well to an exponential model, just like the variable defining interevent time between events.
The main aim of the PhD is to build a probabilistic method in order to evaluate stormwater detention tank 
emission standards. Indicator EN (eventual efficiency) evaluates the long term ratio of events completely 
detained, i.e, the probability of an event to produce or not overflow. Indicator EV (volumetric efficiency) 
reproduces the long term ratio between detained runoff and total produced runoff in the catchment. The 
influence on these indicators of storage volume, flow derived to WWTP and the time sequence of events is 
analysed. A sensitivity analysis is also done, especially for infiltration parameters.
A design abacus for each efficiency indicator is built from results. Their inspection let us conclude that an 
adequate combination of storage volume and flow rate derived to WWTP produces, in the catchment analysed 
in Valencia, eventual efficiencies reaching 90% and volumetric efficiencies about 80%. Another interesting 
issue is that good efficiencies are obtained only if a flow rate to the WWTP is ensured.
Finally, the pollution wash-off during rainfall events and the benefits introduced by the tank are studied from 
a probabilistic approach too. A simple analysis based on Environmental Quality Standards is done starting from 
probabilistic results obtained along the PhD. Nevertheless, the final characterization and adaptation will 
depend on the specific problems of the receiving water body where impacts are established.