Traitement et analyse de séries chronologiques continues de ...

Annexes
Event load calculation
In this last step, TSS and COD event loads are calculated with their standard uncertainties. This
includes two sub-steps which are described hereafter: (i) the determination of the duration of the
hydrologic event and (ii) the calculation of the event load itself for the selected limits.
The duration of the hydrologic event comprises (i) the storm event duration itself, i.e. the time between
the beginning and the end of the rainfall event as measured with the rain gauge and ii) the time needed
for a set of variables (discharge, conductivity, turbidity) to reach again the values they had before the
storm event started. A software tool has been created to facilitate the automatic identification of the
beginning and the end of hydrologic events in separate sewer systems (Métadier and Bertrand-
Krajewski, 2010). The identification of the beginning t b and the end t e of hydrologic events is based on
3 criteria: (i) discharge threshold, (ii) minimum period between 2 successive independent events and
(iii) the maximum duration between the beginning of the storm event and the rising of the discharge in
the sewer. The operator then validates manually the results and may change them according to his/her
expertise if necessary. The tool has been subsequently extended for combined sewer systems by
accounting for the dry weather contributions to both the volume and the total pollutant mass. In this
case, for a given storm event, the “most similar” dry weather pattern can be chosen graphically from a
data base previously established and superimposed to the storm event for both discharge and turbidity
signals. The combined sewer option includes criteria accounting not only for discharge but also for
conductivity and turbidity values. The identification of the end of hydrologic events is more complex
than the identification of the beginning as, in some cases, pre-event dry weather values of conductivity
and/or turbidity are not reached again even many hours after the end of the rainfall event. This is why
the expertise of the operator remains necessary for final validation of the beginning and the end of