10.4122/1.1000001626 Bouteligier, R. Vaes, G. Berlamont, J. The Flemish CSO monitoring network revisited DTU Library, Technical University of Denmark (DTU) 2005 CSOs data analysis monitoring rainfall variability Katholieke Universiteit Leuven, Hydraulics Laboratory en Conference full text 10.4122/1.1000001625 application/pdf 1 In 1998 a combined sewer overflow (CSO) monitoring network was established by the Water division of the Administration of Environment, Nature, Land and Water Management AMINAL (Ministry of Flanders, Department of Environment & Infrastructure). At the end of the measuring campaign 19 CSOs were being monitored. As a result data for five consecutive years are available at a number of CSO locations. This paper presents the results of a research project carried out by the Hydraulics Laboratory of the University of Leuven (K.U.Leuven) in which the monitoring data that were collected throughout the years were evaluated. Analysis of the measured data reveals that, for the larger part of the monitored CSOs, it often not suffices to merely measure the water level in the CSO chamber during overflow events. Also measurements of, or data about, the throughflow at the CSO and the water level in the receiving watercourse are needed in order to thoroughly evaluate the functioning of the CSO in terms of average spill frequency, spill flow rate and spill volume. Long term simulations indicate that, due to the annual rainfall variability, a CSO has to be monitored for several years in order to obtain a reliable, average CSO spill frequency.