Probabilistic Modelling of the Immission of Water and Pollution from Urban Drainage Systems in Receiving Watercourses

This paper presents the results of a recently completed Ph.D. study by Willems, in which a methodology has been worked out to model the current and future states of surface waters considering all sources of water and pollution in the hydrographic catchment in an integrated/holistic way. Both aspects of water quantity and quality have been considered. In this way, the methodology is applicable for river management, both from the perspective of flooding problems and water pollution problems. These problems have internationally been the subject of increasing concern, especially during the last two decades. To solve the problems, water management stategies have been worked out by authorities all over the world. Each of these strategies is often set up only from the perspective of one water or pollution source. In this way, dikes have been built to limit of risk of river flooding and urban drainage systems have been improved to solve urban flooding problems. At the same time, waste water treatment plants (WWTP's) have been built to reduce the cumulative domestic emission loads to the receiving rivers and improved overflow structures, storage and sedimentation basins have been constructed to limit the occasional overflow emissions. Also emission standards have been set up in combination with the latter two types of investments and additional emission standards have been defined for industrial pollution sources and fertilization standards to limit the agricultural pollution. Although all these measures and investments have improved the surface waters' states already significantly, the improvements often did not reach the expected level and/or the measures caused additional problems. In Belgium, this became undeniably clear the last few years by the increased number of floodings in both rural and urban areas. The increasing number of sewer systems and connecting areas and the isolation of natural flooding areas from rivers by dikes partly explain this increase. By the growing insight in the global water system, it has becoming clear that the efficiency of measures and investments based on the analysis of only one water or pollution source is often not optimal. The different water subsystems in a hydrographic catchment have to be considered in an integrated way. This means for instance that immissions (the impact of the emissions on the receiving river) have to be analysed instead of emissions (the discharges and pollution loads from urban drainage systems, industry and agricultural activities). The mathematical modelling of immissions by the holistic modelling of all subsystems causing an increase of water and pollution in the receiving rivers was the subject of the study.