10.4122/1.1000001498 Paoletti, A. Sanfilippo, U. Sessa, D. Innocenti, I. Propagation of pollutants discharged by urban drainage overflows in natural streams in unsteady flow conditions DTU Library, Technical University of Denmark (DTU) 2005 Receiving stream pollution propagation advection dispersion unsteady flow Politecnico di Milano, DIIAR - Sez. CIMI en Conference full text 10.4122/1.1000001497 application/pdf 1 The first aim of the paper is to present a new conceptual approach to simulate the propagation of dissolved pollutants in one-dimensional streams under unsteady flow conditions by means of the superimposition of effects. Such approach consists in the extension to the unsteady flow of an already existing so-called IUP approach, already set up and widely tested for steady flow. The reliability of such an extension to unsteady flow is validated comparing it with the well known OTIS model of USGS, which is based on a set of differential equations that must be solved through finite partial differences. Afterwards, this conceptual model is applied to investigate, through some significant numerical examples in unsteady flow conditions, the propagation along receiving streams of pollutants discharged by intermittent spills coming from the sewer overflows of urban drainage systems. The main result is that in many cases the persistence time above a given concentration threshold in the receiving stream increases progressively along the first sections downstream the spill, but then tends to decrease after an intermediate peak, which locates the critical section from this point of view; and the higher is the longitudinal dispersion value, the less downstream such a critical section is located.