Practical Applications Most models to quantify combined sewer overflow pollution are deterministic [e.g., stormwater management model (SWMM)]. Any given set of model inputs (e.g., rainfall, land use, pipe configuration, etc.) always produces the same discharge. We provide evidence that there can be an element of randomness in CSOs, in which the same rainfall can produce different discharges at specific locations. In this study, a gate controls the flow to the CSO retention tanks.