Multiple versions of mixed-flow models have been developed in recent years with the reported capability to simulate strong transients in stormwater collection or storage systems. However, virtually all these models only simulate single phase flow, considering the water phase but do not explicitly account for air-water interactions during the transition between free surface and pressurized flow. Still the models are represented as demonstrating fidelity to the actual two-phase flow system. Since these are generally complex models with fairly large quantities of output, it is tempting to post-process the predictions to extract quantities such as peal pressures that might be relevant to system design. However, paying attention to details of the prediction results can point to invalid simulations that do not correctly capture the physics of the filling process. This presentation presents a case study where the system geometry presented an opportunity to result in large volumes of air contained in a reach with somewhat limited ventilation capacity. Nevertheless, the model predictions indicated pressure variations that were attributed to water hammer. A careful analysis of the model output indicated that the model did indeed predict water hammer under circumstances where air would have been present to cushion the transient. Various aspects of the model predictions are presented to demonstrate physically unrealistic simulations. While only limited monitoring data was available, examination of the data also fails to validate the predicted water hammer events. This cautionary tale described that it is necessary to understand the actual physics of these events and determine whether or not the model predictions are consistent.
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