Intrusion Modelling and the Effect of Ground Water Conditions

Intrusion of contaminants into water distribution systems due to negative pressures is a complex phenomena that has been theorised and is an active area of research. Work has identified the existence of contaminants in soil and ground water surrounding pipes, and has investigated modelling the risk to human health should contaminants enter water distribution systems. However, there is a lack of understanding regarding the critical interaction between the pipe, the leak aperture and the surrounding ground and water. Typical intrusion models assume a simple orifice relationship, with inflow volumes proportional to the square root of the difference between the pipe pressure and the external hydrostatic pressure. This is shown here through computational modelling to be an overly simple relationship for leak behaviour that does not take into account the existence or properties of a porous media external to the pipe. In this paper the authors will discuss the construction of computational fluid dynamics (CFD) models of the intrusion process due to transient events and describe results that suggest the simple orifice equation is not a sufficient model. In the CFD calculations the surrounding ground water is modelled as a saturated porous media and it is shown that the properties of the media and the leak geometry have a large effect on the relationship between the pressures and flow rate. It is also shown that the risk of intrusion should be considered from contaminants that originate from both above and below the depth of the pipe. Further, the idea of a zone of influence surrounding the leak point, due to the leak size and the magnitude and duration of the negative pressure, is introduced as a possible measure of the level of risk of the intrusion event.