Sensitivity analysis of some critical factors affecting simulated intrusion volumes during a low pressure transient event in a full-scale water distribution system.

Intrusion events caused by transient low pressures may result in the contamination of a water distribution system (DS). This work aims at estimating the range of potential intrusion volumes that could result from a real downsurge event caused by a momentary pump shutdown. A model calibrated with transient low pressure recordings was used to simulate total intrusion volumes through leakage orifices and submerged air vacuum valves (AVVs). Four critical factors influencing intrusion volumes were varied: the external head of (untreated) water on leakage orifices, the external head of (untreated) water on submerged air vacuum valves, the leakage rate, and the diameter of AVVs' outlet orifice (represented by a multiplicative factor). Leakage orifices' head and AVVs' orifice head levels were assessed through fieldwork. Two sets of runs were generated as part of two statistically designed experiments. A first set of 81 runs was based on a complete factorial design in which each factor was varied over 3 levels. A second set of 40 runs was based on a latin hypercube design, better suited for experimental runs on a computer model. The simulations were conducted using commercially available transient analysis software. Responses, measured by total intrusion volumes, ranged from 10 to 366 L. A second degree polynomial was used to analyze the total intrusion volumes. Sensitivity analyses of both designs revealed that the relationship between the total intrusion volume and the four contributing factors is not monotonic, with the AVVs' orifice head being the most influential factor. When intrusion through both pathways occurs concurrently, interactions between the intrusion flows through leakage orifices and submerged AVVs influence intrusion volumes. When only intrusion through leakage orifices is considered, the total intrusion volume is more largely influenced by the leakage rate than by the leakage orifices' head. The latter mainly impacts the extent of the area affected by intrusion.