Gabrielle Ebacher

Transient Modeling of a Full-Scale Distribution System: Comparison with Field Data

The usefulness of transient models depends on their predictive ability. Consequently, their results should ideally be validated with field data. Despite numerous theoretical developments in the area of surge analysis, comparisons between field and modeled data for large distribution systems (DSs) are scarce. Transient low-pressure events at a water treatment plant (WTP) resulted in negative pressures at nu- merous locations in the DS. Three distinct surge events were measured in a full-scale DS and modeled with transient analysis software. The simulated pressure profiles were compared with field data collected from 9-12 sites within the DS. The objective was to apply a commercial transient analysis algorithm to a large and detailed network model (≈15;000 nodes=pipes) to estimate transient pressure variations within the network. Results showed similar trends for the three low-pressure events analyzed: the modeled pressures matched reasonably well with the measured pressures, as long as they remained positive. Whenever the pressures reached negative values, the simulated amplitude was larger than that of the recorded pressures. Modeling parameters and factors that might explain such results were tentatively investigated. The importance of field data in understanding and confirming the model outputs is highlighted. DOI: 10.1061/(ASCE)WR.1943-5452 .0000109.

Negative Pressure Events in Water Distribution Systems: Public Health Risk Assessment Based on Transient Analysis Outputs

Transient analysis of a pump trip was conducted on a full-scale distribution system (DS) equipped with high-speed pressure transient data loggers at the outlet of the water treatment plant (WTP) and at 12 DS sites. Following the calibration of the transient model (~16,000 nodes) with transient pressure recordings, intrusion volume computations were performed considering two intrusion pathways: leakage orifices and submerged air vacuum valves (AVVs). As expected, the estimated intrusion volumes through submerged AVVs are considerably larger than those through leakage orifices. Water quality modeling was conducted in order to evaluate the spatiotemporal dispersion of the intruded water, assumed to be contaminated with Cryptosporidium oocysts. A point estimate of the maximum probability of infection was then computed at each DS node using the negative exponential model. The estimated maximum probabilities of infection were displayed on the DS map and the model assumptions are discussed. This exercise has underlined important risk gradients and the localized occurrence of very high probabilities of infection, suggesting that a global risk analysis might be misleading. This project is the first attempt at quantifying public health risks induced from low pressure events in a large scale system (supplying ~400,000 people) based on actual negative pressure recordings.

Public Health Risk of Microbial Intrusion Events in Distribution Systems

Low or negative pressure events in distribution systems may lead to intrusion of pathogenic microorganisms if (i) there is an external source of contamination, and (ii) there is a pathway for the microorganisms to enter. Adverse health implications may result from such events if there is a lack of detection or intervention to mitigate public exposure.

QUANTIFICATION OF THE RELATIVE IMPORTANCE OF FACTORS CONTRIBUTING TO INTRUSION IN A DISTRIBUTION SYSTEM USING A FULL FACTORIAL DESIGN

Transient analysis of a pump trip event was conducted on a full-scale distribution system (~16,000 nodes) equipped with high speed pressure transient data loggers at the outlet of the water treatment plant (WTP) and at several distribution system (DS) sites. A calibrated transient model was used to perform intrusion volume computations considering two possible intrusion pathways: leakage orifices and submerged air vacuum valves (AVVs). For this pump trip event, a full 3-level 4-factor factorial design (3 4-0 ) with 82 runs was completed to understand the relative impact of the following factors on the total intrusion volume: (1) the external head of untreated water on leakage orifices, (2) the external head of untreated water on the outlet orifice of submerged AVVs, (3) the leakage rate (which translates into orifice size), and, (4) the diameter of AVVs’ outlet orifice. Although some of these factors may be highly uncertain, fieldwork (including installation of piezometers and visits of air valve vaults) is currently being conducted, and realistic values were assigned to these factors. The analysis showed that the factors and interactions associated with AVVs had a significant effect on the total intrusion volume. The importance of the external head on AVVs’ orifice on the total intrusion volume was highlighted in a plot of marginal means. When intrusion through both pathways occurs concurrently, the interactions between these intrusion flows influence the total intrusion volume. The latter can even decrease when the external head on leakage orifices increases creating surge dampening effects large enough to significantly reduce intrusion through submerged AVVs.

Comparison of Pressures Simulated using Transient Analysis with Field Data from a Full-scale Distribution System

Negative pressures were measured in a full-scale distribution system following low pressure events at a water treatment plant. Transient analysis was used to model three downsurge events and compare the simulated pressure profiles with field measurement data. The objective of this work is to assess the source of uncertainty and variability associated with the estimation of intrusion volumes calculated by a transient analysis model. This assessment was conducted by comparing actual field pressure measurements and model outputs under various model settings (e.g., cavitation head, wave speed). For the three downsurge events, the modeled pressure profiles matched reasonably well with the measured pressures, as long as the pressures remained positive at a site. When the pressures reached negative values, the amplitude of the modeled pressures was larger than that of the recorded pressures. The difference between measured and modeled pressure is strongly related to a greater energy dissipation in the real distribution system, which is affected by the uncertain presence of air in pipes, the level of network skeletonization, and the allocation of demand. The estimation of intrusion volumes and risk for public health is directly affected by the pressure results obtained using transient analysis. Comparison to field data is therefore important to evaluate the accuracy of such a process.