Reliability Analysis of a Gravity Adduction of Water Supply System: Case Study

Abstract

The gravitational transfer of drinking water through gravity-fed supply pipes from a departure tankto a receiving tank often follows very complex and winding itineraries. The longitudinal profiles of theseitineraries consist of a succession of singular points (top and bottom points). A common mistake made byinexperienced engineers is to ignore these specific features and focus only on the levels of the departure andreceiving tanks. The problem becomes even more complex when the longitudinal profile forms a siphon, wherethe transferred water undergoes a significant drop to the lowest point before rising again to reach the receivingtank. This paper focuses on the study of a practical case of a siphon in a gravity-fed water supply systemconnecting two tanks over a distance of 15.25 km. By using a Geographic Information System (GIS), a DigitalElevation Model (DEM) was generated along the pipeline itinerary to highlight the terrain's complexity and thesingular points of the pathway. A deterministic analysis was conducted to evaluate the service life of the siphonshaped gravity-fed water supply system. This analysis revealed that uncertainties in the gravity adduction,particularly regarding the internal roughness of the pipeline, can directly impact operating pressures at singularpoints and, consequently, affect the system’s lifetime. Additionally, a reliability-based analysis was performed,accounting for the random variability of roughness. The limit state function is governed by the normal operatingconditions and failure criteria of the water supply system. The failure probabilities of the gravity adduction withrespect to these limit state functions were computed using the classical Monte Carlo simulation method and thencompared to the allowable failure probability for civil engineering structures