Water main decision support system seizes advantages
Ewe Leng Lim, PE
Senior Systems Analyst
Seattle Public Utilities
710 Second Avenue, 9'hFloor
Seattle, WA 98104
Nick Bodnar, PE
Project Director
Roy F. Weston, Inc.
700 Fifth Avenue, Suite 5700
Seattle, WA 98104-5057
Introduction
Seattle Public Utilities (SPU) has the largest water utility in the state of Washington and one of
the largest in the nation. The water utility supplies high-quality water to nearly 600,000 people
in the metropolitan Seattle area through direct retail service and another 625,000 people through
28 wholesale customers. The area served by the regional supply system covers 320 square miles.
The water pipeline system consists of 160 miles of supply mains, 45 miles of feeder mains, and
1,620 miles of distribution mains. The pipeline system ranges in age from newly installed to
over a hundred years old. Various different pipe materials were use throughout the years with
varying installation quality and under many different environmental conditions.
As the water distribution infrastructure ages, it becomes increasingly more challenging to assign
limited capital expenditures to prioritize the repair, replacement, or rehabilitation of pipelines.
There is a growing need by municipalities to find better ways to prioritize their future projects.
Much research has gone into the process of water distribution system rehabilitation planning.
The American Water Works Association (AWWA), American Society of Civil Engineers
(ASCE), and other professional associations have produced numerous publications on the
subject. Results and recommendations from these research have been adopted by water utilities
in varying degrees. Most water utilities have adopted some form of subjective ranking system to
prioritize pipeline rehabilitation. A smaller number of water utilities have completed statistical
analyses to predict pipeline failure and incorporate the results within a cost-benefit analysis.
SPU water engineers and managers have seized upon the advantages of accessing water main
characteristic information through Seattle’s Geographic Information System (GIS) and have
significantly reduced the time spent identi~ing and prioritizing water mains for preventive
maintenance, rehabilitation, or replacement. This paper describes how SPU developed and now
utilizes GIS-based information in day-to-day decision making and capital improvement program
preparation with a focus on the water pipeline infrastructure.
Overview
In order to provide easier access to SPU’S GIS database, the GIS group embarked on a widescale
installation of customized ArcView projects on user desktops. ArcView is a relatively
user-friendly GIS soflware that is available for the desktop users. The customized ArcView
projects allow users to more easily add layers of information with default symbology and other
descriptive information. Among the customized tools in the ArcView project is a fairly smart
address matching capability that would try to match address to the parcel layer first which will
result in a fairly accurate location if the exact parcel is matched. If an exact match cannot be
found, the program will then try to match the street lines that will result in a “within-the-block”
location, depending upon the accuracy of the street network database. The layers of information
available to the users include: water pipeline attributes, such as material, diameter, age, etc.;
historical pipeline breaks; water pressure zones; major water users; soil type; land use and
zoning, such as industrial, commercial, residential, etc.; critical water service connections such as
hospitals, dialysis centers, medical facilities, and others.
With this vast amount of valuable information readily available to the users, they can begin to
analyze the water network and ask questions such as:
- Which pipelines appear to be most vulnerable or susceptible to failure based on their
attributes?
- Which pipelines are most critical or would cause most damage in the event of a failure?
- What is the correlation between historical pipe breaks and a pipe’s attributes or its relative
location to its surrounding conditions, such as soil type or proximity to other features?
- How can we best prioritize which pipelines to replace based on the answers to the above
questions?
To assist the users in making these and other analyses, a custom decision-support application was
developed to leverage the GIS water main characteristic data with statistically based decision
models to improve decision making. Correlation of water main characteristics with time-to-
failure factors identified statistically significant failure prediction indicators. Based on the
identified indicators, decision models were developed for evaluating deterioration, vulnerability,
and service criticality factors. Then, based on evaluation results, main segments can be
prioritized on the probability of their failure during a given time period. Ratings maybe output
to tabular or map reports for further discussion or investigation.
To make replace-versus-repair decisions, SPU staff may apply standard costing data to compare,
for each main segment, the present worth of probable future repair costs against current
rehabilitation or replacement costs.
