Ordering Information:
ORDER NUMBER:  91178
DATE AVAILABLE: Summer 2008

PRINCIPAL INVESTIGATORS:
Donald Ballantyne, Hope Seligson, Karen Damianick, William Heubach, and William Steenberg

OBJECTIVES:
The Seattle Public Utilities water distribution system suffered pipeline damage in three moderate earthquake events over 52 years, the most recent being the February 28, 2001 Nisqually earthquake. The purpose of this project was to quantify the damage of the three earthquakes and pose three mitigation alternatives that would help maintain system operation when a large event occurs. Specifically, the research team sought to evaluate a strategy to mitigate pipeline damage using isolation and control valves to quickly detect damage in a selected pressure zone, and if heavily damaged, remove it from service.

BACKGROUND:
Earthquake damage to pipelines has historically resulted in significant impacts on water systems. Cast iron pipe installed in liquefiable soils is particularly vulnerable. Many cities with a high earthquake risk have large inventories of cast iron pipe; however, replacing pipe is prohibitively expensive.

HIGHLIGHTS:

• The pipeline damage rates encountered in the three Seattle earthquake events were of the same order of magnitude as those proposed by published damage reports from other major earthquakes.
• The lowest pressure zone in the Seattle system, with much of the pipe in liquefiable soils, would hydraulically collapse in the event of a 500-year return (code design) earthquake.
• Three mitigation alternatives including pipeline replacement, system control/isolation, and emergency response could potentially maintain post-earthquake system functionality.
• A simplified system analysis was developed for implementation on smaller systems.
• Lessons learned by regional water purveyors in response to the 2001 Nisqually earthquake were documented.

APPROACH:

The research team conducted the following steps:

• Using GIS tools, quantified earthquake shaking intensity and liquefaction hazards, along with pipe exposure, for the three Seattle earthquakes
• Calculated damage rates
• Modeled the effect of a 500-year return earthquake using EPANET
• Estimated pipeline damage and incorporated network connectivity (pipe breaks) and leakage in the model
• Estimated the cost of replacing all vulnerable pipe in liquefiable areas
• Modeled hydraulically the system performance using an isolation valve strategy
• Evaluated an emergency response approach while considering the delays associated with traffic congestion following a large event

RESULTS/FINDINGS:
Cast iron pipeline damage founded in liquefiable soils can result in hydraulic collapse of the system. Wholesale replacement of all of the vulnerable pipe can be very expensive; in Seattle’s case, replacing all vulnerable pipe in liquefiable soils would cost on the order of $1 billion. System control to isolate the most heavily damaged portions of the system, while maintaining the function of less damaged pressure zones appears workable at very moderate costs. The disadvantage is that loss of service would be planned (e.g., turn off the isolation valves) in some areas rather than loss from pipeline damage due to natural causes. Alternative means for fire protection would need to be provided.

IMPACT:
The work reinforces that water distribution systems are vulnerable to earthquakes and expensive to replace. The proposed system control/isolation alternative provides an approach that could be very effective at a limited cost. The policy of planning to shut down the damaged portion of a system in order to maintain operation of the remaining sections requires careful thought before implementation.

RESEARCH PARTNERS:

• Seattle Public Utilities
• Everett (Wash.) Public Works Department
• San Francisco Public Utilities Commission
• Los Angeles Department of Water and Power
• Greater Vancouver Regional Water District, Burnaby, B.C., Canada
• Tacoma Public Utilities
• St. Louis Water Division
• Thames Water Utilities, United Kingdom

PARTICIPANTS:
The project was sponsored by eight water utilities from the United States, Canada, and the United Kingdom, all with concerns about seismic vulnerability. The evaluation was conducted on the Seattle Public Utilities system.