Three large diameter conduits (Conduits 2, 3 and 4) convey water from theHeadworks at Bull Run Dam No. 2 in the Bull Run Watershed to the City of PortlandReservoirs, located on Mt. Tabor and Powell Butte in southeast Portland.Twenty-two trestles support exposed or above-ground sections of these conduits atcreeks and swale crossings. A recent system vulnerability study concluded that tovarying degrees, most of the trestles supporting Conduits 2 & 3 are vulnerable toearthquake ground shaking with return periods greater than 100 years. Inaddition, the study found that the risk of damage from floods with return periodsof 100 years or greater exists for some of the trestles on all of the conduits,and that localized landslides of 100 to 200 feet in width are possible along theconduit corridors east of the Sandy River. A preliminary evaluation studydeveloped mitigation options designed to reduce the vulnerability of the trestlesto damage from earthquake, flood, and additional hazards such as multiple conduitfailure, operational error, and attractive nuisance. Bureau stakeholders decidedthat mitigation options should at the very least enable trestles to withstandflood and earthquake events with 100-year return periods. 500-year events,however, were established as the most desirable standard for evaluatingmitigation options. These "preferred standards" directly influenced the weight offlood and earthquake criteria in the evaluation of mitigation options. Tworanking methods, Criteria Ranking and Expected Annual Outage (EAT), were comparedand used to develop an integrated priority strategy for sequencing and timing theimplementation of the recommended mitigation options. Although the two rankingmethods yielded generally different results, close inspection revealed a logicalcommonality among groups of projects. The study also explored permittingrequirements for the implementation of the recommended mitigation options. Includes reference, figures.
