Bolton Reservoir Siting Alternative Analysis
Bolton Reservoir Siting Alternatives Analysis
The city of West Linn is proposing to replace the Bolton Reservoir with a new 4-million-gallon reservoir. The 2.5-million-gallon Bolton Reservoir is now 101 years old and was constructed when the city’s population was 1,500. Today, West Linn is home to 25,000 residents with increased water demand.
The city is using a two-phased process to identify a site of the new reservoir.
- Phase 1 is a siting analysis with an engineering firm assessing locations that fit the siting criteria. A site will be recommended for further evaluation at the end of Phase 1 based on numerous technical and cost factors.
- Phase 2 is a geotechnical analysis with an engineering firm conducting a site analysis with geotechnical evaluations, including soil samples and geological evaluations to ensure the recommended site is suitable for the proposed reservoir.
Engineering firm Murray, Smith & Associates, Inc. (MSA) has completed the Phase 1 siting alternatives analysis. This analysis identified all possible locations for a reservoir in the same pressure zone as the current reservoir. A site in the same pressure zone is necessary to meet the existing system hydraulics needed to get water from the South Fork Water Board supply, which draws water from the Clackamas River.
MSA identified approximately 90 properties for investigation that were greater than a minimum size of 1.5 acres. The list was further narrowed down to 28 properties within an elevation range of 430 to 450 feet above sea level for a ground-level reservoir (similar to the Bolton Reservoir but capped) and 370 feet to 430 feet above sea level for an elevated reservoir (similar to the Rosemont Reservoir). Sites at a lower elevation range between 300 to 370 feet were also considered, although it would require a taller tank of upwards of 150 feet tall – the height of a 12-story building. (Current city code limits commercial building heights, with exceptions, at 50 feet.) Site size requirements, based on topography, ranged from 2 to 3.5 acres.
Based on the conceptual siting criteria, MSA determined 74 of the 90 sites were not feasible. The majority were not within the necessary elevation range, with others encumbered by dedicated drainage basins, city-dedicated open spaces or parks, protected Water Resource Areas or stream corridors, slopes too severe for construction or parcel shape constraints.
The final 16 sites determined appropriate for further evaluation:
- 5464 West A St. (West Linn High School)
- Near 2688 Beacon Hill Drive
- 4096 Cornwall St.
- Near 2300 Hammerle St.
- 3200 S. Haskins Road
- Near 19300 Hidden Springs Road
- Near 19701 Hidden Springs Road
- 20001 Larkspur Land/3600 Mohawk Way
- Near 20130 Larkspur Lane
- 4340 S. Parker Road
- Near 2701 Pimlico Drive
- Near 2600 Pimlico Drive
- 6111 Skyline Drive (existing site)
- 2111 Sunset Ave.
- Wilderness Park, north parking lot
- Wilderness Park, east of Clark Street
The remaining 16 sites were assessed in more detail based on potential reservoir placement, elevation, needed tank type, current use and topography. As a result, four sites were determined most suitable for final siting evaluation: the current location on Skyline Drive, two sites within the Wilderness Park boundary and one on the edge of West Linn High School’s property.
The four sites were evaluated based on:
- Topography, property size and slope
- Need for additional utility easements or temporary construction easements
- Proximity to city limits, service area and water transmission lines
- Existing development and land use of the property
- Geotechnical considerations
- Project cost
The assessment of the final four sites and recommendations were:
Site 1 – Existing Bolton Reservoir site Est. Cost: $9m
This 3.23-acre site is the right size and in the right elevation zone for a reservoir. It can accommodate the new larger reservoir as well as the existing pump station. New piping within the site and along Skyline Circle would be required to connect the replacement reservoir to the distribution system and the pump station. The new reservoir would most likely be moved further back from the slope on the property.
MSA recommends this site as the preferred option based on its close proximity to the existing water system infrastructure, the city’s existing use of the site and that it is a lower-cost option compared to other sites.
Site 2 – Wilderness Park, north parking lot Est. Cost: $12.7M*
This location is generally across Skyline Drive from the existing reservoir, inside the park boundary near the parking lot area. Though the parking lot is flat, it is surrounded by a gentle slope. About 2 to 2.5 acres of the 64.8-acre park would be needed to accommodate construction of a buried reservoir at this site.
The current parking area is 0.5 acres. The area needed to excavate for the buried reservoir is about triple the size of the existing paved area – about 1.5 acres total. Between 50 to 100 trees would be removed during the project. Parking would be disrupted at the park for 12 to 15 months.
A sturdier roof would be needed for the reservoir to allow parking on top, and maintain this area as a parking lot, which increases the construction cost.
A major obstacle for this option is that city parks are Charter protected from “non-authorized use” of park property. A reservoir is not an authorized use.
This site is not preferred because there are no advantages over Site 1, the current reservoir location. The site is in a city park that is Charter-protected from “non-authorized use” of park property (a reservoir is not an approved use) and the higher cost to complete construction.
* Cost includes $1.7 million to relocate the pump station
Site 3 – Wilderness Park, east of Clark Street Est. Cost: $13M
This site is about 1,500 feet from the current reservoir, and the needed 2 to 3 acres of the 64.8-acre park is sloped at approximately 20 percent. The reservoir would partially buried in this forested area.
A driveway/access road would be constructed from Clark Street to the reservoir. To minimize pipe depth, piping would be routed through an easement to the northeast across the park, instead of along Clark Street. Between 100 and 200 trees would be removed during the construction process. Also, the existing pump station would be replaced with a new pump station located at Site 3, so another 1,900 feet of piping would be routed through the park and along Skyline Drive to connect to the new pump station to the Horton Pressure Zone.
A major obstacle in this option is the fact that city parks are charter protected from “nonauthorized use” of park property. A reservoir is not an authorized use.
This option is not preferred because the site is in a city park that is charter-protected from “non-authorized use” of park property (a reservoir is not an approved use), it would have a greater impact on the park compared to Site 2 and the higher cost to complete construction.
Site 4 – West Linn High School Est. Cost: $22M
This site is located at the forested western corner of the high school property. Because of the lower elevation, an elevated reservoir with a height of about 90 feet – about the height of a 7-story building – would be needed. The city would need to use 2 to 3 acres of the school property to place the reservoir here.
Technical and aesthetic concerns about the site are: an elevated reservoir of this size would have less useable storage than a ground-level reservoir; access to the site would be through a developed single-family lot along Windsor Terrace; and a stream area may cause site and access constraints.
Additionally, about 1,500 feet of piping would be needed to connect to Bolton pressure zone piping at Skyline Drive and Webb Street. Also, the existing pump station would be replaced with a new pump station located at Site 4, so another 4,500 feet of piping would be routed through the park and along Skyline Drive to connect the new pump station to the Horton Pressure Zone.
This option was designated as not viable because of technical constraints, reduced useable storage, need to acquire private property for access and the total estimated cost to complete the project at this location.
Recommendation
MSA will present its reservoir siting alternatives analysis to the City Council during its Oct. 20 work session and recommend the city proceed with the geotechnical investigation and site-specific seismic hazard study of preferred Site 1. Work will proceed following this meeting.
The geotechnical investigation and site-specific seismic hazard study will be performed to verify that the site is suitable for new reservoir construction. MSA’s sub-consultant will conduct drilling at the site to sample the soil and rock to determine the foundation conditions at the site. They will also analyze and study the soil and rock to identify any unknown conditions that would make the site unsuitable for the reservoir.
