Sammamish Plateau Water and Sewer District

West Beaver Lake Sewer Subbasin Sewer Alternatives

Project Update December 22, 2004

The design of the Gravity Sewer Alternative was completed in Summer 2004 and submitted to the City of Sammamish for permitting review and acquisition of the easements. Discussions between the District and the City for acquisition of the necessary easements are continuing.

The Sammamish Plateau Water and Sewer District is the agency responsible for planning how sewer service will be provided in the Sammamish Plateau’s urban area. As part of the development of the District’s Wastewater Comprehensive Plan, conceptual plans for providing sewer service throughout the urban area have been developed, including the West Beaver Lake Sewer Subbasin. The West Beaver Lake Sewer Subbasin, which is part of the larger North Sunny Hills Basin, is the area located west of Beaver Lake, from West Beaver Lake Drive westward to 248th Avenue SE, and from SE 12th Street (if extended) to Beaver Lake Park.

The conceptual sewer plan includes the future construction of gravity sewer lines along West Beaver Lake Drive and its side streets, and northward from the development of Wesley Park (aka Norris Estates) to SE 13th Street, as shown on the attached North Sunny Hills Basin Plan. These gravity sewer lines would flow toward SE 24th Street. From that point, the sewage would be conveyed to the District’s existing gravity sewer system located on SE 24th Street between 248th and 244th Avenue SE.

Two alternatives have been identified for conveying the sewage from the West Beaver Lake subbasin to the existing sewer system: 1) a gravity sewer through the north portion of Beaver Lake Park, or 2) a sewage lift station located near the intersection of SE 24th Street and West Beaver Lake Drive, and a forcemain from the lift station to the existing sewer system. The two alternatives are described below.

The Gravity Service Alternative (Gravity Service Alternative plan) would be constructed in Beaver Lake Park, between the entrance to the Beaver Lake Park Maintenance Facility and 244th Avenue SE (Beaver Lake Park Exhibit). This alternative can be described in three sections:

Trail Section: The sewer would follow the existing trail between the Maintenance Facility and the BPA (power line) easement. The sewer in this section would be shallow (8-10 feet deep) and would be constructed by open cut (digging a trench, installing the sewer, and backfilling). (Sewer Plan SSMH6 TO SSMH14 and Option E Sewer Plan and Profile). Following completion of construction, a 12 foot wide gravel trail would be constructed over the portion of the sewer line between the Maintenance Facility and the BPA easement. This trail would be used for hiking, biking, and equestrian uses; by the City of Sammamish for park maintenance vehicles; and by the District for maintenance of the sewer. The area along the trail section would be revegetated with native plants and trees (Conceptual Section).

BPA Easement Section: The sewer crossing the BPA easement and the area west of the BPA easement until reaching SE 24th Street would be installed by boring underground to the northwest. This construction would not disturb the ground surface of the BPA easement or the densely-forested area west of the BPA easement (Option E Sewer Plan and Profile).

SE 24th Section: From the northwestern end the BPA Easement section, the sewer line would be constructed along the south side of SE 24th to an existing sewer manhole just east of 244th Avenue SE. This section would be installed using open cut construction. (Option E Sewer Plan and Profile).

The Lift Station Alternative (Lift Station Alternative plan) can also be described in three sections:

Gravity Connection Section: A gravity sewer would be constructed to connect to the existing sewer collection system from Wesely Park, located at 250th Avenue SE and SE 24th Street, eastward along SE 24th Street to the lift station location.

Lift Station Section: A sewage lift station would be constructed at a location near the intersection of SE 24th Street and West Beaver Lake Drive. A site for the lift station would need to be identified and acquired. The lift station would include below-ground facilities for collecting the sewage and housing the pumps, and an aboveground building for electrical equipment, odor control facilities, and a backup generator.

Forcemain Section: The lift station would pump the sewage through a pressure forcemain from the lift station to an existing sewer manhole on SE 24th Street near 248th Avenue SE. The forcemain would be primarily located in SE 24th Street.

Either the Gravity Alternative or the Lift Station Alternative would provide permanent sewer service to the Wesely Park development and would replace the temporary lift station that is currently providing sewer service to the development. Sewer service to some of the facilities in Beaver Lake Park may be provided as part of the selected alternative, although future sewer extensions may be required to extend sewer to other park facilities. For the remainder of the West Beaver Lake Sewer Subbasin, future sewer facilities (shown as future sewer extensions on the attached maps) would be constructed as the property owners in the subbasin desire sewer service, most likely as one or more sewer utility local improvement districts.

A gravity sewage collection system is preferred by the District over pumped sewage systems because of the gravity system’s lower capital and maintenance costs, no electrical requirements for operation, less visibility (underground pipes versus aboveground lift station building), and less potential for generating odors.