November 13, 1998

Ms. Judy Cochran, Project Manager
King County, Department of Natural Resources
Denny Way/Lake Union CSO Control Project
821 Second Avenue, MS-130
Seattle, WA 98104-1598

Re: Denny Way/Lake Union CSO Control Facilities Plan
Phase 2 and 3/4

Dear Ms. Cochran:

The facilities plan, dated July 1998, for the above referenced project has been reviewed. The selected, cost-effective alternative is alternative No. 3, described in Chapter 10.3 of the facilities plan. Alternative 3 is summarized in the Executive Summary of the facilities plan, and in general consists of:

• Connection with Seattle's improved conveyance facilities on the south and east sides of Lake Union. Facilities include a structure located at Valley Street and Fairview Avenue North that will divert high flows to the CSO tunnel, and a pipeline along Valley and Broad Streets to the east tunnel portal.
• South Lake Union Conveyance Facilities. Two diversion/regulator structures and three 54- to 84-inch diameter pipelines to convey flows to a new Mercer Street Tunnel.
• Denny Area Conveyance Facilities. Two diversion/control structures and three pipelines to
  convey flows to the new tunnel.
• Mercer Street Tunnel, a 6,200-foot-long, 14- to 16-foot-inside-diameter storage tunnel extending eastward beneath Mercer Street from Elliott Avenue West to a point near the intersection of Roy Street and Eighth Avenue North. The tunnel storage capacity of the tunnel would be approximately 7.1 million gallons at the lower diameter, 9.3 MG at the upper
  diameter.
• Elliott West CSO Control Facility. A 250-million-gallon-per-day pump station and CSO treatment system, located at the west portal of the tunnel, providing removal offloatable objects, chlorine disinfection, and dechlorination.
• A new 96- to 108-inch-diameter outfall to discharge disinfected CSO flows from the Elliott West CSO Control Facility into Elliott Bay.
• Extension of the existing Denny Way CSO outfall with a 96- to 120-inch-diameter outfall to discharge untreated and non-disinfected CSO flows into Elliott Bay.

In accordance with RCW 90.48.110 and Chapter 173-240 WAC, and Title 40, Code of Federal Regulations (40 CFR), and on behalf of the Department of Ecology, the subject document is hereby approved as a FACILITIES PLAN with the following conditions:

• NPDES permit assumptions for TSS removal, suspended solids monitoring, and settleable solids cited in the Facilities Plan are not to be construed as being approved on the basis of their appearance in the approved Facilities Plan document. They are accepted as professional judgement estimates for the purpose of demonstrating that permit limits are obtainable, and will be required to be confirmed by testing and monitoring protocols established by Ecology in the NPDES permit for West Point or in a separate permit. Permit limits are a function of regulations and policy, and Ecology will write the permit conditions based on regulation, policy, and previously negotiated terms. At this time, it is recommended that compliance with this limit be evaluated on a one to five year cycle. Nothing in this approval shall be interpreted as precluding additional monitoring or testing requirements.
• As discussed in previous correspondence, the diffusers and mixing zones for the Denny Way CSO outfall and Denny Way Regulator outfall will need to be reviewed and approved in their entirety once the design is reviewed. Nothing in the Facilities Plan approval shall preclude requirements of the final mixing zone and outfall analysis as determined in the review of the design.
• Planning data stated in Chapter 10 of the Facilities Plan will be the operative data in cases of conflict with data presented in other chapters. In other instances where the data are inconsistent, or in cases where the data are ambiguous, Ecology will determine which data are operative.

One copy of the approved document is enclosed for your project records.

Please provide prompt notification to the Department of any proposed changes or revisions to the approved document. Any such changes or revisions need to be issued in the form of addenda, technical appendices, or supplemental reports to the original, approved documents. As a reminder, revisions must be approved in writing by the Department before submitting plans and specifications for the proposed facilities to the Department for review and approval.

The construction cost eligibility associated with this special appropriations grant has not been determined at this time. The EPA grant administrator will determine and communicate the eligibility to you under separate cover.

Please note that the Department of Ecology's review and approval authority is limited to assuring compliance and consistency with the appropriate rules, regulations, guidelines, planning and design criteria, terms of a grant or loan agreement, and/or other similar documents. You, as owner, and your engineer retain full responsibility for the technical completeness, accuracy and adequacy of this document, for obtaining other required permits and approvals, and for compliance with environmental laws and regulations during the design, construction and operation of the proposed facilities.

If you have any questions or need any additional information, please contact Kenneth D. Ziebart, Project Manager; at (425) 649-7164 or me at (425) 649-7033.

Sincerely,

JHG:KDZ:ct

Encl: Approved Denny Way/Lake Union CSO Control Facilities Plan-Phase 2 and 3/4 dated July 1998

cc: Brian Yim, EPA-Region 10
Laura Fricke, Ecology-WQ/NWRO
Kenneth D. Ziebart, Ecology-WQ/NWRO
Shara Stelling, Ecology-WQ/FMS