Phased Landsburg Debris Passage Design Services

The City of Seattle is seeking proposals for Phased Landsburg Debris Passage Design Services . Proposals are due no later than 1:00 pm on Monday, April 21, 2025 . All questions are to be submitted through the e-procurement portal at https://procurement.opengov.com/portal/seattle no later than 1:00 pm on Monday, April 14, 2025 .

The City of Seattle Public Utilities (SPU) is seeking a consultant team to provide options analysis, design, permitting, community outreach, and construction support services to address debris passage at the Landsburg Diversion Dam. SPU intends to execute one (1) Phased contract with an estimated value of $6,500,000. Each Phase will be issued by way of Work Assignments. The Options Analysis Phase will be issued at an initial estimated fee of $900,000. SPU does not guarantee any minimum quantity of work under this contract.

Landsburg Diversion Dam is a run-of-river type dam on the Cedar River in eastern King County and is where Seattle Public Utilities diverts approximately 2/3 of the drinking water for its 1.6 million customers. The dam, as built in 1935, included five Tainter gates and diverts water via headworks on the north side of the river. That project replaced a previous weir-type dam that diverted water into two Cedar stave pipes through an abutment on the south side of the river that is still present, though now buried. A project to make the dam passable for migratory fish added a fish ladder on the north side of the dam and replaced one of the Tainter gates with a tip gate in 2004. The dam now serves as a fish passage facility and has served to allow Chinook and Coho salmon returning up the Cedar River to swim up a fish ladder to pass the dam site since modifications to the dam were completed in 2002.

Events in the past two decades demonstrated that the dam is potentially vulnerable to damage from floating large woody debris (LWD) during infrequent high flow events. Every few years, the Cedar River carries enough woody debris from fallen trees that it can stack up against the dam and cause the water level to range dangerously high. During one such event in 1990, the water rose high enough to flow up and over the north side of the dam causing damage to the facility.

SPU is interested in a project to modify the site to allow LWD to safely pass the dam. SPU has historically relied on operators and heavy equipment to manage debris during these events. These methods have allowed SPU to avoid another overtopping situation thus far, but the methods carry high levels of safety risk to personnel and this methodology may not be sufficient to address major debris loading events in the future. Additionally, it is predicted that climate change will increase storm intensities and cause both the quantity and frequency at which large trees fall into the river and reach the dam as floating debris. None of these were considerations when the dam was built in 1934. In other words, SPU is at risk of its own staff being injured or worse and, should the dam fail, losing the dam as the primary source of drinking water for the region.

The following timeline and relevant history of work performed on this issue is provided for additional context:

• 1990 - Debris accumulated during a high flow event. This caused the river to overtop the north abutment and erode the downstream face of the embankment.
• 1997 – SPU completed Landsburg Master Plan (Raytheon 1998). The master plan included prioritization of projects to address debris loading during high flows and fish passage through the dam.
• 1999 – The National Marine Fisheries Services listed Puget Sound Chinook as an endangered species, and it was recognized that Landsburg Dam was a total barrier to fish passage.
• 2000 - The City of Seattle committed to providing passage for fish at Landsburg by 2003 and then to construct a hatchery to mitigate for impacts on Cedar River Sockeye. One implication of this commitment was SPU de-prioritized addressing debris passage until after addressing fish passage.
• 2003 - SPU completed the fish passage project.
• 2005 – SPU completed an options analysis for debris passage at Landsburg (MWH 2005) and selected merging two of the Tainter gates into one single Tainter gate as the preferred option.
• 2007 – SPU stopped design of this preferred option at 90% due to a debris study completed as part of the design (Herrera 2007) that indicated debris would potentially include pieces too large to fit through the proposed double-width Tainter gate. SPU also postponed further effort until 2010 when more budget was expected to become available. (SPU 2007)
• 2010 - SPU completed a new options analysis that recommended a debris boom as the preferred option (URS 2010).
• 2012 - SPU chose to pursue an improved operational approach (i.e., purchased new, larger log loaders and coordinated around-the-clock response lists to activate during high flow events) with an expectation that a future effort would evaluate the effectiveness of this approach.
• 2015 – SPU managed multiple high flow events without incident.
• 2020 – A high flow event included periods when SPU staff were unable to move debris through the dam and the water reached high enough levels to be of substantial concern.
• Present – SPU approves process to begin a renewed options analysis.

Figure 1. Project Location

Figure 2. Cedar River and Landsburg Dam

The eventual design will also include repairing damage to the Tainter Gates, including straightening bent supporting structural pieces and replacing the rubber seals at the lower edges of each of the gates.