NE 93rd Street Culvert Repair Project
Seattle Public Utilities (SPU) will repair the culvert that carries Thornton Creek under NE 93rd Street to reduce the risk of sudden road failure during heavy rains. The culvert is at risk of failing, which would isolate several homes from access to Sand Point Way NE.
The NE 93rd Street culvert carries the lower portion of Thornton Creek into the Matthews Beach Park area, where it flows into Lake Washington and its confluence with the Maple Creek tributary. This is an area formerly occupied by the Lake Washington shoreline prior to the nine-foot lowering of the lake with the opening of the Chittenden Locks. The native soils beneath the culvert are soft and compressible. Over the past century, the culvert site has experienced considerable surface modification as well.
Maple Creek has been channelized through residential yard areas to the extent that the natural channel no longer exists. Thornton Creek here also has been artificially channelized with concrete and rock walls. These changes have altered the creek’s capacity to transport sediments. Flooding of these creeks is directly related to sedimentation of the channel where the gradient flattens appreciably. As the upstream area of the watershed became more developed, creek flows began to exceed the capacity of the culvert and downstream channel, resulting in flooding during heavy storm events.
Construction is expected to begin in 2014, and could take up to three months to complete.
The NE 93rd Street culvert is located about 230 feet east of the intersection of Sand Point Way NE & NE 93rd St., near the entry to Matthews Beach Park, approximately 1000 feet from where the creek flows into Lake Washington. The deteriorating condition of the culvert has reached a point where the risk of sudden road failure makes this a high-priority project for SPU.
SPU has examined different approaches and methods for stabilizing the existing culvert. The plan is to lengthen the life of the structure until a longer-term solution can be developed. The preferred option, which will be taken through design and construction, is to utilize steel beams (H-piles) to support the side of the culvert, which is experiencing displacement due to undermining of the culvert footing.
The preferred option meets the following criteria:
- Limit additional settlement and cracking of the culvert
- Limit additional rotation of the culvert walls
- Provide some support of the culvert if additional channel erosion or undermining of the footing occurs
- Minimal maintenance and debris removal requirements throughout its life cycle.
- Repair the culvert as soon as possible to lessen the chance of a catastrophic failure (note: this means that in-water work is not feasible due to the time it would take to acquire necessary permits).
The long-term vision is to replace the culvert with a larger one and make changes to how Thornton Creek flows. Such a solution would remove a bottleneck in the creek during storms, while protecting downstream properties from the risk of increased flooding. Those plans have to involve the community, Seattle Department of Transportation (SDOT), the Seattle Parks Department, the U.S. Army Corps of Engineers and others.
April 24, 2014 Update
SPU today issued a SEPA Addendum to remove a large big-leaf maple tree (Acer macrophyllum) prior to beginning the project’s excavation. The tree is located in street right-of-way owned and managed by the Seattle Department of Transportation (SDOT). SDOT’s certified arborist has closely examined the condition of the tree and reevaluated the potential impacts of project construction, consistent with the risk assessment requirements of DPD Director’s Rule 16-2008. The SDOT arborist observed the tree has significant decay in portions of the trunk and limbs and is leaning toward private property, and disturbance of the root zone would likely endanger the tree’s stability and health.
As mitigation, SPU shall plant two new trees in the right-of-way at the end of construction. SPU and SDOT are currently negotiating the species, size and placement of those replacement trees.
As lead agency, SPU has reviewed the findings and concluded the project change does not substantially alter the analyses of impacts contained in the March 2013 Environmental Checklist and will not result in any significant environmental impacts. This addendum has been prepared in accordance with the authority provided in SMC 25.05.600 and in accordance with the procedures described in SMC 25.05.625.
November 7, 2013 Update
SEPA Appeal Dismissed
The appeal by an adjacent property owner to the Hearing Examiner’s affirmation of SPU’s SEPA determination was dismissed by the Superior Court on October 11th, without going to trial. The construction is tentatively scheduled to start in Summer 2014.
July 12, 2013 Update
SEPA Appeal #2 – Project Will Be Delayed
The Hearing Examiner’s decision is being appealed to the King County Superior Court. The hearing date for this appeal has not been set, but the anticipated delay is requiring SPU to postpone the project until at least the Summer of 2014.
When more information on timelines is released, we will post the dates here.
SPU will continue to monitor the culvert’s condition, per usual protocols. In the case of a culvert and/or road collapse, the City will be ready to respond to the emergency to limit damage to surrounding homes and utilities. If a failure occurs, access will likely be limited to foot access for residents and park users.
June 21, 2013 Update
There was an appeal to the SEPA checklist we released on March 29th. The appeal was heard by the Hearing Examiner on June 3rd and dismissed; however, it did impact our schedule and budget.
Thornton Creek Study Results Released
Seattle Public Utilities has released the results of a two-year Microbial Trace Study of Thornton Creek (pdf) including this portion of the creek at Matthews Beach.
April 4, 2013 Update
March 28, 2013 Update
The culvert repair project is now in the design and permitting stage. The design has been refined to the point that the team is able to apply for its permits.
The State Environmental Policy Act (SEPA) checklist is expected to be published on April 4th. A link to it will be uploaded to this web site and there will be opportunities to formally comment on the project through April 18th.
View the Survey Map (pdf) used by the engineering team to start the design.
The Geotechnical Memo (pdf) presents the results of subsurface investigations, geotechnical design parameters and project recommendations.
The Design (pdf), currently at about 50%, will be taken to a more detailed level over the next few months.
For more information, or if you would like to be added to our distribution list, please contact Holly McCracken at email@example.com or (206) 386-4195.