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.
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 in 2013 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 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.
Construction may occur during Late-Summer/Early-Fall 2013.
A public meeting will be held in June (well before construction begins) to talk about potential temporary construction impacts.
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.