The Lyon Creek Flood Mitigation Project included flood reduction, environmental restoration, and transportation enhancements. While the overall goal of the project was to protect more than 20 homes, a fire station, state highway and commercial center from frequent flooding, the project was a great opportunity to restore this reach of Lyon Creek. The project began in March 2013, was completed in December 2015, and benefits the community and environment in numerous ways:
Take a look at the project's Floodplain Map and Project/Culvert Map. The Floodplain Map will show you the 100-year floodplain pre-project and post-project. The Project/Culvert Map will show the map of the whole project and the new culvert capacity compared to the old culverts.
Check out the City's Flickr account for photos of the project.
Click here to download the Final Lyon Creek Flood Mitigation Project Plans
The most significant flood event in the City’s history occurred on December 3, 2007. The damage estimate for the single event was more than $4 million. Over 20 homes, the City’s only fire station, SR 522 and the City’s largest commercial center, Lake Forest Park Town Center, flooded and many of the structures were severely damaged. The 2007 flood event was the third in less than 25 years with similar events in 1986 and 1997. The 2007 event prompted the City to initiate a flood reduction planning study to determine the cause and explore a variety of solutions to the flooding problem. The study determined that the cause of the flooding is inadequate conveyance capacity in the lower Lyon Creek systemcausing about 127 cubic feet per second (cfs) of the 212 cfs currently overflows to McAleer Creek. This overflow results in flooding on both Lyon Creek and McAleer Creek and the areas between the creeks.
The solution was simply to keep Lyon Creek flows in Lyon Creek by increasing its conveyance capacity. This project became the highest priority project for the City but it became clear that it was no easy task as it would require coordination with multiple agencies and property owners:
The complexities of the project including the need to complete the project in one summer season, involvement of multiple property owners, utilities, public agencies, traffic and business impacts, environmental considerations it was critical that the project be thoughtfully designed and constructed.
The City has partnered with all levels of government to fulfill the funding requirements for this project.
King County Flood Control District
The length of the project and various project elements required that construction occur in six phases. Much of the project would also require in-stream work that permitting agencies limited to July 1 – September 15. The project team also required that the project be completed in one construction season to provide flood protection for the 2015-16 wet season. WSDOT would allow SR 522 to be closed for one weekend as long as it did not conflict with Seafair or SR 520 closures.
Engineering design of the culverts focused on maximizing constructability and cost-effectiveness without sacrificing functionality. This focus is exemplified by the culvert design. Culverts were designed as a three-sided base with a lid, which allowed for faster installation of streambed materials within the culvert, and therefore reduced closure durations in SR 522 and within the Town Center. The culvert design was subject to several constraints; overall culvert depth was limited by the existing stream profile and surrounding pavement grades, and this depth had to include both a lid designed to carry heavy traffic loads and a culvert cross sectional area of sufficient size to convey the design stream flow.
Culverts and bridge crossings were designed to meet Washington Department of Fish and Wildlife’s water crossing design guidelines, including stream simulation design criteria. Culvert widths were determined based on Lyon Creek’s bankful width, or active channel width, and are wide enough to allow for natural stream meandering and channel formation within each structure.
Where possible, the project reused existing aggregates including streambed substrates, pavement subgrade materials, and rock for retaining walls. These reuse efforts demonstrated a commitment to sustainability by reducing the volume of imported and exported materials, and minimizing emissions resulting from the transportation of materials.
The project also successfully overcame adverse conditions, which could have otherwise negatively impacted the construction schedule. Unsuitable subgrade conditions were encountered at all four culverts and required quick decisions from the project’s geotechnical engineer to amend onsite subgrade with quarry spalls. Many of these decisions had to be made in the middle of the night so the project would not be delayed.
Phase I - Lyon Creek Waterfront Preserve Improvements (June – July 2015): In an effort to begin in-stream work on July 1, 2015, KLB Construction began upland floodplain grading near the mouth of Lyon Creek on June 15, 2015 and mobilized much of the equipment necessary to construct all phases of the project. On July 1, the lower reaches of Lyon Creek were de-fished by electrofishing and bypassed around the work area using a pump at the upstream end of the work zone to move water around the construction area in pipes back to Lyon Creek downstream of the work zone. This dewatering method was used for all phases of the project. Once grading was complete in Lyon Creek Waterfront Preserve the private vehicle bridge immediately upstream of the Preserve was demolished and replaced in 10 working days. The abbreviated bridge replacement schedule reduced the impact to private property access and use.
Phase II – Lake Forest Park Civic Club Bridge Replacement (July 2015): Once grading was complete in Lyon Creek Waterfront Preserve the private vehicle bridge immediately upstream of the Preserve was demolished and replaced in 10 working days to allow use of the property during the prime summer boating season. The aging bridge had been inspected by an King County engineer as part of design of the project and the engineer quickly determined the bridge could not accommodate additional flows and was, in fact, not structurally sound. Demolition was a very simple as the bridge collapsed with a tap of the excavator bucket. The new vehicle bridge at Lake Forest Park Civic Club, threatened by new higher storm flows, was designed to use prefabricated abutments and bridge decking in order to expedite construction and minimize impacts to users. The Civic Club contributed a substantial amount of the cost to replace the bridge.
Phase III – SR 522 Culvert Replacement, Culvert L10 (July 17-19, 2015): The project team’s initial plan was to construct the SR 522 culvert over a period of weeks, moving lane closures with extensive traffic control measures. WSDOT suggested closure of SR 522 for a single weekend as an alternative to reduce costs and construction risks. The project team jumped at the opportunity but scheduling the work was difficult. The SR 522 closure could not conflict with other planned SR 520 closures and could not occur if there was a significant rain event that would overwhelm the stream bypass pumps. The project team and contractor selected July 17-19 as the weekend to perform the work. Lane closures began the evening of Friday, July 17, and by 9:00PM, the highway was fully closed and a detour route through local streets was implemented. The culvert replacement work, which proceeded on a 24-hour work schedule, included temporary stream bypass, temporary de-energizing of high-voltage overhead power lines, demolition and excavation, subgrade preparation, precast concrete culvert installation, stream channel grading, backfill, paving, and channelization. All work was completed in time to re-open the busy highway to traffic by 5:00AM on Monday, July 20. It was an impressive coordination effort between the City of Lake Forest Park, WSDOT, Seattle City Light and CenturyLink, and adjacent property owners including the Lake Forest Park Town Center.
Phase IV – First Town Center Culvert, Culvert L20 (July 13-25): Construction proceeded immediately upstream into the Lake Forest Park Town Center with replacement of culvert L20. The culvert design and installation method was the same for all of the culverts allowing the contractor to gain efficiencies as construction proceeded. The ~40 feet length of channel between culvert L10 and L20 was also regraded, new retaining walls constructed, large woody debris was installed, in-stream refuge pools were excavated and a variety of native plantings to shade the creek were planted. This element of the project required closure of one of the primary entrances to the Lake Forest Park Town Center. Traffic was rerouted around the project area and adaptive management was employed to improve traffic circulation. Construction on the Town Center property was carefully phased to ensure that no more than one entrance to the commercial property was closed at any time during the project.
Phase V – Culvert L30 (July 23 – August 17): This phase included regrading 300 feet of open stream channel, installation of 11 pieces of woody debris, replacement of two long retaining walls, excavation of 7 habitat pools and replacement of the 150 feet of culvert between commercial buildings. The work was primarily on private commercial property requiring the contractor to work in the barely adequate easement areas. Much of the culvert replacement work occurred at night to mitigate impacts to businesses and SR 522 traffic. The combined stream enhancement for this phase was 450 feet of stream channel, the longest of the project.
Phase VI – Culvert L40 (August 15 – September 15): This phase included construction of the final culvert, stream channel regrading, trail construction connecting the Town Center to Whispering Willow Park on top of a flood berm, restoration of wetland in the same park and construction of a stream channel meander area in the park were constructed. During this phase two flash floods forced the contractor to stop work for days at a time. The project team could not identify the source of the increased flows as it wasn’t raining and upstream discharges could not be located. Despite the flash floods the contractor completed in-stream work by September 15 as required by the permitting agencies.
Additional Project Elements: The Lake Forest Park Town Center ownership group requested that the contractor make changes and additions to the property. These additions extended the project by 25 working days and was funded by the property owner. The City also opted to add a sidewalk along SR 522 connecting two bus stops on Ballinger Way NE (SR 104) to bus stops and businesses on SR 522. The sidewalk also increased channel capacity, further protecting SR 522 from flooding, and overlooks of the open channel sections of the project.