Boulevard Park Flood Reduction Project

The Boulevard Park drainage basin has an area of 112 acres and is part of the Miller Creek basin which drains into Puget Sound. The basin begins in the City of Burien with the stormwater generally flowing towards the southeast. Water flows through the back yards of private residences in the Neighborhood before crossing Burien’s City Limits and entering the City of SeaTac at North SeaTac Park located on Des Moines Memorial Drive. The water then flows toward the south before discharging into an open channel stream and forested wetland located south of S 136th Street and west of Sunset Playfields in the City of SeaTac. Water from the stream leaves the wetland through a stream channel to Tub Lake, and Tub Lake outlets to Miller Creek.

Mild flooding had been observed for many years in the Neighborhood, but severe flooding was observed in the recent past. Flooding complaints from citizens that have been collected by the City over the years are depicted on the map as yellow stars. There are two changes of significance that have occurred to stormwater conveyance in the Neighborhood over the last few years:

 2016 Imagery  (Attachment #2)

 2021 Imagery  (Attachment #3)

Because there are sensitive environmental areas such as streams, wetlands, and their buffers that could be affected by any proposed flood reduction improvements, a preliminary assessment was done to characterize the environmental conditions in the study area and downstream to Tub Lake. In the environmental assessment the unnamed water course upstream of S 136th Street was determined to be non-fish bearing, but confirmation by Washington Department of Fish and Wildlife will be required. Three potential wetland areas were also identified in the assessment. The assessment is linked below.

 Environmental Assessment Memorandum  (Attachment #4)

To quantify the frequency, extent, and duration of flooding, a preliminary hydrologic and hydraulic (H&H) model was set up. For this project’s hydrologic analysis, the Boulevard Park basin was divided into 4 subbasins. Subbasins 1, 2, and 3 are entirely within Burien, but subbasin 4 has area within both Burien and SeaTac. The details associated with the model setup can be found in a memorandum attached at the end of this report. The results from the model are summarized in this section.

The H&H model extends between 12th Avenue S and Des Moines Memorial Drive and includes major pipes, open channels, and nodes that represent the stormwater infrastructure in the study area. Links in the model represent pipes and open channels. Open channel links can flood where an open channel floods over its banks, as is the case for issue ID 4. Nodes represent catch basins, transitions between open channels and pipes, or flood storage, such as a floodplain that holds water for some time before discharging into the conveyance system. Flooding associated with a catch basin node in the model represents flooding in the vicinity of that catch basin based on topography. For example, flooding at catch basin 50305 (north of S 124th Street) in the model means that all the areas around the catch basin with an elevation equal to or lower than the rim of the catch basin would have ponded water. In this example, those properties are 12446 12th Avenue S, 1230, 1242, 1254 and 1241 S 124th Street - associated with issue IDs 1, 2 and 3. Water surface elevation in other types of nodes, such as transitions and storage nodes, is used to estimate flooding depth, extent, and duration.

Overall, the existing conditions model shows that areas of frequent flooding correspond closely with areas of low elevation in the basin – the same as those where flooding was reported or observed. The modeled flooding occurs as frequently as once every two years at some of the locations and less frequently (once in ten years and once in twenty-five years) in other locations. Although we know that flooding occurs more frequently, the model was set up to simulate a storm event that occurs with a probability of once in two years. In addition to frequency of flooding, the depth of flooding and hours flooded were also analyzed. Depth and hours of flooding are tabulated in the Hydrologic and Hydraulic Model Results section. Extents of flooding can be seen as polygons on the map. Note these polygons are based on rough topography and are approximated based on the results of the preliminary H&H model.

The open channel that was recently converted to an undersized 8-inch PVC pipe was found to be the primary cause for flooding in issue IDs 1, 2 and 3. The ground elevations of the properties associated with issue IDs 1, 2, and 3 are lower than the inlet invert of the 8-inch pipe. The ground elevations below the pipe invert elevation frequently have ponded water to a depth of over a foot and water stays for over 2 days before it drains out. Although residents have voiced a concern that modifications at house addresses of 1230, 1242, and 1254 124th Street have caused flooding, modeling shows that the reduced stream buffer area on these properties has had a relatively insignificant impact to the flooding compared to the impact of the 8-inch pipe discussed above.

We estimate that issue IDs 4 through 8 used to flood more often and to a larger extent than they have in the recent years, although we don’t have time stamps for when those were reported, or pictures associated with flooding at these locations. The installation of the undersized 8-inch PVC pipe resulted in less volume of water being conveyed into the downstream properties during a storm event which in turn reduced the flooding. However, if the undersized pipe is replaced with a larger pipe, the downstream properties may flood again unless the conveyance improvements are detained or carried further downstream into a large wetland in the City of SeaTac. With the 8-inch pipes, the H&H model shows only a few inches of flooding in the properties associated with issue IDs 4 through 8 and further downstream. Like the 8” pipe there have been other alterations that have had adverse impacts. Examples of alterations observed or discussed include landscaping that incorporates weirs, blocking of pipe ends with boards or other material, replacing open channels with undersized pipes, partially filling open channels with rock and/or other material, and covering pipe ends with dirt/sediment or other debris.

Click on any complaint (yellow star) in the map to find a description of the complaint, probable cause of flooding, and photographs, if available. Click on any node or link (orange star) to find the frequency at which it floods in the H&H model.

Click on the links below to see hydraulic profiles of the stormwater infrastructure, flooding durations, and depth of flooding for storm events of different frequencies.

 2-year storm event  (Attachment #5)

 10-year storm event  (Attachment #6)

 25-year storm event  (Attachment #7)

 Hours of flooding  (Attachment #8)

 Depth of flooding  (Attachment #9)

16 alternatives were analyzed – some of which were modeled and some not. These included considerations for an interim solution as well as comprehensive solutions. All interim solutions were found not to be viable because they push flooding further downstream and there isn’t enough area available to implement a solution that would significantly reduce existing flooding.

The major project constraints used to weigh the alternatives include:

Three major types of solutions were considered: conveyance only, conveyance coupled with detention, and direct discharge to the downstream wetland. Conveyance exclusively within the Neighborhood only will not work because it would push the flooding to downstream properties. Detention can work but the detention volume required is more than 9 acre-ft and it is difficult to find a site for this detention volume. Conveyance improvements all the way to the stream and wetland south of S 136th Street will solve the flooding problems, not create new flooding problems, will require minimal detention (if any), and can meet Burien and King County stormwater design manual criteria for allowable changes of flows to the stream and wetland.

Click the link below to see alternatives considered:

 Flood Reduction Alternatives Considered  (Attachment #10)

Of the 16 alternatives, two viable solutions – Alternative 9 and Alternative 13, were analyzed further to address flooding. Both alternatives will require the cooperation of up to 40 property owners within Burien by way of permanent easements. As these solutions extend into SeaTac and onto Port-owned property, cooperation from the City of SeaTac and Port of Seattle would also be needed.

Both alternatives consist of surface water conveyance main line improvements throughout the Neighborhood, from S 124th Street to Des Moines Memorial Drive, through a new 24-inch conveyance system. For Alternative 9, a detention facility is proposed downstream of the conveyance improvements to provide flow control before eventual discharge into the wetland and stream channel downstream of S 136th Street. There would be no downstream impacts or mitigation requirements for Alternative 9 because the detention facility would release flows to match the existing flow rates and durations. On the other hand, for Alternative 13, the conveyance improvements, piped or open channel, would continue downstream of Des Moines Memorial Drive to discharge into the wetland and stream channel downstream of S 136th Street. Stream and wetland enhancements would be used to mitigate the increased flow rate to the S 136th Street outfall, instead of relying on the large detention facility in Alternative 9. A reduced amount of detention is included in this alternative in case the environmental impacts cannot be avoided or mitigated. The detention would be much smaller and located in the North SeaTac Park near the intersection of Des Moines Memorial Drive and S 136th Street.

Note that the solutions discussed in this report will only address the flooding. The City is in talks with Valley View Sewer District about a potential project that would connect the properties in the Neighborhood into sanitary sewer conveyance infrastructure. This project that would address the septic backup issues that exist.

Click on the link below to see hydraulic profiles of the proposed stormwater infrastructure (Alternative 9 or 13) for a storm event with a probability of occurring once in 25 years.

 25-year storm event with proposed improvements  (Attachment #11)

Costs were developed for both Alternatives 9 and 13. Alternative 13, the direct discharge option, is the recommended alternative because of lower cost and opportunity for environmental enhancements.

The conceptual alignment for stormwater conveyance for Alternative 13 is depicted as a blue dashed line. This option will route flows from the project site under Des Moines Memorial Drive into a new 24-inch conveyance system that will convey the flows under South 136th Street. The flows will then be conveyed into a large wetland downstream of South 136th Street. There are two criteria that govern direct discharge into a wetland. These criteria are:

Criteria 1. Mean Daily Total Discharge Volumes from the Site

Total volume of water into a wetland on daily basis should not be more than 20% higher or lower than the pre-project volumes.

Criteria 2. Mean Monthly Total Discharge Volumes from the Site

Total volume of water into a wetland on a monthly basis should not be more than 15% higher or lower than the pre-project volumes.

Based on a preliminary analysis, both criteria can be met because both the existing conditions and proposed conditions use the same land use. There will be no significant increase in total runoff. However, the timing of the runoff will be altered slightly. Some flood flows will reach the wetland more rapidly in the proposed conditions than under the existing conditions. It is likely that this increase in timing of flows reaching the wetland will be on the order of a few hours to maybe a day or two. The wetland criteria addresses daily and monthly water volumes which are not likely to change more than a few percent.

Due to potential short-term increases in peak flows and peak flow velocities, a detailed analysis of the wetland impacts and mitigation measures downstream of South 136th Street would be required in the design phase of these improvements. Methods to mitigate the impacts can include an energy dissipator at the S 136th Street outfall, channel restoration with added woody material downstream of the outfall, or a small upstream facility designed to reduce flow velocities. Potential environmental enhancements and mitigation opportunities with this solution include daylighting the existing piped system downstream of Des Moines Memorial Drive to S 136th Street.

Note this solution is only conceptual. Elements may be added or removed based on stakeholder feedback, permitting considerations, and funding availability.

Click on the link below to see preliminary costs for Alternatives 9 and 13. The City should consider partnering with Valley View Sewer District to design and construct joint improvements that would address both storm and sanitary sewer issues. The partnership may open doors to additional grant funding opportunities.

 Cost Estimate  (Attachment #11)

Next Steps:

Determine if Council wants to take the recommended solution on as a public project. Or whether this is a private project.

on as a public project. Or whether this is a private project.

If it’s a public project,

Project Timeline: