Update on Edmonds
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Winter is finally here, and it started with a good one! This December, the City experienced, flooding at the intersection of Dayton and SR-104 for the first time in several years. It’s not an unknown condition by any means, but it requires a relatively specific set of circumstances in order to create the conditions. However, with any luck, this was the last time we should see this scene.
Dayton1

The cause of the flooding is a result of several things which go wrong at the same time, most of which come back to a battle against the tides. The City’s storm drain system outfalls to the Puget Sound directly, so when the tide level rises up it can actually raise above our storm drain system. The City has one-way valves, gates, or flaps on most of the outfalls so that the tide is held out when it comes up and does not rush back into the storm drain system. However, the rainfall and surface water cannot drain out of the pipes when the tide is above the outfall (the pressure of the tide holds the water back). So when the tides comes up to extremely high levels, or we have repeat high cycles which do not allow the pipes to drain completely between cycles, the storm system begins to back up the surface water within the pipes. Eventually the pipes get completely full and the pressure build-up pushes the water up until they find the lowest surface point to release at. The low point is currently at the intersection of Dayton and SR-104, and thus we see the flooding that occasionally occurs here.

The same tidal problem occurs at the Edmonds Marsh as well, and we see the problems on Dayton get compounded by some of the other drainage systems which are negatively impacted by the filled Marsh as well. The tidal elevations and repeat lack of drainage between cycles, result in the water level of the Marsh rising as well. Once the Marsh level rises, the drainage system which carries parking lot drainage from Salish Crossing and Harbour Square (shopping centers on the northwest and southwest corners of SR-104 and Dayton) back up as well. The twin WSDOT culverts under SR-104 also become impacted and unable to pass stormwater from east to west under the highway. The runoff from the east side of the highway then turns north to flow along the east side of SR-104 until it eventually spills out into the road way near the intersection with Dayton. Both of these overflows then contribute back to the Dayton flooding, speeding up the accumulation of ponded water.

In addition to high tides, it take a measureable amount of rain to completely fill the drainage system to the point of flooding on the surface. The number of times that the right tidal conditions and rain conditions have occurred at the same time has been somewhat limited for the past couple years, but it only takes about an inch of rain to cause problems with the right tidal conditions.

But there is a plan! The City is now ready to break ground on a new stormwater pump station in January of 2020 (weather permitting). The new pump system will tap into the existing drainage pipes and pump the stormwater up and over the tide elevation to drain freely to the sound. The new pump will be located in the parking lot for the fishing pier, with new storm drain lines stretching out into Dayton Street and down the Olympic Beach walkway, out toward the pier.

The construction estimate for the project is $1.53 million to install the 4,233 gallons per minute (gpm) capacity pump and all of the associated appurtenances, power supplies (including backup power), and control systems. Construction is anticipated to take approximately 4 months with the hope of having most of the park disturbance done before summer. The project will alleviate the flooding of Dayton Street and better manage flooding from the Edmonds Marsh as well.  

For additional questions or information, please contact the City Project Manager Zachary Richardson at zachary.richardson@edmondswa.gov or 425-771-0220.

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