Stanwood to divert stormwater runoff from Irvine Slough

STANWOOD — City leaders have decided to kick off a 10-year project to better manage stormwater and floodwater so downtown doesn’t get inundated when the Stillaguamish River rises.

On Jan. 14 they reviewed a study of options for redirecting the city’s stormwater runoff from the flood-prone Irvine Slough and for speeding up drainage when it does flood. They voted to accept the study and its recommendations, which include two longterm projects expected to cost $18.9 million total.

Design work should start soon on a pump station and stormwater pipe, the first pieces of the projects, city administrator Deborah Knight said.

Downtown Stanwood, seated near the meeting place of the Stillaguamish River and Port Susan, falls entirely within a federally designated floodplain. Irvine Slough is a drainage channel that parallels Highway 532 just south of downtown.

Stormwater runs downhill into Irvine Slough. When the Stillaguamish River overflows its banks, the slough and surrounding farmland flood. That floodwater closes flap gates between the city’s stormwater system and the slough, causing stormwater to back up into downtown streets.

“These floods cause property damage, delay transportation, endanger the public, and are expensive and time consuming to resolve,” according to a city staff report.

A pump station was built more than 35 years ago where the slough and the Stilly meet. It pumps water out of the slough and acts as a levee to keep the slough from flooding every time high tides raise the river level. But the pumps can’t keep up with a major flood.

The city hired RH2 and Northwest Hydraulic Consultants to complete a study of how to divert stormwater and drain floodwater from the slough. They worked with the Stillaguamish Tribe, Diking District 7 and the Stillaguamish Flood Control District. The study compared nine options for redirecting stormwater and 14 options for speeding up the exit of floodwater. Each plan was evaluated based on how it would affect the public, the environment and the city’s pocketbook.

The larger of the two recommended projects is expected to cost $16.7 million. It would keep the existing pump station and build a second one at Douglas Slough, north of Highway 532 at the far west edge of town. The new pump station would be jointly operated by the city and Diking District 7 to handle runoff from downtown and the Douglas Creek Basin.

The plan also calls for installing a gravity main that runs the length of downtown, from the Pioneer Highway to the new pump station, to carry water on the north side of Highway 532 directly to the Stillaguamish River and Port Susan.

This would intercept stormwater that currently runs into Irvine Slough and help prospective developers. They would no longer have to build stormwater retention ponds and instead would pay a fee to help pay back the cost of the pipe and pump station. It would be cheaper for the developers and save them space on their properties, Knight said.

As for managing the flow of floodwaters, the report recommends a $2.2 million project to remove a berm that cuts across the slough at 92nd Avenue NW and build a bypass channel around the existing pump station to speed up drainage.

The city got $785,000 in state money to help with the projects. The first $300,000 was for the study. Another $485,000 is to design the first phase of a new pump station. The City Council is set to vote on formally accepting those funds at the end of the month.

The first designs should be done in about a year, Knight said. After that, officials hope to land funding so they can design the rest of the project. They’ll need at least $600,000 more.

At a workshop next month, the Stanwood City Council plans to talk about how to pay for building the pump station, water main and other pieces of the projects. They’ll likely have to borrow money, raise rates and create a utility local improvement district where downtown property owners pay fees, according to the study.

If the council decides to look into raising any rates, a study could be done in time for the 2017-18 budget, Knight said.

Building the pump station, installing pipe, taking out the berm and creating a bypass channel probably won’t be done for another decade, she said.

“We’re looking out a ways ahead, but it’s a big project,” she said. “We have to get started.”

Kari Bray: 425-339-3439; kbray@heraldnet.com

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