STANWOOD — In December, the Stillaguamish River flooded with winter rains. Its waters rose and poured into the Irvine Slough along Highway 532, then backed up underground pipes and into downtown Stanwood.

A similar event happens every few years, flooding the streets and sometimes homes and businesses.

Next week, the city begins a 10-year plan to end such downtown inundation.

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

Stormwater runs downhill from town 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.

“So all of downtown becomes essentially a bathtub,” city engineer Shawn Smith said.

Floods close roads, delay transportation, swamp yards and cause property damage, he said.

The problem led to talks of moving Stanwood’s City Hall and police station to higher ground.

To fix the issue, the city plans to redirect stormwater straight into the river, rather than the slough.

The plan will take place in six phases.

The first, which begins Aug. 31, will add a water pump right next to the Hamilton smoke stack. Water from the west end of downtown, near Twin City Foods and across Highway 532, will be rerouted to that pump instead of Irvine Slough.

Work should finish in early 2021. When it’s complete, Smith said that portion of downtown should fare better in any floods.

The entire project is estimated to cost about $11 million. The first phase will cost about $900,000, and is covered entirely by a $1 million state Department of Ecology grant.

Stormwater has backed up into downtown several times in the past decade, only skipping a few years, Smith said. But the floods may start happening more often, he said.

“With sea level rise and increased rain events, it is likely we will see an increase in the frequency of these events,” Smith said.

Julia-Grace Sanders: 425-339-3439; jgsanders@heraldnet.com.