Stanwood, Washington

Old Stilly floodgate drains water more quickly to prevent damage

Stanwood sits near the mouth of the Stillaguamish River, and parts of the city fall within a 1-percent annual chance floodplain. In 1996, severe floods hit Stanwood and other communities in Washington and the president declared a major disaster. After the flooding, the city invested in a drainage floodgate that replaced a 100-foot section of the Old Stillaguamish River Levee. The floodgate allows surface water to drain more quickly, preventing damage to local infrastructure, including a railroad and a busy commuter roadway.

Photo by FEMA

Community facts

Population
6,685
Homes
2,644
Businesses
572
Source: US Census Bureau.

Mitigation actions

The ten fiberglass-reinforced drainage gates on the Stillaguamish River levee can be easily opened during a heavy-rain event to allow floodwaters to drain rapidly off local properties and into the river. The improved drainage rate has allowed transportation infrastructure to return to normal more quickly. For example, the local railroad line, often affected by flooding, reopened around 12 hours after the end of rain from one flood event, compared with 3–4 days during previous floods. In addition, Marine Drive, which is frequented by roughly 10,000 commuters per day, was reopened in a day and a half, instead of four days. In prior floods, commuters had been forced to make a 15-mile detour until the floodwaters had cleared.

Benefits of mitigation

Economic benefits
$190,728
Total monetized benefits
Return on investment
16.0%
Estimated return on investment
Benefit-cost ratio
1.16
Benefit-cost ratio of the improved floodgate
Source: FEMA Loss Avoidance Study (January 2017).

Project challenges

Quantifying the benefits of a drainage project was the first hurdle

The geography of the city lent itself to drainage projects that would lessen the extent and duration of flooding. Proving the benefit-cost ratio of a drainage system can be more complicated than for a flood prevention system, according to Vaughn Collins, principal engineer for Northwest Hydraulic Consultants, a contractor for the project. The Stillaguamish Flood Control District made its case based on the economic benefits of reducing flood-related closure times for the railroad and highway systems. “Being able to calibrate and/or validate the results of your hydraulic and flood damage models against actual floods is very important when you are going after grants,” he says.

“When we looked at every hour [the road and railroad] were opened up sooner, it turned out the benefits were pretty substantial compared to the project cost”
Vaughn Collins, principal engineer for Northwest Hydraulic Consultants

Future considerations

Aerial view of the new drainage floodgate
Photo by US Coast Guard
The city is continuing to relocate its most valuable assets and to prepare its critical infrastructure for future floods

The entire downtown area of Stanwood sits in a Special Flood Hazard Area. Given the flood risks, in recent years the local government has discussed the idea of moving key public buildings, including City Hall and the police station, out of the floodplain. Renovating the buildings was ruled out owing to cost concerns as well as the risks that earthquakes pose to raised structures in the region. Moving key public services and vulnerable businesses out of the floodplain is a long-term plan for the city.

Flood mitigation resources

Below is a non-exhaustive list of other flood mitigation information resources.