Irvine Slough Stormwater Separation Study
When the Stillaguamish River experiences 100-year flood events, Irvine Slough and the lowlands south of State Route (SR) 532 are inundated with floodwaters that get trapped inside the Stillaguamish River levees which are designed to keep the Stillaguamish River from flooding the downtown area of the City of Stanwood (City). Floodwater can be pumped out by the Irvine Slough Pump Station (ISPS) and discharged via gravity culverts and the Stillaguamish flood control gates when the peak flood level in the River recedes; however, the discharge rate of the floodwater from the ISPS is significantly less than the rate of flows backing up behind the levees; i.e, the pump station cannot keep up with the flood flows.
In addition to flooding the roads, railroad, fields, and homes in the floodplain, this trapped floodwater closes the flap gates that isolate downtown Stanwood’s stormwater system from Irvine Slough. The closed flap gates cause the City’s stormwater system to back up, flooding ditches, roads, yards, homes, and businesses in downtown Stanwood and exacerbating the flooding from the Stillaguamish River. These floods cause property damage, delay transportation, endanger the public, and are expensive and time consuming to resolve. (For more details regarding the flooding problems, previous studies, and past mitigation efforts, refer to Appendix A (PDF) and Appendix B (PDF).)
The Irvine Slough Stormwater Separation Study
In 2013, the Washington State Legislature approved a $300,000 legislative proviso through the Washington State Department of Ecology (Ecology) and the Stillaguamish Tribe to develop alternatives to reduce lowland flooding in the City caused by the Stillaguamish River. The funding was earmarked to develop alternatives to separate stormwater from floodwater in Irvine Slough and improve flood conveyance south of SR 532.
The two primary goals of this IS4 (PDF) include the following:
- Identify alternatives to separate the City’s downtown stormwater conveyance system from Irvine Slough because Irvine Slough is subject to flooding (Stormwater Separation)
- Develop alternatives to improve flood conveyance in order to accelerate floodwater drainage (Flood Conveyance)
This IS4 report analyzed a series of potential solutions to the City’s stormwater and flooding problems to determine which options would be worth further pursuit.
A wide range of designs was conceived, developed, and assessed with input from community stakeholders, City staff, the City’s Public Works Committee, and the City Council. The goal was to identify sustainable solutions that provide the best value across a range of social, environmental, and financial criteria.
Each alternative was developed, modeled, and evaluated for how effectively it achieved the IS4 goals. Then, all of the alternatives were compared side-by-side for all aspects using an objective, multi-faceted decision matrix option and the best conveyance option and the best separation option were identified for further consideration.
Conclusions & Recommendations
The preferred stormwater separation design is Alternative 4 ($16.7 million), which would benefit both the City and Diking and Drainage District Number 7 (DD7) and would help spur local development. This alternative includes a new pump station jointly operated by the City and DD7 that would handle runoff from downtown and the Douglas Creek basin. With consideration of future development, the pump station would be designed and constructed with sufficient capacity for a fully-developed City, thus providing future developments with direct discharge exemption. This incentive could be tied to a special-use fee to help with long-term financing.
The preferred conveyance design is Alternative E ($2.2 million), which would demolish Larson Dam and construct a bypass channel around the existing pump station. By providing floodwater with a direct path to the Stillaguamish River, this economical, low-impact project would promote faster drainage and help reduce the duration and height of flood events. It is also noteworthy that Alternative E would essentially be the first phase of implementing the second- or third-ranked flood conveyance alternatives, should more funding become available.
The recommendation of this IS4 report is that stormwater separation Alternative 4 and flood conveyance Alternative E be pursued together. If only one option can be selected at this time, Separation Alternative 4 is the preferred alternative to implement first.