Project Summary

The City of Anacortes, Washington recognized that its water treatment plant, located along the Skagit River and serving 56,000 people, was vulnerable to current floods and future climate risks. In 2003, the City recognized the need to update the facility from 21.4 million gallons of water per day (mgd) to a stated capacity of 31.5 mgd. Moving the facility out of the floodplain was deemed cost prohibitive in 2008, so officials decided to rebuild on the existing site. Such a strategy can be risky unless climate projections are taken into account and adaptation strategies implemented to reduce future vulnerability.

Implementation

To determine the plant’s vulnerability, Anacortes officials worked with non-profit organizations to determine the best available climate science and the associated impacts to the plant siting. A variety of climate risks were taken into account including:

• more frequent and intense storms
• saltwater intrusion
• increased sedimentation levels

Climate change impacts were projected through the 2080s and downscaled for the Skagit River area. These vulnerabilities included an expanded 100-year floodplain, an estimated 350% increase in peak suspended sediment load in winter and anticipated upstream migration of the salt water wedge due to the effects of sea level rise.

In design and construction of this plant, officials sought to protect against higher risk of flooding by:

• minimizing penetration below current 100 year flood elevation
• raising critical electrical equipment out of the 100 year flood level
• utilizing water proofing techniques below 40 foot elevation
• designing ring dikes for flood protection

This plant was rebuilt on site at an expected cost of $56 million dollars and improvements to this design better prepare the facility to meet increased service demand as well as projected changes in climate.

How Did They Do It?

The City recognized climate risks and anticipated vulnerabilities.

• Anacortes recognized the vulnerability of the facility to flooding in 2003 and 2008, however they also identified the cost prohibitive nature of moving the facility.
• When upgrading the facility, they received input from scientists and used downscaled climate data to inform decision making.
• Applicable Tool - The U.S. Environmental Protection Agency Coastal Inundation Toolkit can assist utilities in better understanding facility vulnerability by illustrating a range of potential sea level rise and storm surge scenarios.

Next, the City incorporated climate vulnerability projections into the plant's design to adapt to future conditions by:

• Developing a design that acknowledged current vulnerability and seeking to reduce future vulnerability under anticipated future conditions.
• Elevating critical facility structures and including ring dikes and dewatering pump systems to protect against flooding.
• Utilizing water tight construction on the facility and using water proof membrane below 40 foot elevation.
• Designing to have no/minimal penetrations below 100 year flood elevation and having electrical switch gear located above 100 year flood level.
• Raising and strengthening levees near the plant.
• Increasing the sediment removal ability to deal with an expected increase from climate change (although the plant is still trying to quantify the projected increase of sediment).
• Applicable Tool - USEPA’s Creating Resilient Water Utilities Adaptation Strategies Guide can assist utilities in identifying low cost adaptation strategies to incorporate within plant designs.

Finally, the City identified information gaps for future research and found that:

• Preliminary modeling suggests saltwater intrusion is likely to be a future concern; additional analysis is planned.
• Further research is needed to better understand the long term challenges associated with sediment loads.
• Applicable Tool - The Climate Resilience Evaluation and Awareness Tool (CREAT) helps utilities conduct traditional risk based or scenario based vulnerability assessments to better understand information gaps and necessary future research.