County alleviates chronic flooding near Canyon Road

Thursday Oct 29 2009 7:48 PM

For some Pierce County residents and businesses, annual flooding is nothing new. It's not from a raging river that threatens to sweep buildings off their foundations, but from the small creek that meanders through the neighborhood. It floods the road every time there's a major storm, and each time it takes about two days for the water to go down.

This is the case of Canyon Creek, an intermittent stream near Canyon Road that doesn't even show up on some maps. A county Public Works and Utilities project recently corrected drainage problems along the creek between 90th and 84th streets east, just in time to provide flood relief to residents and motorists during the upcoming storm season.

Historically, Canyon Creek was a small, marshy stream flowing through a long, narrow glacier-made valley that stretches from 124th Street East north to Clark's Creek. Urban development caused parts of the original stream channel and wetlands to be filled between what is now 90th and 84th, and culverts beneath roads and driveways eventually became broken and plugged. This resulted in water flowing over the roads and cutting off access to homes a dozen times per year.

To correct the flooding problem and restore degraded habitat, county surface water engineers realigned and reshaped the stream, creating 2,700 feet of new channel and one acre of new wetlands. They lined the channel and new culverts with streambed gravel and designed them to be suitable for fish.

An important consideration of the project was to avoid simply transferring flood water downstream to other properties. To prevent this, a large stormwater pond was constructed near the corner of 84th and Canyon Road, and Canyon Creek was routed through it.

The storm pond is designed to store excess water and release it at its current rate, with no negative impact to downstream areas. It also features constructed wetlands, sediment settling areas for water quality, native plants, and natural-looking islands and logs to mimic natural fish and wildlife habitat.

Particular attention was paid to wildlife habitat as part of the project. There may be resident fish in several upstream ponds, so the new culverts were made larger to allow for fish passage, with flat bottoms and shallow gradients. Special fencing allows wildlife access to the new pond, and islands provide waterfowl nesting areas that are protected from predators.

Salmon downstream in lower Canyon Creek and Clear Creek will benefit from lower water temperature and decreased turbidity made possible by natural vegetation planted in the Canyon Creek project. Restored wetlands will produce and export insects for fish to eat and nutrients that support the growth of even more insects downstream.

The Canyon Creek project was scheduled for construction three years ago, but was postponed as part of a strategy for managing declining funds. Ultimately, this turned out to save county ratepayers an estimated $1.5 million. The project was originally estimated to cost around $4 million, based on past bids. Due to the current economic conditions, bids came in much lower this year, and the combined costs of land, design and construction totaled $2.5 million.

Even a project of this magnitude could not resolve all drainage issues in this vicinity. The stream corridor upstream of 90th was not improved, and private roads that experience flooding there are not within the county's jurisdiction. There is a west fork of Canyon Creek that flows under Canyon Road, but new culverts won't be installed until other improvements are made to Canyon in the next 5 years or so. When that happens, flooding near 90th Street East will be improved even further.

Historically, salmon may have come up Canyon Creek when there was enough water, but several man-made barriers downstream prevent that now. Some barriers are on county property and some are private property, but restoring fish passage appears to be possible should this become a priority in the future.

Meanwhile, residents and drivers near the newly-completed project can expect far less serious flooding from now on.

Contact: Hans Hunger, Pierce County Surface Water Management program director, 253-798-6162, or Hunter George, Pierce County Communications director, 253-798-6606, or hgeorge@co.pierce.wa.us.

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