Week of Nov. 10, 2008

Engineering solutions for flooding is precision work

The road-closure list can be pretty predictable, because historically the same low-lying areas tend to flood first. For example, crews from the King County Road Services Division know to patrol certain roadways along the Tolt and Snoqualmie rivers right away, because those locations are very susceptible to flooding from either rainfall accumulating in low spots or the nearby rivers overflowing their banks.

Even though some locations are well-known as problem areas during floods, there are trouble spots that Road Division engineers can’t, or don’t want to fix because it would only make problems worse.

“You can’t engineer a solution for every flood-prone location,” said Jon Cassidy, maintenance engineering manager for the King County Road Services Division. “We’ve had some great successes in recent years where we have enlarged culverts or built-up low spots to reduce flooding over roadways, but you have to consider the entire drainage system when you do this kind of work.”

Cassidy said the most common reason that a flood-prone area cannot be changed is that the fix would make downstream flooding worse. Also, if a low spot in a floodplain is built up, it usually requires adding fill, which reduces the floodwater storage area. If the storage area is reduced, it will push the water higher in another location.

Sometimes, a modified solution is the best. In 2005, the Road Division raised Northeast 124th Street in the lower Snoqualmie Valley by nine inches. Cassidy said the roadway is still sometimes covered by water, but not as often as it was in the past.

During the drier months of 2008, the Road Division completed more than a dozen culvert replacement projects to reduce localized flooding and improve fish passage. All involved replacing old, failing culverts with larger culverts that allow a greater volume of water to pass under the roadway. This lessens the amount of standing water in these locations during storms and also reduces the velocity of the water. The higher the velocity, the more likely it is that the water will wash away the shoulders of the road or scour out supports.

This summer on Vashon Island, the Road Division completed the last in a series of three culvert projects on Judd Creek. Cassidy said the projects were designed to make improvements in the entire Judd Creek basin and not just at isolated spots. In order not to make flooding in other locations on the creek worse, the Road Division began downstream and worked upstream over several seasons. Another location where multiple projects have been staggered for similar reasons is in the Cherry Valley area between Carnation and Duvall.

The work finished this summer on Vashon is where Judd Creek crosses Southwest 204th Street, a low-lying area that has been prone to flooding. In addition to lessening that problem, the new culvert re-opened the creek to salmon spawning. Just last week, several Coho were seen in the creek as far north as Southwest 204th Street – more than a mile upstream.

“With the roads along Judd Creek, we were able to make a lot of improvements – both for motorists and for fish,” said Cassidy. “But, there are many locations in King County where you just can’t avoid flooding – you can only try to keep it from getting worse.”

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