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Big dig for small creek in Santa Rosa

  • Civil engineering technician Greg Loraditch walks through the construction rehabilitating the Colgan Creek bed along Bellevue Avenue in Santa Rosa on Friday, July 18, 2014. (Conner Jay/The Press Democrat)

Confined to a narrow flood control channel for more than 50 years, Colgan Creek is once again getting some room to roam.

Workers have been sculpting a curvaceous new course for the creek in an ambitious bid to restore the city’s most polluted waterway to a more natural state.

Along a half-mile stretch of Bellevue Avenue near Elsie Allen High School, bulldozers and excavators have been removing hundreds of truckloads of dirt as they reshape and widen the previously flat, straight channel.

“It’s nice to see a project you’ve been working on so long move forward,” said Greg Dwyer, the city’s civil engineer on the project.

Built by the U.S. Army Corps of Engineers around the 1950s, the original channel along Bellevue was about 70 feet wide with steep walls, a largely flat bottom and little vegetation to shade the water.

The new channel profile will be up to 100 feet wider in places with gently sloping banks planted with a variety of native trees, shrubs and grasses. The wider channel will greatly improve the capacity of the creek to carry floodwaters.

Now instead of running in a straight shot, the creek will be allowed take a detour onto farmland the city acquired from an adjacent property owner for the project. Creating a watercourse that curves is important because it mimics the way area creeks would have naturally meandered as they made their way around obstacles on their journey toward the Laguna de Santa Rosa, Dwyer explained.

Such creeks had varied riparian habitat, alternating between shallow riffles, narrow channels of faster moving water, and pools offering safe, cool habitat for fish.

Replicating that variation in water speed in Colgan Creek is a challenge because the creek has low flows most of the year and only drops a few feet in elevation over the half-mile stretch from Dutton Meadow to where the creek crosses Bellevue, Dwyer explained.

The solution was to design a creek so that during low-flow periods, it follows a narrow, curving course within the wider channel, one that passes over and around obstacles such as logs and boulders anchored to the creekbed, Dwyer explained. Outside this deep channel, called a thalweg, the remainder of the new channel will act as a floodplain, giving the excess water an escape route.

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