Colorado: CDOT eyes fix for I-70 bridges in Silverthorne

New drainage system would prevent sand from reaching the Blue River

Traction sand streaming off the I-70 bridge over the Blue River in Silverthorne has resulted in unsightly piles of sand and damaged vegetation along the banks of the Blue River.

By Bob Berwyn

SUMMIT COUNTY — The Colorado Department of Transportation and Silverthorne hope to remediate a long-standing source of Blue River pollution this summer, when the highway department plans to redesign the drainage system on the I-70 bridges crossing the river.

The idea is to try and shunt at least some of the traction sand and the rest of the road gook in some kind of drainage pipe, starting in the middle of the bridges, and catch it in a basin on either end, said CDOT engineer Peter Kozinksi, who acknowledged that most of the sand has been running off the bridges for 40 years — and most of it ends up in the Blue River, a Gold Medal trout streamwhere  Silverthorne and partners have invested hundreds of thousand of dollars in restoration.

Some of it ends up in gritty piles mixed with snow at an angler’s parking area beneath the bridges, right next to a sign touting the restoration work. Plows subsequently push it around, damaging riparian vegetation even in a low-snow year like this one.

“To the best of my knowledge you are seeing the leftover road sand from 40 years of plowing on the bridges over the river,” said Silverthorne public works director Bill Linfield. “When we placed the sign you have in one of your photos, I strategically placed it under the bridge so that it would be protected from the sand which come down along the sides of the bridges,” Linfield said. “We placed the sign here because this was a heavily used parking area for fishermen.  That parking area got smaller with the recent improvements to Wildernest Road, but fishermen still park there.”

Kozinski said that, ideally, the shoulders on the bridges would be wide enough to let the snow melt in place so the sand could be removed. He acknowledged that the unchecked runoff has created a bit of a mess, he said the bridge was built to the required standards of several decades ago.

But since then, community values have changed and environmental awareness has increased. “That’s why the town approached us  and asked if there’s anything we can do,” Kozinksi said, emphasizing CDOT’s partnership approach to tackling these kinds of issues. The transportation agency has for years been working on reducing impacts to nearby Straight Creek with ambitious re-vegetation and sediment-trapping projects.

Up to now, the Blue River’s Gold Medal waters haven’t suffered too much from the sand coming off the highway, said Colorado Parks and Wildlife aquatic biologist Jon Ewert, who monitors fish and the condition of the stream on a regular basis.

But that could change if there are more dry years, when Denver Water can’t release cleansing high flows from Dillon Reservoir.

“The time that I have been here (since 2007) has essentially covered one wet-weather cycle. I think that 2012 was the first year that Dillon has not spilled since I have been here,” Ewert said. 

“As you know, when the tailwater is running several hundred or over 1,000 CFS, the water just rips through there with a lot of power because of the relatively high gradient in that reach, the v-shaped channel and very small or nonexistent floodplain,” Ewert said.

“So as far as I have seen, the Blue is quite resilient to sediment inputs and I have not observed an issue with excessive sediment (in the form of highway sand) accumulation. In fact, the hydrologists that I’ve had conversations with who know more than me about the subject talk about that reach of the Blue as a classic “sediment-starved” reach of river, due to the presence of the dam,” he said.

“Keep in mind, though, that if the Blue stays pegged at 50 CFS for the next fifteen years, it’s probably going to be a very different story. Hopefully we don’t see that. And this isn’t to say that I’m not glad they’re building a retention system – certainly won’t hurt,” he concluded.
Kozinski said he hopes to finalize plans for the improvements and announce a schedule for the work in the next few weeks.

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