PROJECT ALERT NOTICE (CO) Colorado Post Flood Work

Date: Tue, 24 Sep 2013 08:46:46 EDT

Summary: Post-flood survey work to reconstruct the peak flows in the Loveland/Longmont/Boulder, Colorado area.

Over the past weekend, a recon team worked in the Boulder Creek, St Vrain, Big Thompson, and Cache la Poudre basins, evaluating field sites for follow-up surveys that will be used to determine the peaks flows for those USGS streamgages that direct measurement of streamflow was impossible. A large number of roads and bridges were washed out making travel difficult. Along the St Vrain River, a large part of the floodplain was still de-watering resulting in many roads still being partially flooded. Erosion was severe in several areas along the roadways and bridges, with a great deal of damage to underground utilities, including fibre optic lines.

In searching for sufield sites along the St Vrain and Big Thompson, many homes, trailers, and apartments were inundated with high water marks as much as 4 feet in some of these buildings. In addition, this flood carried a great deal of sediment as evidenced by large amounts of deposition. Along Fourmile Creek (trib to Boulder Creek) transport of large boulders and smaller sediment indicated the high velocities present during the flood. In some locations, as much as 3-4 feet of deposits of boulders and finer sediments (along with tree debris) was evident. Preliminary analysis of the depositional evidence indicates this to be water flood and not debris flows. Many private homes, vehicles, and bridges were destroyed along Fourmile Creek, which drains the Fourmile burn area (wildfire of a few years ago).

Numerous landslides were witnessed which did significant damage to Boulder Canyon Road.

Some of the most violent water flood damage was along the Big Thompson Creek, where the field recon team was only able to go partway up Highway 34 to attempt to reach the State of Colorado streamgage at the canyon mouth. An important location to compare the 2013 flood to the devestating 1976 Big Thompson flood that killed hundreds of people.