The High Park Fire of 2012 burned approximately 88,000 acres of Larimer County near and upstream of the City of Fort Collins. The Lewstone Creek watershed sustained significant wildfire damage between Rist Creek and the Cache la Poudre River. As a result, 2012 and 2013 thunderstorms on Lewstone Creek produced exaggerated stormwater runoff volumes and durations, threatening the City’s two main drinking water supply pipelines. The September 2013 flood event further compromised the site and physically exposed one of the pipes.
This project was about arresting the erosion and protecting the City’s water supply, which is conveyed to the City’s water plant through three pipelines. Two of the pipelines convey the City’s first choice of water rights from the Poudre River to the water treatment plant. These two lines cross Lewstone Creek.
City operations crews conduct weekly inspections of the water supply lines and noticed in July of 2013 that the protective covering over the two City waterlines was exposed. The City engaged Ayres Associates and Hydro Construction to design and construct a solution. Subsequent site visits in July and August revealed a deteriorating condition in the stream channel. September brought heavy rainfall that devastated Northern Colorado. This watershed did not experience extreme flows, but the ongoing rainfall tore apart this channel and threatened the existing grade control wall with a potential 6-foot headcut headed upstream.
Ayres designed a partially grouted riprap drop structure starting from the downstream edge of the existing cutoff wall at a 2:1 slope that extended below the predicted scour depth of the headcut. The existing wall was used as a groundwater cutoff wall and was raised in elevation and extended laterally. Due to prevailing groundwater levels, the toe of the partially grouted riprap was protected with a launchable key of very large riprap that will arrest the progress of headcutting when it reaches the protected reach of Lewstone Creek.
Upstream of the cutoff wall, the critical pipe crossing was stabilized and covered with a structural concrete cap to maintain the existing channel bottom elevation, which was only 6 inches above the pipes.