by Jackie Blumberg, Introduction by Brian Rasmussen
Remnant Channel Reconnect Treatment
“Carnage Canyon” is the user’s name for what was once the most challenging four-wheel-drive route in the heavily used Lefthand Canyon Off-Highway Vehicle (OHV) Area. The site is located in the Roosevelt National Forest just outside Boulder Colorado. Due to extreme resource damage to the riparian area, the Boulder Ranger District has closed the route to motor vehicles while keeping other roads in the area open. A local watershed group, the James Creek Watershed Initiative, has been awarded a Section 319 Nonpoint Source grant which is funding a larger watershed restoration effort in the entire OHV Area. This multi-year restoration effort has already proven to be successful at building partnerships with stakeholders and using volunteers to get much of the on-the-ground work done. All of the various drainages within the OHV Area flow into Lefthand Creek. The long term goal of the 319 grant funded project is to improve the water quality of Lefhand Creek for drinking water and aquatic life by reducing sediment loading from the Lefthand Canyon OHV Area. Because “Carnage Canyon” is the main drainage in the heart of the OHV Area and has suffered major damage from motor vehicles, it is one of the most important and technically challenging pieces of the larger watershed efforts.
Brice and Blodgett Classification
The nearly 0.75 mile long stream in Carnage Canyon is small, intermittent and alluvial with bed material comprised primarily of cobbles and boulders. It is located in the mountains in a high relief valley with a wide floodplain relative to channel width. The system is meandering with sinuosity range of 1.2 to 1.8, and the watershed has a high vegetative cover, around 90%.
Channel Restoration Treatment
The intent of this restoration project was to return the Carnage Canyon corridor to its natural form and function. The reference reach approach was employed as the basis of design for the project. Verification of unaltered hydrologic regime and similar land uses was therefore the first step and it was confirmed that impacts in the basin have been minor and localized, and further, some rehabilitation of past OHV impacts has been completed. No activities such as logging or mining have caused significant alterations to the upper watersheds and impacts are primarily associated with recreation, including OHV road networks. Seven remnant channel segments, which were bypassed as OHV traffic altered flow patterns in the canyon, were studied as reference reaches to determine ranges for stable “between drop” slopes and target channel geometry. Given that streamflow gage data were absent for Carnage Canyon, the TR-55 method was selected for flow quantification. TR-55 utilizes site-specific inputs, such as watershed characteristics and local precipitation values, and is a widely accepted method for determining discharge. TR-55 allowed for a series of return period flows to be calculated, down to the 10-year. Return periods of greater frequency, such as the 2-year and 1.5-year, were determined by interpolation. These lower flows were significant in the design process, as the system would ultimately be designed using the “bankfull flow”. The project goal was not absolute containment of smaller flood flows, rather, to return the channel to its natural state, one which floods periodically and with the same frequency as it did prior to disturbance. Hydrologic modeling was used to supplement the reference reach approach. HEC RAS software was utilized to approximate flow depths for frequent storm events, such that channel geometry and berms reconnecting remnant channels could strike the optimal balance between containment and overtopping for floodplain access.
Restoration activities were comprised of three different treatments, based on channel condition. The proposed treatment types are described in greater detail below.
Remnant Channel Reconnect
The first proposed treatment was simply reconnecting the isolated reference reaches. Boulders were embedded at the top of the remnant channel, in an alignment that directed flows back into the historic channel, as seen in Photo 1. The minimum height for the boulder diversions was 1.5 ft, which correlated approximately to the 2-year storm, the overtopping event. Several of the target reference reaches were higher in elevation than the adjacent to the OHV road. Because the road elevation fell away from the remnant channels in these areas, larger boulders were used and additional controls were installed downslope, along the abandoned road, to collect water that overtopped the structure during larger storm events and reroute it back to the remnant channel. Drainage controls for overland flow included outsloping, rolling dips, water bars, and small check structures.
Extensive portions of the stream were completely obliterated by the OHV road. The second treatment type was to restore these sections to their pre-disturbed form and function as the main channel. Grading and rock work included the creation of small boulder drop structures and minor reshaping to encourage low flow concentration, as illustrated by Photo 2. The small drop structures imitated natural conditions documented in the remnant channels and served to lessen the overly steep profile so that a stable gradient could be achieved.
OHV Abandonment and Revegetation
The last treatment type was OHV road abandonment and revegetation. These sections would be abandoned through disconnection from the restored main channel, installation of overland flow controls, and scarification for revegetation efforts. Overland flow controls were similar to those proposed in remnant channel reconnect areas. In addition, a sediment trap was installed at the downstream terminus to dampen any short-term sediment loading from storm events occurring after construction.
Revegetation of the corridor followed a phased plan that started with a backhoe scarifying all compacted soils. Wildland Restoration Volunteers then planted a native seed mix composed primarily of grasses and mulched with certified weed free straw. Several scoured outer bends were targeted for installation of woody vegetation, to ameliorate bank stability. Revegetation efforts within Lefthand Canyon may be seen in Photos 3 and 4.
Over the next two years, thousands of native shrubs will be planted to further stabilize soils and accelerate the re-establishment of woody vegetation. Some of the woody material to be used will be grown from the local, genetic stock that was harvested from the canyon in 2008.