Some of our West Creek residents

Site 11

Site 11 is located immediately upstream of Broadview Road. The extent of the project reach is approximately 900 feet. The Site is characterized by the presence of two low head dams. The dam closest to Broadview Road has been breached. Between the lower breached dam and the upper intact dam, the channel has incised down to bedrock and is primarily a steep, riffle. The stream bed in this segment (between the dams) is largely stable due to the resiliency of the bedrock. However, bank erosion is a problem within the first 75 feet downstream of this structure. A major problem through Site 11 is associated with the presence of the intact dam (approximately 600 feet upstream of Broadview Road). This dam is a major obstacle for the passage of fish. In addition, the dam functions as a sediment trap and acts as a major grade control, forcing the flow to expand wide and flow shallow. As flow becomes shallow and wide, there is greater contact with the boundary surface, which lowers velocities and reduces shear stresses. As a result, sediment is deposited upstream of the dam, as can be observed by the significant mid channel bars that have developed. Two mid channel bars immediately upstream of the dam force flows to their outside bank, causing significant bank erosion and stimulating the lateral migration of the channel. The channel substrate upstream of the intact dam consists primarily of cobbles, with some smaller gravel and sand. A well formed floodplain terrace has developed throughout the length of the reach. Interestingly, the grade and slope of the terrace persist from the upstream to the downstream side of the intact dam.

Full Dam Removal and Site Restoration

Site 11 is primarily impaired due to the presence of the low head dam. One alternative for eliminating the impacts from this dam is to demolish and remove it. The concrete can either be hauled off site or disposed on-site below areas where fill is required. Since sediment has built up behind the dam, removal of the dam alone would likely cause a head cut and instability upstream. Head cuts are defined as erosion of the channel bed that occurs from downstream to upstream. The anticipated extent of a channel head cut is a function of the height of the drop as well as the channel substrate and is unknown at this time. Therefore, complete removal of the dam must be coupled with the installation of grade controls. Due to the natural bedrock character of the stream downstream of the intact dam, this alternative proposes constructing a series of boulder steps and pools. The steps maintain grade and will be constructed to make up the elevation difference from upstream to downstream of the intact dam in small increments. The drop at each step will be designed for passage of all fish. Pools between each step help dissipate energy and offer a refuge for fish to rest. Step pool sequences also allow sediment to move downstream, thus preventing the generation of mid channel bars and ensuing channel migration.

In addition to removal of the intact dam and construction of step pools, minor bank protection or structures are needed to halt erosion on the outside meander bends upstream of the dam and locally through the exposed bedrock segment. Potential options include installing rock vanes, boulder toe protection, bioengineering or some combination. The breached dam immediately upstream of Broadview Road will also be removed. This area will not require grade controls such as step/pools, however, a rock cross vane will protect the banks in this confined section of the valley. The alternative also includes supplemental native plantings to enhance the existing riparian forest. Special consideration will be given to remove and control a patch of Common reed (Phragmites australis) on the north bank and floodplain between the two dams.

The benefit of the dam removal is the elimination of a major fish barrier as well as improved aesthetics. A potential hazard of this approach is instability associated with the disturbance of removal. As stated previously, a stable bankfull terrace has developed both upstream and downstream of this structure. Removing the entire dam may result in adjustments to this feature. In addition, concrete removal, if hauled off site could be costly. However, it is felt that with appropriate considerations, such as removing only the concrete necessary for the channel bankfull width, these potential impacts can be minimized.

Benefits

• Remove fish passage barrier
• Provide fish resting areas and improved habitat
• Enable natural movement of sediments
• Eliminate lateral channel migration
• Reduce bank erosion and sediment yield
• Enhanced riparian forest
• Remove invasive species

Risks and Constraints

• Expense of concrete removal
• Extensive disturbance to floodplain during removal
• Potential floodplain instability
• Litter may collect in the rootwads and newly established vegetation

Maintenance

• Remove debris jams
• Replace the dead or inadequately established vegetation within the first year
• Invasive species management

Project Plans, Maps & Photos

These maps are large, so download time may take a while - especially for dial-up connections. You will need to have the Adobe Acrobat Reader installed on your computer. If you do not already have it, you can download it for free at the Adobe Acrobat website.

West Creek Site Overview Project 11

West Creek site 11 concept plan - Option 1

West Creek Site 11 map - low head dams just west of Broadview

West Creek Site 11 photos

West Creek Site 11 FINAL Engineering Plans

November 2009 West Creek Dam Removal Pics