South Platte River Channel Improvement Project Vegetation Management Plan

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by Tom Browning, Colorado Water Conservation Board

 
Semi-urbanized reach of the South Platte River with healthy riparian vegetation.

The CWCB owns right-of-way along the South Platte River below Chatfield Reservoir from Columbine Valley downstream to approximately Hampden Avenue. The right-of-way resulted when a federally constructed flood control project known as the “Chatfield Downstream Channel Improvement Project” (Project) was completed by the U.S. Army Corps of Engineers (Corps). Maintenance requirements for the Project are the responsibility of the non-federal sponsor, in this case the CWCB, and are specified in the Project Operation & Maintenance (O&M) manual.

The Project currently provides much needed flood protection for citizens and property located in this urbanized reach of the South Platte River. The Great Flood of 1965 emphasized the importance of flood protection in the southern portion of the Denver Metropolitan area.

For many years, the Project has received a “minimally acceptable” rating by the Corps based on their on-going observations and policy guidance with respect to annual Project inspections. The main reason for the low rating is due to vegetative growth along the banks of the river consisting of willows, cottonwoods, and other native species that have naturally thrived in the riparian corridor (Fig.). The CWCB has worked closely with the Urban Drainage & Flood Control District (District) to perform necessary maintenance operations including removal of non-native trees (e.g. Russian Olives) and unwanted vegetation in the channel bottom.

Long-term controversy has existed between the Corps and the local interests regarding willow and tree growth on the channel banks (located between the normal high water line and the top of the banks on either side of the channel). Public perception and input regarding the high value of the vegetation, in terms of environmental and aesthetic benefits at a minimum, has prevented the CWCB and the District from taking extreme action to remove the trees in question. The South Platte corridor also includes one of the most highly utilized trail systems in the state and is enjoyed by thousands of outdoor enthusiasts each year.

In an attempt to address the issue, a technical study was commissioned by the CWCB and the District to determine, through proper hydraulic modeling, what impact the vegetation has on the flood capacity of Project. Results of the technical study were provided to the Corps for review and consideration. The CWCB then requested the Corps to initiate a Section 1135 study that would revise the O&M manual to allow for certain types and quantities of vegetation to exist along the banks of the river. However, the Corps terminated the 1135 effort due to lack of federal interest in revising the O&M manual.

The Corps has acknowledged their intent to categorize the Project condition as “unacceptable” in the near future if tree removal (as described in their previous inspection reports) is not conducted in a timely manner. An “unacceptable” rating would essentially remove the Project from the PL 84-99 program, thereby eliminating future federal funds to repair the Project after an actual flood event happens. It would also trigger potential floodplain mapping and flood insurance consequences by FEMA that are undesirable. The CWCB recognizes the important need to maintain the Project and the flood control benefits that it provides to citizens and vulnerable lands along the river.

In order to avoid negative flood protection consequences that would occur if the Project were to receive an “unacceptable” rating, the CWCB proposes to work closely with the District, the Corps, and a qualified consulting team to develop a phased Vegetation Management Plan (Plan) that would likely take place over a 3 to 5 year span. The Plan would call for select removal of tree and willow growth along the Project reach as prescribed by the Corps, but it would also identify non-floodway areas of the Project lands where replacement vegetation and environmental enhancements could take place to maintain or augment ecosystem benefits and to ensure continued habitat connectivity.

An extensive public outreach component of the tree removal effort is envisioned, and a draft Plan will be posted on the CWCB website in early 2008. Communication with members of the South Platte Working Group and staff from the South Suburban Parks & Recreation District (South Platte Park) has already taken place. In addition, a pilot area within the river has been treated with FDA approved herbicide to evaluate effectiveness of the treatment for future use. Public notices will be posted and a public meeting will be held prior to full implementation of the proposed Plan to provide detailed information to interested parties and to gain public input.

Questions can be directed to Mr. Joe Busto via email at joe.busto@state.co.us or by phone at             (303) 866-4807      .