VMP Final Report 4-30-08 - Laserfiche WebLink

South Platte River, Chatfield Downstream Channel Improvement Project- Vegetation Management Plan I. INTRODUCTION Background The Colorado Water Conservation Board (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 occurred during the federally constructed flood control project called "Chatfield Downstream Channel Improvement Project" (Project) completed by the U.S. Army Corps of Engineers (Corps). Maintenance requirements for the Project are the responsibility of the CWCB, and requirements 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, and the Great Flood of 1965 emphasized the importance of flood protection in the southern portion of the Denver Metropolitan area (CWCB 2007). For many years the Project has received a "minimally acceptable" rating by the Corps based on observations and policy guidance with respect to annual Project inspections. The main reason for the low rating is due to vegetative growth in the channel and along the banks of the river consisting of willows, cottonwoods, and other native species that naturally thrive in the riparian corridor. The CWCB has worked closely with the Urban Drainage & Flood Control District to perform necessary maintenance operations including removal of non- native trees such as Russian Olives (Elaeagnus angustifolia) and unwanted vegetation in the channel bottom. In January 2007, the Corps sent a letter to the CWCB detailing their PL 84-99 Rehabilitation and Inspection Program's Continuing Eligibility Inspection and Federal Performance Evaluation Summary for the Project. The letter detailed the "minimally acceptable" Inspection Report, which was conducted on June 6th, 2006. Deficiencies and unacceptable ratings noted in the report included unwanted levee growth, encroachments, riprap/revetments/banks, culverts, and gates (USACE 2007). The report stated "... our office is recommending that a plan be prepared to remove all trees, brush, and weeds from riprap, weirs, drainage structures, and sandbars. The plan should identifY where the removals will take place and how long it will take to complete, so that inspectors can readily identify progress" (USACE 2007). 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 (CWCB 2007). The South Platte corridor includes one of the most highly utilized trail systems in the state and is enjoyed by thousands of outdoor enthusiasts. In an attempt to address the issue, a technical study was commissioned by the CWCB and the District to demonstrate, through proper hydraulic modeling, that the vegetation does not substantially impact the flood conveyance capacity of Project. As a result, staff requested the Corps to initiate a Section 1135 study that would revise the O&M manual to allow for certain Page 4 Final Report - April 30, 2008