Pagosa Springs San Juan/Delores River Div 7 Response Letter

Home Browse Search

Wildland Hydrology Consultants Ken Jacobsen July 22, 2005 U.S. Army Corps of Engineers Sacramento District 1325 J St. Sacramento, CA 95814-2922 CC: Kara Hellige and Bruce McCloskey Dear Ken, I would like to respond to the public Notice Number 200575106 for the proposed work on the San Juan River near Pagosa Springs. First, I think that recreational boating and fish habitat can be compatible, depending on the nature of the design. The proposed design should meet multiple objectives including flood level issues,bank erosion potential, aesthetics, as well as the fishery and recreational boating emphasis. Second, as a landowner in Archuleta County and a concerned Professional Hydrologist/Geomophologist, I offer a summary of my concerns of the proposed design: 1. Since most of the bed of the San Juan River is bedrock, how are the proposed 4-6 foot boulders going to be anchored"flush"with the bed(channel invert) at the center portion of the proposed U dams? The existing"U"dam that replaced the W-weir in front of the springs needs to be assessed in terms of potential change in flood capacity(after the fact). The invert of this U structure was placed approximately 1.5 to 2.0 feet above the bedrock bed. The flood stage change should be checked not by estimates of stage-varying roughness coefficients using HEC-RAS or other one-dimensional models,but checked against controlled elevations and rating curves tied into bedrock benchmarks of the USGS. I suggest the USGS or a third party survey effort be utilized in this assessment of flood level relations. Validation of the models used to evaluate flood levels for the proposed work should be made against measured velocity/stage/slope relations using data from the form 9-207 (streamflow measurement summary data). This effort would be extremely valuable to the City of Pagosa due to their flood level policies and the very real potential flood risks of the proposed structures. 2. Concreted structures are not necessary on bedrock as our 10 year history of boulder placements on the San Juan River has shown based on the use of strategically placed footers. Over 4,100 boulders were placed on a bedrock- controlled bed and have been extremely stable associated with flood levels larger than those of the spring of 2005. Our monitoring has shown that approximately five boulders have moved a short distance over ten years, which is 0.1 per cent of all rocks placed. If the fishery resource is one of the objectives of this work, concrete is generally not used with native material enhancement of habitat. Since • 11210 N. County Rd. 19 • Ft. Collins, CO 80524 • (970) 568-0002 • FAX (970) 568-0014