Oxbows Reach 30% Design Final Report

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From the Report:
“The Oxbows Reach is located along US 34 approximately 4 miles upstream of Drake, CO in the Big Thompson Canyon (Figure 1). … The drainage area is 182 square miles and two drainages confluence with the Big Thompson River within the project extents: Rabbit Gulch from the south and Long Gulch from the North. Elevations within the project range from 7200 feet on the slopes above the channel to 6875 feet at the downstream reach extent. Portions of the project area were found to be recovering well and were not included in the 30% design.

The condition of the entire project reach will need to be re-evaluated should funds be acquired for a final design phase, including the reaches where no work is proposed, to evaluate how the channel is recovering on its own. The Oxbows Reach was broken into three sub-reaches named Brown Trout Reach, Rainbow Trout Reach, and Forest Service Reach (from upstream to downstream). The US 34 road reconstruction is occurring adjacent to the project and includes a section of the Big Thompson River that overlaps with the Rainbow Trout Reach. The total proposed treatment length is approximately 4000 feet for the three sub-reaches. It’s bounded by a US 34 bridge on the upstream end, and includes several residential homes and two private residential bridges.

Each of the three sub-reaches showed a similar response to flooding – channel expansion and widening causing severe bank failure and degradation. In the time since the flood, the channel has yet to recover effective channel bedforms (e.g, pool-riffle sequences) and remains over-widened. In some locations, vertical, eroding banks continue to supply fine sediments to the channel. In relatively shallower gradient areas, the channel is wide and contains multiple low flow pathways. Steeper gradients show signs of stabilization with groupings of loosely organized larger clasts (i.e., boulders) and some structural large wood. The proposed restoration work is strategically designed to address assets and areas that are not on a trajectory towards self-recovery, while allowing other areas to continue to recover on their own. Assets are addressed by stabilizing failing banks where they are threatening assets and adjusting a high flow path to direct flow back towards the river and away from assets. Other proposed work focuses on strategic grading to locally enhance habitat features, encourage floodplain connectivity, narrow the over-widened channel and encourage bedforms to help moderate fine sediment transportation and deposition and stabilize the channel bed. ”

Download Report Files (.pdfs)

01 BTWC_PriorityReaches_30%_BasisOfDesign_Oxbows_FINAL_20180228
02 Appendix A—30% Design Planset
03 Appendix B—Hydraulic Modeling Results
04 Appendix C—Aquatic Habitat Exhibits
05 Appendix D—Design Calculations
06 Appendix E—Revegetation Restoration Plan Approach and Exhibits
07 Appendix F—Opinion of Probable Costs
08 Appendix G—Monitoring&AdaptiveMgmtStrategy_20180228
09 30signComments_Oxbows_FINAL

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