C150334 Feasibility Study - Laserfiche WebLink

Home Browse Search

Feasibility Study for the Plaza Project - Phase 2: McDonald Ditch Implementation Project 7.2 Selected Alternative for the Wetland The selected alternative for the wetland includes considerations to ensure the wetland meets ecological standards along with road line -of -sight guidelines. The wetland will be contoured to include floodplain benches and a curving water line to increase surface area for wetland vegetation. Vegetation will be introduced in the upland, mesic, and water logged communities. Because upland vegetation will be adjacent to roads on the north, south, and west side of the wetland, species used will be conducive to mowing and other activities to ensure road safety. Additionally, large trees and willows currently in the line -of -sight at a road intersection will be transplanted to other locations in the wetland. The Rio Grande County officials and USACE representatives have reached an agreement on the wetland plans. 7.3 Selected Alternatives for the McDonald Ditch The Stakeholders selected mitigation measures for the McDD diversion and headgate. These mitigation measures included considerations for surrounding infrastructure issues including the Sevenmile Plaza Bridge and power poles owned by the San Luis Valley Rural Electric Cooperative (SLVREC). 7.3.1 McDonald Ditch Diversion The Stakeholders selected Diversion Alternative #3: Hybrid Rock and Concrete Diversion for the McDD diversion replacement. This option will greatly improve diversion efficiency and riparian condition. The diversion will have a sluice, which will move sediment and debris past the headgate and downstream. The angle of the diversion will be changed so it extends across the narrowest part of the river and is perpendicular to the direction of flow. This will improve the channel and streambank stability near the diversion, as the current diversion pushes flow into the banks. Finally, the grouted rock drop structures will allow for fish and boat passage; this will improve the habitat and recreation potential in the reach. 7.3.2 McDonald Ditch Headgate The Stakeholders selected a concrete headgate with two gates. One manual gate and one solar - powered automated gate. The manual gate will be closest to the river and will be used to open or close the headgate to flows, allowing the McDD to close the gate in the winter and protect the automated gate from ice and freezing. Additionally, the manual gate will be used to regulate ditch flows if the automated gate malfunctions. The automated gate will be closest to the ditch and will regulate ditch flows, improving diversion accuracy and accounting. 7.3.3 Sevenmile Plaza Bridge Stability Concerns were shared about the stability of the Sevenmile Plaza Bridge, which is owned by Rio Grande County, in the event the current McDD diversion dam is removed. For decades, the existing diversion has held the channel at a controlled elevation. There is a risk that removing the diversion would lead to increased flow velocity, increased grade in this reach of the river, and instability of the bridge. Although the stakeholders decided to replace the structure, it was uncertain if any changes in the new structure would impact the bridge's stability or influence scouring near the bridge's piers. At one time, the bridge was on the Colorado bridge scour list. The scouring was caused during a high water event when a tree snagged on the middle pier of the bridge. To determine if the McDD diversion is having any effect on the Sevenmile Plaza Bridge, the RGHRP hired Stantec Consulting Services, Inc ( Stantec). Stantec has worked on inspections and scour analysis projects for this bridge. NRCS and RGHRP staff collected survey and cross section data. Stantec used this information to build an existing conditions model. The model will be used to perform HECRAS modeling to determine the impacts of current and proposed structures on the Sevenmile Plaza Bridge. Stantec's final report on the Sevenmile Plaza Bridge will be complete in May 2012. W1