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6 <br /> thalwegs without a hydraulic jump. The center of the W-weir is at 1/2 bankfull stage, and <br /> the water that goes over the middle is "stepped down"through two to three footer rock <br /> steps. This dissipates energy and prevents scour and recirculation eddies. <br /> 5. Pinning Hazards <br /> The W-weirs are not perforated as claimed, and have not ever constituted a hazard for <br /> recreational boating. Vane arms are back filled with gravel/cobble to close gaps to <br /> prevent water from washing under rock structure. <br /> 6. Structural stability <br /> According to the Lacy report, "structures are constructed of one single layer of medium <br /> to large boulders". This is totally untrue and misrepresented. I personally directed the <br /> construction of this project and each boulder had a footer;many footers were much larger <br /> than the surface rock. Since much of the structure was on bedrock,we had to create a <br /> footer step system to prevent flattening the grade,but providing stability for the surface <br /> rocks. Design specifications are shown in the appendix of this report on the location and <br /> nature of footers—the foundation of the structure. To prevent perforations,the vane arms <br /> were backfilled with gravel/cobble mixture to plug any gaps in the vane arms—contrary <br /> also to the Lacy et al.report. Flood flows do not push the structure and shift the rocks— <br /> also contrary to the Lacy et al. report. If that were the case, all of the W-weirs in the <br /> world would have shifted,moved and become "failed structures"as the Lacy et al.report <br /> claims. The report spends a great deal of effort to try to explain the forces at work <br /> associated with erosive failure criterion of the W-weir. What is odd is that even with <br /> 5,000 cfs floods on the San Juan in Downtown Pagosa,the W-structures have not <br /> failed. Nor have they failed elsewhere where 5 years of detailed monitoring by CSU <br /> (Little Snake River,21 miles project) and the USGS monitoring of the same structures on <br /> the Lake fork of the Gunnison have all survived major floods. <br /> Discussion <br /> A comparison between the hydraulic character of the different structures is important to address <br /> the potential problem of flood stage change. It was a requirement before any structures were <br /> installed in the San Juan to ensure by hydraulic computation that the 100 year flood stage would <br /> not be raised(FEMA). The W-weir and the other structures were modeled to prove that they <br /> would not raise flood stage. The stage discharge curve of the USGS verified that the structures <br /> decreased flood stage from post-restoration rating curve data. This structure was not modeled <br /> nor evaluated in terms of flood stage change, yet still was installed under a maintenance permit <br /> associated with W-weirs and other structures. <br /> The U-drop was shown being constructed in Figure 4 of the Lacy report. The invert elevation is <br /> not on bedrock but is being elevated at least 1 to 1.5 feet above the bed in order to create a bigger <br /> drop. Isn't this going to cause backwater, sediment deposition and increased flood stage? I <br /> prefer structures that narrow the channel rather than flatten it and raise the stage to create the <br />