Pagosa Springs San Juan/Delores River Div 7 Response Letter

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4 1998). The isovels show the distribution of higher velocity in the core/thalweg region in the"run"part of the structure. Due to the low departure angle from the bank, recirculation eddies are minimized as they cause bank erosion and can create safety issues due to strong upstream re-circulation currents. The W-weir does not create recirculation eddies(see also the photograph at high water where the currents are directed gently away from the streambank region and directed toward the thalweg,the deepest part of channel). b. Stability. One of the principle objectives in the development and design of the W- weir was to stabilize the bed(grade control) and reduce bank erosion rate. The W-weir reduces near-bank stress by flattening the slope in the near-bank region, while increasing local slope in the thalweg region. The result is reduced, negligible bank erosion. The photograph of the W-weir at high flow demonstrates the low velocity in the near-bank region and the velocity being gently redirected away from the bank. The grade control of the bed is done in a manner different than a check dam. The width/depth ratio is decreased in order to transport coarse high bedload sediment and to decrease flood stage for the same magnitude flow, The thalwegs can be"canted" (one higher than the other),so that during drought, the majority of the flow will be in a smaller width with greater depth for holding cover for fish. The stability of the structure is excellent with little to no maintenance requirements for structures in place for over 10 years on many rivers throughout the United States. Detailed monitoring of these structures was done by the USGS on the Lake Fork of the Gunnison River, Colorado (John Elliot, Denver office published) and the Little Snake River project, Colorado by the Engineering and Fisheries departments for 5 years(report each year, contact Dr. Brian Bledsoe). The Lacy report refers to the"failing W-weir". The photographs taken, even last year in their report at low flow, do not show the W-weir failing. One float rock is setting at the entrance to the left thalweg,which could have been easily removed, rather than replacing the entire structure. The structure was not removed for reasons it was"failing." In comparing the U-drop with the W-weir, the bank erosion potential is increased with the U-drop since the structure is acting as a check dam due to backwater,recirculation eddies and lateral extension of the river. Also the slope is flattened headward on the U-drop,and since sinuosity is inversely proportional to slope, the stream will increase bank erosion to try to increase sinuosity due to the decrease in slope. Backwater also encourages coarse bedload deposition, which causes channel aggradation and increase in flood stage. The photograph in Figure 3 of the Lacy et al. Report shows a backwater effect of the raised bed elevation and water surface stage and an increase in width depth ratio. The design of the W-weir is to decrease width/depth ratio in order to maintain sediment transport, increase shear stress, stream power, sediment transport competence and capacity, and thereby decrease flood stage. c. Safety. Many of the safety issues have been addressed. There are no significant gaps in the vane arms of the W-weir: That is because the water needs to flow over:not through the structure. Thus the design features as demonstrated