Pagosa Springs San Juan/Delores River Div 7 Response Letter

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11. Create maintenance-free diversion structures; 12.Reduce bridge pier/footer scour,road fill erosion and prevent sediment deposition. The use of rip-rap, gabions,concrete lined channels, bin walls, interlocking blocks, groynes, Kelner Jacks, spur dikes, rock jetties,barbs, reinforced revetment, sheet piling, log cribs, concrete check dams, and loose rock check dams are not only expensive but often do not meet the above stated objectives for river structures.A central problem with riprap,gabions,toe rock protection and similar structures is the increase in near-bank velocity,velocity gradient, stream power, and shear stress. These problems often lead to either on-site failures or problems immediately upstream and/or downstream of the structures. This, in combination with their high cost,resultant poor fish habitat and "less than natural"appearance, led to the development in the early 1990's of the Cross-Vane, W-Weir and J-Hook Vane. Description of Structures Cross-Vane General description The design of the Cross-Vane structure is shown in plan,profile and section view in Figure 3. The Cross-Vane is a grade control structure that decreases near-bank shear stress,velocity and stream power,but increases the energy in the center of the channel. The structure will establish grade control, reduce bank erosion,create a stable width/depth ratio, maintain channel capacity, while maintaining sediment transport capacity, and sediment competence. The Cross-Vane also provides for the proper natural conditions of secondary circulation patterns commensurate with channel pattern,but with high velocity gradients and boundary stress shifted from the near-bank region. The Cross-Vane is also a stream habitat improvement structure due to: 1)an increase in bank cover due to a differential raise of the water surface in the bank region;2)the creation of holding and refuge cover during both high and low flow periods in the deep pool;3)the development of feeding lanes in the flow separation zones(the interface between fast and slow water)due to the strong downwelling and upwelling forces in the center of the channel;and 4) the creation of spawning habitat in the tail-out or glide portion of the pool. The Cross-Vane is also a popular boating feature as kayakers routinely do "enders"and "surf' the vane portion of the structures installed on the Lake Fork of the Gunnison River near Lake City, Colorado and the San Juan River in Pagosa Springs, Colorado. The invert portion(center 1/3, see Figure 3)of the structure creates a standing wave,but is associated with a"run" immediately downstream of the invert. As a result the potential development of a dangerous re- circulation pool that traps"swimming paddlers" is eliminated. The structure"chutes"the swimmers and/or their boats into the deep,low velocity pool approximately half a bankfull width below the invert. 5