Pagosa Springs San Juan/Delores River Div 7 Response Letter

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morphological relations for stable natural rivers as described by"reference reach"by stream type (Rosgen, 1998). Structures are often placed in rivers in an attempt to correct some of the adverse effects of channel adjustment due to instability. Unfortunately, many structures are often installed to "patch a symptom"rather than achieve the stable channel form. Appropriately used structures can assist in maintaining the stable dimension pattern and profile(Rosgen, 1996). River engineering structures need to be incorporated with a clear understanding of the river variables that constitute the stable form. Structure failures are generally associated with designs incompatible with the"rules of the river". For example,cross-channel check dams decrease energy slope upstream of the structure. Data from natural rivers indicate a negative power function relation between sinuosity and slope(Figure 1),thus when slope is decreased there is a corresponding increase in sinuosity through lateral migration following bank erosion. Most failures of check dams occur when they are"out flanked"by the river through lateral adjustment upstream of the structure. Consequently, many check dams often accelerate excess bank erosion. Cheek dams generally decrease upstream velocity, slope, and depth, increase roughness, and induce sediment deposition. These changes lead to instability and contribute to the failure of the structure. Structures have also failed due to excessive bedload deposition that leads to a loss of channel capacity and subsequent change in the stable dimension,pattern and profile of the river. Streambank stabilization structures proliferate as bank erosion accelerates. Most of the structures implemented involve "hardening"the banks. Changes in near-bank stress and/or stream power associated with unstable channels can accelerate bank erosion. Work conducted by Parker(1978), and Bathurst(1979),described secondary circulation patterns and the distribution of boundary shear stress in both straight and meandering rivers. Ikeda, et al, (1988), described the erosion and transport of grains from the bank region to the center of the channel as a result of bank erosion. They also described the process of lateral momentum transfer due to turbulence that resulted in eddy diffusion and induced net lateral transport of longitudinal fluid momentum from regions of high momentum to regions of low momentum. These processes resulted in a lateral redistribution of bed shear stress. Secondary cells associated with down welling(high boundary shear stress)and upwelling(low boundary shear stress)occur in the near-bank region creating very high velocity gradients(Bathhurst, et al, 1979). Boundary shear stresses associated with high velocity gradients, can accelerate erosion rates,and are shown in the velocity isovel constructed from vertical velocity profiles(Figure 2). The streambank erosion prediction methodology developed by Rosgen(1996, 2001),utilizes computations of near-bank stress for assessing various erosion rates. Any structures that can reduce near-bank stress will reduce bank erosion by several orders of magnitude. New attempts at similar problems To offset near-bank forces to reduce streambank erosion, Paice and Hey(1989)installed submerged concrete vanes on the outside of meander bends to control secondary circulation and redirect river currents to decrease boundary shear stress in the outer bank region. These attempts were successful in reducing erosion by redirecting erosive currents in the near-bank region. Iowa Vanes(Ogrlaard and Mosconi, 1987)were previously used to redirect currents away from streambanks to reduce accelerated erosion. Submerged vanes were installed and tested by Hey (1992) not only to re-direct velocity distribution but to also provide improved fish habitat. 2