One-dimensional sediment transport modeling of pool recovery along a mountain channel after a reservoir sediment release

A reservoir sediment release along the North Fork Cache la Poudre River (No rth Fork) in northern Colorado resulted in a massive fish kill and channel sedimentation that filled pools critical to fish as overwinter habitat. Rec ognition of the hazards associated with a large influx of sediment into a r iverine ecosystem is critical for a greater understanding of the effects of sediment releases, and hence, future management of sediment within reservo irs. Two one-dimensional sediment transport models. HEC-6 and GSTARS 2.0, w ere evaluated for applicability to predict sediment removal along the steep gradient, bedrock-controlled pool-riffle North Fork. The ability of both m odels to identify flushing discharges that assist channel recovery was also evaluated. Two modeling scenarios representing a low and high flushing dis charge were modeled. Within each scenario, two levels of simulations were c onducted to represent conditions of data availability, a default simulation for limited data input, and a robust simulation that utilized the entire s et of field data, collected over a 1-year period. The models were calibrate d against quantitative measurements of pool bed elevation obtained during f ield resurveys. Preliminary analyses were conducted to identify the appropr iate sediment transport equations. HEC-6 results indicate that long-term, r obust simulations yield the closest agreement between predicted and measure d pool bed elevation change. More than 50% of the actual scour and depositi on within three pools was modeled using HEC-6. Modeling accuracy using GSTA RS 2.0 was considerably lower, and it appears that the present construct of the model does not reflect the physical processes operating along the Nort h Fork. Computer models are useful tools in the sediment management decisio n process, provided adequate data collection and calibration are conducted. In situations where restrictions exist on available flushing discharges, s ediment transport modeling can assist management decisions, and modeling is always preferable to uncalibrated estimates. Copyright (C) 2001 John Wiley & Sons, Ltd.