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

Abstract

A reservoir sediment release along the North Fork Cache la Poudre River (North Fork) in northern Colorado resulted in a massive fish kill and channel sedimentation that filled pools critical to fish as overwinter habitat. Recognition of the hazards associated with a large influx of sediment into a riverine ecosystem is critical for a greater understanding of the effects of sediment releases, and hence, future management of sediment within reservoirs. Two one-dimensional sediment transport models, HEC-6 and GSTARS 2.0, were evaluated for applicability to predict sediment removal along the steep gradient, bedrock-controlled pool-riffle North Fork. The ability of both models to identify flushing discharges that assist channel recovery was also evaluated. Two modeling scenarios representing a low and high flushing discharge were modeled. Within each scenario, two levels of simulations were conducted to represent conditions of data availability, a default simulation for limited data input, and a robust simulation that utilized the entire set of field data, collected over a 1-year period. The models were calibrated against quantitative measurements of pool bed elevation obtained during field resurveys. Preliminary analyses were conducted to identify the appropriate sediment transport equations. HEC-6 results indicate that long-term, robust simulations yield the closest agreement between predicted and measured pool bed elevation change. More than 50% of the actual scour and deposition within three pools was modeled using HEC-6. Modeling accuracy using GSTARS 2.0 was considerably lower, and it appears that the present construct of the model does not reflect the physical processes operating along the North Fork. Computer models are useful tools in the sediment management decision process, provided adequate data collection and calibration are conducted. In situations where restrictions exist on available flushing discharges, sediment transport modeling can assist management decisions, and modeling is always preferable to uncalibrated estimates. Copyright © 2001 John Wiley & Sons, Ltd.