Sediment Transport Evaluation for Dam Removal Scenarios, Fossil Springs Diversion Dam, Arizona

摘要

Fossil Creek is a stream in north-central Arizona that is fed by Fossil Springs. Spring flow, nearly 43 cfs, is characterized by high concentrations of calcium carbonate – which leads to the formation of natural travertine dams in the creek. Flow in the creek has been altered for almost a century due to a hydropower (currently producing 4.2 MW) facility (diversion dam, flume, power plants, channels, tunnels, etc.), which was constructed in the early 1900’s. The facility manager has committed to decommissioning, beginning in early 2005, with a return of full flow to Fossil Creek. There are several major issues related to decommissioning and restoration in the watershed and stream corridor areas. Of concern to many parties is the future of the Fossil Springs diversion dam, sited approximately 1000 ft downstream of Fossil Springs. Due to nearly complete siltation of the reservoir area between the dam and Fossil Springs, the Fossil Springs riparian ecosystem has expanded over the past 90 years to encompass the sediment wedge upstream of the dam. Dam management scenarios have been identified that range from no action to complete removal. The Federal Energy Regulatory Commission, or FERC, as part of its preparation of an Environmental Assessment (EA) of the license surrender application, sought to identify the hydrological, hydraulic and sedimentation impacts related to these scenarios. We considered a project reach of approximately 0.25 miles – that portion of the Fossil Creek drainage in the immediate vicinity of the Fossil Springs dam. As part of our study, we prepared hydrologic, hydraulic and sediment transport models to evaluate probable impacts of the different scenarios. Though the results are approximate, we were able to develop forecasts of system response to return of base flow and to seasonally significant flows, as well as to infrequent high-magnitude storm flows.