Sediment Transport Analyses and Development of a Hydraulic Computer Model for Desiging a Stable Vegetated Channel at Lost Lake

The erosion and transport of mine tailings containing arsenic is occurring at several locations within the Lost Lake watershed in the vicinity of Nevada City, California. The source of the tailings is the abandoned Lava Cap Mine which is a Superfund Site under the jurisdiction of Region 9 of the United States Environmental Protection Agency (USEPA). During an extreme flow event in 1997, the watershed experienced large amounts of surface water runoff which demolished a tailings dam located upstream. As a result, tailings have been transported to several locations within the watershed extending from the upper reaches of Little Clipper Creek downstream to the areas surrounding Lost Lake. Remedial action should be taken to enhance channel stability during extreme runoff events. The primary objective of this project was to develop recommendations to enhance the stability of the stream channel for Little Clipper Creek and Clipper Creek extending downstream to the earthen dam at the outlet of Lost Lake. This objective was accomplished by completing the following tasks: 1) performing a thorough engineering survey of the Lost Lake watershed; 2) collecting and characterizing sediment samples from several locations within the watershed; 3) performing laboratory flume experiments on sediment samples; 4) developing a hydraulic computer model of the watershed using the US Army Corps of Engineers Hydrologic Engineering Center River Analysis System (HEC-RAS) software; and 5) the evaluation of various channel configurations and erosion control strategies that could be implemented to stabilize the stream channels within the Lost Lake watershed.A detailed engineering survey was conducted to obtain channel cross section data for both the upper and lower reaches of Little Clipper Creek and Clipper Creek and the topography of the area adjacent to the stream channels. The sediment samples collected were classified by particle size distribution and mineralogy. The data from the engineering survey were incorporated into a HEC-RAS model with the existing channel geometry to evaluate the hydrodynamic conditions within Little Clipper Creek and Clipper Creek under a range of flow conditions from 2 to 1624 cfs. The hydrodynamic conditions predicted by the HEC-RAS model were compared to the results obtained from the laboratory flume experiments to assess the erodibility of sediments in the streams. The results indicated that sediment transport is anticipated at essentially every location within the Lost Lake watershed even under minimal flow conditions. To enhance channel stability, permanent erosion and revegetation materials (PERMs) could be installed within the main channel of each creek. This material may either consist of a "soft-armored" PERM such as a turf reinforcement mat (TRM) or a "hard-armored" PERM such as rock rip-rap. Soft-armored PERMs are generally more aesthetically pleasing because they incorporate vegetation; however, a combination of both PERMs could be implemented in order to minimize the potential for erosion and sediment transport within the Lost Lake watershed