A PRELIMINARY COMPARISON OF ROCK STRENGTH AND TURBIDITY IN THE COASTAL BELT FRANCISCAN ASSEMBLAGE, SOUTH FORK WAGES CREEK VERSUS SOUTH FORK CASPAR CREEK, USING THE HOEK-BROWN CRITERION, MENDOCINO COUNTY, CALIFORNIA: IMPLICATIONS FOR METHODOLOGY IN UNDERSTANDING TURBIDITY WITHIN HARVESTED WATERSHEDS

Our study compares rock strength between the South Fork Wages Creek watershed and the South Fork Caspar Creek experimental watershed in Mendocino County, California. The SF Caspar Creek watershed has been studied by the USFS and State of California since the 1980’s, researching the effects of historic and ongoing timber management. The SF Wages Creek watershed has been managed for timber resources by various landowners; most recently by the Lyme Timber Company. Published geological maps show both watersheds as being underlain by the same rock unit, the Coastal Belt Franciscan Assemblage, deformed rocks accreted to the western North American coastline. Both watersheds have historically and recently undergone timber harvest activities with similar levels and types of harvest. Turbidity studies conducted after activities show higher turbidity values measured in the SF Caspar Creek. The Generalized Hoek-Brown Criterion, a method used to estimate the strength of a rock mass based on an evaluation of the physical properties of a rock exposure, was used to characterize rock strengths in both watersheds. Rock strength and deformational properties were determined by estimating Uniaxial Compressive Strength, Material Constant, Modulus Ratio, and Geological Strength Index at sample locations using an ESRI based Collector app. Bedding orientations, where present, were also measured. Preliminary results suggest higher rock strengths in the SF Wages Creek watershed where intact turbidites were observed compared to deformed rock in the SF Caspar Creek. Ongoing work includes rebound hammer measurements, laboratory testing including sodium sulfate soundness, slake durability, L.A. abrasion testing, as well as Wolman Pebble Count surveys. Preliminary results emphasize the importance of characterizing rock strength and susceptibility to weathering when evaluating water quality and management activities, and that geologic mapping of the Coastal Belt Franciscan Assemblage along the Mendocino County coastline could be better differentiated.