Delineation of preventative landslide buffers along steep streamside slopes in northern California
This article is part of a larger document. View the larger document here.Abstract
Green Diamond Resource Co (GDRCo) applies tree retention buffers to steep slopes along fish bearing (Class I) and non-fish bearing (Class II) streams that are in addition to the standard riparian management zones associated with timber harvest plans. These Steep Streamside Slope (SSS) buffers were designed to reduce the amount of sediment delivering to watercourses as a result of landslides generated by forest management related operations. Initial default buffers were developed through a landslide study during the planning stages of GDRCo’s Aquatic Habitat Conservation Plan (AHCP). The continued evaluation of streamside landslides across the property is one of the AHCP’s long term research projects, which is aimed at re-defining the SSS “default” prescriptions using a spatially distributed probability based sampling design. Through continued research, our goal is to refine the minimum slope gradients and maximum slope distances associated with the Steep Streamside Slope criteria for each of the 11 Hydrographic Planning Areas (HPA) identified in GDRCo’s AHCP. The HPA to be re-evaluated was the Coastal Klamath HPA, which is a grouping of several watersheds near the mouth of the Klamath River in Humboldt and Del Norte Counties. Sampling consisted of 93 half-mile long stream segments that represent roughly ten percent of the lineal distances of Class I streams and five percent of the lineal distances of Class II streams within this HPA. The sampling method used insured both random selection and spatial distribution of the half mile segments within the study area. The study focused on those landslides that were non road-related, active to historically active, and had observably delivered sediment to a watercourse. The landslide data collected included causal factors, slope characteristics, cross sections, and dimensions of the source and debris, of more than 400 landslides in this region. Using a topographic ruggedness model on a 50 m LiDAR-based DEM and field-based geologic mapping of the area, we divided the area into distinct morphologic units. The results from the analysis of the landslide data provide new slope gradient and distance thresholds for the SSS default prescriptions. These new SSS prescriptions are unique to each of these morphologic units and provide site specific protections to areas prone to streamside landsliding within the Coastal Klamath HPA. A similar approach will be used for the remaining 10 HPAs.