Using Isopleth Maps of Landslide Deposits as a Tool in Timber Sale Planning

Abstract

Timber sales on the Sierra National Forest are encountering increasing landslide hazard. Soil loss from debris flows can lead to long-term losses in forest productivity. Isopleth mapping of landslide deposits is the technique currently employed to estimate relative landslide susceptibility for planning timber sales and harvest activities that minimize soil erosion. Established procedures are used to transform landslide inventory maps into isopleth maps. Relative landslide susceptibility is defined by the percent of a standard area underlain by landslides. Hazard zonation of the timber sale is defined by contours of certain percent values: a) negligible (less than 1 percent), b) low (1–10 percent), c) moderate (10–30 percent), d) high (30–50 percent), e) very high (50–70 percent), and f) extreme (70–100 percent). This quantitative approach permits consistent representation of anticipated landslide hazard between different areas. Isopleth maps identify whether landslide hazard poses a critical concern early in the process of planning a timber sale. For example, twelve of twenty-eight proposed timber sales involved areas with only negligible or low landslide susceptibility. Where susceptibility was greater, specific areas requiring detailed field investigation were readily identified. The degree of landslide activity and related on-site conditions lead to formulation of recommendations for adjusting cut unit boundaries, harvest method, or road design. This timely assessment of landslide hazard reduces the need for last minute adjustments in sale plans or the consequence of landslide occurrence after harvest.