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Landslide Prone Definition

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Landslide Prone Definition Clearwater-Nez Perce National Forest Summary of Landslide Prone Delineation for Forest Plan Revision Clearwater National Forest Introduction The Clearwater National Forest delineation for landslide prone terrain (LSP) is based on a slope break of 55 percent using a digital elevation model (DEM). Polygons delineating the 50 landtype (LT) series, which are the slump/earthflow landtypes mapped in the Clearwater Land System Inventory, were included also in the (LSP) layer. The 50 LT include some mass wasting areas less than 55 percent. Data and published literature from the 1995 and 1996 landslide study, and from the 1973 and 1974 landslide study are summarized below to support the use of the DEM model to break slopes at 55 percent for landslide prone terrain. A paper, written by Dale Wilson, retired Clearwater National Forest soil scientist, on methodology to delineate mass wasting areas on the Clearwater National Forest also supports the delineation of landslide prone areas used for Forest Plan Revision. Summary of the 1995/1996 Landslide Study “Assessment of the 1995 and 1996 Floods and Landslides on the Clearwater National Forest” Part I: Landslide Assessment by McClelland, Foltz, Wilson, Cundy, Heinemann, Saurbier, and Schuster. This summary of the landslide study will only address the data from the literature cited that are supporting documentation for using slopes greater than 55 percent as a break for landslide prone terrain, and the addition of the 50 LT polygons as part of the landslide prone cover for Forest Plan Revision. Only the natural slides will be used in this portion of the summary, as slides associated with roads or timber harvest, have slide origins that are associated with management affects. Origin of occurrence of all landslides, related to management plus natural are discussed in the next section. The landslide 1995/1996 landslide study shows number of landslides by slope steepness and land use. Natural slides occurred at the following rates:17 natural slides occurred on slopes less than 40 percent, 33 slides occurred on 41-50 percent slopes, 14 slides occurred from 51 to 55 percent slopes, 207 slides occurred on greater than 55 percent slopes. This table supports a break at 55 percent for the highest occurrence of natural slides occurring on greater that 55 percent slopes. Natural landslides occurring on breakland slopes Of the 236 natural landslides that occurred in 1995 and 1996 landslide study, around 200 slides out of 236 occurred on breakland slopes and mass wasted slopes. Most of the natural landslides occurred on Idaho Batholith geology (150 slides) and Border Zone geology (99 slides). Most of the slides occurred below 5000 feet (250 slides) and most often on south, southwest and west slopes. This summary supports the fact that breakland landtypes fail naturally more often than other landtypes. Using the 55 percent slopes break to delineate the landslide prone terrain is supported with the slope data from the landslide study. The breakland slopes are delineated at 60 percent and above, but the landslide study shows high occurance down to 55 percent. Occurrence of All Landslides, Management and Natural The following is a discussion of natural plus management related landslide occurrence from the 1995 and 1996 event. When looking at all slides whether they were related to roads and harvest or natural, occurrence was at the highest rates on slopes of 50 percent or greater. The highest occurrence of all slides is on mass wasting or breakland slopes. There was 2.0 slides per1000 acres on slopes greater than 56 percent, which is 2 to 3 times higher than other slopes breaks. The next highest occurrence of slides was on slope 46 to 50 percent at .73/1000 acres. The mass wasted slopes had the highest occurrence at 1.72 slides/1000 acres and breaklands at 1.12 slides per 1000 acres. Landslides occurred on other landforms, but at a lower rate. The data for all slides supports the >55 percent slope break for the highest occurrence of landslides in this study and the inclusion of the 50 mass wasting landtypes where landslides occurred in the 40-60 percent slope range. Table 1. Summary of Five Landslide Prone Factors and Rate of Landslide Occurrence During storm and flood events in the 1995-1996 period, over 860 landslides occurred across the Clearwater National Forest. A survey was conducted to review these landslides and five factors were identified to assess the inherent risk of landslides on the Clearwater National Forest (McClelland et al. 1997). The analysis was based upon an inventory of landslides that occurred on the Forest during storm events in the fall of 1995 and the winter/spring period of 1996. The information reported by McClelland was modified, based on corrections made to the landslide database (Clearwater National Forest 2000). The five factors, which confirmed previous findings by Day and Megahan (1977), are: Border Zone metamorphics (1.06 slides/1000acres) Belt Series meta-sediments (0.56 slides/1000 acres) Geology Idaho Batholith granitics (0.28 slides/1000 acres) Volcanics (0.16 slides/1000 acres) Sediments (0.16 slides/1000 acres) > 56% (2.00 slides/1000 acres) 46-50% slopes (0.73 slides/1000 acres) Slope 51-55% slopes (0.59 slides/1000 acres) 41-45% slopes (0.43 slides/1000 acres) < 35% (relatively few). Mass wasted slopes (1.72 slides/1000 acres) Breaklands (1.12 slides/1000 acres) Landform Stream terraces/valley bottoms (0.70 slides/1000 acres) Colluvial midslopes (0.54 slides/1000 acres) Low-relief hills and frost-churned ridges (few) Aspect South (21.8% of the slides) Southwest (20.8%) West (16.8%) Southeast (14.9%) Northwest, north, and northeast aspects (few) 3001-3500’(1.66 slides/1000 acres) < 2000’ (1.65 slides/1000 acres) 2501’-3000’ (1.48 slides/1000 acres) 3501’-4000’ (1.10 slides/1000 acres) Elevation 2001’-2500’ (0.90 slides/1000acres) 4001’-4500’ (0.85 slides/1000 acres) 4501’-5000’ (0.50 slides/1000 acres) > 5000' (relatively few) Summary of supporting data from the 1973 and 1974 landslide event. Megahan, Day and Bliss, 1978. “Landslide Occurrence in the Western and Central Northern Rocky Mountain Physiographic Province in Idaho” Landslide and Slope Relationships • Geology types where slides occurred are border zone, belt series, and Idaho Batholith. • Other factors that affect landslides are, depth of weathering, relation to faulting, and slope of bedrock when it parallels the slope of the side-slope. • 21 percent of landslides occurred on slopes less than 50 percent in the study. • Slides were most common between 51% and 70% slope and accounted for 47%, most of the landslides. • Slides on 71% - 90% slope accounted for 20% of all landslides. • Landslides were rare above 90 percent, but can occur up to 150 percent slopes. • Quote “We found that most slides occurred on slopes of about 60 percent, while slides are rare on slopes over 90% • Up to 95 percent of the landslides occurred in small watersheds with very little drainage area above the slide origin. • Old mass wasted lands (LT 50) on Clearwater Forest, consisting of large, ancient rotational failures exhibit high landslide failure when disturbed by road construction. • The break-lands (LT 61 and 63) on the Clearwater Forest are the most hazardous for landslides related to road construction. • Micaceous rocks that are deeply weathered are the most unstable. • Landslide occurrences range from high to low for Border Zone, Belt Series, and Batholith rocks. This varies by weathering, fracturing, and strata paralleling hill- slopes. Climatic Influences • Rain-on-snow events such as December 1973 and January 1974 along with warm temperatures over saturated soils and dense snow with high water content produced a high number of landslides. Many more slides occurred during this year than the following two years without major climatic events. • “Climatic events produce an inordinate amount of erosion activity. Responses are exponential, rather than linear, thus, extreme climatic events produce an inordinately high number and volume of landslides” • False sense of security with “normal years, with the risk of climatic events always present.” Landslide Occurrence • 58% of the slides were associated with roads. • Roads, logging and fire combined accounted for 30 percent of the slides. • 88% of the slides were associated in someway with a road in combination with logging or fire. • Nine percent of slides related only to logging. • Only 3 percent of slides were associated with natural slopes • Slide occurrence related to root strength and the time it takes for the roots to rot in soil after tree is cut. One study says 4-10 years, another 3-5 years, in general less than 20 years for roots to rot. Slides occurred in one study 4-10 years after cutting, another study3-5 in Alaska and another study 16 years. Summary of Findings The findings from this paper support the slope break used for the Clearwater National Forest for Forest Plan Revision Landslide Prone GIS layer. The slope break that is used for landslide prone terrain is 55 percent. The break-lands (LT 61 and 63) on the Clearwater Forest are the most hazardous for landslides related to road construction. The study above showed that most slides occurred between 50 and 70 percent slopes. Slides also occur on mass wasting landtypes on slopes below 55 percent. This landslide study is only one point in time tied to storm events, such as heavy rainfall from winter storms that move across northern Idaho from the Pacific Ocean, melting of existing snow due to the heavy rain, rain-on-snow events and triggering of landslide events by water and snowmelt on saturated soils. Natural landslide occurrence is much lower on the Clearwater Forest, and is usually tied to this type of storm when many slides have occurred in a short time period.
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