Clearwater-Nez Perce National Forest Summary of Prone Delineation for Forest Plan Revision

Clearwater National Forest

Introduction

The Clearwater National Forest delineation for landslide prone (LSP) is based on a slope break of 55 percent using a digital model (DEM). Polygons delineating the 50 landtype (LT) series, which are the / landtypes mapped in the Clearwater System Inventory, were included also in the (LSP) layer. The 50 LT include some 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 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 and 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 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 . 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 (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 , 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 and 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) terraces/valley bottoms (0.70 slides/1000 acres) Colluvial midslopes (0.54 slides/1000 acres) Low-relief hills and frost-churned (few) 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 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 , relation to faulting, and slope of 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 area above the slide origin. • Old mass wasted (LT 50) on Clearwater Forest, consisting of large, ancient rotational failures exhibit high landslide failure when disturbed by . • 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 • -on-snow events such as December 1973 and January 1974 along with warm temperatures over saturated and dense snow with high 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 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, and 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 strength and the time it takes for the to rot in soil after tree is . 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 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. The reason that landslide prone risk needs to be considered in Forest Plan revision is because these events occur periodically over time and have occurred 7 times between 1919 and 1996.

Summary of paper by Dale Wilson on Identification of Landslides on the Clearwater Forest

Dale Wilson, 1994, “Subjective Techniques for Identification and Assessment of Unstable Terrain”

This technique for recognizing unstable terrain developed by Dale Wilson supports the idea that the highest risk slopes are the breakland landtypes and the slump/earthflow landytpes. Slope characteristics, drainage dissection, slope shape, soil depth and texture, and slope saturation by water on these slopes can increase the risk of slope instability.

Classification of landslides • Methodology for this model was developed from information from the landslides on the Clearwater, mapped and inventoried in 1974-1976 by Megahan, Day and Bliss. • Rotational mass wasting (slump, -flows) identified as the first mass wasting category. • and debris torrents are the second main type of mass wasting.

Slope Shape, soil depth and slope steepness

• Concave stream basins, steep draws, and long steep, dissected slopes are areas where landslides and slumps start.

• Slump/earth-flow landtypes with deep soil mantles that become saturated by water result in deep-seated rotational failures.

• Slump earth-flow landtypes have the characteristics where mass wasting potential is rated high: slope range 40-60 percent, soils are deep, 60 inch soil depth, highly dissected landforms are common, soil texture are sandy to loamy , and mica is present in the soils.

• Debris high risk rating is based on >60 percent slopes, concave or planar slopes, frequent old slide scars, and soil textures sandy to loamy .

Nez Perce National Forest Landslide Prone Cover

The basis for the Nez Perce National Forest Landslide Prone cover that will be used for Forest Plan Revision analysis and interpretations is based on a slope break of greater than 60 percent using DEM’s and addition of the slump/earthflow polygons from the Nez Perce Landtype Cover, landtypes 50 CUU (landslide, 20-50% slope, weathered rock) and landtype 50EUU (landslide, 45-80% slope, weathered rock) These landtypes are included due to the unstable of the slump/earthflow .

Nez Perce National Forest LISA modeling for Using the LISA Model by Richard Kenedy and Rodney Prellwitz, 1987.

The landtype mass erosion hazard ratings are used as a preliminary screen to identify areas with potentially high, moderate or low mass failure hazard. The slope stability hazard classes were developed for the soil survey by Rod Prellwitz and Rich Kennedy in February, 1987. These soil survey units are large polygons of 5 to more than 1000 acres, and usually include areas that differ from the modal information used to develop the hazard rating. Hazard ratings are based on application of the LISA slope stability model to dominant mid-slope soil, slope, moisture and characteristics of each landtype. Landtypes with a factor of safety less than 1.00 have a High Hazard Rating. Landtypes with a factor of safety from 1.00 to 1.20 have a Moderate Hazard Rating, and landtypes with a factor of safety >1.20 have a Low Hazard Rating. Most of the landtypes classified as landslide prone often have a range of hazard rating from moderate to high. The land areas modeled were assumed to be clear-cut prescription. Landtypes where concentrations of soil moisture (e.g. in RHCAs) increase the hazard from Moderate to High in these local zones, have a rating of High when Wet. These can usually be spatially referenced by the intersection of stream RHCAs and the appropriate landtypes.

The majority of the Forest Land of the Nez Perce Forest that is designated landslide prone is >60 percent slope and includes the 60 LT series. Landtypes 50CUU and EUU include slopes less than 60 percent, with LT 41 and LT 48 also including some slopes less that 60 percent The 50 LT’s are the slump/earthflow landtypes, and are a concern to management due to active slope failure over large areas.

Shown in Table XX below are the Map Unit Number, Slope Range for LT, Average Slope of LT, Mean Factor of Safety and the Hazard Class for the LT. This was calculated using the LISA slope stability model. The following table shows that by breaking out slopes greater that 60 percent, the majority of the landslide prone terrain is identified for interpretive land uses. Slump earthflow landtypes shown are less than 60 percent slope in most cases.

Table 2 Landslide Prone Summary of Analysis and Rating for Nez Perce Landtypes

Map Unit Slope Range Mean Slope Range for Mean Hazard for LT for LT Factor of Factor Class Safety of Safety for the for LT LT 41E67 40-100 65 1.01-2.51 1.56 M 48E67 60-80 70 .93-1.43 1.13 H 48E6P 60-80 75 .99-1.70 1.31 M/H 48E77 60-90 70 .89-1.63 1.21 M/H 50CUU 20-50 40 1.16-3.33 1.77 M 50EUU 45-80 71 .79-1.76 1.12 H 60E3F 50-70 65 .89-1.67 1.19 H 60E48 50-80 65 .79 -1.60 1.12 H 60E67 50-70 65 .99-1.69 1.30 M/H 61E14 60-90 62 1.07-2.11 1.54 M 61E24 60-90 70 .98-1.70 1.36 M/H 61E2E 50-80 64 1.03-2.05 1.44 M/H 61E38 60-90 68 .82-1.48 1.14 H 61E3F 50-70 65 1.02-1.92 1.37 M/H 61E48 60-90 65 .86-1.43 1.11 H 61E67 60-80 62 1.01-1.63 1.30 M/H 61E8B Dry 60-80 65 .95-1.38 1.15 H 61E8B Wet 60-80 65 .72-1.17 .92 H 61EHP 50-70 60 .90-1.63 1.22 M/H 63E38 45-80 60 .84.1.98 1.33 M/H

Literature Cited

Landslide Occurrence in the Western and Central Northern Rocky Mountain Physiographic Province in Idaho, 1989, Walter F. Megahan, Norman F. Day, and Timothy M. Bliss

Assessment of the 1995 and 1996 Floods and Landslides on the Clearwater National Forest, Part 1: Landslide Assessment, December 1996, Douglas E. McClelland. Randy B. Foltz, W. Dale Wilson, Terrance W. Cundy, Ron Heinemann, James A. Saurbier, Robert L. Schuster, USDA Forest Service

Natural Slope Stability Hazard Class for Landtype Limits, November, 1994, Pat Green, Draft, USDA Forest Service, Nez Perce National Forest

Nez Perce National Forest Initial Level I Slope Stability Analysis, February, 1987, Rodney W. Prellwitz, Draft, USDA Forest Service, Nez Perce National Forest

Subjective Techniques for Identification and Hazard Assessment of Unstable Terrain, 1994, Dale Wilson, USDA Forest Service, Clearwater National Forest

USDA Forest Service, Nez Perce National Forest, Nez Perce National Forest Soil Survey, Grangeville, Idaho, 1987.

USDA Forest Service. Slope Stability Reference Guide for National Forests in the United States, Volume I, EM-7170-13, Engineering Staff, Washington D.C. August, 1994

Wilson, Dale; Coyner, Jack, Deckart, Thomas. Land System Inventory of the Clearwater National Forest. Orofino, ID; U.S. Department of Agriculture, Forest Service, Clearwater National Forest; 1983. 400p.