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Figure 2a. Channel networks (solid line), source basins (outlined with solid line), and hollow axes (dashed lines) determined from field work in three landscapes. Maps are portrayed at the same scale with 40-foot contour intervals. (A) A portion of the Rock Creek catchment, central coastal Oregon. Due to the dense old-growth coniferous canopy the fine scale topography is not accurately mapped even in the most recent 1:24,000-scale mapping (US Geological Survey, 1984). Consequently, the contours do not reveal all unchanneled valleys. (B) A portion of the Fern Creek catchment, San Dimas Experimental Forest, southern California. Redrawn from a map made by Maxwell (1960). (c) Southern Sierra Nevada. (from Dietrich and Dunne, 1993). Figure 2b. Field measured extent of channels, areas of estimated thick colluvium and landslide scars in a portion of the Mettman Ridge site shown in Figure 1. Border of area mapped also shown (field work done by D. Montgomery). All landslides occurred between 1987 and 1997 after clear cut logging. It was not always possible to separate failure scar from runout disturbance, so some elongate scars include both. Topographic base map has 5 m contours and is derived from a 2 m grid of the laser altimetry data. Figure 3. Cartoon (from Dietrich et al., 1982) to illustrate the observation that colluvium-mantled bedrock valleys may experience a cycle of accumulation and discharge of colluvium (drawing by L. Reid). Although highly idealized, the essential idea (which is supported with radiocarbon dating, field mapping, and theory) is illustrated: progressive accumulation of colluvium in the valley axis (from B to C) , followed by discharge (A), and then refilling. Instability may result from progressive thickening alone, or be influence by fire, disease, logging, roads or other disturbances. Figure 4. The one-dimensional approximation used in the slope stability model in which the failure plane, water table, and ground surface are assumed parallel. The slope is q, the height of the water table is h, and the thickness of the colluvium that slides above the failure plane is z. Typically, the failure plane is at the colluvium- weathered bedrock or saprolite boundary.Figure 5. Definition of stability fields (from Montgomery and Dietrich, 1984). For this particular example, the angle of internal friction is 45 degrees, and the bulk density ratio is 1.6. Figure 6. The proportion of the colluvium thickness that is saturated at failure (h/z) in the Mettman Ridge study area. Friction angle is 45 degrees, bulk density ratio is 1.6, and contour interval is 5 m. Figure 7. Plan view and cross section of area draining across a contour of length, b. In the cross section, the heavy line depicts the ground surface. The stippled area is the shallow subsurface flow and saturation overland flow with discharge of TMb and udb, respectively. Here q equals the precipitation, p, minus evapotranspiration, e, and deep drainage, r; a is drainage area and h and z are the thickness of the saturated subsurface flow and the thickness of the potential unstable mass, respectively (each measured vertically). In SHALSTAB, the conductivity is assumed to drop significantly below the failure plane and the saturation overland flow, consisting of the product of the mean flow velocity, u, d (measured normal to the ground surface, and b, is not calculated. T is the transmissivity and M is sin q (from Dietrich et al., 1992).Figure 8a. Spatial pattern of the topographic ratio a/(bsin q) that controls the shallow subsurface pore pressure development (h/z) at the Mettman Ridge study site. Contour interval is 5 m and grid size is 2 m. The legend gives the values of the ratio in meters.Figure 8b. Spatial pattern of a/b at the Mettman Ridge study site. Contour interval is 5 m and grid size is 2 m. The legend gives the values of the ratio in meters. sin(theta) is equal to sin q.Figure 9. The proportion of the colluvium thickness (h/z) that is saturated for a given log(q/T) at the Mettman Ridge. Not that the larger the negative number, the smaller the ratio q/T and the smaller the precipitation needed to cause instability. Unit of q/T is 1/meters. Value of log(q/T) is given in the upper right corner of each of the 6 maps. Contour interval is 5 m and grid size is 2 m. Figure 10. Landslide stability fields for the Mettman Ridge area. Value of log(q/T) for each map is given in the upper right corner of each of the 6 maps. Contour interval is 5 m and grid size is 2 m. Figure 11. Relationships among log(q/T), surface slope and a/b for the a bulk density of 1.6 and friction angle of 45 degrees in SHALSTAB. The heavy parallel lines correspond to the log(q/T) value used in various plots on runoff and slope stability. Figure 12a. Log(q/T) values for instability at the Mettman Ridge study area. Contour interval is 5 m and grid size is 2 m. Figure 12b. Log(q/T) values for instability at the Mettman Ridge study area. White areas are cells that are above a threshold of channelization based on drainage and slope (i.e. Montgomery and Dietrich, 1992) and hence represents the channel network. Note that the estimated channel network closely corresponds to areas with log(q/T) <-3.4 . Contour interval is 5 m and grid size is 2 m. Figure 13. SHALSTAB stability field relationships. Particular boundaries correspond to a friction angle of 40 degrees, bulk density ratio of 2 and a T/q of 350 m. The dashed line is the threshold of saturation: for a give slope any site with an a/b value above the line will be saturated (from Montgomery and Dietrich, 1994). Figure 14. Plots of contributing area per unit contour length (a/b) versus slope (tan q) for convergent (circles) and divergent (crosses) elements in (a) Tennessee Valley, California (see Dietrich et al., 1992, 1993); Mettman Ridge (based on 5 m contour data derived from aerial photography before clear cutting began), and Split Creek (based on 5 m contour data derived from aerial photography). The T/q values on the saturation threshold solid lines correspond to log (q/T) values of -3.5, -2.5 and -2.2 from top to bottom in each graph. Dashed lines represent limit to slope stability model(from Montgomery and Dietrich, 1994).Figure 15. Comparison of the spatial distribution of log(q/T) values for instability using 30 m grid data obtained from the USGS and 6 m grid data derived from topographic maps created for the BLM from aerial photographs.. Figure 16. Comparison of the spatial distribution of log(q/T) values for instability using 10 m grid data derived from digitized 40 ft USGS contour lines and from topographic data derived from 2 m grids for the same area based on laser altimetry. Figure 17a. Landslide density (number of mapped landslides divided by the area in the log(q/T) category) for in-unit failures in the Noyo River watershed. Site is in Mendocino County of Northern California and landslides were mapped from 1996 aerial photographs by John Coyle. Categories are in - 0.3 units of log(q/T). Chronic means slope ³ friction angle (45 degrees in this case). Stable sites are too low a gradient to fail even if saturated, so effective precipitation needed for instability increases from left to right on this graph. Random scar size was 800 m2 and the median mapped scar size was 562 m2.Figure 17b. Landslide density (number of mapped landslides divided by the area in the log(q/T) category) for road related failures in the Noyo River watershed. Site is in Mendocino County of Northern California and landslides were mapped from 1996 aerial photographs by John Coyle. Categories are in - 0.3 units of log(q/T). Chronic means slope ³ friction angle (45 degrees in this case). Stable sites are too low a gradient to fail even if saturated, so effective precipitation needed for instability increases from leftto right on this graph. Figure 18. Comparison of stability fields and mapped landslide scars in the Oregon Coast Range. Landslide data were provided by Barry Williams of the Bureau of Land Management and were obtained by field mapping. Stability fields (as shown in Figures 13 and 14) are for a friction angle of 45 degrees and a bulk density ratio of 1.6. A value of log(q/T) = -3.1 effectively separates unstable from stable domains. Figure 19. Cumulative percent of the watershed drainage, number of mapped landslides, and random landslides for given log(q/T) categories for the Noyo watershed, California. Figure 20. Map of a portion of the Noyo watershed in northern California showing the distribution of high hazard (log(q/T² -3.1) in black, moderate to low hazard in gray and stable areas in white. Also shown is the channel network steeper than 5% (slopes less than 5% will not transmit debris flows). Contour interval is 40 ft. Much of this watershed is managed by Louisiana-Pacific for timber production. Figure 21. Cumulative percent of the watershed area in corresponding log(q/T) slope stability categories for 6 watersheds in Northern California. Numbers on the curves record the number of landslides per km2 of the entire drainage basin mapped by John Coyle from 1996 aerial photographs.
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Copyright 1998, William Dietrich and David Montgomery
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