Erosion and Sedimentation in the Pacific Rim (Proceedings of the Corvallis Symposium, August, 1987). IAHS Publ. no. 165. Size and location of colluvial landslides in a steep forested landscape STEVEN L. RENEAU & WILLIAM E. DIETRICH Dept. of Geology and Geophysics, Univ. of California, Berkeley, CA, USA 94720 ABSTRACT An inventory of 61 landslide scars in part of the central California Coast Ranges is used to document sites of instability and to infer conditions necessary for failure. Scar dimensions cluster around widths of 7-10 m, lengths of 10-20 m, and depths of 0.7-1.1 m. Simple theoretical analyses indicate that root strength along the margins of a potential landslide imposes constraints on landslide size; typical scar size may reflect the size of a deposit required for instability at sites with typical vegetation, slope gradients, soil texture, and hydrology. Hollows are the main source of landslides, consistent with the convergence of shallow groundwater flow and the long-term accumulation of colluvium in hollows, but conditions of sufficiently thick soil and high pore pressures for failure are also attained on side slopes. INTRODUCTION A central problem in landslide studies is to determine where and when conditions for failure can be satisfied. In steep, well-vegetated, soil-mantled landscapes, roots provide strength that can greatly increase the stability of a site (e.g., Burroughs & Thomas, 1977; Gray & Megahan, 1981; Ziemer, 1981; Riestenberg & Sovonick-Dunford, 1983; Tsukamoto & Kusakabe, 1984; Wu, 1984). The probability of failure at a site should increase as soils thicken and root penetration into bedrock decreases (Dietrich & Dunne, 1978), and therefore failure should be most likely at sites where soil thickness progres­ sively increases and where recurrent high pore pressures are produced. These conditions are met in hollows, where topographically induced convergence of colluvial debris and shallow groundwater flow occur, and is consistent with the common occurrence of landslides in hollows in many regions (see review in Reneau & Dietrich, 1987). Steps have been made towards quantifying the conditions necessary for failure by relating site stability to increasing soil thickness over time (e.g., Okunishi & Iida, 1981; Shimokawa, 1984). Although instability in shallow soils is generally treated as a one- dimensional problem, approached by an infinite slope analysis, recent workers have recognized that a pure infinite slope model for failure is insufficient because of strength provided by roots along the margins of a potential failure (Burroughs & Thomas, 1977; Wu, 1984). Riestenberg & Sovonick-Dunford (1983), Burroughs (1984), and Tsukamoto & Kusakabe (1984) have included such edge effects in slope stability analyses. Implicit in their stability equations are limitations on the size of a colluvial deposit that is stable under given conditions. Observations of landslide scars suggest that scar dimensions tend to have typical values in any area (e.g., Lehre, 1982; Tochiki, 1985), and this may record prevailing conditions of root strength, soil texture, hydrology, and slope gradient. In this study, the distribution and characteristics of 61 recent landslides in a portion of the central California Coast Ranges are used to document sites of instability and to infer conditions necessary for failure. The dimensions of these landslide scars cluster around characteristic values, and simplified theoretical analyses will be used to address the controls on scar size, and implications for the location and timing of failure. 39 40 Steven L.Reneau & William E.Dietrich STUDY SITE A 3.8 km2 portion of San Pedro Ridge in the Coast Ranges of Marin County, California, was selected for detailed study. The area has a Mediterranean climate, with mean annual precipitation of about 700 mm. Elevations in the study area range from near sea level to 322 m. The vegetation is predominantly a mixed hardwood forest composed of Califor­ nia laurel (Umbellularia californica), coast live oak (Quercus agrifolia), and Pacific madrone (Arbutus menziesii); the vegetation is a native community, and slopes are mainly undisturbed by human activity. Bedrock consists of interbedded arkosic sand­ stone and shale of the Franciscan assemblage (Rice et al, 1976). The sandstone beds vary from 0.1 to at least 15 m thick, and are generally steeply dipping, isoclinally folded, and faulted. Shallow landslides in colluvial soils were abundant in the study area during a major storm on 3-5 January 1982. These landslides mobilized as debris flows, causing exten­ sive erosion in downslope canyons and destroying two houses along one third-order chan­ nel (Smith & Hart, 1982; Reneau & Dietrich, 1987). The storm was a discrete event last­ ing 24 to 36 hours, with 24-hour rainfall totals of from 230 to 380 mm in southern Marin County. A gauge at the Marin County Civic Center, 2 km east of the study area, recorded 258 mm in 24 hours. Antecedent rainfall was high before the storm, with 209 mm falling in the previous 17 days, and 78 mm in the previous 6 days at the Marin County Civic Center. Rainfall intensity-duration-frequency relationships for the January 1982 storm were calculated for the Marin County Flood Control District by J. Goodridge (unpublished data). For two stations with the longest periods of record (26 and 27 years at Mill Valley and San Anselmo), the 12- and 24-hour intensities exceeded the estimated 100-year storm. Shorter duration intensities at these stations were less unusual, and 30-minute to 3-hour intensities did not exceed the 10-year storm. Locally, however, short-term inten­ sities were substantially greater. At Nicasio Dam, 24 km east of the study area, a 30- minute intensity of 37 mm was recorded. This is thrice the 13 mm intensities recorded at other stations, exceeding the estimated 100-year 30-minute intensities for the Mill Val­ ley and San Anselmo stations (26 and 29 mm, respectively). The 1-hour intensity at Nicasio Dam also exceeded the estimated 100-year event. The landslides in the study area all occurred within colluvial soils, and, except in rare cases, the failures completely mobilized as debris flows. The soils are typically gravelly loams, with 5-15% clay and 10-30% gravel. Failure planes were either within the collu- vium or near the bedrock-colluvium interface, as shown in Figure 1. Although roots are abundant in the landslide scarps, failures occurred below the depth of major rooting and few roots were observed crossing failure surfaces. The landslide scars are typically 10-20 m long, 7-10 m wide, 0.7-1.1 m deep, and less than 200 m3 in volume. Data on scar size and topographic setting are shown in Figure 2 and Table 1. Additional landslide data from Marin County, collected by Lehre (1982), are also shown in Table 1, and will be referred to later. Scar dimensions tend to be log-normally distributed (e.g., Lehre, 1982), and this biases average values towards the larger scars. The mean of log-transformed data more accurately represents typical scar size, and both these and arithmetic means are shown in Table 1. Colluvium exposed within eight landslide scars on San Pedro Ridge has been dated using radiocarbon analysis of charcoal, providing ages of 9000-25,000 years BP for the lower portions of these deposits (Reneau et al., 1986, and unpublished dates). Two dated cross sections are shown in Figure 1. The dates place upper limits on the time since major evacuation last occurred at each site, although unconformities may be present and landsliding therefore more frequent (Reneau et al., 1986). Relative soil profile develop­ ment suggests that significantly older colluvium, with redder hues and higher clay con­ tent, is exposed in only three 1982 landslide scars. Colluvial landslides 41 colluvium approximate bedrock surface 11,670 +1680 -1390 yr BP FIG. 1 Scat cross-sections, showing location of radiocarbon dates, (a) Lindenwood Court landslide, and (b) Mosquito landslide (San Rafael 2 and 5 sites of Reneau et al, 1986). Topographic Setting Side Slope (P> <»0°) hoMl ^ Subtle Hollow (p=10-30°) \^<X Distinct Hollow (P =30-50°) _i_@ L_B_ A . m . A , El 0 100 200 300 400 500 600 700 VÏ200 1300 V2600 Scar Volume (m3) Average Scar Width (m) Average Slope Gradient FIG. 2 Histograms of (a) scar volume, (b) scar length, (c) average scar depth, (d) average scar width, and (e) average slope gradient, showing topo­ graphic setting of each scar. Data for 61 landslides from January 1982 storm. Convergence angle, /?, is illustrated, and study area location is shown. 42 Steven L.Reneau & William E.Dietrich TABLE 1 Landslide dimensions from Marin County, California (data in m; scars in brush and grass from Lone Tree Creek basin, Lehre, 1982) Arithmetic Mean Mean of Log-Transformed Data Vegetation n length width 1/w length width 1/w mixed hardwood forest 61 20.2 8.8 2.3 16.8 8.5 2.0 northern coastal scrub 46 19.1 6.8 2.8 14.6 6.2 2.4 grass 22 11.3 5.6 2.0 9.2 5.3 1.7 LANDSLIDE LOCATION In order to provide a quantitative comparison of site topography, an angle of topographic convergence is used here, defined as the angle between the orientation of the hollow axis and the orientation of the adjacent side slopes (Fig. 2). In this paper, hollows are arbi­ trarily grouped as either distinct, with convergence angles of roughly 30-50°, or subtle, with angles of 10-30°. Convergence angles into hollows rarely exceed 50°, and below 10° concavities are very difficult to recognize in the field; as such, slopes with little or no con­ vergence are classed as side slopes. Within the study area, hollows were the most important source of landslides, account­ ing for 62% of the scars by number and 77% by volume.