kChapter 4 GERALD F. WIECZOREK LANDSLIDE TRIGGERING MECHANISMS 1. INTRODUCTION 2.INTENSE RAINFALL andslides can have several causes, including Storms that produce intense rainfall for periods as L geological, morphological, physical, and hu- short as several hours or have a more moderate in- man (Alexander 1992; Cruden and Vames, Chap. tensity lasting several days have triggered abun- 3 in this report, p. 70), but only one trigger (Varnes dant landslides in many regions, for example, 1978, 26). By definition a trigger is an external California (Figures 4-1, 4-2, and 4-3). Well- stimulus such as intense rainfall, earthquake shak- documented studies that have revealed a close ing, volcanic eruption, storm waves, or rapid stream relationship between rainfall intensity and acti- erosion that causes a near-immediate response in vation of landslides include those from California the form of a landslide by rapidly increasing the (Campbell 1975; Ellen et al. 1988), North stresses or by reducing the strength of slope mate- Carolina (Gryta and Bartholomew 1983; Neary rials. In some cases landslides may occur without an and Swift 1987), Virginia (Kochel 1987; Gryta apparent attributable trigger because of a variety or and Bartholomew 1989; Jacobson et al. 1989), combination of causes, such as chemical or physi- Puerto Rico (Jibson 1989; Simon et al. 1990; cal weathering of materials, that gradually bring the Larsen and Torres Sanchez 1992)., and Hawaii slope to failure. The requisite short time frame of (Wilson et al. 1992; Ellen et al. 1993). cause and effect is the critical element in the iden- These studies show that shallow landslides in tification of a landslide trigger. soils and weathered rock often are generated on The most common natural landslide triggers are steep slopes during the more intense parts of a described in this chapter, including intense rainfall, storm, and thresholds of combined intensity and rapid snowmelt, water-level change, volcanic erup- duration may be necessary to trigger them. In the tion, and earthquake shaking, and examples are pro- Santa Monica Mountains of southern California, vided in which observations or measurements have Campbell (1975) found that rainfall exceeding a documented the relationship between triggers and threshold of 6.35 mm/hr triggered shallow landslides landslides. Some geologic conditions that lead to that led to damaging debris flows (Figure 4-4). susceptibility to landsliding caused by these triggers During 1982 intense rainfall lasting for about 32 are identified. Human activities that trigger land- hr in the San Francisco Bay region of California slides, such as excavation for road cuts and irriga- triggered more than 18,000 predominantly shallow tion, are not discussed in this chapter. To the extent landslides involving soil and weathered rock, possible, examples have been selected that illustrate which blocked many primary and secondary roads landslide damage to transportation systems. (Ellen et al. 1988). Those landslides whose times 76 FIGURE 4-1 Landslide blocking State Highway 1 near Julia Pfeiffer-Burns State Park, California: debris slides and flows from toe and flanks of reactivated landslide. Intense storm Februdry 28—March 1, 1983, triggered many debris flows that blocked primary and secondary roads along Big Sur coastline. G.F. WIF(70REK, MARCh 125, 933 FIGURE 4-3 Landslide blocking State Highway 1: excavation of cut of estimated 6.1 million .. RV m3 in 6-m-wide ' 7 W!i1; •3.: ,,'. '' D ? ; 4t benches extending about 300 m above roadbed made this the largest highway repair job ';: undertaken in California history. Highwayropennd in April 1984 (Works 1984). CALIFORNIA 1)EPARTMENT OF TRANSPORTAThON FIGURE 4-2 (above left) Landslide blocking State Highway 1: May 1, 1983, massive rock slide of 1.2 million m3 incorporated entire hillside. Exceptionally heavy rainfall during winters of 1981-1982 and 1982-1983 was responsible for raising groundwater levels and triggering slide. During excavation, groundwater flow of approximately 378,000 Llday was collected and drained from cut (Works 1984). CALIFORNIA DEPARTMENTOFTP,ANSF'ORTATION 78 Lands/ides: /nvestigation and Mitigation FIGURE 4-4 700 Cumulative rainfall at selected recording 25 gauges in Santa 600 Monica and San z E Gabriel Mountains, ' 20 500 -J —j southern California. —j —I 4 4 Known times zLI. U. of debris flows SAN DIMAS 400 Z 15 F— TAN BARK FLAT 4 indicated by heavy cc dots. Steepness of F- LECHUZA PT Ui — I- STAFC3528 300 > cumulative rainfall ------------- ---- -- -- I- 10 --- 4 line indicates BELAIRFC 108 intensity of rainfall SEPULVEDA DAM 200 (modified from F BURBANK VALLEY \ ' LOS ANGELES CIVIC CTR PMPPLT Campbell 1975). 100 FLINTRIDGE FC 2808 il 1''11 11 111 11 111 1 I 111111111111111, 11111111.1111 E BEEEE EE E EEE EEEE BEEBE EEEE EE E EEEE E EE EEEE BE EE E E 0.00 aa.o d.a ,i 0 d.0.o 0.0.000.0. d d.d.o , d.Q.0 QQ 000.0.00 ä.ä. a d. j coc.awcqWCJ wc.Jwccoc.JwrJ C0CgC0NWOJWCWCIW1WC €004 ,00JCOCSJ tgWCvWOJw'cucOOJ w JANUARY 1969 FIGURE 4-5 Rainfall thresholds that triggered abundant landslides in San Francisco Bay region, California. Thresholds for high E and low mean annual L Threshold for high MAP 1 20 precipitation (MAP) . I- Threshold for I 0wMAP areas are indicated as C curves representing got combination of /1 I C 0.4 rainfall intensity and S duration (modified _110 from Cannon and 02 S. Ellen 1985). 10 15 20 25 30 35 40 45 Duration (hours) could be well documented were closely associated sures, is generally believed to be the mechanism by with periods of most intense precipitation; which most shallow landslides are generated dur- this documentation permitted identification of ing storms. With the advent of improved instru- landslide-triggering rainfall thresholds based on mentation and electronic monitoring devices, both rainfall intensity and duration (Figure 4-5) transient elevated pore pressures have been mea- (Cannon and Ellen 1985). Such thresholds are sured in hillside soils and shallow bedrock during regional, depending on local geologic, geomorphic, rainstorms associated with abundant shallow land- and climatologic conditions. sliding (Figures 4-6 and 4-7) (Sidle 1984; Wilson The rapid infiltration of rainfall, causing soil and Dietrich 1987; Reid et al. 1988; Wilson 1989; saturation and a temporary rise in pore-water pres- Johnson and Sitar 1990; Simon et al. 1990). Landslide Triggering Mechanisms Loose or weak soils are especially prone to land- 1.50 30 slides triggered by intense rainfall. Wildfire may — INTENSITY — -------- 1.00 -- produce a water-repellent (hydrophobic) soil layer 20 below and parallel to the burned surface that, to- gether with loss of vegetative cover, promotes rav- eling of loose coarse soil grains and fragments at UZ :::: the surface. Increased overland flow and nIl for- mation then lead to small debris flows (Wells 1987). On the lower parts of hill slopes and in NEST 2 stream channels, major storms generate high sedi- ment content in streams (hyperconcentrated flows) or large debris flows (Scott 1971; Wells et al. 1987; Weirich 1989; Florsheim et al. 1991). 75 cm DEPTH Shortly after midnight on January 1, 1934, an ----150cm DEPTH intense downpour after more than 12 hr of rainfall -100 resulted in debris flows from several recently 100 burned canyons into the La Canada Valley of NEST 3 southern California and caused significant prop- erty damage and loss of life (Troxell and Peterson 1937). Following an August 1972 wildfire north of Big Sur in central coastal California, storms -- - IOcmDEPTH with intensities of 19 to 22 mmfhr triggered two 75 cm DEPTH episodes of debris flows. During the second, more 120 cm DEPTH -100 devastating storm on November 15, 1972, large debris flows reached Big Sur about 15 min after in- 100 NEST 4 — 75 cm DEPTH tense (22-mmfhr) rain (Johnson 1984). Debris ----125 cm DEPTH flows blocked California State Highway 1 with mud, boulders, and vegetative debris; the flows 1r\r\ -I, , '1 ,' - •'—. '\ partly buried, heavily damaged, or leveled struc- I I I tures and caused one fatality (Jackson 1977). .s, '. I • — ..I In and regions, intense storms can trigger debris - flows in thin loose soils on hillsides or in alluvium 0 24 48 72 96 120 144 168 in stream channels (Woolley 1946; Jahns 1949; TIME(h) Johnson 1984). On September 14, 1974, an in- tense thunderstorm passed over Eldorado Canyon, FIGURE 4-6 Nevada, and although the duration of the rainfall Response of pore was short (generally less than an hour), the inten- August 1981 a wildfire had removed all vegetation, pressure to rainfall in sities were very high—from 76 to 152 mm/hr for exposed bare soil, and converted 15 percent of the shallow hillside soils 30 min. The intense rain eroded shallow soils, burned area to a hydrophobic condition; by June of northern California. leaving nills on some of the sparsely vegetated hill- 1982 reseeded grasses were establishing themselves Positive peaks of pore pressure correspond sides, and the high runoff scoured unconsolidated because of the wet winter of 1981-1982. In a recording gauge 1.2 km away, rainfall of 46 mm in to periods of high alluviuin from the larger stream channels. The rainfall intensity; initial debris-flow surge, heavily laden with sedi- 6 mm, 76 mm in 18 mm, and 101 mm in 27 mm negative pore ment and with the consistency of fresh concrete, was measured during the height of the storm. This pressures indicate emerged from the canyon with a high steep front, intense rain triggered a debris flow by sheet and nIl soil tension in partly damaging a marina and killing at least nine people erosion from shallow soils and from erosion of al- saturated soil at beginning of storm (Glancy and Harmsen 1975).