Soil Slips, Debris Flows, and Rainstorms 

in the Santa Monica Mountains and 

Vici11ity, Southern California 

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 851 

Soil Slips, Debris Flows, and Rainstorms 

in the Santa Monica Mountains and 

Vicinity, Southern California 

By RUSSELL H. CAMPBELL 

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 851 

Southern California residents have suffered death, injury, 

and property damage from debris flows generated by 

soil slips that occur during heavy rains; the process is 

a recurring major natural geomorphic agent in the 

region. D~fenses and warning are possible but require 

speical engineering and proced--ures 

UNITED STATES GOVERNMEN''T PRINTING OFFICE, WASHINGTON: 1975 
DEPARTMENT OF THE INTERIOR 

DONALD PAUL HODEL, Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

First printing 1975 

Second printing 1985 

For sale by the Distribution Branch, U.S. Geological Survey 

604 South Pickett Street, Alexandria, VA 22304 
CONTENTS 

Page 

~bstract -------------------------------------------------- 1 

Introduction ---------------------------------------------- 1 ~cknowledgments _ _ _ _ _ _ _ ___ _ _ _ _ _ _ ______ _ _ _ _ __ _ _ _ _ _ _ _ _ __ _ _ 4 

Soil slips and other landslides------------------------------ 5 

Soil slips: antecedent slope conditions ______________________ 5 

Slope angles and channel gradients ____________________ 6 

Parent material, colluvial soil, and ravine fill____________ 7 

Vegetation, fire, and soil moisture ______________________ 9 

Soil slips: failure conditions and mechanism ________________ 10 

Slope ------------------------------------------------ 11 

Failure and rainfall____________________________________ 12 

~echanism____________________________________________ 18 

Rainfall intensity and duration ________________________ 20 

Recurrence intervals ---------------------------------- 22 

Change of state____________________________________________ 24 

Debris flows ---------------------------------------------- 25 

Downslope transport ---------------------------------- 25 

Page 

Debris flows-Continued 

Deposits ------~--------------------------------------- 27 

Remedial measures and warning systems ------------------ 29 

The value of warning ---------------------------------- 30 

Contingency plans _ _ _ _ _ _____ _ _ _ _ _ _ _ _ _ _ ____ _ _ _ ___ _ _ _ ___ 30 

~warning system ------------------------------------ 30 

Supplemental data on associations of debris flows with heavy 

rainfall of recent years ------------------------------ 32 

Storms of February 7-19, 1962 ------------------------ 33 

Storms of November 14 to December 30, 1965 ---------- 36 

September 1965 ---------------------------------- 36 

September 19 through November 13, 1965 ---------- 37 

Storm period of January 18--25, 1969____________________ 39 

Storm period of February &-25, 1969-------------------- 40 

Case descriptions at three selected sites of soil-slip-debris-flow 

events -------------------~----------------------------- 42 

References cited ------------------------------------------ 50 

ILLUSTRATIONS 

Page 

FIGURE 1. Index map showing locations of areas of figure 5 and general-purpose geologic mapping project in the central Santa 

~onica ~ountains, southern California __________________________________________ --------------------------- 2 

2. Prestorm and poststorm photographs showing new soil-slip scars, Liberty and Las Virgenes Canyons -------------- 4 

3. Photograph of soil-slip scar and debris-flow channel above house in Old Topanga Canyon--------------------------- 6 

4. Photographs of soil-slip 'scars and debris-flow fans, Las Virgenes Canyon area ---------------------------- --------- 6 

5. ~ap showing localities of debris flows discussed in this report and of recording rain gages used to relate rainfall and 

soil slips _____________________________________________________________________________ ----- _ _ _ _ _ _ _ _ _ _ _ _ _ __ 8 

6. Photograph showing clumps of sod on soil-slip deposit ----------------------------------------------------------- 10 

7. Photograph showing large tree emplaced in kitchen ofresidence in Old Topanga Canyon by soil slip-debris flow ______ 11 

8. Diagram showing approximate relations of natural slopes to soil-slip failur~s ------------------------------------- 12 

9. Graph showing cumulative rainfall and times ofnearby superficial slope failures, January 18--2-8,.1969 ------------ 13 

10. ~aps showing distribution of rainfall (radar) at times associated with soil slips ----------------------------------- 14 

11-15. Photographs showing: 

11. Soil-slip scar behind house in ~andeville Canyon damaged by debris flow --------------------------------- 16 

12. Soil-slip scar and damage to residence in Sherman Oaks caused by soil slip-debris flow about 6:00a.m., January 25, 1969 _ __ _ _ _ _ _ _ _ __ _ _ _ _ _ _ _ _ __ _ _ _ _ _ _ __ __ _ _ __ __ _ _ _ _ _ _ __ _ _ __ __ _ _ _ _ _ _ _ _ _ _ ___ _ _____ ____ _ _ __ __ _ _ _ _ 16 

13. Scars of soil slips above residence in Thousand Oaks ----------------------------------------------------- 16 

14. Residence in Old Topanga Canyon damaged by soil slip-debris flow between 1:00 and 9:00a.m., January 25, 

1969 _______________________________________________________________________________________________ 17 

15. Scar indicating soil-slip origin of debris flow that destroyed residence in Highland Park--------------------- 17 

16. Diagram showing buildup of perched water table in colluvial soil during heavy rainfall --------------------------- 18 

17-20. Graphs showing: 

17. Relation of failure in some typical soils to ground-water content and slope angle --------------------------- 20 

18. Cumulative rainfall, February 7-19, 1962, and times of associated debris-flow activity --------------------- 22 

19. Cumulative rainfall, November 14-December 30, 1965, and times of associated debris-flow activity __________ 23 

20., Cumulative rainfall, February &-25, 1969, and times of associated debris-flow activity ---------------------- 24 

III 
IV CONTENTS 

FIGURE 21-28. Photographs showing: Page 

TABLE 

21. Boulders, garage, and car transported by debris flow; Newton Canyon, December 29, 1965 ___________________ 27 

22. Compacted-fill slope with surface-drainage interceptors rendered ineffective as a result of small soil slips______ 28 

23. Soil-slip scars and debris-flow deposits impounded behind road fill; Lobo Canyon area ----------------------- 28 

24. Debris flow through drainage that does not yield enough surface runoff of water to require a culvert __________ 29 

25. Damage by debris flow of December 29, 1965, Newton Canyon area --------------------------------------- 30 

26. Damage by debris flow of January 25, 1969, west side of Topanga Canyon --------------------------------- 31 

27. Hole punched through house at 975 Old Topanga Canyon Road, debris flow of January 25, 1969 ____________ 32 

28. Three views of soil-slip scar and damage to Silverado Canyon Fire Station --------------------------------- 34 

29. Map showing drainage basin of Newton Canyon and location of house destroyed by debris flow of December 29, 

1965 ______________ ~-------------------------------------------------------------------------------- 43 

30. Geologic map of upper Newton Canyon, showing path of debris flow of December 29, 1965, and similar scars in 

nearby uninhabited areas --------------------------------------------------------------------------------- 44 

31. Photograph of upper part of scar of ravine-fill failure on south face of Castro Peak-----------------------------____ 45 

32. Profile of Newton Canyon debris flow of December 29, 1965, comparing gradients of trunk canyon and tributary _ __ _ _ _ 45 

33. Map showing soil slip-debris flow of January 25, 1969, at 3221 S. Topanga Canyon Blvd ___________________________ 46 

34. Map showing location of several damaging debris flows in Old Topanga Canyon ----------------------------------- 48 

35. Photograph of scar above house at 869 Old Topanga Canyon Road ----------------------------------------------- 48 

36. Sketch map and profile of soil slip-debris flow at 87 4 Old Topanga Canyon Road _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 49 

TABLE 

Page 

1. Viscosities of common Newtonian and non-Newtonian (Bingham) fluids ___________________________________________ 26 
SOIL SLIPS, DEBRIS FLOWS, AND RAINSTORMS IN THE 

SANTA MONICA MOUNTAINS AND VICINITY, 

SOUTHERN CALIFORNIA 

By RussELL H. CAMPBELL 

ABSTRACT 

On the record of the past decade, debris flows generated by soil slips 

during rainstorms present a greater risk of death and injury to southern California residents than all other kinds of slope failure combined. 

During the years 1962-71, 23 people in the greater Los Angeles area 

died as a direct result of being buried or struck by debris flows that 

probably originated as soil slips. Soil slips are shallow failures of 

colluvial soil and ravine fill. They have in common several characteristics and associations with rain storms that set them apart from 

other classes oflandslides, such as rotational slumps and block glides. 

The latter, for example, depend more upon deep percolation of ground 

water and may not respond to the effects of heavy rainfall until long 

after a storm. In contrast, the shallow soil slips occur during, and only 

during, heavy rainfall. Soil slips appear to occur only on steep slopes, 

whereas other classes of landslides may occur on low slopes as well. 

Moreover, unlike the more deeply rooted landslides that damage 

structures situated on them by differential movement of the foundations, damage is due chiefly to inundation by, or high-velocity impact 

of, debris flows generated by the shallow slides. Debris flows are 

generated when the initial movement of slabs of soil and wedges of 

ravine fill causes a reconstitution of the sliding masses into viscous, 

debris-laden mud, which then flows down available drainage courses 

(in some accelerating to avalanche speed) until reaching gradients 

gentle enough for deposition to begin. 

Soil slips require a combination of three conditions: (1) A mantle of 

colluvial soil or a wedge of colluvial ravine fill, on (2) a steep slope, 

where (3) soil moisture is equal to or greater than the liquid limit of 

the remolded colluvial soil. The most common range of slopes for soil 

slips that give rise to destructive debris flows is from about 56° (150 

percent) to about 27°(50 percent). Slopes steeper than 56° generally do 

not have a continuous mantle of colluvium; most commonly they are 

bare bedrock. Soil slips on slopes of less than 27° are less common; 

moreover, the debris flows that they generate do not tend to accelerate 

downslope, though flows originating on steeper slopes above may be 

transported across with little or no loss of destructive power. The 

lowest slope on which a soil slip has been reported in coastal California is 15°. Most of the debris flows begin to deposit their coarser 

material on slopes of 12° (about 20 percent) or less, as indicated by the 

many fanhead slopes of from 10° to 12°, though transport on much 

gentler slopes is common. The soil moisture is almost entirely a direct 

result of seasonal rainfall. Because of the long dry season, the soil 

moisture at the beginning of the rainy season is generally well below 

field capacity. Once field capacity has been reached, further increase 

in the moisture content of the slope mantle requires rates of rainfall 

high enough so that water is added at a faster rate than it can drain 

away through the underlying subsoil. 

The exceptional storm period of January 18-26, 1969, presented an 

unusual opportunity to determine the times of occurrence of numerous debris flows, establish their origin from soil slips, and compare the 

times of those events with rainfall records from an extensive network 

of continuously recording rain gages, as well as with a sequential set 

of radar weather maps. An empirical association between soil slips 

and rainfall was noted for that storm period; the same association 

seems applicable to the less severe storms of February 1969, February 1962, November 1965, and December 1965, during which smaller 

numbers of soil slips occurred. In all cases where the times of soil slip 

failure could be documented or reliably inferred, the rainfall intensity 

at nearby recording gages exceeded 0.20 inch per hour, and nearly all 

exceeded 0.25 inch per hour. Moreover, there were few reports of 

debris flows, interpreted to have originated as soil slips, that occurred 

before the total seasonal antecedent rainfall has reached 10 inches. A 

10-inch antecedent rainfall appears to represent the total required to 

bring most of the colluvial soil of the area to field capacity, and a 0.25 

inch per hour intensity apparently represents the minimum rate at 

which surface infiltration exceeds subsoil drainage for most of the 

colluvial soils of the area. With radar weather maps showing the 

distribution of high-intensity rainfall and slope maps showing the 

distribution of slopes of the most susceptible steepness, the empirical 

association may provide a means of recognizing areas where the 

hazard is greatest at any given time during a storm, and warnings to 

residents could be more specific and reliable. 

INTRODUCTION 

The exceptionally heavy rainstorm of January 18 to 

26, 1969, covered a large part of coastal southern 

California. The area affected includes the Santa Monica 

Mountains the southernmost ofthe east-west-trending 

Transvers~ Ranges. The central Santa Monica Mountains has been the subject of a continuing program 

(since 1961) of general-purpose geologic mapping by the 

U.S. Geological Survey in cooperation with the Department of the County Engineer, Los Angeles County 

(fig. 1). A poststorm reconnaissance of the project area 

was made in early February 1969 to assess the effects of 

the storm on slope stability. Although many different 

1 
2 SOIL SLIPS, DEBRIS FLOWS, AND RAINSTORMS IN THE SANTA MONICA MOUNTAINS, SOUTHERN CALIFORNIA 

\·. 

: 80-. l'<r'O¥-"P't 

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4 5MILES 

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FIGURE 1.-Index map showing locations of areas of figure 5 and general-purpose geologic mapping project in the central Santa Monica 

Mountains, southern California. 
INTRODUCTION 3 

kinds of landslide activity were in evidence, from rockfalls to deep rotational slumps, it was immediately obvious that hundreds of shallow scars had resulted from 

the mass failure of colluvial soil cover of steep hillsides 

and colluvial fill in steep ravines (Campbell, 1969). 

Most scars were on natural slopes (fig. 2), though some 

also occurred on manmade slopes. 

Investigation of the downslope effects of these shallow failures indicates that, in many instances, the initial movement of slabs of soil and wedges of ravine fill 

caused reconstitution of the sliding wet masses into 

flowing, viscous, debris-laden mud, which then flowed 

down available drainage courses (accelerating to avalanche speed in some) until reaching a gradient gentle 

enough for deposition to occur. Structures in the paths 

of these flows were either inundated or subjected to 

high-velocity impact (fig. 3), and sometimes both. On 

Saturday morning, January 25, 1969, 8 debris flows 

("mudslides" in the press vernacular) of this probable 

origin caused 12 fatalities among residents of the Santa 

Monica Mountains and nearby hilly areas. One month 

later, on the morning of February 25th, two more debris 

flows, probably of the same origin, resulted in eight 

more fatalities-five in the Santa Ana Mountains and 

three in the San Gabriel Mountains. Further study 

added three more fatalities to the list of the past 

decade-two during a storm in February 1962 and 

another during a storm in December 1965. On this record, these shallow, relatively small landslides present 

a greater risk of bodily injury to southern California 

residents than the more slowly moving deep rotational 

and block-glide landslides. Furthermore, this hazard is 

not unique to coastal southern California. Debris flows 

that have apparently resulted from storm-related soil 

slips have also caused extensive damage in Brazil (Vargas and Pichler, 1957), Japan (Oka and Katsurajima, 

1971), coastal Alaska (Bishop and Stevens, 1964; Swanston, 1969), and other parts of the world. 

The initial failures are slab or wedge shaped, with 

length per thickness ratios generally in excess of 10:1. 

In the scheme of Skempton and Hutchinson (1969), see 

especially p. 295), they are probably best characterized 

as "slab slides"; in coastal California they are more 

commonly called "soil slips" (Kesseli, 1943; Bailey and 

Rice, 1969). The masses that continue downslope as 

flows are probably best termed "debris flows"- a relatively broad class that may be interpreted to include 

subclasses such as "mudflows" and "silt flows" of more 

specific grain size. "Semiarid mudflow" and "debrisavalanche" are terms proposed by Sharpe (1938, p. 

57-63) that would include the flowing masses; however, 

Sharpe's emphasis was clearly on events of larger volume and, although he inferred an origin by "slippage" 

(1938, p. 61) for debris-avalanches, mudflows and 

debris-avalanches can occur m semiarid regions in 

some circumstances that do not require concurrent 

heavy rainfall nor restrict the originating slippage to 

the surficial mantle. For the purposes of this report it 

seems preferable to use the compound term "soil 

slip-debris flow" to specify debris flows that are known 

or reliably inferred to have originated from soil slips. 

Many soil slip-debris flow events have been referred to 

as "mudslides" in press reports and other nontechnical 

accounts; however, "mudslide" has been applied to 

other events of such diverse character that its connotations are too broad to be appropriate here. 

The January 1969 storm also revealed that soil slips 

and debris flows may have more widespread 

significance in the evolution of local landforms than 

previously suspected. The removal of soil cover in slabshaped masses preserves the inclination of steep slopes 

and tends to preserve a sharp break at the crown. The 

downslope transport by flow ensures removal of the 

material to lower gradients where deposition takes 

place, preserving the slope break at the foot and accounting for the buildup of large "alluvial fans" at the 

mouths of short, steep drainage basins (fig. 4). That 

debris flows contribute to the formation of some fans has 

long been recognized (Blackwelder, 1928; Sharpe, 

1938), and the significance of soil slips in the erosion of 

steep slopes in the San Gabriel Mountains has recently 

been noted (Bailey and Rice, 1969, p. 176; Rice and 

Foggin 1971). However, the widespread, perhaps dominant influence of this mechanism in the natural evolution of the landforms has gone largely unrecognized 

owing to the long recurrence intervals-75-150 years 

(Simpson, 1969, p. 14)-between storms of the magnitude ofthat of January 1969. Of course, soil slips occur 

in smaller numbers much more frequently than 

record-breaking storms and pose a recurring 

debris-flow hazard to many hillside residents. 

The exceptional storm of January 18-26, 1969, provided a unique opportunity to examine the relations 

between rainfall and the debris-flow hazard in the Los 

Angeles area because: The wide areal distribution of 

heavy rainfall ensured that representatives of the full 

range of slope angles, soil types, bedrock type, and vegetation were subjected to rainfall intensities, durations, 

and totals that were closely monitored by a net of continuously recording rain gages; and the affected region 

included several populated areas where the severe 

damage and injuries drew attention to the times of 

failure causing many events to be reliably reported by 

the press or by other investigating agencies (fig. 5). The 

comparisons permit some approximations of limiting 

slope angles, and some rough qualitative observations 

ofthe effects of geologic soil type, soil thickness, parent 

material (bedrock or other), and vegetation. In addition, 
4 SOIL SLIPS, DEBRIS FLOWS, AND RAINSTORMS IN THE SANTA MONICA MOUNTAINS, SOUTHERN CALIFORNIA 

A , Prestorm; photograph taken November 1968. 

an empirical correlation between rainfall total and intensity and the times of observed slope failures leads to 

interpretations ofthe probable frequency of recurrence 

during lesser storms and to suggestions for minimizing 

the hazard to residents. 

Summary accounts of the associations of events of 

debris-flow activity with rainstorms in the past 10 years 

in coastal southern California are included in a supplement at the end of the report. Events that are interpreted as of soil slip-debris flow origin are listed, with 

time, location, and the association of each event or 

group of events with rainstorm activity recorded at 

nearby continuously recording gages. Nearly all the 

pre-1969 reports of times, places, and nature of origin 

were taken from newspaper accounts-chiefly from the 

Los Angeles Times. The general sources of data for the 

1969 storms are noted in "Acknowledgments." Lowaltitude oblique aerial photographs supplied by the Los 

Angeles County Engineer and the Department of Building and Safety (City of Los Angeles) were particularly 

FIGURE 2.-Views northeast across 

useful in recognizing scars of soil slips and freshly 

scoured ravines. 

ACKNOWLEDGMENTS 

My thanks to A. G. Keene, Head Engineering 

Geologist, and to S. H. Miller, Regional Engineer, of the 

Department of the County Engineer, Los Angeles 

County, for information on location and extent of damage by landslides in areas under their jurisdiction. 

Thanks, too, to C. A. Yelverton, Engineering Geologist, 

Department of Building and Safety, City of Los 

Angeles, for information on the location, character, 

time, and extent of damage at the sites of fatal landslides in the City ofLos Angeles, and particularly, for an 

excellent photographic record of those failures. The personnel of the Los Angeles County Sheriffs Department, 

the Los Angeles County Fire Department, and the Los 

Angeles County Road Department also helped me to 

acquire information about the extent of damage and 
SOIL SLIPS: ANTECEDENT SLOPE CONDffiONS 5 

B , Poststonn; scars formed during storm of January 18--26, 1969; photograph taken July 1969. 

Liberty <r:r::>d Las Virgenes Canyons. 

mjury. The Los Angeles County Flood Control Di:::otrict 

kindly furnished rainfall records that supplemented 

those published by the ESSA (Environmental Science 

Services Administration) Weather Bureau; and the 

ESSA Weather Bureau Office (Radar) at Palmdale, 

Calif., generously loaned their set of radar maps of the 

distribution of precipitation that they made almost 

hourly during the 1969 storm periods. Much valuable 

advice and counsel was received from colleagues in the 

U.S. Geological Survey. 

SOIL SLIPS AND OTHER LANDSLIDES 

Shallow failures of colluvial soil and ravine fill have a 

number of characteristics and storm associations in 

common that set them apart from other classes oflandslides, such as rotational slumps and block glides. The 

latter, for example, depend upon deep percolation of 

ground water and may not begin to move until many 

days or weeks after a storm. In contrast, soil slips occur 

only during heavy rainfall, and new ones do not appear 

after the rain cease:::;. (At higher alLitudes, ::>ud: as Lr.e 

higher parts of the San Gabriel Mountaim;, the water 

input into the soil may be provided by rapidly melting 

snow, instead of rain.) Because soil slips are generally 

limited to steep slopes, the kinds of landslides more 

commonly seen on gentle slopes are rotational slump, 

block glide, failure by lateral spreading, or liquefaction 

of sand and sensitive clay. The damage that may result 

is also different; differential movement of foundations is 

the major cause of structural damage by the more 

deeply rooted landslides, whereas inundation and lateral impact by flowing debris are the chief causes of 

damage that result from soil slip:::;. 

SOIL SLIPS: ANTECEDENT SLOPE CONDITIONS 

The association of soil slips with rainstorms is clear 

evidence that slope-mantle materials that are stable 

under "normal" conditions become unstable during 

rainfall of sufficient duration and intensity. The antecedent conditions on the slopes that fail are probably 
6 SOIL SLIPS, DEBRIS FLOWS, AND RAINSTORMS IN THE SANTA MONICA MOUNTAINS, SOUTHERN CALIFORNIA 

FIGURE 3.-Scar of soil slip and debris-flow channel above house in 

Old Topanga Canyon. Note train of deposits downslope from house. 

Slide of January 25, 1969; photograph taken within a few days by 

the Department of the County Engineer, Los Angeles County. 

best seen by examining the slopes adjacent to soil-slip 

scars. Although the parts that fail and slide off must be 

less stable than those remaining, movement generally 

alters the soil structure so thoroughly that its nearest 

representative is the material that remains behind. 

SLOPE ANGLES AND CHANNEL GRADIENTS 

Although the total relief of the Santa Monica Mountains is not much more than 2,900 feet, many slopes 

exceed 30°, and slopes of 40° and more are common. 

Precipitous c)iffs are present but are generally limited 

to the risers of flat-lying ledges of thick individual beds 

of resistant sandstone or volcanic rocks or to steeply 

dipping surfaces of resistant sandstone beds or volcanic 

rocks. Generally, slopes steeper than about 56° (ranging 

from about 45° to about 60°, depending on the character 

of the bedrock and the type of vegetation) are bare 

FIGURE 4.-Soil-slip scars and debris-flow fans, Las Virgenes Canyon area. Photographs taken February 4, 1969. 
SOIL SLIPS: ANTECEDENT SLOPE CONDITIONS 7 

bedrock, too steep to retain a continuous mantle of colluvium. Generally, though by no means invariably , 

profiles of slopes ofless than about 27° (50 percent) tend 

to be rounded, and profiles of steeper slopes tend to have 

relatively long straight segments. Depositional slopes 

of about 11 o (20 percent) are common in the heads of 

many fans; consequently, the upper edges of depositional slopes are commonly marked by sharp breaks 

with steeper erosional slopes above. The more distal 

parts of fans have more gentle slopes. Alluviated valleys are more common on the north and south flanks of 

the mountains than in the mountain core where most of 

the canyons are V -shaped, and deposits in the bottoms 

are periodically flushed out by floods. 

PARENT MATERIAL, COLLUVIAL SOIL, AND RAVINE FILL 

The storm of January 18-26, 1969, was accompanied 

by soil slips in terrain underlain by every bedrock unit 

in the Santa Monica Mountains. Scars of similar shape 

and size were formed in the colluvial soils overlying 

granitic and slaty basement rocks, as well as the sandstone, shale, and volcanic rocks of the entire superjacent 

sequence. (For summary descriptions of the "basement" 

and "superjacent" rocks see Yerkes and others, 1965, p. 

20-46.) The only evident controls by parent materials 

are indirect: (a) Generally, but not invariably, units 

containing more resistant rocks support steeper and 

longer slopes, and (b) colluvial soils developed over the 

volcanic rocks tend to be thinner and less continuous, 

perhaps because they do not generally support so dense 

a vegetal cover as the other bedrock types. 

Almost all the soils on slopes steeper than about 11 o 

are colluvial. They are derived from the parent material 

that underlies the slope (whether bedrock or surficial 

deposit, including artificial fill) by weathering and 

gravitational creep. Root wedging by vegetation, burrowing and walking by animals, and alternate swelling 

and shrinking of clays with changes in moisture contribute much to the breakup of the parent material and 

the downslope creep of the detritus. Rilling and other 

evidence of sheetwash by surface runoff are not common 

on the well-vegetated slopes, and raindrop impact is 

generally cushioned by the canopy of grass or chaparral. 

It seems probable that in years when rainfall intensities 

are low to moderate, soil creep on vegetated slopes during the dry season exceeds movement during the wet 

season, as reported by Anderson, Coleman, and Zinke 

(1959) and Krammes (1965) for slopes in the San Gabriel Mountains. 

FIGURE 4.-Continued. 
8 SOIL SLIPS, DEBRIS FLOWS, AND RAINSTORMS IN THE SANTA MONICA MOUNTAINS, SOUTHERN CALIFORNIA 

34°L__ 

Shaded re lief base by U.S. Ge o logical Survey : 1968 

0 3 MI L ES 

I,., '' 'I I 

2 

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ANAH£1'<1 ''' 0 3 4 KILOMETRES 

X 

EXPLANATION 

DEBRIS FLOWS 

eaJ ~~ • ti 

Hunt ngtO!'l beat • 

Numbers refer to descriptions in text and 

to times on rainfall curves 

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RDPI GRQ\'( 

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SANTA AHA • 

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...... Feb. 10-11, 1962 Dec. 29, 1965 Jan. 25, 1969 Feb. 25, 1969 

0 

Flows that caused fatalities 

Represents a total of 23 fatalities 

<I> 

Jan. 22, 1969 Jan. 25, 1969 

Flows that <:aused damage but not injury 

Area of most numerous scars in Santa 

Monica Mountains and vicinity. fonned 

during storm of Jan. 18-26, 1969 

,.L 

IIIII/I Glendora-Azusa area of extensive damage (Jan. 22, 1969) and later heavy 

rainfall 

Recording rain gage, showing initials of gage name 

L, Lechuza Pt FC; S, Sepulveda dam; BA; Bel Air FC; BK, Burbank; Los Angeles Civic Center; F, Flintridge 

FC; TF, San Dimas Tanbark flat; MB, Mount Baldy; SF, Santa Fe dam; SO, Santiago dam; T, Trabuco; H, 

Hansen 

FIGURE 5.-Locations of debris flows discussed in this report and of recording rain gages used to relate rainfall and soil slips. 
SOIL SLIPS: ANTECEDENT SLOPE CONDITIONS 9 

The thickness to which colluvium accumulates on the 

slopes generally ranges from 1 to 4 feet, and the full 

range is from a few inches to an unknown upper limit 

that may be as much as several tens offeet. The ravines 

at the foot ofthe slopes serve as repositories for detritus 

from both adjacent slopes, and 2-10 feet is the most 

common range of thickness ofthe ravine fill. The retention of a colluvial soil mantle on slopes steeper than 34° 

(the common angle of repose for loose, dry colluvial 

materials) is probably best attributed to the cohesive 

effects of vegetation roots and soil moisture. 

VEGETATION, FIRE, AND SOIL MOISTURE 

Although vegetation has long been regarded as effectively retarding hillside erosion, grass and chaparral 

seem much less effective in preventing slabs of soil from 

sliding than in retarding grain-by-grain erosion. The 

vegetal canopy and litter reduce susceptibility to 

grain-by-grain erosion by raindrop impact and sheetwash, but the ratio of infiltration to runoff is thereby 

increased, leading to a more rapid and thorough saturation of the soil mantle. The consequence may be an 

increased susceptibility to soil-slip erosion, depending 

on specific site conditions. Vegetal cover has not prevented large numbers of soil-slip events on steep slopes 

covered by all kinds of grass and chaparral; however, 

the rooting character of the plants probably has a 

significant effect on susceptibility to failure by soil slip. 

Spreading, inlocking roots tend to bind relatively shallow parts of the soil together, and roots that penetrate 

deeply into the subsoil (including cracks in bed-rock) 

tend to bind the soil and subsoil zones together and 

wedge bedrock blocks apart. Detailed studies in the San 

Dimas Experimental Forest near Glendora (Corbett 

and Rice, 1966; Rice and others, 1969; and Rice and 

Foggin, 1971) have demonstrated that the frequencies 

and areas of soil slips are three to five times greater for 

grass-covered slopes than for brush-covered ones. A 

significant correlation between vegetation and the 

minimum angles at which soil slips occurred was also 

noted by Corbett and Rice (1966, p. 4-6) and Rice and 

Foggin (1971, p. 1488 Table 1, p. 1493, 1496), who found 

that the minimum angles for failure were less for grassy 

cover than for most chaparral vegetation. No comparable quantitative data are available for the Santa 

Monica Mountains area, where similar correlations 

would be in accord with general impressions but might 

be biased by the greater visibility of scars on grassy 

slopes. The rooting characteristics of the various kinds 

of vegetation also probably affect the degree to which 

the soil structure in different zones is remolded by 

movement. 

Studies in the nearby San Gabriel and San Bernardino Mountains indicate that woody shrubs, such as 

chamise and scrub oak, have the deepest penetrating 

root systems (about 25 feet) of the chaparral vegetation, 

and that subshrubs such as California buckwheat, 

white and black sage, and chaparral yucca have maximum penetrations of about 5 feet (Hellmers and others, 

1955). Many of the same species occur in the Santa 

Monica Mountains, where the storm of January 18-26, 

1969, left the scars of soil slips on slopes dominated by 

both kinds of chaparral cover as well as by grass and 

mustard. Where the plants were the spreading, 

shallow-rooted type, the deposits below some scars included clumps of sod (fig. 6), suggesting that the nearsurface zone of interlocking roots tended to hold together better than the deeper zones that were more 

completely remolded to 'fluid mud. Where more deeply 

rooted plant varieties predominated, some of the soil 

slips left the tattered remnants of bushes dangling from 

the scars by a few root strands or holding small islandlike clumps of soil in place. Generally, however, the 

deeper rooted chaparral shrubs did not have sufficient 

binding effect to prevent large numbers of soil slips. 

Locally even large trees moved downslope together with 

the slabs of soil containing their shallow spreading 

roots (fig. 7). 

The density and variety of vegetation may also affect 

the width and shape of slab failures. Where the slopes 

were covered with grass, mustard, and sage, the scars 

from the January 1969 storm were as much as several 

tens of feet wide, but where the stronger chaparral 

plants prevailed, the scars were generally only a few 

tens of feet wide and relatively longer, tending to resemble the failure of ravine fill. Where the chaparral 

plants were widely spaced and not mixed with grass, as 

in many areas underlain by the volcanic rocks, soil slips 

were less abundant. So many interrelated factors are 

involved, however, that these associations are not satisfactorily documented. The distribution of the grassy 

vegetation, for example, is controlled partly by slope 

(many low slopes have been cleared and pasture grasses 

encouraged), by the recency of destruction of the larger 

bushes by fire, and by the chemistry of the soils developed over the various parent materials. Grassy vegetation is particularly common over shaly bedrock units, 

and the steep slopes developed over shaly bedrock tend 

to be shorter than those developed over sandstone strata 

where chaparral predominates. The relatively long, 

narrow aspect of the slides in chaparral, therefore, may 

be as much (or more) dependent on slope length as on 

the variety of vegetation. 

The effects of the states of growth of various types of 
10 SOIL SLIPS, DEBRIS FLOWS, AND RAINSTORMS IN THE SANTA MONICA MOUNTAINS, SOUTHERN CALIFORNIA 

FIGURE 6.- Clumps of sod on soil-slip deposit. Note trace of drainage of excess water from deposit toward the left, across the cultivated field, 

and the similar trace in the foreground that comes from another soil slip out of sight to the right. 

vegetation are not thoroughly known. However, the 

January 1969 storms struck when many ofthe annual 

grass seeds had germinated and green shoots, from a 

few millimeters to several centimeters long, had appeared on many slopes. These, of course, could have 

helped rainwater penetrate the surface of the soil. 

From the historical record, it seems clear that during 

heavy rainstorms, watersheds that have been recently 

burned over yield greater amounts of debris than those 

that have not been burned (for example, see Simpson, 

1969, p. 21). Fire destroys the vegetal canopy and some 

of the shallower roots, thereby exposing the surface to 

greater erosion by rain impact and sheetwash. Dryseason sliding is intensified (Krammes, 1965), probably 

resulting in much faster accumulation of channel-bed 

material (ravine fill). These conditions should not be 

expected to increase the probability of slab failures on 

the slopes but could increase the likelihood of ravine-fill 

failure and mobilization of bed material in lower channel reaches. Soil slips do occur on burned-over slopes, for 

Scott (1971) found that the 1969 debris flows generated 

in the burned watersheds above Glendora resulted from 

mobilization of channel-bed material, triggered, at 

least in part, by surficial slope failures. There were no 

large burned-over areas in the Santa Monica Mountains at the time of the January 1969 storms, so no 

comparative studies could be made there. 

Loss of soil moisture during fire and, perhaps, 

drought conditions also may promote dry sliding because air-water surface tensicn in the soil interstices 

contributes to the cohesiveness of the colluvial soil. (At 

the other end of the scale, air-water surface tension is 

also reduced when excess water displaces the air in the 

interstices.) 

As Krammes and DeBano (1965) report, many soils in 

chaparral watersheds have hydrophobic properties that 

appear to be associated with an organic coating on soil 

particles. The "nonwettability" may be intensified by 

the effects of brush fires. Hydrophobic properties are 

locally restricted to depth zones within the soil mantle 

where they can cause variations in moisture content 

and rate of infiltration in layers that are unrecognized 

without specific testing. 

SOIL SLIPS: FAILURE CONDITIONS 

AND MECHANISM 

It has long been accepted that debris flows are associated with the same kinds of rainstorms that gener-
SOIL SLIPS: FAILURE CONDITIONS AND MECHANISM 11 

FIGURE 7.-Large tree emplaced (nearly upright) in kitchen of residence in Old Topanga Canyon by soil slip-debris flow. Note doorframe 

on left strained from rectangle to parallelogram. 

ate flood flows in the major streams of semiarid regions 

(Blackwelder, 1928); however, opportunities to document the association between rainfall and debris flows 

generated by soil slips, such as offered by the January 

18--26 storm, are rare. The storm also enabled correlations between rainfall and soil slips that corroborate the 

applicability of the general mechanism suggested by 

Kesseli (1943). 

SLOPE 

Various observations on the relations of slope (or 

ravine gradient) to the characteristics of soil slips and 

their consequent debris flows are summarized in figure 

8. Because both the slab-shaped slides of colluvial slope 

mantle and the elongated wedges of ravine fill are virtually identical in longitudinal section, the mechanism 

offailure is believed to be the same. The limiting angles 

appear to be the same for both, though significant differences may not yet have been detected. Of course, 

slope is only one of many parameters that affects the 

accumulation of colluvial soil mantle, its susceptibility 

to failure, and the behavior of consequent debris flows; 

therefore, the limits suggested on the diagram must be 

recognized as approximations and subject to large variations. 

One requirement for slab failure of soil mantle is, of 

course, a soil cover of sufficient surface area, relative to 

its thickness, to be treated as a slab geometrically. This 

requirement is not met by most slopes steeper than 

about 55°, where bare bedrock predominates and soil 

accumulates only in small pockets and crevices. (Rockfalls are the most common failures from these precipitous slopes during rainstorms.) From about 34°-the 

common angle of repose for loose, dry colluvial soil- up 

to the steepest soil-mantled slopes, the area of soil cover 

decreases as the stability of surficial material is decreased. For most of the Santa Monica Mountains, the 

angle at which the lack of continuous soil mantle 

decreases the likelihood of soil slip is estimated to be 

about 45°. 

Soil slips were common in the Santa Monica Mountains on slopes from about 25° to about 45° during the 

storm of January 18--26, 1969, and the lowest slope that 

failed was about 22°. For comparison, after the storm of 

November 21-25, 1965, Corbett and Rice (1966, p. 6) 

found that in the San Dimas Experimental Forest, in 
12 SOIL SLIPS, DEBRIS FLOWS, AND RAINSTORMS IN THE SANTA MONICA MOUNTAINS, SOUTHERN CALIFORNIA 

Ratio 

Q) 

a. Percent .2 

CJl 

o<' 7>,,o<:-

"o<"«' "q;.e" 

~o"" ?-" 

i~ 

o"' Q' 

(000// / co~ / 

Degrees 

-

(0(00<'~ / unc o ::11' 

1

0 n slopes ranging from 45° to 55° , 

areas covered by soil mantle decrease in 

L ikelihood of 

slab soil slips 

UnliKe y~ ('('o<' proportion to increasing areas of bare 

00co\"0 bed rock . Slopes steeper than about 55° 

e\eta.'-i 0 (\ are generally bare, and rockfalls (rather 

r . . ~p.cc than soil slips) are the chief hazard Common effects 

on flow velocity 

DepOSition 

L-------------~ L-------------------------------------------------------------------------~ 

FIGURE 8.- Approximate relations of natural slopes to soil-slip failures. 

the San Gabriel Mountains, no soil slips occurred on 

slope areas classed as less than 40 percent (about 22°), 

and most were in areas of slope-class 70 percent (about 

35°) or greater. They add, however, that the slips reported in slope classes gentler than 70 percent are usually on small areas of steeper terrain within the slopeclass boundary, and that most occurred on slopes of 80 

percent (about 39°) or steeper. In contrast, Rice and 

Foggin (1971, p. 1489) found that the storm of January 

18-26, 1969, caused soil slips on slopes as low as 60 

percent (31 °) in the same area. 

In the Coast Ranges of central California, Kesseli 

(1943) reported numerous soil slips on slopes of from 

18°- 25° and one that originated on a slope as gentle as 

15°. He referred to these as "disintegrating soil slips" 

because the slabs generally disintegrated into clods and 

blocks of sod that were scattered in discontinuous trains 

over the slopes below the failures. Although some were 

associated with lobate tongues of mud, indicating that 

parts of some slabs had liquefied and flowed, neither 

channel scour nor distant transport of material was 

noted, suggesting that any flows that formed underwent 

little or no acceleration as they progressed downslope. 

Gentler slopes, for which slab failures attributed to the 

same general mechanism have been reported (Skempton and DeLory, 1957, p. 379-381), have inclinations of 

about 10°; however, these are in London Clay, and 

movement was relatively slow and constant. Hutchinson and Bhandari (1971, p. 353) attribute a "mudslide" 

on a slope as low as 3.9° to a special circumstance involving pore water pressures above hydrostatic at the 

slip surface, caused by the rapid loading of the headward parts of the slide; here, too, the material involved 

in the failure was stiff fissured clay (Hamstead Beds). 

FAILURE AND RAINFALL 

Even though most of the soil slips occurred in uninhabited areas where they were not observed, the poststorm reconnaissance established that a vast multitude 
SOIL SLIPS: FAILURE CONDITIONS AND MECHANISM 13 

+ 9 EXPL A N AT ION /7 -------------- -------~ 

25 Po int on c u mul at•ve n11 n tall curve for 

spe<:o fic gage at known tome of n eurbv 

debris ftow Number s • efer to localities 

on f igu r e 5 and notes in "Supplemental 

data" on failure 

K n own t ime of debris flows for which 

no single gage w&s cl o se en ough to be 

co n sidered rep resenta t ive of ram h11 at 

t he lite of hilure. P roba bl y b est repre -

sen ted b y a curve in term ediate between t he end points of the t im e l ines 

C/J 

w 0 i5 20 

z 

~ 

Part of c u r ve c o ver on g tome range of 

debri s-flow activity in nea rby areas. 

Debris-flow origin attributed c h iefly to 

so ot slops 

Circle marks t ime at w hlc" sea1onat to ut 

p a ned 10 inch es at 1 specofoc gage. Not 

show n for Fltntridgc 

...J 

...J 

:;: 15 

z SAN DIMAS TANBARK F L AT 

~ 

a: 

w 

> 

~ 10 

~ P'!!!f''"'i'-'""'-':~"'-""; 

~ 

:J 

u 

5 

0 

E E E E E E H o ur ci " ~ 0. ci ~ ~ "' "' N 

Day 18 

E E E ~ 0. ci 

"' N 

19 

E E E E E E E E E E E E 

• ~ 0: ci " N ci 0: " ~ 0. ci 

"' "' N "' "' N "' "' N 

20 21 22 

E E E E E E E E E E ~ E E E ~ E E E E E E E E E 

" ~ 0. ci " ~ 0. ci " ci " ~ 0. ci " ~ 0. 0: " ~ ci ci 

"' "' N "' "' ~ "' ~ "' N "' "' N "' "' N "' "' N 

23 24 25 26 27 28 

JANUARY 1969 

FIGURE 9.-Cumulative rainfall a t selected continuously recording gages in and near the Santa Monica and San Gabriel Mountains, showing times of nearby slope failures, January 18-28, 1969. 

of scars had appeared during the generalS-day period of 

storm activity. Of the many slides that occurred in inhabited areas, the best documented as to time are those 

that resulted in injuries and property damage. (See fig. 

5.) These times are markedly clustered into two periods, 

a 9-hour period between midnight and 9:00 a.m., 

January 22, and an 8lfz-hour period between 12:30 a.m. 

and 9:00a.m., January 25, and undoubtedly represent 

two climaxes of soil-slip activity in the area shown (fig. 

9). The relationships suggest that few, if any, soil slips 

occurred at other times during the 8-day storm. Consequently, for the purpose of comparison with the time 

and distribution of rainfall of varying intensity, the 

times of the documented debris flows have been taken as 

representative of the time span of the great majority of 

the soil slips. Indeed, there seems little room for doubt 

that these failures occurred while rain was falling, and 

that failure was caused by dynamic conditions- conditions that were created by the storm, continued to build 

toward failure during periods of rainfall, and ceased 

when rainfall ceased. 

The correlation in time between failure and heavy 

rainfall is strongly supported by comparisons of the 

documented slides with the hourly rainfall distribution 

maps made by the ESSA Weather Bureau Office 

,(Radar), Palmdale, Calif. , using FAA (Federal Aviation 

Administration) Air Traffic Control Radar. Figure lOA 

shows areas of rainfall at 11:30 p.m., January 24, 1969, 

12:30 a.m, January 25, 1969, and 1:30 a.m., January 25, 

1969; the dot shows the location of a debris flow in 

Brentwood at 12:30 a.m., January 25, 1969. (Figure 11 

shows the residence and the slide scar.) Figure lOB 

shows rainfall at 2:26a.m. and 4:26a.m., January 25, 

1969. The dot shows the location of a debris flow in the 

Topanga Canyon area (see pages 46-48 for description) at about 3:00 a .m., and the pattern shows the 

general area of many other slides that reportedly occurred in that vicinity at about the same time. Figure lOC 

shows rainfall at 5:29 a.m. and 6:26 a.m., January 25, 

1969. The first of two fatal debris flows in Sherman 

Oaks occurred at 6:00 a.m. (Figures 12A and 12B show 

the residence and slide scar where one fatality occurred.) Figure lOD shows rainfall at 6:26 a.m. and 8:38 

a.m., January 25, 1969. Fatal debris flows occurred in 

Glendale at 6:30a.m., in Sherman Oaks at 6:50a.m., 

and in Thousand Oaks at 7:45 a.m. (Figure 13 shows 

slide scars above the residence in Thousand Oaks.) Figure lOE shows rainfall at 8:38 a.m. and 10:15 a.m., 
14 SOIL SLIPS, DEBRIS FLOWS, AND RAINSTORMS IN THE SANTA MONICA MOUNTAINS, SOUTHERN CALIFORNIA 

0 10 MI L ES 10 MILES 

A hi •-+1-.'•''"• f-'-..L.,r~--' 0 5 10 KILOMETRES 10 KILOM ETR ES 

EXPLANATION EXPLANATION 

Light Moderate to heavy 

/' ~"-....~ "'<--~~ ~ 

(about 0.02-0.2 inch per hour) (0.2 inch per hour or more) 

Rainfall at 11:30 p .m.P.s.t., January 24,1969 

Light Moderate to heavy 

Rainfall at 2 :26 a.m.P-s.t. , January 25, 1969 

~I< 

""~'~",~' Light Moderate to heavy 

Rainfall at 12:30 a.m.P .s.t., January 25, 1969 

Moderate to heavy 

Rainfall at 1:30 a.m.P.s.t., January 25, 1969 

6 BA 

Gage station "Bel Air" 

e3 

Site of soil slip-debris flow in Mandeville Canyon, about 

12:30 a .m .P.s.t., January 25, 1969, one fatality 

Moderate to heavy 

Rainfall at 4 :26 a.m.P.s.t., January 25, 1969 

&L 

Gage station "Lechuza Pt." 

.s 

Gage station "Sepulveda dam" 

::-.... 4 

Site of several soil slip-debris flows in Topanga Canyon and 

Old Topanga Canyon areas at about 3:00 a .m.P.s.t., January 

25, 1969, one of which was fatal to three persons 

0 5 10 MILES 

I·' "•I 'I I I 0 5 10 KILOMETRES 

EXPLANAT ION c 

Light Moderate to heavy 

Rainfall at 5:29 a.m .P.s.t., January 25, 1969 

Light Moderate to heavy 

Rainfall at 6 :26 a.m.P.s.t ., January 25, 1969 

.s 

Gage station 

"Sepulveda dam" 

e5 

Site of soil slip-debris flow in Sherman Oaks 

at about 6:00 a.m.P.s.t., January 25, 1969, one 

fatality 

B 

FIGURE 10.- Distribution of rainfall (radar) at times associated with soil slips. (Rainfall compiled from U.S. Weather Bureau maps made by 

FAA/ESSA Radar Unit, Palmdale, Calif.) 
SOIL SLIPS: FAILURE CONDITIONS AND MECHANISM 

10 Kl LOMETRES D EXPLANATION 

~ ....-?'A 

'<-'""'-' //).,.., 

Light Moderate to heavy Light Moderate to heavy 

Rainfall at 6:26 a.m.P.s.t., January 25, 1969 

"'F 

Rainfall at 8:38 a.m.P.s.t., January 25, 1969 

Gage station "F1intridge" 

... 

"'L 

Gage station "Lechuza Pt" 

.s 

Gage station "Sepulveda dam" Site of soil slip-debris flow in Sherman Oaks at about 

6:50 a.m.P.s.t., January 25, 1969, one fatality 

e7 

Site of soil slip-debris flow in 

Thousand Oaks at about 7:45 a.m. 

P.s.t., January 25, 1969, one 

fatality 

e9 

Site of debris flow of probable 

soil slip origin in Glendale at 

about 6:30 a.m.P.s.t., January 25, 

1969, two fatalities 

10MILES 

10 KILOMETR ES 

EXPLANATION 

Light 

( 

Moderate 

~ to heavy 

Rainfall at 8:38 a.m.P.s.t., January 25,1969 

"'F 

Gage station "Flintridge" 

.s 

Moderate to heavy 

Rainfall at 10:15 a.m.P.s.t., January25,1969 

"'LA 

Gage station "Los Angeles" 

Site of soil slip-debris flow in Highland Park at about 9:00 a.m. 

P.s.t. , January 25, 1969, two fatalities 

FIGURE 10.-Continued. 

E 

15 
16 SOIL SLIPS, DEBRIS FLOWS, AND RAINSTORMS IN THE SANTA MONICA MOUNTAINS, SOUTHERN CALIFORNIA 

FIGURE 11.-Soil-slip scar behind house in Mandeville Canyon damaged by debris flow; one fatality. Photograph taken February 13, 

1969, by the Department of Building and Safety, City of Los 

Angeles. 

A, Scar on hillside behind house. Note rills eroded by rainwash on 

bare scar area. Photograph taken February 13, 1969, by Department of Building and Safety, City of Los Angeles. 

January 25, 1969, bracketing the 9:00 a.m. time of a 

slide in Highland Park. The time of a debris flow in the 

Old Topanga Canyon area (see pages 48--50) could be 

determined only as between 1:00 a.m. and 9:00 a.m., 

January 25, 1969; however, neighbors reported that 

many debris flows occurred, beginning about 1:00 a.m., 

and we£e particularly frequent for the next 4 or 5 hours. 

Figure 14 shows the damage to the residence. 

FIGURE 13.----,Scars of soil slips above residence in Thousand Oaks. 

About 7:45a.m., on January 25, 1969, a debris flow generated by the 

slip on the left broke into the rear of the house, killing a sleeping 

guest in the back bedroom. 

B, Damage to upslope side of house, showing broken wall of room 

where sleeping resident was killed. (Photograph by Hollywood Citizen News, courtesy of Department of Building and Safety, City of 

Los Angeles.) 

FIGURE 12.-Residence in Sherman Oaks damaged by soil slip--debris flow about 6:00 a.m., January 25, 1969; one fatality. 
SOIL SLIPS: F AlLURE CONDITIONS AND MECHANISM 17 

A, Flow broke through rear wall of lower story of a two-story house 

after "ski jumping" from top of vertical cut about 8 feet high. Upper 

story splattered (note broken window) but not entered by main flow. 

B, View through house from the front. Refrigerator and other heavy 

appliances, carried from kitchen in rear and through partition into 

living room in front, trapped and killed one occupant. All coarse 

debris remained inside, only muddy water flowed out the front. 

FIGURE 14.-Residence in Old Topanga Canyon damaged by soil slipdebris flow between 1:00 a.m. and 9:00a.m., January 25, 1969; one 

fatality. 

The maps (fig. 10) document that rain was falling at 

the sites offailure when failure occurred, and they indicate that the rainfall at the sites offailure was "moderate to heavy" (about 0.2 in/hr or greater) at the times of 

failure. The lack of "moderate to heavy" rain covering 

the Mandeville Canyon site at 12:30 a.m., January 25, 

1969 (see fig. lOA) may be partly attributable to radar 

FIGURE 15.-Scar above site of residence destroyed by debris flow 

indicates origin by soil slip; Highland Park, two fatalities at about 

9:00 a.m. January 25, 1969. Photograph by City of Los Angeles, 

Department of Building and Safety. 

characteristics1 rather than to an actual absence of rain 

over the area at 12:30 a.m. 

Some uncertainty in the correlation of the radar maps 

with ground effects arises from the transfer ofthe radar 

data from compilation sheets at about 1:2,000,000 scale 

to the more detailed bases needed for plotting soil-slip 

locations (about 1:500,000 scale, at a minimum). Although the radar-scope image is sufficiently precise to 

warrant such enlargement (John W. Fassler, Chief 

Radar Meteorologist, ESSA/FAA Radar Unit, Palmdale, California, oral commun., September 9, 1970), the 

enlarged base for the radar maps was difficult to register to the more detailed maps on which soil-slip locations were plotted. 

All the foregoing time-documented debris flows are 

believed to have originated from soil slips. I examined 

all but two of the sites (Glendale and Highland Park) 

and found distinctive scars in the source areas of each. 

Photographs of the Highland Park site by the City of 

Los Angeles, Department of Building and Safety, clearly show a slide scar in the source area (fig. 15). Only 

newspaper accounts of the debris flow at the Glendale 

site have been examined, and although they do not 

explicitly indicate the presence of slide · scars in the 

1Radar at the Los Angeles [nternational Airport, the one nearest the site of failure, was 

temJX>rarily operating with circuits that are designed to increase sensitivity to aircraft by 

removing effects of precipitation but that prevent detection of weather activity within 30 

miles of the radar site (Benner and Smith, 1968, p. 18). 
18 SOIL SLIPS, DEBRIS FLOWS, AND RAINSTORMS IN THE SANTA MONICA MOUNTAINS, SOUTHERN CALIFORNIA 

zone 

Seepage parallel 

to slope in 

saturated zone 

Colluvial soil; 

relatively 

permeable 

FIGURE 16.-Diagram showing buildup of perched water table in colluvial soil during heavy rainfall. Bedrock subsoil is shown here; however, soil slips also occurred over compacted fill slopes where the transition from parent material to colluvial soil is more obviously 

gradational. The significance of the piezometric head (h) in reducing the shearing resistance (S) is discussed in the text, with regard to 

the formulaS= c + (p - hw) tan cf> (Terzaghi, 1950). 

source area, a soil-slip origin may be inferred from the 

reports of many "mudslides" that occurred in this 

neighborhood at about the same time as the fatal one. 

MECHANISM 

The strong correlations between debris-flow activity 

and rainfall of moderate to high intensity (fig. 9) support the hypothesis ofKesseli (1943, p. 34 7) attributing 

the disintegrating soil slips of the California Coast 

Ranges to the buildup of water in the regolith when 

infiltration at the surface takes place at a greater rate 

than deep percolation-a dynamic imbalance that can 

occur only when rainfall intensity exceeds the rate of 

deep percolation. 

The mechanism is illustrated by figure 16, which 

shows shallow-rooted vegetation with a thin mulch of 

dead leaves and grass growing in a regolith of colluvial 

soil, the upper part of which contains abundant living 

and dead roots as well as animal burrows. When the 

rate of infiltration into and through the upper layers is 

equal to or less than the capacity of the bedrock to 

remove it by deep percolation, the water moves toward 

the permanent water table far below, and the stability 

of the surficial material on the slope is not affected. 

When infiltration through the regolith exceeds the 

transmissive capacity of the rocks below, a temporary 

perched water table is formed. Its head will continue to 

increase as long as infiltration continues at the high· 

rate until the whole surficial zone is saturated, at which 

time all the rainfall in excess of the transmissive capacity of the bedrock is distributed as surface runoff and 

downslope seepage within the saturated surficial zone. 

In this manner, the slope approaches the special condition described by Haefeli (1948, p. 59-60) of seepage 

flow parallel to the slope surface. 

Even long after the warning of Hacker (1940), many 

lists of causes oflandslides include "overloading," without qualification as to the geometry of the load distribu-
SOIL SLIPS: FAILURE CONDITIONS AND MECHANISM 19 

tion, and state or imply that failure can be caused by an 

added downslope driving force derived from the increase in weight of the soil mantle through the 

infiltration of water (for example, Bishop and Stevens, 

1964, p. 6; Corbett and Rice, 1966, p. 1; Nilsen, 1971; 

and Putnam, 1971, p.194). Mechanically, this concept is 

inappropriate, for if the weight is added uniformly, as 

might be expected in the case of vertical rainwater 

infiltration, it should increase the normal force of the 

soil mantle on the underlying slope (the frictional force 

that resists sliding) as well as the tangential force . 

(downslope shearing force of the slab) in such a way that 

the ratio of the two remains constant. The criterion for 

failure of a slab is that the ratio of the tangential and 

normal forces must exceed a critical number-a number 

that is different for different materials and determined 

by experiment. That ratio is independent of the weight 

of the material in the slab. Specifically, the only part of 

the !!added weight" of the water that contributes to 

failure is the weight of the column of interstitial water 

above the potential slip surface, and this weight serves 

to increase the pore fluid pressure. 

The effect of the addition of water in changing a slab 

of slope mantle from stable to unstable may be illustrated by reference to the familiar Terzahgi (1950, p. 92) 

formula for resistance to shear: 

S = c + (p-hw) tan cf>, 

where, at a point on a potential surface of sliding, sis the 

shearing resistance per unit of area, cf> is the angle of 

sliding friction for the surface of potential sliding, hw is 

the pore pressure from the unit weight of water (w) and 

the piezometric head (h), p is the pressure due to the 

weight of the solids and water, and cis the cohesion per 

unit area. Note what happens to the equation when a 

water-saturated zone forms above the slip surface: (1) 

The component of cohesion. (c) that is derived from intergranular air-water surface tension is reduced as 

water replaces air in the interstices; and (2) (p -hw) is 

decreased as the piezometric head increases-both lead 

to a reduction in shearing resistance (S). 

T. L. Youd, U.S. Geological Survey, suggested (written commun., 1969) that the formula developed by 

Skempton and DeLory (1957, p. 379) for the condition 

that ground-water flow is parallel to the slope at shallow depth might serve to cast the Terzaghi equation into 

terms of more readily measured soil parameters. It may 

be written in the form: 

F _ c' + (y - my w) z cos2B tancf>' 

- y z sinB cosB 

where F is the factor of safety, c' is the cohesion intercept, z is the vertical depth of the slip surface, B is the 

slope angle, y is the unit weight (density) of the soil, 'Yw 

is the unit weight of the water, m is the fraction ofz such 

that mz is the vertical height of the ground-water table 

above the slip surface, and cf>' is the angle of shearing 

resistance. (See fig. 17.) As m approaches 1,F decreases 

to a minimum value dependent upon -y, z, and tan cf>' for 

any given slope angle. 2 

If cohesion (c') may be neglected, the formula for the 

safety factor may be written: 

myw tancf>' 

F = (1 --,-)tan B · 

And, where F = 1 (failure criterion): 

myw 

tan B = (1 - --) tancf>'. 

'Y 

A graph can then be drawn showing a family of curves of 

F = 1, with varying combinations of measured soil 

parameters y (unit weight) and tan cf>' (fig. 17). Thus, 

from the graph (fig: 17) a slope overlain by a colluvial 

soil mantle having an angle of shearing resistance (cf>') 

of 46° and a unit weight of90 lb per cu ft will be stable in 

the field to the left of curve number 3, and unstable (atF 

= 1) to the right of it. Thus, on a slope of30°, failure can 

be expected when the piezometric surface rises to a 

proportion (m) of0.63 of the thickness ofthe soil above 

the potential slip surface. For soils with higher unit 

weights, the value of m required to achieve F = 1 is 

higher. Thus, for -y = 132 lb per cu ft (curve 4), the 

piezometric surface must rise to 95 percent of the thickness of the zone above the potential slip surface. The 

intercepts of the curves with the ordinate m = 1.0 imply 

that failure by this mechanism should not occur on 

slopes gentler than the intercept values even if the soil 

is fully saturated to the ground surface. 

A set of similar curves, prepared for the soil and slope 

2Swanston (1970), working on the mechanics of similar soil slips in the shallow till soils of 

southeast Alaska, used the "method of slices," in which the formula takes the form: 

}.: [CAL + (6-W n + Qn - uALJ cos a tan~] 

F = }.:(6-Wn + Qn> sina 

where Cis the intercept cohesion (assumed to be 0 for shallow granular soils) and Qn is the 

"surface loading" and is assumed to be negligible, 6-W n is the weight of the soil in each slice, 

IlL is the slope width of each slice, u is pore water pressure, ~(same as</>') is effective angle of 

internal friction, and a (same asB) is the slope angle. Now, for a slab having a length(£) that 

is large with respect to a relatively uniform depth (z), lAL = L L 

and '£6-Wn = Wn. Further, from the diagram we see that 

Wn = Lty = ·yLz cosB, where y is the saturated unit weight 

of the soil. Moreover, because y is measured according 

to unit horizontal cross section, the weight of yz on 

the larger inclined surface is reduced to yz cosB 

and Wn = Lyz cos2B. Similarly, following Skempton and Hutchinson 

(1969, p. 319~ for the case of piezometric surface parallel to the slope, u = Ywmz cos2B. 

Integrating equation 1 for the slab geometry and substituting yields: 

F= CL + (Wn - uL) cosa tan<ti 

Wn sina 

C'L + (Lyz cos2B + Lyw mz cos2B) cosB tan<f/ 

Lyz cos2B sin B 

C' + (y - mywl z cos2B tan</>' 

yz sinB cosB 

Thus, for cases whereL/z is large (that is, slab slides), formula (1) transforms to the formula of 

Skempton and DeLory (1957). 
20 SOIL SLIPS, DEBRIS FLOWS, AND RAINSTORMS IN THE SANTA MONICA MOUNTAINS, SOUTHERN CALIFORNIA 

.....l 

<( 

1-

0 

1-

w 

J: 

ILL 

0 

E. 

z 

Q~ 1-....J 

Uen 

0.8 

~~ LL 0 0.6 

<(en 

enw 

<(~ 

WLL 

zo 

O.en 

Nen 

Cl w 0.4 wZ 

~--~ 

<(U 

~i 1-1-

<( 

en 

~ 0.2 

en 

en 

w 

z 

~ 

u 

F = c' + ('y-m'Y .. l z cos 2 8 tan 0' 

'YZ sin 8 cos 8 

atfailure: F= 1, and ifc'=O, 

then: tan 8= (1-m 'Yoh) tan 0' 

for example, 

if 'Y=90 lb per cu ft, 'Y./'Y=0.70 

if 'Y=132 lb per cu ft, 'Y./'Y=0.47 

Modified from Skempton and 

Delory (1957) 

J: 

1- 0.0~--------~--------~--~----~--------~--~~--~------~ 

0 10 20 30 40 50 60 

SLOPE ANGLE, IN DEGREES 

FIGURE 17.-Relation of failure in some typical soils to ground-water content and slope angle. Computed curves for F = 1 (failure criterion) 

at selected values of y (unit weight) and ¢' (angle of shearing resistance). The curves for most natural nonclayey soils lie between 

curves 1 and 4. Fields to the left and right of each curve are stable and unstable, respectively. 

parameters of a given site, should permit a preliminary 

evaluation of recurrence interval for failures due to 

rainstorms. If the thickness and infiltration rate are 

known for the regolith on a slope of known angle, the 

recurrence interval for values of m at F = 1 can be 

approximated by using the recurrence intervals of rainfall of sufficient intensity and duration. If infiltration 

rates are very low, duration of rainfall should be the 

dominant factor; if they are high, rainfall intensity 

should be more important. Either way, the buildup of a 

temporary perched water tat1e requires that the 

infiltration rate for the soil mantle be greater than the 

infiltration rate for the underlying parent material. 

RAINFALL INTENSITY AND DURATION 

From the foregoing mechanism, the minimum conditions for failure would seem to be: an initial period of 

enough rainfall to bring the full thickness of the soil 

mantle to field capacity (the moisture content at which, 

under gravity, water will flow out as fast as it flows in), 

followed by rainfall intense enough to exceed the 

infiltration rate of the parent material underlying the 

soil mantle, and lasting long enough to establish a 

perched ground-water table of sufficient proportional 

thickness (m, fig. 17) to cause failure. (Note that rainfall 

intensity need not exceed, nor even equal, the maximum infiltration rate for the soil; therefore, surface 

runoff may not be in evidence on a slope about to fail.) 

Evaluation of the data from the storm of January 18-26, 

1969, suggests that, in the greater Los Angeles area, 

these minimum conditions were reached when the sites 

of failure had received a total of about 10 inches ofrain, 

after which they were subjected to rainfall intensities of 

about 0.25 inch per hour or more. (See fig. 9.) The numerical values are, of course, preliminary and subject to 

revision as more data are examined. 

Certainly, the threshold total rainfall should be expected to be less for thin soils than for thick soils, and 

the 10-inch antecedent total suggested here must reflect 

conditions for colluvial soil mantles of average thickness. Any loss of soil moisture by evaporation or transpiration during dry interstorm intervals should tend to 

increase the minim urn threshold for a following storm; 

however, comparisons of the records for the storm seasons of1961-62, 1965--66, and 1968-69 suggest that the 

threshold may be more closely associated with antecedent rainfall for the season than the immediately preceding hours of storm. 

Damage accounts from the winter seasons of 1961-62 

and 1965--66 appear to support the suggested minimum 
SOIL SLIPS: FAILURE CONDITIONS AND MECHANISM 21 

for intensity of 0.25 inch per hour (figs. 18, 19), although at a few sites, failure may have occurred at 

intensities as low as 0.20 inch per hour. They do not, 

however, give unequivocal support to the inferred 

minimum antecedent total. Debris flows did occur well 

before that total was reached. Although most of these 

early flows may be attributed to causes other than soil 

slips, such as surface runoff in association with soilfalls, 

rockfalls, and rockslides, the possibility that some 

originated in soil slips has not been eliminated. Such a 

!!premature" failure by soil slips can be reconciled with 

the proposed mechanism only if the soil slabs that failed 

were significantly thinner than the !(average" thickness 

of colluvial soil, or if the soil zone was already wetted 

from an agency other than rainfall. A full evaluation of 

the significance and numerical value of a !!minim urn 

antecedent season total" would require additional details and documentation, particularly with regard to 

time of occurrence, classification (whether clearly a debris flow or muddy floodflow or slide), and whether there 

was clear evidence of soil-slip origin (such as a scar) or 

origin by some other means. 

The curves for the storm of January 18--26, 1969 (fig. 

9) show varying areal, orographic, and episodic characteristics of the storm. The high totals for the gages at 

San Dimas Tanbark Flat and Lechuza Pt. Station represent the orographic effects of their relatively high 

altitudes in the San Gabriel and Santa Monica Mountains, respectively. Most of the episodes of moderate- to 

high-intensity rainfall during the storm period were 

recorded by all the gages. However, during the highintensity rainfall along the front of the San Gabriel 

Mountains on January 22 (as represented by the San 

Dimas and Flintridge gages), only light rain was falling 

in the Santa Monica Mountains and vicinity. As the 

curves indicate, there were early episodes of highintensity rainfall during which soil slips did not occur. 

Rainfall of similar, in part greater, intensity than that 

associated with the failures of January 22 and 25 occurred during earlier episodes on January 19 and 21 

when no significant number of debris flows were observed. This lack of sliding during the earlier storm 

episodes seems to indicate that at those times the total 

antecedent rainfall had not been sufficient to bring the 

regolith up to field capacity. 

The 10.5-inch ((threshold total" indicated in figure 9 is 

probably an empirical representation of the range of 

minimum field capacities for the slope mantle materials 

of the general Los Angeles area. The numerical value of 

the !!threshold total" may be different for different 

storms, depending upon the soil moisture content at the 

beginning of the storm. Soil moisture at the time the 

storm of January 18--26, 1969, began was probably relatively low for that time of year because the total rainfall 

through January 17 was only a little over half of the 

seasonal normal to that date (Simpson, 1969, p. 12). 

This condition contrasts markedly with the antecedent 

conditions at the beginning of the storm series of February, 1962, which began after seasonal totals of 6-11 

inches, approximately the !!normal to date" (fig. 18), had 

already been attained. 

In February 1962, the storm precipitation totaled less 

than 2 inches when the first debris flows were reported. 

The seasonal totals, however, generally exceeded 10 

inches (with possible exceptions as low as 9.8 inches in 

the Hollywood area, and as low as 9.2 inches in the 

Monterey Park area, providing the Bel Air and Los 

Angeles recording gages, respectively, are representative). The storm series of February 1962 also records 

debris-flow activity associated with heavy rainfall of 

relatively short duration. On February 12, 15, and 19, 

heavy rainfall lasting as little as 2 hours apparently 

caused numerous debris flows. 

The significance of antecedent totals for events associated with the storms of November and December 

1965 is difficult to evaluate. (fig. 19). From newspaper 

accounts, (see section on !(Storms of November 14 to 

December 30, 1965") it appears likely that debris flows 

occurred in several areas after antecedent totals of as 

little as 3 inches; however, their origins are not clearly 

established. Some were reportedly associated with active construction projects. Others may have originated 

as soilfalls or rockfalls from road and highway cuts. 

Only a few were clearly associated with circumstances 

where soil slips may have been the most likely origin. 

Even there, the possibility that the antecedent soil 

moisture was partly a result of irrigation cannot be 

discounted. Seasonal totals reached 10 inches at most 

gages on November 22, during widespread heavy rainfall that was associated with debris flows in several 

places. The best documented soil slip--debris flow event 

occurred on December 29, during an episode of heavy 

rainfall at the beginning of which all gages showed 

seasonal antecedent totals of over 10 inches (fig. 5, no. 

14). 

Damaging debris flows that occurred in the greater 

Los Angeles area during February 1969 began well 

after season totals had exceeded 10 inches. As indicated 

in figure 20, high-intensity rainfall in the San Gabriel 

Mountains was accompanied by renewed debris-flow 

activity in the Glendora area on February 6, after only 

2-4 inches of antecedent storm rainfall, and again on 

February 23-25, after relatively little antecedent storm 

rainfall. According to Scott (1971, p. C244), damage 

from these later storms was minor compared with that 

of the January 18--26 storm period. Simpson (1969, p. 

187) ascribes the smaller debris production of the February storms to lesser rainfall intensities. 
22 SOIL SLIPS, DEBRIS FLOWS, AND RAINSTORMS IN THE SANTA MONICA MOUNTAINS, SOUTHERN CALIFORNIA 

"' w 

J: 

C/) 20 ~ 

~ ~ (.) :i z ~5 

~ ~ 

-l 15 -l 

<( 

u.. 

z 

~ 

a: 

~ 10 

i= 

<( 

-l 

:::> 

::2! 

:::> 

u 5 

a: 

6.91 .......... Season totals to February 6, 1962 

0 

Hour E E E E E E E E E ci .; "' ci ci .; "' ci ci 

~ <D <D "' <D <D N 

Day 7 8 

E E E E E E E E E E .; N ci ci .; N ci ci .; <D <D N <D <D "' "' <D 

9 10 11 

E E E E E ci ci .; ~ ci <D N <D <D 

12 

E 

2 

> 

, 

"' 

ci E 

ci 

~ ~ 

E E E .; "' ci <D <D 

15 

E 

ci 

~ 

"~~~;~----:. 

~--------- \;i:-0«' 

1----f 62~,..·-------- 1------

E 

ci 

~ 

Heavy delhts indiclte parts of curves 

covering time ranges af reported .. mud· 

~ides;" hnvy dots indicm parts of -

curves covering time ranges poaibfyllsocilttd with nported .. mudslide" ec· 

tivity, but for which the identification 

is doubtful or for !~Which tht represent· 

ltivtnts&ofthenearestgageisdoubtful. 

Small circles mark times at which Ill· 

sonal totals passed 10 inches at the 

respective gages. Numbers refer to notes 

in ''Supplemental dati" concerning the -

reportedfailuns,thesuitlbilityofpge 

recordsasrepmentativeofconditions 

atthesittsoffailure,andtheaource, 

method, and rtliability of dltlrminations of time of occurrence end charec· 

tar of failures. Numbers 1211Mf 13are 

locatedinfigure5 

E E E E E E E .; ~ ci ci .; "' ci <D <D "' <D <D 

18 19 

E 

ci 

"' 

FEBRUARY 1962 

FIGURE 18.-Cumulative rainfall at several continuously recording gages in the vicinity of the Santa Monica 

Mountains, February 7-19, 1962, showing times of associated debris-flow activity. 

The intensity of rainfall at the times of nearby soil 

slips, as shown by figure 9, was nowhere significantly 

less than 0.25 inch per hour. Long periods of lighter 

rainfall (see, for example, curves for January 24, 1969) 

apparently were not accompanied by any debris flows. 

Again, during the storm of February 23-25, 1969, the 

gage records (fig. 20) in the Santa Ana Mountains and 

San Gabriel Mountains indicate that the minimum intensity at the times of soil-slip failures was about 0.25 

inch per hour. A minimum intensity at failure as low as 

0.20 per hour is suggested by a few parts of the curves 

for the storm series of February 1962 (fig. 18), but items 

62-2a, 13, and 62-7 (fig. 18) are subject to further interpretation as to the representativeness of the curves 

on which they are plotted. Items 2a and 13 refer to 

events in Monterey Park; the nearest continuously recording rain gage is that at the Los Angeles Civic 

Center. A comparison with the nonrecording gage at 

San Gabriel Fire Station, closer to the sites of failure in 

Monterey Park, indicates that the Monterey Park area 

may have received as much as 0.43 inch more total 

rainfall on February 9, 1962. If, as seems most likely, 

most of it fell during the same 3- to 4-hour period as the 

heavy rainfall at the Los Angeles gage, the intensity 

would have been in excess of 0.30 inch per hour. On 

February 11, 1962, (fig. 18, item 13,) the Los Angeles 

station received a total of 1.24 inches, while the San 

Gabriel Fire Station received 2.43 inches, nearly twice 

as much, indicating that some significant storm activity 

in the Monterey Park area was not represented on the 

Los Angeles gage. Item 7 (fig. 18) may be reasonably 

well represented by the Bel Air gage. 

RECURRENCE INTERVALS 

The recurrence interval for storms of the magnitude 

of that of January 18-26, 1969, has been reported at 75 

to 150 years by Simpson (1969, p. 14). Some residents 

have interpreted news reports that this was an 80-year 

storm to mean that they need not expect another such 

storm and its associated hazards for the next 80 

years-a dangerously misleading belief. Perhaps only 

an H80-year storm" could cause such widespread and 

numerous soil slips, but it may be more important to 

note that during the storm period of January 18-26, 

1969, a great many of the soil slips occurred well before 

the end of the storm, before the rainfall had reached its 

record and near-record totals. Debris flows probably 

caused by soil slips have resulted in fatal injuries during at least four storms in the decade 196~70 

-February 1962, December 1965, January 1969, and 

February 1969-and have occurred, with less disastrous results, during some of the other storms of the 

1960's. 

The recurrence interval for rainfall intensities of over 

0.25 inch per hour in the greater Los Angeles area may 

be inferred to be less than 1 year. The t(Rainfall frequency atlas of the United States" (Hershfield, 1961) 

indicated (p. 23) that 1-hour periods of rainfall ranging 
(/) 

w 

25 

~ 20 

z 

~ 

_.i 

....J 

~ 15 

z 

~ 

a: 

w 

> 

~ 10 

....J 

::J 

~ 

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u 

5 

0 

10 

12 24 36 48 

TIME, IN HOURS 

Small c1rcles mark times 

65.3 

I 

~---~ 

~AM 

LOS ANGELES CIVIC CTR 65.7 

65 5 65.6 

Numbered horiZontal 11nes represent ap 

prox1mate timespans of report pen ads 

covered by newspaper accounts of 

damage, numbers keyed to summar.es 

in text 

-~-----------1 

-

65.8 65.9 

,-//1 

:2 

> 

, 

Approximate time Interval during wh1ch 

fatal debris flow oc65.10 

65.12 

65.11 

e E E E E e E E E e E E E E E E EE E E E E E E E E E e e E E E E E E E E E E E E E EE E E E E E E E E E E E E 

Hour ~ ~ ~ : ; ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ; ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ 

Day 14 15 16 17 22 23 24 25 9 10 28 29 

NOVEMBER 1965 DECEMBER 1965 

FIGURE 19.-Cumulative rainfall at selected continuously recording gages in and near the Santa Monica and San Gabriel Mountains, November 14 through 

December 30, 1965, showing times of associated debris-flow activity. 

30 

~ 

F 

00. 

t""' 

~ 

~ 

F 

~ 0 

0 

~ =3 

0 

z 

00. 

~ t:l 

s= 

t?=.:l 

0 

~ 

~ 

1:\.J 

~ 
24 SOIL SLIPS, DEBRIS FLOWS, AND RAINSTORMS IN THE SANTA MONICA MOUNTAINS, SOUTHERN CALIFORNIA 

(/) 

UJ 

I "' u 15 "' z 

Note Numbers refer to notes 1n 

Appredtx A 69 (p ) concern 

tng the reported failures, the 

suttabtlity of gage records as 

representative of condttions at 

the Sites of fatlure, and the 

source, method, and reliablltty 

of determ tnations of ttme of 

TRABUCO CANYON 

Trme of fatal debris 

flow at Mount Baldy 

Village ~\0'\.e A"' 

010G:'~ Renewed debtts-flow ~eco~ _... 11 a 

actiVIty on south _.-• 

flank of San Gabr•el 

Ttme of fatal debris 

flow at Silverado 

Total, January 1-31, 1969, 10-77 1n 

(no record February 1 20, 1969) 

SANTIAGO DAM 

0 

Hour E E E E E E E E E E E E E E E E E E E E E E E E E E E E E ci ..; N ci ci ..; N ci ci ci ~ ci ci ..; ~ ci ci ..; ~ ci ci ~ ~ ci ci ..; ~ ci ci 

~ lC CD N CD CD ~ ~ lC CD ~ CD CD N "' lC N lC N lC CD N 

Day 5 6 21 22 23 24 25 

FEBRUARY 1969 

FIGURE 20.-Cumulative rainfall at continuously recording gages at Mount Baldy, Trabuco 

Canyon, and Santiago Dam, February 5-25, 1969, showing times of associated 

debris-flow activity. 

from 0.40-0.80 inch normally recur each year. There-

.fore, the inferred minimum intensity to trigger soil slips 

(0.25 inch per hour) may be expected at least once each 

year, and some years twice. The observed recurrence is 

less frequent, perhaps because the soil moisture content 

derived from antecedent rainfall must exceed the field 

capacity when high-intensity rainfall occurs. 

The normal annual rainfall at the Los Angeles Civic 

Center is 12.63 inches (U.S. Environmental Science 

Services Administration, 1969B, p. 44), nearly all of 

which falls during the October through March rainy 

season. Most nearby hillside areas probably have 

higher ((normals" than the Los Angeles station because 

of the pronounced orographic effect so common in 

storms in the area, therefore, in most ((normal" years 

most hillsides will have received the inferred threshold 

((seasonal to date" total of about 10.0 inches by the latter 

part of the season. Combinations of minimum antecedent rainfall and rainfall intensity that may cause a few 

scattered soil slips, particularly at higher altitudes, 

probably recur nearly every year. 

Some inferences may also be made about the recurrence intervals of more severe storms. The data for the 

storm of January 18--26, 1969 (see fig. 9) show that a 

significantly large number of soil slips had taken place 

by the time the storm total had reached about 13 inches. 

Miller (1964, p. 20-21) indicates that the return interval for a 7 -day period of rainfall totaling 10 inches is 10 

years, and that for a return interval of25 years as much 

as 15 inches may be expected during a similar period (7 

days). It would seem, therefore, that storms capable of 

causing numerous soil slips in the Santa Monica Mountains and vicinity may be expected to recur at intervals 

of between 10 and 25 years. 

CHANGE OF STATE 

When a relatively rigid slab changes to a viscous fluid 

(a characteristic also observed by Bishop and Stevens, 

1964, p.11), the conditions of resistance to downslope 

movement change from sliding friction to viscous flow. 

This change helps to explain how, on a given slope, a 

mass that has just become unstable (under conditions of 

sliding friction) can accelerate to avalanche speed 

rather than move at a steady slow velocity. Although 

the exact manner in which the change of state begins 

has not been observed or reported, some reasonable 

speculations may be made on the basis of the postulated 

mechanism for the slab failures (p. 18--20) together with 

the phenomenon of ((spontaneous liquefaction" (Terzaghi and Peck, 1967, p. 108). For instance, the change 

should begin in the saturated zone adjacent to the slip 

surface of the slab failure and quickly encompass all the 

saturated zone. If an upper zone of the soil remains 

unsaturated (but at field capacity), it, too, may become 

fluid if field capacity provides enough moisture to surpass the liquid limit and if the soil structure is reworked 

thoroughly enough to effect remolding. That many such 

zones did not become completely fluid is shown by 

clumps and blocks of sod found well downstream from 

their scars of origin (fig. 6). 
DEBRIS FLOWS 25 

DEBRIS FLOWS 

.Once a slab of soil becomes detached at an underlying 

shp surface and at the margins and begins moving, part 

or all of the mass is effectively remolded by its own 

motion, and it changes from a rigid slab to a viscous 

fluid. As it moves downslope, the material flows together into a relatively narrow stream, moving down 

established drainages-ravines, gullies, swales, and so 

on-as a discrete slug. The speed of the flow depends on 

its fluidity and on the length and gradient of the channel. 

A comprehensive discussion of the mechanics of 

downslope flow of debris slurries is beyond the scope of 

this report. In general, the tendency of the flows to 

accelerate on steep slopes seems to indicate that the 

dominant conditions are those of viscous flow. Because 

visible effects of viscous or plastic creep of the soil mantle are not commonly associated with the storm-related 

slab failures in the Santa Monica Mountains, it seems 

necessary to consider elements of plastic or quasiplastic 

deformation (Leopold and others, 1964, p. 31) only on 

gentle slopes where flows decelerated to slow speeds just 

before depositing. Bagnold (1968, 48-51) and Johnson 

(1970, p. 461-534) have recently discussed the 

mechanics of viscous slurry flow (mudflows). 

The same processes may operate on the bedload of 

flooding drainage channels. Discrete debris flows resulting from individual slope failures can cause 

~i?~ificant but temporary increases in volume upon 

JOining streams of flowing floodwater and can cause 

discrete surges in their flow. During the January 1969 

storm, Scott (1971, p. C245) noted surges in flow from 

several drainage basins in the San Gabriel Mountains 

that were apparently generated by small slope failures. 

In so_me drainage channels he observed (Scott, 1971, fig. 

3) high-water marks above debris-flow levees in the 

lower reaches of drainage channels but no evidence of 

water flowing above the debris flows in the upper 

reaches. The proportions of slurry flow (bedload?) to 

floodwater flow ranged from 100 percent (where no 

overflowing floodwater was evident) to 70 percent. 

DOWNSLOPE TRANSPORT 

Many flows clearly moved at avalanche speeds; damage caused by high-velocity impact attests to this. Other 

flows reportedly oozed slowly down relatively gentle 

slopes, building up against the upslope walls of structures until a window or some other weak point gave way 

under the lateral pressure, permitting fluid mud to inundate the interior. Judging from the appearance of the 

deposits, the character of the damage, and newspaper 

accounts of witnesses, most of the slower flows probably 

moved at not much less than 1 ft/s (foot per second), and 

the extremely rapid flows probably moved at not much 

more than 40 ft/s. Slower and faster flows are, of course, 

not excluded by the mechanism, and the estimated 

range of rates cannot be expected to apply where other 

conditions of slope angle, slope length, soil character, 

and climate prevail. It is interesting, however, to compare the estimated rates with those measured and computed for mudflows (debris flows) elsewhere. For example: ,..hnson and Hampton (1969, p. 6.4-6.5) computed 

velocities for mudflows in channels that cross a small 

alluvial fan in the Panamint Range and found them to 

range from about 12 to 40 ft/s. D. M. Morton (written 

commun., 1971) measured velocities of channeled 

mudflows ranging from 2 to 12 ft/s at Wrightwood (1969 

mudfiow activity), where they moved through the 

channel in the alluviated canyon just above the apex of 

the fan. In the same part of the Wrightwood area, 

Gleason and Amidon (unpub. data)3 measured velocities (for 1941 mudflow activity) ranging from 4 to 

14.5 ft/s. (These computed and measured velocities are 

for flows in confined channels with gradients of about 

17° or less.) 

The flows were generally laminar in appearance; 

however, considerable turbulence must have occurred 

in at least two general circumstances: (1) Where relatively wide slabs were funneled into relatively narrow 

ravines downstream; and (2) where the roughness of a 

channel bottom imposed a cascading, plunge-and-pond 

character to the flows. 

No direct measurements of viscosity were made during the debris-flow activity of January and February 

1969. Debris-flow consistencies, however, have often 

been likened to wet concrete (for example, Rantz, 1970, 

p. B10). (The subject of visual estimates of viscosity has 

been discussed by Van Wazer and others, 1963, p. 

38-39, who comment that it is difficult for the eye to 

distinguish differences in viscosity in the range of0.001 

to 0.1 poise. In contrast, relatively small differences in 

viscosity are clearly recognized in the range from 1 to 

100 poises. The eye again has trouble distinguishing 

between viscosities greater than 1,000 poises because 

Newtonian fluids of such high viscosity do not pour and 

appear (<solid.") Surprisingly, very few viscosity measurements on wet concrete have been reported. According to Reiner (1960, p. 345, 352), V. P. Lobonov measured Bingham4 viscosities of 24 poises and 34 poises, 

respectively, for fresh cement paste and for fresh cement mortar. Table 1 lists viscosities for a number of 

Newtonian and non-Newtonian fluids to aid in comparative judgments. 

3Gleason, C. H., and Amidbn, R. E., 1941, Landslide and mudflow, Wrightwood, California: 

California Forest and Range Experiment Station, RI-CAL, July 21, 1941, 7 p. 

4Viscosities for single-phase liquids are commonly calculated using formulae derived from 

a "Newtonian" mathematical model, whereas most slurry flows are thought to be better 

represented by the "Bingham" model. Although both ways of calculating viscosity yield 

values expressed in poises, the numbers are generally significantly different. 
26 SOIL SLIPS, DEBRIS FLOWS, AND RAINSTORMS IN THE SANTA MONICA MOUNTAINS, SOUTHERN CALIFORNIA 

TABLE I.-Viscosities of common Newtonian and non-Newtonian 

(Bingham) fluids 

Newtonian fluid Viscosity 

(poises! 

Water, 20°C ______________ 0.01 

Machine oil, light, 15.6°C __ 

Machine oil, heavy, 15.6°C 

Glycerine, 14.3°C _________ _ 

Glycerine, 2.8°C _________ _ 

Asphalt, 47°C ___________ _ 

1.14 

6.61 

13.87 

42.20 

10.4 X 103 

Pitch, 15°C ________________ 13.0 x 109 

Non-Newtonian <Bingham! fluid 

Experimental kaolin 

slurry _________________ _ 

Oil well drilling muds, 

from ___________________ _ 

to _____________________ _ 

J<etchup _________________ _ 

Mustard _________________ _ 

Mayonnaise _____________ _ 

Viscosity 

(poises) 

0.30 

O.Ox 

>3.0 

0.83 

2.94 

6.33 

Wet cement paste ________ 24 

Wet cement mortar________ 34 

Natural mudflows at 

Wrightwood: 

Muddy water between 

mudflow surges _____ _ 1.0 

Mudflows, range of 

45 measurements ___ _ 400-1,000 

Natural mudflow at 

Wrightwood ___________ _ 760 

Basaltic lava (25 percent 

crystal slurry) _________ _ 6,500 

Source 

Hodgeman and Holmes, 

1942,p. 1638-1641. 

Do. 

Do. 

Do. 

Do. 

Shaw, Wright, Peck, and 

Okamura, 1968, p. 

239. 

Hodgeman and Holmes, 

1942, p. 1644. 

Source 

Johnson, 1970, p. 509. 

Sutter, in LeRoy, 

1950, p. 715. 

Weltmann, 1960,p. 241. 

Do. 

Do. 

Reiner, 1960, p. 345,352. 

Do. 

D. M. Morton, 1972, 

written commun. 

Johnson, 1970, p. 513. 

Shaw, Wright, Peck, and 

Okamura, 1968, p. 

248. 

The ability of the debris flows in the Santa Monica 

Mountains to deepen their own channels by erosion 

appears related to the steepness of the gradient and the 

thickness and moisture content of old channel-fill deposits. Most of the flows began to deposit debris upon 

reaching fanhead gradients of 10°-12°, and many passed with little or no scour through reaches with gradients as steep as 27°. This is inferred to reflect a relation of gradient to acceleration (as indicated in fig. 8) 

such that, generally, flows accelerated down channel 

gradients steeper than 27° and flowed at relatively constant speed through transitional reaches with gradients 

between 27° and 11 o • At gradients of about 11 o and less, 

the flows began to decelerate, and debris deposition 

began. 

These inferred limits must be regarded cautiously, 

and only used as tentative guides because soil character, soil moisture, storm duration and intensity, and 

debris-flow velocity and viscosity can cause deviations 

of many degrees. (For example, the erodability of unconsolidated channel-fill debris depends on its degree of 

saturation-the more nearly saturated the easier it is 

mobilized by a passing debris flow and incorporated into 

the flowing mass.) Moreover, the numbers are generalized, for the most part, from observations of relatively small single-pulse events (virtually one slope 

failure generating one debris flow at a particular time), 

generally depositing at the foot of a first- or secondorder drainage. Where debris flows move down higher 

order drainages, fed by other tributaries with additional debris flows or with surface runoff water, both 

velocity and duration of flow will probably be increased 

with increasing size. In the larger drainage areas, there 

may also be runoff water available for addition to the 

unconsolidated deposits below the flow, and scour in 

relatively gentle gradients might be expected. Scott 

(1971) found evidence that debris flows generated in 

small drainage basins along the south flank of the San 

Gabriel Mountains during the storms of January and 

February 1969 had scoured into channel-bed material 

of higher order (third- to fourth-order) drainages in 

reaches with gradients as low as 4°. 

The debris flows appear capable of transporting· the 

largest boulders available. Except for some very large 

blocks, obviously derived by rockfall from adjacent canyon walls, most of the coarse-cobble- to boulder-size 

clasts in the ravines and canyons were probably emplaced by debris flows (including the mobilized bedloads 

of flooding streams in only a few of the larger canyons) 

rather than by running water (floodflow ). Generally, 

the coarsest material is found in flows derived from 

failure (or mobilization) of ravine fill, apparently because the fill includes coarse rockfall debris that did not 

come to repose upslope. The soil mantle of the slopes 

does not generally contain material coarser than cobble 

size, though there are important local exceptions. 

Figure 21 shows size range of boulders deposited in 

the bottom of a fifth-order channel from a debris flow 

that issued from a third-order tributary in December 

1965. The flow destroyed a residence (killing the occupant) at the confluence of the two drainages and had 

sufficient carrying power to completely remove a separate garage structure, together with the car inside it, 

and deposit the wreckage about 100 yards downstream 

from the original site. 

The size of the clasts available for transport by the 

debris flows depends greatly on the character of the 

bedrock units that have contributed debris. Thus, thick 

beds of resis~ant sandstone or volcanic rock commonly 

contribute large blocks and boulders, whereas thin, friable sandstone and mudstone may contribute clasts no 

larger than pebble size. 
DEBRIS FLOWS 27 

FIGURE 21.-Boulders, garage, and car transported by debris flow; Newton Canyon, December 29, 1965. 

DEPOSITS 

Except where influenced by manmade impediments, 

debris-flow deposits are found in two principal forms: 

debris fans (including many "alluvial fans") and debris 

trains. 

Fans, coalesced into aprons in many places, are most 

common in the broader valleys and on the flanks of the 

mountains where steep, low-order drainages on the 

hillsides descend abruptly to slopes of relatively gentle 

gradient (figs. 2 and 4). Extensive fans have been built 

at the mouths of some remarkably small drainages, 

including first- and second-order drainages so small 

that they show no bare, stream-washed channels even 

after very severe rainstorms. This condition suggests 

that surface runoff in many of the small drainage basins 

is not sufficiently powerful, even during the severest 

rainstorms, to have eroded and transported all the debris now deposited in the fans. Fans of this kind, therefore, indicate a history of recurring debris flows. Moreover, their abundance indicates that, at least within the 

recent geologic past, erosion by slab failure of colluvial 

soil, transport of the debris by slurry flow, and deposition from slurry flow are major elements in the geomorphic processes that formed the present landscape. 

Debris trains are found in the bottoms of mountain 

canyons in drainages of intermediate and higher order 

(generally fifth or higher) at and immediately downstream from the mouths of tributaries of lower order 

(commonly first to third). The trains are elongate, extending down the trunk canyons for distances of from a 

few tens of feet to as much as 2,000 feet from the upstream ends, which commonly mark the junction of the 

trunk canyon with the tributary that served as a channel for the flowing debris (see fig. 21). Postdepositional 

settlement, winnowing, sheetwash, and trunk-stream 

erosion have, in some places, resulted in localized distinctive drainage and morphologic features. The most 

commonly recognized feature is an irregular, rough, 

bouldery surface, including large generally angular 

boulders of resistant rock types that crop out in the 

drainage of the source tributary. In several places, debris trains have blocked the original junctions of 
28 SOIL SLIPS, DEBRIS FLOWS, AND RAINSTORMS IN THE SANTA MONICA MOUNTAINS, SOUTHERN CALIFORNIA 

FIGURE 22.-Compacted-fill slope with surface-drainage interceptors 

rendered ineffective as a result of small soil slips. Liberty Canyon 

area, storm of January 18-26, 1969. 

tributaries and trunk drainages, forcing them to migrate downstream and leaving boulder-covered medial 

ridges in the canyon bottoms. 

Where the debris flows strike manmade structures, a 

wide variety of effects result , depending on the kind of 

structure and the size and velocity of the flow. For example, in January 1969, many paved drainage interceptors on engineered slopes were dammed by the deposition of relatively small amounts of debris, causing 

undesirable diversion of surface runoff across slopes 

below, which resulted in erosion by sheetwash (fig. 22). 

Debris flows may quickly fill the basins behind small 

check dams, rendering them ineffective in controlling 

subsequent surface runoff. Similarly, road and highway 

fill may impound large amounts of debris in basins 

formed when culverts become blocked (fig. 23). Drainages so small and gentle that no culverts were installed 

where roads cross them may disgorge tons of muddy 

debris onto roadways (fig. 24). 

The effects on relatively small residential dwellings 

(commonly 1- or 2-story frame construction) range from 

quiet inundation to complete destruction by highvelocity impact. Debris flows of sufficient volume and 

momentum h:we smashed such structures into pieces 

and moved the remains off their foundations (figs. 25, 

26). Debris flows of relatively small volume but high 

B, Impounded debris, part of the scoured bedrock debris-flow chute, 

A, Soil-slip scars in headwall drainage of ravine crossed by road fill. and lower parts of some of the soil-slip scars. 

"!i'IGURE 23.-Soil-slip scars and debris-flow deposits impounded behind road fill; Lobo Canyon area, storm of January 18--26, 1969. 
REMEDIAL MEASURES AND WARNING SYSTEMS 29 

FIGURE 24.-Debris flow through drainage that does not yield enough surface runoff of water to require a culvert; Mulholland Highway, 

storm of January 18--26, 1969. Road has been cleared after inundation. 

momentum have punched holes into and even completely through structures (figs. 14A, 27). Some structures show little evidence of damage other than a hole in 

the wall (fig. 14A ), but many have been distorted from 

their normal erect positions (fig. 7). Some buildings 

have had even layers of muddy debris deposited inside 

them, commonly accompanied by little structural damage. Apparently, the debris flows were moving at relatively low velocities; the flows entered dwellings 

through open doors or pushed laterally through windows and doorways and quietly flooded the interiors. 

REMEDIAL MEASURES AND WARNING SYSTEMS 

The hillside sites where soil slips may generate future 

debris flows are so small, numerous, and widely scattered that the safety of all downslope residents cannot 

be ensured merely by the construction of defensive 

works such as check dams, debris basins, and levees. 

Preventive measures, particularly the careful control of 

all surface and subsoil drainage, are generally practicable only where an entire slope area is carefully 

engineered. Many hazardous sites, however, lie downslope from natural, undeveloped areas, where access is 

difficult; moreover, the slopes on which the debris flows 

originate may be divided among several owners. Both of 

these factors may complicate the installation, operation, and maintenance of preventive and protective 

works. In some locations, where the anticipated flows 

would be of sufficiently low volume and velocity, and 

where the probable paths of debris flows can be predicted, relatively simple protective structures might be 

erected. However, these would have to be carefully located after hillsides above vulnerable dwellings had 

been thoroughly studied. The extensive literature on 

snow-avalanche defenses may suggest some kinds of 

structures of possible practical value (for example, U.S. 

Dept. Agriculture, 1961; Bucher, 1956; Flaig, 1955; 

Fuchs, 1955; and Roch, 1956). Of particular interest 

might be the design, bracing, and reinforcement of 

upslope walls of dwellings so as to deflect or resist lateral pressures and impacts from material moving 

downslope. In general, however, it does not seem 

economically feasible to prevent debris flows from forming, nor to protect all dwellings within the immediate 
30 SOIL SLIPS, DEBRIS FLOWS, AND RAINSTORMS IN THE SANTA MONICA MOUNTAINS, SOUTHERN CALIFORNIA 

FIGURE 25--Wood-frame house, right side caught by edge of debris flow from the right, crushed and spun off foundation by impact. One fatality at this site. Newton Canyon area, debris flow of December 29, 1965. 

future. On the other hand, the injuries caused when 

those dwellings are damaged or destroyed by debris 

flows must be classed as preventable because with 

warning people cart avoid the relatively small flowing 

masse8. 

THE VALUE OF WARNING 

During the early morning hours of January 25, 1969, 

there were 12 fatalities from 8 debris flows in the Santa 

Monica Mountains and adjacent areas. All the victims 

were inside residential structures that were damaged 

by impact or inundation by flowing debris. Eight-six 

adults and two children-were in their own bedrooms 

when crushed beneath collapsing walls or buried by 

muddy debris. The mother of the two children had reportedly awakened and was on her way to their bedroom 

when the house was crushed and part of it was pushed 

into the flooding stream below, resulting in the death of 

all three (fig. 26). Two of the victims who were awake 

were also small children, playing in one room of their 

home while their mother worked in another. Only one 

fatally injured adult was awake and fully clad. He had 

evacuated his family and had just returned to his house 

when it was struck by a high-velocity debris flow, which 

trapped and killed him (fig. 14). 

The stories of many who escaped injury when their 

homes were damaged or destroyed are also enlightening. If an adult of the household was alert to problems 

outside the house (chief concerns were storm-runoff 

drainage, including small mudflows), almost invariably 

the approaching hazard was recognized in time to 

evacuate the remaining occupants-though very hastily in several instances (for example, the residents ofthe 

dwellings shown in figs. 7, 27). 

The value of advance warning, therefore, seems clear. 

Residents who are notified that storm conditions have 

reached a point where debris flows may be generated by 

soil slips if high-intensity rainfall continues should be 

alert, should be prepared to recognize approaching 

danger, and should move quickly out of harm's way. 

Small children, invalids, and elderly people might be 

evacuated at such a time, but general evacuation of 

whole neighborhoods should not be necessary. The records show that even without advance planning, many 

people were able to react in ways that. saved them from 
REMEDIAL MEASURES AND WARNING SYSTEMS 31 

FIGURE 26.-Debris flow from above left crushed wood-frame dwelling 

and pushed most of it downhill into the floodwaters of Topanga 

Creek. Photograph taken after partial excavation for rescue operations. Three fatalities at this site. West side of Topanga Canyon, 

debris flow of January 25, 1969. 

injury. Obviously, advance planning would provide for 

quicker and better protective response. 

CONTINGENCY PLANS 

Because each hillside dwelling is in a unique position 

and orientation with respect to the slopes above, each 

household needs an individualized set of contingency 

plans. When residents are warned that critical rainfall 

conditions have been reached in their area and are 

likely to be exceeded, some may elect to leave immediately, others to evacuate only small children and 

invalids; still others to remain at home, trusting in their 

ability to keep a sharp lookout, recognize approaching 

danger, and evacuate only when and if the hazard becomes an immediate threat. Those who remain after 

being alerted should be prepared for a round-the-clock 

vigil un~il the end of the storm. Their preparations 

should provide for a nighttime illumination of the 

slopes above them. 

The preparations for each adult and older child 

should also include a careful look at the slopes above 

and below their homes during clear weather. They 

should take careful note of slope angles and of the locations of small gullies (and even gentle swales) which 

may come the channels for flowing mud. Note should 

also be taken of prestorm soil moisture conditions that 

might advance the time of the threat to their property 

from upslope locations. Such conditions might be recognized in irrigated gardens or in natural springs and 

seeps. Downslope areas should also be examined for 

similar conditions, and care should be taken in design of 

landscaping and draina,ge so as not to increase the hazards to residents below. Evacuation routes and destinations should be planned, and alternatives studied. 

Evacuation centers should be carefully selected to avoid 

the sort oftragedy that occurred in Silverado Canyon on 

February 25, 1969, where a fire station being used to 

shelter about 60 storm refugees was struck by a debris 

flow, killing 5 persons and injuring 20 (fig. 28). 

Satisfactory contingency planning by individuals 

clearly requires effective and timely public education. 

Given a timely warning and a well-planned response, 

alert adults can expect to avoid injury, even though they 

may not be able to prevent damage to their homes. 

A WARNING SYSTEM 

The many variables that influence the origin of each 

individual debris flow make the prediction of small soil 

slips in specific places extremely difficult. Prediction 

might be possible if the geologic properties of entire 

slope areas were studied in detail and a network of 

instruments capable of continuously monitoring soil 

moisture were installed. Such extensive studies and 

instrumentation may be feasible during construction of 

a large subdivision but are not generally economic for 

one or two small residences at the foot of a steep slope 

area covering several acres. 

A means by which the general time of greatest 

debris-flow hazard may be recognized is suggested by 

the empirical association of soil slips with a threshold 

total of about 10 inches of rainfall and a minimum 

intensity of 0.25 inch per hour. Although these specific 

empirical numbers should be tested further and may 

need considerable revision, the basic association appears established. Moreover, it should be possible to use 

slope maps or topographic maps, in conjunction with 

hourly radar weather maps showing areas of moderateto high-intensity rainfall, to determine areas subject to 

the greatest hazard at particular times during a storm. 

A warning system, therefore, could be constructed of 

three major elements, each of which is partly or wholly 

operative at the present time: (1) A system of rain 
32 SOIL SLIPS, DEBRIS FLOWS, AND RAINSTORMS IN THE SANTA MONICA MOUNTAINS, SOUTHERN CALIFORNIA 

FIGURE 27.-Hole punched through house of975 Old Topanga Canyon Road, debris flow of January 25, 1969. Occupants reportedly escaped 

without injury. Soil-slip scar above house is shown in figure 3. Photograph by Department of the County Engineer, Los Angeles County. 

gages, recording total rainfall on an hourly basis; (2) a 

weather-mapping system capable of recognizing centers of high-intensity rainfall in the storm area and, at 

frequent intervals, plotting the locations of these centers with respect to locations of gages with adequate 

registry for accurate transfer to slope maps or topographic maps; and (3) an administrative and communications network to collate the data, recognize when 

critical factors have been exceeded in a particular area, 

and inform the residents there. Such a system is probably well within the capability of existing technology. 

SUPPLEMENTAL DATA ON ASSOCIATIONS OF 

DEBRIS FLOWS WITH HEAVY RAINFALL 

OF RECENT YEARS 

The following summary reports are intended to supplement _text discussions of the association of debris 

flows with rainstorms in coastal southern California 

over the past several years. The times and places of 

events that are interpreted as of soil-slip-debris-flow 

origin, and the association of each event or group of 

events with rainfall recorded at nearby continuously 

recording gages, need to be further examined in order to 

test any general hypothesis of rainfall-soil-slip relations. Nearly all ofthe pre-1969 reports oftimes, places, 

and nature of origin were taken from newspaper 

accounts-chiefly from the Los Angeles Times. (Sources 

of information are listed with each description.) The 

general sources of data for the 1969 storms are noted in 

the "Introduction" and "Acknowledgments" sections of 

this report; low-altitude oblique aerial photographs 

supplied by the Los Angeles County Engineer and the 

Department of Building and Safety (City of Los 

Angeles) were particularly useful in recognizing scars 
SUPPLEMENTAL DATA ON ASSOCIATIONS OF DEBRIS FLOWS WITH HEAVY RAINFALL OF RECENT YEARS 33 

of soil slips and freshly scoured ravines. 

The chief drawback to the use of news accounts is the 

lack of rigorously defined descriptive terminology. For 

example, the most widely used term is "mudslide"; 

there is however no widely accepted, scientifically 

characterized definition for the term. Although "mudslide" has been recognized and used in a few technical 

and semi technical papers (for example, Hutchinson and 

Bhandari, 1971; Rantz, 1970), the term has been applied to events of such diverse character as rapidly 

flowing debris and slowly creeping coherent soil mantle. 

Southern California newspapers have been generally 

consistent in using "mudslide" to describe soil slips and 

debris flows generated by soil slips. However, the same 

term is also frequently used to refer to other events such 

as very muddy floodflows of streams, rockfalls and 

slumps from steep highway cuts and bluffs (especially 

when they occur during heavy rains), and even to some 

deep-rooted rotational or block-glide types oflandslides 

that may be in motion at varying (but generally slow) 

rates. Many news stories contain enough background 

information on which to base a reliable inference concerning origin, but many others do not. 

The reader is referred to figures 9, 18, 19, and 20 for 

cumulative curves of recording rain gages that illustrate the times and distributions of episodes of heavy 

rainfall during four storms in 1962, 1965, and 1969. 

Numbers on various parts of the curves refer to individual events or to groups of events described. The locations of selected events are shown in figures 5 and 10. 

Notes include an evaluation of the probability that 

nearby gages represent the rainfall at the sites examined, as well as a discussion of factors that bear on 

the interpretation that the events began as soil slips. 

STORMS OF FEBRUARY 7- 19, 1962 

The storms of February 7-19, 1962, deposited their 

rainfall on ground that had already received seasonal 

totals of from 6.9 to 11.2 inches. The next previous 

rainfall had occurred in January, and had ended on the 

22d or 23d, about 2 weeks earlier. The early February 

soil moisture had accumulated from brief storms in late 

November and early December, and in mid-January, 

from which no severe damage by "mudslides" had been 

reported. 

A comparison of the various gage records (fig. 18) 

reveals some interesting patterns of distribution of individual episodes of the February storms. Episodes of 

heavy rainfall along the north side ofthe Los Angeles 

basin were not matched in intensity in the San Fernando Valley (Sepulveda dam and Burbank gages) on 

February 8 and 9. In contrast, the intensities in the 

valley and in the Santa Monica Mountains were nearly 

equal on February 10 and February 11, while the Civic 

Center gage indicated lower intensitiP-s in the central 

Los Angeles basin. The latter, howe·;er, may be anomalous with respect to nea .·by areas, as discussed on page 

22. 

Damage estimates published by the Los Angeles 

Times of February 17, 1962, totaled about 4 million 

dollars, about evenly divided between public and private property. (This amount is total storm damage, including flood damage and traffic accidents as well as 

"mudslide" damage.) 

The followir.g summary account is derived chiefly 

from newspaper reports published in the Los Angeles 

Times. The numbers refer to episodes of storm activity 

as indicated in fig·.1re 18. 

62-1: The report of several tons of mud and rock on the 

Pacific Coast Highway near Corral Beach (Los Angeles 

Times, February 9, 1962, p. 25) is probably better attributed to rockfall than to debris flow. The cliff above 

the highway cut just west of Corral Beach has been the 

site of many such failures. Activity reported as "numerous rock and mud slides" above Sunset Strip on Sunset 

Plaza Drive, on Hollywood Boulevard west of Laurel 

Canyon Boulevard, on Creston Drive in Laurel Canyon, 

on Mulholland Drive, and along the Angeles Crest 

Highway (op. cit., 27) probably included some debris 

flows of unknown origin. The Bel Air gage record is 

probably the most representative of the rainfall at the 

Sunset Plaza Drive, Hollywood Boulevard, Laurel Canyon, and Mulholland sites. It is, however, probably a 

minimum, as many of the sites are at higher altitudes 

than the gage and the storms commonly show a strong 

orographic effect. 

62-2: Reports of damage from mud in motion in the 

Laurel Canyon area-2611 Laurel Pass Avenue; 2227 

Nichols Canyon Road (north of Hollywood; Los Angeles 

Times, February 10, 1962)-and in the Topanga Canyon area (19963 Observation Drive); both are parts of 

the Santa Monica Mountains. Rainfall at those sites is 

best represented by Bel Air gage record, though that is 

probably a minimum because of the expected orographic effect of the storm at the generally higher altitudes of the sites of damage. The times of the events 

were not precisely determined. The descriptions permit 

(but do not require) soil-slip origin. 

62-2a: Damage from mud was reported (Los Angeles 

Times, February 10, 1962) at three places in Monterey 

Park (916 East Monney Drive; 923 County Road; 985 

County Road). The nearest recording gage is at Los 

Angeles. A comparison with the nonrecording gage at 

San Gabriel Fire Station, which is located much closer 

to the sites of failure in Monterey Park, indicates that 

the Monterey Park area may have received as much as 
34 SOIL SLIPS, DEBRIS FLOWS, AND RAINSTORMS IN THE SANTA MONICA MOUNTAINS, SOUTHERN CALIFORNIA 

0.43 inch more total rainfall on February 9 than the Los 

Angeles gage. If, as seems most likely, most of the 

excess fell during the same 3- to 4-hour period as the 

heavy rainfall at the Los Angeles gage, the intensity 

would hiwe been more than 0.30 inch per hour. The 

times of occurrence were not precisely determined; they 

were probably, but not demonstrably, debris flows of 

soil-slip origin. 

62-3: "Mudslides" were reported in the Bel Air, Mandeville Canyon, and Brentwood areas (Los Angeles 

Times, February 11, 1962). The nearest recording gage 

is at Bel Air and is probably representative of the 

minimum total and intensities at sites of failure. The 

descriptions permit (but do not require) soil-slip origin. 

12 : Fatal debris flow occurred "early Sunday morning" 

February 11, 1962, killing one child "in bed" while 

damaging a residence in Sherman Oaks (Los Angeles 

Times, February 12, 1962, p. 1, photographs p. 3). The 

nearby recording gage record at Sepulveda dam is probA, Fire station, located on the flood plain of a major stream at the foot 

of a long steep slope drained by low-order tributary gullies. 

ably representative of the minimum total and intensity 

at the place and time offailure. Descriptions and photographs permit and suggest (but do not require) origin by 

soil slip. (Address: 3519 Camino de la Cumbre, Sherman Oaks.) (See figs. 5, 18). 

13 : A debris flow crushed the rear of a house in Monterey Park about 1:00 a.m. Sunday February 11, 1962, 

killing one child in bed (Los Angeles Times, February 

12, 1962, p. 1, 2, and photographs p. 3.) The nearest 

recording gage is at Los Angeles Civic Center, and 

that record is probably not fully representative of storm 

conditions at the time and place of the failure. A comparison with the nonrecording gage at San Gabriel Fire 

Station, which is located closer to the sites offailure in 

Monterey Park, shows that on February 11 the Los 

Angeles station received a total of 1.24 inches while the 

San Gabriel Fire Station received 2.43 inches, nearly 

twice as much. It is evident that some significant storm 

activity in the Monterey Park area was not represented 

B, Slope above rear of fire station. Note head of scar in gully at upper 

right and mouth of gully directly upslope from the rear wall of 

the station. 

FIGURE 28.-Three views of Silverado Canyon Fire Station. Debris flow of February 25, 1969, broke through back wall an swept the 

taken August 2, 
STORMS OF FEBRUARY 7-19, 1962 35 

at downtown Los Angeles. The debris flow was very 

probably of soil-slip origin. (Address: 2011 Emerald 

Way, Monterey Park.) (See figs. 5, 18.) 

62-5: Many "mudslides" reported (Los Angeles Times, 

February 13, 1962, p. 1, photographs p. 3). Hollywood 

hills "hard hit"; area ordinarily best represented by 

recording gage at Bel Air, which was not operating at 

the time. Other gages, such as Sepulveda Dam, Burbank, and Los Angeles, give fair indication of conditions 

at times and places offailures. (Note on fig. 18 that the 

area of storm activity at that time apparently did not 

extend as far west as the Lechuza gage.) Descriptions 

and photographs show damage from debris flows; they 

were probably, but not necessarily, of soil-slip origin. 

Damage described as: 

1. 2934 Beechwood-shoved off foundation 

2. General area of Beechwood ran mud and debris; 

Cahuenga Boulevard partly closed. 

3. Photograph (op. cit., p. 3) shows cars mired in 

mud in vicinity of Beech\yood. 

62-6: The other area "hard hit" on the morning of the 

12th was the Burbank-Sierra Madre area (Los Angeles 

Times, February 13, 1962, p. 1, photographs p. 3), probably best represented by the gage record at Burbank. In 

Sierra Madre, a "mudslide" buried to its eaves a residence at 660 Canyon Crest Drive. The photograph (op. 

cit., p. 3) shows no muddy marks or water marks on the 

exterior of the house above the level of the firm mud 

deposit, a strong indication that the mud was emplaced 

as a debris flow. It was probably (but not necessarily) of 

soil-slip origin. The time was not precisely reported, but 

the event was reportedly associated with the "final fury 

of the storm" which occurred at about 4:00a.m. Monday 

(February 12). Also in Sierra Madre, at about the same 

time, a house at 1440 Carriage House Road was flooded 

with mud nearly to the ceiling, and a swimming pool 

was filled with mud. 

In Burbank, at the same time, floodwaters, mud, and 

tons of rocks reportedly crashed down a narrow canyon 

road and into nearly every home along a mile-long 

stretch of Country Club Drive, swamped swimming 

pools and yards, and mired 30 cars and 3 boats on trailers. A photograph (Los Angeles Times, February 13, 

1962, p. 3) shows the interior of one house with the mud 

deposited inside. The lack of mud or water marks above 

the top of the deposit strongly indicates emplacement 

by debris flow. The flow was probably at least partly of 

soil-slip origin. 

C, Natural levee deposits on steep, small debris-flow fan, the toe of which has been undercut to increase the fiat area where the station 

was built. 

length of the refugee-filled building, killing five. Note relative lack of severe structural damage to side walls and doors. Photographs 

1969, after cleanup-_ 
36 SOIL SLIPS, DEBRIS FLOWS, AND RAINSTORMS IN THE SANTA MONICA MOUNTAINS, SOUTHERN CALIFORNIA 

No new debris flows or slope failures were reported for 

the 2-day lull of February 13-14. The Los Angeles 

Times (February 15, 1962), however, did report continued movement of landslides in the Alginet Drive 

area of Encino that, from the descriptions (including 

cracks beginning to open a month earlier), were clearly 

not soil slips nor did they develop debris flows. 

62-7: The Los Angeles Times for Friday, February 16 

(p. 1, 30), reports that the storm of Thursday, February 

15, was of short duration and heavy rainfall-heaviest 

in Montibello and Santa Monica. A garage wall in West 

Los Angeles (9780 Pea vine Drive) was punched through 

when "boulders tumbled from hillside at rear." (This 

description could fit either a rockfall or soil-slipdebris-flow event.) The Bel Air gage is probably near 

enough to be representative of the rainfall at the site of 

the failure. 

Other reports covering this storm period deal cbefly 

with larger, more slowly moving landslides and their 

continued movement. John Lambie, County Engineer, 

Los Angeles County, announced that although street 

and road flooding were widespread, the landslide problems in hill and canyon areas were not so severe as those 

that had resulted from the earlier storm. 

No new debris flows or other slope failures were reported for the 2-day lull of February 16-17. The Los 

Angeles Times (February 18, 1962, section K, local 

classified, San Gabriel Valley, p. 2) printed a photograph of the debris flow at 2011 Emerald Way, Monterey Park; (see number 13, fig. 18). It was fatal to a 

9-year-old child whose parents were watching the slope 

through a window from al.'1ther room when the slide 

suddenly crashed down into the boy's bedroom. The 

suddenness of the event indicates origin by soil slip. 

62-8: In East Los Angeles a residence (3908 Eagle 

Street) was abandoned when a slide washed mud and 

water through the kitchen (Los Angeles Times, February 20, 1962) during severe storm on the morning of 

February 19. The recording gage at Los Angeles probably yields a fair representation of the intensity and 

duration of rainfall at the site offailure. The slide was 

possibly (but not necessarily) of soil-slip origin. 

(The "big" weather news for February 19 was the 

occurrence of damage from tornados in Northridge and 

Santa Ana.) 

The Los Angeles Times, February 21 and 22, reports 

minor damage from two more short, localized flurries 

that are not plotted in figure 18. The first occurred on 

the evening of February 20, and apparently it caused 

some minor debris flows that partly blocked roads in the 

Benedict Canyon area of the Santa Monica Mountains. 

The second, on late Wednesday, February 21, was 

heaviest in the San Gabriel Valley-Whittier area 

where minor landslides were quickly removed by street 

maintenance crews. The Bel Air gage would provide a 

representative record of rainfall at the Benedict Canyon 

site, but it was apparently not operating on February 

20. The Los Angeles gage is the nearest of those plotted 

(fig. 18) to the San Gabriel Valley-Whittier area, but 

judging from the higher totals registered at nonrecording gages in the most affected area, the storm intensities there may have been as much as two or three 

times as great as in downtown Los Angeles. Some 

damaging debris flows occurred, but they were probably 

not all of soil-slip origin. 

STORMS OF NOVEMBER 14 TO DECEMBER 30, 1965 

The rains ofthe 1965--1966 winter season began unseasonably early-in September 1965. A few scattered 

light showers fell on September 5 and 6. More scattered 

showers fell on September 16 and 17, but on the 18th the 

showers turned into extremely heavy (as much as 1 inch 

per hour) rains of relatively short duration and irregular distribution. (For example, Los Angeles Civic 

Center reported 1.39 inches for the same day that Bel 

Air reported no precipitation. The rain tapered back to 

scattered, light showers on the 19th. Although Elford 

(1966, p. 314) reported damage from "mud flows" as well 

as flooding, an examination of newspaper accounts of 

the storm damage indicates that debris flows of soil-slip 

origin probably did not occur. During the remainder of 

September, throughout October and through the first 

two weeks of November, only traces of precipitation 

were reported in the area. The series of storms of 

November 14 through December 30 is illustrated by 

figure 19. The numbered horizontal lines (fig. 19) indicate the approximate timespans covered by newspaper 

reports that are summarized and keyed to those numbers in the following text. 

September 1965: No storm damage was reported in the 

Los Angeles Times until September 19, when a story by 

Eric Malnic (p. B) listed reports of street blockage by 

flowing mud, debris, and water (presumably a mixture, 

rather than discrete spurts of each) in Highland Park. 

Also in Highland Park, Malnic (Los Angeles Times September 19, 1965, p. B) related reports of mud oozing into 

a backyard of a residence at 442 West Avenue 37 from a 

road construction project above. The reports do not mention "mudslides" or "landslides," and probably do not 

refer to debris flows of soil-slip origin. The first incident 

is probably best interpreted as surface-runoff flooding. 

While the second incident probably depicts a slow debris 

flow, its origin in an area of active construction complicates any interpretation of mechanism of origin. Although the intensity of rainfall was high (the Malnic 

story reports over 1 inch per hour locally), 
STORMS OF NOVEMBER 14 TO DECEMBER 30, 1965 37 

soil-slip-debris-flow activity was, at most, extremely 

sparse and slight, and it may have been non•3xistent. 

The Times of September 20 recapped the unusually 

early storm with emphasis on the early snow in the 

mountains, extraordinarily high total rainfall for September, and traffic deaths on rain-slickened streets, 

without mention of landslides, mudslides, mud or debris. 

September 19 through November 13, 1965: From September 19 through November 13, only trace amounts of 

precipitation were ·recorded in the area. The Los 

Angeles Times for the morning of November 15 provides the first indication of storm damage, referring to 

rains that fell on the 14th and, perhaps, including rain 

of the early hours of the morning of the 15th (fig. 19). 

Most reports were clearly of flood damage. The only 

account even vaguely resembling a debris flow was that 

of a ~~river of mud" in the 3800-3900 block of Bluff 

Street, Palos Verdes Hills, whose origin was attributed 

to ~~runoff waters in a uncompleted subdivision." 

65.1: On the morning of November 16, the Los Angeles 

Times, reporting events chiefly of November 15, noted: 

(a) A mudslide from a hillside construction project in 

Torrance invaded the yards of homes at 3429 and 4255 

Newton Street. Its origin was attributed to water runoff 

(Los Angeles Times, November 16, p. 1). (b) In the Santa 

Monica Mountains, expensive homes in the 1800 block 

of Laurel Canyon Boulevard were ~~threatened" (op. cit., 

p. 1), and small slides were noted along Beverly Canyon 

Drive by the Bureau of Street Maintenance (op. cit., p. 

3). From these accounts, the events in Torrance and in 

Laurel Canyon probably included debris flows, and 

those on Beverly Canyon Drive possibly included debris 

flows. It seems unlikely that the origin of the Torrance 

events involved soil slips. Soil slips may have contributed to the Laurel Canyon and Beverly Canyon 

events but are not required by the data in the reports. 

Specific times for the events were not reported; however, there was at least one significant flurry of highintensity rainfall at nearly all the recording gages between 6:00p.m. and 11:00 p.m. on November 15 during 

the report period. 

65.2: On the morning of November 17, the Los Angeles 

Times noted that continuing rainfall had caused additional problems from mud and debris: (a) In the Palos 

Verdes Hills section of Torrance at Newton and Bluff 

Streets (p. 1, cc) there was possibly but not necessarily a 

debris flow. (b) In the hills of Glendale, Chevy Chase 

Boulevard between Glenoaks and Linda Vista was 

blocked by mud and debris (p. 1); the blockage was 

probably, but not necessarily from a debris flow. (c) In 

the Santa Monica Mountains, Topanga Canyon 

Boulevard was listed as impassable (p. 1), and there 

w~re problems with mud and debris above Hollywood 

at 1825 Prospect Drive (p. 1, cc). The material probably, 

but not necessarily, includes some debris flows. (d) To 

the north, U.S. Highway 99 and Interstate 5 were partly 

blocked by ~~mudslides" in the Welden Canyon area, and 

the Santa Susana Pass Road was closed by a uslide"·(p. 

1). These blockages probably, but not necessarily~ include some debris flows. Many flooded areas were noted 

also. Soil slips may have contributed to some of the 

probable debris flows, but none are required by the data 

in the reports. Specific times for events were not reported; however, all gages examined (fig. 19) recorded 

steady rainfall beginning before noon on November 16, 

climaxed by a general high-intensity rainfall between 

about 10:00 p.m., November 16, and about 3:00 a.m., 

November 17, during the probable report period. 

65.3: On the morning of November 18, the Los Angeles 

Times reported widespread damage from high water 

and flood erosion. In the vicinity of the Santa Monica 

Mountains: (a) ~~Mudslides" were reported clogging 

Beachwood Drive in Hollywood and Sunset Boulevard 

in Pacific Palisades (Los Angeles Times, November 18, 

1965, p. 1), and thick deposits of silt and mud were 

reported (op. cit., p. 36) in the Hollywood hills at 1836 

Laurel Canyon Boulevard and in Mandeville Canyon. 

These deposits probably include some debris flows. Mud 

deposited on properties on Prospect Drive and Laurel 

Canyon Boulevard was attributed to runoff water that 

cut a deep channel in an unpaved road in the area (op. 

cit., p. 1). (b) In the Elysian Park hills, Glendale 

Boulevard between Riverside Drive and Waverly Drive 

was barricaded because of ·~slides" (p. 1); the account 

does not distinguish whether loss of roadway was due to 

moving slide mass or blockage by slide debris. (c) Along 

the east side of the San Fernando Valley, ~~landslides" 

were reported in the vicinity of Little Tujunga and 

Pacoima Canyons (op. cit., p. 36), and the mud and 

debris that reportedly ~~rolled out of Stetson Canyon" 

above Sylmar (op. cit., p. 1) probably included some 

debris flows. (d) On the north side ofSan Gabriel Valley, 

~~landslides" reportedly closed Fish Canyon Road above 

Duarte, and in Temple City, pavement was eroded by 

floodwaters (op. cit., p. 36). (e) Floods were reported in 

the Simi Valley, and the Thousand Oaks area was troubled with high water (op. cit., p. 1). (f) In Santa Susana 

Pass the road was blocked by rocks and mud, and in 

nearby Chatsworth, some retaining walls collapsed in 

Box Canyon (op. cit., p. 1). (g) ((Mudslides" were reported 

as occurring in the San Diego area. 

Some of the foregoing are probably best assigned to 

~~debris flows," but for many others the assignment is 

questionable. Although soil slips may have contributed 

to some of them, none is required by the data in the 

reports. Specific times for the above events were not 
38 SOIL SLIPS, DEBRIS FWWS, AND RAINSTORMS IN THE SANTA MONICA MOUNTAINS, SOUTHERN CALIFORNIA 

reported; however, the curves (fig. 19) show that the 

steady rainfall that had begun on November 15 had 

continued until nearly midnight on November 17. The 

report period covers two parts of the storm in which 

high-intensity rainfall occurred. Many events associated with the general high-intensity flurry of the 

early morning hours of November 17 were probably not 

reported until the morning of the 18th. The highintensity phases of the evening of November 17 were 

somewhat lower and less pervasively distributed than 

the previous one, and it shows a stronger orographic 

effect with higher intensities at the higher stations. 

November 18 through 21 was relatively clear with a 

few sparsely scattered showers. The only new site of 

storm damage reported was the closing of Sepulveda 

Boulevard east of Torrance, between Normandie and 

Western, because of mud, debris, and floodwaters (Los 

Angeles Times, November 19, 1965, p. 3). The closing 

was probably caused by a flood runoff problem rather 

than debris flow, judging from the low relief of the area. 

By the morning of November 22, showers had begun, 

and heavier rains were predicted to follow (Los Angeles 

Times, November 22, 1965). Storm totals through 

Thursday, November 18, in adjacent areas were 13 

'inches at Lake Arrowhead in the San Bernardino 

Mountains (Los Angeles Times, November 19, 1965, p. 

3) and 15 inches in the Simi Valley area of Ventura 

County (Los Angeles Times, November 22, 1965) 

65.4: On the morning of November 23, the Los Angeles 

Times reported: (a) There was extensive damage as 

eight homes along Laurel Canyon Boulevard were invaded by mud and water, attributed to drainage from a 

subdivision under construction uphill from the damage 

(Los Angeles Times, November 23, 1965, p. 26). A 

photograph (op. cit., p. 3) shows mud flowing around the 

garage at 1836 Laurel Canyon Boulevard; because 

there is no free water, it is very probably a debris flow, 

possibly, but doubtfully, of soil-slip origin. (b) In Woodland Hills, Topanga Canyon Boulevard was closed by a 

ttmudslide" between Ventura Boulevard and Mulholland Drive (op. cit., p. 26); it was very probably caused 

by a debris flow, and was possibly but not necessarily of 

soil-slip origin. (c) In Thousand Oaks, Meadows School 

was closed for fear that an access road would be blocked 

by a ttmudslide." (d) In Pacific Palisades a ttslide" closed 

the Pacific Coast Highway (op. cit., p. 3). J. T. McGill 

(written commun., 1972) assigns it to soilfall-debris 

avalanche class. (e) Minor ttmudslides" were reported 

along Rosecrans Avenue in Fullerton, Orange County 

(Los Angeles Times, November 23, 1965, p. 8). 

Some of the foregoing are clearly debris flows. Soil 

slips may have contributed to some, but no unequivocal 

assignment to that origin can be made. Specific times 

for the events were not reported; however, the curves 

(fig. 19) show that rainfall of high intensity was continuous throughout much of the probable reporting 

period, beginning about 4:00 a.m. and continuing to 

about 10:00 p.m. on November 22. 

65.5: On the morning of November 24, the Los Angeles 

Times reported no further storm damage in the Los 

Angeles area but reported (p. 3) flash floods and umudslides" farther south including three fatalities in Tijuana, Mexico where two children were killed when 

their family residence (a uflimsy shack") was crushed by 

debris and an elderly woman was killed at her home ina 

ttmudslide." 

65.6: On the morning of November 25, the Times' 

stories featured discussion of mopping-up, fears of more 

damage if new rains should come, disaster assessments, 

and financial aid programs. Although there was a relatively short flurry of moderate to heavy rainfall on the 

morning of November 24, between about 3:00 a.m. and 

12:00 noon it received no specific mention, and there 

was no report of new damage. 

65.7: On the morning of Friday, November 26, the Los 

Angeles Times' weather news emphasized high winds 

and a tornado in Pomona and noted that weather was 

clearing. Although there had been a short general 

flurry between about 6:00a.m. and 8:00a.m., November 

25, no new damage was reported aside from that caused 

by the tornado. 

During the 13-day lull that followed, there were no 

reports of new damage that could be interpreted as 

debris flow. The lull was broken by a couple of short, 

locally heavy rain flurries beginning about 7:00p.m. on 

December 9, 1965. 

65.8: On Friday morning, December 10, the Los Angeles 

Times (p. 1, 30) reported mostly light to moderate rain, 

locally heavy rains with thunder and lightning, but no 

major flooding or slides in the Los Angeles area. In San 

Diego, however, there were heavy rains and floods with 

damage and evacuations, and at Needles, heavy rains. 

65.9: On Saturday morning, December 11, the Los 

Angeles Times (p. 1) reported that the storm was fading, 

but that some clouds remained. There were no reports of 

floods or slope failure. 

The 17-day lull that followed was broken by only a 

few scattered showers. One ttslide" that occurred in 

Pacific Palisades on December 25, about 2:00 a.m., 

blocked the Pacific Coast Highway between Chautauqua and Sunset Boulevards (Los Angeles Times, December 28, 1965, p. 3). It was very probably a soilfall 

from the coastal bluff. There were no other reports of 

new events that could be interpreted as being caused by 

debris flows. 

65.10: On Wednesday morning, December 29, the Los 
STORM PERIOD OF JANUARY 18-25, 1969 39 

Angeles Times reported showers falling and predictions 

of rain, heavy at times for the Los Angeles area, from a 

north Pacific storm already pounding northern California. There were no reports of new events that could be 

interpreted as being caused by debris flows. 

65.11: On Thursday morning, December 30, the Los 

Angeles Times reported extensive damage from heavy 

rains. Flooding was reported in the San Fernando Valley (p. 1), in the Encino area (p. 3), and in Ventura 

County in the Simi Valley, and at Oak View (p. 3) as the 

storm swept southeast through Los Angeles. Several 

road washouts were noted (op. cit., p. 1). ~~Mudslides" 

and flooding were reported north of Newhall (op. cit., 

p. 3); ~~mudslides" were reported in the Jackson Lake 

area, near Big Pines (op. cit., p. 3); and many highways 

in the mountains and canyons were closed by landslides, including 1-15 at Cajon Pass (op. cit., p. 3). In the 

Santa Monica Mountains, a woman was reported missing (op. cit., p. 3) when her residence at 1400 Latigo 

Canyon Road was destroyed. 

This last event has been documented as a debris flow 

of soil-slip origin (seep. 42-46) and is shown in figures 5 

and 19 as item 14. Of the other reported events that 

might have been of soil-slip--debris-flow character, the 

one near Big Pines was sufficiently close to one of the 

plotted recording gages-the San Dimas Tanbark Flat 

gage-to provide an acceptable basis for the evaluation 

of storm conditions. Specific times for the events were 

not reported, but the contexts contain no implications 

that any of them occurred before or after the general 

period of high-intensity rainfall between about 3:00 

a.m. and about 2:00p.m. on December 29. 

65.12: On Friday morning, December 31, the Los 

Angeles Times (p. 1, 3) reported that a search of debris 

for the missing woman continued. The recovery of her 

body was reported in the Los Angeles Times the following morning (January 1, 1966, p. 2). Mud (of unreported 

origin) was photographed on Chevy Chase Drive in 

Glendale, where it blocked the street (Los Angeles 

Times, December 31, 1965, pt. 2, p. 1). The Los Angeles 

Times of Saturday morning, January 1, 1966, also reported a home above the Sunset Strip as wrecked by a 

landslide (p. 2). This landslide was probably not a debris 

flow. 

The end of the high-intensity rainfall on the afternoon of December 29 apparently marked the end of the 

severe storm activity that had begun on November 14. 

STORM PERIOD OF JANUARY 18-25, 1969 

ments" of this report. The numbers following the prefix 69 refer to locations plotted on the map (fig. 5) 

and times plotted on the cumulative rainfall curves 

(fig. 9). 

69.1. Glendora-Azusa area, rrmorning," January 22, 

1969: First spate of debris flows inundated some residential areas. Total rainfall and intensities in tributary watersheds are probably well represented by the 

gage record for (!San Dimas Tanbark Flat." Kevin Scott 

(1971, p. C242-C24 7) attributes origin, in large part, to 

superficial slope failures in the short steep drainage 

basins on the south flank of the San Gabriel Mountains; 

he cites scars as evidence. 

69.2 Glendora-Azusa area, rrmorning," January 25, 

1969: Debris flows again inundate some residential 

areas, many of the same ones damaged by the flows of 

January 22. Scott again attributes origin largely to 

superficial slope failures. Rainfall total and intensity 

probably well represented by gage ~~san Dimas Tanbark 

Flat." 

69.10. Glendale, Chevy Chase Canyon area, 7:00a.m., 

January 22, 1969: Glendale News-Press reports that a 

residence at 3086 Chevy Chase Drive damaged by 

~~mudslide" at 7:00 a.m.; occupants escaped without injury. The approaching slug was heard, and described as 

a ~~crash" or a ~~roar." A soil-slip origin is inferred from 

its description as a single-pulse episode; no photograph 

or description of scar. Reports also mention other 

storm-related activity that includes problems clearly of 

surface-runoff origin, such as washouts and gullying of 

roads. 

69.9. Glendale, Chevy Chase Canyon area, 6:30 a.m., 

January 25, 1969: Two persons were killed at 3048 

Buckingham Road when walls of bedroom were crushed 

by a ~~mudslide." A soil-slip origin is inferred from the 

apparent single-pulse episode as described in Glendale 

News-Press report dated January 25. (See Sunday 

Mercury-News, San Jose, California, for photograph of 

rescue operations at site, credited as Associated Press 

Wirephoto). No photographs or description of scar. Gage 

~~Flintridge FC" is probably best representative of rainfall character at site of Hmudslide." Much other ~~mud-

·slide" activity was reported (Glendale News-Press) 

in the same general neighborhood at the same time. 

Some descriptions indicate soil-slip--debris-flow character rather clearly; however, damage from other types of 

landslides and from muddy floodwaters has also been 

included. 

The following paragraphs summarize episodes of 69.8. Highland Park, 9:00a.m., January 25, 1969: Two 

mudflow activity in specific areas and specific children were killed when their home at 1279 El Paso 

debris-flow events as reported by the general sources Drive was crushed by a debris flow at about 9:00 a.m., 

listed in the sections (!Introduction" and ~~Acknowledg- January 25, 1969 (time and address from report in Los 
40 SOIL SLIPS, DEBRIS FLOWS, AND RAINSTORMS IN THE SANTA MONICA MOUNTAINS, SOUTHERN CALIFORNIA 

Angeles Times for January 26, 1969). A soil-slip origin 

is documented by photographs (Photograph by Associated Press Wirephoto on p. 16, San Jose MercuryNews for January 26; photograph by Los Angeles City 

Department ofBuilding and Safety, see fig. 15). Rainfall 

characteristics at site of failure probably best represented by gage ((Los Angeles Civic Center." 

69.5. Sherman Oaks, 6:00a.m., January 25, 1969: One 

person was killed by a debris flow that crashed into the 

back bedroom of a residence at 15421 Deerhorn at 6:00 

a.m., January 25 (C. A. Yelverton, oral commun., July 

27, 1969). A soil-slip origin is indicated by the scar. 

Rainfall at the failure site is probably well represented 

by the characteristics of the curve for gage ((Sepulveda 

Dam." (See fig. 12.) 

69.6 Sherman Oaks, 6:50 a.m., January 25, 1969: One 

person was killed by a debris flow that broke through 

bedroom wall of a residence at 3830 Sherview at 6:50 

a.m., January 25 (C. A. Yelverton, oral commun., July 

20, 1969). A soil-slip origin is indicated by the scar. The 

rainfall at the failure site is probably well represented 

by the characteristics of curve for gage ((Sepulveda 

Dam." 

69.3 Brentwood, Mandeville Canyon area, 12:30 a.m., 

January 25, 1969: A debris flow C(mudslide") crashed 

into back of a house killing one man in bed at 2077 

Mandeville Canyon Road. About 45 minutes later, six 

firemen attempting to rescue him were temporarily 

trapped in the house by a second slide. A photograph 

(fig. 11), courtesy of C. A. Yelverton, Los Angeles (City) 

Department of Building and Safety, shows the soil-slip 

scars from which the flows were derived. (C. A. Yelverton, oral commun. and photograph, July 29, 1969.) 

69.4 Topanga area, about 3:00a.m., January 25, 1969: 

Three people were killed and one seriously injured 

when a debris flow crushed the rear of a house about 

0.15 mile north ofthe Greenleaf Canyon bridge. Part of 

the house was pushed into the floodwaters of Topanga 

Creek, normally about 15 feet below, and carried downstream. Scars establish the soil-slip origin. The time of 

the event is tentatively established by neighbors who 

were awakened by less damaging slides on their own 

property; they heard a noise that they believe to have 

marked the destructive slide nearby at about 3:00a.m. 

(See case description, p. 46-48.) 

None of the continuously recording gages is sufficiently 

close to be satisfactorily representative of the site of 

failure; however, the site is about midway between the 

gage locations at ((Sepulveda Dam" and ((Lechuza 

Point," so that the two gages probably bracket the rainfall at the failure site (fig. 9). 

69.4a. Old Topanga Canyon area, between 1:00 a.m. and 

9:00 a.m., January 25, 1969: One man was killed by 

debris flow at 87 4 Old Topanga Canyon Road. The impact of the debris broke a hole in the back wall of the 

house through which the flow entered at high velocity. 

The soil-slip origin was indicated by slide scars (ravine 

fill). The time was established from accounts of neighbors. The neighbors also described many other debris 

flows, some causing serious damage, beginning between 

midnight and 12:30 a.m., January 25, and reaching a 

climactic frequency about 3:00 a.m. (See case description, p. 48-50.) 

69.7. Thousand Oaks, 7:45a.m., January25, 1969: One 

person was killed by debris flow which crashed into the 

back bedroom of the residence at 818 Combes Road. The 

soil-slip origin is documented by scars (see fig. 13) and 

by an eye-witness account published in the Thousand 

Oaks News-Chronicle of Sunday, January 26, 1969. 

Rainfall characteristics are not well represented because there is no nearby continuously recording gage; 

however, reasonable maximum and minimum totals 

and intensities may be inferred to be somewhere between the curves for gages ((Lechuza Point" and 

((Sepulveda Dam." C(Lechuza Point" is at about the 

same meridian, but much higher; ((Sepulveda Dam" is 

about the same altitude, but far to the east.) 

STORM PERIOD OF FEBRUARY 5--25, 1969 

The following summary of episodes of storm and 

debris-flow activity is taken largely from accounts in 

the Los Angeles Times covering the stated period. The 

numbers following the prefix 69 that head the paragraphs on the events of February 25 refer to locations 

plotted on the map (fig. 5) and times plotted on t.he 

cumulative rainfall curves (fig. 20). 

On Wednesday morning, February 5, the Times reported (p. 1) that scattered showers fell in advance of the 

storm, more showers were expected during Wednesday, 

and storm rains by Thursday. 

On Thursday morning, February 6, the Times reported (p. 1, 32) thunderstorms, but no floods or landslides were noted. 

On Friday morning, February 7, the Times (p. 1) 

reported ((giant mudslides" in the Glendora area and 

other storm damage, all apparently occurring on 

Thursday. Reports included a page-1 photograph of debris deposited in a suburban neighborhood in Glendora. 

Flooding and high winds-including tornadoes-were 

reported at several locations. These events are at 

localities not well represented by the gages plotted in 

figure 20, but the general period of high-intensity rainfall indicated on the morning of Thursday, February 6, 

is the most probable association. Although the Mount 

Baldy gage is at a much higher altitude, whPre the 
STORM PERIOD OF FEBRUARY 5-25, 1969 41 

orographic effect usually produces heavier rainfall, a 

visual comparison of the hourly records of that gage 

with those of the gage at San Dimas Tanbark Flat in the 

mountains directly above Glendora (U.S. Environmental Science Services Administration, 1969A) establishes that the general periods ofhigh-intensity rainfall 

for the two gages were virtually identical. 

On the morning of Saturday, February 8, the Times 

noted only a slow-moving landslide at Princess Park 

Estates, in the Newhall area. The weather was clear 

and remained so until Tuesday, February 11. Photographs of some flood damage in Big Tujunga wash, from 

the storm of February 6, were published in the Times for 

Sunday, February 9 (section L, p. 1). 

On the morning of Wednesday, February 12, the 

Times reported that a storm front had reached Los 

Angeles Tuesday night and that heavy rains might be 

expected. 

On Thursday morning, February 13, the Times reported that the expected storm had brought only a few 

showers. Reports and photographs of damage from 

landslides on the Pomona Freeway (p. 1), at Mount 

Washington, and in Laurel Canyon (p. 3) show very 

clearly that they are probably of the slump-earthflow 

class, involving deep rotational slip surfaces and large 

volumes of material. 

On Friday morning, February 14, the Times reported 

no new damage, yo new rainfall; however, a photograph 

(p. 1) shows a good view of the slide on the Pomona 

Freeway. Judging from the crown scar and deposit 

head, the failure was a deep rotational slump in weak 

late Tertiary sedimentary rocks. The toe has the steepfronted lobate appearance of a very viscous debris flow, 

and discrete blocks are few, but accounts of one witness 

who ((rode it out" in his car indicates that the movement 

was very rapid. 

On Saturday morning, February 15, the Times reported that scattered light showers fell Friday night; 

clouds and rain were forecast. 

On Sunday morning, February 16, the Times reported a short period of heavy rain, leaving about Ya inch 

at the Civic Center between 6:00p.m. and 7:00p.m. on 

Saturday; snow was reported at Mount Baldy and a 

chance of showers was forecast. No new slides were 

reported. Mention was made of continued movement of 

the slide in the Mount Washington area which overturned a house. 

No rain or storm damage was reported on Monday, 

February 17, or Tuesday, February 18. 

The light rainfall that began on Tuesday, February 

18, continued through to the evening hours of Wednesday, February 19, but generally did not exceed 0.10 inch 

per hour except at the highest altitudes, and even there, 

it rained only for an hour or two on the evening of 

February 19. The maximum hourly intensities recorded 

in the Los Angeles area during that period were 0.22 

inch for the hour ending at 7:00p.m., February 19, at 

the Mount Wilson gage and 0.18 inch for the hour ending 10:00 p.m., February 19, at the San Dimas Tanbark 

Flat gage. 

On Wednesday morning, February 19, the Times reported (p. 1) that several homes in the Encino area were 

evacuated when mud threatened to slide into two of 

them. (The relatively slow rates of movement implied 

suggest that the slide may have been a slump of some 

sort, although the soil-slip-debris-flow class cannot be 

excluded.) In the Castellammare area of Pacific 

Palisades, a slow-moving slide was reported (Los 

Angeles Times, February 19, 1969, p. 22) in an area long 

plagued with slumps on complex surfaces of weakness 

in highly deformed bedrock. No unequivocal debris 

flows were reported even though scattered rains of light 

to moderate intensity fell during the probable report 

period (Tuesday, February 18, and the first few hours of 

Wednesday, February 19). 

On Thursday morning, February 20, the Times reported (p. 1) slides in Laurel Canyon, Encino, and Highland Park occurring on Wednesday. A photograph (op. 

cit., p. 3) of the slide in Encino shows deep cracking that 

affects a house on the slide mass. The slide seems to be a 

large slump. The description (op. cit., p. 3) of the Laurel 

Canyon slide suggests a massive slump rather than a 

soil slip. At Highland Park, the slide description (op. 

cit., p. 3) mentions a utwisting fissure" beginning to 

((buckle" two houses. This seems most likely to be associated with slump failure rather than soil slip. Two 

descriptions on page 3 possibly, but not necessarily, 

represent debris flows: one mentions the ucaving in" of 

the rear part of an abandoned house, and at another 

locality a garage was reported ubuckled." However, no 

unequivocal debris flows were recorded. 

No rain fell on Thursday, February 20, over most of 

the area, and no new damage was reported in the Times 

on the morning of Friday, February 21. 

On Saturday morning, February 22, the Times reported that rain had begun Friday night and was expected to continue until Monday, with possible heavy 

rains forecast for Sunday. The brief, heavy rainfall of 

2:00a.m., February 22, (fig. 20) resulted in no reports of 

debris-flow activity. 

On Sunday morning, February 23, the Times reported scattered showers falling Saturday night. A 

umudslide" was reported blocking Mount Baldy Road 

about 9 miles above Claremont and umudslides" were 

reported blocking Valencia Road and Ortega Highway 

in Orange County. 

On Monday morning, ~ebruary 24, the Times reported heavy rain (see also fig. 20) and snow in the 
42 SOIL SLIPS, DEBRIS FLOWS, AND RAINSTORMS IN THE SANTA MONICA MOUNTAINS, SOUTHERN CALIFORNIA 

mountains. A ((mudslide" was reported (op. cit., p. 1) on 

Glen Oaks Boulevard in Pasadena in the San Rafael 

Hills. On page 3 it was noted that although there were 

no new areas with earthslides in the Los Angeles area, 

areas affected in the previous weeks were still unstable; 

and at Newhall there was a renewed threat of landslides. 

The storm episode that began the afternoon of Saturday, February 23; extended through the morning hours 

of Tuesday, February 25, with little respite. On the 

morning of Tuesday, February 25, the Times reported 

widespread flooding, earthslides, and mudslides. In the 

Santa Ana Mountains, two damaging ((mudslides" were 

reported in Silverado Canyon and another in Mojeska 

Canyon (op. cit., p. 2). ((Mudslides" contributed to the 

closingofnumerous roads in the State (op. cit., p. 2). The 

large slump at Mount Washington continued to move 

causing more damage (op. cit., p. 1, 20), but conditions in 

other areas of active slumping, including Highland 

Park and Laurel Canyon, remained unchanged (op. cit., 

p. 2). 

On Wednesday morning, February 26, the Times reported the continuation of widespread flood and ((mudslide" damage through the morning of Tuesday, February 25. Two ((mudslides," one in the Santa Ana Mountains (see 69.11b, below) and one in the San Gabriel 

Mountains (see 69.11a, below) resulted in a total of 

eight fatalities. In addition, there was a damaging 

Hmudslide" in Eagle Rock, at 1114 Oak Grove Drive, on 

Tuesday morning (op. cit., p. 13), and another in Sherman Oaks (op. cit., p. 13), at 3733 Loadstone Drive. 

Many major roads and highways throughout southern 

California were closed, and ((mudslides" were prominently mentioned as causes (op. cit., p. 3, 13) along with 

floods and heavy snow in the mountains. 

At the Eagle Rock site, residents evacuated on Tuesday morning, early enough so that they were still in 

their nightclothes. Although the time is not precisely 

reported, and nearby gages have not been plotted, there 

appears to be a general correlation with a local peak in 

the rainfall intensity. Visual inspection of the Hourly 

Precipitation Data (U.S. Environmental Science Services Administration, 1969) shows that, within a longer 

period of light to moderate rainfall, a flurry of intensity 

greater than 0.20 inch per hour lasted from 4:00 a.m. to 

6:00 a.m. at the Los Angeles Civic Center and from 4:00 

a.m. to 7:00a.m. at Santa Fe Dam. An average of those 

two gages is probably a fair representation of the rainfall intensity at the time and place of failure. 

At the Sherman Oaks site the time is not even approximately reported. Inspection of the Hourly Precipitation 

Date (U.S. Environmental Sciences Services Administration, 1969), however, shows that, within the longer 

period of light to moderate rainfall, a flurry ofintensity 

greater than 0.20 inch per hour lasted from about midnight to 4:00a.m. hours at the nearby gage at Sepulveda 

Dam. Time correlation is permitted but is not required 

by the circumstances. 

69.11a Mount Baldy Village, 8:00 a.m., February 25, 

1969: (Number following the prefix 69 refers to location 

in fig. 5 and time plotted in fig. 20.) A slide killed three 

members of one family in a residence at Mount Baldy 

Village, San Gabriel Mountains, at about 8:00 a.m., 

February 25 (Los Angeles Times, February 26, p. 13). 

The nearby gage at Mount Baldy (fig. 20) was, unfortunately, not operating at the time of failure; however, the 

gage at San Dimas Tanbark Flat, less than 10 miles 

away and at a slightly lower altitude, recorded 0. 73 inch 

of rain for the hour ending 8:00 a.m. and 0.69 inch for 

the hour ending 9:00 a.m., February 25. Therefore, the 

failure was almost certainly associated with a rainfall 

of high intensity. The event was very probably a soil 

slip-debris flow, however, the phrasing of the report 

would also permit interpretations of origin as a rockfall 

or rockslide. 

69.11 b. Silverado Canyon, 11:00 a.m., February 25, 

1969: (Number following the prefix 69 refers to location 

in fig. 5 and time plotted in fig. 20.) A ((mudslide" swept 

into the back and through the length of a fire station in 

Silverado Canyon, Santa Ana Mountains. Fire engines, 

crew, and many of the 60 flood refugees who had sought 

shelter there were carried out through the doors into the 

road in front of the station (Los Angeles Times, February 26, p. 1, 13). Five of the people were killed and 

about 20 others were injured. The incident occurred 

suddenly at about 11:00 a.m., February 25. The newspaper description of the debris flow and personal examination of the scar establish its character as a debris 

flow of soil-slip (ravine fill) origin. Figure 28 shows 

three views of the station after cleanup and placing the 

structure back into service. Note the scar of the soil slip 

and course of the debris flow. Nearby gages at Santiago 

Dam and in Trabuco Canyon approximately bracket the 

failure site, and the storm conditions there at the time of 

failure are probably represented by a curve somewhere 

between those. (See fig. 20.) 

CASE DESCRIPTIONS AT THREE SELECTED 

SITES OF SOIL-SLIP-DEBRIS-FLOW EVENTS 

The sites of the five fatal injuries from soil-slipdebris-flow events that have occurred within the past 10 

years (1962--71) in the area ofthe Los Angeles County 

Cooperative project were examined in more detail. 

Case description, soil slip-debris flow of December 29, 1965, 

in Newton Canyon, Santa Monica Mountains, California 

1. Reference number: Figure 5, No. 14. 
CASE DESCRIPTIONS AT THREE SELECTED SITES OF SOIL-SLIP-DEBRIS-FLOW EVENTS 43 

Base from U.S. Geological Survey 

Point Dume, 1950 

Photoreviscd 1 96 7 0 

0 

1 MILE 

.5 1 KILOMETRE 

CONTOUR IN TERV AL :25 FEET 

DATUM IS MEAN SEA LEVEL 

FIGURE 29.-Drainage basin of Newton Canyon, showing location of house destroyed by debris flow of December 29, 1965, at junction of 

third-order tributary with fifth-order trunk stream. Rank of tributary shown by number. 

2. Location: Point Dume 7.5' quadrangle, sec. 17, T. 1 

S., R. 18 W., 1400 Latigo Canyon Road, Malibu, 

Calif. (fig. 29). 

3. Landslide type: Soil slip (ravine fill}-debris flow. 

4. Date and time of occurrence: December 29, 1965, 

after about 11:00 a.m. (probably shortly after, 

certainly well before 6:00 p.m. when the destruction of the house was reported on a news broadcast). 

a. How established: Neighbor spoke to victim on 

telephone in late midmorning. 

5. Landslide dimensions: (fig. 30). 

a. Slope length: Scar, 3,100 feet (fig. 31). Debris 

train, 600 feet. 

b. Width: Scar, about 60 feet wide at widest part 

of the head, generally no more than 20-25 

feet wide through most of its length. Debris 

train, variable, 60-80 feet wide at upper 

end, diminishing downstream rapidly below 

crossing of Latigo Canyon Road. 

c. Thickness: Scar.-Judging from thickness of 

material remaining adjacent to scar, colluvium was 1-3 feet thick in headwall and 

on flanks of channel. May have been as 

much as 15 feet thick in the bottom of a few 

parts of the gentler reaches of the ravine, 

but was probably generally less than 5 feet 

thick; Deposit.-Highly irregular piles of 

boulders and brush from a few inches to as 

much as 6 feet (mostly brushy), but mostly 

less than 4 feet high. Much of deposit reworked by bulldozer during search for the 

missing resident. Much of silt and fine sand 

probably continued downstream as muddy 

floodflow. 

d. Volume~ Estimate approximately 6,000-8,000 

cubic yards maximum removed; probably 

only 3,000-4,000 cubic yards retained in deposit. 
44 SOIL SLIPS, DEBRIS FLOWS, AND RAINSTORMS IN THE SANTA MONICA MOUNTAINS, SOUTHERN CALIFORNIA 

Base from U.S. Cculugkal Survey 

Point Dumc 1.24 .000. 1950: 

Photurevised 1967 

0 1000 2 000 3000 FEET Geology modified from 

Campbell and others. 1970 

0 200 400 600 800 METRES CONTOUR INTERVAL 25 FEET 

DATUM IS MEAN SEA LEV EL 

EXPLANATION 

Unconsolidated deposits 

{\ Scar of soil slip-debris flow 

Dashed where approximately located. A II scars shown may have [armed 

during the same storm period (December 29, 1965), ·but only the 

Newton Canyon event provides a record of date and approximate 

time 

I cid~ :.1 Debris trains 

Gravel, debris, and sand deposited from debris flows. The debris train 

in Newton Canyon is mainly that from the December 29,1965, event. 

The debris trains in the other drainage, further east, include deposits 

overgrown with brush as well as fresh deposits, showing the recurring character of the events 

t1ds~ ) 

't.Y Landslide deposits 

Boundaries approximately located, queried where doubtfully recognized 

ds, debris slide; rs, rotational slump; queried where classification 

uncertain. Chiefly Holocene, possibly older in part; none have historic 

movement. Arrows indicate apparent direction of movement 

B 

Alluvium 

Stream-deposited silt, sand, and gravel 

_j___ 

Bedrock 

B 

Upper Topanga Formation of Durrell (1954) 

Thin-bedded shale, sandy shale, and volcaniclastic beds. Middle Miocene. Synvolcanic and postvolcanic in age 

G Lower Topanga Formation of Durrell (1954) 

Thick-bedded sandstone and mudstone, minor shale. Middle Miocene. 

Prevolcanic in age 

Vaqueros Formation 

Thick-bedded sandstone and mudstone, separated from overlying Lower 

Topanga by distinctive molluscan faunas. Early Miocene 

Sespe Formation 

Very thick bedded continental sandstone and conglomerate, locally 

red. Age may range from late Eocene to early Miocene 

0 

Llajas (?)Formation of McMasters (1933) 

Very thin bedded to medium-bedded turbidite sandstone and sandy 

shale. Middle Eocene 

Contact, showing direction of dip 

Dashed where approximately located, and 

where inferred from faunal control 

High-angle fault 

---.,___ ·· ~ 

Low-angle fault 

Dotted where concealed. A"ows show 

direch'on of apparent lateral displacement 

~ Dotted where concealed. Sawteeth 

on upper plate also point downdip 

FIGURE 30--Geologic map of upper Newton Canyon, showing path of debris flow of December 29, 1965, and similar scars in 

nearby uninhabited areas. 
CASE DESCRIPTIONS AT THREE SELECTEDSITES OF SOIL-SLIP-DEBRIS-FLOW EVENTS 45 

FIGURE 31.-Upper part of scar of ravine-fill failure on south face of 

Castro Peak. 

6. Distance moved: Material from the head of the scar 

moved at least 3,100 feet. 

7. Original slope at failure: Steepest gradient in 

headwall about 50 degrees. Gentlest reach 11 o 

(fig. 32). 

8. Estimated rate of movement: Very rapid to extremely rapid-probably greater than 10 ft/s. The 

impact damage at the head of the debris train 

indicates momentum from high velocity of debris. 

9. Precipitation conditions: Prolonged heavy rams 

preceded and accompanied the failure . 

10. Subsurface water: Prior to seasonal rains, there 

were no springs or other indications that a permanent water table in the bedrock intersected the 

ground surface in the drainage that failed, 

~ 

t- c , ~ 0 w 3000 .c w 

u. " . . ~ ~ g> 

" ~ ~ z' 0 0 " 0 " a: 0 

f: 2500 . 

c c 

0 . <l: > ·~ 

c > ~ 

w u ull'/, patfl o ...J \rioutal''t' g 11o w 2000 40 

1700 

0 1000 2000 

though small seeps might have gone unrecorded. 

Close association of failure and seasonal rains 

(heavy rainfall, at that) suggests rapid, nearsurface circulation-probably entirely within the 

soil zone. 

11. Monetary damage: House, garage, car, and landscaping destroyed. Road partly blocked, partly 

eroded, requiring repairs. Emergency crew and 

equipment for most of2 days searching debris for 

remains of victim (figs. 21, 25). 

12. Injury: Elderly widow, the sole resident, and her 

pet dog were killed. 

13. Slide material: Colluvial soil and ravine fill, chiefly 

derived from the Vaqueros Formation, including 

numerous large boulders of sandstone that 

clearly originated from that formation. Deposited 

on alluvium of Newton Canyon composed chiefly 

of silt, sand, and gravel derived from the shale 

and sandy shale of the Upper Topanga Formation, overlying bedrock sandstone of the Sespe 

Formation (fig. 30). 

14. Cause: Concluded to be increases in pore pressure 

in soil owing to dynamic imbalance ofinfiltra~ion 

and deep percolation during heavy rainfall. Incremental loading from the rear of the sort described by Hutchinson and Bhandari (1971) may 

have contributed to the mobilization of material 

below the steepest part of the slope; however, the 

narrowness of the debris flow channel and the 

persistent low level of the "mudline" that marks 

the depth of flow precludes piecemeal, randomly 

distributed ponding and flow erosion ofthe ravine 

fill. 

15. Treatment: Event not predicted and proceeded too 

rapidly to permit erection of defenses. Recognition of potential for event and evacuation during 

high-intensity rainfall would have saved resident 

Crown of scar 

Castro Peak 

(Eiev. 2824) -~ 

.,, 

~0 

. {\o\N i deb ! IS 

3000 4000 5000 

DISTANCE , IN FEET 

FIGURE 32.-Profile of Newton Canyon debris flow of December 29, 1965, comparing gradients of trunk canyon and 

tributary. Note that there is no vertical exaggeration. 
46 SOIL SLIPS, DEBRIS FLOWS, AND RAINSTORMS IN THE SANTA MONICA MOUNTAINS, SOUTHERN CALIFORNIA 

EXPLANATION 

\_.., 

Apparent course of surface runoff 

Trees. that survived in good condition to form 

firmly rooted islands with debris piled on 

uphill sides 

Bush or small tree in scar, in place, undisturbed 

or only partly disturbed 

0 

0 

50 100 

10 20 30 

CONTOUR INTERVAL 5 FEET 

DATUM IS MEAN SEA LEVEL 

150 FEET 

40 METRES 

Base from Survey Division, Department of the County Engineer, Los Angeles 

County, 1964 

Geology by R. H. CampbeJI. 1969 

FIGURE 33.-Soil slip-debris flow of January 25, 1969, at 3221 South Topanga Canyon Boulevard, Topanga, Calif. 

from death or injury by the slide. Recognition of 

hazardous location and removal of residence from 

most probable path of debris flows by relocation of 

only a few tens offeet should have been possible. 

Defense by debris dam possible, but with maintenance costs included, would probably not have 

been economical. Defense by bracing the dwelling 

to resist lateral stresses might have added to protection but would also require ties of dwelling to 

foundation capable of resisting impact which, in 

this case, was sufficient to remove the building 

from its foundation. 

Case description,soil slip-debris flow of January 25, 1969, in 

Topanga, Santa Monica Mountains, California 

1. Reference number: Figure 5, No. 4. 

2. Location: Topanga 7.5' quadrangle, sec. 7, T. 1S., R. 

16 W., 3221 South Topanga Canyon Boulevard 

(529 North Creek Trail), Topanga, Calif. 

3. Landslide type: Soil slip-debris flow. 

4. Date and time of occurrence: January 25, 1969, 

probably about 3:00a.m. Survivor reported having been asleep in an upstairs bedroom and 

awakened by the mother of two children asleep 

downstairs, who reported hearing a loud rumbling noise. At this time the house collapsed, and 

the next he knew he was in the hospital. A neighbor reported being awakened by a loud noise at 

about 3:00a.m.; he presumes that was the sound 

ofthe destructive debris flow; immediately afterward, a small mudflow entered his back window. 

5. Landslide dimensions: 

a. Slope length: Scar, about 130 feet. Debris 

train, about 50 feet minimum, with toe having been removed and reworked by 

floodwaters of Topanga Creek. 

b. Width: Scar, as much as 75 feet wide in places. 

(See fig. 33.), 

c. Thickness: Material at edges of scar ranges 
CASE DESCRIPTIONS AT THREE SELECTED SITES OF SOIL-SLIP-DEBRIS-FLOW EVENTS 47 

from about 6 inches to as much as 3 feet 

thick. 

d. Volume: Probably no more than 600 cubic 

yards removed from scar. Probably no more 

than a few tens of cubic yards deposited, 

mostly on pad of destroyed residence and on 

road below; remainder apparently removed 

by stream erosion of flooding Topanga 

Creek. Bedrock is exposed in the scar only 

where the remaining coating of colluvium is 

cut through by rills that are probably best 

attributed to rainwash erosion of the postfailure scar. The nearly uniform coating of 

colluvium that remains in the scar area indicates that failure took place within the soil 

zone rather than exactly at the bedrockcolluvium interface. Scar area may represent more than one failure-small nondamaging failures may have occurred both 

before and after the larger damaging failure. Moreover, the scar was deepened and 

rilled by rainwash subsequent to the slab 

failure. These factors tend to reduce the estimate of the volume of the damaging slab 

failure. 

6. Distance moved: Material from the head ofthe scar 

moved at least 130 feet. 

7. Original slope at failure: Overall slope in scar area 

about 40°; some local variations as gentle as 35°. 

8. Estimated rate of movement: Very rapid to extremely rapid, probably greater than 1 ft/s. Failure of awake adults to escape from house during 

collapse indicates that it happened very suddenly, suggesting high-velocity impact rather 

than the slower application oflateral pressure by 

inundation. 

9. Precipitation conditions: Prolonged heavy rains 

preceded and accompanied the failure . Interpolation of records from gages at Sepulveda Dam and 

Lechuza Point Station indicates that rainfall intensity was between 0.40 and 0.50 inch per hour 

at the time of failure. (See fig. 9.) 

10. Subsurface water: Prior to the seasonal rains, there 

were no reported springs or other indications that 

a permanent water table in the bedrock intersected the ground surface above the level of Topanga Creek. Close association offailure and seasonal rains suggests rapid, near-surface 

circulation-probably entirely within the soil 

zone. 

11. Monetary damage: Complete destruction of house 

and contents (fig. 26). Emergency crew and 

equipment were employed for much of 2 days 

searching the debris for bodies of victims. 

12. Injuries: Three residents were killed-a mother 

and two children. Another was seriously injured 

(hospitalized). 

13. Slide material: The material that failed consisted 

almost entirely of colluvial soil derived from the 

sandstone and shale of the underlying "Upper 

Topanga Formation" (of Durrell, 1954). The interbedded sandstone and shale dip gently into the 

hillside; this resistant obsequent geometry may 

account for the general steepness of the slope. 

The bedrock is chiefly sandstone, very coarse to 

medium grained, moderately to poorly indurated; 

beds 1 inch-2 feet thick, thicker beds seem to 

predominate at this site; interbedded with thin 

partings to 3-inch beds of laminated silty shale. 

The attitude of the beds is nearly flat, with a 

slight dip into the hillside. There are some cross 

joints perpendicular to the bedding, and their 

steepness may contribute to maintaining the 

general steepness of the slope. Bedrock is exposed 

in the scar only where eroded by subsequent rill 

and sheetwash. The coating of colluvium that remains in the scar area indicates that failure took 

place within the soil zone rather than exactly at 

the bedrock-colluvium interface. 

The cover, as represented by exposures at the 

edges of scar, consists of colluvial soil, generally 

2--4 feet thick. The top 1-3 inches commonly is 

gray in color from admixed organic material; it is 

commonly root filled and friable. Rodent burrows 

are abundant in the zone 3-6 inches below the 

surface. Below about 6 inches slabby, angular 

clasts of sandstone, from small pebbles to large 

boulders in size, are common. One clast in the 

debris deposit was as much as 3 feet across and 1 

foot thick. The vegetation is predominantly tall 

grasses and mustard, with scattered thorny 

bushes (Ceanothus?) as much as 15 feet high. The 

bushy plants are more deeply rooted and appear 

to have prevented the failure of a couple of "islands" now surrounded by scar. 

14. Cause: Concluded to be from increase in pore pressure in soil owing to dynamic imbalance of 

infiltration and deep percolation during heavy 

rainfall. Sudden failure oflarge mass is indicated 

by the sudden and complete destruction of the 

residence downslope. 

There is evidence that surface runoff entered 

one part of the scar area from uphill. Its contribution to the failure is unknown; however, its location indicates that it could not be solely responsible. The scar extends significantly higher on the 
48 SOIL SLIPS, DEBRIS FLOWS, AND RAINSTORMS IN THE SANTA MONICA MOUNTAINS, SOUTHERN CALIFORNIA 

0 500 1000 1500 FEET 

0 100 200 300 400 METRES 

CON TOUR INTERV AL 250 FEET 

D A TU M IS MEAN SEA L EVEL 

FIGURE 34.-Map of NW14 sec. 12, T. 1 S., R. 16 W. , Topanga 7.5' 

quadrangle, showing location of several damaging debris flows in 

Old Topanga Canyon. Numbers refer to street addresses. 

slope than the point at which the runoff channel 

entered (fig. 33). Moreover, it is possible' that the 

runoff did not occur until after the slope failure. 

15, Treatment: The event was not predicted and took 

place too rapidly to permit erection of defenses. 

Recognition of the potential for the event could 

have led to evacuation during high-intensity 

rainfall, removing residents from harm's way. 

Recognition that the particular site had greater 

potential for hazard than other sites on the same 

hillside would have been possible only with extremely detailed field and laboratory study of the 

entire hillside and does not appear to have been a 

practical economic possibility. Therefore, relocation of the house on the property would not have 

been an effective practical defense. It would appear that the best defenses for such structures in 

such sites would be: deeply anchored revetments 

upslope to divert the force of the moving mass, 

some sort of avalanche-shed structure to pass the 

moving mass over the top of the residence, or 

massive bracing of the upslope walls to resist 

lateral pressure and impact, combined with 

stronger anchoring of the structure to the bedrock. 

Case description, soil slip-debris flow of January 25, 1969, 

in Old Topanga Canyon, Santa Monica Mountains, California 

1. Reference number: Figure 5, No. 4a. 

2. Location: Topanga 7.5' quad., NW14 sec. 12, T. 1 S., 

R. 17 W. 874 Old Topanga Canyon Road, Topanga, Calif. (fig. 34). 

3. Landslide type: Soil slip (ravine fill)-debris flow. 

4. Date and time of occurrence: January 25, 1969, 

probably after about 1:00 a.m., before 9:00 a.m. 

a. How established: According to interviews with 

residents, only a few small mudflows had 

occurred in the neighborhood prior to about 

1:00 a.m. After that time, nearly all were 

awake and alert to their own danger, having 

been awakened by the storm. Event occurred prior to about 9:00 a.m., at which 

time the victim's body had been recovered 

and seen by one of the interviewed neighbors. The residents at 905 Old Topanga Road 

reported hearing the slide that damaged 869 

Old Topanga Road (figs. 7, 35) at 3:00 a.m. 

5. Landslide dimensions: See sketch plan and profile, 

fig. 36. 

a. Slope length: Scar about 250 feet (by visual 

estimate). Debris train, about 30 feet (inside 

dimension of house). 

FIGURE 35.- -Scar above house at 869 Old Topanga Canyon Road (roof 

of house at 87 4 Old Topanga Canyon Road, on near side of canyon in 

foreground). Brush-covered colluvial soil above lip of cliff (bedding 

surface of steeply dipping sandstone) failed and cascaded over bedrock cliff into back of frame dwelling below. (See fig. 7.) 
CASE DESCRIPTIONS AT THREE SELECTED SITES OF SOIL-SLIP-DEBRIS-FLOW EVENTS 

Colluvial soil 1.5-2 ft thick 

PLAN VIEW OF SCAR AREA 

AND HOUSE LOCATION 

0 20 

0 

N 

40 60 

10 20 

Approximate scale 

80 

~ 

0 

100 FEET 

30METRES 

.... 

0 

"C 

(0 

Q) 

I 

1. u. . ·""- . ·. 

49 

I ...... · .. -~ 

• : "' <:>. • 

. 5.1·::-... ~0- 0~· .• 

.• o·, ·. o ·. ·. ~ 0 

... '?J"" . . 0 . . ' ........... c 

.. : .0:~~~6,0(\~o o··><;·.·._.- o .. ·o .• · .. _· • ........._o 

. . . 0<-a~ "'- . . . • "- . . . . 

. ·. \!>-" . " - .. 0 .• " .•.. 0 

... l·y .. ·. \· ..... .\ ... > . -~- . . -~· o ··.\.· ··""".·.a· 

Retaining wall 8-10ft high 

Profile of scar area 

and house location 

., . ;, . . 0 0 . . \ •... ' 0 . • ' . \. •. . • ' ' . "-. • 

• · · · -._ 0<''- '2-q . o · . · \' · · · · \ · • · · · "- . ( 3 0.' • . . . · o · . . . . ,· . -a90 9 0 . • \ . . 0 . • . • . . . • 

House 

. • . P.."0( ·. . • • \ · . • \ . · . o . . \-

~·oO·.·._\~ •• · .. o_., \ ·_o 0 . -\·.·.·.·.· 

.. •· : .. · . . . \\.. . • . 0 .••.•. \ . . . • 0 ..... \ . 0 \ • . . 

r------L---'-'-=~:_:_:_.::..L..J • . • • . \ . • 0 • 

;_ ~ Debris deposit on first floor ... ·. \. o ·. ·. \ · \ -=- .,__.,_ =----...- / . . • • • . • • \ "OLD TOPANGA - ~-<:>-=-- . ' • \ · 0 .·' · \' • Q · . 

CANYON ROAD" o. ':, -· ~ • ~ ·~ ~-1'?~ . . . . . . . . . . . ·. 0 '· . .' · > _·. ·\-- ' . ~/·.o··· \ . --~;;· ·. ·. o··. 

FIGURE 36.-Soil slip-debris flow at 874 Old Topanga Canyon Road. 

b. Width: Scar, fairly constant width of 10-12 

feet. Deposit, no more than about 30 feet 

(inside dimension of house). 

c. Thickness: Judging from thickness of material 

remaining adjacent to scar, colluvium 18 

. inches -2 feet thick in headwall, ravine fill 

apparently no more than 3 feet thick; deposit in house estimated 6 inches -3 feet 

thick (fig. 14B). 

d. Volume: Estimate about 200 cubic yards removed from scars; probably not more than 

70 cubic yards deposited in discrete deposit 
50 SOIL SLIPS, DEBRIS FLOWS, AND RAINSTORMS IN THE SANTA MONICA MOUNTAINS, SOUTHERN CALIFORNIA 

inside house, remainder probably continued 

downstream as muddy floodflow, leaving no 

distinct deposit in vicinity. 

6. Distance moved: Material from head of scar moved 

the full length of scar-about 250 ft. 

7. Original slope at failure: Average gradient in upper 

part of ravine 31°, may steepen to 35° at the fork 

in ravine drainage where bedrock is exposed. Average gradient in lower part of ravine about 20°. 

(See profile, fig. 36.) 

8. Estimated rate of movement: Very rapid to extremely rapid, probably greater than 10 ft/s. The 

impact damage to the house and contents indicates momentum from high velocity of debris. 

9. Precipitation conditions: Prolonged heavy rains 

preceded and accompanied the failure. (See fig. 9.) 

10. Subsurface water: There were no springs or seeps 

reported in the drainage prior to the seasonal 

rains, nor were there other indications that a 

permanent water table in the bedrock intersected 

the ground surface, though small seeps might 

have gone unrecorded. The use of a cistern (fig. 

36) to store some surface runoff suggests that, on 

the contrary, the permanent water table was 

beyond easy reach by a dug well. The close association in time of failure and heavy seasonal rainfall suggests rapid near-surface circulationprobably mostly within the soil zone. 

11. Monetary damage: Virtually total loss of 2- or 

3-bedroom frame dwelling and contents. Questionable whether battered shell can be restored. 

12. Injury: One adult man killed. 

13. Slide material: Colluvial soil and ravine fill, derived chiefly from underlying sandstone and conglomerate sandstone assigned to the Sespe Formation. Deposited on the building site, which is 

underlain by stream terrace deposits of Old Topanga Creek. 

14. Causes: The principal cause is concluded to be increases in pore pressure in the colluvial soil owing 

to the dynamic imbalance of rates of shallow circulation and deep percolation during heavy rainfall. The narrowness of the debris-flow channel 

and the relatively constant depth of erosion (and 

apparent depth of flow), seem to preclude 

piecemeal, randomly distributed ponding and 

flow erosion of the ravine fill. The characte~ of the 

damage below is a strong indication of sudden and 

complete failure of all or nearly all of the relatively small mass of debris. It is unlikely that 

progressive, piecemeal failure could have produced a single surge with enough momentum to 

break through the wall of the house at chest 

height and move large appliances around violently inside. 

15. Treatment: The general danger of the situation was 

apparently recognized by many residents of the 

neighborhood, including those at this house, 

where the family had been evacuated and only 

one man remained behind. It is difficult to determine whether or not the residents were aware of 

the specific hazard of high-velocity debris flows. 

Recognition of a hazardous location and removal 

of the house from the most probable path of debris 

flow should have been possible by relocation of 

only a few tens of feet. Defense by debris dam 

would be possible, but with consideration of 

maintenance costs, may not have been economical. Defense by bracing the dwelling to resist lateral stresses might have added to protection, but 

would probably not have been sufficient at that 

site. The site might have been protected by diverting the drainage above or around by an avalanche 

shed or a levee system. 

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