Current landslides in southern California

Geologic mapping and El Niño in southern California:

Active debris flows in southern California

Observations made on debris flow activity in the laguna beach area, 3/5/98, by D.M. Morton and M.L. Dawson

We visited the Laguna Canyon area, Orange County, where debris flows decending a small drainage adjacent to Castle Rock Road resulted in the death of three people during the night of Feb. 23, 1998. Here we refer to this drainage as Castle Rock canyon. As we did not wish to potentially interfere with emergency services, we waited until 3/5 to inspect Laguna Canyon.

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Observations made on Route southwestern San Gabriel Mtns- Santa Clarita- Ventura- Carpenteria- Santa Barbara- Lake Cachuma- San Luis Obispo and return, 2/28-3/1, D.M. Morton and R.L. Morton.

Relatively common debris flows in the Tertiary sedimentary rocks, Miocene and younger, north of Sunland. Most of the debris flows in the Miocene units were on very steep grass-covered slopes at the head of small drainages. Up Little Tujunga Canyon debris flows were relatively common on steep faces mostly at the heads of minor drainages; some slumps. Landslides appeared to be more common in the part of the Saugus Formation with abundant redbeds; debris flows occurred on both grass-covered and brush-covered slopes. Rare small landslides in the granitic basement on the east side of the canyon. At the junction of Interstate 5 and highway 126 debris flows and thin landslides common in the Saugus(?) Formation; one quite impressive bedding plane failure at the northeast of 5 and 126.
No noticeable changes in landslides was observed along 126 east to Ventura. One or more small debris flows had occurred on the lower part of the La Conchita landslide.
North of Carpenteria very rare debris flows on the steep, brush covered slopes underlain by Eocene(?) rocks. North of the Santa Barbara Polo Club grounds is an interesting small debris flow(s) which which appeared to have originated in Sespe Formation, descended down a step chute through Eocene sandstone completely clearing brush out of the chute before traveling some distance onto gently inclined slopes.
Grass-covered steep and moderatly steep slopes west from Santa Barbara had scattered small slumps and debris flows. In the vicinity of highway 1(to Lompoc) and 101 junction scattered soil slips, slumps, and debris flows. West of 101 on 1 debris flows are very uncommon. About 1.5 mi west of 101 rather large slump has taken out part of highway 1(which remains closed as of 3/1/98. Just to the east of the 1 & 101 junction two small debris flows occurred at the head of small drainages on steep, densely tree and brush covered slopes. Along 101 for several miles north of the 1 & 101 junction debris flows are relatively common on steeper grass-covered slopes. Debris flows are almost absent in the gentle topography north of 101 and highway 154 junction to the vicinity of San Luis Obispo and Los Osos.
East of 101 on 154 through Santa Ynez Valley area debris flows are rare except on relatively low steep slopes in Plio-Pleistocene sedimentary rocks on the north side of the valley - north of Los Olivos. Further east debris flows are common on the north side of Bradbury Dam-Lake Cachuma. Some steep slopes, especially at the heads of minor drainages are abundant debris flows. Debris flows are common along steep banks along the edge of Lake Cachuma, especially on the north side of the lake. Moderate sloping grass-covered slopes along 154 contain about the same number of slumps and debris flows. Slopes appear marginal to produce debris flows, but rainfall has been sufficient to produce common slumps.

Observations made on 2/27/98 by D.M. Morton and Kelly Ruppert

Route along Ortega Highway from Interstate 5 at San Juan Capistrano east to Elsinore. No new (since 2/23 - i.e. before the heavy rainfall the night of the 23th and morning of the 24th) debris flows were observed in the Miocene sedimentary rocks east of San Juan Capistrano. No new debris flows were observed on La Pata Road off of Ortega Highway.
Few, very few post 2 PM 23th debris flows in the Eocene sedimentary rocks further to the east. Only sizeable increase in debris flows was in the Trabuco Formation near the contact with basement rocks. Although the Trabuco is conglomerate, it contains a fair amount of clay which may explain it's relatively debris flow prone nature. Debris flows in the Trabuco were limited to grassy steep slopes at the headway parts of minor drainages.
Only two debris flows were noted from the highway within granitic rocks at higher elevations on the west face of the Santa Ana Mtns. Both of these debris flows were in soil covered, very steep slopes. No debris flows were seen on the steep east face of the Santa Ana Mtns.

Observations made on 2/24/98 by D.M. Morton, M. Dawson, M. Klinefelter - route San Timoteo Badlands - northern end of Santa Ana Mtns - southeastern Puente-Chino Hills - San Dimas - Glendora - Azusa - Thousand Oaks - Las Posas Hills - Ventura - Wheeler Canyon - Santa Clara Valley - Piru Canyon - Santa Clarita - Sunland.

Heavy rainfall was widespread over most of southwestern California from PM 2/23 into AM on 2/24; rainfall for Feb. set a new record for many areas of southern California.

Most significant change in increase in soil slips-debris flows was in the San Timoteo Badlands area between Moreno Valley and Beaumont. Evening of 2/23 only about 100 debris flows were estimated to have occurred in the San Timoteo Badlands; by 6 AM 2/24 debris flows were estimated to number in the 1,000s. Most were located on grass covered slopes at the steep heads of minor drainages and lesser numbers appeared on very steep side slopes.
In the southeastern Puente-Chino Hills, northwest of Corona, there appeared to be about a 30% increase in the number of debris flows overnight. Essentially all debris flows seen were on steep, grass-covered slopes.
No landslides were seen in San Jose Hills, hills west of San Dimas, or that part of the San Gabriel Mtns north of San Dimas that are underlain by landslide prone Tertiary sedimentary rocks and Miocene volcanic rocks. The south side of the San Gabriel Mtns underlain by basement rocks between Glendora and Pasadena were essentially devoid of any landslides seen from U.S. 210. Only one small debris flow was seen north of the Monrovia area on a steep brush-covered slope. No landslides were seen in the hills south of Glendale or in the north side of the Santa Monica Mtns. In the vicinity of Las Virgenes Road and U.S. 101 small debris flows are sparse, but about double in number since 2/22. These small debris flows appear to be restricted to grass-covered steep slopes. Brush-covered hillsides, mainly underlain by Miocene volcanic rocks west of Las Virgenes Road to base of Canejo grade appeared devoid of any landslides.
The low Las Posas Hills north of U.S. 101 had common, widespread new debris flows; on 2/22 the south side of the Las Posas Hills appeared devoid of any landslides.
In eastern Ventura some increase in the number of debris flows, such as in the Hidden Valley area. In Aliso Canyon, east of Ventura, where we have repeated ground photography, there was a significant increase in debris flows. In Wheeler Canyon there likewise was a significant increase (perhaps 50% to double) in the number of debris flows in the "weak" Pleistocene-Pliocene marine sedimentary rocks. Some new, small slump-flow type of landslides. Some of the more gentle, grass-covered slopes contain large areas of incipient soil slips, which had the appearance of buckled carpet. The south side of South Mountain, from west of Santa Paula to Piru, appeared to have a significant increase in debris flows since 2/22, but it is hard to estimate the amount of increase.
Piru Canyon had at least a doubling of the number of debris flows since 2/22. Access was restricted to the wash-out of the road to Lake Piru. Piru Creek was still in impressive flood-stage, but well below the level which washed out the road and bridge across Piru Creek. Parts of Piru still under water and highway 126 still closed across Piru Creek.
East from Piru to Santa Clarita significant increase in the number of landslides and debris flows including a nice bedding failure on the northeast corner of the intersection of US 5 and 126. South of McBean to junction of US 5 and 14 scattered debris flows on oak tree vegetated slopes; some debris flows incorporated brush, but none appeared to have displaced trees.
In the Sylmar-Sunland area 50 or so debris flows on steep grass-covered slopes underlain by Tertiary sedimentary rocks; no debris flows were seen here on 2/22; no debris flows were seen in the landslide-prone fractured granitic rock north of the sedimentary rocks. No landslides were seen east of the Sunland area to Pasadena.

In summary, heavy rainfall on PM 23rd-AM 24th produced sizable increases in the number of debris flows in areas with existing debris flows; a number of areas without debris flows on the 22nd, the 23rd rainfall exceeded the rainfall thresholds to produce debris flows. The frequency and areal distribution of debris flows is still way below the number and areal distribution of debris flows produced during the winter of 1968-69.

Today, 2/25, with clouds cooperating, 7 of the 11 debris flow inventory aerial photography transects will be reflown. For transect #1, Ventura and to the east to Fillmore area, it will be the 3rd flight, so we should be getting some interesting data. No ground recon is planned for today, 2/25.

Observations made on 2/23/98 - Riverside-Corona-Puente Hills-San Juan Capistrano-Ortega Highway-Elsinore-Corona-Riverside-San Timoteo Badlands

Along highway 71 north of highway 91 no noticeable changes in the number of debris flows in the southeastern Puente-Chino Hills. Few scattered debris flows along the steep face of the northernmost part of the Santa Ana Mtns above highway 91. No debris flows observed until area of La Pata Road along Ortega Highway. Relatively landslide prone sediments contain several relatively deep-seated slimps; some slumps are reactivated last year(?) landslides; a few appear to be new. Scattered debris flows on steep grass-covered slopes, mostly at the head of small drainages and along over-steepened channel faces. Further east in the older rocks along lower part of the Santa Ana Mtns were a few small debris flows on extremely steep debris covered slopes. Although heavy rain obscured visibility, higher into the Santa Ana Mtns slopes underlain by granitic rocks, regardless of the steepness of slope apparently were devoid of any landsliding except for minor sloughing along the highway. San Juan Creek was high and all the minor drainages were flowing.
Between the Santa Ana Mtns and east to the San Timoteo Badlands no landsliding was observed.
In the San Timoteo Badlands, along Highway 60 and the old Jackrabbit Trail steep grass-covered faces, mainly at the head of small drainages , contains sparse debris flows. These flows were not observed on 2/18/98.

Observations made on 2/22/98

San Timoteo Badlands east of Riverside. Rare short distance run-out soil slips and debris flows limited to over-steepened faces at the head of minor drainages. Limited to grass-covered slopes
Northeastern part of the Santa Ana Mtns. No debris flows observed on steep brush-covered slopes. Few soil slips-debris flows along grass-covered steep road cuts.
Southeastern Puente-Chino Hills. North along highway 71 a few new scattered soil slips and few debris on flows on steep grass-covered slopes at head of minor drainages and along oversteepened grass-covered faces along channels.
No soil slips-debris flows observed in the area north of San Dimas-Glendora-Azusa. Single minor debris flow on steep face along San Gabriel Canyon below Morris Dam.
No soil slips-debris flows observed along south front of the San Gabriel Mtns between Azusa and Sunland. The landslide prone areas north of Sunland was devoid of soil slips-debris flows in both the sedimentary and plutonic rocks.
Along Interstate 5 from the junction highway 14 to the Santa Clarita area were scattered soil slips debris flows localized on steep grass-covered faces.
Along the Santa Clara River valley along highway 126 west from Santa Clarita area steep grass covered slopes were scattered soil slips-debris flows. Common landslides along steep cut-bank faces on the south side of the Santa Clara River. Few debris on very steep, brush covered faces. Other than along the banks of the river most of the debris flows were at the over-steepened head of drainages.
In the Riru Creek area heavy runoff, but few soil slips-debris flows up Piru Canyon. Debris flows were only common on the steepest slopes.
Scattered soil slips-debris flows occurred on steep grass-covered slopes between Piru and Santa Paula. On the extremely steep north-faces of canyons in the western part of South Mtn were the sites of extensive debris flows; at the head of some canyons debris flows covered an estimated 40% of the surface. These were not apparent on 2/18/98. In the relative incompetent sedimentary rocks in Wheeler Canyon and Alsio Canyon area east of Ventura debris flows were common by 2/18/98; there appeared to be little change between 2/18/98 and 2/22/98. Most noticeably was more extensive erosion along Wheeler Creek. No noticeable change in the scattered relatively deep-seated slumps in the area.
No noticeable change since 2/18/98 along the hillside west of Aliso Canyon through Ventura. Santa Clara and Ventura Rivers flowing relatively high.
Pacific Coast Highway north of Ventura to Pt Rincon area few additonal soil slips since 2/18/98.
East from Ventura along Highway 101 to the Thousand Oaks area no soil slips-debris flows seen. In the area of Las Virgenes(sp) Road 2-3 minor soil slips on steepest grass-covered slopes at the head of small drainages. Scattered soil slips along steep grass-covered road cuts. Ground looks like will readily fail with any sizable additional rain.

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http://geology.wr.usgs.gov/wgmt/elnino/scampen/daily.html, 25 February 1998, Contact: El Niño Web Team