Geologic mapping and El Niño in southern California:

Detailed maps of past debris-flows and areas susceptible for future flows

The U.S. Geological Survey's debris-flow maps for Southern California describe areas prone to damage during high-rainfall seasons, such as El Niño events. These maps can be compared to actual debris flows that occurred during past high-rainfall winter seasons.

Val Verde 7.5-minute quadrangle, northwest of the San Fernando Valley

Technical description...

Location of the map:
Click on the image for a larger shaded relief map of SCAMP

The Val Verde quadrangle includes a bit of Lake Piru along the upper left side. Val Verde Park lies in it, as does the town of Castaic. The Santa Clara River flows through the lower part of the quadrangle.

What does the map show?

• The different shades of gray show a digitally created effect of shaded relief showing mountains, canyons, and plains
• Bright yellow areas indicate slopes that are prone to possible debris flow activity.
• Bright red shows debris flows active during the winter of 1968-1969.
  

About the past debris flows:

During the exceptionally rainy winter season of 1968-1969, thousands of debris flows took place in hills throughout greater Los Angeles. The debris flows were mapped in part of the area by D. M. Morton (USGS) using aerial photographs taken a few months later. The flows can be easily recognized from the air as bright-colored scars in the green hills because they remain devoid of vegetation for more than a year. [Photograph of debris-flow scars]

Conclusion:

The debris-flow susceptibility maps prepared by terrain modeling methods do a good job of showing the areas subject to potentially dangerous debris flows. Actual debris flows are generally active in much narrower, restricted parts of the bigger area shown (in yellow) as susceptible to debris flows, but the specific narrow areas cannot be predicted using terrain data currently available.

Download JPEG files for the Val Verde quadrangle

Val Verde quadrangle [256 kb jpg]

• The areas colored yellow are alluviated canyon floors.
• The map on the left shows, in red, the "deep-seated," relatively thick, landslides that developed over a long period of time under unknown conditions, but prior to 1968.
• The map on the right shows, in red, the debris flows that were produced by heavy rainfall during January and February, 1969. In contrast to the long time over which the "deep-seated" landslides were produced, these debris flows originated during two storms. Also unlike the "deep-seated" landslides these debris flows leave much smaller deposits.

Higher resolution image of maps (177 kb)

Getting the hazard maps: Smaller, low resolution files.

How the maps are made

Photos of debris flows and landslides

For further information, contact David Miller

Technical details: The conditions and processes that form landslides are varied and complex. Landslides generally form when some triggering event, such as an earthquake or rainfall, destabilizes earth materials. Gravity causes the destabilized materials to slide or flow down slope. Without the triggering event the earth materials are in an essentially stable condition. The triggering event produces a short-lived condition, such as ground shaking or saturation, that temporarily destabilizes the earth materials. Landsliding tends to stabilize the earth materials during the transitory triggering conditions.

When discussing landslides, distinctions should be made as to the type of landslide. For example, Figure ? shows two landslide maps for the Val Verde 7.5' quadrangle in Ventura County. The map on the right shows the landslides that developed over a long period of time under unknown conditions. The landslides outlined on this map, commonly referred to as "deep-seated" landslides, are relatively thick. Due to their substantial thickness they remain a visible part of the landscape for hundreds to thousands of years. These landslides are most easily identified soon after they originate, and are progressively less well defined and less readily identified with time as they are modified and eventually removed by erosional processes. These "deep-seated" landslides generally reduce the hillside slope angle except in their headwater part, which commonly remains relatively steep.

The map on the left shows the debris flows that were produced by heavy rainfall during January and February, 1969. In contrast to the long time over which the "deep-seated" landslides were produced, these debris flows originated during two storms. Also unlike the "deep-seated" landslides these debris flows leave much smaller deposits. Most of what is visible after their occurrence is a thin veneer of debris along the course of their flow. The recognizable presence of these debris flows is very ephemeral. A few years after their origin little is left to record their occurrence. Unlike the "deep-seated" landslides, the debris flows do not reduce the slope angle of the hillside where they originate. As soon as adequate "soil" forms on the site of their origin, the setting is once again ready for the development of debris flows when the material is properly saturated.

Both maps were made by interpreting aerial photographs taken soon after the 1968-69 winter rains. The "tracks" or "scars" of the debris flows were readily observable on the aerial photographs. The map of the debris flows was made by simply determining the location of the flows on the photographs and their corresponding location on the Val Verde 7.5' quadrangle.

The "deep-seated" landslides were identified through recognition of a variety of physiographic features indicative of such landslides which sets them apart from the hillsides not underlain by landslides. The degree of confidence in the detection of landslides generally becomes more difficult and problematic with increasing age of the landslides.
Back to the maps...

http://geology.wr.usgs.gov/wgmt/elnino/scampen/valverde.html, 6 February 1998, Contact: El Niño Web Team