California
Division of Mines and Geology
California
Division of Mines and Geology
Geologic mapping and El Niño in southern California:
Maps describing landslides and debris-flows susceptibility
The U.S. Geological Survey's Southern California project is using knowledge of damage from past El Niño events to produce maps that describe areas more prone to damage from landsliding and debris flows.
Debris-flow occurrence maps
The extreme El Niño condition of winter 1997-98 may result in prolonged and intense rainfall that will produce debris flows. To help identify locations most subject to debris flows, generalized maps are being made that outline those areas most likely to generate debris flows during periods of long-duration and high intensity rainfall (critical rainfall). These debris-flow occurrence maps are being made for the more populated upland-areas of southern California, extending from San Diego to Santa Barbara and through the coastal mountains to include the "Inland Empire" area. Index map of the area studied.
Two different debris-flow maps are being made.
Maps show the areas prone to debris flows based on observed relationships with rainfall thresholds and landscape in past debris flow events. If threshold values of the duration and intensity of rainfall are met, debris flows may result on certain landscapes, particularly on and below steep slopes. The generalized debris-flow occurrence maps show those areas that have slopes steep enough to generate debris flows under proper rainfall conditions.
The debris-flow occurrence maps show those areas of hillsides that have slopes of 26 degrees or greater, which are areas where debris flows originate under critical rainfall conditions. Most of the debris flows will stop moving on gentle hillsides. Some debris flows with life-threatening potential will reach the base of the hillside, where if man-made structures exist, they can produce extensive damage. A few debris flows will run out onto gentle slopes before halting. If channelized, large debris flows can travel distances of a mile or more.
Details... A few debris-flow inventory maps were made for previous winters that generated numerous debris flows to learn where and when debris flows occur. The largest of these inventories included over 36,000 individual debris flows in an area of Ventura and Los Angeles counties. [Example map]. Two smaller inventories, one north of San Fernando Valley and the other east of Riverside, were also made. The debris-flow inventory maps show that for areas underlain by sedimentary rocks and fractured basement rocks, essentially all the debris flows generated were on hillsides with slopes of 26 degrees or steeper. However, relatively unfractured basement rocks (primarily granitic rocks, such as those in much of the Peninsular Ranges) outside the inventory areas rarely generate debris flows.
The slope maps are made from Digital Terrain Models (DTMs) that are derived from Digital Elevation Models (DEMs). Each DEM and DTM covers an area of 7.5 minutes of latitude and longitude and consist of square cells 10 or 30 meters (10m or 30m) on a side. These cells contain topographic information that is used to calculate the slope. The most detailed topographic information is contained in the DEMs with 10m resolution.
The DEMs are of variable quality and the resulting slope maps also vary in quality. For part of the area mapped, 10m DEMs with excellent topographic definition exist; these DEMs produce the highest quality debris-flow occurrence maps. For the remaining areas, lower topographic-definition 30m DEMs were used. The DTMs derived from the 30m DEMs were resampled to generate mathematically derived 10m DTMs, which contain greater topographic detail than do the original 30m DTMs but do not contain the topographic detail of the best10m DTMs.
Not included as high potential for generation of debris flows are upland areas that are underlain by essentially unfractured basement rocks, mainly granitic rocks. The exclusion of these areas is based upon past observations that slopes underlain by relatively unfractured basement rock rarely generate debris flows. Also not included are the areas of large alluvial fans located below large drainages along the front of major mountains, such as the south side of the San Gabriel Mountains. These alluvial fans are composed of debris-flow deposits mixed with debris deposited by water. Most of the large drainages, which are the conduits for debris flows, are contained by flood-control dams, which should be effective at containing major debris flows originating in unburned areas.
Future directions... Limited data now preclude development of more refined and detailed debris-flow occurrence maps. A more detailed map can give probabilities of debris flow generation as a function of a variety of physical conditions in the upland areas. With detailed data pertaining to the generation of debris flows, such as repeated debris flow inventories during the intense storms and more detailed knowledge of the physical setting, detailed debris-flow occurrence maps can be made.
Wildfires in upland areas of southern California, especially brush covered slopes, generally create loose soils that are unable to absorb rainfall. Essentially all rainfall in these areas runs downslope into stream channels, so even small storms can create large stream flow, which in turn can fluidize huge amounts of channel-filling debris and form large, devastating debris flows. In cases of extreme rainfall in a recently burned area sufficiently large debris flows can overwhelm flood-control facilities at the heads of alluvial fans. More information on fires and debris flows. A recent occurrence was in the area north of the city of San Bernardino where a localized September, 1997 storm produced about 1.5 inches of rainfall over a recently burned area. Debris flows generated in the burned area overwhelmed debris catchment basins and flowed down streets for distances over 2 miles below the catchment basins, causing considerable damage. Photograph of the damage
A map showing perimeters of 1997 southern California wildfires that burned more than 300 acres has been prepared. The wildfire perimeters enclose areas that can undergo extreme surface run-off from hillslopes. Areas downslope from the burned areas could potentially experience flooding and debris flows in the event of significant rainfall and if large debris flows overwhelm flood control facilities. Data for the wildfire areas were obtained from the California Division of Forestry, California Office of Emergency Services, USDA Forest Service, and Orange, Los Angeles and Ventura Counties.
The debris-flow occurrence maps, for both the burned and naturally vegetated areas, should be used as:
These maps are not intended nor are they suitable for any site-specific purposes. The maps should only be used for purposes commensurate with a map scale of 1:100,000. (Note: one inch on the maps covers a distance of almost 1.5 miles on the ground.) The maps should only be used for undeveloped natural hillsides. Not considered in the development of the maps is whether the hillsides are developed or remain natural.
Detailed maps: Comparing past
debris flows and the predictions.
Index map: For what areas are we
making debris-flow hazard maps?
Photos of debris
flows and landslides
Geologic
mapping in southern California
For further information, contact David Miller
http://geology.wr.usgs.gov/wgmt/elnino/scampen/products.html, 09 March 1998, Contact: El Niño Web Team
