Introduction This group
of maps shows relative susceptibility of hill slopes to the initiation
sites of rainfall-triggered soil slip-debris flows in southwestern California.
As such, the maps offer a partial answer to one part of the three parts
necessary to predict the soil-slip/debris-flow process. A complete prediction
of the process would include assessments of where, when,
and how big. These maps empirically show part of the where
of prediction (i.e., relative susceptibility to sites of initiation of
the soil slips) but do not attempt to show the extent of run out of the
resultant debris flows. Some information pertinent to when
the process might begin is developed. When is determined mostly
by dynamic factors such as rainfall rate and duration, for which local
variations are not amenable to long-term prediction. When
information is not provided on the maps but is described later in this
narrative. The prediction of how big is addressed indirectly
by restricting the maps to a single type of landslide processsoil
slip-debris flows. The susceptibility
maps were created through an iterative process from two kinds of information.
First, locations of sites of past soil slips were obtained from inventory
maps of past events. Aerial photographs, taken during six rainy seasons
that produced abundant soil slips, were used as the basis for soil slip-debris
flow inventory. Second, digital elevation models (DEM) of the areas that
were inventoried were used to analyze the spatial characteristics of soil
slip locations. These data were supplemented by observations made on the
ground. Certain physical attributes of the locations of the soil-slip
debris flows were found to be important and others were not. The most
important attribute was the mapped bedrock formation at the site of initiation
of the soil slip. However, because the soil slips occur in surficial materials
overlying the bedrocks units, the bedrock formation can only serve as
a surrogate for the susceptibility of the overlying surficial materials. The maps
of susceptibility were created from those physical attributes learned
to be important from the inventories. The multiple inventories allow a
model to be created from one set of inventory data and evaluated with
others. The resultant maps of relative susceptibility represent the best
estimate generated from available inventory and DEM data. Slope and aspect values used in the susceptibility analysis were 10-meter DEM cells at a scale of 1:24,000. For most of the area 10-meter DEMs were available; for those quadrangles that have only 30-meter DEMs, the 30-meter DEMS were resampled to 10-meters to maintain resolution of 10-meter cells. Geologic unit values used in the susceptibility analysis were five-meter cells. For convenience, the soil slip susceptibility values are assembled on 1:100,000-scale bases. Any area of the 1:100,000-scale maps can be transferred to 1:24,000-scale base without any loss of accuracy. Figure 32 is an example of part of a 1:100,000-scale susceptibility map transferred back to a 1:24,000-scale quadrangle. |
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There are 7 plates that accompany this report, provided below as zipped Encapsulated PostScript files and the corresponding digital databases
Quadrangle
Name*
|
EPS
file
|
PDF
file
|
Digital
Database
|
Santa Barbara | plate1.zip | plate1.pdf | sbarb_sus.tar.gz |
Los Angeles | plate2.zip | plate2.pdf | la_sus.tar.gz |
San Bernardino | plate3.zip | plate3.pdf | sbern_sus.tar.gz |
Long Beach | plate4.zip | plate4.pdf | lb_sus.tar.gz |
Santa Ana | plate5.zip | plate5.pdf | sa_sus.tar.gz |
Oceanside | plate6.zip | plate6.pdf | ocs_sus.tar.gz |
San Diego and El Cajon | plate7.zip | plate7.pdf | sd_sus.tar.gz |
readme file | |||
FGDC-compliant metadata for this report as an ASCII text or as a .met file |
*The maps include all or parts of the 1:100,000 quadrangles listed above
For questions about the content of this report, contact Doug Morton
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