Preliminary Soil-Slip Susceptibility Maps, Southwestern California

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 process soil 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.