Sayre Report - Hazard Mitigation Web Portal - State of California

the infiltration may be affected by the development of hydrophobic soils.
Related is the increase in runoff resulting from reductions in interception of
the rain fall by the vegetation cover and infiltration due to the hydrophobic soil
conditions. Hydrophobic soils develop where the waxy substances released
by plant materials during hot fires follow thermal gradients into the soil and
congeal as continuous surfaces/layers. (Further explanation is in the Soil
Report.)
Values at Risk near or within stream channels and those located adjacent to
the basin spillways that outlet to the streets may be affected by significant inchannel
floods, hyper-concentrated floods, debris torrents, and debris flows.
It is our understanding that the USDA Forest Service, in concert with the US
Geological Survey and CAL FIRE are developing models to assess the
potential magnitudes of post-fire events.
Houses and other features may be affected by local floods and slope failures
as identified below. The magnitude of post-fire damage will be determined by
the intensity and duration of rain storms that impact the area.
B. Describe Condition of Values at Risk
In addition to the sites previously identified by USDA Angeles National Forest
Burned Area Emergency Response (BAER) Team. and the CAL OES Sayre
Fire BART survey team observed residences and other sites within and down
slope from the burn area. The purpose of this survey was to evaluate
potential risks from near-site debris flow, rock fall, floods, and other geologic
hazards that may not have been identified in the regional evaluation. Of the
thirty four (34) Values at Risk summary identified (see Appendix 1) by the
survey team, only 2 sites (Pacoima Dam Office Building and Reservoir) were
found at risk from the direct geologic impacts of rock fall and slides, and 26
sites (chiefly the dam reservoir and debris basins) from debris flow or mud
torrent flood inundation. The remaining sites were evaluated not to have a
geologic impact.
The Pacoima Dam Office is situated near the toe of a steep rock slope and is
already subject to a significant rock fall hazard. A masonry wall, two timber
lagging wall, and chain link fence have been placed at or near the toe of the
slope to partially mitigate the hazard. The rock fall hazard potential has been
increased due to the Sayre fire from denuding of the slope and thermal
effects of the fire on the exposed rock face. Recent rock fall debris and dry
ravel, along with the burnt timber lagging, and an impact scar to the masonry
wall were observed during our reconnaissance of the site.
The Pacoima Dam Reservoir is also already subject to a significant rock fall
and landslide hazards from the adjacent slope areas. These potential risk
have become increased due to both the Sayre Fire on the western side and