Sayre Report - Hazard Mitigation Web Portal - State of California

potential for erosion are the silty clay and sandy loams with a low water (< 2
in/hr). The Los Angeles County, West San Fernando area soil survey indicates
the soils in the Sayre Fire are in the loam category and has a moderately high
susceptibility to erosion and will produce moderate sediment flows.
B. Describe Condition of Values at Risk
Evaluations of the Los Angeles County Public Works Debris Basins indicate
some initial signs of increased sediment load. Small amounts of sheet and rill
erosion was observed in the local area surrounding the basins. During the onsite
evaluation of the Sombrero debris basin, Los Angeles County Public Works
was in the process of removing up to 13 feet of sediment that was reportedly
produced during the post-fire storm event (11/25 and 11/26) of up to 2 inches of
rain. The topography in the local area of the basins ranged for 33 to 75% slope.
Erosion, flooding and mudflows are considered naturally occurring phenomenon
during pre-fire conditions. However, burn severity combined with slope
steepness accelerates the process.
The access road at the Pacoima Reservoir was reported to have had up to 3 feet
of erosion sediment blocking the main access to the Dam. The sediment
originated from the area adjacent to the road and was the result of recent Marek
Fire (October 12, 2008).
The six-foot box culvert under Gavina Avenue was observed to have up to twofeet
of sediment in the flow line of the structure. The amount of blockage is
moderate at this time, but could easily become completely blocked if a
combination of debris and sediment passed through the channel.(see hazard
point in appendix 1)
Multiple residential debris basins and small drainage areas behind a number of
homes were evaluated with minor to no erosion sediment observed from recent
storm events.
III.
IV.
Emergency Determination
The Values at Risk considered in this soils assessment include the possible loss
of life and property due to erosion sediment flow and flooding from increased
surface water runoff. In general, there is a low risk of loss of life and moderate
risk for loss of property due to increase runoff from a 4.1 - 4.7 inch – 24 hour rain
event. In this case there is no Emergency action recommended for property and
structures, but recommendations have been made for treatments and monitoring.
.
Discussion/Summary/Recommendations
Landscape level erosion control treatments are impractical for treating the entire
fire area, but mulching, fiber rolls, and straw bales with silt fencing may be used