Sayre Report - Hazard Mitigation Web Portal - State of California

The VARs considered in this assessment include the possible loss of life and property
due to landsliding, debris flow, rock fall, debris flow, mud torrents, and flooding from
increased surface water runoff. In general, 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 potential for these processes to be exacerbated by
fire is primarily dependent upon burn severity and slope steepness, both of which are
highly variable in the Sayre Fire area.
Treatments to Mitigate the Emergency
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Pacoima Dam Office Building
• Mitigate rock fall to protect life and property.
Pacoima Dam Reservoir
• Mitigation of slopes is not considered economically feasible.
• Restore and maintain reservoir capacity and function by removal of
sediment load.
Other debris basins
• Mitigation of the debris flow, mud torrents, flood waters into and
overtopping the basins is paramount to reduce the potential loss of
property. This can be accomplished by monitoring and maintaining
the debris basins. The potential loss of life is minimal due to the
ability to evacuate when necessary by elevated public awareness
and early warning of pending events.
The site-specific observations included with the Technical Specialist Geology report
should be considered in any emergency response plan develop by the local emergency
agencies and stakeholders. Each area should undergo a more thorough analysis by a
suitable licensed professional to address the specific hazard(s).
Hydrology
The Sayre Fire consumed large areas of vegetative material and large woody debris in
multiple watersheds. The Soils Burn Severity Map indicates that 24% of the area burned
at a low level and 60% at moderate level and only less than 1% at high level. The
hydrophobicity tests ranged from low to moderate with small patches of high confirming
the burn severity map.
Peak flows equivalent to 193% to 243% pre-fire peak flow for a 2- year storm event
were estimated on drainage areas of major the debris basins and for the Pacoima
reservoir. The sediment loads on all debris basins were estimated to be comparable to
their maximum capacity; at Wilson, May-1 and Sombrero debris basins, there is a 2-5%
probability that their maximum capacity would be exceeded.
Evaluations of the major debris basins in the area indicate some initial signs of sediment
build up. The Pacoima reservoir has high risk of sediment build up, and the inlet/outlet