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Thomas debris flow

Post-fire effects

Debris flows in Montecito

On January 9, 2018, 3:30 AM, PST, 0.54 inch (13.7 mm) of rain in 5 minutes was reported at Montecito ^[1]. Heavy rain on burned hill slopes above the city resulted in rapid erosion, mud flow and debris flow (mass wasting) of soil and stream channels, causing catastrophic damage in Montecito Creek and San Ysidro Creek.

As of January 21, 2018, 21 fatalities, 2 missing persons, 129 destroyed residences and 307 damaged residences, attributed to debris flows, were reported by the inter-agency, storm-response team ^[2] in Santa Barbara County ^[3].

Writer T.C. Boyle, whose Montecito home was within the proximity of both the fire and mudslides, documented the collective trauma in The New Yorker magazine.^[4].

Debris flows are generated in the following general sequence of events: Fire reduces the organic soil layer to ash and char. Water-repellent, organic hydrocarbons condense in shallow, mineral soil, enhancing runoff. Locally heavy rain, exceeding the infiltration rate of the soil, generates surface runoff. Eroded unconsolidated sediment and ash is suspended and dissolved, increasing the density of the flow. Pore water pressure developed in unconsolidated soil and sediment causes liquefaction. Erosion increases as gravity drives mud flow through the drainage basin in confined channels. Mud flows mobilize woody debris and larger sedimentary particles as debris flow, which continues to accumulate as smaller channels combine with larger channels. As the size of the debris flow increases, larger particles are entrained, channel erosion occurs, landslides and vegetation are incorporated. Multiple debris flows may be generated in the eroding, mountainous portion of the drainage basin. Debris flows typically surge, as precipitation rate and sediment supply vary, hydraulic jumps occur, landslide dams in the channel form and collapse. Upon exiting the steeper mountain canyon, as the stream gradient is reduced, the debris flow spreads laterally and begins to deposit larger particles on the alluvial fan. Where the stream channel is plugged with debris, lateral channel migration occurs, building upon the existing alluvial fan deposits. Deposition also occurs down-stream of the alluvial fan, where stream gradient decreases, on the floodplain, alluvial plain, coastal plain and river delta. As precipitation, subsurface flow, runoff, stream flow and supply of sediment and debris decreases, the stream reverts to low-flow characteristics. It takes years for disturbed channels to achieve relative stability as bed load sediment adjusts and vegetation is reestablished.

Vegetation

Native chaparral burned extensively to charred stems and ash. Chaparral is a fire-adapted ecosystem that burns frequently, catastrophically in Santa Ana wind. Coastal chaparral plant species readily re-sprout or regenerate from serotinous seed in the Mediterranean climate of southern California.

Fire history

The Inter-agency Thomas Fire Command mapped at least 50 chapparal fires of various sizes that burned in the Thomas Fire area since 1950 ^[5]^[6]. The 1985 Wheeler Fire burned the central portion of the Thomas Fire.