**2.1 Study area**

Located 8 km east of Santa Barbara, Santa Barbara County, California (USA) (**Figure 1**), Montecito is an unincorporated community and a census-designated place. The population was estimated in 2018 to be 9145 residents, and Montecito is regularly ranked by the Forbes magazine as one of the wealthiest in the United States (2016, 2017). According to the magazine, it is 21 of the 100 most influential public figures in the United States are known to own property (2017). The climate is characterized by warmer winters and cooler summers, compared with places further inland, because of the ocean's proximity [5, 28]. Located at the foot of the Santa Ynez Mountains, which are mostly of sedimentary origin, the peak relief has an altitude of 981 m. Several creeks span approximately 3 km between the mountain front and the Pacific Ocean, intersecting State Route 192 and Highway 101. Since distances between mountains and bay are shorter, steep terrain presents hillslope gradients: ~37% of the terrain exceeds 35 degrees, and creek-bed gradients are ~12% [5]. Under these conditions, surface runoff and waters rapidly flowed south into a series of creeks with gradients of ~5%, and drain south through the residential city of Montecito, with construction in a series of alluvial fans [29, 30]. The urbanized piedmont plain contains steeply sloping alluvial-fan landforms generally north of State Road 192 and Highway 101 and gently sloping alluvial-fan landforms near the coast. And to protect the high density of roads and structures on the alluvial fans, four sediment-retention basins have been built along the main paths: Cold Spring (1964), San Ysidro (1964), and Romero (1971), as recorded by Santa Barbara County.

### **2.2 Awareness of the risk of debris-flows**

Before the 2018 event, Montecito had a high level of situational awareness prior to the storm [5]. Historical and damaging debris flows (1926, 1934, 1964, 1969, 1971, 1990, and 2002) have been already registered [31, 32], and debris flows became a

**Figure 1.** *Location of the five studied catchments (with damage extracted from 31).*


**Table 1.**

*Indicators confirming the high probability for a postfire debris flow in Montecito just before the 9 January 2018.*

topic of research since the 1934 debris flow in Montrose that killed over 40 people [29, 30]. Some researchers documented the sediment-laden water flow following the 1990 Painted Cave fire, and they have proven that the first few months following wildfire are of most concern [29, 30]. Awareness of the debris flow risk was emphasized by coordinated efforts between county, state, and federal agencies that included: (1) a determination of the soil burn severity as a good indicator of flooding and debris-flow potential [33]; (2) the debris-flow hazard assessment that shows the high likelihood and potential volume of debris flows from the burned areas, in response to design storms [34]; a warning system that predicted significant to the extreme potential for debris flow in the 4 days leading up to the storm [35, 36]; and (4) a proactive emergency community that coordinated evacuation orders to reduced casualties [5]. However, despite these efforts, it remains hard to anticipate such events: (1) assuming the annual probability of a wildfire in this area (following recovery from a previous fire rated at 0.10), and that the probability of a short duration high intensity storm is 0.02, the conditional probability is 0.002 [29, 30]; (2) prior to the 9 January 2018 rain event, the U.S. Geological Survey (2018) had predicted a 62–70% probability of debris flows from the five catchments located in upstream Montecito (**Table 1**) for a design rainstorm of I15 = 24 mm/hr. [34], but forecasting such intensity remains difficult in real time.

#### **2.3 Characteristics of the January 2018 event and hydrological data**

The Thomas fire above Montecito burned from the ridge crest of the Santa Ynez Mountains on December 2017 (from the 14th until the 27th), to approximately the apex of the urbanized alluvial fans [5, 6, 29, 30]. Under these conditions, the occurrence of debris flows was feared, and rainfalls were watched [37]. Early on the morning of 9 January 2018, a heavy rain characterized by high intensity (13 mm in 5 min, e.g. 157 mm/h) at 3.45 a.m. (recorded by the Montecito rain gauge) caused flows consisting of mud, boulders, and tree branches up to 15 feet (up to 5 m) in height, and moving at estimated speeds of up to 20 miles per hour (around 30 km/h) into the downstream creeks, valleys, and lower areas of Montecito [38]. A few minutes later, at 3:47 a.m., the Montecito Fire Department received the first calls of a major explosion on San Ysidro Creek–East Mountain Drive [5]. Flows on Cold Spring Creek began at 3:49 a.m. (this was confirmed by the security camera footage). Inundations began on the northern plains of Montecito around 3:50 a.m., and estimated lag times are estimated ranging from 4:06 to 4:10 a.m. [39], with the seismic signature of debris flows. The rainfall threshold for the occurrence of debris flows decreased obviously due to the Thomas Fire and the percent of burned areas, ranging from 49% within Hit


**Table 2.**

*Data collected during the 2018 Montecito event [5, 6, 34, 37].*

Spring Creek to 85% in San Ysidro Creek (**Table 2**). No floods occurred on 9 January 2017 (for cumulative precipitation of 16.5 mm), whereas several debris flows occurred on 9 January 2018, with cumulative precipitation of less than 13.7 mm in 5 min [28]. And the sediment basins do not limit the violence of flows.

The 30-foot wall of boulders, mud, and debris flows ran down hillsides at 15 miles per hour, injuring dozens and causing 21 prehospital deaths and 2 missing persons (two children of 17- and 2-year old's). 163 people were hospitalized for injuries [40] and a retroactive review conducted of 24 victims from the debris flows presenting to Cottage Health showed that most part of symptoms referred to soft tissue injuries (100%), hypothermia (67%), craniofacial injuries (67%), corneal abrasions (53%), and orthopedic injuries (47%), as well as the loss of an immediate family member (73%) because of the incident [40]. Around half of the victims who perished were migrants from working-class families [28]. Hopefully, crews rescued at least 50 people by air, dozens more from the ground, and 300 people stocked in their homes along the Romero Canyon neighborhood after impassable roads halted rescue operations, the disaster caused \$177 million of insured property damage, \$7 million in emergency responses, and \$43 million in cleaning costs [41, 42], with a final cost approaching more than \$250 million in 2019 [5].

Although creeks are incised by more than 5 m into the surrounding terrain (**Figure 2**), the debris flows overflowed the valleys, often at bridge crossings, and carried boulders into the neighboring residential areas [5]. The debris flow deposits cover around 7 km<sup>2</sup> and the cumulative amount of sediment are ranging from 297,000 m3 [33], 680,000 m3 (including boulders up to 6 m. in diameter) to up to 880,000 m3 [34]. Damage was concentrated within the 500-meter-wide flow path in numerous areas, and was mostly pronounced along the Montecito and San Ysidro Creeks, as 80% of the 408 damaged and destroyed homes were located in them. Additionally, flow bifurcated approximately 0.7 km downstream of the mountain front and traveled in a southwest direction from San Ysidro Creek, along El Bosque Road [37]. Finally, debris flows resulted in damage to at least 163 structures and complete destruction of an additional 92 structures.
