**5. Discussions**

#### **5.1 Insights related to previous flash floods collected data**

Additionally, we compared simulation results with various data collected in other studies. Human damage was really dramatic at the final outlet of Montecito and San Ysidro Creeks: along the 1st 1 km of these two catchments from the apex, debris-flow stages were several meters above the channel banks, and at some locations exceeded 10 m above the initial thalweg [5]. By the way, our simulations demonstrate the good performance of RuiCells© model, which exhibits high morphological efficiency and the role played by morphological signature. Due to the intense efficiencies and the burned surface, conducive to hortonian runoff, debris flows arrived suddenly and violently at the interface between the final outlets and first homes located at the apex. The high burned areas (respectively, 79 and 85%) added to morphological efficiencies have also aggravated the quick responses (**Table 1**). Complementary simulations with RuiCells© [49] also proved that the rainfall intensity played a tiny role in the increase in runoff flows and volumes (1.4 to 1.86, if comparing 2017 and 2018 rainfall with no fire impacts). By injecting the 2018 rainfall and the magnitude of the burned areas (from 83 to 79% in San Ysidro, Montecito, and Romero), we estimate that fire and morphology combined with rainfall increase peak flows by orders of magnitude ranging from 9.7 to 10.3. On the other hand, other geomorphological indicators also confirm that the "cauliflowers effects" are not fictive. Large boulders (with a-axis > 1 m) were transported nearly the entire length of San Ysidro, Montecito, and

Romero creeks. But numerous locations of boulder deposits coincide with human infrastructure, roads, clogged culverts, bridge underpasses, or variations in the channel slopes. But surely all these damages are related to high water velocities and capacities, so indirectly to high concentration. And finally, topography really seems to be the key factor to assess postfire debris flows [29, 30, 34, 40].

### **5.2 Challenges face in the current era of extremes**

The outcome of the 2018 postfire debris flow event that took place in Montecito was devastating but could have been worse if no coordination had taken place between the local, State, and Federal Agencies and if no early warning had been issued by the NWS or Santa Barbara OEM. However, our new simulation results can be used to produce a simple heuristic approach and be relevant to the existing "Duck, Cover, and Hold" for earthquakes. We might use "wildfire – intense rain – move uphill," or "Ready" (be prepared in advance to evacuate if necessary), "Set" (monitor fire burned areas and postfire precipitation in preparation to evacuate), and "Go" (evacuate when directed or if you are uncomfortable). These terms are in the Santa Barbara County hazard education program [50], and drills are currently being tested in elementary schools in Montecito as part of hazard education [29, 30]. More suitable, during the 2018 event, many people did not comply with the evacuation order required by local stakeholders, and errors detected in the past were not taken into account by many actors. Understanding why people do not comply with evacuation orders concerning debris flows is key to knowing how to better communicate the risks in ways that may lead to better disaster preparedness and response. And even a slight delay in starting your evacuation will result in significantly longer travel times as traffic congestion worsens [50]. Therefore, the challenge still remains in identifying the exact timing and location where intense convective cells might develop (isolated or within a larger system). The 2018 event featured a north–south oriented atmospheric river with two moisture bands interacting with a closed low-pressure system. The main AR had moved southeast by the time of the debris flows. While the NCFR drove the high rain rates that produced the debris flow, the AR helped transport moisture into the area. Across the Santa Ynez and Topatopa Mountains, approximately 2−5+ inches of rain fell over a 2-day period. This value indicates a moderate storm for the region in terms of precipitation totals. However, the NCFR produced periods of intense rain. The 15-minute rains observed correspond to a 25−50 years event according to NOAA, while the FS model reported a 15 min total of 0.86 inches, a 100-year event [51].
