Post-wildfire mudflows, in which more than half of the solids are sand or smaller, destroy the watershed environment, life, and infrastructures. The surficial soil particles turn hydrophobic due to the deposition of combusted organic matter during wildfires. Initiated by raindrops splash, runoff and erosion grow into devastating mudflows, quickly blasting obstacles on the way, and carrying large boulders and debris. The internal composition of post-wildfire mudflows has recently become of interest, intending to understand better mechanisms and transport differences between post-wildfire mudflows and non-post-wildfire mudflows. This paper shows critical new insights into how the air got entrapped during the early stage of mudflow and how air entrapment affects the properties of post-wildfire mudflows as a mixture of air bubbles, water, and hydrophobic sand. This paper proposes and experimentally investigates a new paradigm in which a significant amount of air remains entrapped in post-wildfire mudflow via hydrophobic-particle-air attraction. The mudflow mixture’s internal structure depends on the physical state of small liquid marbles, which are small air bubbles covered by hydrophobic sand particles. This paper quantifies the amount of air trapped under different sand-water volumetric concentrations, the effects of mixing speeds (energy), mixing duration, and sand particle size on the final mudflow internal structure. In addition, this paper proposes an empirical estimation of density reductions due to air entrapment in the mixture during the mixing process.

Post-wildifre mudflows are devastating to watershed environment, life, and infrastructure. Burned scars tend to form catastrophic mudflows when rained upon shortly after fires, flow very fast, quickly blasting obstacles on the way and carrying large boulders and debris. Internal composition of post-wildfire mudflows has recently become of interest, with a goal to understand better mechanisms and differences between post-wildfire and natural mudflows flow and transport. This paper shows critical new insights into how air entrapment affects the properties of rain-induced post-wildfire mudflows as a mixture of air bubbles, water, and hydrophobic sand. The idea of mudflows' internal structure containing trapped air bubbles is novel. Such mixtures can flow down slopes at incredible speeds, quickly blasting obstacles on the way and carrying large stone boulders and objects. The surficial soil particles turn hydrophobic due to the deposition of combusted organic matter during wildfires. Afterward, raindrops, splash, and erosion form devastating mudflows. We propose and experimentally investigate a new paradigm in which a significant amount of air remains entrapped in post-wildfire mudflow via hydrophobic particle-air attraction. Specific findings quantify the amount of air trapped within sand-water volumetric concentrations, the effect of intermixing energy, gravity, and sand particle size on outcome mudflow internal structure. As a result, little agglomerates of sand particles covering air bubbles characterize the mudflow mixture's internal structure.

After three foreshocks the day before (M5.0, M4.7 and M4.1, respectively), a strong M6.4 Petrinja earthquake occurred on December 29, 2020, followed by thousands of aftershocks (the strongest was a January 6 M5.0 earthquake). This paper presents a unique multihazard sequence of complex events resulting in numerous cover-collapse sinkhole failures. Although the area heavily impacted by the earthquake was larger than 1,000 km2, all 91 sinkholes appeared within a 4 km2 area surrounding Mečenčani and Borojevići villages located 20–25 km SE of the epicentral area, during the three months following the main earthquake. That area was also previously prone to seldom sinkhole appearances, as evidenced by 45 documented fossil sinkholes. All 91 sinkholes opened as post-seismic events; the first one (the second biggest, 10.8x9.8 m in diameter and 3.6 m deep) started to open six hours after the strongest earthquake. The biggest sinkhole, 25x23 m in diameter and 11.7 m deep, opened seven days after the main earthquake and one day before the strongest aftershock; its total volume is larger than volume of all other 90 new sinkholes combined. The Mečenčani and Borojevići villages surroundings is the only area where a 4–15 m thick sequence of Holocene soil built of unsaturated low plasticity clays with gravel and sand interlayers and lenses covers the heavily karstified carbonate bedrock composed of alternating highly porous Miocene limestones and calcarenites. The unconfined aquifer within a soil is underlain by a well-permeable confined karst aquifer in which the water pressure during wet periods becomes subartesian to artesian, enabling significant erosion and formation of numerous caverns at the soil–limestone contact. Continuous removal of eroded sediment by groundwater flow through karstified systems in carbonates gradually expands cavernous space until a final cover-collapse. The 2020–2021 Petrinja earthquake sequence significantly accelerated these processes, resulting in 91 cover-collapse sinkholes opened during a three-months period, instead of usually one sinkhole opened every few years as reported by local people. It is interesting to note that during the strongest earthquake the water level in the unconfined aquifer was very close to the surface, and in the underlying karst aquifer artesian conditions prevailed.

Post-wildfire mudflows have intensified in recent years due to extreme wildfire occurrence, causing significant damage and infrastructure threats. However, despite recent advancements, across-scale geotechnical characterization of mudflow onset and flow behavior remains a challenge. We present a novel experimental and theoretical understanding of the sand type and rain intensity roles on mudflow onset and composition, integrating micromechanics and laboratory experiments. The analysis shows that hydrophobic fine sand, a consequence of wildfires, significantly enhances raindrops’ downhill velocity and splash due to Cassie-Baxter-type surface, as opposed to medium or coarse sand, which affects raindrops as Wenzel surface wettability model. We use micromechanical and single-drop interactions with sand particles to explain erosion on the intermediate scale laboratory tests. Raining experiments on hydrophobic sloped flumes evaluate different slope failure mechanisms in fine, medium, and coarse hydrophobic sand as erosion patterns and seepage induced infinite slope failure in the case of embedded hydrophobic layers. The sand type also affects the spatio-temporal dynamic of erosion onset and distribution of eroded material and overflown rainwater. Surprisingly, we detected a possible equilibrium state where the eroded surface roughness changes affect water overflow and lead to an equilibrium state with very little subsequent erosion under constant rain intensity. On the other hand, erosion gradually increases after the rain starts, reaches a peak, and then subsides very quickly in coarse sand. In contrast, fine sand erosion continues for a longer time but decreases as the surface roughness increases. Furthermore, micromechanical investigation of mixtures of hydrophobic sands, water, and air gives an insight into air entrapment during flow and transport of mudflows. Hydrophobic sand particles attach to air bubbles and form agglomerates, contributing to the mixture heterogeneity and affecting flow and transport properties. Sand particle size, due to gravity, also plays a role in the amount and size of resulting agglomerates. Covering air bubbles with attached sand particles decreases the post-wildfire mudflow density up to 33% in laboratory conditions.