Wildfires striking vegetated hillslopes appear to increase the hazard towards rainfall-induced landslides. One mechanism little investigated in the literature consists in the formation of Wooden Embers Cover (WEC) following the wildfire. This layer has very peculiar thermohydraulic properties and may affect the interaction between the atmosphere and the subsoil. The paper presents an experiment conducted in an outdoor lysimeter filled with pyroclastic silt (SILT) up to 75 cm covered with 5 cm of WEC. Water storage in the SILT layer, soil water content, suction, and temperature were recorded for several years, initially under bare (no-WEC) condition (4 years), then vegetated (no-WEC) condition (5 years) and, finally, with a WEC placed on the top of the SILT (SILT+WEC condition; 3 years). The hydrological effect of the WEC was assessed by comparing the response of the SILT+WEC with the SILT under bare or vegetated conditions. The WEC reduces water losses by evaporation, thus increasing the average water content in the underlying SILT, an effect that is detrimental to slope stability. To discriminate whether the barrier effect was associated with the lower thermal or hydraulic conductivity of the WEC, a numerical simulation was carried out by considering the case of a WEC with its real thermal and hydraulic properties and the case of a fictitious top layer placed on the top of the SILT having the same hydraulic properties of the WEC but the thermal properties of the SILT. It is concluded that the barrier effect of the WEC is mainly associated with its hydraulic properties, i.e. the WEC acts as a capillary barrier. To demonstrate the practical implications of this findings, a case study of rainfall-induced landslide has been reanalysed by simulating the presence of a WEC layer having the same thermohydraulic properties as the material characterised in this study. It is shown that a WEC can substantially reduce the severity of the triggering rainfall event, thus increasing the vulnerability of the slope to rainfall-induced failure.

中文翻译：

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木质余烬覆盖对粉质火山覆盖层热水文响应的影响及其对野火后边坡稳定性的影响


袭击植被山坡的野火似乎增加了降雨引发山体滑坡的危险。文献中很少研究的一种机制是野火后木质余烬覆盖物（WEC）的形成。该层具有非常奇特的热水力特性，可能会影响大气与底土之间的相互作用。本文介绍了在户外蒸渗仪中进行的实验，该蒸渗仪充满了高达 75 厘米的火山碎屑粉砂 (SILT)，并覆盖了 5 厘米的 WEC。记录 SILT 层的储水量、土壤含水量、吸力和温度数年，最初在裸露（无 WEC）条件下（4 年），然后在植被（无 WEC）条件下（5 年），最后，将 WEC 放置在 SILT 顶部（SILT+WEC 条件；3 年）。通过比较 SILT+WEC 与裸露或植被条件下 SILT 的响应来评估 WEC 的水文效应。 WEC 减少了蒸发造成的水损失，从而增加了底层淤泥中的平均含水量，这种效应不利于边坡稳定性。为了区分屏障效应是否与 WEC 较低的热导率或水力导率相关，通过考虑 WEC 的真实热力和水力特性以及放置在 WEC 上的虚构顶层的情况，进行了数值模拟。 SILT 顶部具有与 WEC 相同的水力特性，但具有 SILT 的热特性。结论是WEC的屏障效应主要与其水力特性有关，即WEC起到毛细屏障的作用。 为了证明这一发现的实际意义，通过模拟与本研究中表征的材料具有相同热水力特性的 WEC 层的存在，重新分析了降雨引起的滑坡的案例研究。结果表明，WEC 可以大大降低触发降雨事件的严重程度，从而增加边坡因降雨引起的破坏的脆弱性。