Lahars impose significant secondary hazards on highly populated volcanic islands by remobilizing volcanic ash deposits. Karthala, on Grande Comore Island, is a relatively young and poorly eroded basaltic shield volcano with sporadic occurrence of ash-forming phreatic eruptions. In 2005, two mildly explosive episodes emplaced tephra blankets on the summit area. Heavy precipitation subsequently triggered the repetitive occurrence of small-volume secondary lahars up to 2012. These lahars damaged roads and hundreds of houses, affecting thousands of inhabitants at the foot of Karthala volcano, but were poorly documented at the time of their occurrence. Their future hazard remains unclear as well. This study aims at gaining insights into the main characteristics of these lahars, as well as testing and comparing the effectiveness of two numerical tools to simulate the extent of these small-volume lahars. To understand the physical characteristics of the lahars, we first documented the spatial extent and characteristics of the debris deposits at the foot of Karthala volcano and in the ravines that guided the flows. Our observations suggest that the debris were emplaced by small-scale (volumes ≤ 10⁵ m³), rain-triggered and predominantly low sediment concentration lahars. The spatial extent of the deposits served to calibrate and compare numerical lahar simulations from the widely used volume-limited LaharZ model with simulations from Q-LavHA, a probabilistic flow model originally developed for lava flows. Q-LavHA mitigates some limitations of LaharZ, such as its ability to simulate flow bifurcations and the transition from constrained to unconstrained flow but comparison demonstrates that Q-LavHA typically yielded lower simulation accuracies compared to LaharZ simulations. The obtained accuracy values represent a rather poor performance for both models compared to existing studies on larger-volume lahars on stratovolcanoes, and are inferred to result mostly from difficulties in delineating lahar flow paths on the smooth, poorly eroded topography of the volcanic edifice. We therefore also evaluated the potential to increase simulation accuracy by updating a 10 m resolution Digital Elevation Model (DEM) with channel topography measurements. By using such updated DEMs, the correctly delineated area improved for both models. This approach, however, did not prevent simulations to sometimes miss the hazard-prone position of settlements which were actually affected by the hazard in the past. Our study shows the limitations of numerical volcanic flow simulation strategies on young and poorly eroded volcanic edifices, such as active basaltic shields. The results indicate that accurate topographic representations and detailed documentation of spatial extent of the impacted area and lahar deposit thickness are needed to produce accurate lahar simulations, as well as the further adaptation of existing numerical simulation tools to better suit these particular environmental settings.

Lahars通过转移火山灰沉积物，对人口稠密的火山岛造成了重大的次生危害。格兰德·科莫尔岛（Grande Comore Island）上的卡尔塔拉（Karthala）是一个相对年轻且侵蚀较弱的玄武岩盾构火山，偶发地形成灰烬的潜水喷发。2005年，在山顶地区放置了两次轻度爆炸性的特发拉毯子。暴雨随后触发了小批量次生拉哈尔的重复发生，直到2012年。这些拉哈尔破坏了道路和数百所房屋，影响了喀他拉火山脚下的数千名居民，但在发生时记录在案。它们的未来危害也仍然不清楚。这项研究旨在深入了解这些Lahar的主要特征，以及测试和比较两个数值工具的有效性，以模拟这些小容量拉哈尔的范围。为了了解拉哈尔的物理特征，我们首先记录了喀他拉火山脚下和引导水流的沟壑中碎片沉积的空间范围和特征。我们的观察结果表明，这些碎片是小规模放置的（体积≤10⁵ m ³），降雨引发的沉积物含量低的拉哈斯。沉积物的空间范围用于校准和比较广泛使用的有限体积LaharZ模型的数值拉哈尔模拟与Q-LavHA的模拟，Q-LavHA是最初为熔岩流开发的概率流动模型。Q-LavHA减轻了LaharZ的一些局限性，例如它模拟流分支的能力以及从约束流到无约束流的过渡，但是比较表明，与LaharZ模拟相比，Q-LavHA通常产生较低的模拟精度。与现有的关于平流火山大流量拉哈尔的研究相比，两种模型所获得的精度值均表示相当差的性能，并且推断出，这主要是由于在光滑，火山大厦的地形侵蚀不佳。因此，我们还评估了通过更新10 m分辨率的数字高程模型（DEM）和通道形貌测量值来提高仿真精度的潜力。通过使用这种更新的DEM，两个模型的正确描绘区域都得到了改善。但是，这种方法并不能防止模拟有时会遗漏过去实际受灾害影响的定居点易发生灾害的位置。我们的研究显示了数值火山流动模拟策略对年轻且侵蚀较弱的火山建筑物（如活动玄武岩盾构）的局限性。结果表明，要进行精确的拉哈尔模拟，需要准确的地形图表示以及受影响区域的空间范围和拉哈尔沉积物厚度的详细文档，