Abstract
Lahars impose significant secondary hazards on highly populated volcanic islands by remobilising volcanic ash deposits. Karthala, on Grande Comore Island, is a 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 impacted the settlements at the foot of Karthala, damaging roads and hundreds of houses, affecting thousands of inhabitants. This study aims at gaining insights into the effectiveness of two simple tools to reproduce the lahars’ extent in this data-poor context. To understand the physical characteristics of the lahars, we first documented the 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 was 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 results from Q-LavHA, a recent probabilistic flow model originally developed for lava flows. Our comparison demonstrates that LaharZ outperforms Q-LavHA for small-scale lahar simulations. While Q-LavHA has features that mitigate some limitations of LaharZ, such as its ability to simulate flow bifurcations and the transition from constrained to unconstrained flow, it typically underestimates the actual inundated area. Nevertheless, both models show rather poor performance, encountering difficulties in delineating lahar flow paths due to the smooth topography of the volcanic edifice. We therefore also evaluated the potential to increase simulation accuracy by updating a 10-meter resolution Digital Elevation Model (DEM) with channel topography measurements. While this approach led to improved simulation accuracies for both models compared to using un-adapted DEMs, it did not prevent simulations to miss the hazard-prone position of settlements actually affected by the hazard. This study helps inform numerical volcanic flow simulation strategies on young and poorly dissected volcanic edifices, such as basaltic shields, and forms a basis for further developing numerical simulation tools more adapted to these particular environmental settings.
Highlights
Lahars are sudden and highly mobile mixtures of water, sediment and rock debris originating from volcano slopes (Smith and Lowe, 1991)
Our model comparison demonstrates that LaharZ outperforms Q-LavHA for small-scale lahar simulations
While Q-LavHA has features that mitigate some limitations of LaharZ, such as its ability to simulate flow bifurcations and the transition from constrained to unconstrained flow, it typically yielded lower simulation accuracies compared to LaharZ simulations
Summary
Lahars are sudden and highly mobile mixtures of water, sediment and rock debris originating from volcano slopes (Smith and Lowe, 1991). A critical requirement for the formation of a lahar is the availability of erodible material in the rainwater accumulation zone or along the flow path, allowing an increase of both the initial lahar volume and a multiplication of the discharge by several times while flowing downstream (Scott et al, 2005; Doyle et al, 2011; Vallance and Iverson, 2015). This volume increase process is known as bulking and can lead to intensive channel and bank erosion upstream and incremental infilling of channels downstream of break-in-slope. By reducing the channel conveyance capacity, the infilling can induce overflows during, and after the event(s) if no action is undertaken to improve the channel conveyance capacity (Pierson, 2005)
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