Estimating Sediment Export over Two Decades Following a Volcanic Eruption: A Case Study at Mount Unzen, Japan

Abstract

Rainfall-triggered volcanic debris flows (VDFs) occur because of reduced infiltration capacity after a volcanic eruption and are the primary source of sediment export. Even after the infiltration capacity of volcanic slopes has recovered, VDFs can persist for several decades. However, no models have been developed to estimate long-term sediment export more than a decade after an eruption of any volcano. We developed generalized linear models to estimate sediment export (Se) over a long term in the Tansandani and Gokurakudani gullies at Mount Unzen, which erupted between 1990 and 1995. The variable Se, calculated using digital elevation models from 2003 to 2022, was set as the dependent variable. The rainfall index, decline indicator for sediment export, proxy of constant sediment supply, and gully difference were set as independent variables. In addition to 40 rainfall index patterns, we tried three decline indicators: the normalized difference vegetation index for the growing and non-growing seasons (Model 1a and 1b) and the cumulative dates (Model 2). Based on stepwise processes, the rainfall index and decline indicator were selected for all models. The gully difference was selected for Models 1b and 2. For all models, the estimated Se well followed the observed Se, although the estimation error was the smallest for Model 2, followed by Models 1b and 1a. Thus, we successfully estimated Se over two decades using easily obtainable variables. Further studies would be effective to confirm the broader applicability of our models.