Study on methods for assessing sediment disaster inundation zone in regions with insufficient data: case study of the Aranayake disaster in Sri Lanka

Abstract

In Sri Lanka, landslide disasters and floods occur frequently and have caused much damage. So, landslide susceptibility has been mapped and published as the Landslide Hazard Zonation Map (LHZM). Although the LHZM shows the susceptibility of landslide initiation, it does not show the potential inundation zone of landslides. As a characteristic of land use in Sri Lanka, plantation farms for cultivating Ceylon tea, natural rubber and other products are located on slopes. And, many people live on slopes. Therefore, it is necessary to assess the potential inundation zone not only in the deposited area but also the runout area below hillslopes. Recently, diverse numerical simulation models have been developed for describing landslides and debris flows. These numerical simulations are effective tools for addressing landslide problems in Sri Lanka. However, to use numerical simulation, we have to input and validate a variety of environmental variables and, unfortunately, adequate information on past disasters and environmental conditions is still lacking. Therefore, we studied methods for assessing the potential sediment disaster inundation zone in regions where empirical methods cannot be applied due to insufficient past disaster records. We applied the "HyperKANAKO" debris-flow model for describing inundated zone due to recent debris flow in Sri Lanka. We tested the sensitivity of input conditions such as the (1) percentage of fine grains (fluid density and sediment concentration), (2) landslide volume, (3) input hydrograph (peak flow rate and duration), and (4) representative sediment particle diameter. We confirmed that, for regions with insufficient disaster records, the potential inundation zone due to debris flow can be assessed using numerical simulation. However, we also found that the calculation result is strongly controlled by (1), (2), and (4), so it is necessary to set these parameters appropriately.