Effects of Artificial Structures on Grain Size and Characteristics of the 2018 Anak Krakatau Tsunami Deposits

This paper presents the first detailed analysis of grain size parameters and tsunami deposit of the 2018 Anak Krakatau tsunami, considering artificial structures such as buildings and roads. The deposition of tsunamis on land is extremely complex, involving contributions from both the hydrodynamics of tsunami flow propagation and the characteristics of the tsunami, as well as topographical conditions. In both local and detailed contexts, artificial structures can affect the formation of tsunami deposits. In this study, we focused on the effect of artificial structures on the characteristics of the 2018 Anak Krakatau tsunami deposits. Field observations showed that the remains of dikes, houses and asphalt roads significantly influenced the source, thickness, structure of tsunami deposits and grain size parameters. Tsunami deposits consisting of fine-grained material originated from the sediment on the seafloor. Tsunami deposits were thicker in areas closer to the part of the sea unaffected by artificial structures and exhibited a thinning trend inland. In general, the grain size of tsunami deposits is coarser than that of pre-tsunami deposits. Tsunami deposits predominantly exhibit an upward fining trend. Our study explains the key role of artificial structures in the formation of tsunami deposits and the sediment structure produced in coastal areas. A drastic change in the characteristics of the grain size parameters results from the presence of artificial structures. Overall, the tsunami deposit layers could have been better sorted. Significantly different sortation values were observed at sites located above the asphalt road. Similarly, kurtosis values showed an anomaly. The skewness value was inversely proportional to the average grain size in areas affected by artificial structures.