[A-CG48] Water and sediment dynamics from land to coastal zones

[ACG48-03] Quantifying the longshore sediment transport from residual luminescence signal along the Teradomari coast

*Hirona Homma^1,2, Toru Tamura^2,3, Masaaki Shirai¹ (1.Graduate School of Urban Environmental Sciences, Tokyo Metropolitan University, 2.Geological Survey of Japan, AIST, 3.Graduate School of Frontier Sciences, The University of Tokyo)

Keywords:luminescence, feldspar, beach sand, sediment transport, bleach experiment

Detecting and quantifying the longshore sediment transport is fundamental for coastal preservation. We considered a way of quantifying the duration of longshore transport of beach sand based on residual luminescence signals along the Teradomari coast, central Japan. A diversion channel, constructed in 1922, has supplied a huge amount of fluvial sediment to the coast, resulting in remarkable beach progradation nearby the channel mouth. The progradation has also been observed in multiple pocket beaches northeast, suggesting the presence of longshore transport beyond the headlands. We collected beach sand samples at 7 sites along the Teradomari coast for measurements of residual dose of K-feldspar luminescence. Residual doses were determined from the MET-pIRIR measuring protocol, which is able to measure multiple signals from a specific aliquot. We measured pIRIR_{150, 225, 290} signals, all of which show similar trends. For example, residual dose of pIRIR₂₉₀ was 72±22 Gy at the channel mouth and 18±4 Gy at the point 15 km from the channel mouth, revealing an increasing duration of sunlight exposure of sand grains during the longshore transport. However, at 23 km from channel mouth it was 23±4 Gy, suggesting supply from another beach sand source. Then, we attempted at quantifying the duration of the longshore transport between the channel mouth and 15 km point based on a bleach experiment of luminescence and beach envelope model. In the bleach experiment, we exposed the channel mouth sample to artificial sunlight for different durations and measured residual dose. We then fitted the relationships between exposure duration and residual dose with a regression equation. This regression enables calculation of the duration of bleaching with the artificial sunlight from observed residual doses. We converted the duration of the bleaching to that of exposure to the average solar irradiance in the Teradomari coast based on the irradiance of the artificial sunlight. A beach envelope model was also introduced to assess the probability of the sunlight exposure of sand grains during the longshore transport. In the model, a mixing layer is assumed to range from the backshore to surf zone and to the depth of 50–150 cm. The estimated transport duration from the diversion channel to the 15 km point was given by the exposure duration divided by the exposure probability. Three signals, pIRIR₁₅₀, pIRIR₂₂₅, and pIRIR₂₉₀, revealed 13–38, 12–35, and 10–29 years, respectively, depending on the mixing layer depth assumed. While these signals characterized by different rates of bleaching, they all show consistent results, which suggest sand grains are transported over a distance of 15 km in several decades along the Teradomari coast.