Quantitative analysis of land transformation in a Holocene delta: An example from the Tama River lowland, central Japan

Human activities, such as the expansion of agricultural and urban land use on subaerial deltas and landfilling of subaqueous deltas have large impacts on Holocene deltas. Here, we quantified the volume of the Holocene deltaic deposits in the Tama River lowland located in the Tokyo Metropolitan Area by analyzing 3872 existing borehole sections using GIS and evaluated artificial land transformation. The total volume of deltaic deposits accumulated during the past 9000 years was estimated to be 3925 × 10⁶ m³, of which artificially deformed near-surface material (AD) (676 × 10⁶ m³) occupies approximately 17%. This ratio is much larger than that in the Nobi Plain (4.6%) in the Nagoya metropolitan area, suggesting that the impact of anthropogenic disturbance on the delta has been greater in Tokyo than in the Nagoya metropolitan area. Approximately 71% of AD is distributed in the seaward area, where landfills were created mainly after the Taisho period (1912–1926); the source of the AD is considered to be dredged soil, construction waste soil, and mountain sand. The remaining 29% of AD occurred in the landward area (floodplain and delta plain) and likely originates from the in situ disturbed sediments of the delta plain and floodplain in addition to earth fill. The seaward AD deposits (including redeposition by dredging within the delta) accumulated at a rate of 4.79 × 10⁶ m³/year, which is much higher than that of the accumulation of natural deltaic deposits during the Holocene (0.36 × 10⁶ m³/year). This clearly shows that anthropogenic forcing has become a strong influence on geomorphological and sedimentological processes within the delta.