Characteristics of modern flood deposits in the Drainage basin of Mi River, Shandong Province and the reconstruction of hydrodynamic processes

With the aggravation of global climate change, severe weather processes and resulted flood disasters become more and more serious nowadays. Understanding the characteristics of modern flood deposition, not only provide theoretical guidance for flood mitigation, but also offer evidence and referential benchmarks for the identification of paleoflood deposits and reconstruction of paleoenvironment. In the mid-August of 2018, two typhoons hit the drainage basin of the Mihe River in the northern Shandong Province and caused serious damage and economic loss. Based on the surveys of the flooded areas, a 21.5-cm-long sediments core labeled MH1, including modern soil, was drilled in the high floodplain of downstream area where flood deposits are well preserved, and the grain size, pollen, magnetic susceptibility, and loss-on -ignition are analyzed. The results suggest that flood layer deposits are mainly composed of clay and fine silt, and the average content of sand is only 1.7%, indicating that the flood intensity was quite low. Two environmentally sensitive grain size components, 7.9 μm and 30 μm respectively, are identified in the flood sediment layer. According to the variation characteristics of sediments, the flood process can be divided into five stages: initial flooding, first flood peak, high-water level period, second flood peak, and flood recession period. The loss-on-ignition of flood sediment layer is also significantly higher than that of soil layer, with a lower magnetic susceptibility value. There is no significant difference in pollen characteristics between the flood layer and soil layer, suggesting that the pollen assemblages of flood plain soil and flood sediment can both be used to define the vegetation of the watershed. However, the pollen abundance of flood accumulation layer (7313.96 grains/g) is much higher than that of underlying soil layer (1562.65 grains/g). The experimental results suggest that the flood has the function to enrich the sporopollen to certain extent, and the magnetic susceptibility value may reflect the generation and accumulation of magnetic minerals in the process of soil formation. Therefore, they can be used as a reference index to identify palaeoflood and palaeosoil layers. However, the geospatial scope of their use needs to be further investigated.