Sedimentary characterization and provenance analysis of the 2018 flooding along the Dan River, Shandong, and the hydrodynamic process reconstruction

Sedimentary characterization of modern floods helps to reconstruct the hydrologic flood process and spot the source area of sediment erosion. It is of great referential value both to the design of flood-control works and the identification of paleo-flood layers. In mid-August 2018, two typhoons battered the Dan River Basin in northern Shandong, leading to flooding disasters. Based on the research of the flooded area, the boreholeDH1 of 21 cm long was obtained from the downstream with well-preserved flood sediments. The analyses on its grain size, loss on ignition, magnetic susceptibility, and sporo-pollen features, show that the sedimentary cycle under flooding is characterized by fine grains in the lower part and coarse grains in the upper part. Modern soil is developed at a depth of 11.5~21.0 cm; a typical flood silt layer is developed at a depth of 0~9.0 cm, which is relatively coarse in grain size, dominated by medium-to-coarse-grained silt, and with a sand content of 14.7% on average; and a fine-grained sedimentary layer is developed at a depth of 9.0~11.5 cm during the initial period of flooding. According to the various traits of sensitive component contents, the flooding process can be divided into two stages: the relatively weak hydrodynamic force at the earlier stage and the significantly accelerated flow velocity at the later stage; The former is characterized by severe vicious flow and deposition of clays under the combined action of natural conditions and human activities, which can be further divided into two sub-sections of the rising water level and the significant acceleration of flow velocity; The latter result in typical silty deposits from flooding. Carbonate, organic matter content, and sporo-pollen abundance all are negatively correlated with grain size, indicating that the weak hydrodynamic environment is conducive to its deposition and accumulation. The sporo-pollen assemblage of the surface soil can better indicate the distribution of vegetation in the study area, and the sporo-pollen assemblage of the silty layer from flooding can better reflect the overall condition of vegetation in the basin, which has revealed that the river flood is much greater than wind in transporting sporo-pollen. In addition, the sporo-pollen assemblage features in the clayey layer from flooding are in good agreement with the distribution of vegetation in the study area, and obviously different from those of the silty layer from flooding, according to which it is speculated that the primary cause of the water level rise during the initial period of flooding lies in precipitation incorporating with local surface runoff; therefore, the sediment and sporo-pollen are derived from within the study area, and meanwhile the influx of tourists in the later period has brought in more sporo-pollen from mountainous vegetation. The sporo-pollen deposition records from borehole DH1 show that the sporo-pollen assemblage features are in good response to the propagation process and hydrodynamic force, and have the potential to identify the source area of sediment erosion. The magnetic susceptibility mainly reflects the intensity of pedogenesis, and its value for both the clayey and silty layers from flooding is steadily low, significantly lower than that for the soil contact layer. Therefore, it can be used as a reliable indicator for identifying flood deposits, but the geospatial scope of their use needs to be further discussed.