The Arrival of Seeds into Sand Bars after Several Floods in River Channels

This paper presents the results of field survey on the arrival of plant seeds into surface layers of sand bars after several floods during 2016 in Kinugawa River, Japan. The seed arrival could be an onset of the secondary succession of plant communities on sand bars, leading to their well-vegetated states after several decades that cause river management issues both on flood disaster prevention and riverine ecosystem alteration. Kinugawa River had the largest flood on record in September 9-10, 2015. It resulted in the levee failure and the corresponding flood disaster in Joso City, located in the downstream part of Kinugawa River. It also had made the enormous impact on the riverine vegetation communities, resulting in making many sand bars and gravel beds be bare surface states. For investigating the very initial state of the seed arrival into the created bare surfaces by small to medium floods, 3 channel sections with 6 observation points of Kinugawa River in total were selected and observed during the rainy season in 2016. In order to measure the depth of active surface layers on the sand bars in situ, a steel ling with a pile was used during the flood events. The sediments in the active surface layers were sampled to draw the grain size accumulation curve as well as to count the number of seeds within the sample sediments. In addition, a numerical simulation model, iRIC software, was used for evaluating bed shear stresses at the peak discharges of the flood events. The results showed that the number of seeds in active surface layers seemed to be large on the order of 10² to 10³ in a unit volume (1.0×1.0×0.1 m³), and its majority were included in the sediments with smaller mean diameters, i.e., fine sand, rather than those with the larger mean diameters. In addition, the number of seeds decreased with the small percentile, around 15^th, in particle diameter rather than the mean diameter. Moreover, relations in terms of hydraulics and sediment dynamics were discussed in detail between the number of seeds, the depth of the active layers, and the shear stresses on the riverbeds calculated by the numerical simulation model.