5:15 PM - 6:45 PM
[HGM03-P09] Erosion and sedimentation on a sand and gravel bar flooded by rainfall events
Keywords:River, Sand and gravel bar, Radiocaesium, Bedform
In Japan, many rivers have sand and gravel bars due to the steep longitudinal profiles. The bars show wide grain size distribution from boulders to clay. Although such sediments may contain fine-grained components that would not be deposited at that flow velocity, sufficient data have not been obtained on the conditions under which the fine-grained fractions are captured. In this study, we continuously observed a sand and gravel bar artificially smoothed by restoration work after being damaged by typhoon Hagibis in 2019, and aimed to obtain new knowledge on the distribution and structure of new sediments after rainfall events.
The observation site is a river park in the Niida River, which flows through Minamisoma City in Fukushima Prefecture. Since this park had been damaged by the flood in 2019, restoration work had been conducted between 2020 and 2021 in the park. The sand and gravel bar, which was located on the other side of the park, was artificially flattened using heavy machinery. We took photographs on September 9 in 2022, August 22, and November 22 in 2023 using a UAV (Phantom4 RTK, DJI) and obtained DSM (digital surface model) using the UAV-SfM method. To record the flooded area during rainfall events, interval cameras were placed on the opposite bank of the bar and photographs were taken around the bar every 10 minutes. Sediment samples were corrected on the same days of UAV flight, and then they were sieved using a sieve with a 2 mm mesh aperture, and the grain size distribution and radioactivity concentration of Cs-137 were measured for the fraction less than 2 mm.
Comparison ortho mosaic images indicated the following topographic change. 1) A branch flow channel on the sand and gravel bar was abandoned between Sep. 2022 and Aug. 2023. 2) New bedforms were formed between Aug. 2023 and Nov. 2023. For example, sand ribbons following the flow line were formed at tens-centimeter intervals, and a microdelta-like topography was observed. Based on interval camera images and water level data from the Haramachi water level station, located several kilometers downstream from the river park, it was estimated that the bedforms were formed by a rainfall event on September 8-9, 2023.
In the poster, we will report the results of the grain size distribution and Cs-137 concentration measurements of the November 2023 sample, which are currently being measured, together with a discussion of the extent to which fine particles are present in the new sediments.
The observation site is a river park in the Niida River, which flows through Minamisoma City in Fukushima Prefecture. Since this park had been damaged by the flood in 2019, restoration work had been conducted between 2020 and 2021 in the park. The sand and gravel bar, which was located on the other side of the park, was artificially flattened using heavy machinery. We took photographs on September 9 in 2022, August 22, and November 22 in 2023 using a UAV (Phantom4 RTK, DJI) and obtained DSM (digital surface model) using the UAV-SfM method. To record the flooded area during rainfall events, interval cameras were placed on the opposite bank of the bar and photographs were taken around the bar every 10 minutes. Sediment samples were corrected on the same days of UAV flight, and then they were sieved using a sieve with a 2 mm mesh aperture, and the grain size distribution and radioactivity concentration of Cs-137 were measured for the fraction less than 2 mm.
Comparison ortho mosaic images indicated the following topographic change. 1) A branch flow channel on the sand and gravel bar was abandoned between Sep. 2022 and Aug. 2023. 2) New bedforms were formed between Aug. 2023 and Nov. 2023. For example, sand ribbons following the flow line were formed at tens-centimeter intervals, and a microdelta-like topography was observed. Based on interval camera images and water level data from the Haramachi water level station, located several kilometers downstream from the river park, it was estimated that the bedforms were formed by a rainfall event on September 8-9, 2023.
In the poster, we will report the results of the grain size distribution and Cs-137 concentration measurements of the November 2023 sample, which are currently being measured, together with a discussion of the extent to which fine particles are present in the new sediments.