11:45 AM - 12:00 PM
[HDS08-11] Characteristics of grain size distribution along a debris flow torrent
Keywords:Debris flow, Sediment size, UAV photogrammetry, BASEGRAIN
Debris flow is a common geohazard in mountainous regions that poses a serious threat to people's lives and property, necessitating extensive investigation to explore different aspects of it. This study aims to automatically measure grain size distributions in the debris flow torrent of the Ohya landslide in central Japan. The approach involves UAV-based Structure from Motion (UAV-SfM) photogrammetry for capturing aerial images and granulometric analysis using BASEGRAIN software.
Multiple clipped images that are exported from orthomosaic generated by UAV-SfM are analysed using BASEGRAIN software for the detection of spatial change in grain size along the debris flow torrent. BASEGRAIN object detection algorithm was used to obtain the region properties of each grain's top-view area such as long axis, short axis, area value etc. Due to the UAV's greater distance (over 50 m), only larger sediments with a long axis above 15 cm were detectable, making them suitable for the analysis of boulders. Rigorous visual tests were conducted to eliminate errors from poor image quality and adverse conditions such as vegetation. For the validation check, Grid Sampling Technique was used, where length and breadth of each sample was measured which later on compared with the result from BASEGRAIN data. Comparative BASEGRAIN analysis across different channel deposit sections clarifies variation in boulders ratio between the upper and lower channels as well as temporal variations within the same section following debris flow and sediment supply events. The ratio of boulder-size sediments varied due to the heterogeneity of the depositional and erosional processes.This study implies that the information gathered on grain size will be useful in the future for estimating other parameters, such as the velocity, energy, and volume of debris flows, which can be used to support the development of early warning systems as well as hazard mitigation and risk reduction plans.
Multiple clipped images that are exported from orthomosaic generated by UAV-SfM are analysed using BASEGRAIN software for the detection of spatial change in grain size along the debris flow torrent. BASEGRAIN object detection algorithm was used to obtain the region properties of each grain's top-view area such as long axis, short axis, area value etc. Due to the UAV's greater distance (over 50 m), only larger sediments with a long axis above 15 cm were detectable, making them suitable for the analysis of boulders. Rigorous visual tests were conducted to eliminate errors from poor image quality and adverse conditions such as vegetation. For the validation check, Grid Sampling Technique was used, where length and breadth of each sample was measured which later on compared with the result from BASEGRAIN data. Comparative BASEGRAIN analysis across different channel deposit sections clarifies variation in boulders ratio between the upper and lower channels as well as temporal variations within the same section following debris flow and sediment supply events. The ratio of boulder-size sediments varied due to the heterogeneity of the depositional and erosional processes.This study implies that the information gathered on grain size will be useful in the future for estimating other parameters, such as the velocity, energy, and volume of debris flows, which can be used to support the development of early warning systems as well as hazard mitigation and risk reduction plans.