Author(s): Tomoko Kyuka; Yasumitsu Kato; Satomi Yamaguchi; Kunihiro Tomita; Hideto Kon; Yasuyuki Shimizu
Linked Author(s): Yasuyuki Shimizu, Tomoko Kyuka
Keywords: No keywords
Abstract: On September 6,2018, a magnitude 6.7 earthquake occurred in the eastern part of the Hokkaido Iburi Region, causing more than 6,000 surface landslides in the Azuma River basin. There are concerns about the impact of fine sediment runoff from the landslides and sedimentation on riverbeds of shishamo spawning sites. With these backgrounds, we investigated the suitable physical parameters for shishamo spawning based on long-term monitoring data conducted by Hokkaido Government. Subsequently, using a two-dimensional flow and bed deformation model, the impact of various discharge scales and high concentration silt on shishamo spawning sites were identified. The results showed that common spawning sites for shishamo are located at depths of 0.3 to 0.7 m and bottom velocities of 0.2 to 0.5 m/s, where the occupancy ratio of coarse grain sediment is high. While bottom velocities faster than 0.6 m/s and high silt sedimentation are not suitable as a spawning ground. Numerical simulation explained that the optimum locations of spawning become widespread when the flow rate is higher than the average discharge in winter month (spawning season) of 5.0 m 3 /s. Most fine-grain sediment is transported to the river mouth without sedimentation in the low flow channel, suggesting that the effects of finegradient sediment on spawning may be small so far. It should be noted, however, that sedimentation tends to occur on the inner banks of meanders, in large areas of the river width, and along the riverbanks. The Azuma River has low discharge during the winter months, so it is necessary to maintain as much flow as possible.