Characteristics of Flood Flow with Active Sediment Transport in the Sozu River Flood Hazards at the Severe Rainfall Event in July 2018
Daisuke Harada, Naoko Nagumo, Yousuke Nakamura, and Shinji Egashira
International Centre for Water Hazard and Risk Management (ICHARM) under the Auspices of UNESCO, Public Works Research Institute (PWRI)
1-6 Minamihara, Tsukuba, Ibaraki 305-8516, Japan
This study reveals the characteristics of flood flow with active sediment transport that caused the Sozu River flood disaster in July 2018. The results of field surveys revealed that the basin contains steep mountains and flat areas, facilitating the rapid transport of sediment produced in the mountains to downstream areas during the disaster. Two-dimensional numerical simulations of flood flow with sediment transport are conducted to realize the flood flow characteristics. As a result of the simulation, a huge amount of sediment deposition in the channel near the top of the alluvial fan increases the inundation damage at downstream areas. The results indicate that within two flood peaks during the disaster, the inundation in the second flood peak is more severe than that in the first peak, whereas the discharge of the second peak is much less than that in the first peak, caused by the reduction in the capacity of the original channel owing to the sand deposition during the first and second flood peaks.
-  D. Harada and S. Egashira, “Flood flow characteristics with fine sediment supply and drift woods – analysis on the Akatani River flood hazards in July,” J. of Japan Society of Civil Engineers, Ser. B1 (Hydraulic Engineering), Vol.74, Issue 4, pp. I_937-I_942, 2017 (in Japanese).
-  D. Harada, S. Egashira, A. Yorozuya, and Y. Iwami, “Characteristics of flood flow with riverbed deformation in the Omoto river in 2016 flood disaster,” Advances in River Engineering, Vol.23, pp. 43-48, 2017 (in Japanese).
-  Hiroshima Prefecture, “Damage report by the rainfall disaster in 2018,” Ver.46, 28th July, 2018.
-  T. Sayama, G. Ozawa, T. Kawakami, S. Nabesaka, and K. Fukami, “Rainfall–runoff–inundation analysis of the 2011 Pakistan flood in the Kabul river basin,” Hydrological Sciences J., Vol.57, pp. 298-312, 2012.
-  Y. Shimizu, “A method for simultaneous computation of bed and bank deformation of a river,” River Flow 2002: Proc. of the Int. Conf. on Fluvial Hydraulics, 2002.
-  Y. Shimizu et al., “Advances in Computational Morphodynamics Using the International River Interface Cooperative (iRIC) Software,” Earth Surface Processes and Landforms, DOI: https://doi.org/10.1002/esp.4653, 2019.
-  K. Ashida and M. Michiue, “Study on hydraulic resistance and bed-load transport rate in alluvial streams,” Proc. of the Japan Society of Civil Engineers, No.206, pp. 59-69, 1972 (in Japanese).
-  S. Egashira, K. Miyamoto, and T. Itoh, “Constitutive equations of debris flow and their applicability,” Proc. 1st Int. Conf., Debris-Flow Hazards Mitigation, 1997.
-  H. Takebayashi, “River configuration in middle-lower reach of river basin,” J. of Japan Society of Fluid Mechanics, Vol.24, No.1, pp. 27-36, 2005.
-  W. W. Rubey, “Settling velocity of gravel, sand, and silt particles,” American J. of Science, Vol.25, No.148, pp. 325-338, 1933.
-  E. W. Lane and A. A. Kalinske, “The relation of suspended to bed material in rivers,” Eos, Trans. American Geophysical Union, Vol.20, Issue 4, pp. 637-641, 1938.