INFLUENCE OF SEDIMENT DISCHARGE AND DRIFTWOODS ON NORO-GAWA DAM AND RIVER FLOODING IN JULY 2018 DURING HEAVY RAIN

Abstract

 The heavy rain occurred in July 2018 induced huge damage west Japan area including Hiroshima, Okayama, Ehime, Yamaguchi. Noro-gawa River basin in Hiroshima Prefecture was also damaged by the heavy rain. The synergistic disaster between land failure disasters and flood disasters can be found in Hiroshima Prefecture. This disaster was a typical example of synergistic disasters. This paper reports the consideration of the influence of sedimentations and driftwoods on Noro-gawa dam, flooding in the downstream basin region of Noro-gawa River.

 The main results of this paper are summarized as follows: 1) The damage of Noro-gawa dam, Noro-gawa River and Nakahata-gawa River were enhanced by the accumulation of sedimentations and driftwoods. On the other hand, the river blockade by driftwoods at the bridge in Nakahata-gawa River induced the flooding. 2) the estimated maximum inflow discharge to Noro-gawa dam and outflow discharge from Noro-gawa dam were 179m³/s and 173m³/s, respectively. 3) The inflow discharge to Noro-gawa dam was estimated, excluding influence of sediment discharge and driftwoods. 4) The estimated river discharges during the flooding were 180m³/s at the downstream of Noro-gawa dam, 230m³/s at Fujinami water level observation station and 430m³/s behind the connection point of two branch rivers.