西谷川および熊野川谷川流域における降水と流砂量の関係性

我が国では，近年，森林の荒廃により，降水時に土砂生産・流入や崩壊危険度が増大している流域が存在するため，降水や森林荒廃が土砂生産等に影響を及ぼす程度を明確にする必要がある．本研究では，和歌山県の二級河川富田川支川の西谷川および熊野川谷川流域で，降水，河川水位，流砂量の観測を行うとともに，富田川流域に隣接する一級河川熊野川右支川の音無川流域における流出土砂推計値の検討を行った結果，降水量と流砂量の間に，雨量の閾値，土砂流出に有効な降雨強度および一次関数の関係式を見出せた．また，得られた関係式に基づく流出土砂量の予測により効率的な河川管理を行い，治水安全度の向上に資する可能性について考察を行った．

Due to forest dilapidations in recent years, sediment yields from a slope and slope collapsing risk are increased. In addition to devastating sediment disasters such as happened in Kii Peninsula flooding in 2011, continuous sediment runoff following to such disasters causes riverbed aggradation and revetment damage under normal flood period,which seriously threatens safety level of flood control. Thus it is important to clarify the relationship between precipitation and sediment discharge.
In this study, we analysed the relationship in the Otonashi River basin, a tributary of the Kumano River in Kii Peninsula, and tried to derive a critical hourly precipitation, that is effective to yield sediment discharge. From the analyses, the critical hourly precipitation can be found around 15mm/h or 20mm/h. Moreover, by applying the critical hourly precipitation as a cut-off value to the accumulated precipitation, a linear function between the adjusted precipitation and sediment discharge becomes clearer. These concepts were also applied to the observational results in the Nishidani and the Kumanogawadani river Basin, tributaries of the Tonda River adjacent to the Kumano River, then we showed the proposed linear function can estimate sediment discharge in accordance with a rainfall event reasonably well. Although the coefficients of the function vary from one river basin to another, the concept can be useful to estimate sediment deposition within a river section after normal flood roughly but quite easily.

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