河川工事のための出水予測における降水予報誤差分析とその活用
安全な河川工事のためには，出水が発生する前に作業員や建設資機材を退避させることが求められる。出水の危険がある際にアラートを配信する「出水警報システムT-iAlert® River」では，降水予報を用いて退避に必要なリードタイムを確保している。一方で，予報降水量には誤差があり，予報が実測降水量よりも過小であると予測水位も過小となり，出水を見逃す懸念がある。本研究では，水位予測における出水の見逃しを減らすことを念頭に予報降水量の誤差分析を行い，その誤差を考慮した水位予測方法を検討した。熊野川流域を対象とした誤差分析の結果，予報降水量が2～8 mm/h程度の比較的少ない場合に過小となりやすい特徴が確認できた。また，誤差を考慮する関数によって補正した降水予報を用いて水位を予測することで，警報水位を超える出水の見逃し回数を低減する効果が得られたため，河川工事における出水の見逃し防止のための判断指標としての可能性が示された。
キーワード：降水予報，出水予測，誤差分析，河川工事，MSM
＊１ 技術センター 社会基盤技術研究部 水理研究室
Error Analysis of Precipitation Forecasts and Its Application to Water Level Prediction for River Works
To assure safety in river work projects, workers and construction equipment must be evacuated before flooding occurs. The T-iAlert River, a warning system that delivers alerts when there is a risk of such a water level increase, uses precipitation forecasts to ensure the necessary lead time for evacuation. However, precipitation forecasts may have errors. If a forecast is less than the observed precipitation, the predicted water level will also be less than the actual level, which may result in signs indicating the risk of flooding being overlooked. In order to reduce the overlooking of such signs indicating the risk of flooding, the error analysis of precipitation forecasts was conducted, and a method for predicting water levels that takes such errors into account was studied. The error analysis results for the Kumano River basin showed that the precipitation forecast tends to be underestimated when the forecast is relatively small, ranging from 2 to 8 mm/h. Also, the prediction of water levels using a precipitation forecast corrected by an error-correcting function was effective in reducing the number of water level increases exceeding the warning level that were overlooked. Thus, the results indicate the method’s potential as a decision indicator for reducing the number of water level increases that are overlooked in river works.
Keywords: precipitation forecast, water level prediction, error analysis, river work, MSM
*1 Hydraulic Research Section, Infrastructure Technology Research Department, Taisei Advanced Center of Technology