The runoff from heavy rainfall reaches urban streams quickly, causing them to rise rapidly. It is therefore of great importance to provide sufficient lead time for evacuation planning and decision making. An efficient flood forecasting and warning method is crucial for ensuring adequate lead time. With this objective, this paper proposes an analysis method for a flood forecasting and warning system, and establishes the criteria for issuing urban-stream flash flood warnings based on the amount of rainfall to allow sufficient lead time. The proposed methodology is a nonstructural approach to flood prediction and risk reduction. It considers water level fluctuations during a rainfall event and estimates the upstream (alert point) and downstream (confluence) water levels for water level analysis based on the rainfall intensity and duration. We also investigate the rainfall/runoff and flow rate/water level relationships using the Hydrologic Engineering Center's Hydrologic Modeling System (HEC-HMS) and the HEC's River Analysis System (HEC-RAS) models, respectively, and estimate the rainfall threshold for issuing flash flood warnings depending on the backwater state based on actual watershed conditions. We present a methodology for issuing flash flood warnings at a critical point by considering the effects of fluctuations in various backwater conditions in real time, which will provide practical support for decision making by disaster protection workers. The results are compared with real-time water level observations of the Dorim Stream. Finally, we verify the validity of the flash flood warning criteria by comparing the predicted values with the observed values and performing validity analysis.

Due to recent occurrence of regional heavy rain events, the amount of life loss and property damages is increasing every year. In case of South Korea, it is necessary to have a pretty accurate weather forecasting system to predict regional heavy rain events during rainy season in summer since typhoons or heavy rains result in urban inundation. In this study, we select 12 government districts out of the important districts and metropolitan cities and determine the accuracy of heavy rain forecasts by comparing the observed rainfall data and heavy rain forecast announced by KMA based on the old and current two criteria for issuing heavy rain forecast which were revised in June of 2011. Also, by comparing the regional design rainfall for each return period and heavy rain warning, it may contribute to improve the accuracy of heavy rain forecast and suggest a way of the application for each region.

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The flow of driftwood and soil into drainage from agricultural areas accelerates sedimentation and inflicts overflow damage after rainfall events due to insufficient discharge capacity, causing flooding on agricultural land. However, there have been few efforts to develop a driftwood capture trellis for agricultural drainage ditches, except for some suggested design criteria. In this study, we developed a driftwood capture trellis to capture driftwood in agricultural drainage ditches and evaluated its performance based on hydraulic characteristics. The facility was designed considering criteria for drainage and driftwood control barriers, as well as the properties of driftwood found near agricultural drainage ditches. Performance evaluation was conducted through hydraulic experiments. Driftwood capture trellises were installed in 400 mm drainage pipes and a total of 216 experimental runs were conducted: six runs each in six different velocity variations and six water depth variations. The results showed that the driftwood capture efficiency of the facility exceeded 60% at a velocity of 0.144 m³/s. Limited conditions for hydraulic experiments should be considered. The driftwood capture trellis for agricultural drainage ditches developed in this study could contribute to a reduction in overflow damage caused by driftwood sedimentation.

In this study, a regression equation for drought occurrence is developed using past drought damage data from Asia. To estimate the regression equation for drought occurrence, Asian droughts are converted into standard scores according to yearly damage. Standard scores of drought occurrence are analyzed using the drought damage data for Asia from 1900 to 2018 to utilize the dimensionless index of normal distribution. Standard scores of drought occurrence in Asia are classified into seven grades. Quantitative indicators of the number of drought occurrences as well as the standard scores of tributaries are presented. Also, the regression equation for drought occurrence in Asia is calculated to predict drought damage considering the climate change effect. The drought outlook for the continent is expected to be the same in Central and Western Asia in 2040 compared with 2018, while the occurrence of drought in East Asia, Southeast Asia, and South Asia is increasing.