Abstract
Flood warning systems can provide an effective and low-cost solution to protect urban residents especially in developing countries suffering from severe flood damages caused by weak capacity of flood defense structures. Rainfall Threshold for Flood Warning (RTFW) is an essential key of accurate flood warning system. This study proposed an easy-to-use mathematical equation for RTFW for small urban watersheds based on computer simulations. First, a coupled 1D–2D dual-drainage model was used to simulate the flooded area in nine watersheds of Seoul, Korea corresponding to 540 scenarios of various synthetic rainfall events and watershed imperviousness. Then, the results of the 101 simulations that caused the critical flooded depth (0.25 m–0.35 m) were used to develop the equation that relates the value of RTFW to the rainfall event temporal variability (represented as coefficient of variation or CV) and the watershed imperviousness (represented as NRCS Curve Number or CN). The results suggest that (1) RTFW exponentially decreases as the rainfall CV increases; (2) RTFW linearly decreases as the watershed CN increases; and that (3) RTFW is dominated by CV when the rainfall has low temporal variability (e.g., CV < 0.2) while RTFW is dominated by CN when the rainfall has high temporal variability (e.g., CV > 0.4). For validation, the proposed equation was applied for the flood warning system with two storm events occurred in 2010 and 2011 over 239 watersheds in Seoul. The system showed the hit, false and missed alarm rates at 69% (48%), 31% (52%) and 6.7% (4.5%), respectively for the 2010 (2011) event.
Highlights
Floods have posed serious challenges to urban areas, with high concentration of population and properties
The results show that there is great uncertainty residing on the relationship between flood depth and rainfall intensity, the regression lines follow a major trend that the line corresponding to higher curve number (CN) value is located on upper side of the line of the lower CN
This study introduced a method of estimating RTFW based on numerical simulations to be used as a core of flood warning system for urban areas. 1D-2D dual drainage hydrodynamic model (XPSWMM)
Summary
Floods have posed serious challenges to urban areas, with high concentration of population and properties. Urban floods in history often caused widespread devastation, economic damages, and loss of human lives across the globe (Braud et al, 2012; Hammonda et al, 2015; Miller et al, 2014;2017; Sarmah et al, 2017). The magnitude of damages from the urban floods have been increasing at a rapid rate (O’Driscoll et al, 2010; Chen et al, 2015; Miller et al.,2017) due to following reasons. In 2008, a research from UN showed that half of the world's population lived in urban areas, and it was projected to rise to 60% in 2030 and. This rapid urbanization rate is resulting in the increase of imperviousness and runoff in these areas, thereby leading to an increase in damage from the urban flood. 100-year flood event could become 2-5 year flood event in some areas leading to increase the flood risk and associated damages (Milly et al, 2002)
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