The consideration of storm durations in hydrological frequency analysis and design storm determination

     For engineering practice, the determination of design storms mainly applies Intensity-duration-frequency method or revised ranking method with fixed given duration based on the long-term rainfall record. However, this kind of method does not address the influence of storm duration. Both long or short storms are combined together in performing frequency analysis. As a result, in some cases, the peak intensity of short duration design storm might be overestimated by traditional methods. Consequently, how to incorporate the concept of duration to improve current frequency analysis is essential to improve design storms are the two main purposes of this research. With this concern, this study tries to find better definitions of the rainfall characteristics based on different durations, and generate revised design storm in the consideration of more comprehensive rainfall patterns. For the purpose of flood control, this study tries to proposed the improved method based on a conceptual watershed with known drainage capacity. In the watershed, when the runoff exceeds this capacity, the accumulation of the surface runoff will become the flood water, and keep increasing with raining time. Considering this situation, the duration of storm plays an important role in the severity of flooding. For this concern, this study tries to incorporate two parameters, the peak intensity, and time that rainfall exceeds drainage capacity, to indicate the severity of storm. Following, to avoid the under/over-estimation of rainfall intensity without considering the influence of duration, we propose the data weight index to composed design storm by combining different duration rainfall events. More specifically, if the duration of rainfall event is similar to the design storm, the parameters of data will get a higher weight index to shape the rainfall characteristics of design. With this process, we can obtain the suitable parameters for our design storms and compare the risk of flood with any two storms with different duration in a same drainage capacity. Based on the equivalent risk of storms, we could transform the parameters of the rainfall with the same risk level even for different duration. By this way, this study will propose a method of design storms which offers a better application for flood protection plan and management.

 

Keywords: Design storm, Urban rainfall design, Hydrologic frequency analysis