Evaluation of Changing Characteristics of Temporal Rainfall Distribution within 24-Hour Duration Storms and Their Influences on Peak Discharges: Case Study of Asheville, North Carolina

Abstract

Design storms are used for the sizing of urban drainage systems and for delineating floodplains. Recorded rainfall extremes are commonly used to develop design storms with the assumption of stationarity. However, the projections provided by climate models indicate a continued increase in the magnitude and frequency of rainfall extremes. Therefore, a continuous revision of design storm distribution is required. The objective of this study is to propose a framework for the development of site-specific temporal rainfall distributions (design storm distributions) from the historical rainfall data, using a clustering technique. The k-means for the longitudinal data (KmL) package on the R platform was used to cluster the storm distributions prepared from the historical rainfall data. The effects of temporal rainfall patterns and the antecedent moisture condition (AMC) on the peak discharges were assessed by comparing them with the results obtained from the standard Soil Conservation Service (SCS) Type II curve. The proposed framework was successfully implemented using the long-term hourly rainfall data obtained from the Asheville gauging station in North Carolina. It was found that 14, 61, and 25% of the storm distributions were categorized under the lower, middle and upper cluster curves, respectively. The clustered curves, which represent the ranges of severe storm events, can be considered design storm distributions for the study area. For the same hydrologic engineering center-hydrologic modeling system (HEC-HMS) model setup, the difference in peak discharges obtained from the SCS Type II curve and the cluster curves were as high as 179.1 m3/s. This result indicates that the use of the SCS Type II curve for estimating the design flood can cause the overdesign of structures. The difference in the peak flows is even more pronounced for wetter initial soil moisture conditions. In consideration of the percentage change in the peak flows and the preceding storms, the antecedent wetness needs to be included in estimating design floods.