Distribution of excessive rainfall amounts over an urban area

Abstract

Data recorded over a 10-year period from a network of 11 recording rain gages was used in a study of the distribution characteristics of excessive rainfall amounts over the 10-square-mile urban area of Champaign-Urbana, Illinois. All storms were used in which one or more rain gages recorded an amount equalling or exceeding the 2-year return period value of point rainfall for durations of 30 minutes, 1 hour, 2 hours, 3 hours, 6 hours, 12 hours, and 24 hours. Factors investigated include: the frequency of excessive rainfall amounts within the area compared with the frequency at a given observation point within the area; the percentage of the urban area experiencing excessive rainfall amounts during individual storms and the frequency distribution of this percentage; the adequacy of a single point to define the frequency distribution of areal mean rainfall within a small area; the frequency with which excessive amounts for 30-minute to 24-hour periods occur within the same storms; and urban influences on the distribution of heavy storm rainfall. Based on the 10-year sampling period, results indicate that twice as many excessive rainfall amounts occur within a 10-square-mile area compared with the number recorded at specific points; on the average, the percentage of the 10-square-mile area experiencing excessive rainfall amounts increases with increasing storm duration; the majority of the excessive amounts for duration of 30 minutes to 24 hours occur in the same storms; a point rainfall record is a satisfactory index of the areal mean rainfall frequency distribution in a 10-square-mile area; and urban influences, if present, are not of practical significance.