Effects of August 1995 and July 1997 Storms in the City of Charlotte and Mecklenburg County, North Carolina

Prepared in cooperation with the City of Charlotte and Mecklenburg County

WHAT IS A RECURRENCE INTERVAL?

Statistical techniques, through a process called frequency analysis, are used to estimate the probability of the occurrence of a given event. The recurrence interval (sometimes called the return period) is based on the probability that the given event will be equalled or exceeded in any given year. For example, there is a 1 in 50 chance that 6.60 inches of rain will fall in Mecklenburg County in a 24-hour period during any given year. Thus, a rainfall total of 6.60 inches in a consecutive 24-hour period is said to have a 50-year recurrence interval (see table below). Likewise, using a frequency analysis (Interagency Advisory Committee on Water Data, 1982) there is a 1 in 100 chance that a streamflow of 15,000 cubic feet per second (ft³/s) will occur during any year at Little Sugar Creek at Archdale Drive (site 54, fig. 1). Thus, a peak flow of 15,000 ft³/s at site 54 is said to have a 100-year recurrence interval. Rainfall recurrence intervals are based on both the magnitude and the duration of a rainfall event, whereas streamflow recurrence intervals are based solely on the magnitude of the annual peak flow.

Ten or more years of data are required to perform a frequency analysis for the determination of recurrence intervals. More confidence can be placed in the results of a frequency analysis based on, for example, 30 years of record than on an analysis based on 10 years of record.

The rainfall recurrence intervals presented in this fact sheet were developed almost 40 years ago (Hershfield, 1961). The USGS is currently (1998) collecting data and developing software to re-evaluate the rainfall recurrence intervals for Mecklenburg County by using more recent, locally collected data. These recurrence intervals may become better defined as more data become available for analysis.

Recurrence intervals for the annual peak streamflow at a given location change if there are significant changes in the flow patterns at that location, possibly caused by an impoundment or diversion of flow. The effects of development (conversion of land from forested or agricultural uses to commercial, residential, or industrial uses) on peak flows is generally much greater for low-recurrence interval floods than for high-recurrence interval floods, such as 25- 50- or 100-year floods. During these larger floods, the soil is saturated and does not have the capacity to absorb additional rainfall. Under these conditions, essentially all of the rain that falls, whether on paved surfaces or on saturated soil, runs off and becomes streamflow.

Rainfall estimates for selected durations and recurrence intervals in Mecklenburg County [Values are inches of rainfall, plus or minus 0.04 inch. Values interpolated for Mecklenburg County from Hershfield, 1961]
Recurrence interval, in years	Duration, in hours
	0.5	1	2	3	6	12	24
1	1.05	1.34	1.60	1.77	2.09	2.51	2.86
2	1.29	1.57	1.94	2.15	2.54	3.00	3.48
5	1.60	2.07	2.46	2.77	3.24	3.87	4.39
10	1.85	2.38	2.92	3.18	3.71	4.39	5.04
25	2.18	2.77	3.31	3.60	4.38	5.00	5.88
50	2.40	3.09	3.76	3.97	4.78	5.71	6.60
100	2.73	3.39	4.08	4.44	5.40	6.22	7.09