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

DOES A 100-YEAR STORM ALWAYS CAUSE A 100-YEAR FLOOD?

No. Several factors can independently influence the cause-and-effect relation between rainfall and streamflow.

When rainfall data are collected at a point within a stream basin, it is highly unlikely that this same amount of rainfall occurred uniformly throughout the entire basin. During intensely localized storms, rainfall amounts throughout the basin can differ greatly from the rainfall amount measured at the location of the raingage. Some parts of the basin may even remain dry, supplying no additional runoff to the streamflow and lessening the impact of the storm. Consequently, only part of the basin may experience a 100-year rainfall event; for example, in July 1997 only 30 percent of the McAlpine Creek Basin experienced rainfall amounts greater than or equal to a 100-year event (table 1).

Existing conditions prior to the storm can influence the amount of stormwater runoff into the stream system. Dry soil allows greater infiltration of rainfall and reduces the amount of runoff entering the stream. Conversely, soil that is already wet from previous rains has a lower capacity for infiltration, allowing more runoff to enter the stream.

Another factor to consider is the relation between the duration of the storm and the size of the stream basin in which the storm occurs. For example, a 100-year storm of 30-minutes duration in a 1-square-mile (mi²) basin will have a more significant effect on streamflow than the same storm in a 50-mi² basin. Generally, streams with larger drainage areas require storms of longer duration for a significant increase in streamflow to occur. These and other factors determine whether or not a 100-year storm will produce a 100-year flood.