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 flooding.
Extent of rainfall in the watershed: In the case of Escambia County, there are two primary drainage basins: the Escambia Basin and the Perdido Basin. Within those two primary basins, there are 40 sub-basins. During a heavy rainfall event, it is highly unlikely that the same amount of rain occurrs uniformly throughout the entire County or even throughout a primary basin. During intensely localized storms, rainfall amounts throughout the basin can differ greatly from the amount measured at the location of the rain gauge. Some parts of the basin may even remain dry, supplying no additional runoff and lessening the impact of the storm.
Soil saturation before the storm: Existing conditions prior to the storm can influence the amount of storm water runoff and flooding. This phenomenon was exemplified in April 2014, when an extremely wet month – estimated at 200 percent to 600 percent over normal – exacerbated the damage caused the record rain on April 29. Dry soil allows greater infiltration of into the ground. Conversely, soil that is already wet from previous rains has a lower capacity for infiltration.
Relation between the size of the watershed and duration of the storm: Another factor to consider is the relation between the duration of the storm and the size of the 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 than the same storm in a 50-mi2 basin. Generally, larger drainage areas require storms of longer duration for a significant increase in flooding to occur. These and other factors determine whether or not a 100-year storm will produce a 100-year flood.
*Source: The United States Geological Survey Water Science School.
