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

HOW CAN THE SAME STREAMFLOW BE A 100-YEAR FLOOD AT ONE LOCATION AND ONLY A 50-YEAR FLOOD AT ANOTHER?

Recurrence intervals are based on the probability of the peak streamflow occurring at a given location in any year. As water flows downstream from point "A" to point "B" and the drainage area increases, the volume of streamflow increases. Given this, it may seem reasonable to think that peak flows would increase in the same manner, but this is not necessarily true. The upstream-downstream relation that might be expected is illustrated in figure 7. During the July 1997 flood, as Little Sugar Creek flowed downstream from Medical Center Drive to Archdale Drive, significant increases occurred in both total and peak streamflow. A somewhat different situation is illustrated in figure 8. During the same flood, a higher peak flow was measured at the upstream gaging station at Irwin Creek near Charlotte than was measured at the downstream gaging station at Sugar Creek at NC 51 near Pineville (Irwin Creek becomes Sugar Creek at the confluence with Taggart Creek).

It is apparent from figure 8 that Irwin Creek rose and fell much more quickly than did Sugar Creek at the streamflow gaging locations. While Sugar Creek did not have a higher peak flow, it did have a higher volume of streamflow for the event as the streamflow remained elevated for a longer period of time. This phenomenon, known as peak attenuation, can be attributed to several variables. A narrow, efficient stream channel will allow the water to pass quickly, resulting in a nearly instantaneous increase in peak flow. At locations where the stream channel widens or may contain heavy vegetation, the water velocity may decrease. Also, as the peak flow moves downstream, water may move into the floodplain where it is stored until the water level begins to recede. As the water level recedes, the stored water in the floodplain will slowly re-enter the stream. These combined factors explain why the peak flow may be less in magnitude but longer in duration as the flood progresses downstream.