Flooding at Peachtree Creek
Peachtree Creek WaterWatch
Stream stage at Peachtree Creek
The U.S. Geological Survey (USGS) uses the term "stream stage", or stage or gage height, measured in feet above an arbitrary altitude, to refer to how high streams are. During dry periods (baseflow conditions), Peachtree Creek typically is at a stage of about 3 feet. During a typical flood, Peachtree Creek might reach a stage of 17 feet. At 17 feet, streamflow is almost 150 times greater (about 5,750 cubic feet per second (ft3/s)) than at 3 feet. On March 8, 1998 stream stage reached 20.16 feet, which means that about 8,800 ft3/s of water was flowing. It is easy to imagine the magnitude of this flood when, at a stage of 3 feet, only 67 ft3/s of water is flowing.
Comparison of low flow and high flow
The pictures below help show the difference in streamflow during baseflow and a flood. The picture on the left shows Peachtree Creek at a stage of about 3 feet. The picture on the right was taken during the September 2009 flooding, when stage peaked at 23 feet.
Effects of floods
When Peachtree Creek overflows its banks, water floods nearby streets and can inundate yards and basements of nearby houses, as well as clog streets with mud and debris. High water can also cause problems at bridges. This picture shows a huge pile of debris on the upstream section of the bridge at Northside Drive.
The debris contains whole tree trunks that must weigh tons. If enough debris piled up above the bridge then detrimental effects, such as scouring out of the bridge piers, could eventually occur, or the pipes holding back the debris could break. Local governments have to bring in cranes and heavy equipment to remove this debris.
Largest floods at Peachtree Creek
Peachtree Creek reacts very quickly when heavy rains occur. As is typical with smaller streams in urban areas, a heavy rain can cause the stream to rise in a matter of hours or even minutes. Also, as is typical with smaller urban streams, high water peaks quickly and then falls quickly; thus, streamflow at Peachtree Creek can go from base flow to flooding and back to near base flow in a single day.
The chart below shows the highest peak streamlows at Peachtree Creek, using historical USGS records. Peak streamflow is the maximum instantaneous measure of the flow of water. Some of the values in the chart are estimates. This chart goes only to about 2005; note that the flood of September 2009 reached a gage height of 22.91 feet (the hightest stage on record) with a streamflow peak of about 9,050 ft3/s. This storm was unique in that the excessive flooding was not caused by rainfall that fell in the watershed of Peachtree Creek, but rather was caused by rainfall that fell west of Atlanta. This rainfall caused historic flooding and high river levels on the Chattahoochee River in the Vinings area. The Chattahoochee River got so high that the water from Peachtree Creek, which flows into the Chattahoochee River essentially "backed up" and could not drain into the Chattahoochee River. So, much of the flooding caused by Peachtree Creek was from water that could not flow where it normally did, so it flowed outward, until the Chattahoochee River dropped enough to let Peachtree Creek flow out.
Table of peak flows at Peachtree Creek (new window).