Flooding at Peachtree Creek

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 on April 6, 1964, when stage peaked at 17.3 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. The picture below 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 10 highest peak streamflows for Peachtree Creek. Peak streamflow is the maximum instantaneous measure of the flow of water. Some of the values in the chart are estimates.

[D] - Descriptive link of data for this chart.

Table of peak flows at Peachtree Creek (new window).

Peak streamflow and daily mean streamflow

Don't confuse peak streamflow, as shown in the chart, with daily mean streamflow, which is the average streamflow for a single day. For small urban streams, typical peak streamflows are significantly higher than the daily mean streamflow. A good example of this comparison is the flood of March 17, 1990. The peak streamflow that day was 9,650 ft³/s, but the mean streamflow for the day was 6,060 ft³/s.

Find out how much more water can flow during a stormy day than during a base flow day.