In Georgia, flooding is common. More than 75% of the disaster losses in Georgia since 1990 have been a result of flooding.
The United States Geological Survey (USGS) has called the flooding in Georgia "epic" in proportion. According to the USGS, the rivers and streams had flood stages so great that the odds of it happening were less than 0.2 percent in any given year. In other words, there was less than a 1 in 500 chance that parts of Cobb and Douglas counties were going to be hit with such an event. That makes the Georgia flooding a 500-year flood.
President Obama declared Georgia a major disaster zone meaning federal disaster aid was made available for the State of Georgia. Counties included in the list of eligible counties are Carroll, Chattooga, Cherokee, Cobb, Douglas, Paulding and Stephens counties.
A high amount of concrete is a surprising source for the floods. As buildings and roads cover larger portions of the ground, the amount of infiltration of precipitation into the ground is reduced causing excess water to build in small regions. Gutters and drains also affect the movement of that water. The natural flow of water in a rain event will often flow in unnatural directions due to an increased number of water channeling structures.
It is important to remember that if you live on or near a creek, stream or river it is not IF it will flood-- it is just a matter of when, how often and how bad. You must remember what the purpose of a creek or river is... nature put it there to gather and transport storm water!
Our tearing down trees and paving over grass and soil serves to exaggerate and accelerate that gathering and transporting of rain water serving to exacerbate the problem. All the concrete and asphalt prevents rain from having to travel through the soil more slowly before reaching creeks and streams, instead it flows rapidly right to sewer systems that dump it immediately and efficiently into the rivers and creeks making for rapid and exaggerated rises.
Fall is normally dry in Georgia but not last year. Rain began to fall on Monday, September 14, 2009 and continued to fall for over a week. During that time, several major flood records were set. Some areas got over 20 inches of rain as reported by the rain gauge network. One record on the Chattahoochee River had not been broken since 1919. Flooding was so bad that 20 river level gauges were broken! 14 separate creeks and streams in the Atlanta area set new all time record flood levels.
The storm system was a result of a low-pressure zone that persisted in the Southeast. Warm, moist air from the Gulf of Mexico was pulled in to create a long-lasting rain event. Ground saturation was at a high level due to previous rain events meaning the fresh rains had nowhere to go. The rainwater caused runoff into streams, rivers and creeks in and around Atlanta.
See the weather maps that show surface and upper level weather features were perfect to pull moisture out of the Gulf of Mexico and lift it up into rain and thunderstorms over Georgia and adjacent states. As low pressure and fronts stalled across the South, an El Nino enhanced southern jet stream-- more typical of spring or winter was locked in place from September 14th, 2009 to September 26th, 2009 when a final round of widespread area wide heavy rain hit. The area wide nature of the rain was an important factor because it meant all the area lake and river drainage basins were all being soaked at the same time leading to rises-- even before the heaviest rain storms hit.
The air mass was tropical like, and the light upper airflow meant heavy storms were slow moving or stationary over the same areas and repeated for almost 7 days at various intensities. It's worth noting the antecedent conditions: The week of rain in September followed on top of what had already been a fairly wet summer -- with 13 inches of rain. AUGUST rain had been 67% above average. Also of note is that the week of September rains were widespread, soaking all of North and Central Georgia. This meant soils were saturated and lakes and creeks were already higher.
Another flood report from the Atlanta Chapter of the AMS:
"The Epic Floods of September 2009 in North Georgia: One Year Later." Kent Frantz, President of the chapter and Senior Service Hydrologist at NWS Peachtree city, started the presentation with a perspective from the National Weather Service. An upper level low over Texas was dominant for a few days leading up to the event and helped to feed tropical moisture into Georgia - thus the ground was moist and some flash flooding was experienced across the Southeast during the week prior to September 21. With the low level southerly flow across Georgia, the rainfall was enhanced across the flooded areas due to upslope. In addition, the FFC sounding showed a moist adiabatic profile with decent instability (~1500 J/kg). Another factor that played a role in the flooding event was a boundary from the coast that worked its way westward into the Atlanta metropolitan area. The 18-hour rainfall total at 9am on September 21, 2009 was 16 inches across portions of Douglas County. This epic flood event was a greater than 1000 year flood. The flooding caused a total of 10 deaths across North Georgia, nearly 100 water rescues and more than $300 million dollars in damage. All of the deaths except for one occurred overnight and all except for two deaths involved an automobile. A few of the lessons learned by the National Weather Service in Peachtree City were shared. First, it is important to have the rainfall frequency chart readily available. Second, when issuing river flood warnings (FLW) include major in the headline, i.e. major flood warning, to help emphasize the severity of the event. Another lesson learned was to inform emergency managers when stream gages fail. Since 20% of the stream gages failed during this event, other methods need to be in place to take stream height measurements - one option is to install staff gages. In August 2010, one such staff gage was installed at Sweetwater Creek where a record crest of 30.8 feet occurred during the September 2009 flood.
The second guest speaker, Brian McCallum from the United States Geological Survey (USGS), was involved in documenting the extent of the flood. In addition, the USGS was also responsible for fixing the stream gages that failed during the event. The USGS has 324 surface water gages in Georgia, 257 of which have rain gages. These gages transmit hourly through the GOES-8 satellite. The importance of these gages spans a variety of reasons: flood warnings/forecasting, flood control, drought monitoring, state water planning, water wars, safe bridge and roadway design and more. By Monday afternoon (September 21), almost 50 stream gages were above the highest point ever measured before and by that evening, more than 15 stream gages were not reporting. Since many stream gages failed, the USGS had to go out into the field to take measurements and fix the stream gages. Within five days of the flooding, USGS crews made nearly 75 discrete flood measurements, flagged high water marks at approximately two dozen locations and repaired or installed temporary real-time gages at all 20 destroyed gages. 16,981 dwellings were impacted from this epic flood (FEMA) and 3,482 businesses were impacted (SBA). Similar to the NWS, the USGS also shared lessons learned from the event. First, the gages did their job. Second, investments in stream gage networks paid off. Lastly, there is still room for improvement regarding the communication of flood threat/risk. The USGS has a variety of tools to monitor flooding:
Â· Water Watch - http://water.usgs.gov/waterwatch
Â· USGS StreaMail - http://water.usgs.gov/hif/streamail/index.html
Â· Water Alert - http://water.usgs.gov/wateralert
Â· Inundation Mapping - http://water.usgs.gov/osw/flood_inundation
The last speaker of the evening was Jason Milhollin, Douglas County Emergency Manager. Douglas County received the most rainfall from the epic flood event and was one of the counties that sustained the most damage. At one station, over 21 inches of rain fell in a 24 hour period, with a peak rainfall of 5.53 inches in one hour. Over 130 sections of the roadway infrastructure were closed during and immediately after the flood event and only two roads remain closed today. At one point there was no way to get in or out of the county and Highway Five was the only road that allowed for north/south travel within the county. In total, there was more than 30 water rescues from cars (does not include rescues from houses), seven fatalities in the county, and over 968 tons of debris. Debris removal of a vehicle was a challenge and Blackhawk helicopter had to be used to remove the car from the water. Similar to the two other speakers, the EMA shared lessons learned from Douglas County. The county did not know the National Weather Service needed information from the county and has since learned the importance of sharing information. In addition, it is important to share information with the media so it can be passed along to the public. Since the public is accustomed to moving to the lowest level of their home when outdoor warnings sirens alert, the decision was made not to sound the sirens as the county did not want people going to the lowest floor, instead people needed to seek higher ground. In order to be able to alert citizens, the county now has a reverse 911 system to notify residents of dangerous situations. During the epic flood a makeshift EOC was used and the county is now in the process of making a permanent EOC. Another challenge during the epic flood event was the fact that there were not enough boats available.
Floods of a widespread or serious nature in Atlanta usually occur in the Spring or Winter, with more hit and miss flooding in the summer months from localized isolated thunderstorm downpours. September and October are usually very dry months. If the slide show maps do not show full size click on it to zoom. Click here and click on maps to see close up of weather maps.