Water Resources Research Act Program

Details for Project ID 2017GA373B

Quantifying the relative contributions of the physical mechanisms responsible for the Atlanta 2009 flood

Institute: Georgia
Year Established: 2017 Start Date: 2017-03-01 End Date: 2018-02-28
Total Federal Funds: $18,000 Total Non-Federal Funds: $40,440

Principal Investigators: Marshall Shepherd, Neil Debbage

Abstract: Due to a complex combination of both social and physical factors, urban areas are particularly vulnerable to flooding. The pressures of urbanization have likely increased flood zone occupancy throughout the state of Georgia, particularly amongst vulnerable individuals, while simultaneously altering the physical surface properties that largely govern rainfall-runoff processes. A striking example of the fragility of Georgia’s urban environments to extreme rainfall occurred in the Atlanta metropolitan area during late September 2009, as it suffered from a record-breaking flood event. Stream gauges in the greater metro area approached or exceeded the discharge associated with a 100-year flood and many were rendered inoperative during the flood peak. A total of 11 fatalities were attributable to the flood and damage claims approached $220 million. Previous research has qualitatively identified the probable meteorological mechanisms responsible for the flooding: 1) a weak cut-off low that stalled over the lower Mississippi Valley and established moist antecedent conditions, 2) persistent S/SE flow that advected moisture into the greater Atlanta region, 3) a quasi-stationary convective system likely amplified by the regional topography, and 4) potential amplification of the precipitation event due to interactions with the urban environment. Due to the multifaceted nature of the flood, it is challenging, if not impossible, to discern the relative importance of these various factors from analyzing observational records alone. This project aims to disentangle the various mechanisms responsible for the 2009 flood that devastated much of the Atlanta metro through the usage of a physically-based meteorological model. Utilizing a modeling framework is appropriate since it enables the isolation of each contributing component and a quantitative description of their relative importance. The study will focus particularly on the influence of urbanization, topography, and antecedent moisture conditions on the flood response by running various WRF-ARW modeling experiments that replace urban development with natural land cover, remove the natural topography, and alter soil moisture characteristics. Collectively, these experiments will provide a more quantitative understanding of the degree to which urbanization and the other meteorological mechanisms influenced the hydrometeorological characteristics of the 2009 flooding event. Ultimately, such results will provide guidance for more sustainable urban development policies throughout Georgia, which are particularly necessary given that extreme rainfall events will likely become more common throughout the state due to climate change. If it is discovered that urbanization had a minimal influence on the 2009 flood, this will highlight the critical importance of the “make space for water” planning paradigm for the Atlanta metropolitan region since it suggests that urban development, even of a more sustainable character, will likely be damaged during extreme flooding when located in flood prone areas. Conversely, if urbanization is very influential, then more sustainable patterns of development that better mimic natural hydrologic processes may hold the potential to reduce the damages incurred even during extreme flooding events.