Magnitude and Frequency of Floods for Urban and Small Rural Streams in Georgia, 2008

Abstract

A study was conducted that updated methods for estimating the magnitude and frequency of floods in ungaged urban basins in Georgia that are not substantially affected by regulation or tidal fluctuations. Annual peak-flow data for urban streams from September 2008 were analyzed for 50 streamgaging stations (streamgages) in Georgia and 6 streamgages on adjacent urban streams in Florida and South Carolina having 10 or more years of data. Flood-frequency estimates were computed for the 56 urban streamgages by fitting logarithms of annual peak flows for each streamgage to a Pearson Type III distribution. Additionally, basin characteristics for the streamgages were computed by using a geographical information system and computer algorithms. Regional regression analysis, using generalized leastsquares regression, was used to develop a set of equations for estimating flows with 50-, 20-, 10-, 4-, 2-, 1-, 0.5-, and 0.2-percent annual exceedance probabilities for ungaged urban basins in Georgia. In addition to the 56 urban streamgages, 171 rural streamgages were included in the regression analysis to maintain continuity between flood estimates for urban and rural basins as the basin characteristics pertaining to urbanization approach zero. Because 21 of the rural streamgages have drainage areas less than 1 square mile, the set of equations developed for this study can also be used for estimating small ungaged rural streams in Georgia. Flood-frequency estimates and basin characteristics for 227 streamgages were combined to form the final database used in the regional regression analysis. Four hydrologic regions were developed for Georgia. The final equations are functions of drainage area and percentage of impervious area for three of the regions and drainage area, percentage of developed land, and mean basin slope for the fourth region. Average standard errors of prediction for these regression equations range from 20.0 to 74.5 percent.

Flood-inundation maps for the Yellow River from River Drive to Centerville Highway, Gwinnett County, Georgia

“…Flow-change points were added into the HEC-RAS model at stream tributaries in each reach where the drainage area increased abruptly. The ratio of the drainage area at these points, with the drainage area at each streamgage raised to the power of 0.54 multiplied by the flow at the streamgage, is the model flow at these points (Gotvald and Knaak, 2011). Tables 2 and 3 show model flow values for selected locations.…”

Flood-inundation maps for the Withlacoochee River From Skipper Bridge Road to St. Augustine Road, within the City of Valdosta, Georgia, and Lowndes County, Georgia

“…During some events, the peak flow was higher at the downstream streamgage, and at other events, it was lower. In most hydrologic settings, peak flows increase with the ratio of the drainage area (Gotvald and Knaak, 2011), but the wetlands and (or) the close interaction with groundwater in the study area complicate the hydrology and could be reducing peak flows downstream. Table 2 shows the peak-flow values for each Withlacoochee River streamgage, the date of the peaks, and the ratio of the downstream peak-flow value to the upstream peak flow value.…”

Magnitude of flood flows for selected annual exceedance probabilities for streams in Massachusetts

“…Regional flood flow studies (Robbins and Pope, 1996;Southard, 2010;Gotvald and Knaak, 2011;Feaster and others, 2014) have found that IMPERV is an important predictor of the magnitude of flood flows. In general, the relative increase in the annual maximum discharge in urbanized basins has been shown to be more substantial for small frequent floods than for large infrequent floods (Hollis, 1975;Konrad and Booth, 2002;Konrad, 2003).…”