Methods for estimating the magnitude and frequency of floods for urban and small, rural streams in Georgia, South Carolina, and North Carolina, 2011

Abstract

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Hydro-meteorologic Assessment of October 2015 Extreme Precipitation Event on Santee Experimental Forest Watersheds, South Carolina

“…The estimated preliminary peak flood discharges of 17.3 m 3 /s and 33.9 m 3 /s measured on the watersheds WS 80 and WS 79 on October 4, 2015, with an excess of 250 mm rainfall within 24 hours on the top of 5-day antecedent rainfall of 170 mm+ at the study site, are significantly higher than the 200-yr flood estimates of 2.3 m 3 /s and 15.3 m 3 /s obtained as preliminary numbers by Amatya and Radecki-Pawlik (2007) using the Pearson Type-III flood frequency curves with only 8 and 13 years, respectively, of the historic data (1964)(1965)(1966)(1967)(1968)(1969)(1970)(1971)(1972)(1973)(1974)(1975)(1976) (Table 3). These October 4 extreme flood discharge estimates are also much higher than both the estimates obtained by using empirical equations with only drainage area as a parameter (Feaster et al, 2009) and two additional parameters (percent imperviousness and 50-yr 24hr rainfall intensity) besides the area (Feaster et al, 2014) for rural basins in coastal South Carolina. We speculate that these preliminary estimates of high flood discharges at the peak of the extreme event may have possibly exceeded the 500year flood discharge at these locations.…”

“…The fact that the estimated high peak discharges as a result of the extreme event were found to exceed the estimated design discharges of even 500-yr return periods obtained using recent USGS empirical relationships (Feaster et al, 2014) and other similar methods for low-gradient coastal watersheds and our own flood-frequency analysis (Amatya and Radecki-Pawlik, 2007) for these watersheds suggest a need for reassessing the capacity of existing gauging stations and other cross-drainage structures to minimize the risk of submergence and flooding in the future at this and similar other site. However, the predictions developed by Amatya and Radecki-Pawlik (2007) using only 13 years of data should be re-evaluated using longer periods of observed data for more accurate predictions.…”

“…2) Three parameters (drainage area, impervious area, and 50-yr 24-hr maximum rainfall) formula (Feaster et al, 2014) also for 100-yr and 500-yr return periods…”

“…Hydrologists are concerned with high-intensity rainfall and peak runoff rates for stormwater infrastructure design, post-event assessments, and mitigation of environmental impacts (Keefer et al, 2015). This is because most regions of the country have stormwater systems and cross-drainage structures designed for specified design return period storms based on tolerable risks for the given system/infrastructure (Obeysekara and Salas, 2016;Hutton et al, 2015;Marion et al, 2013;Feaster et al, 2014). Increases in the return period of large storms, as predicted by many global climate models, may stress existing stormwater infrastructure, depending on the design criteria, (Hutton et al, 2015) and thereby force consideration of replacement with larger size structures to accommodate peak discharges with more frequent return intervals (Marion et al, 2013).…”

Proctor Creek Ecological Model (PCEM) : Phase 2 - benefits analysis

“…Hydrologic change due to increased impervious area in cities represents one of the most important changes in stream and riparian functions resulting from urbanization (Fischenich 2005). Piedmont streams provide a classic example of urban hydrologic change, with watersheds generally exhibiting increased peak flows and reduced time of concentration (i.e., a "flashy" hydrograph) (Inman 2000;Gotvald and Knaak 2011;Feaster et al 2014). Flashy conditions result in a reduction in baseflow discharge and depth which adversely affects aquatic biota (e.g., suppressed dissolved oxygen, increased water temperature, reduction in "living space", limited access to habitat types).…”