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Maps and flood profiles for the 100-year flood plain were prepared for Pen Branch, Steel Creek, and their selected tributaries at the Savannah River Site in Aiken, Allendale, and Barnwell Counties, South Carolina. The maps and profiles provide water-resource managers of the Savannah River Site with a technical basis for making floodplain management decisions that could minimize future flood problems and provide a basis for designing and constructing drainage structures along roadways. The results are provided in tabular and graphical formats. A hydrologic analysis was made that verified that the South Carolina upper Coastal Plain regional ruraland urban-regression equations for peak flow were applicable for the Pen Branch and Steel Creek drainage basins. The equations were verified by an analysis of the flood-frequency data collected from U.S. Geological Survey gaging station 02197342 on Fourmile Branch and by comparing predominant soil types in the Fourmile Branch, Pen Branch, and Steel Creek drainage basins. Cross sections were surveyed throughout the basins, and other pertinent data such as flow resistance and land use were collected. The surveyed cross sections and computed 100-year recurrence-interval flows were used in a stepbackwater model to compute the 100-year flood profile for Pen Branch, Steel Creek, and their selected tributaries. The profiles were used to delineate the 100-year flood plain on topographic maps. INTRODUCTION In 1951, the U.S. Department of Energy (USDOE), formerly the Atomic Energy Commission, created the Savannah River Site (SRS) to produce nuclear materials for national defense. The SRS is located in parts of Aiken, Allendale, and Barnwell Counties, S.C. The operation of the first nuclear production reactor, in area R, began in 1953 (fig. 1), after which four other nuclear reactors were activated in areas C, K, L, and P. Reactors in areas R and P were permanently deactivated in 1964 and 1991, respectively. The remaining reactors have been placed on stand-by since the late 1980's, except for the restart testing of K reactor in 1991. Other areas on the SRS include reactor materials (area M), separation (areas F and H), waste management (areas E, F, H, S, Y, and Z), heavy-water processing (area D), administration (areas A, B, and CS), the Savannah River Ecology Laboratory, and the Savannah Technology Center (Arnett and others, 1992). In 1992, the U.S. Geological Survey (USGS), in cooperation with the USDOE, initiated an investigation to determine the areal extent of the inundation caused by the 100-year recurrence-interval flow (100-year flow) for the Savannah River along the SRS boundary and for the major streams and their tributaries on the SRS, with the exception of Lower Three Runs. Introduction 1 Purpose and Scope This report documents the approximate boundaries of the 100-year flood plain on the SRS for Pen Branch, Steel Creek, and their selected tributaries (fig. 2). Computed 100-year flows at selected locations, and tabular and graphical profiles of the 100-year flood are presented. Descriptions and elevations of benchmarks utilized for the Pen Branch and Steel Creek Basins are listed in the appendix. The USGS/Federal Highways Administration one dimensional step backwater computer model (WSPRO) (Shearman and others, 1986; Shearman, 1990), and the USGS A-526 culvert-flow model (Bodhaine, 1968) were used to compute water-surface profiles for each reach. Less-than-detailed methods, which require less cross-sectional definition, were used with the step-backwater and culvert flow models to compute the 100-year flood-plain boundaries. In particular, cross sections were surveyed primarily at road crossings or other easily accessible locations. Intermediate cross sections were interpolated by using the surveyed cross sections and 7.5-minute topographic maps (U.S. Geological Survey, 1963-65b). This method was requested by the USDOE, because extreme accuracy was not warranted. Description of Study Area The SRS occupies more than 300 mi2 along the Georgia-South Carolina border in parts of Aiken, Allendale, and Barnwell Counties, S.C. The southwestern boundary of the SRS is formed by the Savannah River. The five major streams that drain into the Savannah River from the SRS are Upper Three Runs, Fourmile Branch, Pen Branch, Steel Creek, and Lower Three Runs (fig. 2). The SRS is located in the upper Coastal Plain physiographic province of South Carolina; the province encompasses about 20 percent of the state (fig. 1). The general topography of the upper Coastal Plain consists of rounded hills with gradual slopes; however, some areas of highly irregular terrain exist in the province, and some elevations exceed 700 ft above sea level. The highest elevation on the SRS is approximately 420 ft above sea level, near Tims Branch and the northwest boundary of SRS (fig. 2). The land-surface elevation at the boundary of the upper and lower Coastal Plains, located southeast of the SRS, is usually less than 200 ft above sea level. Upper Coastal Plain stream slopes range from 5 to 20 ft/mi, and many of the streams are bordered by swamps with wide flood plains relative to the size of the stream (Zalants, 1990). The study area consists of Pen Branch and Steel Creek drainage basins (pi. 1). Pen Branch originates near Road F (fig. 1, pi. 1), and flows into the Savannah River south of Augusta, Ga., at river mile 141.5 (pi. 1). Pen Branch enters the Savannah River flood plain approximately 30,300 ft upstream from its confluence with the Savannah River (river mile 148.8); downstream from this point, Pen Branch becomes braided and is undefinable during high-flow conditions. Because of the effect of backwater from the Savannah River, the downstream study limit of the 100-year flood-plain and profile computations begins upstream from where Pen Branch enters the Savannah River flood plain (fig. 1). The total drainage area of Pen Branch before it enters the Savannah River Swamp is 22.2 mi2 . There are two upstream study limits. The upstream study limit of Pen Branch is Road C (fig. 1, pi. 1); this point was selected because there is no industrialization or road crossings in the drainage area upstream of Road C. An unnamed tributary flows into Pen Branch at Station 75150. The upstream study limit of this tributary is Road 6 (fig. 1, pi. 1). This point was selected because there are no road crossings upstream from this point and the drainage area upstream of Road 6 is less than 1.0 mi2 . Steel Creek originates near Road F (fig. 1, pi. 1), and flows into the Savannah River south of Augusta, Ga., at river mile 141.5 (pi. 1). Steel Creek enters the Savannah River flood plain approximately 8,200 ft upstream from its confluence with the Savannah River (river mile 143.4); downstream from this point, Steel Creek becomes braided and is undefinable during highflow conditions. Because of the effect of backwater from the Savannah River, the downstream study limit of the 100-year flood-plain and profile computations begins upstream from where Steel Creek enters the Savannah River flood plain (fig. 1). The total drainage area of Steel Creek before it enters the Savannah River Swamp is 35.1 mi2 . There are three upstream study limits. The upstream study limit of Steel Creek is the toe of L lake dam (fig. 1, pi. 1). Meyers Branch, a tributary to Steel Creek, flows into Steel Creek at Station 22700. The upstream study limit of Meyers Branch is Old Dunbarton Road (pi. 1). An unnamed tributary to Meyers Branch flows into Meyers Branch at Station 21800. 2 Determination of the 100-year Flood Plain on Pen Branch, Steel Creek, and their Selected Tributaries, Savannah River Site, South Carolina, 1996 81°50' 81°40' 81°30'

Determination of the 100-year flood plain on Pen Branch, Steel Creek, and their selected tributaries, Savannah River Site, South Carolina, 1996