Floods in Central Texas, September 7–14, 2010 texaswaterjournal.org An online, peer-reviewed journal published in cooperation with the Texas Water Resources Institute Volume 3, Number 1 2012 TEXAS WATER JOURNAL http://texaswaterjournal.org Editor-in-Chief Todd H. Votteler, Ph.D. Guadalupe-Blanco River Authority Editorial Board Kathy A. Alexander, Ph.D. Robert Gulley, Ph.D. Robert Mace, Ph.D. Texas Water Development Board Todd H. Votteler, Ph.D. Guadalupe-Blanco River Authority Ralph A. Wurbs, Ph.D. Texas Water Resources Institute Texas A&M University TEXAS WATER JOURNAL Volume 3, Number 1 2012 ISSN 2160-5319 texaswaterjournal.org THE TEXAS WATER JOURNAL is an online, peer-reviewed journal devoted to the timely consideration of Texas water resources management and policy issues. The jour- nal provides in-depth analysis of Texas water resources management and policies from a multidisciplinary perspective that integrates science, engineering, law, planning, and other disciplines. It also provides updates on key state legislation and policy changes by Texas administrative agencies. For more information on TWJ as well as TWJ policies and submission guidelines, please visit texaswaterjournal.org. The Texas Water Journal is published in cooperation with the Texas Water Resources Institute, part of Texas AgriLife Research, the Texas AgriLife Extension Service, and the College of Agriculture and Life Sciences at Texas A&M University. Managing Editor Kathy Wythe Texas Water Resources Institute Texas A&M Institute of Renewable Natural Resources Layout Editor Leslie Lee Texas Water Resources Institute Texas A&M Institute of Renewable Natural Resources Website Editor Ross Anderson Texas Water Resources Institute Texas A&M Institute of Renewable Natural Resources Copy and Social Media Editor Forrest Burnson The University of Texas at Austin School of Journalism Cover photo: Located in far east Texas and stretching into Louisiana, Caddo Lake is known for its extensive forests of baldcypress trees draped with Spanish moss. This famous lake is home to a rich ecosystem and a wide variety of wildlife. The cover photo was taken during normal water levels, but in 2011 the lake’s levels dropped significantly during the drought. Photo credit: Texas Water Resources Institute http://texaswaterjournal.org http://texaswaterjournal.org Texas Water Journal, Volume 3, Number 1 14 Texas Water Resources Institute Texas Water Journal Volume 3, Number 1, Pages 14 –25, July 2012 Floods in Central Texas, September 7–14, 2010 Abstract: Severe flooding occurred near the Austin metropolitan area in central Texas September 7–14, 2010, because of heavy rainfall associated with Tropical Storm Hermine. The U.S. Geological Survey, in cooperation with the Upper Brushy Creek Water Control and Improvement District, determined rainfall amounts and annual exceedance probabilities for rainfall resulting in flooding in Bell, Williamson, and Travis counties in central Texas during September 2010. We documented peak streamflows and the annual exceedance probabilities for peak streamflows recorded at several streamflow-gaging stations in the study area. The 24-hour rainfall total exceeded 12 inches at some locations, with one report of 14.57 inches at Lake Georgetown. Rainfall probabilities were estimated using previously published depth-duration frequency maps for Texas. At 4 sites in Williamson County, the 24-hour rainfall had an annual exceedance probability of 0.002. Streamflow measurement data and flood-peak data from U.S. Geological Survey surface-water monitoring stations (streamflow and reservoir gaging stations) are presented, along with a comparison of September 2010 flood peaks to previous known maximums in the periods of record. Annual exceedance probabilities for peak streamflow were computed for 20 streamflow-gaging stations based on an analysis of streamflow-gaging station records. The annual exceedance probability was 0.03 for the September 2010 peak streamflow at the Geological Survey’s streamflow-gaging stations 08104700 North Fork San Gabriel River near Georgetown, Texas, and 08154700 Bull Creek at Loop 360 near Austin, Texas. The annual exceedance probability was 0.02 for the peak streamflow for Geological Survey´s streamflow- gaging station 08104500 Little River near Little River, Texas. The lack of similarity in the annual exceedance probabilities com- puted for precipitation and streamflow might be attributed to the small areal extent of the heaviest rainfall over these and the other gaged watersheds. Keywords: flood, Hermine, Texas, 2010, annual exceedance probability 1U.S. Geological Survey, Austin, Texas Karl E. Winters, P.E.1 Citation: Winters KE. 2012. Floods in Central Texas, September 7–14, 2010. Texas Water Journal. 3(1):14-25. Available from: https:// doi.org/10.21423/twj.v3i1.3292. © 2012 Karl E. Winters. This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this license, visit https://creativecommons.org/licenses/by/4.0/ or visit the TWJ website. https://doi.org/10.21423/twj.v3i1.3292 https://twj-ojs-tdl.tdl.org/twj/index.php/twj/about#licensing Texas Water Journal, Volume 3, Number 1 15 INTRODUCTION Severe flooding occurred in the greater Austin metropolitan area in central Texas September 7–14, 2010 because of heavy rainfall associated with Tropical Storm Hermine. Storm totals exceeded 12 inches near Georgetown, Texas. More than 10 inches fell in parts of Austin, Texas. Numerous homes were damaged along Brushy Creek and Lake Creek in William- son County (Rasmussen 2010). Flood-related deaths were reported in Austin, Georgetown, and Killeen (Associated Press 2010). One of these deaths occurred as 2 vehicles were swept into Bull Creek at Farm Road 2222 in Austin (Aus- tin American-Statesman 2010). The U.S. Geological Survey, in cooperation with the Upper Brushy Creek Water Control and Improvement District, determined rainfall amounts and annual exceedance probabilities for rainfall resulting in flood- ing in central Texas in Bell, Williamson, and Travis counties in September 2010. They documented peak streamflows and the annual exceedance probabilities for peak streamflows mea- sured at several Geological Survey’s streamflow-gaging stations in the study area (Figure 1). PURPOSE AND SCOPE This report documents Tropical Storm Hermine-associated rainfall during September 7–8, 2010, and runoff during Sep- tember 7–14, 2010, near Austin, and selected statistical char- acteristics of these data. Rainfall and runoff in Bell, Travis, and Williamson counties in central Texas are described. The report gives rainfall data from various sources and estimates annual exceedance probabilities for 24-hour rainfall totals at selected stations for September 7–8, 2010. The report presents hyetographs of rainfall data collected from 2 rain gages near Georgetown. It documents stage (height of the water surface in a stream above an established datum), streamflow, and mean velocity measurements made during the flood along with peak streamflows computed by the slope-area indirect method. The report presents peak stage and streamflow data for selected Geological Survey streamflow-gaging stations along with the estimated annual exceedance probabilities for peak streamflow for selected gages. CONDITIONS LEADING TO THE FLOOD As Tropical Storm Hermine approached the Texas Gulf Coast on September 3, 2010, rainfall of about 1 to 2 inches fell in the study area, with the larger amounts falling in cen- tral and western Travis County. An additional quarter-inch fell near the Travis-Williamson County line on September 4. No measurable precipitation fell during September 5–6. Tropi- cal Storm Hermine made landfall about 30 miles south of Floods in Central Texas, September 7–14, 2010 Brownsville, Texas on September 6 at 9 PM with peak winds of 69 miles per hour and a minimum pressure of 989 mil- libars. With a forward speed of 18 miles per hour, the center of circulation reached San Antonio, Texas at 1 PM September 7. Light rain (about 0.14 inch per hour) fell between 4:30 AM and 6 PM on September 7. The heaviest rain fell between 6 PM September 7 and 4 AM on September 8. During this period, rainfall rates were as much as 1 inch per hour in parts of Williamson County. Rainfall during the 24-hour period ending September 8 at 6 AM exceeded 12 inches at some locations in the study area, with one report of 14.57 inches at Lake Georgetown. Rainfall quickly diminished after 6 AM September 8 as Tropical Storm Hermine moved out of the study area (NWS 2010). Widespread flooding occurred Sep- tember 7–14, 2010. RAINFALL DEPTHS AND ANNUAL EXCEEDANCE PROBABILITIES Rainfall depth contours were determined using the Nation- al Weather Service-gridded rainfall data (NWS 2010) for the 24-hour period ending at 6 AM September 8, 2010. These data are based on Next Generation Weather Radar estimates (NWS 2010). The data have a spatial resolution of about 2.5 miles (4 kilometers). The 24-hour rainfall totals are shown in Figure 2. Rainfall data collected by Upper Brushy Creek Water Con- trol and Improvement District (Dustin Mortensen, Civil Engi- neer, Freese and Nichols, Inc., written communication 2010), Geological Survey (USGS 2012), and 2 local airport stations (FAA 2012) were used to verify the isohyetal contours (Jain and Singh 2005) derived from the National Weather Service- gridded rainfall data. Rainfall data collected by the Geological Survey were measured at selected Geological Survey surface- water monitoring stations (Table 1). The 24-hour rainfall totals for most of the stations listed in Table 1 compare favor- ably with the isohyetal contours of National Weather Service- gridded rainfall data shown in Figure 2. However, the 24-hour totals recorded by 4 of the water control and improvement district rain gages (sites 46, 51, 54, and 56) near Round Rock, Texas (Figures 1 and 2), differed appreciably from the Nation- al Weather Service-gridded rainfall data (Figure 2). These sites are where the isohyetal contours are close together, indicat- ing that large differences in rainfall amounts occurred over a small area. Sites 51, 54, and 56 are less than 5 miles apart and recorded similar 24-hour rainfall totals (0.91, 0.98, and 0.91 inches), respectively, indicating that the National Weath- er Service-gridded rainfall totals might not be accurate near these gages. The largest rainfall totals for the 24-hour period ending 6 AM September 8, 2010 (more than 12 inches), were measured west of Georgetown, at sites 5, 42, 49, and 58 (rain Texas Water Journal, Volume 3, Number 1 16                   -97°30' -98° 31° 31° 30°30' 30°30' -97°30' -98°    EXPLANATION 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 4849 50 51 52 53 54 55 56 57 58 59 TEXAS Study Area 0 10 20 0 10 20 MILES KILOMETERS  Base from U.S. Geological Survey digital data Albers equal area conic projection North American Datum of 1983 Standard Parallel: 29.5 Standard Parallel: 45.5 Longitude of Central Meridian: -96 Latitude of Projection Origin: 23 Station and site identifier UBCWCID dam and raingage (table 1)38 U.S. Geological Survey streamgage (tables 2 and 3)9 U.S. Geological Survey lake gage (table 3)5 U.S. Geological Survey (table 1)rain gage10 Airport rain gage (table 1)59 Brownsville San Antonio       Area enlarged 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37                                                                                    B ELL C O U N TY TR AV IS CO UN TY WILLIAMSON COUNTY  Figure 1. Map showing the locations of selected rain gages, reservoir gages, streamflow-gaging stations, and Upper Brushy Creek Water Control Improvement District dams in the study area of Bell, Williamson, and Travis counties, Texas. Floods in Central Texas, September 7–14, 2010 Texas Water Journal, Volume 3, Number 1 17                   -97°30' -98° 31° 31° 30°30' 30°30' -97°30' -98°    B ELL C O U N TY TRAVIS COUNTY Line of 24-hour total rainfall, in inches Interval 1 inch EXPLANATION 12 12 1 0 8 6 4 2 1 0 8 6 4 2 8 6 4 8 6 12 TEXAS Study Area 0 10 20 0 10 20 MILES KILOMETERS  Base from U.S. Geological Survey digital data Albers equal area conic projection North American Datum of 1983 Standard Parallel: 29.5 Standard Parallel: 45.5 Longitude of Central Meridian: -96 Latitude of Projection Origin: 23 WIL LIAM SON COU NTY Figure 2. Map showing the 24-hour total rainfall ending 6 AM September 8, 2010, in Bell, Williamson, and Travis counties, Texas (modified from NWS 2010). Floods in Central Texas, September 7–14, 2010 Texas Water Journal, Volume 3, Number 1 18 Table 1. Rainfall totals and associated annual exceedance probabilities based on depth-duration frequency of rainfall by Asquith and Roussel (2004). [--, not applicable; nd, not determined; Upper Bushy Creek Water Control and Improvement District (UBCWCID); U.S. Army Corps of Engineers (USACE)] Rainfall depth (inches) Site number (Fig. 1) Station number Station name 24-hr period ending 6 AM 9/8/2010 Sliding 24-hr maximum1 Annual exceedance probability 5 08104650 Lake Georgetown near Georgetown, Texas2 12.07 12.66 0.002 8 08105095 Berry Creek at Airport Road near Georgetown, Texas2 11.43 11.45 0.003 10 08105600 Granger Lake near Granger, Texas2 0.47 0.64 -- 13 08154700 Bull Creek at Loop 360 near Austin, Texas2 9.67 9.77 0.008 38 -- UBCWCID dam 13 9.8 9.84 0.008 39 -- UBCWCID dam 23 9.73 9.84 0.008 40 -- UBCWCID dam 33 11.61 11.81 0.003 41 -- UBCWCID dam 43 10.87 10.91 0.004 42 -- UBCWCID dam 53 12.16 12.45 0.002 43 -- UBCWCID dam 63 nd4 nd nd 44 -- UBCWCID dam 73 10.67 10.79 0.005 45 -- UBCWCID dam 83 11.02 11.26 0.004 46 -- UBCWCID dam 93 1.77 1.97 -- 47 -- UBCWCID dam 113 6.73 7.09 0.036 48 -- UBCWCID dam 123 10.51 10.94 0.004 49 -- UBCWCID dam 13A3 12.01 12.28 0.002 50 -- UBCWCID dam 143 6.42 6.97 0.038 51 -- UBCWCID dam 153 0.91 1.14 -- 52 -- UBCWCID dam 163 4.96 5.51 0.143 53 -- UBCWCID dam 173 5.23 5.55 0.125 54 -- UBCWCID dam 183 0.98 1.70 -- 55 -- UBCWCID dam 193 4.37 4.73 0.217 56 -- UBCWCID dam 203 0.91 0.95 -- 57 -- UBCWCID dam 213 3.3 3.66 0.333 58 KGTU Georgetown airport5 11.12 12.31 0.002 59 K5R3 Lago Vista airport5 9.64 9.83 0.011 60 -- USACE rain gage near Lake Georgetown 14.576 nd nd 1Determined by sliding (moving) a 24-hour window through successive values of incremental rainfall data; the first 24-hour window began at 12 AM on September 7, 2010, and the last window began at 12 AM on September 8, 2010. 2Data obtained from the U.S. Geological Survey National Water Information System (USGS 2012). 3Data obtained from Dustin Mortensen, Civil Engineer, Freese and Nichols, Inc., written communication, 2010. 4The rain gage at dam 6 was damaged during the September 2010 storm. 5Data obtained from the Federal Aviation Administration (2012). 6For a 24-hour period ending 8 AM on September 8, 2010. Floods in Central Texas, September 7–14, 2010 Texas Water Journal, Volume 3, Number 1 19 gages at the Geological Survey’s surface-water monitoring sta- tion 08104650 Lake Georgetown near Georgetown, the Water Control and Improvement District’s dam 5 and 13A, and the Georgetown airport, respectively; Figures 1–2; Table 1). These 24-hour rainfall totals agreed within about 10% with the National Weather Service-gridded rainfall data. Cumulative 24-hour rainfall totals for sites 5 and 42 are shown in Figure 3. A rain gage operated by the U.S. Army Corps of Engineers, about 0.5 mile north of Georgetown Lake (site 60, Figure 1; Table 1), recorded 14.57 inches during the 24-hour period ending at 8 AM September 8, 2010 (John Rael, Hydraulic Engineer, U.S. Army Corps of Engineers, written communi- cation 2012). Rainfall annual exceedance probabilities for the September 2010 flood were estimated using depth-duration frequency maps for Texas (Asquith and Roussel 2004). Annual exceed- ance probability is the reciprocal of the “x-year rainfall.” When describing flood frequency, annual exceedance probability is the reciprocal of the “x-year flood.” For example, a 50-year flood has an annual exceedance probability of 1/50 = 0.02, equivalent to a 2% chance of occurring in any given year. The “x-year flood” terminology is no longer preferred, as it is often 14 12 10 8 6 4 2 0 C U M U LA T IV E R A IN FA LL , IN I N C H E S UBCWCID rain gage at dam 5 (site 42, table 1, fig. 1) U.S. Geological Survey rain gage at Lake Georgetown (site 5, table 1, fig. 1) 2010 SEPT 7 6:00 12:00 18:00 0:00 6:00 EXPLANATION SEPT 8 Figure 3. Cumulative rainfall for 24-hour period ending 6 AM September 8, 2010, at Upper Brushy Creek Water Control and Improvement District dam 5 and U.S. Geological Survey surface-water monitoring station 08104650 Lake Georgetown near Georgetown, Texas. 0 2 4 6 8 10 12 14 0.010.11251020305070809095989999.999.99 0.5 ANNUAL EXCEEDENCE PROBABILITY, IN PERCENT 2 4 -H O U R R A IN FA LL , IN I N C H E S Figure 4. Annual exceedance probabilities for 24-hour rainfall totals in Williamson County, Texas, derived from Asquith and Roussel (2004). Floods in Central Texas, September 7–14, 2010 Texas Water Journal, Volume 3, Number 1 20 misunderstood to imply an interoccurrence period between events (Holmes and Dinicola 2010). To determine rainfall annual exceedance probabilities for Williamson County, the 24-hour rainfall totals from maps of various return periods (Asquith and Roussel 2004) were interpolated to develop the relation shown in Figure 4. The annual exceedance probability values listed in Table 1 were computed using the maximum 24-hour rainfall amount and depth-duration frequency of rainfall by Asquith and Roussel (2004). This maximum rain- fall was determined by sliding (moving) a 24-hour window through successive values of (primarily 5-minutes) incremen- tal rainfall data; the first 24-hour window began at 12 AM September 7, 2010, and the last window began at 12 AM Sep- tember 8, 2010. The maximum intensities typically occurred during a 24-hour window ending at 4:30 AM September 8, and these values are only slightly larger than those recorded for the 24-hour period ending at 6 AM September 8 (Table 1). The rainfall recorded at sites 5, 42, 49, and 58 (Figures 1–2, Table 1) had an annual exceedance probability of 0.002, a 1-in-500 chance of occurring in any year. PEAK STREAMFLOWS AND ANNUAL EXCEEDANCE PROBABILITIES Peak streamflow values are generally computed from stage- discharge rating curves (Kennedy 1983, and Rantz and oth- ers 1982). Measurements of streamflow are used to define stage-discharge rating curves, and measurements made dur- ing floods are especially necessary for reliable computation of peak streamflow (Turnipseed and Sauer 2010). Streamflow measurement data from 19 Geological Survey streamflow-gag- ing stations and flood-peak data from 35 Geological Survey streamflow-gaging stations and 2 reservoir gages were evalu- ated; peak streamflows measured during the September 2010 runoff event were compared to previous known maximum flood peaks from the period of record for each station. All Geological Survey data were obtained from its National Water Information System (USGS 2012). When it is logistically impossible to measure the peak streamflow because of difficulties accessing the site at the time of the peak or because of rapid changes in stage, it is often possible to indirectly compute the peak streamflow “after- the-fact,” using methods based on principles of open-channel hydraulics. The slope-area computation method incorporates channel cross-section geometry and roughness (a measure of frictional resistance to flow) to compute the peak streamflow associated with a flood profile defined from interpretation of high-water marks (Dalrymple and Benson 1967). For selected peaks associated with the September 2010 flood, slope-area computations were performed using the Geological Survey slope-area computation program (Fulford 1994). Six slope- area computations of peak streamflow made following the September 2010 flood are included in Table 2. Selected streamflow measurements made September 7–8, 2010 are listed in Table 2. The streamflow of 50,700 cubic feet per second measured at site 3 (Geological Survey streamflow- gaging station 08104500 Little River near Little River, Texas [hereinafter Little River gage]) was the largest discharge mea- sured, and this measurement was made near the peak of the flood. Slope-area computations were performed at sites 8, 12, 13, 29, 34, and 36 (Table 2). These indirect measurements of peak discharge are probably less accurate compared to direct measurements of streamflow. For example, the slope-area com- putation for site 29 (Geological Survey streamflow-gaging sta- tion 08158819 Bear Creek near Brodie Lane near Manchaca, Texas) differed by 11% from the discharge estimated from the stage-discharge rating curve in use for this site, which is based in part on a direct measurement from 2004 of 6,900 cubic feet per second (stage 12.40 feet). The peak streamflow at a location divided by the con- tributing area upstream from it, (cubic feet per second per square mile), described here as unit runoff, is a measure of the intensity of a watershed’s response to a storm and is use- ful for comparing peak discharges from different sites (Fon- taine and Hill 2002; Rowe and Allander 2000). The drainage area for each Geological Survey streamflow-gaging station is available in its 2010 annual data report (USGS 2010). Peak stages, streamflows, and unit runoff for the September 2010 flood are shown in Table 3, along with data from the previous known maximum flood. Only streamflow from unregulated drainage areas was considered; if dams were present, unit run- off was based on the drainage area of the unregulated part of the basin. On September 8, 2010, site 6 (Geological Survey streamflow-gaging station 08104700 North Fork San Gabriel River near Georgetown [hereinafter North Fork San Gabriel gage]) recorded the highest peak streamflow (7,330 cubic feet per second) since regulation of streamflow at this site began in 1980. Site 13 (Geological Survey streamflow-gaging sta- tion 08154700 Bull Creek at Loop 360 near Austin [herein- after Bull Creek at Loop 360 gage]) recorded the highest peak streamflow in its 32-year history. In addition to sites 6 and 13, the September 2010 flood was the highest recorded flood at 9 other sites (8, 9, 12, 19, 21, 23, 24, 25, and 34) in the study area, although none of these 9 sites had more than 7 years of record. Streamflow hydrographs for site 7 (Geological Survey streamflow-gaging station 08104900 South Fork San Gabriel River at Georgetown) and site 13 are shown in Figure 5. The relation between peak streamflow and unregulated drainage area for 35 Geological Survey streamflow-gaging stations September 7–8, 2010, in Bell, Williamson, and Tra- vis counties is shown in Figure 6, along with selected flood peaks used to define an envelope of maximum floods for a Floods in Central Texas, September 7–14, 2010 Texas Water Journal, Volume 3, Number 1 21 Table 2. Data from selected streamflow measurements made at U.S. Geological Survey streamflow-gaging stations during September 7–8, 2010. [mi2, square miles; ft, feet; ft3/s, cubic feet per second; ft/s, feet per second; nd, not determined] Site number (Fig. 1) Station number Station name Drainage area (mi2) Date and time (24-hr) Stage (ft) Measured stream- flow (ft3/s) Mean velocity (ft/s) 3 08104500 Little River near Little River, Texas 5,228 9/8/2010 1330 40.51 50,700 3.0 8 08105095 Berry Creek at Airport Road near Georgetown, Texas 71.4 9/8/2010 0305 28.72 25,9001 4.9 9 08105505 Willis Creek near Granger, Texas 57.8 9/8/2010 1747 10.68 697 3.2 12 08105886 Lake Creek at Lake Creek Parkway near Aus- tin, Texas 2.18 9/8/2010 0035 8.59 3,5101 6.7 13 08154700 Bull Creek at Loop 360 near Austin, Texas 22.3 9/8/2010 0140 14.97 16,9001 13.4 15 08155240 Barton Creek at Lost Creek Blvd near Austin, Texas 107 9/8/2010 1113 9.54 6,280 5.5 16 08155300 Barton Creek at Loop 360, Austin, Texas 116 9/8/2010 1311 10.83 6,990 6.0 18 08155541 West Bouldin Creek at Oltorf Road, Austin, Texas 1.77 9/7/2010 1305 2.13 40.7 2.4 19 08156675 Shoal Creek at Silverway Drive, Austin, Texas 5.59 9/7/2010 1405 3.49 51 1.0 20 08156800 Shoal Creek at W 12th Street, Austin, Texas 12.3 9/7/2010 1245 3.87 325 3.8 24 08158035 Boggy Creek at Webberville Road, Austin, Texas 3.44 9/7/2010 0917 1.28 84.5 nd 25 08158045 Fort Branch Boggy Creek at Manor Road, Austin, Texas 1.47 9/8/2010 0900 3.33 18.8 3.3 28 08158600 Walnut Creek at Webberville Road, Austin, Texas 51.3 9/8/2010 0930 13.45 2,870 2.9 29 08158819 Bear Creek near Brodie Lane near Manchaca, Texas 23.8 9/8/2010 0025 11.92 5,3301 6.5 32 08158860 Slaughter Creek at Farm Road 2304 near Austin, Texas 23.1 9/8/2010 1147 3.53 357 1.6 34 08158927 Kincheon Branch at William Cannon Blvd, Austin, Texas 6.73 9/8/2010 0015 5.05 2,3401 5.7 35 08158930 Williamson Creek at Manchaca Road, Austin, Texas 19 9/7/2010 1830 5.73 700 3.1 36 08158970 Williamson Creek at Jimmy Clay Road, Austin, Texas 27.6 9/8/2010 0200 17.87 4,8601 4.2 37 08159000 Onion Creek at U.S. Highway 183, Austin, Texas 321 9/8/2010 1300 16.93 7,580 3.1 1Peak streamflow computed using slope-area method (Fulford 1994). Floods in Central Texas, September 7–14, 2010 Texas Water Journal, Volume 3, Number 1 22 Table 3. Flood-peak data at selected U.S. Geological Survey surface-water monitoring stations in Bell, Williamson, and Travis counties, Texas. [mi2, square miles; ft, feet; ft3/s, cubic feet per second; --, not applicable] C h a ra ct e ri st ic s o f sy st e m a ti c re co rd S e p te m b e r 2 0 1 0 fl o o d P re v io u s k n o w n m a x im u m 1 S it e n u m b e r (F ig . 1 ) S ta ti o n n u m b e r S ta ti o n n a m e D ra in a g e a re a (m i2 ) Le n g th (y rs ) P e ri o d (w a te r yr s) D a te T im e P e a k st a g e (f t) P e a k st re a m - fl o w (f t3 / s) U n it r u n o ff (f t3 / s/ m i2 ) A n n u a l e x ce e d a n ce p ro b a b il it y D a te P e a k st a g e (f t) P e a k st re a m - fl o w (f t3 / s) 1 08 10 25 00 Le on R iv er n ea r B el to n, T ex as 3, 58 2 56 19 55 1 – 20 10 9/ 8/ 20 10 09 00 5. 36 1, 33 0 11 52 0. 88 3/ 6/ 19 92 9. 74 10 ,2 00 1 2 08 10 41 00 La m pa sa s R iv er n ea r B el to n, T ex as 1, 32 1 34 19 67 1 – 20 10 9/ 8/ 20 10 04 00 11 .8 3 2, 14 0 26 52 0. 49 6/ 26 /2 00 7 18 .9 9 6, 39 01 3 08 10 45 00 Li tt le R iv er n ea r Li tt le R iv er , Te xa s 5, 22 8 48 19 63 –2 01 0 9/ 8/ 20 10 14 00 40 .5 8 50 ,7 00 12 92 0. 02 5/ 17 /1 96 5 42 .8 5 79 ,6 00 4 08 10 46 46 60 N or th F or k Sa n G ab ri el R iv er a t R ea ga n B lv d ne ar Le an de r, Te xa s 21 0 2 20 09 –2 01 0 9/ 8/ 20 10 00 45 15 .2 6 13 ,7 00 65 .2 -- 10 /2 2/ 20 09 17 .2 2 18 ,3 00 5 08 10 46 50 La ke G eo rg et ow n ne ar G eo rg et ow n, T ex as 24 7 31 19 80 –2 01 0 9/ 14 /2 01 0 01 30 79 8. 65 -- -- -- 3/ 4/ 19 92 83 5. 86 -- 6 08 10 47 00 N or th F or k Sa n G ab ri el R iv er n ea r G eo rg et ow n, T ex as 24 8 31 19 80 1 – 20 10 9/ 8/ 20 10 01 00 14 .1 5 7, 33 0 4, 73 0 0. 03 3/ 4/ 19 92 13 .0 5 6, 07 01 7 08 10 49 00 So ut h Fo rk S an G ab ri el R iv er a t G eo rg et ow n, T ex as 13 3 43 19 68 –2 01 0 9/ 8/ 20 10 03 45 21 .9 8 24 ,5 00 18 4 0. 10 6/ 27 /2 00 7 31 .6 5 57 ,5 00 8 08 10 50 95 B er ry C re ek a t A ir po rt R oa d ne ar G eo rg et ow n, T ex as 71 .4 7 20 04 –2 01 0 9/ 8/ 20 10 03 05 28 .7 2 25 ,9 00 36 3 -- 6/ 27 /2 00 7 23 .0 5 12 ,4 00 9 08 10 55 05 W ill is C re ek n ea r G ra ng er , Te xa s 57 .8 2 20 09 –2 01 0 9/ 8/ 20 10 07 00 23 .1 6 10 ,0 00 17 3 -- 9/ 11 /2 00 9 22 .2 0 8, 87 0 10 08 10 56 00 G ra ng er L ak e ne ar G ra ng er , Te xa s 73 0 31 19 80 –2 01 0 9/ 10 /2 01 0 10 00 51 3. 75 -- -- -- 3/ 5/ 19 92 53 0. 11 -- 11 08 10 57 00 Sa n G ab ri el R iv er a t La ne po rt , Te xa s 73 8 31 19 80 1 – 20 10 9/ 8/ 20 10 10 45 4. 87 8. 3 1. 12 -- 3/ 5/ 19 92 21 .8 6 7, 54 01 12 08 10 58 86 La ke C re ek a t La ke C re ek P ar kw ay n ea r A us ti n, T ex as 2. 18 1 20 10 –2 01 0 9/ 8/ 20 10 00 35 8. 59 3, 51 0 1, 61 0 -- -- -- -- 13 08 15 47 00 B ul l C re ek a t Lo op 3 60 n ea r A us ti n, T ex as 22 .3 32 19 79 –2 01 0 9/ 8/ 20 10 01 40 14 .9 7 16 ,9 00 75 8 0. 03 5/ 13 /1 98 2 11 .9 6 13 ,7 00 14 08 15 52 00 B ar to n C re ek a t St at e H ig hw ay 7 1 ne ar O ak H ill , Te xa s 89 .7 29 19 76 –2 01 0 9/ 8/ 20 10 02 55 15 .7 7 7, 56 0 84 .3 0. 24 7/ 2/ 20 02 22 .8 2 25 ,3 00 15 08 15 52 40 B ar to n C re ek a t Lo st C re ek B lv d ne ar A us ti n, T ex as 10 7 22 19 89 –2 01 0 9/ 8/ 20 10 06 40 10 .8 1 8, 45 0 79 .0 0. 21 5/ 28 /1 92 9 -- 39 ,4 00 16 08 15 53 00 B ar to n C re ek a t Lo op 3 60 , A us ti n, T ex as 11 6 35 19 76 –2 01 0 9/ 8/ 20 10 07 55 12 .5 1 8, 79 0 75 .8 0. 21 5/ 28 /1 92 9 -- 39 ,4 00 17 08 15 54 00 B ar to n C re ek a bo ve B ar to n Sp ri ng s at A us ti n, T ex as 12 5 12 19 99 –2 01 0 9/ 8/ 20 10 09 00 14 .0 2 5, 77 0 46 .2 0. 25 7/ 2/ 20 02 18 .2 1 17 ,2 00 18 08 15 55 41 W es t B ou ld in C re ek a t O lt or f R oa d, A us ti n, T ex as 1. 77 3 20 08 –2 01 0 9/ 7/ 20 10 22 40 3. 48 35 1 19 8 -- 9/ 12 /2 00 9 5. 06 98 7 19 08 15 66 75 Sh oa l C re ek a t Si lv er w ay D ri ve , A us ti n, T ex as 5. 59 3 20 08 –2 01 0 9/ 7/ 20 10 23 45 9. 59 3, 19 0 57 1 -- 5/ 23 /2 00 9 7. 94 2, 21 0 20 08 15 68 00 Sh oa l C re ek a t W 1 2t h St re et , A us ti n, T ex as 12 .3 36 19 75 –2 01 0 9/ 8/ 20 10 00 30 16 .9 5 6, 25 0 50 8 0. 18 5/ 24 /1 98 1 23 .2 2 16 ,0 00 21 08 15 69 10 W al le r C re ek a t Ko en ig L an e, A us ti n, T ex as 1. 09 3 20 08 –2 01 0 9/ 7/ 20 10 19 40 4. 79 50 1 46 0 -- 9/ 4/ 20 09 4. 29 39 2 22 08 15 80 00 C ol or ad o R iv er a t A us ti n, T ex as 39 ,0 09 11 33 18 98 3 – 20 10 9/ 8/ 20 10 03 30 27 .6 7 37 ,7 00 14 82 0. 09 4/ 29 /1 94 1 23 .5 54 47 ,6 00 1 23 08 15 80 30 B og gy C re ek a t M an or R oa d, A us ti n, T ex as 1. 67 3 20 08 –2 01 0 9/ 7/ 20 10 19 50 5. 65 63 8 38 2 -- 4/ 27 /2 00 8 5. 32 57 3 24 08 15 80 35 B og gy C re ek a t W eb be rv ill e R oa d, A us ti n, T ex as 3. 44 3 20 08 –2 01 0 9/ 7/ 20 10 23 20 3. 52 66 3 19 3 -- 4/ 27 /2 00 8 2. 82 46 7 25 08 15 80 45 Fo rt B ra nc h B og gy C re ek a t M an or R oa d, A us ti n, T ex as 1. 47 3 20 08 –2 01 0 9/ 7/ 20 10 23 30 5. 58 42 6 29 0 -- 9/ 4/ 20 09 5. 59 37 0 26 08 15 82 00 W al nu t C re ek a t D es sa u R oa d, A us ti n, T ex as 26 .2 17 19 75 –2 01 0 9/ 8/ 20 10 01 55 19 .6 6 9, 66 0 36 9 0. 10 5/ 25 /1 98 1 26 .2 0 21 ,6 00 27 08 15 83 80 Li tt le W al nu t C re ek a t G eo rg ia n D ri ve , A us ti n, T ex as 5. 22 9 19 83 –2 01 0 9/ 7/ 20 10 19 50 8. 93 3, 13 0 60 0 0. 17 9/ 14 /1 98 5 11 .9 0 3, 49 0 28 08 15 86 00 W al nu t C re ek a t W eb be rv ill e R oa d, A us ti n, T ex as 51 .3 45 19 66 –2 01 0 9/ 8/ 20 10 00 40 21 .4 3 8, 79 0 17 1 0. 20 1/ 13 /2 00 7 26 .3 0 16 ,4 00 29 08 15 88 19 B ea r C re ek n ea r B ro di e La ne n ea r M an ch ac a, T ex as 23 .8 7 20 04 –2 01 0 9/ 8/ 20 10 00 25 11 .9 2 6, 01 05 25 2 -- 11 /2 2/ 20 04 12 .4 0 6, 90 0 30 08 15 88 27 O ni on C re ek a t Tw in C re ek s R oa d ne ar M an ch ac a, Te xa s 18 1 7 20 04 –2 01 0 9/ 8/ 20 10 01 25 15 .3 3 5, 48 0 30 .3 -- 11 /1 7/ 20 04 23 .7 2 19 ,2 00 31 08 15 88 40 Sl au gh te r C re ek a t Fa rm R oa d 18 26 n ea r A us ti n, T ex as 8. 24 33 19 78 –2 01 0 9/ 7/ 20 10 22 45 10 .2 2 3, 71 0 45 0 0. 14 12 /2 0/ 19 91 10 .6 8 6, 33 0 32 08 15 88 60 Sl au gh te r C re ek a t Fa rm R oa d 23 04 n ea r A us ti n, T ex as 23 .1 13 19 79 –2 01 0 9/ 8/ 20 10 01 55 8. 18 4, 28 0 18 5 0. 20 6/ 11 /1 98 1 12 .4 0 8, 34 0 33 08 15 89 20 W ill ia m so n C re ek a t O ak H ill , Te xa s 6. 3 22 19 79 –2 01 0 9/ 7/ 20 10 22 40 9. 49 2, 51 0 39 8 0. 17 5/ 18 /1 99 2 9. 97 4, 75 0 34 08 15 89 27 K in ch eo n B ra nc h at W ill ia m C an no n B lv d, A us ti n, T ex as 6. 73 3 20 08 –2 01 0 9/ 8/ 20 10 00 15 5. 05 2, 34 0 34 8 -- 9/ 12 /2 00 9 1. 97 40 3 35 08 15 89 30 W ill ia m so n C re ek a t M an ch ac a R oa d, A us ti n, T ex as 19 21 19 76 –2 01 0 9/ 7/ 20 10 23 35 14 .6 5 4, 72 0 24 8 0. 15 6/ 11 /1 98 1 16 .0 0 8, 49 0 36 08 15 89 70 W ill ia m so n C re ek a t Ji m m y C la y R oa d, A us ti n, T ex as 27 .6 15 19 75 –2 01 0 9/ 8/ 20 10 02 00 17 .8 7 4, 86 0 17 6 -- 6/ 11 /1 98 1 17 .2 5 14 ,1 00 37 08 15 90 00 O ni on C re ek a t U .S . H ig hw ay 1 83 , A us ti n, T ex as 32 1 35 19 76 –2 01 0 9/ 8/ 20 10 06 00 20 .3 7 9, 54 0 29 .7 0. 37 9/ 9/ 19 21 38 .0 0 13 8, 00 0 Floods in Central Texas, September 7–14, 2010 1 F or p er io d si nc e st re am flo w r eg ul at io n be ga n. 2 F or u nr eg ul at ed p ar t o f t he b as in . 3 R eg ul at ed b y M an sfi el d D am s in ce 1 94 1. Th e 36 -y ea r pe ri od 1 97 5- 20 10 is m or e ty pi ca l o f c ur re nt d am o pe ra tio ns a nd w as u se d in d et er m in in g an nu al e xc ee da nc e pr ob ab ili ty fo r th e Se pt em be r 20 10 fl oo d. 4 A dj us te d to p re se nt d at um . 5 B as ed o n st ag e- di sc ha rg e ra tin g cu rv e ex te nd ed to a m ea su re m en t m ad e in 2 00 4. Texas Water Journal, Volume 3, Number 1 23 Figure 5. Streamflow hydrographs for U.S. Geological Survey streamflow-gaging stations 08104900 South Fork San Gabriel River at Georgetown and 08154700 Bull Creek at Loop 360 near Austin. Figure 6. Relation between peak streamflow and unregulated drainage area at 35 U.S. Geological Survey streamflow-gaging stations September 7–8, 2010, in Bell, Williamson, and Travis counties and selected flood peaks used to define an envelope of maximum floods documented in the United States by the U.S. Geological Survey. 0 5,000 10,000 15,000 20,000 25,000 30,000 0:00 12:00 0:00 12:00 0:00 08104900 (site 7, fig. 1, table 3) 08154700 (site 13, fig. 1, table 3) 2010 SEPT 7 SEPT 8 S T R E A M FL O W , IN C U B IC F E E T P E R S E C O N D EXPLANATION UNREGULATED DRAINAGE AREA, IN SQUARE MILES 1 10 100 1,000 1 10 100 1,000 10,000 100,000 P E A K S T R E A M FL O W , IN C U B IC F E E T P E R S E C O N D 08105700 San Gabriel River at Laneport, Tex. (site 11, fig. 1, table 3) 08104700 North Fork San Gabriel River near Georgetown, Tex. (site 6, fig. 1, table 3) 08105886 Lake Creek at Lake Creek Parkway near Austin, Tex. (site 12, fig. 1, table 3) Figure 6. Relation between peak streamflow and unregulated drainage area during September 7 8, 2010, in Bell, Williamson, and Travis Counties, Texas, and selected flood peaks used to define an envelope of maximum floods documented in the United States by the U.S. Geological Survey. at 35 U.S. Geological Survey streamflow-gaging stations 1,000,000 Flood of September 7 8, 2010, in Central Texas Extraordinary Floods in the United States (Costa and Jarrett, 2008) 10,000 08104700 08105886 08105700 EXPLANATION range of drainage areas documented in the United States by the Geological Survey (Costa and Jarrett 2008). Asquith and Slade (1995) developed envelope curves for maximum peak streamflows in Texas. These were not considered for this study because the areal extent of the 2010 flood is at the convergence of 3 regions with different maximum peak streamflow charac- teristics as described in Asquith and Slade (1995). In Figure 6, the peak streamflow of 7,330 cubic feet per second recorded at site 6 is plotted versus the unregulated drainage area of this site (1.55 square miles). Because releases from Lake George- town did not begin until September 14 (USACE 2011), the peak streamflow recorded for site 6 is the runoff from the unregulated area downstream from the dam. The peak dis- charge for site 6 plots just below the data for the envelope of maximum floods (Figure 6); the centroid of the unregu- lated part of the basin between Lake Georgetown and site 6 is about 0.5 mile from the reported 24-hour rainfall of 14.57 inches at the Corps Georgetown Lake office. The peak stream- flow at site 12 (Geological Survey streamflow-gaging station 08105886 Lake Creek at Lake Creek Parkway near Austin) was 3,510 cubic feet per second; the drainage area for this site is 2.18 miles, (Figure 6, Table 3). Site 11 (Geological Sur- vey streamflow-gaging station 08105700 San Gabriel River at Laneport), 4 miles downstream from Granger Lake, recorded a peak streamflow of 8.3 cubic feet per second (Figure 6). The unregulated part of the drainage area of site 11 received only 2 inches of rain (Figure 2) and the water-surface elevation at Granger Lake did not reach the spillway. The annual exceedance probabilities listed in Table 3 for peak streamflows were computed for 20 streamflow-gaging stations in the study area, based on the annual flood peaks for the period of systematic record. Because many of these sta- tions have dams and/or substantial development within the basin, annual exceedance probabilities were based strictly on the systematic record without consideration of regional flood- frequency equations (e.g., Asquith and Roussel 2009). Annual exceedance probabilities were computed using methods out- lined in Bulletin 17B (Interagency Advisory Committee on Water Data 1982). Calculations were made using the Geologi- cal Survey program Peak flow FreQuency (PeakFQ) (Flynn et al. 2006). For stations where the streamflow is regulated, peak streamflows for the period prior to when regulation began were not used in the analysis. For site 22 (Geological Survey streamflow-gaging station 08158000 Colorado River at Aus- tin) (Figure 1, Table 3) the period 1975–2010 was used in the analysis, as annual peak streamflows during this period appear to reflect consistent reservoir operations. The annual exceedance probability was 0.03 for sites 6 (North Fork San Gabriel gage) and 13 (Bull Creek at Loop 360 gage) (Table 3). The annual exceedance probability for site 3 (Little River gage) was 0.02. Generally, annual exceed- ance probabilities for 24-hour rainfall were lower than for peak streamflows. The lack of similarity in the annual exceedance probabilities computed for precipitation and streamflow could be partly attributed to the small areal extent of the heaviest rainfall over the gaged watersheds (Figure 2). Peak stream- flows on Brushy Creek are not known; however, much of the basin received more than 10 inches of rainfall, and the annual exceedance probability was less than 0.01 at several rain gages Floods in Central Texas, September 7–14, 2010 Texas Water Journal, Volume 3, Number 1 24 (Table 1). Additionally, the distribution of streamflow-gaging stations by drainage basin size is not uniform across the study area. The geometric mean of the drainage areas for streamflow- gaging stations in Travis County is 22.4 square miles, while that for Williamson County, where the most intense rainfall occurred, is 89.5 square miles. Only one site (site 12, Geologi- cal Survey station 08105886 Lake Creek at Lake Creek Park- way near Austin) in Williamson County had a drainage area less than 50 square miles; however, none of the streamflow- gaging stations for the smaller basins in Williamson County have sufficient record length to compute annual exceedance probabilities for peak streamflow. The lack of stream gages on smaller watersheds in Williamson County limits the under- standing of peak streamflows (and associated annual exceed- ance probabilities) for the September 2010 flood. SUMMARY Heavy rainfall associated with Tropical Storm Hermine Sep- tember 7–8 resulted in widespread flooding September 7–14, 2010, in Bell, Williamson, and Travis counties near the Austin metropolitan area in central Texas. The U.S. Geological Sur- vey, in cooperation with the Upper Brush Creek Water Con- trol and Improvement District, determined rainfall amounts and annual exceedance probabilities for rainfall resulting in flooding in central Texas in Bell, Williamson, and Travis coun- ties during September 2010 and documented peak streamflow amounts and the annual exceedance probabilities for peak streamflows measured at several streamflow-gaging stations in the study area. Total 24-hour rainfall exceeded 12 inches at some locations, with one report of 14.57 inches at Lake Georgetown. Annual exceedance probabilities of rainfall were estimated using depth-duration frequency maps for Texas. At 4 sites in Williamson County where more than 12 inches of rain fell in 24 hours (as recorded by rain gages at the Geo- logical Survey surface-water monitoring station 08104610 Lake Georgetown near Georgetown, the Water Control and Improvement District dam 5 and 13A, and the Georgetown airport), the 24-hour rainfall had an annual exceedance prob- ability of 0.002. Streamflow-measurement data from 19 Geological Survey streamflow-gaging stations are presented, including slope-area computations of peak streamflow. Flood- peak data from 35 Geological Survey streamflow-gaging sta- tions and 2 reservoir gages are presented, along with previous known maximums. The peak streamflow at site 6 (North Fork San Gabriel River gage) approached the envelope of maxi- mum floods for a range of drainage areas documented in the United States. The annual exceedance probability for peak streamflows were computed for 20 streamflow-gaging stations in the study area. The annual exceedance probability was 0.03 for the peak streamflow at site 6 and at site 13 (Bull Creek at Loop 360 gage). The annual exceedance probability was 0.02 for the peak discharge for site 3 (Little River gage). The lack of similarity in the annual exceedance probabilities computed for precipitation and streamflow could be partly attributed to the small areal extent of the heaviest rainfall over the gaged watersheds. Additionally, the distribution of streamflow-gaging stations by drainage basin size is not uni- form across the study area. The lack of stream gages on smaller watersheds in Williamson County limits the understanding of peak streamflows (and associated annual exceedance prob- abilities) for the September 2010 flood. REFERENCES Asquith WH, Roussel MC. 2004. Atlas of depth-duration fre- quency of precipitation annual maxima for Texas. Austin (Texas): U.S. Geological Survey. Scientific Investigations Report 2004-5041. 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