Floods in Central Texas, August 1978 - rt • -r .- r .,.... ... :-< ~ i'f tit ""' • f:• .... .!..J ~ 'tc.J· .... ''.' t r [ ~ U.S. GEOLOGICAL SURVEY ::;;: ?l Open-File Report 79-682 0.. 0.. (1) :::: ~ I I 'Tl ~ 0 0 ... - 0.. V> . ~R_AI.!J}ALL _ 1-'• ::s (") (1) ::s .,rt - ?l ! ~ (/) 20 w ~ u z V> rt Prepared in cooperation with the State of Texas and other agencia Cover photograph, Brazos River in flood at Graham, by Randy Black, Dallas, Texas. Floods in Central Texas, August 1978 By E.E. Schroeder, B.C. Massey, and Kidd M. Waddell U.S. GEOLOGICAL SURVEY Open-File Report 79-682 Prepared in cooperation with the State of Texas and other agencies April 1979 Reproduced by the Texas Department of Water Resources as part of the continuing program of cooperation in water-resources investigations between the Department of Water Resources and the U.S. Geological Survey. Copies of this report may be obtained from the U.S. Geological Survey Federal Building 300 East 8th Street Austin, TX 78701 CONTENTS Page Abstract---------------------------------------------------------- 1 Introduction-----------------------------.------------------------- 2 Purpose and scope of this report----------------------------- 2 Definitions of terms and abbreviations----------------------- 2 Metric conversions------------------------------------------- 3 Description of the storm-----------~------------------------------ 3 Description of the floods----------------------------------------- 4 Nueces River basin------------------------------------------- 4 Guadalupe River basin---------------------------------------- 4 Medina River basin-------------------------------------- 6 Colorado River basin----------------------------------------- 11 Pedernales River------------------:_____________________ 11 Llano River-------~------------------------------------- 11 Brazos River basin----------~-----~----------------~--------- 11 Magnitude and frequency of the floods----------------------------- 12 Flood damage------------------------------------------------------ 14 -Water-quality changes in the Brazos River basin--~---------------- 18 Water-level changes in the Edwards aquifer------------------------ 26 Selected references----------------------------------------------- 29 iii ILLUSTRATIONS Page Figure 1. Map showing locations of streamflow-data sites--------- 5 2. Discharge hydrographs for Guadalupe River at Comfort and Guadalupe River near Spring Branch------- 7 3. Log Pearson type III frequency curve for Medina River near Pipe Creek-------------------------------- 8 4. Photograph showing cypress tree uprooted by floodwaters on the Medina River---------------------- 9 5. Photographs showing Medina River near Pipe Creek, before, during, and after the flood------------------ 10 6. Discharge hydrographs for Hubbard Creek below Albany and Hubbard Creek near Breckenridge----------- 13 7. Photograph showing inundation of State Highway 173 and the Medina River bridge at Bandera----------- 15 8. Photograph showing State Highway 290 flooded by Spring Creek near Fredericksburg--------------------- 16 9. Photograph showing aerial view of the Brazos River in flood at Graham----------------------------------- 17 10. Map showing locations of water-quality data- collection sites on Hubbard Creek Reservoir---------- 19 11. Map showing locations of water-quality data- collection sites on Possum Kingdom Reservoir--------- 20 12. Map showing locations of water-quality data- collection sites on Whitney Lake--------------------- 21 13. Map showing locations of water-quality data- collection sites on the Brazos River----------------- 22 14. Profiles of specific conductance and density for Hubbard Creek Reservoir during June and Aygust 1978------------------------------------------------- 23 15. Profiles of specific conductance and density for Possum Kingdom Reservoir during June and August 1978------------------------------------------------- 24 16. Profiles of specific conductance and density for Whitney Lake during June and September 1978---------- 25 17. Graphs showing daily specific conductance ~nd instantaneous discharge for five sites on the Brazos River downstream from Whitney Lake, July and August 1978-----------------------·--------------- 27 18. Water-level changes in wells in the Edwards aquifer, July 10-August 25, 1978--------------------- 28 iv TABLES Page Table 1. Summary of flood stages and discharges------------------ 30 2. Locations of discontinued stream-gaging stations and miscellaneous discharge-measurement sites--------- 35 3. Water-quality data for Hubbard Creek, June 9 and August 29, 1978--------------------------------------- 36 4. Water-quality data for Possum Kingdom Reservoir, June 13.and August 30, 1978--------------------------- 38 5. Water-quality data for Whitney Lake, June 23 and September 5-6, 1978----------------------------------- 40 6. Station descriptions and discharge data----------------- 42 v FLOODS IN CENTRAL TEXAS AUGUST 1978 By E. E~ Schroeder, B. C. Massey and Kidd M. Waddell ABSTRACT Catastrophic floods, which resulted in millions of dollars in prop­ erty damages and the loss of 33 lives, occurred in Central Texas during August 1-4, 1978, as a result of intense rainfall produced by the remnants of tropical storm Amelia. Rainfall in excess of 30 inches was unofficially reported at several locations, while the highest 24-hour amount recorded by the National·Weather Service was 29.05 inches at Albany in Shackelford County. Major flooding occurred on the Medina River and triputaries above Medina Lake and on the Guadalupe River and tributaries above Canyon Lake. Minor to severe flooding occurred on the tributaries of the Nueces River, on the Clear Fork Brazos River and tributaries, and on the Llano and Peder­ nales Rivers, which are tributaries of the Colorado River. Peak discharges at several streamflow stations exceeded the historic peaks, and the flood magnitude and frequency data for the Guadalupe River above Canyon Lake, the Medina River near Pipe Creek, and Clear Fork Brazos River indicate that the August 1978 flood had a recurrence interval in excess of 100 years. The highest unit discharge observed during this flood was 3,010 cubic feet per second from a 14.1-square-mil.e drainage area of Spring Creek, which is tributary to the Pedernales River. -1- INTRODUCTION Purpose and Scope of This Report The purpose of this report, which was prepared in cooperation with the State of Texas and other agencies, is to present the available flood data in a form that should be of value in assessing the risks involved in developing the flood plains of streams that are subject to chronic flood­ ing. The report presents a description of the storm, a description of the flood by basins, peak stages and discharges, discharge-hydrograDh data, flood-frequency estimates, damage estimates, a discussion of the changes in water quality in selected streams and reservoirs in the Brazos River basin, and a discussion of water-level changes in observa.tion wells in the Edwards aquifer. Definitions of Terms and Abbreviations Technical terms and abbreviations, as used in this report, are defined as follows: Gaging station is a particular site on a stream, canal, lake, or reservoir where systematic observations of gage height or discharge are obtained. When used in connection with a discharge record, the term is applied only to gaging stations where a continuous record of discharge is obtained. Crest-stage station is a particular site where limited streamflow data on peak stages are collected systematically over a period of years for use in hydrologic analyses. Cubic foot per second (ft3/s) is the rate of discharge representing a vol­ ume of 1 cubic foot passing a given point during 1 second. This rate is equivalent to a 24-hour volume of 86,400 cubic feet, 1.983471 acre-feet, or 646,317 gallons. Cubic foot per second per square mile [(ft3/s)/mi2 ] is the average number of cubic feet of water flowing per second from each square mile of area drained, assuming that the runoff is distributed uniformly in time and area. Acre-foot is the quantity of water required to cover 1 acre to a depth of 1 foot and is equivalent to 43,560 cubic feet, or 325,851 gallons. The term is generally used in relation to storage and volume of runoff. Runoff, in inches, is the depth to which a drainage area would be covered if all the runoff for a given time period were uniformly distributed on its surface. Drainage area of a stream at a specified location is that area, measured in a horizontal plane, that is enclosed by a topographic divide so that direct surface runoff from precipitation normally would drain by gravity into the stream above the specified point. Drainage area is expressed in square miles. Contents is the volume of water in the reservoir or lake and is expressed in acre-feet. Volume is computed on the basis of a level pool and does not include bank storage. Time of day is expressed in 24-hour time; for example, 12:30 a.m. is 0030, 1:30 p.m. is 1330. All times noted are Central daylight time. Specific conductance is a measure of the ability of water to conduct an electrical current and is expressed in micromhos per centimeter at 25°C. Because the specific conductance is related to the number and chemical -2- types of ions in solution, it can be used to determine the approximate concentration of dissolved solids in the water. Commonly, the concentra­ tion of dissolved solids (in milligrams per liter) is about 65 percent of the specific conductance (in ~icromhos per centimeter at 25°C).