Flood of September 1946 at San Antonio, Tex
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UNITED STATES DEPARTMENT OF THE INTERIOR J. A. Krug, Secretary. GEOLOGICAL SURVEY W. E. Wrather, Director. CIRCULAR 32 November 1948 FLOOD OF SEPTEMBER 1946 AT SAN ANTONIO, TEX. By Seth D. Breeding Prepared in cooperation with the TEXAS BOARD OF WATER ENGINEERS WASHINGTON, D. C. FLOOD OF SEPTEMBER 1946 AT SAN ANTONIO, TEX. By Seth D. Breeding CONTENTS / Page Page Introduction.............................. 1 Stagers and discharges at gaging stations Acknowledgments........................... 2 In 3an Antonio River Basin Continued. Rainfall.................................. 2 San Antonio River at Qoliad............. 15 General features of the flood............. 8 Medina River near San Antonio........... 16 Olmos Dam................................. 9 Cibolo Creek near Falls City............ 17 Measurements t>f peak discharge............ 11 Rainfall and runoff....................... 18 Stages and discharges at gaging stations Damages................................... 18 in San Antonio River Basin............... 13 Map showing 1946 and 1921 flooded areas San Antonio River at San Antonio........ 13 in San Antonio......................... 18 San Antonio River at Palls City......... 14 ILLUSTRATIONS Page Figure 1. Map of San Antonio River and tributaries in vicinity of San Antonio, showing drainage basins and total rainfall, September 8-10, 1921...................... 2. Isohyetal map of pao?t of San Antonio River Basin in vicinity of San Antonio, showing total rainfall September '26-27, 1946,................................. 4 3. Isohyetal nap of storm of September 26-27, 1946, in vicinity of San Antonio...., 5 4. Graph of discharge at stream-gaging station on San Antonio River at South Alamo Street crossing in San Antonio. ....-....................... > ' < 10 5. Graphs of discharge at various stream-gaging stations in San Antonio River Basin, 1946...................................»............. .. 12 6. Map of San Antonio, showing 1946 and 1921 flood plains.......................... 19 TABUS Page Table 1. Rainfall at said near San Antonio, September 7-1Q, 1921............................... 6 2. Daily rainfall September 25-29, 1946, at points near San Antonio..................... 6 3. Maximum stages in San Antonio River Basin resulting from storm of September 26-27,1946 9 4. Maximum discharges of flood of September 27, 1946, in streams in vicinity of San Antonio...........................................;........ *«« « H INTRODUCTION off but did saturate the ground enough to increase the rate of runoff of the subsequent A flood occurred in the streams in and near heavy rains. Heavy rainfall began about San Antonio, Teat., during the early morning 8 p. ». September 26, and within a short time hours of September 27, 1946, as a result of San Pedro Creek and its tributaries (Alazan, heavy rains falling during the previous night. Martlaez, and Apaohe Creeks) were in flood and Much property damage occurred in San Antonio all reached peak stages about 1 to 2 a. m. and below, and four lives were lost. It is September 27. The floods in these streams the purpose of the present report to describe caused the greater part of the damage in San this flood and its relation to the flood of Antonio. San Antonio River flows through the September 10, 1921 the greatest of record heart of the city, but did not overflow its , since 1819.3L/ banks upstream from San Pedro Creek. The fact that San Antonio River did little- damage The heavy rainfall during the night of Sep through the city can be attributed to the tember 26, 1946, was preceded by a moderate presence of Olmos Dam on Olmos Creek, the main rain during the late afternoon of September tributary to the river, which held back the 25, when from 1 to 3 inches fell on the drain waters that otherwise might hare caused a age basins of streams'that flow through San major flood on .the stream. Considerable dam Antonio. This rainfall produced little run- age was done in the downtown area by rainwater ponded by overloaded storm sewers. tell sworth* C. E., The Floods in Central Texas in September 1921: IT. S. Qeol., Tbe streams flowing through San Antonio in Surrey Water-Supply Paper 4&S f p. 63. general have small open channels and oompara- tively wide flood plains greatly encroached this area. An excellent record was obtained upon by residences, business buildings, and from a standard rain gage at the State Api numerous bridges. San Antonio River below culture Farm in the Calaveras Creek Basin 11 San Antonio has a well defined channel of miles southeast of the San Antonio post office. small capacity and a wide flood plain. Al This record shows that 0.55 inch fell between though extensive flooding occurred downstream 6 and 10 p. m. September 26, and 16.67 inches from the city, damages were relatively light fell between 10 p. m. and 9 a. m. the next as the flood plain is not well developed. morning. The fact that the flood crest in Calaveras Creek at a point about 2 miles Immediately after the storm, the Weather below the State Apiculture Farm occurred be Bureau obtained a great many miscellaneous fore daylight indicates that most of this records of rainfall. These data, together rain fell in 6 to 8 hours as at other places. with records of rainfall at the Weather Bureau The most intense rainfall reported was at the stations in the vicinity and an isohyetal map, San Antonio Nursery in the southern part of Were published in the Weather Bureau Hydro- the city where 4.15 inches fell during the logic Bulletin, Lower Mississippi-West Qulf 1-hour period 9 to' 10 p. m. September 26. A District, September 1946. total of 13.03 inches fell at this place. In February 1947 the Texas State Board of The rain causing the damage to the* city of Water Engineers in cooperation with the Geo San Antonio varied from 16 inches in south logical Survey published a report with limit eastern San Antonio to about 7 inches at the ed distribution giving rainfall and runoff headwaters of the streams that flow through data for the storm. Analyses Of rainfall- the city. The rainfall in September 1921 was runoff relations included in that report were much heavier over the upper drainage basins based on an isohyetal map furnished by the of these streams than in September 1946. For Weather Bureau and records of runoff obtained the purpose of comparing the patterns of rain at gaging stations. In the present report fall in 1921 and 1946, figure 1 and table 1, the isohyetal map has been redrawn mainly to showing total rainfall over San Antonio River give more weight to a record of rainfall of Basin in the vicinity of San Antonio for 10.75 inches caught in a standard 8-inch gage Sept. 8-10, 1921, are reproduced from Wa,ter- at 201 Mulberry Street in north-central San Supply Paper 488. The rainfall lines shown Antonio. on figure 1 were taken from figure 1 in the account by Jarboe.j2/ The following comments ACKNOWLEDGMENTS also come from the same source: Base data used in this report were obtained FifiliUfflilJi AP ffflB JJfl^filAff'" ^ drought of in cooperation with the Texas State Board of two months 1 duration was broken when a Water Engineers and the Corps of Engineers, shower of 0.53 inch fell between 6 and 7 a. m. War Department. The Corps of Engineers Sept. 8. Seventeen hours later, between furnished special help in obtaining field 12 midnight and la. m. on the 9th, steady data for the computation of maximum discharges rains began and continued until shortly after by slope-area method listed in table 4. The 11 p. m.--a period of about 23 hours. The map showing the flooded areas in San Antonio crest of the flood came through the city two and the.statement of damages resulting from hours after the precipitation ended. the 1946 flood are reproduced from a report on that flood prepared by the Corps of Engi* The amounts of precipitation, aa measured neers. The Engineering Department of the at the United States Weather Bureau (near the city of San Antonio made the surveys outlin center of the city3 are aa followss ing the flooded areas and also supplied the figure for maximum elevation of stored water Inches in Olmos reservoir. Rainfall data were furnished by the United States Weather Bureau, September 8, 7 a. m. 0.53 7 p. m. .01 RAINFALL September 9, 7 a. m. 3.48 7 p. ». 1.90 Records show that the heavy rainfall began September ID, 7 a. m. 1.46 about 8 p. m. September 26. Crest stages 7 p. m. Trace occurred about 1 to 2 a. m. September 27 in the small streams in San Antonio and in the- Total. .... 7.38 early morning hours before daylight in those near San Antonio. It is evident that the Heavy rains shown.- Study of the accompa greater part of the rain fell in the 6 or 8 nying map [[fig. 1 ] shows the rapid increase hours following 8p.m. 'September 26. A of rainfall north and west of the Weather Bureau . record obtained by a U. S. Weather Bureau station. Two miles north, 9.50 inches were re recording rain gage at San Antonio Municipal corded, with 3 inches after 7 p. m. on the 9th, Airport shows that, of the total rainfall of as compared to 1.4.6 inches at the Weather Bureau 6.93 inches, 6.61 inches fell between 8 p. m. station, further study of the map brings out September 26 and 4 a. m. September 27. The the fact that approximately 3 miles north and period of heaviest rainfall at this place was west from the city more than 10 inches Bust have between 11 p. m. and 1 a. m. when 2.38 inches fallen.