CIRCULAR 32

UNITED STATES DEPARTMENT OF THE 

INTERIOR J. A. Krug, Secretary.

GEOLOGICAL SURVEY W. E. Wrather, Director.

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 

Introduction.............................. 1

Acknowledgments........................... 2

Rainfall.................................. 2

General features of the flood............. 8

Olmos Dam................................. 9

Measurements t>f peak discharge............ 11

Stages and discharges at gaging stations 

in San Antonio River Basin............... 13

San Antonio River at San Antonio........ 13

San Antonio River at Palls City......... 14

Page

Stagers and discharges at gaging stations 

In 3an Antonio River Basin Continued. 

San Antonio River at Qoliad............. 15

Medina River near San Antonio........... 16

Cibolo Creek near Falls City............ 17

Rainfall and runoff....................... 18

Damages................................... 18

Map showing 1946 and 1921 flooded areas 

in San Antonio......................... 18

ILLUSTRATIONS

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,.................................

3. Isohyetal nap of storm of September 26-27, 1946, in vicinity of San Antonio....,

4. Graph of discharge at stream-gaging station on San Antonio River at South Alamo 

Street crossing in San Antonio. ....-....................... > ' <

5. Graphs of discharge at various stream-gaging stations in San Antonio River

Basin, 1946...................................»............. .. 

6. Map of San Antonio, showing 1946 and 1921 flood plains..........................

Page

4

5

10

12

19

TABUS

Table 1.

2.

3.

4.

Page 

Rainfall at said near San Antonio, September 7-1Q, 1921............................... 6

Daily rainfall September 25-29, 1946, at points near San Antonio..................... 6

Maximum stages in San Antonio River Basin resulting from storm of September 26-27,1946 9 

Maximum discharges of flood of September 27, 1946, in streams in vicinity of San

Antonio...........................................;........ *«« « H

INTRODUCTION

A flood occurred in the streams in and near 

San Antonio, Teat., during the early morning 

hours of September 27, 1946, as a result of 

heavy rains falling during the previous night. 

Much property damage occurred in San Antonio 

and below, and four lives were lost. . It is 

the purpose of the present report to describe 

this flood and its relation to the flood of 

September 10, 1921 the greatest of record 

since 1819.3L/

The heavy rainfall during the night of September 26, 1946, was preceded by a moderate 

rain during the late afternoon of September 

25, when from 1 to 3 inches fell on the drainage basins of streams'that flow through San 

Antonio. This rainfall produced little runtell sworth* C. E., The Floods in Central 

Texas in September 1921: IT. S. Qeol., 

Surrey Water-Supply Paper 4&S f p. 63.

off but did saturate the ground enough to 

increase the rate of runoff of the subsequent 

heavy rains. Heavy rainfall began about 

8 p. ». September 26, and within a short time 

San Pedro Creek and its tributaries (Alazan, 

Martlaez, and Apaohe Creeks) were in flood and 

all reached peak stages about 1 to 2 a. m. 

September 27. The floods in these streams 

caused the greater part of the damage in San 

Antonio. San Antonio River flows through the 

heart of the city, but did not overflow its , 

banks upstream from San Pedro Creek. The fact 

that San Antonio River did little- damage 

through the city can be attributed to the 

presence of Olmos Dam on Olmos Creek, the main 

tributary to the river, which held back the 

waters that otherwise might hare caused a 

major flood on .the stream. Considerable damage was done in the downtown area by rainwater 

ponded by overloaded storm sewers.

Tbe streams flowing through San Antonio in 

general have small open channels and oompara-
tively wide flood plains greatly encroached 

upon by residences, business buildings, and 

numerous bridges. San Antonio River below 

San Antonio has a well defined channel of 

small capacity and a wide flood plain. Although extensive flooding occurred downstream 

from the city, damages were relatively light 

as the flood plain is not well developed.

Immediately after the storm, the Weather 

Bureau obtained a great many miscellaneous 

records of rainfall. These data, together 

with records of rainfall at the Weather Bureau 

stations in the vicinity and an isohyetal map, 

Were published in the Weather Bureau Hydrologic Bulletin, Lower Mississippi-West Qulf 

District, September 1946.

In February 1947 the Texas State Board of 

Water Engineers in cooperation with the Geological Survey published a report with limited distribution giving rainfall and runoff 

data for the storm. Analyses Of rainfallrunoff relations included in that report were 

based on an isohyetal map furnished by the

Weather Bureau and records of runoff obtained 

at gaging stations. In the present report 

the isohyetal map has been redrawn mainly to 

give more weight to a record of rainfall of 

10.75 inches caught in a standard 8-inch gage 

at 201 Mulberry Street in north-central San 

Antonio.

ACKNOWLEDGMENTS

Base data used in this report were obtained 

in cooperation with the Texas State Board of 

Water Engineers and the Corps of Engineers, 

War Department. The Corps of Engineers 

furnished special help in obtaining field 

data for the computation of maximum discharges 

by slope-area method listed in table 4. The 

map showing the flooded areas in San Antonio 

and the.statement of damages resulting from 

the 1946 flood are reproduced from a report 

on that flood prepared by the Corps of Engi* 

neers. The Engineering Department of the 

city of San Antonio made the surveys outlining the flooded areas and also supplied the 

figure for maximum elevation of stored water 

in Olmos reservoir. Rainfall data were 

furnished by the United States Weather Bureau,

RAINFALL

Records show that the heavy rainfall began 

about 8 p. m. September 26. Crest stages 

occurred about 1 to 2 a. m. September 27 in 

the small streams in San Antonio and in theearly morning hours before daylight in those 

near San Antonio. It is evident that the 

greater part of the rain fell in the 6 or 8 

hours following 8p.m. 'September 26. A 

record obtained by a U. S. Weather Bureau 

recording rain gage at San Antonio Municipal 

Airport shows that, of the total rainfall of 

6.93 inches, 6.61 inches fell between 8 p. m. 

September 26 and 4 a. m. September 27. The 

period of heaviest rainfall at this place was 

between 11 p. m. and 1 a. m. when 2.38 inches 

fell. The crest stage in San Antonio River 

at the gaging station at South Alamo Street 

bridge occurred at 2 a* m. This crest was 

caused by the runoff from rain falling in 

San Antonio below Olmos Dam. ,

The center of this storm (rainfall greater 

than 16 inches) severed an area of about 130 

square miles that extended from the southern 

part of San Antonio south and southeast for a 

distance of about 20 miles. Humorous miscellaneous records of rainfall were obtained in

this area. An excellent record was obtained 

from a standard rain gage at the State Apiculture Farm in the Calaveras Creek Basin 11 

miles southeast of the San Antonio post office. 

This record shows that 0.55 inch fell between 

6 and 10 p. m. September 26, and 16.67 inches 

fell between 10 p. m. and 9 a. m. the next 

morning. The fact that the flood crest in 

Calaveras Creek at a point about 2 miles 

below the State Apiculture Farm occurred before daylight indicates that most of this 

rain fell in 6 to 8 hours as at other places. 

The most intense rainfall reported was at the 

San Antonio Nursery in the southern part of 

the city where 4.15 inches fell during the 

1-hour period 9 to' 10 p. m. September 26. A 

total of 13.03 inches fell at this place.

The rain causing the damage to the* city of 

San Antonio varied from 16 inches in southeastern San Antonio to about 7 inches at the 

headwaters of the streams that flow through 

the city. The rainfall in September 1921 was 

much heavier over the upper drainage basins 

of these streams than in September 1946. For 

the purpose of comparing the patterns of rainfall in 1921 and 1946, figure 1 and table 1, 

showing total rainfall over San Antonio River 

Basin in the vicinity of San Antonio for 

Sept. 8-10, 1921, are reproduced from Wa,terSupply Paper 488. The rainfall lines shown 

on figure 1 were taken from figure 1 in the 

account by Jarboe.j2/ The following comments 

also come from the same source:

FifiliUfflilJi AP ffflB JJfl^filAff'" ^ drought of 

two months 1 duration was broken when a 

shower of 0.53 inch fell between 6 and 7 a. m. 

Sept. 8. Seventeen hours later, between 

12 midnight and la. m. on the 9th, steady 

rains began and continued until shortly after 

11 p. m.--a period of about 23 hours. The 

crest of the flood came through the city two 

hours after the precipitation ended.

The amounts of precipitation, aa measured 

at the United States Weather Bureau (near the 

center of the city3 are aa followss

September 8, 7 a. m.

7 p. m. 

September 9, 7 a. m.

7 p. ». 

September ID, 7 a. m.

7 p. m.

Inches

0.53 

.01 

3.48 

1.90 

1.46 

Trace

Total. .... 7.38

Heavy rains shown.- Study of the accompanying map [[fig. 1 ] shows the rapid increase 

of rainfall north and west of the Weather Bureau . 

station. Two miles north, 9.50 inches were recorded, with 3 inches after 7 p. m. on the 9th, 

as compared to 1.4.6 inches at the Weather Bureau 

station, further study of the map brings out 

the fact that approximately 3 miles north and 

west from the city more than 10 inches Bust have 

fallen. Between 5 and 6 miles north and west from 

the city's center, the rainfall undoubtedly reached 14 inches over a considerable portion of the 

,drainage area. Hine stations show 15 inches or 

more.

The heaviest rainfall probably occurred 8 to

2Jarboe, J. H., The San Antonio flood of 

Sept. 10, 1921: U. S. Weather Bureau 

Mo. Weather Rev* vol 49, pp. 494-496, 

September 1921.
10 Bll»« ttorthwaat of S*a Antonio. Beyond this 

sei)Kt the lainfall was aot »c heavy, aa abeam by 

' the d-inch gage located 1? ntlec attrtiweat, 

where 10 inches were recorded.

It is possible, bat hardly probable, that 

ace* aeetioas of these drainage basina received 

30 inches of precipitation in the atom preceding the flood. Two stations ehow thla amount, 

but much evidence would be necessary before those 

faniliar with rainfall data could accept the 

figures.

It will be not«d. from the last paragraph 

quoted above that the Weather Bareau did aot 

aeoept recorda of rainfall amounting to 20 

inches or more* although at one place, location Ho. 5, figure 1, 21.0 incites was measured 

it a standard rain gage. In the light of 

evidence gathered since 1921, a rainfall of 

21 inches in 24* hours is not uaooanon in 

central Texas and it is probable that the 

record would be accepted at this time.

The statement quoted above that "steady 

rains fell for a period of abwt 23 hours" 

and the distribution indicated by the record 

for the Weather Bureau gage at San Antonio 

show that although the total rfcin in 1821 was

great»r than ftny recorded in 1946, its in- . 

tensity was leas in S*n Antonio and probably 

less elsewhere than that experienced in the 

aectiena of heavy rainfall in 1946. In 1S21 

the greatest rainfall for a 2-hour period at 

the Ban Antonio Weather Bureau gage w*s 1.52 , inohM between 1 and 3 a. m. Sept. 9, compared 

with 2.38 incites for 2 houre in 1946 j and the 

great?eat rainfall for an 8-hour period was 

3,5$ inches for the 8 hours preceding 8 a.< m. 

Sept* 9, compared with 9.81 inches for 3 hours 

in 19*6. .Greater peak discharge and total 

runoff would be expected from a rainfall of 

greater intensity if other factors were equal. 

In addition to the greater intensity of rain* 

fall tn 1946 there was considerable ante* 

oedent rain in the vicinity of San Antonio, 

rather large rains occurring Aug. 28, 29, 

Sept. 15 and 26, whereas the storm in 1921 

ended a drought that had lasted for two v 

monttas. Records of rainfall for Sept. 25-29, 

1944* furniahtfd in greater part by the Weather 

Bureeai, are listed in table 2* The isehyetal 

nap for San Antonio and vicinity (fig. 2J and 

ithe larger map covering the San Antonio River 

Baslm above Falls City exeept for Medina 

River (fig* 9) are based largely oa records 

of rainfall furnished by the Weather Bureau* 

As Inspection of figure 2 will show that a

 n"

O Standard rain g*g« 

4- Improved rain gage

totaWeintext

Figure l f Map of Saji Antonio River and tributaries in vicinity of Sam Antonio,'

and total '
I SIream-gaging station 

Places at which p*ok 

discharges only wtre 

measured

(Numbers are those 

referred to in Table 2)

Pigurte j-t Antonio, --' ."' "<- - , -

-" »» ^T.*^^j|-'i iw)r?T,Tw'-^"j-''''i"-v-'-' "**('." * t-"*-*'..'"'"'' -* ",V" -v*-''"^ '"""i 1 '

^f^/^\^VV:^^^^.*^^ft;^fe^ii^^^^
Figure 3. laohy&tal map of storm of September 26-3J? f 1946, 'in vicinity of Son Antonio.
Table ! Rainfall at and near San Antonio, September 7 to 10, 1921 

(The Locations given are those shown on fig. l.)

£0«>Ation

1

2

3

4

5

6

7

8

9

10

11

12

a

b

c

d

e

f

8

h

Inches

15.0

17*5

20.0

18.0

81.0

a 25.0

13.0

13.18

12-15

8. 55

9.5

6.84

12.0

10*0

15.0

18.0

10.0

11.0

7.0

10.14

Period covered or details

Sept. 8, midnight, to Sept. 9, midnight.

Sept. 8, midnight, to Sept. 9, midnight.

Sept. 9, 7 a.m., to Sept. 10, 7 a.m. .

Sept. 8, midnight, to Sept. 9, midnight, 17.0 inches.

Sept. 9, 7 a.m., to Sept. 10, 7 a.m.

Sept. 8, 7 p.m., to Sept. 9, 6 p.m., 10 Inches; Sept. 9, 6 p.m., to Sept. 10,

7 a.m., 15 inches.

Sept. 9, '7 a.m., to Sept. 10, 7 a.m.

Sept. 7, midnight, to Sept. 8, 8 p.m., 0.07 inchj Sept. 8, 8 p.m., to Sept* 9,

11:30 p.m., 13.11 Inches.

Sept. 9, 5 p.m., to Sept* 10, 7 a.m.

Sept. 7, 6 a.m. to 7 a.m., 0*60 inch; Sept. 8, 9 p.m. to Sept. 9, 9 a.m.,

3.60 inches; Sept. 9, 9 a.m. to 4 p.m., 1.30 inches; Sept. 9, 4 p.m. to

11:30 p.m., 3.05 inches.

Sept. 8, midnight, to Sept. 9, midnight.

Sept. 8, midnight, to Sept. 9, 6 p.m.* 6 inches; Sept. 9, 6-9 p.m., 3 inches;

Sept. 9, 9-12 p.m., 1 Inch.

Sept. 8, midnight, to Sept. 9, midnight.

Sept. 8, midnight, to Sept. 9, midnight.

Sept. 8, midnight, to Sept. 9, midnight.

Sept. 8, mldadght, to Sept. 9, midnight..

Sept. 8, midnight, to Sept. 9, midnight.

Sept. 8, n6on, to Sept. 9, noon, 4.62 inches; Sept. 9, noon, to Sept. 10,

_ a.m., 3.74 Inches; Sept. 10, _ a.m., to noon, 1.05 Inches; Sept.. 10. noon

to evening, 0.73 inch.

a Note that Weather Bureau gives 15 inches as total for this station and not 25 inches, as 

shown here.

Note.- Numbered gages used by United States Weather Bureau In drawing rainfall lines shown 

on fig. 1* Records at lettered stations and notes regarding distribution furnished by United 

States Engineer Corps, Fort Sam Houston, Tex.

Table 2. Daily rainfall, in inches, September 25-29, 1946, at points near San Antonio

Station

U. S, Weather Bureau

Boerne l/~

Bulverde 2/

Devlne 2/

Palls City (near) I/

Pischer' s Store JL/

Ploresville 2/

Ooliad I/ 

Hindes S/ 

Hondo 2/

Jourdanton J2/

Earnes City I/

la Vernia 2/

New Braunf els I/

Nixon I/

Poteet I/

Randolph Field 3/ 

Riomedlna \f

Runge I/ 

San Antonio 3/

Seguln 3/

Whitsett I/

Supplementary

Adkins, 1.6 miles south of

Adklns, 4 miles south of

Bergheim, 2 miles east of

Converse

Converse, (near)

I*t.

e i

29 49

29 45

29 09

28 57

29 69

29 08

28 40 

28 43 

29 20

28 55

28 53

29 22

29 42

29 16

29 03

29 32 

29 26

28 '53 

29 32

29 33

28 38

29 23

29 20

29 49

29 31

29 31

Long.

o t

98 45

98 27

98 54

98 04

98 16

98 09

97 23 

98 47 

99 09

98 33

97 54

98 06

98 06

9? 46

98 34

98 17 

98 53

97 43

98 28

97 58

98 16

98 15

98 15

98 33

98 19

98 22

September

26

,

2.17

_

1.40

-

_

.83 

.44

.17

1.01

1.53

1.40

.29

.08

1.14 

.29

3.27

.79

-

m

0.11

.37

.65

.

.75

.48 

.60

.11

-

.78

.

.06

.19

.20

1.00 

.24

.56

3.80

.87

1.00

-

27

3.51

6*32

2.18

.70

3.00

.71

.93 

.33 

1.12

5.40

-

2.59

1.61

1.37

5.25

.99

3.05

,65 

3.13

.57

».

b 4.30

12.5

c 3.20

b 3.60

a 5.75

28

-

. ..

_

.10

.75

.82

-

-

.67

-

_

.08

 »

.02

-

29

0.22

.45

-

*

«

_

.65

-

.

.11

.11

-

.07 

.06

_

-

Remarks

-

6.61 in. fell in' 8 .

hours following 8 p.m.

Sept. 26

15J- miles east of

San Antonio

Began 8:30 p.m. 26th,

ended 6 a.m. , 27th

12 miles ENE of San

Antonio 

Cade Juernsey Farm

See footnotes at end of table
"Table S. 'Dally rainfall, in inches. September 25-29, 1946, at points near San Antonio Continued

\ - ; " ~ ' - 

Station

, Supplementary Continued

Elmendorf , 3 miles north of

Ploreaville, 6J- miles NW of

Floresville, 9 miles HI of

Leon Springs, at

New Qratrafels, 19 miles WHW of -

"

Haodona, 2 miles north of

Pleasanton, 2 blocks south of

Pleasanton, south of

(at MoQee Ranch)

Pleasanton, 10 miles WBW of

Pleasanton, 10$- miles north of

Pleasanton, 15$ miles NNW of 

Foteet, 5 miles north of

Rossville, li miles SE of

Saint Hedwig

Saint Hedwig

San Antonio:

5$ miles south of

9 miles south of

/

7 miles south of

5.3 miles SSE of

12 miles south of

327 Wildrose Are.

542 E. Huisaohe Are.

414 Hansford Ave.

2605 Pleasanton Rd.

815 4th St.

152 Grape land

436A Sot 'Wells Blvd.

1611 Sehley 

; 11 miles SE of

12& miles east of, (at Sayers)

,

7f miles east of

5$ miles ESE of

6* miles SE of

5f miles EKE of

8 miles ENE of 

&| miles NE of

18$ miles NNW of 

l&t miles north of 

12 miles north of

12 miles north of 

15 miles NNW of 

15 miles HW of 

, 15 miles NW of 

18 miles WNW of '

14 miles west of

14 miles 3W of ,

327 Blalne Ave.

> 201 Diokson St.

1012 Florida St.

201 Mulberry St.

Lat.

0 >

29 16

29 13

29 14

2*40

29 48

29 22

28 67

28 53

28 59

29 07

29 ,10 

29 07

29 05

29 25

29 25

29 20

29 18

29 19

29 22

29 15

29 £9

29 28

29 23

29 21

29 21

29 22

29 22

26 24 

29 ,lt

29 23

29 25

29 24

29 23

29 27

29 29 

29 31

29 42 

29 42 

29 36

29 38 

29 38 

29 34 

29 33 

29 30

29 24

29 14

29 26

29 22

29 24

29 27

Long;

o t

98 22

98 13

98 OS

98 38

98 25

98 41

98 29

98 24

98 39

98 30

98 36 

98 36

98 40

98 12

98 12

98 29

98 29

r

98 29

98 28

98 28

98 28

98 24

98 29

98 30

98 31

98 SO

98 27

98 27 

98 21

98 18

98 22

98 24

98 24

98 24

98 23 

98 22

98 27

98 30 

98 31

98 29 

98 35 

98 41 

98 43 

98 47

98 44

98 35

98 35

98 25

98 28

98 30

September

25

.80

,'

i

,31

^26^

*

27

. ,

to 16/

to 6.5

to 2.6

a 4.66

3.07

to 3.50

a 8.37

c 4*5

o 5.5

c 15.00

to 10.00 

to 8,0/

0 7.05

c 6.66

,c 5.87

a 13.03

to 12.04

o 12.5/

to 16. 46/

0 IS/

d 6.73

d 7.10

0 13. 5/

to 12.00

12 .76/

to 15.61

0 16.12

14. 5/ 

a 17*22

o 12.2

to 13.0

c 16.72

14.6

to 12.5

12. 4/ 

to 5.75

to 6,0/ 

o 7.46 

to 7.5/

6.0 . 

0 3.0 

6.1 

o 6.8 

to 4.50

to 6.70

o 10.70

c 8.00

to 14. 5/

a 11.66

a 10.76

28

.80

.13

/

,29

29

.75

.02

,

Remarks

W. W. Owen dairy 1

Standard 8- inch gage

Intersection of Highways U. S. 281 &

Texas 46

13 ailes WSW qf San

Aritonio

Dr. B. B. Gillctt has

13 years record

At Amphion

At Hamilton 1 s Dairy ,

17-| miles east of San

Antonio

Do.

Recording gage at

nursery

Standard 8-inoh gage.

Began 8 p.m. 26th,

ended 8 a.m. 27th.

Began 10:30 p.m. 26th,

ended 7i 30 a.m. 27th 

Heavy rain started

9 p.m. 26th

\

Began 9 p.m. 26th,

ended 8 a. m. 27th

Very heavy rain 8 - 11

p.m. 26th

Standard 8-inoh gage

at State Apiculture

Farm 

Began 8:30 p.m. 26th,

ended 6 a.m. 27th

Began 8 p.m. 26th,

ended 5 a.m. 27th

At Kirtoy 

Standard 8- inch gage.

Rain 9 p.m. to 4 a.m. 

At Emil's Place 

At Circle Arrow Ranch

On U. S. Highway 281 

At Camp Bttllis 

At Moonlight favern 

Sleepy Hollow Ranch 

Culetora Store

Standard 8-inoh gage.

7:30 p.m. 26th to 7:30

a.m. 27th

Standard 8-inoh gage

See footnotes at end qf table.
Table 2. Daily rainfall, in inches, September 25-29, 1946, at points near San Antonio Continued

Station

 * 

San Antonio:

175 Sherwood Dr.

918 Broadbent St. 

203 Jennings St.

509 North St.

at Longhorn Cement, Plant

735 Delmar St.

at Kelly Field

Saspamco 

Seguin, 11 miles SW of

Somerset

Sutherland Springs 

Von Ormy **~

Wetmore

Zuehl

-

Lat.

-S r

29 29

29 25 

29 23

29 26

29 32

29 24

29 23

29 14 

29 28

29 11

29 16 

29 17

29 54

29 50

Long. -5 r

98 32

98 28 

98 31

98 2fl

98 24

98 27

98 34

98 18 

98 04

98 41

98 03 

98 39

98 25

98 10

September

25

V

1.29

2.00

26 27

b 6.45/

b 15. O/ 

b 13. 25/

a 10.12

0 6.50

o 12.60

b 5.80

12. 5/ 

b .50

o 4.50

c 5.68 

o 3.5

b 7*26

b 3.25

28 29 Remarks

' .'','

Cont&iner overflowed 

by 1:30 a.m. 27tn

Standard 8-inch gage

' ' '

At New Berlin (

12| miles southwest of

San Antonio

10$- miles northeast of

San Antonio

13 miles southwest of

Seguin

1 Measured in the afternoon.

2 Measured in the morning.

3 Measured at midnight.

4 This estimated, record was disregarded in favor of the good record at Leon.Springs.

Note.- All records furnished by U. S. Weather Bureau except that for Leon Springs. 

All distances glTen are from post office named.

Accuracy of supplementary records: a Excellent, b Good, e Pair, d Poor* 

Amounts shown in supplementary records on Sept* 27 show total rain for storm of night of

Sept. 26. 

Figures for rainfall followed by plus sign (/) indicate, that container used as gage overflowed.

movement of the storm's center 10 miles northwest would have produced, on the basins that 

drain through San Antonio, a rainfall, of 

about 16 inches instead of 7 or 8 inches. 

Had this shift in the storm's center occurred, 

the damage to San Antonio would probably have 

been many times greater than that which occurred.

GENERAL FEATURES OF THE FLOOD

All streams flowing through San Antonio 

apparently experienced higher stages in 1921 

than in 1946, except possibly in the vicinity 

of the junction of San Pedro, Alazan, and 

Apache Creeks where the maximum stage was re* , 

ported to be about the same for 1921 and 1946* 

The maximum stage at the gage on Saa Antonio 

River at South Alamo Street bridge was 20,14 

feet in 1921 and 15.32 feet in 1946, However, 

these stages are not comparable as the Olmos 

Retention Dam was built on the main tributary 

of San Antonio River between those dates. 

During the storm of September 1946 the Olmos 

Reservoir caught and held the runoff from 

32.4 square miles. Thus the peak stage at 

the San Antonio River gage at San Antonio was 

caused by the runoff from 9.4 square miles 

below the dam. However, residents on Olmos 

Creek at Blanc o Road crossing about 2j| oniles 

above Olmos Dam pointed out hlghwater marks 

of the 1921 flood showing that flood to have 

been about 6 feet higher than the flood of , 

September 1946,

In 1921 San Antonio River caused much 

greater damage t^o the business section of 

San Antonio than in 1946, ' The 1921 flood

overflowed across the divide to the west and 

joined with San Pedro Greek* In 1946 the 

river did not overflow its banks through the 

business sections of San Antonio. On the' 

basis of local information, Martinez Creek at 

Frederioksburg Road crossing was about 3 feet 

higher in 1921 and available evidence indicates that Alaaaa and Apache Creeks also were 

higher in 1921 than in 1946. In order to 

present a record of the maximum stages reached 

by the flood In 1946, the altitude of the 

crest of the flood above mean sea level is 

given In table 3 at the various stream-gaging 

stations whose records are1 included in this 

report, and at places in San .Antonio where, 

only the maximum discharges were measured* 

Although this flood in San Antonio River at 

the gaging station in San Antonio was not as 

great as previous floods, its comparative 

size increased as it entered the area of 

heavy rainfall below San Antonio until it 

reached the vicinity of Elmendorf and the 

mouth of Calaveraa Creek, where it was many 

feet higher than ever known before. A tremendous flood from Calaveras Creek did much ' 

to cause record-breaking stages in the river 

below Calaveras. At Calaveras,bridge on San 

Antonio River the stage in 1946 was about,12 

feet higher than in 1921. At the gage near 

Falls City the maximum stage on Sept. 29, 

1946, was 6.4 feet higher than.that reached 

by the fj.ood of October 1913, which was the 

greatest flood previously known. \ By\ the time 

the flood reached Goliad its crest had 

flattened to such an extent that it was no : 

^Longer & notable flood.. .

.The flood in San Antonio River at .the mouth
Table 3. Maximum stages in San Antonio River Baain resulting from storm of September 26-27, 1946

Location

Olmos Reservoir at San Antonio ......

San Antonio River at Geological Survey 

, gage at San Antonio. ..........

San Antonio River at Geological Survey 

gage near Falls City ..........

San Antonio River at Geological Survey

San Pedro Creek at intersection of Flato 

Street and HcKaskill Ave. in San Antonio 

Alazan Creek 600 ft. below oenterllne of

Alazan Creek at oenterllne of San Fernando

Mart Inez Creek at centerllne of Woodlawn 

Ave., in San Antonio ..........

Apache Creek at oenterllne of Sabinas 

Street in San Antonio. .........

Salado Creek at Ritterman Road crossing,

Clbolo Creek at Geological Survey 

gage near Falls City ..........

Latitude

o , »

OQ OR OO

OQ OA. "^K

28 57 05

23 39

29 23 30 

OQ oa i R

OQ OK 1 O

OQ an i K

29 24 50

OQ OQ f)R

OQ O1

Longitude

o , »

QQ OO Of*

QQ pa 4.0

98 03 55

on 0*

98 30 06

QQ *"| »/"!

98 SO 30

98 31 16

98 31 20

98 25 00

97 66

T

Sept

Sept

Sept

Oct.

Sept

Q*hM4k

Sept

Sept

Sept

Sept

Sept

ime

. 27

yrt

, 29

. 27 

an

9*7

tyn

. 27

P7

. 28

of crest

,, 2. a.m.

10 p,m.

, about 2 a,m

1 0 0 «

,1-2 a.m.

,1-8 a.m.

,1-2 a.m.

, 3:30 p.m.

Altitude 

above mean 

sea level

Feet 

712

627.6

319.3

133.8

607.1

a co o

640.8

682.5

640.9

666.8

286.8

of Medina River was sufficiently high to cause 

6 feet of backwater .at the gage on Medina River 

5*2 miles upstream from its mouth, as determined by a current-meter measurement made of 

Medina River at the crest of the flood. Because of this backwater, the IM>,XJ.IPW" stage at 

the Medina River gage was 2*3 feet higher than 

the stage during the flood of Aug. 29, 1946, 

but the maximum discharge was considerably 

less.

OLMOS DAM

Following the flood in September 1921 the 

Olmos Dam was built to provide a reservoir for 

flood control on Olmoa Creek, the main tributary of San Antonio River above San Antonio* 

Its capacity according to the capacity curve 

is 15,500 acre-feet at elevation 728 feet, 

mean sea level, which is top of dam.3/ The 

dam is provided with, six gates, the minimum 

dimensions of the outlet tunnel from each 

gate being 5 feet 9 inches wide and 7 feet 

10 inches high. The elevation of the bottom 

of the gates is 679.53 feet.4/ The gates are 

left open and the reservoir empty except during periods of flood runoff when the gates 

are closed In order to reduce the flood crest 

in San Antonio River as it passes through the 

city. The operation of the gates during the 

flood of September 1946 was the responsibility 

of the San Antonio Fire Department.

The gates In Olmos Dam are reported by city 

officials to have been closed at the beginning of the storm on Sept. 26, 1946, storing 

the water to a maximum elevation of 712 feet 

above mean sea level. This represents a max-

"3Crecellus, S^. P., Report of investigations 

preliminary to the design of Olmos Creek 

Detention Dam, San Antonio, Tex., 1924.

4 Plans for Olmos Creek Detention Dam, 

Office of Flood Prevention Engineers, 

San Antonio, Tex., July 1925.

imum storage of 4,600 acre-feet, according 

to the capacity curve previously mentioned. 

Release of water stored in Olmos Reservoir 

was begun during the morning Sept. 27, and 

city officials reported that releases were 

completed by late afternoon Sept. 28. Released water began reaching the gage on San 

Antonio River at South Alamo Street bridge at 

8:30 a. m. on the 27th in sufficient volume to 

check the rather rapid recession of the stream 

then in progress and cause a rise in gage 

height from 7.35 feet to 9.45 feet at 11 a. m. 

The stage then fell except for minor fluctuations until essentially all released water 

had passed the gage by midnight on the 28th. 

A computation of the amount of water released 

from Olmos Reservoir on the basis of the continuous record of the flow of the river at 

the gage shows a total of 4,470 acre-feet 

which compares favorably with, the 4,500 acrefeet shown stored In the reservoir by the 

capacity curve at maximum elevation of 712 

feet. These data are shown graphically in 

figure 4.

Unfortunately for the purpose of a complete 

study of the flood flow from Olmos Creek, no 

records of the rise and fall In the water 

stored in Olmos Reservoir were obtained, nor 

are any complete records of gate operations 

available. Had a continuous record of the 

rate of rise in Olmos Reservoir been available, 

a complete hydrograph of the flow of Olmos 

Creek into the reservoir could have been developed. This hydrograph would have supplied 

data needed for a better study of the effectiveness of the reservoir in reducing the 

flood crest in the San Antonio River as it 

passes through the city. A complete record 

of the gate openings during the time the water 

was being released from the reservoir or a 

record of the rate of fall of the water level 

in the reservoir would have supplied data 

needed to compute a hydrograph of the released 

water.
10

Peak 5,740 sec. ft

Shaded area shows 

from Olmos Reservoir 

to 4,470 acre-feet

released 

amounting

"20QQ 

a

Estimatec recession hydrograph of 

runoff below Olmos Dam 

6-p.m. 6 a.m. ' 6p.m.

Sept.26,1946

Figure 4.*--Graph of discharge at-rftreftffi'-gaging station on San Antonio River at South Alamo Street

crossing in San Antonio.
1*

MEASUREMEHTS OP PEAK DISCHARGE

In addition t<rt'he complete record of flow 

obtained at the gage on the San Antonio River 

at San Antonio* measurements of maximum discharges only were obtained by slope-area 

method at seven places on streams in and near 

San Antonio. Results; of these measurements 

are given in table 4 and'the locations are 

shown in figure 2. Conditions for these 

measurements were considered fairly good on 

San Fedro and Calavera* Creeks and only fair 

at the other sites.

Measurements of the maximum discharge of 

the flood of 1921 on San Pedro, Alazan and 

'Apache Creeks are given in the report cw that 

flood .§/ These measurements were made at or 

near the sites of the measurements made in 

1946 and uniformly show a much greater discharge. As these measurements were made by 

individuals not employed by the Geological 

Survey, the base data are not available for 

comparison with the data obtained in 1946. 

Certainly the discharge of these streams in 

September 1946 was not nearly so great as 

that shown for 1921.

The flood in Salado Creek in September 

reached about the same stage as the flood in 

September 1921 at the place of measurement 

at Ritterman Road oreasing about 1 mile 

downstream from U. S. Highway 81, according 

to information from local residents. The 

flood of 1913, th» greatest known by residents of this vicinity since about 1853, 

reached a stage 5 or 6 feet higher than that 

reached by the flood in 1946.

The flood of 1946 on Calaveras Creek was 

particularly outstanding. Residents who had 

lived near the site of measurement for 60 

years stated that this flood^as much higher 

than known before. Buildings many years old 

were washed away* The computed maximum discharge of 58,000 second-feet was produced 

by an area of 24*6 square miles and amounted 

to a discharge of 2,360 second-feet per 

square mile. The rainfall over this basin 

varied from about 10 inches to 17 inches with 

an average of about 14 inches. The maximum 

rainfall measured for this storm was 17.22 

inches at a place about,2 miles upstream from 

site of measurement of maximum discharge. It 

should be kept in mind that most of this rain 

fell in a period of 6 to 8 hours.

Table 4. Maximum discharges of flood of September 27, 1946* in streams 

in the vicinity of San Antonio ,

Ho. on 

fig. 2

1

2

3

4

5

6

7

-,

8

Streams *

San Antonio River at

gage at South Alamo

Street bridge

San Pedro Creek,

1500 ft. above mouth

Alazan Creek above

Mart Inez Creek, between Zarzamora and

Navidad Sts.

Alazan Creek below

Mart Inez Creek, between San Luis and

El Paso 'Sts.

Mar tine z Creek, between Huisache and

Woodlawn Ave.

Apache Creek, between

Navidad and Trinity

Sts.

Salado Creek near

San Antonio, 1 mile

, below U, S. Highway

81 crossing

Calaveras Creek near

Elmendorf , 4,000 ft.

below Hondo Creek

latitude

0 t N

29 24 55

29 23 30

29 26 15

29 25 10

29 27 15

29 24 50

29 29 05

29 18 00

Longitude

0 i «

98 29 40

98 30 05

98 31 30

98 30 30

98 31 15

98 31 20

98 25 00

98 19 40

Width

(feet)

-

554

_

392

39O

377

431

566 ^

867

Area

(So. ft)

.

3,130

1,430

2,060

1,170

2,210

5,680

9( ,060

Mean 

velocity

(fpa)

7,26

4.12

5.05

3.38

3.80

8.80

6.40

Discharge

(ofs)

5,740

22,700

5,900

10,400

3,950

8,400

50,000

58,000

Drainage 

area

(Sq miles}

b 9.4

44.5

1

8.8

17.2

6.3

21.5

161

24.6

Runoff

(ofs per 

sq mile)

611

510

 ,

,

670

605

6S7-

391

311

2,360

a Names of streams agree with those given in Water-supply Paper 488, p. 8, !ghe_Floods it 

Central Texas in September* 1921, by C. E. Bllswort%, and supersede those given"on u< 

Geological Survey Topographic hap, San Antonio Quad., 1901-1903.

b Area between Olmos Dam and the gage.

Note.- All discharges, except that for San Antonio .River, were measured by slope-area 

method. Figures for width, area, and mean velocity were obtained by averaging two cross 

sections. Discharge for San Antonio River was obtained from rating curve extended above 

2,200 seoond-fe&t.

S.

5Sllswqrth, C. E., The Floods in Central 

Texas in September 1921; U. S. Oeol. 

Smrvey Wattr*fihi$>#ly Paper 480, p. 43.
45

Peak 47.400

Peak 29,40

Medina River near 

San Ant.onio,Tex. 

Peak 24,800

Antonio River

near Falls Gity.Tex.

at Goliad.Tex.

/-Giboto Greek near 

/ Falls Gity,Tex 

*"- Peak 10,

Son Antonio River I 

at San Antonio

5.-'-Graphs of dlacliarge at various atjream-gaging stations in San Antonio Ri^er B
13

STAGES AND DISCBA8G1® A£ GAGIHG STATIONS "XX SAJ« ASJOHIO RIVER BASIN

Oa the following page* are presented stage and dlsehaiese reo«rd« *t all gaging stations In San 

Antonio Hlver Ba*in particularly affected by this storm* $he*e re*o*a* consist of « station 

description, a table of dally mean discharges and daily wad total ruaoff for the flood period, 

and a table of discharge* at indicated tinea during the. «.ood in sufficient detail for a reasonably reliable delineation of the hydrograph.

The tables snowing stag* and discharge at indicated ttiates are designed to present the rise and 

recession of the flood in detail, as the rate of rise aatt fall is usually so rapid that dally 

mean values do not define the hydrograph adequately* HjOrographs of discharge plotted from these 

tables are shoim in figure 5. ,

San Antonio River at Saa. Antonio

Location.- Lat. 20°24 I 35", long. 98°29'40", at South Alaaw. £& ** &**&».'*» Sam Antuaie, Baxar 

County, 2.1 miles upstream from San Pedro Creek. Datwi of gage la 612.3 feet above mean sea 

level, datum of 1989.

Drainage area*- 42 square miles (1946 revision). 

Gage-height record.- Water-stage <reeorder graph.

Discharge record.- Stage-discharge relation defined by cttrreret-meter measurements up to 2,200 

second-feet and extended above to 5,740 second-feet. Oage heights used to half-tenths between 

2.7 and 4.9 feet; hundredths below and tenths above tfeess llaalts*

Maxima,- 1946: Discharge, 5,740 second-feet 2 a.' m. Sept* 27 (gage height, 15.32 feet)*

1915-29, 1959-46* Discharge, 15,300 second-feet Sept. 10, 1921 (gage height, 20,14 feet, 

from floodmark), by slope-area method.

Flood of July 5, 1819 equalled or exceeded that of Sept* 10, 1921.

Remarks.- Olmos Reservoir (eapacity, 15,600 acre-feet) about 8% miles above station stored all 

 flood water, amounting to 4,600 acre-feet, during night of Sept. 26. This water was released 

Sept* 27-28, after crest from natural runoff below dam had passed.

Mean discharge. In second-feet, and runoff, In acre-feet, 1946

Day

September 

26 27' 

28 

29 

30

Secondfeet

330 

2,690 

1,370

180 

156

Runoff, Sept. 26-30

Acrefeet

665

5,340 

2,720 

357 

313

9,380

Gage-height, in feet, and discharge, in second-feet, at indicated time, 1946

Hour

Sept. 26

1 a.m«

4

11

3 p.m.

7

8:30

9

10

11

12

Sept, 27

1 a.m. 

2

3

4

5

Feet

4.10

2.68

1.87

1.73

1.74

2.10

2.90

6.50

8.16

11.90

14.50 

15.32

14.75

14.05

12.65

Second- feet

514

217

99

81

82

130

254

1,200

1,800

3,580

5,220 

5,740

5,410

4,890

4,000

Hour

Seat. 27 

Con.

6 a.m.

7

8:30

10

11

1 p.m.

3

5

7

9

Sect. 28

1 a.m.

6

11:30

2 p.m.

Feet

10.95

9.00

7.37

8.85

9.42

9.10

8.95

9.14

8,65

8.33

8.07

7.78

7.45

7.64

Second-feet

3,, 060

£,100

1,500

2,020

0,280

,2,140 2:, 100

7,140

1,950

1.830

1,760

1,640

1,500

1,570

Hour

Sent. 28 

Con.

4 p.m.

5t30

7

8

9

10

Sept. 29

1 a.m.

8s30

11

8 p.m. 

11

Sept. 30

6 a.m.

12 p.m.

Feet

7.47

7.66

6.58

5.15

 3.85

3.22

2.65

2.50

2.39

2.35 

2.36

2.32

2.28

Second-feet

1,530

1,610'

1,230

810

454

310

, 212

188

171

165 

166

161

155
14

San Antonio River near Falls City.

Location.- Lat. 28°57'05% long. 98°03'55 W , at highway bridge 0.9 mile upstream froa Scared Dog 

 Creek and 3.6 miles southwest of Palls City, Karnes County. Datum of gage is 285.5 feet above 

mean sea level, datum of 1929.

Drainage area." 2,071 square miles (1946 revision).

Gage-height record.- Water-stage recorder except for Sept. 26, 1946, when record was uncertain 

and Sept. 29, when recorder was submerged and record obtained by levels to floodmarks and to 

stakes set at intervals by gage observer.

Di a charge record,.- Stage-discharge relation defined by current-meter measurements. Gage heights 

used to half-tenThs between 3.0 and 4.4 feet; hundredths below and tenths above these limits.

Maxima.- 1946: Discharge, 47,400 second-feet 9 a. n. Sept. 29 (gage height, 33.80 feet).

1925-46: Discharge, 18,500 second-feet July 6, 1942j gage height, 28.3 feet June 13, 1935, 

affected by backwater.

Flood of October 1913 reached a stage of 28.36 feet, from information supplied by local 

residents.

Remarks,- Flow at high stages affected by natural storage in wide flood plain. 

Mean discharge, in second-feet, and runoff, in acre-feet, 1946

Day

September 

26 

27 

28 

29 

30

Seeoadfeet

552 

1,830 

21 ,100 

42,200 

21 .300

Runoff, Sept

Aerefeet

1,090 

3,630 

41,850 

83,700 

42,250

. 26 to 0

Day

October 

1 

2 

3 

4

ct. 4 . . .

Secondfeet

5,340 

1,380 

1,040 

925

Acrefeet

10*590 

2,740 

2,060 

1,830

189 ,7O6

Gage-height, in feet, and discharge, in second-feet, at indicated time, 1946

Hour

Sept. 26

1 a.m.

1 p.m.

Sept. 27

1 a.m.

6

12 m.

4 p.m.

8

10

12

Sept. 28

2 a.m.

4

6

8

10

12 m.

2 p.m.

4 

6

8

101-2

Feet

1.85

2.05

2.43

2.66

3.05

3.50

4.70

5.65

7.70

10.50

13.60

16.40

18.70

20.45

21.90

23.85

26.55 

28.95

30,35

31.20

32.05

Second-feet

424

546

798

973

1,320

1,760

2,980

3,920

5,840

7,960

10,600

13,200

15,500

17 ,200

18,700

. 20,900

25,200 

30,300

34,200

36 ,700

39 ,500

Hour

Sept. 29

2 a.m.

4

6

8

9

10

12 m.

2 p.m.

4

6

8

10

12

Sept. 30

2 a.m.

4

6

8

10

12 m. 

2 p.m.

4

6

8

10

Feet

32.90

33.50

33.72

33.78

33.60

33.78

33.66

33.30

32.60

31.70

31.00

30.25

29.40

28.60

27.80

26.95

26.03'

25.00

24.00 

22.75

21.40

19.90

18.45

17.00

Second-feet

43,200

46,000

46,900

47,400

47,400

47,400

46,400

45,000

42,000

38,400

36,000

33,600

31,400

29,400

27,600

25,900

24,100

22 ,600

21,200 

19,700

18,200

16,700

15,200

13,800

Hour

Sect. 30 

Con.

12 p.m.

Pet. 1

2 a.m.

4

6

8

10.

12 m.

4 p.m.

8

12

Oct. 2

6 a.m.

12 m.

6 p.m.

12

Pot. 5 

12 m.

12 p.m.

Oct. 4

12 m.

12 p.m.

Feet

15.45

13.90

12.40

10.65

8.75

7.10

5.88

5.05

4.00

3.65

3.28

3.07

2.93

2.87

2.72

2.68

2.60

2.54

Second- feet

12,200

10,900

9,540

8,040

6,690

5,340

4,220

3,300

2,260

1,910

1,560

1,320

1,210

1,150

1,020

989

925

. 880
15

San Antonio River at

Location.- Lat. 28°39', long. 97°23', at bridge on State Highway 29, 1.3 mile a aoutbeaat of 

courteous* In Goliad, Goliad County, and 10 miles upatrcam from ManahuiUa 0*eefc f Datum of 

gage is 91*1 feet above mean sea level, datum of 1929, Houston supplementary adjustment of 1943.

Drainage area.- 3,918 square miles (1946 revision). 

Page-height record.- Water-stage recorder graph.

Discharge reeor<l.- Discharges obtained from loop curves defined by current-meter measurements. 

Gage heights used to half-tenths between 3.7 and 4.8 feet; hundredths below and tenths above 

these limits.

Maxima.- October 1946: Discharge, 29,400 seoond-feet at 10 p. m. Oct. 2 (gage height 42.67 feet), 

1924? 28, 1939-46: Discharge, 33,800 seoond-feet July 9, 1942 (gage height 44.9 feet). 

Floods of October 1913 and June 15, 1935, reached about the same stage as that of July 9, 

1942.

Remarks.- Plow at high stages affected by natural storage in wide flood plain. 

Mean discharge, in second-feet, and runoff, in acre-feet, 1946

Day

September 

26 

27 

28 

29 

30

Seoondfeet

820* 

1,440 

3,760 

6,490 

10 ,300

Acrefeet

1,630 

2,860 

7,460 

12,870 

20 .430

Day

October 

1 

2 

3 

4

Seoondfeet

17,000 

27,800 

25 ,600 

12,500

Acrefeet

33,780 

65,140 

50,580 

24,790

Day

October 

Con. 

5 

6 

7

Seoondfeet

2,510 

1,590 

1,460

Runoff, Sept. 26 to Qet. 7 .......

Aorefeet

4,980 

3,150 

2,900

220,500

Gage-height, in feet, and discharge, in seeond-feet, at indicated time, 1946

Hour

Sect* 26

6 a.m. 

12 m. 

3 p.m. 

6 

9 

12

Sept. 27

3 a.m. 

6 

9 

12 m. 

2 p.m. 

4 

6 

8 

10 

12

Sept. 28

4 a.m. 

8 

12 m. 

4 p.m. 

8 

12

Sept. 29

4 a.m. 

8 

12 m. 

4 p.m.

Feet

6.33 

6.23 

6.36 

6.65 

7.48 

8.40

9.06 

9.32 

9.06 

8.65 

8.60 

9.00 

9.80 

10.80 

11.80 

12.66

14.20 

15.65 

17.05 

18.12 

19.10 

20.04

20.98 

21.93 

22.82 

23.80

Second-feet

725 

706 

744 

782 

763 

1,160

1,330 

1,370 

1,330 

1,210 

1,210 

1,300 

1,490 

1,750 

2,030 

2,260

2,680 

3,080 

3,870 

4,320 

4,700 

5,080

5,480 

5,920 

6,360 

6,900

Hour

Sept. 29 

Con. 

8 p.m. 

12

Sept. 30

4 a.m. 

8 

12 m. 

4 p.m. 

8 

12

Pet. 1 

4 a.m. 

8 

12 m. 

4 p.m. 

8 

12

Oct. 2 

4 a.m. 

8 

12 m. 

6 p.m. 

10 

12

Oct. 3 

6 a.m. 

12 m.

Feet

24.86 

25.92

27.00 

28.02 

29.00 

29.92 

30.70 

31.50

32.35 

33.30 

34.55 

36.05 

37.60 

39.00

40.20 

41.15 

41.85 

42.50 

42.67 

42.65

42.30 

41.63

Second-feet

7,520 

8,170

8,920

9,620 

10,400 

11,100 

11,900 

12,800

13,600 

14,800 

16,200 

18,500 

21,000 

23,400

25 ,400 

27,6QO 

28,900 

29 ,300 

29 ,400 

29,200

27,600 

25,900

Hour

Oct. 3~

Con. 

4 p.m. 

8 

12

Oct. 4

4 a.m. 

8 

12 m. 

4, p.m. 

8 

12

Pet. 5 

4 a.m. 

8 

12 m. 

4 p.m. 

8 

12

Oet T 6 

6 a.m. 

12 m. 

6 p.m. 

12

Oct. 7 

12 m. 

12 p.m.

Feet

40.95

40.10 

38.90

37.50 

35.80 

33.20 

30.36 

27.20 

23.86

20.28 

16.90 

14.34 

12.70 

11.79 

11.37

10.95 

10.64 

10.39 

1O.18

9.95 

9.81

Seoond-feet

24,300 

22,600 

20,200

17,80O 

15,400 

12,200 

9,750 

7,450 

5,330

3,620 

2,600 

2,100 

1,860 

1,750 

1,680

1,630 

1,580 

1,650 

1,500

1,460 

1,440
16

Medina River near San Antonio

Location,- Lat. 29°15', long. 98°28', at bridge on U, S. Highway 281, 5.2 miles upstream freta 

mouth, and 9 miles south of San Antonio, Bexar County.

Drainage area.- 1,225 square miles (587 square miles is above dam forming Medina Lake). 

Gage-height reoord.- Water-stage recorder graph.

Discharge record,- Stage-discharge relation defined by current-meter measurements. Discharges 

obtained from loop curve above 3,000 second-feet because of backwater from San Antonio River. 

For balance of period gage heights used to half-tenths between 3.3 and 5.4 feet, hundredths 

below and tenths above these limits. Shifting-control method used Sept. 26-27, below 3,000 

second-feet.

Maxima.- September 1946: Discharge, 24,800 second-feet 4 to 5 p. m. Sept. 27 (gage height, 

41.57 feet, affected by backwater from San Antonio River.)

1939 - August 1946: Discharge, 31,800 second-feet Aug. 29, 1946 (gage height, 39.23 feet). 

Maximum stage known, about 55 feet in 1912 (prior to construction of Medina Dam, 60 miles 

upstream), from information by State Highway Department.

Remarks.- All flow during this flood period from runoff below Medina Dam. 

those above 3,000 seeond-feet which are fair.

Records good except

Mean discharge, in second-feet, and runoff, in acre-feet, 1946

Day

September 

26 

27 

.28 

29

Secondfeet

126 

15,900 

5,730 

756

Runoff, S

Acrefeet

 250 

31,540 

11,370 

1,500

ept. 26 t

Day

September-- 

Con. 

30 

October

1 

2

:> Oct. 2 .- .

Secondfeet

414

321 

268

Aereff«t

821

637 

532

46,650

Gage-height, in feet, and discharge, in second- feet, at indicated time, 1946

Hour

Sept. 26

4 a.m.

7

9

11

12 m. .

4 p.m.

9

10

12

Sept. 27

1 a.m.

2

3

4

5

6 

7

8

9

10

11

Feet

3.22

3.26

3.75

4.30

4.27

3.62

3.36

3.67

5.80

6.74

6.87

10*00

11.50

18.00

25.20 

31.50

35.50

37.35

38.23

38.70

Second- feet

87

90

130

185

185

120

98

124

378

541

558

1,130

1 ,460 .

3,040

7,400. 

11,500

14,400

16 ,400 '

17,700

18,500

Hour

SeDt. 27 

Gon.

12 m.

1 p.m.

2

4

5

6

8

10

12

Sept. 28

2 a.m.

4

6

8

10 

12 m.

2 p.m.

4

6

8

Feet

39.05

40.25

40.95

41.55

41.57

41.47

40.80

39.05

36.00

32.50

28.90

24.90

21.65

19.35 

17.60

16.06

14.80

13.77

12,82

Second-feet

19,100

21,500

23,300

24,800

24,800

24,700

24,200

22 ,900

20,200

17,000

13,200

8,350

5,050

3,450 

2,680

2,240

1,960

1,690

1,450

Hour

Sent. 28 

Con.

12 p.m.

Sept. 29

6 a.m.

12 m.

5 p.m.

6>

12

Sept.. 30

6 a.B.

12 m.

6 p.m.

12

Oct. 1 

12 m.

12 p.m.

Oct. 2

12 m.

12 p.m.

Feet

11.58

10'. 17

9.37

8.95

8*80

8.17

7.81

7.48

7.23

7.12

6.89

6.67

6.47

6.35

Second-feet

1 ,170

870

722

645

620

512

456

405

370

352

321

294

268

254
17

Cibolo Creek near Fallg City .

Location.- I*t* 29°01», long. 97°66», at bridge on State Highway 123, 6.6 miles northeast of 

Palls City, Karnes County, and 9 miles upstream from mouth. Datum of gage IB 864.3 feet above 

mean sea level, datum of 1929, Houston supplementary adjustment of 1943.

Drainage area,- 831 square miles*

Page-height record.- water-stage recorder graph,

Diaobarg ord.- Stage-discharge relation defined by current-meter measurements. Oage heights 

used to naif-tenths between 2.8 and 4.8 feet; hundredths below and tenths above these limits.

Maxima.- September 1946: Discharge, 10,400 second-feet 3:30 p. m. Sept. 28 (gage height, 22.52 

feet ) . : 

1930 - August 1946: Discharge, 33,600 second-feet July 6, 1942 (gage height, 34.4* feet). 

Flood in October 1913 reach a stage about one-half foot.higher than that of July 6, 1942.

Remarks.- The upper part of the Cibolo Creek Basin, having an area of about 280 square miles, 

does not produce much surface runoff because much of the surface flow enters sink holes and 

caverns in the Glen Rose and Edwards limestones*

Mean discharge, in second-feet, and runoff, in acre-feet, 1946

Day

September 

26 

27 

28 

29 

30

Secondfeet

872

2,970 

8,760 

3,640 

601

Runoff, Sep

Acrefeet

1,730 

6,890 

17,380 

7,020 

994

b, 26 to

Day

October 

1 

2 

3 

4

Jot, 4. . .

Secondfeet

282 

187 

146 

124

Acrefeet

659 

371 

288 

246

34,480

Page-height, in feet, and discharge, in second-feet, at indicated time, 1946

Hour

Sept. 26

2 a.m.

3

6

8

9

10

12 m.

2 p.m.

4

6 

8

10

12

Sept. 27

2 a.au

3

4 

5

6

8

10

11

12 m.

2 p.m.

4

6

8

10

Feet

2.15

2.17

1.99

1.97

2.00

2.60

4.80

6.65

8.00

9.10 

10.05

10.85

11.45

11.82

11.86

11.81 

11.69

11.40

10.31

9.25

9.05

10.25

11.75

12.90

13.80

15.52

16.16

Second-feet

100

102

84

82

86

150

510

926

1,320

1,680 

1,990

2,280

2,510

2,670

2,710

2,670 

2,630

2,510

2,100

1,710

1,650'

2,060

2,670

3,110

3,480

4,320

4>750

Hour

Sept. 27 

Con.

12 p.m.

Sect. 28

2 a.m.

4

6

8

10

12 m. 

2 p.m.

3

4

6

8

10

12

Sept. 29

2 a.m.

4

6

8

10

12 m.

2 p.m.

4

6

9

Feet

17.02

17.93

18.90

19.87

20.78

21.58

22.13 

22.45

22.52

22.52

22.35

21.97

21.33

20.50

19.57

18.62

17.52

16.20

14.63

12.82

10.65

8.45

7.38

6.49

Second-feet

5,300

5,930

6,760

7,700

8,600

9,460

10,000 

10,300

10,400

10,400

10,300

9,900

9,130

8,300

7,400

6,490

6,650

4,750

3,840

3,070

2,210

1,460

1,140

900

Hour

Sept, 29 

COB.

12 p.m.

Sept. SO

6 a.m.

12 m.

6 p.m.

12,

Oct. 1 

12 m.

12 p.m.

Oct. 2

12 m.

12 p.m.

Oct. 3 

12 m.

12 p.m.

Oct. 4

12 m.

12 p.m.

Oct. 5

'I? m*

9 p.m.

12

Feet

5.95

5.21

4,62

4.25

4.00

3.63

3.17

2.92

2.72

2.56

2.42

2.37

2.28

2,28

2.23

2.69

Second-feet

780

594

470

405

360

282

221

187

164

145

129

'

124

114

114

88

161

The drastic reduction in peak discharge at Qoliad from that at Falla City was due to temporary 

storage in wide flood plain between these stations. San Antonio River also has a wide flood plain 

above Falls City., The rainfall), pattern indicates that the maximum discharge in the river probably 

occurred just below the mouth of Calaveras Creek, about 30 miles above Falla City, and probably 

was greatly in excess of the 47,400 seoond«"fe<et measured at Palis City.
18

RAINFALL AND RUNOFF

Analyses of the rainfall for the night of 

Sept. 26, 1946, and the runoff records have 

been made for. three areas t Olmoa Creek above 

Olmos Dam, San Antonio River Basin between 

Olmos Dam and the gage at South Alamo Street 

bridge, and San Antonio River Basin above the 

gage near Falls City but excluding the basin 

of Medina River above the gage near San 

Antonio. These analyses have been made for 

the purpose of showing the relation between 

storm rainfall and the resulting direct runoff. The differences between rainfall and 

runoff give an indication of the volume of 

water retained in the drainage basins; this 

retention is an important factor in flood 

control and other problems*

For the purpose of this study the rainfall 

during the night of September 26 and the 

resulting direct runoff were used. The total 

runoff shown in the discharge tables for San 

Antonio River at Saa Ant©Hie *ad near Falls 

City was adjusted for antecedent flood runoff 

and base flow. The total rainfall for the 

Olmos Creek Basin and for the San Antonio 

River Basin above San Antonio was measured 

on the isohyetal map (fig. 2). The rainfall 

on the basin above Falls City'gage was 

measured on the map in figure 3.

Records at the San Antonio Weather Bureau 

station show that a rainfall of 3.27 inches 

fell in late afternoon September 25. This 

rain produced only small rises in the streams 

but no doubt conditioned the soil to cause 

greater maximum discharges and total runoff 

from the rain that began about 26 hours later.

The rainfall for the night of Sept. 26, 

1946, over Olmos Creek Basin above Olmos Dam 

varied from about 6 to about 10 inches, and 

the average was 7*4 inches. Runoff from this 

area of 32.4 square miles, as obtained from 

capacity curve for Olmoa Reservoir, was 4,500 

acre-feet, or a depth of 2*6 inches* This 

shows that the retention averaged 4.8 inches* 

Olmos Creek Basin is 11 miles long above 

Olmos Dam with an average width of slightly 

less than 3 miles. It heads in the southern 

edge of the Edwards Plateau at an elevation 

of about 1,050 feet. Elevation of the stream 

bed at Olmos Dam prior to construction of the 

dam was about 670 feet. Tb» upper part of the 

basin is crossed by a number of faults in the 

geological structure which may permit the 

entrance of some surface flow into underground reservoirs. The area ia entirely 

rural except for a very small part just above 

the dam. A small part of the area is in cultivation but the much greater part is in 

pasture land and covered with brush and timber. 

The topography is rolling to hilly.

The rainfall on San Antonio River Basin between Olmos Dam and the gage at South,Alamo 

Street bridge varied from about 10 to about 

11 inches and the average was 10.5 inches. 

Storm runoff from this area of 9.4 square 

miles was 3,520 acre-feet, as measured at the 

gage, representing total flow Sept. 26-30 of 

9,380 acre-feet (less 4,470 acre-feet measured 

as water released from Olmos Reservoir, and 

less 1,390 acre-feet antecedent flow and base 

flow frpm springs), The net runoff amount® 

to a depth of 7.0 inches, leaving a retention 

of 3.5 inches. This is largely aa urban ar«a

and includes a large part of the main business 

section of San Antonio. The elevation of the 

stream bed at Olmos Dam Is about 670 feet and 

at the gage, 612 feet. The distance between 

these places, following the meanderings of the 

channel, is 8& miles*

The average rainfall over the San Antonio 

River Basin above Falls City, exclusive of 

the area in the Medina River Basin above gage 

near San Antonio* was 8.7 inches. Storm runoff from this area of 846 square miles aa 

measured at the gage near Falls City for the 

period September 26 to October 4, 1946, waa 

134,000 acre-feet, equivalent to a depth of 

3.0 Inches, leaving a retention of 5.7 inches. 

Within this basin the rainfall was 16 Inches 

or more over an area of 105 square miles, more 

than 12 Inches over 268 square miles, more 

than 8 inches over 409 square miles, and more 

than 4 Inches over 690 square miles. The 

£v*s*ge animal: r«i*fail ov»r ttal» biwts is 

about 28 inches* The headwater streams originate in and drain some 60 square miles of 

the Edwards Plateau. They are crossed by a 

number of fault zones in their upper reaches 

where part of the surface flow may be taken 

into* underground reservoirs. However, possible diversion,of surface flow would have 

very email effect on the rainfall-runoff 

relation under study as nearly all of the 

rain fell below the faulted zone.

DAMAGES 

The direct damages that were caused by the 

flood of September 1946 on San Antonio River 

and its tributaries are estimated at $3,636,330 

as shown below:

Within' City of San Antonio

San Antonio River..$ 117,700 

Martlnez, Alazan,

Apache, and San

Pedro Creeks...... 2.488.630

$2,606,330

Outside City of San Antonio

San Antonio River. 

Tributaries...%...

Total direct damages

880,000 

1SO>000,

1,030,000

13,636,330

The above figures do not include indirect' 

damages nor damages due to ponded rainwater 

caused by Inadequate storm sewers. Indirect 

damages include losses resulting from the 

Interruption of traffloe and communications, 

the interruption to normal business and 

commerce , wage loss , and the relief and care 

of flood victims and their rehabilitation*

Map showing 1946 and 1921 flooded areas 

la Ban Ant onfo"

A map of the city of San Antonio, figure 6, 

is included in this report to show the extent 

of the flood plains of the floods of Sept, 10, 

1921, and Sept. 27, 1946. The flooded area 

in 1921 is outlined only within the original 

city limits , whereas the area for 1946 is 

outlined for some distance upstream and downstream. It will be noted that in 1921 the 

flood- waters of tb* San -.Antonio River covered 

a much greater area than in 1946.
19

Figure 6. Map of San Antonio, showing 1946 and 1921 flood plains.