GEOLOGICAL SURVEY CIRCULAR 365

A FLOOD-FLOW FORMULA FOR 

CONNECTICUT
A FLOOD-FLOW FORMULA FOR CONNECTICUT

By B. L. Bigwood and M. P. Thomas

GEOLOGICAL SURVEY CIRCULAR 365

Washington, D C., 1965

Free on application to the Geological Survey, Washington 26, D. C.
United States Department of the Interior

STEWART 1. UDALL, SECRETARY

Geological Survey

THOMAS B. NOLAN, DIRECTOR

REPRINTED 

1963

Free on application to the U.S. Geological Survey, Washington 25, 0. C.
PREFACE

Recognizing that a flood-magnitude-and-frequency 

relation for Connecticut streams, based on actual flood 

events recorded at gaging stations on typical streams 

within the State or closely bordering territory, would 

in all likelihood provide reliable estimates of flood 

probabilities for ungaged sites on State streams, the 

State Board of Supervision of Dams and the State Highway Department, in cooperation with the United States 

Geological Survey, have sponsored the study described 

in this report. The investigation is especially needed 

in connection with regulative functions of the Board of 

Supervision of Dams and for purposes of hydraulic design in the State Highway Department. The derived 

flood-flow formula also might be expected to answer 

design needs of private engineers and consultants 

throughout the State and, as well, those outside the 

State who might have interest in Connecticut hydraulic 

projects.

A formula was envisioned which would incorporate 

drainage -basin characteristics that could readily be 

scaled from standard topographic maps. A highly desired attribute of the formula would be simplicity in 

form and application. Involved analysis, resulting in 

a profusion of terms having but little weight in final 

results, was to be avoided. In brief, a formula of few 

and simple terms, giving results of reasonable practical accuracy, was the goal. These desired general 

requirements influenced the derivation of the formula 

set forth in this study.

The U. S. Geological Survey has planned a nationwide flood-frequency study, which is to be issued separately for sections of the country corresponding to the

parts in the annual surface water series of the watersupply papers. The first one to be published will be 

that for part 1A, comprising virtually all New England, 

including Connecticut. This first volume is now in 

preparation but will not be forthcoming immediately; 

because of the nature of statistical studies, it may lead 

to a different form of analysis than is developed here.

Cooperating State agencies, namely, the State Board 

of Supervision of Dams under the chairmanship of Mr. 

Richard Martin, succeeded by Mr. William S. Wise; 

and the State Highway Department, Mr. William J. Cox, 

Commissioner, succeeded by Dr. G. Albert Hill, rendered most valuable assistance. Acknowledgments are 

due also to individual members and staff personnel of 

the Board of Supervision of Dams and to the engineering 

staff of the State Highway Department for helpful cooperation.

The basic flood discharge records and some other 

hydro logic data used in this study have been collected 

and compiled largely under the continuing surfacewater investigational programs maintained--in Connecticut, in cooperation with the State Water Commission, 

the Hartford Department of Public Works (succeeding 

the Hartford Flood Investigation & Improvement Commission), the New Britain Board of Water Commissioners, and other agencies; in New York, in cooperation with the Westchester County Department of Public 

Works; and in Massachusetts in cooperation with the 

State Department of Public Works.
CONTENTS

	Page 

Abstract ........................................................................................... 1

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

Method of analysis .................................................................................. 2

Flood magnitude and frequency relation ............................................................... 2

Influence of topographic characteristics ............................................................... 11

Summary of procedure............................................................................... 13

Example of application of formula .................................................................... 15

Literature cited..................................................................................... 16

ILLUSTRATIONS

Page

Figure 1. Map of Connecticut showing location of gaging stations used .................................. 3

2. Chronological record of gaging stations .................................................... 8

3. Flood magnitude and frequency curve, ratios to mean annual flood ............................ 11

4. Relation of mean annual flood to drainage area.............................................. 13

5. Relation of mean annual flood to product of drainage area and basin slope...................... 14

6. Topographic characteristics of a typical stream............................................. 16

TABLES

Page 

Table 1. Index of gaging stations used in Connecticut flood-frequency report ........................... 4

2. Annual peak discharges, before 1929. ...................................................... 5

3. Annual peak discharges, 1929 to 1953 ...................................................... 6

4. Characteristics of streams ............................................................... 9
A FLOOD-FLOW FORMULA FOR CONNECTICUT

By B. L. Bigwood and M. P. Thomas

ABSTRACT

A study of the frequency and magnitude of floods 

within the Connecticut area is contained in this report. 

Annual flood discharges for 44 stream-gaging stations 

whose records range in length from 10 to 40 years are 

presented and a regional flood-frequency relationship 

developed based upon the ratios of all floods to the mean 

annual flood at each of these locations. Definition of 

this curve for floods of larger recurrence intervals is 

based upon historical studies of extraordinary floods 

which have occurred within the area. For ungaged 

areas, the mean annual flood value may be determined 

by use of a flood-flow formula based upon the topographic characteristics of drainage area and basin slope. 

The development of this formula also is included.

INTRODUCTION

Problems relating to flood flows in river channels 

have been the concern of man ever since he chose to 

settle in river valleys where rich soil was available 

for agriculture, where grades for highways and rail - 

roads were moderate, and where abundant water was 

available. Through the years he has learned to his 

dismay that rivers are not always friendly. At times 

they overflow their banks in uncontrolled fury and 

ruthlessly destroy everything before them. In order 

to control flood flows properly and to design and locate 

structures, such as dams, bridges, culverts, highways, 

railroads, residential and industrial buildings, watersupply and sewage-disposal works, and other facilities, 

it becomes necessary to evaluate the magnitude and 

probable frequency of recurrence of floods. Where loss 

of life or great property damage is involved if the structure be overtopped or destroyed, the "design flood" 

must be of such magnitude that it may never be exceeded. 

In general, however, it is adequate and economical to 

design for a flood which may be expected to occur on 

an average of from once in 10 to once in 100 years, 

depending upon the type of structure involved.

Before 1900, engineers attempted to evaluate the 

flood-producing potentialities of rivers by devising the 

flood-flow formulas which abound in our engineering 

literature. Most of these are general in character, 

based on data collected over relatively short periods 

of time, and apply largely to single stream or localized 

areas. These formulas usually involve only the use of 

the drainage area and coefficients, the values of which 

vary within a wide range. The final value of discharge 

obtained was largely a matter of judgment in selecting 

the coefficient in the formula, and for that selection 

there was little or no basis. The results obtained 

usually were in the form of maximum values, base data 

being so meager that anything but a very rough approximation of frequency was impossible.

Enveloping curves of maximum known discharge per 

square mile were also commonly used in the design of 

hydraulic structures. These curves likewise ignored 

the element of frequency and the effect of the physical 

characteristics of the drainage basin other than the 

drainage area, which have an important bearing upon 

the problem.

In 1913, the late Weston E. Fuller introduced the 

concept of a magnitude-frequency relationship based 

upon statistical methods applied to floods in the United 

States, following a line of reasoning that was introduced 

into studies of riverflow about the same time by the 

late Alien Hazen. From that time on, as streamflow 

records have become more extensive and reliable, the 

concept of maximum flood values for hydraulic design 

based on limited experience has been superseded by 

magnitude-frequency relationships computed on the 

basis of methods involving the use of rainfall-runoff 

relationships or physical characteristics of the drainage basins.

It has long been recognized that equal amounts of 

rainfall of similar intensities would produce widely 

varied peak discharge rates in this region. In 1944 the 

Geological Survey (Kinnison and Colby, 1945) completed 

a study to relate the magnitude of flood peaks of various 

frequencies on Massachusetts streams to certain significant drainage-basin characteristics. Short discharge 

records necessitated the use of rainfall-runoff studies 

through unit-hydrograph relationships for flood frequencies greater than 15 years. The resulting formulas are 

free of coefficients or estimated terms and are designed 

to give the peak discharge for minor (15 -year), major 

(100-year), rare (1, 000-year), or maximum floods on 

any stream in or near the Massachusetts area.

In Connecticut, also, there has developed a pressing 

need for a flood-flow formula for hydraulic design, particularly for streams having drainage areas under 100 

square miles. Previous reliance on outmoded formulas 

has resulted in a wide divergence of results. It was, 

therefore, conceived that by means of an analysis of 

existing streamflow records alone, a formula might 

be developed to serve as a basis for estimating expected 

flood flows on any stream in the Connecticut area. This 

formula, it was recognized, should not necessarily 

give results of minute theoretical exactness but rather 

of reasonable practical accuracy to serve as a guide 

in hydraulic design, particularly on ungaged streams 

where no observational data exist for evaluating flood 

flows. Some desirable requirements were (l) that the 

formula be simple in form and application so that even 

the inexperienced might be encouraged in its use, (2) 

that the physical characteristics used be readily

1
CONNECTICUT FLOOD-FLOW FORMULA

determinable from standard topographic maps, and (3) 

that the results exhibit a standard error no higher than 

about 20 percent. A further desired requirement-- 

that acceptable results might be obtained for drainage 

areas under 5 square miles--cculd not be expected 

inasmuch as the base data provided only 3 of 44 gagingstation records for streams under 10 square miles and 

none under 4 square miles. This basic lack of acceptable streamflow records for sites having drainage areas 

of less-than 10 square miles points up the urgent need 

for programming additional hydrologic investigations 

on small streams. At the other extreme, the formula 

would not be expected to apply to streams draining more 

than about '1, 500 square miles.

METHOD OF ANALYSIS

Derivation of a flood-magnitude-frequency formula 

for the Connecticut area, incorporating the desirable 

features mentioned, presented two separate and distinct 

phases: the development of a state-wide magnitudefrequency relation, and the evaluation of the effects 

that the important physical characteristics of each drainage basin individually might have in modifying this relation. Methods employed in the analytical attack on 

these phases in the development of the overall formula 

are outlined below.

FLOOD MAGNITUDE AND FREQUENCY RELATION

In analyzing flood frequency on a regional basis two 

aspects are presented: geographic sampling and time 

sampling. For the first, records for a large number 

of gage sites is required to sample a wide diversity of 

terrain. For the second, a large number of years of 

record is required, not necessarily at one station.

Experience has shown that within fairly wide regions, 

flood-frequency curves for all streams will be essentially the same, if floods are expressed in dimensionless terms. This is accomplished by expressing each 

flood as a ratio to an index flood at each station, called 

the mean annual flood.

The degree of similarity of such individual curves 

may be tested by a statistical "homogeneity" test. All 

the curves within a region thus demonstrated as being 

homogeneous may then be combined to obtain a basic 

flood-frequency curve for the region. The State of 

Connecticut was found to be homogeneous in this respect 

and a single curve for the region was found possible. 

The method of combining increases the reliability of the 

basic relationship for the available period of record; it 

does not increase the period of time.

To increase the flood experience in respect to time, 

it might be thought that the station-year method of analysis could be applied profitably to Hood-frequency studies as it has to rainfall-intensity frequencies. By this 

method, for example, 5 station records of 20 years each 

would be combined to obtain a 100-year record, thereby 

increasing the accuracy of flood prediction by reducing 

the sampling errors. Such a method requires that flood - 

frequency characteristics be comparable, and that the 

data be entirely independent. Although the former hold's 

true in Connecticut, the latter does not. For this reason, 

it was not considered possible to combine Connecticut 

records using the station-year method.

There remained only the possibility of extending 

time experience through study of historical records. 

It is known, for example, that no flood equal to or in 

excess of the 1936 flood peak has been observed within 

the past 320 years of recorded history on the Connecticut 

River. Historical studies from Connecticut sources 

indicate that this is probably also true with regard to Mae 

1938 hurricane flood on some of the streams in eastern 

Connecticut. Although historical data on floods are admittedly meager, nevertheless major floods are usually 

recognized and often related to one another and perhaps 

to others within the period of record, so that by use of 

such data it is possible to improve the frequency plotting 

of extreme floods.

There are two types of flood data which may be considered in flood-frequency analysis, the annual-flood 

series and the partial-duration series (called floods 

above a base). An annual flood is defined as the highest 

momentary peak discharge in a water year. Only the 

greatest flood in each year is used. An objection most 

frequently met with regarding the use of the annual-flood 

series is the very fact that only one flood in each year 

is employed. Infrequently, other floods in a given year, 

which the method omits, may outrank many annual floods.

This objection is met in the partial-duration series, 

by listing all floods above a selected base regardless of 

the number within any given period. The base is generally selected as equal to the lowest annual flood so that 

at least one flood in each year is-included. However, 

this may result in listing an excessive number of floods 

in some years. Therefore, in a long record, the base 

may be raised so that on the average only three or four 

floods a year are included. An objection to the use of 

the partial-duration series is that the floods listed may 

not be fully independent events, that is, one flood sets 

the stage for the next.

There is an important distinction in meaning between 

recurrence intervals by the annual-flood-series method 

and by the partial-duration-series method. In the annualflood series the recurrence interval is the average interval in which a flood of given size will recur as an annual 

maximum. In the partial-duration series, the recurrence 

interval is the average interval between floods of a given 

size regardless of their relationship to the year or any 

other period of time. This distinction remains, even 

though for large floods the recurrence intervals are 

closely the same by both methods. From .statistical 

principles there is a definite relationship between results 

by the two methods, and the following table shows comparative values of recurrence intervals:

Recurrence Intervals in Years

Annual floods

16

58

00

54

5.52

10. 

20. 

50.

100.5

Partial-duration series

0.5

1.0

1.45

2.0

5.0

10

20

50

100
FLOOD MAGNITUDE AND FREQUENCY RELATION

The two methods give essentially identical results 

for intervals greater than about 10 years. As most designs are based on floods of rarer frequency than this, 

it is apparent that, from a practical standpoint, either 

method is satisfactory, and the simplicity of the annualflood-series method makes it more attractive. For this 

study the annual-flood-series method was used.

Since 1928 when the State-Federal cooperative streamgaging program was inaugurated in Connecticut, the U. S. 

Geolpgical Survey has maintained an increasing number 

of stream gaging stations within the State, nearly all of 

them equipped with automatic water-stage recorders. 

Currently, 37 of these station records are of adequate 

length for us© in a flood-flow analysis. Only one station in Connecticut, Burlington Brook near Burlington,

has a drainage area of less than 5 square miles. However, two records in the State of New York, Beaver 

Swamp Brook at Harrison and Blind Brook at Rye, having 

respectively 4. 7 and 9. 2 square miles of drainage area 

in close proximity to the southwest corner of Connecticut, 

could be used in the flood study. Five other stations in 

Massachusetts, located on tributaries of streams that 

flow into Connecticut, also could be used. In all, a total 

of 44 streamflow records were available covering a 

range in size of drainage area from 1, 545 down to 4. 1 

square miles. Seven of these records began previous 

to 1929. Figure 1 shows the location of the gage sites 

throughout Connecticut and contiguous areas. The 

numbers refer to the stations listed in table 1.

Figure 1. --Map of Connecticut showing location of gaging stations used.
CONNECTICUT FLOOD-FLOW FORMULA

Table 1. --Index of gaging stations used in Connecticut flood-frequency report

Index no.

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

Gaging station

Beaver Swamp Brook at Harrison, N. Y.

Blind Brook at Rye, N. Y.

Burlington Brook near Burlington, Conn.

East Branch Eightmile River near North Lyme, Conn.

West Branch Eightmile River at North Plain, Conn.

West Branch Farmington River near New Boston, Mass. 

West Branch Farmington River at Riverton, Conn. 

Farmington River at Rainbow (Tariffville), Conn. 

Five Mile River at Killingly, Conn. 

Hockanum River near East Hartford, Conn.

Hop River near Columbia, Conn. 

East Branch Housatonic River at Coltsville, Mass. 

Housatonic River at Falls Village, Conn. 

Housatonic River at Gaylordsville, Conn. 

Housatonic River near Great Barring!on, Mass.

Housatonic River at-Sttfvenson, Conn. 

Leadmine Brook near Thomaston, Conn. 

Little River at Buffumville, Mass. 

Menunketesuck River near Clinton, Conn. 

Moosup River at Moosup, Conn.

Mount Hope River near Warrenville, Conn. 

Natchaug River atWillimantic, Conn. 

Naugatuck River near Naugatuck, Conn. 

Naugatuck River near Thomaston, Conn. 

Park R ver at Hartford, Conn.

North Branch Park River at Hartford, Conn. 

South Branch Park River at Hartford, Conn. 

Pequabuck River at Forestville, Conn. 

Pomperaug River at Southbury, Conn. 

Quinebaug River at Jewett City, Conn.

Quinebaug River at Putnam, Conn. 

Quinebaug River at Quinebaug, Conn. 

Quinebaug River at Westville, Mass. 

Quinnipiac River at Wallingford. Conn. 

Salmon River near East Hampton, Conn.

Saugatuck River near Westport, Conn. 

Scantic River near Broad Brook, Conn. 

Shepaug River near Roxbury, Conn. 

Shepaug Pi ver at Woodville, Conn. 

Shetucket River near Willimantic, Conn.

Still River near Lanesville, Conn. 

Tenmile River near Gaylordsville, Conn. 

Willimant-'c R v _r near South Coventry, Conn 

Yantic R ver at Yantic, Conn.

Annual flood discharges were tabulated from the 

streamflow records for these 44 gage sites. For all 

stations in Connecticut, these values were revised if 

necessary in accordance with the latest rating information, and some that were known to be affected by regulation or breaching of dams upstream were adjusted 

as nearly as possible to natural flow. The final results 

are shown in tables 2 and 3. Examination of these 

tables, or figure 2, shows a very slow rate of increase 

in the establishment of usable station records from 1 

station in 1901 to 8 by 1928, followed by a more rapid 

and steady accumulation to 44 stations by 1944.

Mean annual flood values lor each station were based 

on either an 18 year (1936-53) or a 25-year (1929-53) 

record, depending upon the comparative lengths of station records. The annual flood values given in tables 

2 and 3, were arranged in order of magnitude from highest to lowest for each station, assigning order numbers 

and computing corresponding recurrence intervals from 

the ratio N + 1, where N is the total number of events,

M

and M is the number of an event in order of magnitude. 

For a few year*; in some series for which no records 

were available, estimated order numbers were assigned 

to the missing events, based upon records for nearby
FLOOD MAGNITUDE AND FREQUENCY RELATION 

Table 2. --Annual peak discharges, before 1929, in cubic feet per second

Water 

year

1901

1902

1903

1904

1905

1906

1907

1908

1909

1910

1911

1912

1913

1914

1915

1916

1917

1918

1919

1920

1921

1922

1923

1924

1925

1926

1927

1928

Station no.

6

__

__

__

__

--

__

--

--

--

--

__

__

._

3,090

2,420

1,900

1,660

1,010

2,360

2,280

2,350

1,960

1,580

3,390

1,830

1, 530

1,620

6,490

8

__

--

__

--

--

__

--

--

--

--

__

_.

8, 100

9,300

13, 500

6,700

7, 100

4,900

10, 500

6,900

7,200

5,500

7,400

21,000

6,900

4,200

4,300

24, 500

10

__

__

__

--

--

--

--

._

--

--

__

__

--

--

--

__

--

--

--

1,860

623

--

--

--

--

__

--

--

13

__

__

__

--

--

__

--

_.

__

--

__

__

--

8,830

--

6,960

6,000

4,220

4.320

7,950

5,490

5,230

5, 570

8,390

7,410

4,210

5,500

11,700

14

17, 700

31,000

12,300

12, 900

13, 700

10, 400

6,690

16, 900

11, 500

13, 200

8,530

12, 100

13, 200

14. 900

--

._

..

--

..

--

-_

-_

-_

16, 000

--

__

-_

21,000

15

__

--

__

__

--

_-

--

--

--

--

__

--

--

5,070

4,110

5,300

4.200

2.670

2,490

4,450

4,860

3,900

4,650

4,900

4,350

3,370

3,860

7,910

23

__

__

__

__

--

--

--

--

--

--

__

--

--

--

--

__

--

--

--

8,470

6,290

14, 200

6,490

21,900

--

__

--

26, 000

30

__

__

__

._

--

__

--

--

--

--

__

--

-_

--

--

__

--

--

7,350

12, 100

5.950

11,600

7,480

9,800

6,820

5,770

3,050

12,500

40

__

--

__

__

--

__

--

--

--

--

__

__

__

--

--

__

.--

--

--

11,000

8,900

--

--

--

--

-_

--

--

streams, in order to rank the observed flood events 

over the base periods of 18 and 25 years. After plotting the annual flood discharges for each station against 

the corresponding computed recurrence intervals on 

Powell-Gumbel type frequency charts and drawing a 

curve to average the data, the mean annual flood was 

taken therefrom as the flood of a 2. 33-year recurrence 

interval. The results are given in table 4. Although 

experience has demonstrated that the mean of the annual-flood series for one station may not usually be compared with the mean for another station for a different 

period of record, it was found by a study of the longer 

records that the mean annual floods for the 18-, 25-, 

or 40-year periods used in the analysis lie within a 

range of about 8 percent and further adjustments were 

not made.

The mean annual flood values thus determined can 

be said, therefore, to be analytically comparable in 

time. Dimensionless flood ratios were obtained by 

dividing each annual flood value for each station by its 

corresponding mean annual flood value. The dimensionless ratios for floods of the same rank may be averaged 

for any number of stations within a homogeneous region 

to produce an average frequency curve.

An 18-year composite frequency curve was then constructed for all 44 stations, an 18-year and 25-year 

curve for 29 of these stations, and a 25-year and 40-

year curve for the 7 stations with records exceeding 

25 years. A composite frequency curve was then developed representing a 40-year record (1914-53) for 

44 stations by adjusting the 18-year frequency curve 

in successive steps by the relationship between the 

18-and 25-year frequency curves for 29 stations and 

the 25-and 40-year frequency curves for 7 stations.

In order to obtain additional flood data beyond the 

40-year periods as previously discussed, recourse 

was had to historical flood accounts in mewspaper files 

and town histories. Although the information secured 

was relatively meager and largely descriptive, it did 

serve to improve the relative plotting positions of the 

extraordinary floods of record in eastern and western 

Connecticut.

Jahns in 1947 (p. 132) states:

"From a combination of historic records of meteorologic and hydrologic conditions in the Connecticut 

River .drainage basin, together with evidence furnished 

by the great flood deposits and materials associated 

with them, the conclusion seems valid that no flood as 

great as or,greater than that of March 1936 has visited 

the lower reaches of the Connecticut since civilization 

began in the valley, and probably not for an additional 

period of several hundreds of years prior to that time. '
CONNECTICUT FLOOD-FLOW FORMULA 

Table 3. Annual peal: discharges, 1929 to 1953,in cubic feet per second

Index 

no. 

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

Water year

1929

_ _

--

--

...

--

2,160

--

7,220

--

1,190

__

--

6,850

9,100

4,740

12,700

--

--

--

--

...

--

5,730

--

--

_-

--

--

6,750

--

--

--

--

2.810

1,190

--

--

6,110

-_

--

--

1930

__

--

--

--

--

920

1,990

3,250

321

._

2,890

4,000

1,820

5,530

--

--

--

__

--

2,480

--

--

--

--

3,030

1,180

--

--

940

__

378

3,710

__

980

--

1931

__

--

--

--

--

1,310

3,130

5,640

350

__

4,320

7,300

2,520

12,700

750

--

--

--

-_

2.520

3,420

1,910

--

--

5,450

3,930

--

818

1,770

__

613

2,720

5.140

__

1,980

2.320

1932

_ _

--

101

--

--

1,840

5,040

4,860

--

410

__

^-

5,290

7,900

2,940

11,900

925

--

--

--

__

2,680

3,970

2,620

--

_-

--

-_

--

6,160

2,320

1,200

--

1,280

2,330

__

664

1,210

--

5,620

605

2,050

1,160

2,400

1933

..

--

395

--

--

3,640

9,720

7,610

--

500

1,970

--

8,160

10,800

6,380

21,700

1,880

--

--

1,460

3,270

8,470

3,800

--

-.

--

--

2,980

7,090

3,830

1,800

--

1,270

1,840

1.230

877

3,650

--

7,000

950

3,090

1,260

2,560

1934

..

-_

434

--

--

2,080

6,180

7,390

1,920

1.700

--

5,400

13,000

2,590

26,400

3,080

--

--

1,520

__

4,150

10,500

5,000

--

__

--

--

5,300

9,800

6,830

2,310

--

1.810

2,560

3,100

1,670

4,150

--

9,720

730

4,000

4,420

2,500

1935

..

--

346

--

--

2,750

6,600

6,800

--

810

1,500

--

4,560

7,500

3,690

14,700

1,880

--

--

2,470

__

4,560

7,820

4,450

--

__

--

-_

1,570

13,100

5,600

2,140

--

1,100

3,030

788

1,290

3,950

8,600

476

1,580

2,220

3,100

1936

..

533

--

--

9,830

19,900

26,900

--

1,810

3,640

6,000

14,500

26,000

8,990

55,000

-2,680

--

--

4,080

__

12,900

23,300

6,590

5,400

2,800

2,380

--

5,990

29,200

17,100

10,500

3,240

6,250

5,310

1,820

7,480

4,070

23.900

3,930

10,200

7,880

6,300

1937

 .

--

288

--

--

2,990

7,330

6,990

--

710

1,720

1,930

5,520

8,200

2,960

13,600

1,000

--

--

1,460

__

4,050

6,490

3,050

1,620

900

635

--

2,200

8,520

4,350-

2, 100

1,080

2,640

1,420

733

2,420

1,500

7,140

710

2,080

1,660

2,600

1938

 .

--

676

2,950

1,810

18,500

37, 100

29,900

2,480

5,160

6,450

6,400

19,900

37,000

11,520

59,500

3,050

-

--

4,100

__

27,200

25,300

10,100

5,650

3,000

3,600

3,800

7,420

25,000

20,900

14, 100

--

5,230

12,400

4,420

5,130

10,500

6,000

52,200

4,410

12,500

15,500

13,500

1939

_ _

--

243

560

495

2,250

5,070

6,190

552

1,320

1,600

3,390

7,790

10,300

5,180

17.100

1,740

--

--

795

__

2,530

8,800

3, 100

1,830

1,100

950

--

2,000

5,770

2,940

1,700

1,690

1,900

1,330

1,110

2,920

1,280

4,980

770

2,630

2,100

1,240

1940

..

--

293

670

705

2,310

6,240

9.000

552

1,330

2,330

1,380

9, 360

15,000

4,290

25-, 100

1,500

516

-,

1,290

__

4,900

9,590

4,010

3, 490-

1,460

2,430

3,660

9,400

5,710

2,550

1,450

2,680

3,470

3,400

1,290

3,190

1,500

9,240

1,730

5,280

3,400

2,500

1941

..

__

189

790

645

1,450

4,500

4,900

480

1,600

2,880

832

2,590

5,370

1,400

30, 500

880

168

--

1,490

950

4,790

11,900

2,160

2,960

950

2,330

5,960

7,150

2,560

1,280

559

2,320

4,120

3,830

1,110

3,190

1,150

10,000

3,800

2,370

2,660

3,570

He also states (p. 64, 65):

"The 1936 flood, although of extraordinary magnitude, 

was relatively most serious along the main river The 

hurricane flood (of September 1938), though of short 

duration as-a great flood along the Connecticut, was of 

record-breaking intensity on many tributary streams, 

particularly those draining the central upland area of 

Massachusetts. "

In most places in the Connecticut area the 1938 flood 

exceeded in magnitude the flood of 1936 and it seems

reasonable to assume, then, that the 1938 flood was 

the greatest flood of historical time in part of the 

Connecticut area, with a recurrence interval in excess 

of 300 years.

There is an apparent tendency toward independence 

of flood events in eastern and western Connecticut; 

therefore, in assembling information from historical 

and other sources, each half of the State was considered 

separately. It was assumed that, though the flood descriptions were usually very local in coverage, floods 

of such magnitudes were probably general over at least
FLOOD MAGNITUDE AND FREQUENCY RELATION 

Table 3. --Annual peak discharges, 1929 to 1953, in cubic feet per second--Continued

Index 

no.

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

Water year

1942

__

--

268

630

615

2,500

7,520

7,200

534

620

1,660

1,450

4,720

8,590

2.830

14,100

1,480

502

390

1,320

814

4,470

10,100

3,900

1,680

850

800

1,340

2,230

8,800

4,430

2,070

820

1,370

2,440

1,150

611

2,830

2,050

8.160

730

1,600

1,880

2,430

1943

__

--

144

750

880

1,810

4,750

5,100

686

1,060

2,180

1,720

4,960

7,890

3,000

16,800

1,000

264

720

2,150

814

3,880

7,640

2,610

2,630

1,380

1,800

1, 190

2,340

8,660

3,130

1,340

740

2,170

3,470

1,790

1,370

3,470

880

8,610

850

2,640

2,040

3,050

1944

140

1,330

242

525

585

2,610

5,900

5,460

512

1,020

2,580

1,280

4,000

7,210

2,550

11,800

1,040

337

400

1,020

950

3,150

5,840

2,020

1,220

560

1,250

1,280

2,340

6,250

2,950

i; 420

748

2,580

2,860

1,860

920

2,090

1,100

6 590

850

2,640

2,120

1,720

1945

88

614

442

578

555

2,450

6,960

5,660

435

960

1,480

1,970

6,100

8,830

3,570

13,600

2,120

331

510

975

767

2,930

11,500

4,450

1.910

1,500

990

1,980

3,340

6,010

2,790

1,560

546

1,180

2,320

1,120

945

2,330

1,300

6,030

1,110

1,950

1,510

1,670

1946

137

-834

374

542

425

1,500

3,990

5,900

353

427

815

1,190

5,570

7,890

3,450

10,800

840

334

330

825

469

1,610

4,880

1,770

1,420

755

795

730

1,520

4,870

2,490

1,490

712

1,180

1,380

1,930

550

1,360

753

3,670

660

1,210

910

1,560

1947

93

409

205

250

326

1,850

4,910

5,420

336

300

785

1,730

5,840

10,800

4,090

14,700

1,180

255

184

680

486

1 , 710

7,140

2,380

1,060

545

615

805

2,060

4,130

2,130

1,080

625

770

1,240

1,240

475

2,330

1,470

3,670

825

4,410

1,010

785

1948

96

502

256

595

580

3,180

7,710

11,200

795

880

1,700

1,970

10,100

16,500

5,420

26 000

920

448

870

1,660

767

4, 100

6,700

2,700

1,870

1,160

1,110

1,090

1,960

9,460

5,290

2,370

1,500

1,690

2,480

2,000

1,580

3, 100

1,500

7,570

1,100

3,310

2,240

1,790

1949

112

534

591

542

455

11,700

22,000

25,200

254

351

960

5,700

23,900

32,300

12,200

51,800

2,700

181

510

582

444

1,720

28,500

10,200

2,100

1,010

1,190

3,260

5,600

3,950

1,820

1,020

497

2,960

1,590

1,360

500

7,010

5, 160

3,850

1,110

6,360

1,070

1,520

1950

30

119

131

490

335

1,270

2,530

3,840

339

700

900

1,070

3,790

5,630

2.670

9,450

840

208

151

720

646

1,780

6,100

1,470

1,400

900

625

1,060

2,920

4,250

1,780

955

569

648

1,050

441

610

1,970

1,000

3,740

528

1,410

838

1,710

1951

130

964

287

610

550

10,100

17,400

9,900

483

1,000

1,940

3,280

7,270

11,600

4,290

21,000

1,440

520

340

1,320

1,170

3,090

11,300

5,000

2,880

1,750

1,600

1,970

3,160

8,900

5,180

2,940

1,000

1,880

3,080

3,220

1,040

3,980

1,430

7,010

1,080

2,630

2,240

2,500

1952

141

770

191

730

650

4,060

7,140

10,100

502

940

1,860

2,080

7,170

12,400

3 790

22,700

900

356

370

1,350

915

3,020

7,880

3,200

2,160

1,260

1,380

1,170

2,500

8,080

3,570

1,990

827

2,270

4,390

2,350

995

5,080

2,400

6,080

1,730

3,450

1,800

2,830

1953

140

1,240

335

690

650

3,240

6,780

9,680

582

1,150

2, 180

1,630

7,790

13,800

3,970

25, 300

1,770

518

770

1,470

890

3,330

15,200

6,150

2,970

1,660

2,220

1,340

3,400

10,200

5,180

2,240

1,220

2,020

2,800

4,710

1,520

4,530

1,900

7,430

1,420

5,390

2,300

2,280

the half of the State in which they were reported. Results of the search through town histories, newspaper 

files, and the like could not be considered completely 

satisfactory, but such information is deemed essential 

to a comprehensive flood-flow analysis. Its use involved 

the exercise of a considerable amount of judgment, but 

it has served as a valuable guide to final conclusions.

In western Connecticut the search was more rewarding, particularly in the central part around Waterbury. 

The earliest flood of significance which was recorded in

the accounts consulted occurred in February 1691. From 

the historical information listed in this study, it seems 

probable that this flood had not been exceeded between 

1634 and 1691 because the period is less than the normal 

life span of some of the hardier inhabitants. This flood 

therefore, considered in relation to a period of 320 years 

(1634-1953), was doubtless higher than the 1938 flood in 

the western part of the State and probably the highest in 

the 320-year record. It was, therefore, given an order 

number of 1 with other floods following it in descending 

order in accordance with the numbering system which
CONNECTICUT FLOOD-FLOW FORMULA

WATER YEAR 

Figure 2. --Chronological record of gaging stations.

appears below. The floods of November 13, 1853,and 

April 30, 1854, both within the climatic year 1854, 

seem to be very closely of the same magnitude and 

were given places 2a and 2b in the annual-flood series. 

Many more flood events were recorded but are not mentioned here inasmuch as all appeared to be of lesser 

magnitude than the November 1927 floodi

The historical information obtained in western Connecticut resulted in the following list of floods which 

are arranged in an approximate order of magnitude, although the relative positions of any but the recorded 

floods of 1927. 1936, 1938, and 1949 are important 

only as they affect the relative position in magnitude of 

the recorded floods. Descriptive material used in deciding the order number of these floods is also given.

1. February 1691. Bronson (1858, p. Ill) states: 

"In February 1691, happened the Great Flood, so called. 

Owing to rains and the sudden melting of the snows, the 

river left its banks and covered the meadows, rising to 

a height never known before or since. " Also, Anderson 

(1896, v. 1, p. 229) prints an "Extract from collections 

of Rev'd. Mr. Prince, at Boston, anno. 1872- " as follows: "In February 1691, There was a remarkable Flood 

in this town. The meadows were all under water and 

the ground so soft and the stream so rapid that it tore 

away a great part of the meadows and almost ruined 

them. The frost came out very quick and the rain fell 

apace, which made the ground uncommonly soft. The 

tbwn did not recover from the damage it received by 

this deluge for many years. Some of the inhabitants 

were grately discouraged, and many drew off, and the 

town was almost ruined* "

2a. November 13, 1853. Bronson (1858, p. 112) 

states that this flood was the largest "seen by oldest 

person living. " Cothern (1854, v. 1, p. 798) calls this 

"by far the largest and most destructive freshet that the 

town has suffered since its first settlement 3 feet

higher than was known by the 'oldest inhabitant' and 3 

feet higher than 'the old Indian marks'. " Campbell, 

Sharpe and Bassett (1902, p. 14) state that in the Naugatuck River at Seymour, Conn., "the water rose 18 

feet 11 inches. The south part of the railroad bridge 

was carried away with the abutment. The bridges atBeacon Falls, Pinebridge, and Ansonia were carried 

away. "

2b. April 30, 1854. Sharpe (1879, p. 84) states that 

"Water rose 8 or 10 inches higher than in the November 

freshet. Great damage was done throughout the valley. 

Derby Avenue was washed out from Broad Street to Pine 

Street to a depth of three feet. The water at Derby was 

19 feet 85 inches above the low water mark. " Campbell, 

Sharpe and Bassett (1902, p. 14) state: "Apr. 30, 1854, 

there was a rise of water 19ft. 5 in. and Derby Avenue 

was washed out to a depth of three feet or more and boats 

were used in the avenue. " Bronson (1858, p. 112) states 

that this flood was 18 inches lower than that of Nov. 13, 

1853, at Waterbury. "At Derby, owing to a greater 

freshet in the Housatonic, the water was highest in 

April. " The Hartford Courant, Tuesday, May 2, 1854, 

states: "At Simsbury, the Farmington River rose very 

rapidly and at its greatest height oh Sunday evening was 

about one foot higher than in the flood of 1801. In New 

Hartford, a saw mill and the west end of Merrills 

Bridge were carried away. The Naugatuck was higher 

at Seymour than at any previous freshet. " The Hartford 

Courant, Wednesday, May 3, 1854: at Ansonia, Conn., 

"The water has never been known to be as high. It is 

from 12 to 15 inches higher than on the 13th of November 

last. " The Hartford Courant, Saturday, May 6, 1854: 

At GaylorcT^s Bridge on the-Housatonic River "the water 

is said to have been some 6 feet higher than at any point 

previously known, by the oldest inhabitant. " A U. S. 

Geological Survey observer, G. H. Monroe, of Gay lords - 

ville, Conn., in his notebook for 1902 states: ' "Highest 

water mark known. Heifhtrot water between piers 21 

feet 3 inches. Got the measure Aug. 16, 1902. " If 

this is correct, runoff in the Housatonic River here was 

about 50 cubic feetper second per square mile. In The 

History of the Old Town of Derby (Orcutt, 1880, p. 343) 

is found a flood height of 19 feet 8? inches in the Housatonic River at Derby (then Birmingham), the highest during the period 1853 to 1879, according to John Whitlock, 

the observer.

3. January 22, 1891. On the 50th anniversary of 

this flood a Derby newspaper published the following: 

"Just 50 years ago *** water poured 12 feet over crest 

of Derby dam *** the largest flood ever to reach these 

towns on the lower banks of the Housatonic. At 5 p. m. 

Jan. 22 the gate tender reported 12 feet of water on the 

crest of the dam. Capstones started going off at 10-11 

p. m. and eastern half of the dam was torn down to its 

foundations. Probably the greatest flood the two towns 

ever did or ever will witness. "

4. March 1801. Perley (1891, p. 159), says this 

storm began March 18 and continued as a 4-day rain to 

produce the greatest flood ever known on the Farmington 

River. "No other freshet has equalled it. " Bronson 

(1858, p. 112), calls it the "most recent of the great 

floods, previous to the two last" (Nov. 13, 1853, and 

Apr. 30, 1854).

5. September 21, 1938. From Geological Survey 

Water-Supply Paper 867: Upper dam of Farmington 

River at Collinsville (p. 553), 10. 5 feet over'342. 5-foot 

crest; Derby dam on Housatonic River (p. 554), "7.6 feet
FLOOD MAGNITUDE AND FREQUENCY RELATION 

Table 4. --Characteristics of streams

Index

no.

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

Mean

annual

flood

(cfs)

(1)

140

1,150

300

650

600

2,400

7,500

8,000

550

1,000

1,850

2,000

7,000

11,000

4,000

20,000

1,400

400

500

1,400

1.000

3,500

9,000

3,600

2,400

1,250

1.450

1,700

2,850

8,000

4,000

1,900

 850

1,800

2,800

1,800

1,100

3,400

1,600

7,200

1,000

3,000

2,100

2,500

Drainage

area

(sq mi)

(2)

4.71

9.20

4. 1

22.0

18.6

92.0

216

584

58.2

745

76.2

57.1

632

.994

280

1,545

24.0

27.7

11.6

83.5

29.1

169

246

71.9

74.0

25.3

40.6

45.2

75.3

711

331

157

93.8

109

105

77.5

98.4

133

38.0

401

68.5

204

121

88.6

Noncontributing

area

(sq mi)

(3)

0

0

0

0

2.5

17

29

129

26.5

17

7.5

4

53

69

35

160

.8

9.5

0

1.5

0

4

10

3

10

1

9

10

2

96

57

19

17

7

7

33

0

0

0

18

10

27

6

5

Effective

drainage

area

(sq mi)

(4)

4.71

9.20

4.1

22.0

16.1

75.0

187

455

31.7

57.5

68.7

53.1

579

925

245

1.385

23.2

18.2

11.6

82.0

29.1

165

236

68.9

64.0

24.3

31.6

35.2

73.3

615

274

138

76.8

102

98

44.5

98.4

133

38.0

383

58.5

177

115

83.6

Main

channel

slope

(ft /mi)

(5)

12.8

38.4

67.3

34.5

45.8

48.7

37.4

12.6

15.7

15.1

15.4

47.9

8.0

9.3

12.7

8.5

57.5

36.8

44.2

17.3

51.2

16.9

17.6

28.4

9.4

14.6

14.6

54. 1

23.6

7.8

14.1

16.1

9.4

7.4

16.3

27.4

14.1

28.4

33.6

14.4

15.7

11.0

15.9

22.2

Tributary

channel

slope

(ft /mi)

(6)

17.4

85.3

143.3

49,5

67.5

87.7

65.4

46.2

46.2

55.0

62.2

68.7

29.8

18.8

36.7

26.4

87.0

45.7

67.2

29.6

43.5

27.0

57.4

73.3

17.4

16.4

18.4

54.6

57.1

21.4

25.8

34.4

34.4

33.0

41.3

36.8

22.0

35.6

42.5

13.9

25.2

34.2

35.7

23.8

Average

channel

slope

(ft /mi)

(7)

15.1

61.8

105.3

42.0

56.6

68.2

51.4

34.4

31.0

35.0

38.8

58.3

18.9

14.0

24.7

17.5

72.2

41.2

55. T

23.4

47.8

22.0

37.5

50,8

13.4

15.5

16.5

54.4

40.4

14.6

20.0

25.2

21.9

20.2

28.8

32.1

18.0

32.0

38.0

14.2

20.4

22.6

25.8

23.0

Effective

area

X

Average

slope

(4) x (7)

(8)

71.1

569

432

924

911

5,120

9,610

15,400

983

2,010

2.670

3,100

10,900

13,000

6,050

24,200

1,680

75fr

646

1,920

1,400

3,630

8,850

3,500

858

377

521

1,920

2,960

8,980

5,480

3,480

1,680

2,060

2,820

^ 060

1,770

4,260

1.950

8,210

1,190

4, 000

2,970

2,460

Basin

coefficient

CB

(9)

2.0

2.0

.85

.85

.85

.85

.85

.55

.55

.55

.85

.55

.55

.85

.55

.85

.85

.55

.85

.85

.t5

.&5

.85

.85

3.0

3.0

3.0

.85

.85

.85

.85

.55

.55

.85

.85

.85

.55

.55

.85

.85

.85

.85

.85

.85

Computed

mean

(8) x (9)

(10)

142

1, 140

367

785

774

4,350

8,170

8,470

541

1,110

2,270

1,700

6,000

11,000

3,330

20,600

1,430

412

549

1,630

1,190

3,090

7,520

2,980

2,570

1,130

1,560

1,630

2,520

7,630

4,660

1,914

924

1,750

2,400

1,750

974

3,620

1,660

6,980

1,010

3,400

2,520

2,090
10 CONNECTICUT FLOOD-FLOW FORMULA

over top of about 0. 5 foot of flashboards on 675-foot 

spillway. " Average peak runoff from 20 Geological 

Survey gages used in this analysis, 102 csm (cubic 

feet per second per square mile); Housatonic River at 

Falls Village, Conn., 34. 4 csm; Housatonic River 

near Barrington, Mass., 47. 0 csm; Naugatuck River 

near Naugatuck, Conn., 107 csm.

6. January 8, 1874. From Geological Survey Water-Supply Paper 798, p. 458: Derby dam, 7. 8 feet 

over 636- or 637-foot spillway. Orcutt (1880, p. 343) 

lists a flood height of 17 feet 4| inches, in the Housatonic 

River at Derby (the Birmingham), determined by 

John Whitlock. Campbell, Sharpe, and Bassett, (1902, 

p. 15) state that water rose 17 feet 6 inches at Seymour on the Naugatuck River.

7. October 4, 1869. Known as "The Pumpkin Flood. " 

Campbell, Sharpe, and Bassett (1902, p. 15) say that 

the water rose 15 feet 9 inches at Seymour on the 

Naugatuck River. Orcutt (1880, p. 343) lists a flood 

height of 16 feet in the Housatonic River at Derby (then 

Birmingham), determined by John Whitlock. Sharpe 

(1879,p. 102) .states: "The Naugatuck rose to the 

highest point reached in 15 years" (since 1854). From 

Geological Survey Water-Supply Paper 798,p. 457; 

Upper dam on Farmington River at Collinsville,Conn., 

10 feet over 329.6-foot crest. Porter (1880, p. 72) 

states: 'In the great storm-of Oct. 3 and 4, 1869, the 

water poured over the Collinsville dam 10 feet deep. 

This storm was a most remarkable one and caused 

widespread damage in New England. It appeared to be 

central 2 or 3 miles east of New Hartford, where a 

downfall of 12. 35 inches was recorded. The amount 

was above 8 inches for the entire Farmington River 

Basin. " Also, "While (Derby) dam was in process of 

construction, and indeed nearly complete, a violent 

storm which swept over this part of the country caused 

a heavy freshet in the Housatonic; water poured 13 feet 

deep over the partially finished dam, undermined and 

destroyed 160 feet of its length and scoured out an immense cavity 20 feet deep on the river bed immediately 

below. " (Probably 13-foot depth was over partly 

finished portion since height of river at Birmingham 

did not'indicate such an extraordinary stage over the 

actual spillway). Hartford Courant, Friday, October 

8, 1869, says of the Farmington River at Collinsville, 

"At 4 p. m. Monday, the water had reached a higher 

point than at any time since the great ice flood of 1857."

8. January 1, 1949. Average runoff from 20 Geological Survey gages, 72. 9 csm; Housatonic River at 

Falls Village, Conn., 41.3 csm; Housatonic River 

near Great Barrington', Mass., 49.8 csm; and Naugatuck River at Naugatuck, Conn., 120. 8 csm.

9. March 12, 1936. From Geological Survey Water-Supply Paper 798, p. 457: Upper dam of Farmington 

River at Collinsville 9.0 feet over 342. 5-foot crest. 

Derby dam on Housatonic River, 7. 4 feet over 675-foot 

spillway (natural peak). Average peak runoff from 20 

Geological Survey gages, 79.1 csm; Housatonic River 

at Falls Village, Conn., 25.0 csm; Housatonic River 

near Great Barrington, Mass., 36.7 csm; and Naugatuck River at Naugatuck, Conn. 98.7 csm.

10. January 1770. Perley (1891, p. 78), relates 

that this storm began January 7, 1770, "the greatest 

freshet perhaps that ever occurred in New England. " 

Ice was present. Farmington River high. At Simsbury, 

iron works of Richard Smith were carried away.

11. November 1927. From Geological Survey WaterSupply Paper 798, p. 457, 458: Upper dam of Farmington 

River at Collinsville, 9. 0 feet over 342. 5-foot crest; 

Derby dam on Housatonic River, 5. 4 feet over 675-foot 

spillway; Housatonic River at Falls Village, Conn., 

20. 2 csm; Housatonic River near Great Barrington, 

Mass., 32. 3 csm; and Naugatuck River at Naugatuck, 

Conn., 131 csm.

In computing recurrence intervals for the recorded 

floods, a period of 320 years (1634 to 1953) was used 

for the September 1938 flood, for it was considered extraordinary enough so that none probably equaled or exceeded it during the period 1634 to 1691. For the other 

floods of November 1927, March 1936, and January 

1949, the base period of 262 years from 1691 to 1953 

was used, for it is possible any one of these floods 

might have been exceeded before 1691. The following 

table gives the original plotting positions for these 4 

floods based upon 41 years of record and the new plotting 

positions based upon historical data (order of magnitude 

in parenthesis):

Ratio to mean

Recurrence interval

Flood of annual flood 41 years Historical

November 1927 2.188 10.5(4) 23.9(11)

March 1936 2.440 14.0(3) 29.2(9)

September 1938 3.106 42.0(1) 64.0(5)

January 1949 2.593 21.0 (2) 32.9 (8)

In eastern Connecticut historic flood information was 

not so complete, but the following list of floods arranged 

in approximate order of magnitude was used to position 

the two outstanding floods of March 1936 and September 

1938. The floods of November 1927 and January 1949 

were not outstanding floods in eastern Connecticut.

1. September 21, 1938. From Water-Supply Paper 

867, p. 553: Greenville Dam, on the Shetucket River at 

Norwich, 14. 5 feet over 401-foot crest.

2. February 7, 1807. Caulkins (1874, p. 353) states 

that "The Shetucket River rose 18 or 20 feet.*** For 

many years no such inundation had been known. " Many 

bridges were lost. Perley (1891, p. 176), confirms 

the fact that the Shetucket River rose from 18 to 20 feet 

and also infers that this flood was greater than that of 

1801 on the Shetucket River.

3. March 26, 1876. From Water-Supply Paper 798, 

p. 553: Greenville Dam on the Shetucket River at 

Norwich, 12 to 14 feet over a 326-foot crest (approximately equivalent to 11. 5 feet over a 1938 crest length 

of 401 feet, assuming same crest coefficients). Porter 

(1880, p. 20), states that "The Shetucket River rose to 

such a height as to run 10 feet deep over the Taftville 

Dam, and 12 to 14 feet deep over that at Greenville. *** 

Through the preceding week heavy rains had filled all 

the reservoirs and during Saturday night (March 25) 

drenching showers carried the streams out of their beds. 

The storm was general and disastrous in Connecticut, 

Rhode Island and Massachusetts; it was stated that in 

the vicinity of Providence 4. 06 inches of rain fell during Saturday and Saturday night making 7. 66 inches in 

Sundays. The losses by this freshet were estimated to 

amount to several hundred thousand dollars in eastern 

Connecticut. "
INFLUENCE OF TOPOGRAPHIC CHARACTERISTICS 11

4. March 19, 1936. From Water-Supply Paper 798, 

p. 451: Greenville Dam on She tucket River at Norwich, 

11. 1 feet over crest partly due to dam failures upstream 

(approximately equivalent to 10. 5 feet over 401-foot crest, 

adjusted to natural flow).

The 1938 flood was considered the greatest in eastern 

Connecticut in the historic period of 320 years (1634 to 

1953) and probably was not exceeded for many years before 1634. Its recurrence interval was therefore taken 

as 321 years instead of 36. 0 years as indicated by the 

35-year period of record. The 1936 flood was considered 

the fourth-ranking flood in a period of 216 years, 1737 

to 1953, as no adequate accounts of flood events before 

1737 were found; its recurrence interval was, therefore, 

taken as 54. 3 years instead of 18. 0 years as indicated 

by its second place in the 35-year period of record.

The new plotting positions obtained for outstanding 

floods of record in eastern and western Connecticut by 

analysis of the foregoing historical data define very 

well the position of the frequency curve above a recurrence 

interval of 6 years, and they were used in drawing the final 

frequency curve which is shown on figure 3. Its position 

seemingly cannot be further improved on basis of currently available information. It is possible, however, that

as time progresses and added information becomes available, a flatter curve may be indicated, and the recurrence 

interval of the 1938 flood, in eastern Connecticut at least, 

here taken as 321 years, may be found too low. Until 

such time, the curve used in this report may give ratios 

to mean annual floods too high for frequencies greater 

than perhaps once in 10 years, but such results are doubtless on the safer side in dealing with the vagaries of future flood events.

INFLUENCE OF TOPOGRAPHIC CHARACTERISTICS

Mean annual flood discharge at any given site on a 

stream results from the integrated influences of innu - 

merable meteorologic, topographic, and geologic factors 

upon the precipitation which falls on the drainage basin 

above it. It is impractical, if not impossible, to evaluate them all in a study of this kind, and the problem resolved itself into finding the major factors of correlation 

and resolving them into as simple a relationship as possible that would give results of an acceptable accuracy. 

Simplicity of relationship was emphasized and striven 

for from beginning to end of this study.

1938. 

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RECURRENCE INTERVAL, IN YEARS

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Figure 3. --Flood magnitude and frequency curve, ratios to mean annual flood.
12 CONNECTICUT FLOOD-FLOW FORMULA

Meteorological factors, such as evaporation and 

transpiration, are probably relatively uniform in a 

homogeneous area such as Connecticut. Their effects 

in causing variations in mean annual flood flows were, 

therefore, considered negligible. Geological facv.-rs, 

too, are probably relatively uniform, as the eastern 

and western Highlands are similar in that both are 

made up of old eroded metamorphic rock overlain by 

debris. Most of the streams in the State on which records are available rise in these Highlands. There are 

probably some contrasting effects from the geology of 

the Central Lowland upon the mean annual floods of 

streams flowing through that area, but for this study it 

seems probable that geologic factors cause relatively 

small variations, if any, in the mean annual flood discharge in the Connecticut area.

Factors classified under the term topographic do not 

exhibit the uniformity of metorologic or geologic factors. They may be grouped under such headings as (1) 

surface area land, water, swamp, (2) altitude--maximum, minimum, mean, or median, (3) slope of land, 

of main stream channel, of tributary stream channels, 

(4) shape of basin--length, width, form factor (length 

divided by width), and (5) surface cover wooded, 

grassed, cultivated. Of these factors, investigation in 

dicated that the effect of variations of altitude within the 

Connecticut area was negligible, and the types of vegetative cover averaged so closely similar on all basins 

that their effects could be neglected. The remaining 

factors of area, slope, and shape of the basin received 

concentrated study. Basin shape was found insignificant.

For years it has been recognized fay hydrologists 

and engineers that one of the major factors of correlation in a flood-flow formula is the drainage area of the 

basin above the point of measurement. The form of the 

relation is generally Q =CAn, where Q is the peak discharge in cubic feet per second, C is a constant, and 

A is the drainage area in square miles. The exponent, 

n, varies from 0. 5 to 0.8. Correlation of mean annual 

flood with drainage area (data in table 4) for the Connecticut area resulted'in the formula Q = 55A^-^ for the 

average curve based on the data in this report. That 

the area factor was not the sole factor in the problem, 

however, was indicated by the fact that the plotted data 

scattered about the curve in an enveloping band ranging 

about 50 percent low and 125 percent high of the mean 

curve, disregarding one point outside the band. As a 

much closer correlation was necessary other factors 

were investigated, among them average fall, total fall 

and length of longest watercourse, and length and width 

of basin. Also investigated were slope factors, such as 

the total and average slope of longest watercourse; 

shape factors, such as length and width, stream density, 

form factor of basin; and the relation between the drainage area at the average basin altitude and the total drainage area. The use of none of these various factors 

seemed to improve the relationship already established 

between mean annual flood and drainage area.

Further study of the relationship between mean 

annual discharge and drainage area (fig. 4) disclosed 

that plotted data for some groups of stations along the 

main stems of the larger streams, such as the Housatonic, Quinebaug, and Shetucket Rivers, seemed to define two separate straight-line relationships of the form 

Qcc KA . These relationships are indicated in figure 

4 by dashed lines connecting the plotted positions for 

the 3 lower stations at Falls Village (13), Gaylordsville

(14), and Stevenson (16) on the Housatonic River; the 4 

stations at Westville (33), Quinebaug (32), Putnam (31), 

and Jewett City (30) on the Quinebaug River; and the stations on the Willimantic River near South Coventry (43) 

and Shetucket River near Willimantic (40). Assuming 

this relationship between mean annual discharge and 

drainage area to be true, it remained to discover the 

major basin characteristics represented by the factor 

K. It became evident that this factor was a function of 

slopes of channels in the basin; therefore, various slope 

factors were tried, such as total slope of main channel, 

average slope of main channel, and finally the weighted 

slope of the main channel obtained by dividing the difference between the average altitude of the main channel 

and the altitude at outlet (point of study) by half the 

length of the same watercourse. This slope value is 

weighted in favor of the slope of the lower reaches of 

the stream channel. Correlation of the products of 

drainage area and this weighted main-channel slope, 

SM» with the mean annual flood showed a considerable 

improvement, with the values for the station groupings 

on each major stream still plotting as simple relationships, QocSjyi, with exponents of unity. Then, some 

further improvement was discovered by modifying the 

main channel slope factor, Sj^, by averaging it with 

the average slopes similarly determined for major 

tributaries, defined as those draining 10 percent or 

more of the total effective drainage area. In the absence 

of these major tributaries to the main channel, its 

uppermost reach above a point where 10 percent of the 

total effective watershed area is drained should be considered as a tributary. Slope factors for each station 

are shown in tab'e 4.

Using the data shown in table 4, the final correlation, 

Q vs ASavg, for the 44 gaging station locations (fig. 5), 

resulted in what may be analyzed as 3 lineal gr.oupings 

of plotted data, tie major one applying to most streams 

in the area, characterized by watersheds largely rural 

with a general similarity of physical features, designed 

"normal" for the State. Of the two other groupings, one 

applies to streams, otherwise normal, but having abnormal amounts of channel storage, and the second includes 

streams draining urban residential areas. Two stations, 

1 and 2, are intermediate in position between the normal 

and the urban-residential curves and probably represent "suburban" conditions. The primary expression 

for mean annual discharge, that for streams of socalled normal characteristics, applicable to a large 

percentage of streams in the State, took the form 

Q = 0.85AS, in which the coefficient may reduce to 

0. 55 in places of unusual channel storage capacity. For 

streams draining urban residential watersheds a multiplier of 3.0 is indicated. Estimated mean annual floods 

for all stations by use of this formula are shown in 

table 4. The standard errors of estimate were found 

to be 13.2 percent for the 29 stations on streams having 

basins with normal characteristics, and 13. 1 percent 

for the 10 stations on streams affected by an excess of 

channel storage. Values of the mean annual flood plotted within about 20 percent of their respective curves 

of relationship. Therefore, the maximum error to be 

expected from use of these formulas for rural conditions 

is about 20 percent, providing that the extent of channel 

storage is properly evaluated. The standard error, 

which includes 68 percent of all cases, is about 13 percent. In those places where the degree of urbanization 

must be estimated, unknown and possibly larger errors 

may be involved.
SUMMARY OF PROCEDURE 13

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EFFECTIVE DRAINAGE AREA, IN SQUARE MILES 

Figure 4. --Relation of mean annual flood to drainage area.

In using these curves, it must be remembered that 

the "normal" curve is that best defined by the bulk of 

the data, and the condition which will be represented 

by most design locations. Other curves should be considered as guides for the engineer in extending the relationships to conditions of abnormal slope or of urbanization, rather than as defining specific conditions.

Caution should be exergiggd in applying the formula 

to streams draining areas of less than 10 square miles, 

and particularly those Of less than 5 square miles. The 

curves of figure 5 are not adequately defined for a product of drainage area times channel slope of less than 

300. Application of the formula to streams of greater 

drainage area than 1, 500 square miles also should be 

made with caution. The formula is not intended to apply to the Connecticut River within Connecticut, the 

drainage area of which is around 10, 000 square miles. 

These limitations obviously must be established because of the inadequacy of data beyond these extremes.

SUMMARY OF PROCEDURE

The step-by-step procedure in the application of 

this flood-frequency formula is as follows:

1 On topographic maps, outline and measure (with 

a planimeter or by other means) the drainage area above 

the selected outlet point or point of study on the stream.

2. Outline and measure the portions of the drainage 

area not contributing to flood volumes. Such portions 

of the drainage area include the watersheds of ponds 

and reservoirs on headwater streams having a relatively large surface area with resp'ect to the drainage area 

and of other ponds and reservoirs elsewhere of a size 

that would impound a considerable proportion of the 

flood volume, and thus delay the runoff from those parts 

of the basin to the extent that their contribution to the 

flood crest at the outlet is unimportant.
CONNECTICUT FLOOD-FLOW FORMULA

100 100 1000 10,000 50,000

EFFECTIVE DRAINAGE AREA X BASIN SLOPE 

Figure 5. --Relation of mean annual flood to product of drainage area and basin slope.

3. Note whether the watershed comprises two or 

more tributaries of comparable basin size that have 

confluence closely upstream from the outlet point. If 

so, each tributary basin must be treated separately and 

their respective peak flows added to determine peak 

flow at the outlet.

4. Determine the effective drainage area above the 

outlet point as the total drainage area less the combined 

areas of the noncontributing portions.

5. Select the main channel, usually the longest continuous water-course, and determine the mean altitude 

along its profile by graphical or arithmetical means. 

Subtract from this value the altitude of channel at the 

outlet point and divide the result by one-half the length 

of the channel in miles. This is the so-called slope of 

main channel, expressed in feet per mile. Source of a 

stream may be defined as the upstream beginning of its 

main channel as shown on a standard topographic map.

6. Select all tributary streams which drain more 

than about 10 percent of the effective drainage area at 

the outlet. Determine the slope of each tributary (main 

stem only) by the method outlined for the main channel 

in paragraph 5 above. Average the slopes for all such 

tributaries. This value is the so-called tributarychannel slope. If there are no tributaries draining 

more than about 10 percent of the total effective drainage area, the slope of the uppermost reach of the main 

watercourse that drains 10 percent of the total effective 

drainage area should be used.

7. Determine the basin channel slope as the average of the main channel slope and the tributary channel 

slope. This is the slope (S) in the derived formula for 

mean annual flood.

8. From personal knowledge or from an inspection 

of the effective drainage area on topographic maps, classify the basin as normal, suburban, or urban residential
EXAMPLE OF APPLICATION OF FORMULA 15

and decide whether or not the stream course in fJ >od 

may contain abnormal channel storage. Abnormal 

channel storage is usually evidenced by a meandering 

stream course and flat stream profile in the middle or 

lower reaches of the river. Urban residential classification includes watersheds of intensive residential 

and street development, whereas suburban areas show 

substantially less development of similar type.

&.. Compute the mean annual flood from the formula 

QM s °- 85AS, if the basin has normal characteristics. 

If there is an abnormal amount of channel storage, a 

coefficient approaching 0.55 must be selected. For 

streams in semisuburban areas, the basin coefficient 

might range between 1. 0 and 1.5, for suburban areas 

from 1.5 to 2.5, and for urban residential areas from 

2. 5 to 3. 0, the upper limit of definition by the base 

data employed herein.

10. Select the recurrence interval desired from consideration of the type of structure, its economic life, 

or whether its destruction might result in undue damage 

or loss of life.

11. Apply the selected recurrence interval to the 

frequency curve (fig. 3) to determine the ratio to mean 

annual flood corresponding to the selected recurrence 

interval.

12. Compute the design flood rate as the product of 

the mean annual flood value and the ratio to the mean 

for the selected frequency.

In essence, of course, the several procedural steps 

given above amount to the determination of the factors 

in the complete flood-flow formula:

Q = R(CBAS)

in which R is the design ratio from the frequency curve, 

and CgAS is the mean annual flood based on the watershed characteristics.

EXAMPLE OF APPLICATION OF FORMULA

As an example of the application of the method outlined above, an analysis is presented for the existing 

gage site on Leadmine Brook near Thomaston, Conn. 

From observation and reference to the topographic map, 

this basin appears as one of normal type, the stream 

draining rural farmland and wooded areas and having 

a channel rather rough and steep., with no abnormal 

amount of channel storage along its course.

An outline of the basin above the gage on the topographic map (fig. 6), encloses the entire drainage area, 

which was found to be 24. 0 square miles. Inspection 

indicated that the drainage areas above the outlets of 

two ponds in the headwaters probably were not effective 

in contributing to peak flows at the gage site, as their 

surface areas are large in proportion to their drainage 

areas, and flood runoff from these areas would be considerably delayed by storage in the ponds. The total 

noncontributing drainage area was 0. 8 square mile, 

leaving a total effective basin area of 23. 2 square miles. 

Although the adjustment here involved is only about 

3f percent, and virtually negligible, it is possible to 

find reductions in other watersheds of much greater 

proportion. Inspection of the map shows that the stream 

above the gage site comprises a single main stem with 

2 major (10 percent) tributaries, and further subdivision 

of the area is unnecessary for computation purposes.

Plots on figure 6 of the profile of the main stem and 

the two tributaries that drain more than 10 percent of 

the effective drainage area indicate that, for the main 

stem, the slope factor is 57. 5 feet per mile, whereas, 

for the two tributaries, the values are 81. 0 and 93. 0 

feet per mile, respectively, or an average of 87. 0 feet 

per mile. Computing basin channel slope as the average of the main channel slope of 57. 5 feet per mile and 

the average tributary slope of 87. 0 feet per mile results 

in a value of 72. 2 feet per mile for substitution in the 

formula.

From the topographic map it is evident that the basin 

above the gage site is entirely rural, and that, since the 

main stem has no abnormally flat reaches or ponds along its course, the stream may be classified as of 

normal type with a formula coefficient of 0. 85.

The mean annual flood for this gage site is then computed as.the product of the basin coefficient (0. 85), the 

basin channel slope (72. 2 ft per mile), and the effective 

drainage area (23.2 sq mi), or 1,430 cfs which, in this 

case, is within 2 percent of the observed median annual 

flood of 1, 400. cfs for this site.

For selected recurrence intervals of 15, 50, and 100 

years, factors of 1. 85, 2.9,and3. 7, respectively, are 

found from the flood-frequency curve (fig. 3). These 

factors, multiplied by the computed mean annual flood 

of 1, 430 cfs, result in peak flood flows expected to 

recur once every 15,50, or 100 years, namely, 2,600, 

4,200, and 5, 300 cfs, respectively.
16 CONNECTICUT FLOOD-FLOW FORMULA

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Figure 6. Topographic characteristics of a typical stream.

LITERATURE CITED

Anderson, Joseph, 1896, The town and city of Waterbury, Conn.: v. 1, New Haven, The Price & Lee 

Co.

Bronson, Henry, 1858, The history of Waterbury, 

Conn.: Waterbury, Bronson Bros.

Campbell, H. A., Sharpe, W. C., and Bassett, F. G., 

1902, Seymour, past and present: Seymour, W. C. 

Sharpe.

Caulkins, F. M., 1874, History of Norwich, Conn.: 

Hartford, Case, Lockwood & Co.

Cothren, William, 1854, History of ancient Woodbury, 

Conn.: v. 1, Waterbury, Bronson Bros.

Jahns, R. H., 1947, Geologic features of the Connecticut Valley, Massachusetts, as related to recent 

floods: U. S. Geol. Survey Water-Supply Paper 996.

Kinnison, H. B., and Colby, B. R., 1945, Flood formulas based on drainage basin characteristics: 

Am. Soc. Civil Engineers Trans. Paper 2248.

Orcutt, Samuel, 1880, History of old town of Derby,

1642 to 1880: Springfield Mass., Springfield

Printing Co. 

Perley, Sidney, 1891, Historic storms of New England:

Salem, The Salem Publishing & Printing Co. 

Porter, Dwight, 18,80, Report on the water-power of

the region tributary to Long Island Sound in 10th

Federal Census, v. 16, Reports of the water-power

of the United States: U. S. Census Office, 1885,

163 p. 

Sharpe, W. C., 1879, History of Seymour, Conn.:

Seymour, Record Print. 

United States Geological Survey, 1937, The floods of

March 1936, part 1, New England Rivers: U. S.

Water-Supply Paper 798. 

____ 1940, Hurricane floods of September 1938: U.S.

Water-Supply Paper 867.

HR.-OUP. SEC.. MSH.. D.C 29150