floods in utah, * magnitude and frequency

Floods in Utah,

* Magnitude and Frequency

GEOLOGICAL SURVEY CIRCULAR 457

Floods in Utah, Magnitude and FrequencyBy V. K. Berwick

GEOLOGICAL SURVEY CIRCULAR 457

Washington 1962

United States Department of the InteriorSTEWART L. UDALL, SECRETARY

Geological SurveyTHOMAS B. NOLAN, DIRECTOR

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

CONTENTS

PageAbstract >................-......... 1Introduction. __-_----__------_----_-. 1Cooperation and supervision__________ 1Description of the area___-_-_____._-- 1

Physical features.__-------_----_-. 2Colorado River basin ____________ 2The Great Basin...._____________ 2Snake River basin __-_-__-___-___ 3Major rivers_______ _____--._-_ 3

Climatic features._________________ 3Flood - frequency relations ____________ 3

Flood frequency at a gaging station __ 4Basic data ______________________ 4Flood series ____________________ 4Station frequency curve __________ 4

Regional flood frequency......______ 4Regional sampling............... 13Selection of comparable floods ____ 13Homogeneity ____________________ 13

PageFlood-frequency relations Continued

Regional flood frequency ContinuedComposite frequency curve _______ 13Regional analysis for summer

floods _______________________ 13Derivation of the mean annual flood._ 15

Basin characteristics .___._._____ 15Mean annual flood relations.______ 15

Major rivers in Utah.._____________ 18Cloudburst floods and mud-rock

flows. ...-.__________________Application of flood-frequency

curves ________________________Method _________________________Use of flood-frequency analysis on

major river s__ _______ ______ ._Limitations _______________________

ILLUSTRATIONS

Page Figure 1. Map of Utah showing location of gaging stations _____________________________ 5

2. Map of Utah showing flood regions.....____________________________________ 123. Composite flood-frequency curves, regions A and B _________________________ 144. Composite flood-frequency curves, regions C and D _________________________ 145. Map of Utah showing hydrologic areas__-_______.___________________________ 166. Variation of mean annual flood with drainage area and mean altitude.__________ 177. Colorado River, discharge of 50-year flood____________________________---__ 188. Green River, discharge of 50-year flood____________________________________ 189. Duchesne River, discharge of 50-year flood.________________________________ 18

10. Strawberry River, discharge of 50-year flood__________________________-_--_ 1811. Price River, discharge of 50-year flood___-___-______----_---_-------_--_-_ 1812. San Rafael River, discharge of 50-year flood.....___________________________ 1913. San Juan River, discharge of 50-year flood _________________________________ 1914. Bear River, discharge of 50-year flood. ____________________________________ 1915. Weber River, discharge of 50-year flood ___________________________________ 1916. Provo River, discharge of 50-year flood________.___________________________ 1917. Sevier River, discharge of 50-year flood ___________________________________ 19

TABLES

Table 1. Peak discharges at gaging stations used in frequency analysis 2. Main stem gaging-station data_____________________________

Page6

20

HI

Floods in Utah, Magnitude and Frequency

By V. K. Berwick

ABSTRACT

This report presents a procedure for estimating the mag nitude and frequency of floods, within the range of the base data, for any site, gaged or ungaged. From the relation of an nual floods to the mean annual flood, a composite frequency curve was derived for recurrence intervals of 1.1 to 50 years. For regions of similar hydrologic characteristics, curves were developed by multiple correlation to express the relation of mean aunual flood to drainage area and mean altitude. The record^ of gaging stations having 5 or more years of record were used as base data when the natural conditions of streamflow are not affected by works of man. For major rivers where the flow is affected by diversion or regulation, separate analyses were made for each stream. The results may be applied to any area in Utah, except the Great Salt Lake Desert and a small area of the State in the Snake River basin.

INTRODUCTION

The proper design of structures in or near a stream should include the determination of the magnitude and frequency of floods. Where flooding may endanger human life, structures should be designed to withstand the greatest probable flood magnitude. In most instances, however, the risk to human life from flooding is small, and structures are designed to with stand a flood of some selected frequency of occurrence based on economics.

Floods result from the combined effects of climatic events and physiographic charac teristics of a basin. Some of the principal physiographic factors affecting flood flows are: drainage area, altitude, geology, basin shape, slope, aspect, and vegetal cover.

Flood data for an individual site show what has occurred at that site during a definite period of time but could lead to erroneous results in the prediction of future events, even at that site, if the record is not representative of the long-term average. A composite flood- frequency curve based on many gaging-station

records adjusted to a common time base is a logical means for predicting future flood ex pectancy anywhere within a homogeneous flood region. This report (a) outlines the homoge neous flood regions in Utah and presents a composite flood-frequency curve for each, and (b) outlines hydrologic areas and presents graphs .showing the variation of mean annual flood with drainage area and altitude.

COOPERATION AND SUPERVISION

This report was prepared by the Geological Survey, in cooperation with the Utah Depart ment of Highways and the U.S. Bureau of Public Roads. It was prepared in the office of the Geological Survey at Salt Lake City, Utah, under the direction of M. T. Wilson, district engineer. Technical guidance was furnished by the Floods Section, Water Re sources Division, Geological Survey, Wash ington, D. C.

DESCRIPTION OF THE AREA

High, rugged mountain ranges are the most distinguishing features of the Utah landscape. Their presence influences the flow of air masses and therefore has a major affect upon precipitation and resulting streamflow. The State is divided by the Wasatch Mountains, a high range that runs in a north-south direc tion. On both sides of this divide are extensive deserts or arid regions with the exception of a humid section in the northern mountain ranges. The Uinta Mountains, one of the few mountain ranges in the Unites States with an east-west axis, extend eastward from the north-central part of the State almost to the Utah-Colorado State line.

FLOODS IN UTAH, MAGNITUDE AND FREQUENCY

The absence of a usable water supply in the arid regions of Utah has discouraged the col lection of enough streamflow records to de fine flood-frequency relations; therefore, the Great Salt Lake Desert has not been included in this analysis.

PHYSICAL FEATURES

COLORADO RIVER BASIN

The Colorado River basin within Utah con tains approximately 41,000 square miles, with plateaus and mountains ranging in altitude from 3,000 to 13,000 feet above sea level. The Uinta Mountains bisect the Green River basin near the Utah-Wyoming State line, forming the northern boundary of the Utah portion of the basin. The Wasatch Mountains and High Plateaus form the western boundary. Other mountain ranges and plateaus are in terspersed throughout the area. Some of the more prominent ranges are the LaSal, Abajo, and Henry Mountains. Another orographic barrier in the Colorado River basin, known as the Book Cliffs, extends in a general east- west direction near the central part of the State. Drainage is predominately by streams with deep canyons and relatively steep'slopes. Drainage areas of streams used in the study for the Colorado River basin range from 2 to 77,000 square miles.

The vegetal cover within the region varies considerably in kind and in growth depending largely upon the amount and seasonal distri bution of precipitation. Parts of the Colorado River basin have an extremely shallow soil mantle with some large areas of exposed sandstone. Where these conditions exist the vegetal cover ranges from sparse to almost nonexistent.

THE GREAT BASIN

That part of the Great Basin considered in this study (approximately 18,000 square miles) includes the Wasatch Mountains and the western portions of the High Plateaus. The Wasatch Mountains extend southward from the Utah-Idaho State line and connect with the High Plateaus to form a north-south divide ending near the southwest corner of the State.

The geology of the Great Basin is complex and is not discussed here except in general terms. The Wasatch Mountains are charac terized by rugged topography with high peaks and deep canyons. The mountains are long narrow ridges which rise abruptly from the valley floors with relatively small foothill areas. Several streams, including the Bear, Weber, and Provo Rivers, are classed as relatively old and have cut through the high mountain ranges in deep canyons. Many of the smaller streams in the area are younger in origin, and for the most part are respon sible for much of the more recent erosion.

The High Plateaus consist of three parallel strips of tabletop formations that are com mon to both the Great Basin and the Colorado River basin. The altitudes range from 5,000 to 10,000 feet above sea level. The streams originating in the High Plateaus have steep slopes leading to gently sloping valley floors. When a stream reaches the mouth of a can yon, where the gradient is lower, the flow often spreads out on the valley floor. In some places, the surface flow disappears as the streams cross the alluvial fans at the en trance to mountain valleys. These conditions are more prevalent in the Great Basin, but apply to some degree in the Colorado River basin.

The vegetal cover in the Great Basin is similar to that in the Colorado River basin; however, in the northern part of both basins, the average annual precipitation is higher, and at some altitudes the forest is denser. Drainage areas of streams used in this study for the Great Basin range from 10 to 8,000 square miles.

The modifications of peak discharge by different geological formations are noticeable in both the Colorado River basin and the Great Basin hydrologic areas. One example of this modification is in the Logan River basin where extensive limestone deposits have a retarding effect on major floods by absorbing some of the surface flow. In other areas, the exposure of impervious formations hastens the rate of runoff. In general, the streams materially affected by geology are widely scattered, and because the flood data do not define these effects sufficiently, it is considered impractical to separate these

FLOOD-FREQUENCY RELATIONS

drainage basins from the larger hydrologic areas.

SNAKE RIVER BASIN

Flood-frequency relations for the Snake River basin in Utah, a small area in the north west corner of the State, are not included in this report. A flood-frequency report (Thomas, Broom, and Cummans, 1961) for the Snake River basin includes that area of Utah.

MAJOR RIVERS

The discharges of the main stems of the larger rivers are so modified by diversion and storage that only extreme floods would reflect the general characteristics of geology, topography, and other natural factors. Fur thermore, the large rivers drain more than one flood region. Studies of frequency and magnitude for the larger rivers are, there fore, considered independently of the flood regions. The slopes of most streams, in cluding the main stem of the larger streams, are relatively steep and for most channel reaches, floods of 50-year recurrence inter val are confined within their natural banks.

CLIMATIC FEATURES

The climate of the State of Utah ranges widely from arid to humid over rather short distances. The average annual temperature is about 48°F. Extreme temperature obser vations recorded are a maximum of 116°F at St. George and a minimum of -50°F at Wood ruff. These temperature variations have a considerable effect on the accumulation of snow. The annual snowfall ranges from less than 6 inches at St. George to 346 inches at Silver Lake in the Wasatch Mountains.

Utah is a region of relatively low rainfall with an average annual precipitation of about 12.6 inches. During an average year there are only 57 days on which 0.01 inch or more of precipitation falls anywhere in the State.

Variations in precipitation during two sea sons of the year have a major effect on floods. During the winter months, when Pacific

storms frequent the region, precipitation oc curs in the form of snow. The precipitation during this season is the most consistent and becomes the major source of streamflow. In the northern part of the State, most streams are fed chiefly from perennial snowfields and the amount of runoff is a function of the snow fall. The southern part of the State is also subject to precipitation in the form of snow, and snowmelt floods may be significant at times. However, the annual maximum floods are usually caused by thunderstorms, and only on rare occasions is the annual maxi mum flood produced by a combination of rain and snowmelt. The second season is during the late summer and early fall when thunder storms occur. These storms have a different source than the winter storms; the moisture, usually following a high-pressure system, is carried from the Gulf of Mexico by unstable air masses over the arid regions of south western United States. Occasionally, a storm from the Gulf of California extends into the State from the southwest, and some floods have been caused by this condition.

Some of the thunderstorm activity develops over the desert areas without regard to change in elevation and are triggered by con vection currents. The amount of moisture available and the ambient temperatures play important roles in the summer storms. An important factor influencing the paths of thunderstorms is the proximity of the storm to mountains. Thunderstorms usually affect relatively small areas; therefore, a flood at a site will often be caused by a high-intensity storm covering only a part of the drainage basin above that site.


Methods developed by engineers of the Water Resources Division of the Geological Survey and others were used to compute magnitude and frequency relations for streams in Utah. Steps taken in the analysis were: (a) computation of a flood-frequency curve for each gaging station, (b) the com bination of individual curves to define regions of homogeneous flood-frequency relations, and (c) computations of the mean annual flood from the related drainage basin character istics.

FLOODS IN UTAH. MAGNITUDE AND FREQUENCY

FLOOD FREQUENCY AT A GAGING STATION

BASIC DATA

Streamflow records from 167 gaging sta tions (fig. 1) with 5 or more years of record were used in the analysis. Selection of records from 118 stations was based on the condition that the flood peaks resulted from natural flow. Records from the remaining 49 stations are for the major rivers and are af fected by diversions or storage.

The effect of diversions or storage on flood flow may be inconsequential for extreme floods but may be significant for lesser floods. If peak flow was not affected by more than 10 percent, it was assumed that the streamflow record could be used without ad justment. Table 1 contains a list of the gag ing stations for which records were used in the flood-frequency analysis. The table shows the maximum known flood and other pertinent data for each station.

To place all records on a comparable basis, it is desirable that they be adjusted to the same time period. The period 1938 57 was used for 71 stations in flood regions A and B (fig. 2). Except in flood regions C and D, the order of magnitude of the. known peaks in shorter records was adjusted to this base by correlation with p<^aks at nearby stations.

FLOOD SERIES

Two methods for analyzing floods are by the annual flood series and by the partial- duration series. In an annual flood series, only the maximum instantaneous discharge for each water year is listed. In a partial- duration series, all floods equal to or greater than an arbitrarily selected base are listed. It should be noted that, in the annual flood series, no consideration is given to secondary floods, which in some years may be greater than the maximum for other years. On the other hand, in the partial-duration series, some water years may have no flood as great as the selected base discharge.

The recurrence interval in an annual flood series is defined as the average interval of time within which the given flood will be equaled or exceeded once as an annual maxi mum. The recurrence interval in a partial- duration series denotes elapsed time without

regard to the water year or any other selec ted time unit. There is a statistical relation between recurrence intervals computed by the two methods, as shown in the following table by Langbein (1949):

Recurrence intervals, in years

Annual flood series

Partial- duration series

1.16.1.58.2.00.2.54.5.52.

10.5...20.5...50.5...

100.5...

0.51.01.452.05.0

102050

100

The annual flood series was used in this report. As will be noted from the table, re currence intervals are essentially the same in both series for periods greater than 10 years. For those desiring information on the basis of the partial-duration series, it is suggested that results be computed by meth ods described in this report and conversion made by use of the table.

STATION FREQUENCY CURVE

The annual peak discharge for each station was listed in order of magnitude with no. 1 as the largest. The time scale, designated as the recurrence interval (T) and plotted as the abscissa on special probability paper (Gumbel, 1941; Powell, 1943), was fitted to the data by the formula

n + 1 T = ~^~

where a is the number of years of record and a is the relative magnitude of the event with the largest as 1. Corresponding peak dis charges were plotted as ordinates, and a smooth curve was then drawn on the basis of the plotted points. The use of extreme value probability paper tends to cause the plotted points for many stations to fall in a straight line.

REGIONAL FLOOD FREQUENCY

A composite frequency curve based on rec ords for many gaging stations is a better tool for estimating the magnitude of future floods


114* Itt*

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Figure 1. Map of Utah showing location of gaging stations.

Tabl

e 1.

Peak

disc

harg

es a

t gag

ing

stat

iMs

used

in fr

eque

ncy

anal

ysis

p

Sta

tion

No.

Gag

ing

stat

ion

Dra

inag

e ar

ea

(sq

mi)

Per

iod

of

reco

rd

Mea

n al

ti

tude

(f

eet)

Are

alQ

2.33

(cfs

)

Max

imum

flo

od

s

Dat

eD

is

char

ge

(cfs

)

Rat

io

to

are

al

Q2.

33(c

fs)

Flo

odre

gio

n

and

hydro

- lo

gic

are

a

CO

LO

RA

DO

R

IVE

R

BA

SIN

9-18

10

1820

1830

1840

1855

1860

1865

1870

1875

2185

2200

2205

22fi

O

2265

2275

2330

23

40

26

40

Trib

utar

ies

betw

een

Dol

ores

Riv

er a

nd G

reen

Riv

er

Oni

on C

reek

nea

r M

oab,

Uta

h___

____

___

____

____

Cas

tle

Cre

ek a

bove

div

ersi

on

s, n

ear

Moa

b, U

tah-

Court

house

Was

h nea

r M

oab,

U

tah

____

____

____

_M

ill

Cre

ek n

ear

Moa

b, U

tah

____

____

____

____

___

Hat

ch W

ash

nea

r L

a S

al,

Uta

h __

____

____

____

___

Indi

an C

reek

nea

r M

on

tice

llo

, U

tah.

___

____

____

_In

dian

Cre

ek a

bove

. C

otto

nwoo

d C

reek

nea

rM

on

tice

llo

, U

tah.

Cot

tonw

ood

Cre

ek n

ear

Monti

cell

o,

Uta

h __

____

__In

dian

Cre

ek a

bove

Har

ts D

raw

, nea

rM

onti

cell

o,

Uta

h. Gree

n Ri

ver

basin

Bla

cks

Fork

nea

r M

illb

urn

e, W

yo _

____

____

____

_E

ast

Fo

rk o

f S

mit

h F

ork

nea

r R

ober

tson,

Wyo

,.-.-

Wes

t F

ork

of

Sm

ith

Fo

rk n

ear

Ro

ber

tso

n,

Wyo

___

Hen

ry's

F

ork

nea

r L

onet

ree,

Wyo

____

____

__- _

Mid

dle

Fo

rk B

eav

er C

reek

nea

r L

onet

ree,

Wyo

__

Wes

t F

ork

Bea

ver

Cre

ek n

ear

Lonet

ree,

Wyo

____

Cart

er

Cre

ek n

ear

Man

ila,

Uta

h __

____

____

____

_C

arte

r C

reek

at

mou

th,

nea

r M

anil

a, U

tah

____

__A

shle

y C

reek

bel

ow T

rout

Cre

ek n

ear

Ver

nal

, U

tah.

18.8

7-. 5

8

150

76 370 4.

531

.2

115

257

156 53 37

.256 28 23 19 MO 27

1950-5

5;

1959

1950

-55;

1957

-59

1949

-55

1914

-19;

1949

-59

1950

-59

1949

-57

1949

-59

1949

-57

1949

-57

1939

-59

1939

-59

1939

-59

1942

-59

1948

-59

1948

-59

1948

-54

1946

-55

1943

-54

5,81

0

9,48

0

4,81

07,

170

6,55

09,

620

7,13

0

7,21

06,

580

10,2

7010

,250

9,79

010

,270

10,4

8010

,680

10,2

809,

010

9,93

0

677 34

2,21

01,

070

939 21 93 340

2,10

0

1,14

049

732

552

232

930

222

956

534

3

Aug

. 29

, 19

51

Jun

e 7,

19

52

Aug

. 5,

19

57A

ug.

21,

1953

Aug

. 4,

19

59A

ug.

6,

1955

July

20

, 19

55

July

10

, 19

53A

ug.

30,

1957

Jun

e 7,

19

57Ju

ne

13,

1953

May

30

, 19

50Ju

ne

13,

1953

Jun

e 12

, 19

53Ju

ne

13,

1953

Jun

e 3,

19

52Ju

ne

4,

1952

May

19

, 19

48

2,10

0 23

12,3

005,

110

3,21

012

258

2

2,14

03,

120

2,53

01,

200

920

1,86

066

341

715

392

863

0

3.10 .67

5.57

4.78

3.42

5.84

6.26

6.29

1.49

2.22

2.41

2.83

3.56

2.02

1.38 .67

1.64

1.84

D9

D8

D9

D9

D8

D8

D8

D8

D9

Al

Al

Al

Al

Al

Al

Al

Al

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1939

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1942

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105

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57Ju

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7, 1957

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23

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52Ju

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14,

1921

Apr.

28

, 19

52M

ay

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1950

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11

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57

May

Mar

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ne

May

May

May

8,

1952

18,

1947

7,

1957

4,

1952

11,

1952

18,

1950

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---.

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M

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10 FLOODS IN UTAH, MAGNITUDE AND FREQUENCY

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alin

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tah _

____

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____

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00

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6 57

621

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365

481

261

317

352

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May

2

0,

1958

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19

52

July

17

, 19

32M

ay

4,

1952

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e 6,

19

57Ju

ne

4,

1957

May

3,

19

45Ju

ne

13,

1953

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19

51

May

26,

1922

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19

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17,

1955

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27,

1953

May

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19

52

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19

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9,

19

36

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0

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0

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3,88

0 9930

424

6

1,49

0

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1

2,6

50

509

386

2,9

10

1.27

3.67

2.27

3.14

3.30

4.08

5.08

2.36

3.37 .6

42

.62

4.34

2.4

0

3.61

1.67

5.51

1.95

1.22

8.27

B4

B4

B4

D6

C6

C5

C6

B4

B4

B4

B4

B4

D6

D6

D6

D6

D6

D6

C6


'its* HCX" % in*

4

40-

2S 0 25 90 =

STATUTE MILES

3T112" ^ A 111'

Figure 2. Map of Utah showing flood regions.

110*

FLOOD-FREQUENCY RELATIONS 13

than a curve based on records for any one site. To combine records, the relative mag nitude of floods must be computed on a dimensionless basis. The ratio of each flood to the mean annual flood, or simply the flood ratio, is a means of satisfying this premise. Floods used in computing composite flood- frequency curves are expressed as the me dian flood ratio for each recurrence interval.

REGIONAL SAMPLING

Sampling for a flood-frequency study can be accomplished by two methods time and areal sampling. Time sampling involves the collection of records at a few long-term sta tions, while areal sampling requires a large number of stations to evaluate the various characteristics of the region, without regard for the length of record. Streamflow records at the present time do not satisfy the re quirements of either method, since the long- term records or the density of stations nec essary to complete a statistical approach do not exist. An analysis of existing records will be helpful until such time as additional information can be obtained. To combine records from all the stations, it is advanta geous to place the flood on a comparable basis as to time and physical characteristics of the drainage basin.

SELECTION OF COMPARABLE FLOODS

The peak flows of a stream at a given point integrate all of the flood characteristics of the drainage basin. A suitable index for the effect of physical features of a basin is the mean annual flood. Theoretically, a peak discharge with a recurrence interval of 2.33 years is the same as the arithmetical mean of an infinitely long series. An estimated value of the mean annual flood(^2.33)may be obtained graphically from the flood-frequency curve for each individual station. All of the floods are placed on a dimensionless basis by dividing the recorded floods by the mean annual flood; thus, flood records for all gag ing stations within the region are comparable irrespective of flood magnitude.

HOMOGENEITY

Before a group of stations can be combined on a regional basis, a test of homogeneity is

necessary to show that all of the records are from a region of similar flood-frequency characteristics. A statistical test is applied to the slope of the individual frequency curves to determine whether or not the deviations from an average slope are due to chance alone. The selection of the flood regions shown in figure 2 was based on the results of the homogeneity test with consideration given to geographic differences.

COMPOSITE FREQUENCY CURVE

When the requirements of the homogeneity test were satisfied, the stations were grouped together and flood ratios for several recur rence intervals were computed. Ratios of floods from each record for the selected re currence intervals were listed, and the me dian flood ratios were obtained from this tabulation. Each median flood ratio was plotted to its corresponding recurrence in terval and a curve was fitted smoothly through these points (figs. 3 and 4). These curves, with the flood discharge expressed in terms of the ratio to the mean annual flood, were based on all of the significant discharge rec ords and represent the most likely flood- frequency relation for each respective re gion. Ratios to. the mean annual flood for any recurrence interval are obtained from the composite flood-frequency curve. At any desired recurrence interval, the magnitude of the discharge is estimated by multiplying this ratio by the mean annual flood.

REGIONAL ANALYSIS FOR SUMMER FLOODS

For flood regions C and D, the length of Streamflow records range from 5 to 10 years with a few records of longer periods for large drainage basins. Floods in these re gions are generally caused by summer thun derstorms. The extension of records and the determination of the order of magnitude of floods by correlation methods are unsatis factory in these regions because each flood occurrence may be entirely independent of any other flood. A flood may occur on one stream while no flow would be experienced on a nearby stream.

If a station record of 1,000 years were available, it would be reasonable to assume that 20 floods having a recurrence interval of 50 years or more may be experienced.


1.5 2 345 10 RECURRENCE INTERVAL, IN YEARS

Figure 3. Composite flood-frequency curves, regions A and B.

20 30 40 50

7o O O

_» 4<ID'

< LU

7

i

1.1 1.5 2 345 10

RECURRENCE INTERVAL, IN YEARS

Figure 4. Composite flood-frequency curves, regions C and D.

20 30 40 50


Within regions C and D, there are records of 690 annual peaks. If the same analogy is ap plied, then 2 percent of these annual peaks should be 50-year floods or greater. From an envelope curve drawn on a plot of annual floods versus drainage area to exclude 2 percent of the floods, it is possible to obtain an estimated 50-year flood for each of the 47 stations. The estimated flood selected from the envelope curve is used as a guide to extend the individual flood-frequency curves. Although it is difficult to meet all of the conditions set forth in an accepted re gional analysis, the two composite frequency curves (fig. 4) should be a suitable substitute for predicting peak discharges until additional hydrologic information is available.

DERIVATION OF THE MEAN ANNUAL FLOOD

To use the regional flood-frequency curve, a means must be provided for the determi nation of the mean annual flood.

BASIN CHARACTERISTICS

Characteristics of the basin as they affect floods are numerous, and many are difficult to define or separate. The ones selected and investigated in this study are as follows:

1. Drainage area. Noncontributing area is normally deducted from the total.

2. Mean altitude. The mean altitude of a basin is the average altitude above a given point.

3. Slope. The slope of a stream is the total fall between any two points divided by the stream length between those points. The points selected were at 0.2 and 0.8 of the total stream length above the site.

4. Shape. The shape of a basin is approx imated by the length-width ratio.

5. Aspect. The aspect of a basin is the compass direction of the main stream and is a measure of the usual storm pattern in the area as well as exposure to prevailing winds and sunshine. This characteristic is signifi cant when considering the peak discharge that results from snowmelt.

The above five characteristics were tested for effect on the mean annual flood in order to determine which were significant. Curves were derived by a multiple correlation of the mean annual flood with the two most signifi cant characteristics, drainage area and mean altitude of the basin. Use of the remaining three characteristics did not significantly improve the statistical correlation. Slope, shape, and aspect probably have some effect on the mean annual flood, but the effect, if any, cannot be adequately defined on the basis of available streamflow records.

MEAN ANNUAL FLOOD RELATIONS

The relationship of the mean annual flood to basin characteristics was correlated from the records of 188 gaging stations. Bound aries of the selected hydrologic areas (fig. 5) are somewhat arbitrary; however, the pres ent program of data collection will aid in the verification of these boundaries. For each hydrologic area, a family of curves was drawn (fig. 6). The use of the curves is limited by the range of the basic data.

For all hydrologic areas except 3, and 9, the mean annual flood was found to be directly related to drainage area and mean altitude of the basin. In area 3 the effects of limestone deposits are known to change the character istics of streamflow when compared with that from surrounding drainage basins where limestone deposits are not present or pre dominant. Also, in area 3, mean altitude was not used as a parameter because of relatively few gaging-station records. In hydrologic area 9, where the peak discharge generally results from summer storms, the mean an nual flood is directly related to the drainage area; however, it is inversely related to the mean altitude of the basin. One condition which could contribute to this fact is the marked change in vegetal cover with respect to altitude. Vegetal cover is almost nonex istent at the lower altitudes, whereas at the higher altitudes the cover is sufficient to re tard runoff. In area 9, winter precipitation in the form of snow at the higher altitudes aids the growth of a vegetal cover. Frequently, summer storms are triggered before reach ing the higher altitudes, thus causing larger floods to occur at lower altitudes.


25 0 2MP"I

STATUTE MILES

114" 11 112* 'N 111*

Figure 5. Map of Utah showing hydrologic areas.

no* __l 37«


§ 1000

0

2

8 10°o_l

Xx

i>"ff^

ARE

X

A 3

Li_ 10

_J< 3000

Z Z

1000Z

100

UJ>,,. \

-.'*

100 300

1000

2000

1000

10.000

1000 10 100

DRAINAGE AREA, IN SQUARE MILES

1000

1000 5000

Figure 6. Variation of mean annual flood with drainage area and mean altitude.


MAJOR RIVERS IN UTAH

Inmost reaches of the major rivers, nearly complete development of the water siipply for

o

1°< Q* O i ftrt

fro uj 80

u!'s>

*

I* - _

EXPLANATION

Gaging stationa

1

L

c

IS

I5 i

120 150 180 210 240 270 300 33

DISTANCE, IN MILES, ABOVE UTAH-ARIZONA STATE LINE

Figure 7. Colorado River, discharge of 50-year flood within the State of Utah.

50-YEAR FLOOD, IN THOUSANDS OF CUBIC FEET PER SECOND

9 8 S S 8 S

> Green River

> n= 'o

CLx «> «o>

1

c «lr=

iEXPLANATION

Gaging stationa

C

«ST,c o « o

ill

ity or town

200 300 400 500

DISTANCE, IN MILES, ABOVE MOUTH

50-YEAR FLOOD, iN THOUSANDS OF CUBIC FEET PER SECOND 3

*- H- ro M P

^o 01 o 01 e5 '"re

8. Green River, discharge of 50-year flood below Wyoming -Utah State line.

5

K

&In*

! -

20

S.c u a Q

ir Tabioni

EXPLAr

City o

!;!Liiil

4ATION 1

i r town

40 60 80 100 120 14

"

10 20 30 40 50


Figure 10. Strawberry River, discharge of 50-year flood.

50

30

3"- 20

2

1

Near We

\

^^

'ellington

yNearV Vellingtor

XS. «

3-JL

CL « SC. «

k^a ^«X

EXPLA4

i _ Gaging

City c

2

\j

NATION

station.i r town

20 40 60 80 100 120 14


Figure 9. Duchesne River, discharge of 50-year flood.


Figure 11. Price River, discharge of 50-year flood.

irrigation and power is reflected by the ab sence of homogeneity with the tributary streams. The major rivers are not homo- geneoiis among themselves; therefore, the use of a composite ratio curve for estimating flood frequencies similar to methods used for the smaller streams is unsatisfactory. Each major river was treated individually for es timating the magnitude and frequency of floods. Table 2 contains a list of gaging sta tions for which records were analyzed on the above basis. The data shown for each station are drainage area, length of annual peak rec ord, and estimated discharge for the 50-year flood. The discharge for the 50-year flood at each gaging site was plotted against the cor responding river miles above the mouth (figs. 7 17). With the regimen of the rivers affected by regulation and diversion, it was impracti cal to compute the magnitude of the floods for recurrence intervals of less than 50 years.


50-YEAR FLOOD. IN THOUSANDS OF

CUBIC FEET PER SECOND

m 5 5 8 8 £,N

Green Ri

1\\

EXPLANATI

Gaging stat

\"»

r Castle I

^

3N

»n

0 20 40 60 80 100 120 140 DISTANCE, IN MILES, ABOVE MOUTH

Figure 12. -San Rafael River, discharge of 50-year flood below Perron Creek.

iia 8gs* Sz «-

pi

00 120 140 160 180 200 220 240

DISTANCE. IN MILES, ABOVE MOUTH

13. San Juan River, discharge of 50-year flood below Shiprock, N. Mex.

50-YEAR FLOOD, IN THOUSANDS OF

CUBIC FEET PER SECOND * o» oo o M J

b

ao

I£

to

^I

I

c

-CM\

EX

C

»'i «

PLANATI

ging stati a

ity or tow

%

I }"TO

DN

on

n

40 24080 120 160 200


Figure 14. Hear River, discharge of 50-yearflood.

10

8

6

4

2

8 5Near PI

nU.S. Am - disch

regul

nio

^*^

a*~*~*

itj

EXPLANATION

Gaging stationa

City or tov

-1

ly Engin urge unc >tion for

vers esti er propo flood co

m

mated rr sed rese ntrol.

,I

1 _aximum rvoir

,~~\

1

halk Creek (flows into reservoi

Coalvllle'

Echo Reservoir, c1 tl

a

1:!

1 | inRockyport Res*

10 20 30 40 50 60

DISTANCE. IN MILES, ABOVE MOUTH

70

Figure 15. Weber River, discharge of 59-year flood and U.S. Army Engineers'estimated maximum discharge under proposed flood-control project below Rockport Reservoir.

0

iiis ax "1- 1C

O ".io ft oa

i

50-YEAR FLOOD, IN THOUSANDS OF 3 CUBIC FEET PER SECOND *§

- 10 ea " 0

.,^4

tt ^^

i

|I

EXPLAf

ftsI

ATION

Gaging, station a

City or town

!

LJI0

ri

-1-

S 10 IS


16. Provo River, discharge of 50 -year flood below Deer Creek Reservoir.

EXPLA

- Gagini

Crtye

M'5

C-4

NATION

ir town

-*

b

1

i J

Gunnison

s iFi -I *1 %U-lL? r

fc e

J 1

LJ

11

0 40 80 120 160 200 240 280DISTANCE, IN MILES, ABOVE MOUTH

Figure 17. Sevier River, discharge of 50-yearflood below Piute Reservoir.


Table 2. Main stem gagwfl-station data

Station No.

Gaging stationDrainage

area (sq mi)

Period of record

50-year flood (cfs)

COLORADO RIVER

9-1635__ Colorado River near Colorado-Utah State line___. 1805__ Colorado River near Cisco, Utah_______________.

3350__ Colorado River at Kite, Utah _._._.__...__.___..

GREEN RIVER

2255__ Green River near Linwood, Utah ________________2345._ Green River near Greendale, Utah ______________2610__ Green River near Jensen, Utah _________________

3070__ Green River near Our ay, Utah__________________

3150__ Green River at Green River, Utah_______________

DUCHESNE RIVER

2740__ Duchesne River near Hanna, Utah _______________

2770.. Duchesne River at Hanna, Utah .................2775.. Duchesne River near Tabiona, Utah __.___.._....2795__ Duchesne River at Duchesne, Utah ______________2950__ Duchesne River at Myton, Utah ....._......_....3020._ Duchesne River near Randlett, Utah_____________

STRAWBERRY RIVER

2850._ Strawberry River near Soldier Springs, Utah..... 2885.. Strawberry River at Duchesne, Utah____________.

PRICE RIVER

3115__ Price River near Scofield, Utah.._..............

3130__ Price River near Heiner, Utah._________________3135__ Price River near Helper, Utah__________________

3140__ Price River near Wellington, Utah ______________3145__ Price River at Woodside, Utah._________________

17,90024,100

76,600

1951-59 1914-17, 1923-59 1947-58

61,00076,000

123,000

14,30015,10025,400

35,500

40,600

1929-591951-59

1904, 1906,1947-591948-55,1957-591895-1905,1906-59

23,00024,00059,000

71,000

70,000

78

230352660

2,7503,920

1922-23,1946-591954-591919-591918-591900-591943-59

1,640

2,8602,5504,500

14,00018,000

2121,040

1943-561909-10,1914-59

3,0503,600

163

455530

8501,500

1918-20,1926-27,1929-31,1939-44,1946-591935-591905-06,1908-11,1913-341950-581909,1946-59

1,320

12,50012,500

21,80030,000


Table 2. Main stem gagmg-sfation data Continued

21

Station No.


area (sq mi)

Period of record

50-year flood (cfs)

SAN RAFAEL RIVER

9-3280.. San Rafael River near Castle Dale, Utah.-..-... 3285.. San Rafael River near Grten Rivtr, Utah - --,

SAN JUAN RIVER

3680.. San Juan River at Shiprock, N. Mex __,---_____, 3795.. San Juan River near Bluff, Utah................

BEAR RIVER

10- 440__ Bear River at Harer, Idaho......._, .,..,.....905.. Bear River near Preston, Idaho.____ _________.

1180,. Bear River near Collinston, Utah ,___.._._..__.

WEBER RIVER

1295.. Weber River near Wanship, Utah______.________

9271,690

1948-591909-18,1946-59

7,10027,000

12,90023,000

1928-591915-17,1927-59

99,00085,000

2,7804,300

6,000

1914-591890-98,1900-02,1904-16,1944-591890-1959

4,4507,650

11,600

1305.1320.1335.1360.1365.

1370.1410.

Weber River near Coalville, Utah. Weber River at Echo, Utah-_____. Weber River at Devils Slide, Utah. Weber River near Morgan, Utah . Weber River at Gateway, Utah.-..

Weber River at Ogden, Utah ______Weber River near Plain City, Utah

320

438732

1,100

1,610

2,060

1951-55,19581927-591927-581905-551951-551890-93,1895-99,1901,1921-591951-581905-59

4,8006,5008,0009,800

9,20010,000

PROVO RIVER

1595.1605.1610.1630.

Provo River below Deer Creek Dam, Utah Provo River near Wildwood, Utah____ ___Provo River at Vivian Park, Utah ________Provo River at Provo, Utah _____________

560590600680

1954-591939-491912-591903-05,1934,1937-59

2,8703,0303,2002,550

SEVIER RIVER

1915.

1940.

Sevier River below Piute Dam, near Marysvale,Utah.

Sevier River above Clear Creek, near Sevier, Utah

2,440

2,700

1912-59

1914-16, 1939-55

2,700

2,700

22 FLOODS IN UTAH. MAGNITUDE AND FREQUENCY

Table 2. Main stem gaging-station data Continued

StationNo.


area (sq mi)

Period of record

50-year flood (cfs)

SEVIER RIVER Continued

10-1955__2050_.2170__

2190..2240_.

2280..2315__

Sevier River at Sevier, Utah. ___________________Sevier River near Sigurd, Utah _________________Sevier River below San Pitch River, near

Gunnison, Utah. Sevier River near Juab, Utah ___________________Sevier River near Lynndyl, Utah _________ _______

Sevier River near Delta, Utah __________________Sevier River at Oasis. Utah ____________________

2,8503,3404,880

5,1206,270

7,3808.080

1917-291915-591918-59

1912-591914-19,1943-59 1913-191913-27

2,9002,5002,700

2,2001,740

1,6001.580

CLOUDBURST FLOODS AND MUD-ROCK FLOWS

Cloudburst floods and mud-rock flows are common to the southern parts of both the Colorado River basin and the Great Basin in Utah. These phenomena occur at less fre quent intervals in other areas of the State. Most of the annual flood peaks in the southern regions occur during the summer storm period, as previously described, and are in cluded in the flood-frequency relations for that region. In contrast to the annual floods observed in the snowmelt regions, occasional peak flows resulting from cloudburst storms in the central and northern parts of the State may have yields of 5,000 cfs (cubic feet per second) per square mile or more from small drainage basins. This is not to imply that these excessive flows occur more frequently in any one section of the State; instead, it is to acquaint the reader with the fact that these events have been observed. Although cloud burst storms may occur on many days in one season and may be distributed over a rather wide area, the high-intensity rainfall is lim ited to very small areas, often less than 1 square mile. Observations of rainfall inten sities from unofficial observers have shown that as much as 7 inches fell in less than 1 hour during one of these storms. The prob ability of a cloudburst or high-intensity rain fall recurring in the same small drainage area during consecutive years is rather re mote. Experience shows that in some drain age basins, the frequency of floods may vary from three in one year to one in each of three consecutive years to none in several years.

Some drainage basins are subject to more cloudburst floods than others in the same general locality because of physical features such as topography, aspect, vegetal cover, and other contributing factors. One example is Pleasant Creek near Mount Pleasant where a large alluvial cone covering more than 12 square miles is evidence of frequent cloud burst flooding.

A cloudburst flood may occur without pro ducing a mud-rock flow. Although mud-rock flows may be associated with cloudburst floods, the presence of certain soil conditions are required to produce them. Mud-rock flows are flows of mud, rock, debris, and water, mixed to a consistency similar to that of wet concrete. A wide variety of these flows has been observed from those carry ing a small load of sediment to others moving large amounts of mud, rock, and other debris. Some flows have just enough water to lubri cate the mass of moving material and usually travel at a relatively slow velocity. Because of infrequent observation of these flows, it is difficult to estimate the probable recurrence interval at any given site. It has been noted by Wooley (1946) that the total discharge of debris and water can exceed 2,000 cfs per square mile.

The cost of replacing bridges, culverts, and sections of highway and other economic- factors will determine whether design should be for rare floods of the above type which may have a recurrence interval in excess of 50 years.

LIMITATIONS 23

APPLICATION OF FLOOD-FREQUENCY CURVES

The application of flood-frequency curves to any drainage basin within the scope of this report required analysis of the basin char acteristics vfrhich effect peak discharge. The significant characteristics are those most readily available from topographic maps.

METHOD

To obtain the magnitude and frequency of floods for any basin within the study area, except on major rivers, the procedure is as follows:

1. Determine the drainage area above the site by planimeter or other acceptable pro cedure.

2. Locate site on figures 2 and 5 and de termine the flood region and hydrologic area involved.

3. Determine the mean altitude for drain age basin above the selected site, unless ba sin is in hydrologic area 3. In this report the mean altitude was determined by using a grid system of rectangular coordinates as an overlay on topographic maps. From a list of the average altitudes of each square, an ar ithmetical mean was computed. The squares were of equal size and small enough to give a representative sampling, but not of a known scale. Contour maps of the Army Map Series, scale 1:250,000 were used.

4. Determine the mean annual flood for the site from hydrologic area curve selected in step 2 (fig. 6). One exception is that to de termine the mean annual flood of the Dirty Devil River near Kite, Utah (9-3335), use hy drologic area 8 curve. This exception applies only to the main stem and should not be used for the tributary streams of the Dirty Devil River outside of area 8.

5. Determine the flood ratio to the mean annual flood for the selected recurrence in terval from the appropriate flood-frequency curve obtained in step 2 (figs. 3 and 4). When the drainage area above the site is near an other flood region or lies in two regions with widely different flood ratios, it is suggested that a weighted average of the two flood ra tios be used.

6. Estimate the magnitude of the flood by multiplying the mean annual flood (step 4) by the flood ratio (step 5).

A complete frequency curve for a site may be derived by repeating steps 5 and 6 for various recurrence intervals.

USE OF FLOOD-FREQUENCY ANALYSIS ON MAJOR RIVERS

To determine the magnitude of the flood for a 50-year recurrence interval at a given location on a major river, scale the mileage of the main stem from a known reference point as indicated on each graph (figs. 7 17). Use of the main stem analysis will require some knowledge of local conditions and the operation of any reservoirs involved. The Weber River is the only major stream that has a specific flood-control plan. The pro posed magnitude of the controlled flow is shown(fig. 15) with the flood experience prior to the initiation of this plan.

LIMITATIONS

The results of this study may be used to estimate the magnitude and frequency of floods, with recurrence intervals up to 50 years, for any drainage area in the State with the exception of the Great Salt Lake Desert and the Snake River basin.

The relationships of the hydrologic fea tures of the drainage basins were defined from the records on streams with natural flows. To obtain the magnitude of any flood on any regulated stream, adjustments should be made for manmade development. Also, the varying effects of geology and vegetal cover were not given detailed consideration in this study. An investigation and evaluation of these factors may be justified in some spe cific areas.

The results obtained from the flood- frequency study are limited by the basic data available. The degree of confidence that can be placed in the results is closely related to the quantity of data used in defining the var ious curves. Extrapolation of curves beyond the limits shown is not recommended, and values obtained by such practice may be greatly in error.


SELECTED BIBLIOGRAPHY

Bodhine, G.L., and Thomas, D.M., 1960, Floods in Washington, magnitude and frequency: U.S. Geol. Survey open-file report.

Fisher, R. A., 1954, Statistical methods for research workers: New York, Hafner Pub lishing Co.

Gumbel, E. J., 1941, The return period of flood flows: Annals Math. Statistics, v. 12, no. 2, p. 163-190.

Langbein, W. B., 1949, Annual floods and the partial-duration flood series: Am. Geo- phys. Union Trans., v. 30, p. 879-881.

Linsley, R. K., Jr., Kohler, M. A., and Paulhus, J. L. H,, 1949, Applied hydrology: New York, McGraw-Hill Book Co.

Powell, R. W., 1943, A simple method of esti mating flood frequency: Civil Eng. v. 13.

Thomas, C. A., Broom, H. C., and Cummans, J. E., 1961, Magnitude and frequency of floods in the United States, Part 13, Snake River basin: U.S. Geol. Survey Water- Supply Paper 1688 (in press).

Woolley, Ralph, R., 1946, Cloudburst floods in Utah, 1850-1938: U.S. Geol. Survey Water-Supply Paper 994.

INT.DUP..D.C.62-^1*6

floods in utah, * magnitude and frequency

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