U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 9

Magnitude and Frequency of Floods 

in the Suwannee River Water Management 

District, Florida

Prepared in cooperation with the

Suwannee River Water Management District

6-4176
U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 9

Magnitude and Frequency of Floods in the 

Suwannee River Water Management 

District, Florida

Prepared in cooperation with the

SUWANNEE RIVER WATER MANAGEMENT DISTRICT

6-4176

By G.L. Giese and M.A. Franklin

Tallahassee, Florida

1996
U.S. DEPARTMENT OF THE INTERIOR

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY

Gordon P. Eaton, Director

Any use of trade, product, or firm names in this publication is for descriptive purposes only

and does not imply endorsement by the U.S. Geological Survey.

For additional information write to: Copies of this report can be 

purchased from:

District Chief U.S. Geological Survey

U.S. Geological Survey Branch of Information Center

227 North Bronough Street, Suite 3015 Box 25286

Tallahassee, Florida 32301 Denver, CO 80225-0286
Contents iii

CONTENTS

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

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

Purpose and Scope....................................................................................................................................................... 2

Previous Studies .......................................................................................................................................................... 3

Physical Setting ........................................................................................................................................................... 3

Station Analysis................................................................................................................................................................. 3

Regional Analysis ............................................................................................................................................................. 4

Improved Frequency Estimates By Use Of Weighting ..................................................................................................... 8

Gaged Sites .................................................................................................................................................................. 8

Ungaged Sites on Gaged Streams................................................................................................................................ 8

Summary ........................................................................................................................................................................... 9

References ......................................................................................................................................................................... 9

Appendix 1. Station, regional, and weighted T-year flood estimates for the Suwannee River Water 

Management District .................................................................................................................................................... 14

Figures

1. Map showing location of Suwannee River Water Management District and stream-gaging stations used 

for measuring peak discharges ................................................................................................................................ 2

2. Map showing physiographic regions within the Suwannee River Water Management District ............................ 4

3. Graphs showing elation of flood discharge to drainage area for selected frequencies on the 

Suwannee River....................................................................................................................................................... 5

4. Graphs showing relation of flood discharge to drainage area for selected frequencies on the 

Santa Fe River ......................................................................................................................................................... 5

Tables

1. Flood-frequency regression model for the Suwannee River Water Management District ..................................... 6

2. Probability that a flood of a given recurrence interval will be exceeded during indicated time period ................. 7

CONVERSION FACTORS AND ABBREVIATIONS

Multiply By To obtain

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

square mile (mi2

) 2.590 square kilometer (km2)

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3

/s)
SYMBOLS

iv Symbols List

Symbol Meaning

Ag Drainage area for gaged site, in square miles

Au Drainage area for ungaged site, in square miles

B1T, B2T, B3T Partial regression coefficients

CT Regression constant

Cv Coefficient of variation

DA Drainage area, in square miles

EY Accuracy for a flood estimate, in equivalent years of record

KT Pearson Type III deviate

LE Channel length, in miles

LK Total area of lakes and ponds, in percent of drainage area

M Mean of logarithms of the annual peaks

N Number of items in a data set

n Time interval, in years

P Exceedance probability

PN Probability of at least one exceedance within the specified time interval

Qg Estimate from log-Pearson Type III distributions of T-year flood, in cubic feet per second

QR Estimate from regression equation of T-year flood for gaged site, in cubic feet per second

Qru Regional estimate of T-year flood from regression equation for ungaged site, in cubic feet per second

QT Estimate of the T-year flood from log-Pearson Type III distribution in cubic feet per second

Qu Adjusted estimate of T-year flood for ungaged site, in cubic feet per second

Qwt Weighted estimate of T-year flood at gaged site, in cubic feet per second

R Multiple correlation coefficient

S Standard deviation of the logarithms of annual peaks

SEP Standard-error of prediction

SL Channel slope, in feet per mile

T Recurrence interval, in years

X1, X2, X3... Independent variables in linear regression
Abstract 1

Magnitude and Frequency of Floods in the Suwannee

River Water Management District, Florida

By G.L. Giese and M.A. Franklin

ABSTRACT

Flood-frequency statistics for 2-, 5-, 10-,

25-, 50-. 100-, 200-, and 500-year recurrence

intervals, based on three methods of analysis, are

presented for 25 continuous-record and seven

peak flow partial-record gaging stations in the

Suwannee River Water Management District. The

first method, for gaged stations, utilizes station

records; the second method, for ungaged sites,

utilizes regional regression analysis; and the third

method uses a weighted combination of the

station and regional values. Because the weighted

values utilize two more or less independent

estimates of the peak flow statistic, they are

considered more accurate than the station

estimates or the regression estimates alone. Also,

the use of another weighting scheme to improve

estimates of flood frequency statistics at ungaged

sites is demonstrated.

The karstic nature of much of the

Suwannee River Water Management District

significantly attenuates flood peaks in some

streams by providing substantial subsurface

storage when river stages are high. At such times,

springs discharging into rivers may reverse flow

temporarily and become sinks.

INTRODUCTION

In recent years there has developed a better

awareness that natural systems such as wetlands,

flood plains, native ecological communities, and

aquifer recharge areas within the 7,640-square-mile

SRWMD (Suwannee River Water Management

District) (fig.1) provide vital water-related functions. These functions include water- quality treatment, water supply, flood water conveyance and

attenuation, fish and wildlife habitat, and recreational and economic uses. These systems depend

on the maintenance of the natural variability of the

hydrologic cycle as reflected by the magnitude,

duration, and timing of changing streamflow, rising

and falling water levels of lakes, rivers, and aquifers, and interaction of surface and ground waters.

Alterations to the natural hydrologic regime by

human activities may have adverse effects on the

natural systems and their functions. However, many

aspects of natural-system requirements are poorly

known and must be better understood in order to

establish minimum flow and water-level requirements that will allow adequate water to balance

present and future needs of the natural system with

those of the human population. Quantification of the

natural hydrologic regime that has shaped the current natural system is basic to developing this

understanding and achieving this balance.

Accordingly, in 1994, the USGS (U.S. Geological Survey) and the SRWMD entered into a

cooperative agreement wherein the USGS agreed to

provide, in the course of a long-term program of

investigation, the hydrologic information or tools

needed for the SRWMD to establish minimum flow

and water-level requirements for surface and

ground waters of the SRWMD. This report, dealing

only with flood magnitudes and frequency, is one of

a planned series of studies intended to accomplish

this goal.

The primary motivation for this particular

report is to document the high-flow regime of the

natural hydrologic system of the SRWMD. This

information is available to researchers studying minimum inundation requirements for wetlands, estuar-
2 Magnitude and Frequency of Floods in the Suwannee River Water Management District, Florida

ies, riparian communities, and other ecosystems in

the SRWMD. These needs are beyond the traditional

needs for flood-frequency information in the design

of drainage structures and bridges, flood-plain zoning, and flow regulation. These traditional needs will

also be served by the information in this report.

Figure 1. Location of Suwannee River Water Management District and streamgaging stations used for measuring

peak discharges.

LOCATION OF

SUWANNEE RIVER

WATER MANAGEMENT

DISTRICT FLORIDA

HIGH-FLOW PARTIAL-RECORD

STATION—And number

STREAM-GAGING

STATION—And number

MISCELLANEOUS SITE—And number

SUWANNEE RIVER WATER

MANAGEMENT DISTRICT

BOUNDARY

EXPLANATION

02314200

02313400

02321446

84°00′ 30′ 83°00′ 30′ 82°00′

30°00′

30°30′

29°00′

30′

GEORGIA

G U L F

O F

M E X I C O

02323500

02323000

02319000

02326300 02317630 02315000

02315550 02319500

02321600

02321000

02321700

02322000

02321800

02315200

02326000 02324400

02324500

02325000

02320000

02317620

02320500

02322500 02324000

02322016

02321500 02320700

02326512

02326500

0

0

20

20

40 MILES

40 KILOMETERS

02315500

02314200

Lake

City

Suwannee River

Suwannee River Aucilla

River

Santa Fe River

Econfina River

Waccasassa River

Steinhatchee River

Fenholloway River

02321446

02313400

02320900

02326250

Purpose and Scope

This report is presents flood-frequency statistics

for unregulated gaged sites in the SRWMD for which

10 years or more of peak flow data were available,

provides means of improving estimates at gaged sites,
Station Analysis 3

and presents methods for estimating peak-flow characteristics at ungaged sites. Flood magnitudes are presented for 25 continuous-record stations and 7 partialrecord gaging stations for recurrence intervals of 2, 5,

10, 25, 50, 100, 200, and 500 years, utilizing log-Pearson Type III distributions. Recurrence interval here

can be defined as the average number of years between

flood peaks greater than, or equal to, a specified magnitude. This analysis utilized records up to and including the 1994 water year. Regression equations for

estimating flood-frequency characteristics at ungaged

sites are also presented. Finally, this report includes

methods for improved estimates at both gaged sites

and ungaged sites on gaged streams.

Previous Studies

Franklin and others (1994) listed and ranked

annual peak flows and various consecutive-day high

flows for the SRWMD but did not assign frequencies

of occurrence. Stamey and Hess (1993) presented

flood-frequency characteristics of Georgia streams and

techniques for estimating magnitude and frequency of

floods in rural basins in Georgia. That report included

station analyses and regional regression equations for

streams in the upper Suwannee River basin in Georgia.

Bridges (1982) presented flood frequency analyses for

Florida stations gaging natural flow, including many of

the stations in the SRWMD. That study also contained

regional regression equations for estimating flood-frequency characteristics of ungaged streams. Bridge’s

report, which utilized records only through the 1978

water year, is superceded by this report for stations in

the SRWMD. The U.S. Army Corps of Engineers

(1974) delineated the areal extent of major Suwannee

River flooding for the large floods of April 1948 and

April 1973.

Physical Setting

The SRWMD, located in the north-central part

of Florida, is one of five water management districts

created by the Florida Legislature through the passage

of the Water Resources Act of 1972. The SRWMD is

the smallest in area and the most sparsely populated.

The SRWMD is in the Southeastern Coastal Plain

physiographic province of the United States, as delineated by Fenneman (1938). The SRWMD is covered

by three physiographic regions of this province--the

Northern Highlands, the Gulf Coastal Lowlands, and

the River Valley Lowlands (fig. 2). The areal extent of

overbank flooding is generally least in the Northern

Highlands and greatest in the River Valley Lowlands.

The SRWMD has a humid subtropical climate.

Rainfall averages about 56 inches per year. July and

August are typically the wettest months; late spring

and early fall are the driest. The largest floods, however, have occurred in March or April as the result of

cumulative effects of several consecutive broad frontal-type rainfall events over the basin.The largest

floods of record on the Suwannee River occurred in

March and April of 1948, March 1959, and

April 1973.

STATION ANALYSIS

Frequency curves for individual gaging stations

were developed following the guidelines described in

Bulletin 17B, Interagency Advisory Committee on

Water Data (1982). A log-Pearson Type III distribution function was used to fit annual peak discharges to

log-probability curves. The distribution is defined by

the equation:

logQT=M+KTS, (1)

where

QT is the peak discharge for a selected recurrence

interval T, in cubic feet per second;

M is the mean of the logarithms of the annual

peaks;

KT is the Pearson Type III frequency factor

expressed in number of standard deviations

from the mean for a selected recurrence

interval, T; and

S is the standard deviation of the logarithm of

the annual peaks.

The computer program J407 (Kirby, 1979) was

used to fit the log-Pearson Type III distribution to the

annual maximum discharges at each of 25 continuousrecord and 7 peak-flow partial-record gaging stations

(app. 1). User judgment was exercised in the use of

this program in determining historic peaks, whether to

use station or regional skew coefficents, and interpretation of low and high outliers.

Figures 3 and 4 were constructed to examine the

consistency of flood peaks of different frequencies up

to 100 years in a downstream direction on the Suwannee and Santa Fe Rivers. Weighted values of station
4 Magnitude and Frequency of Floods in the Suwannee River Water Management District, Florida

and regional flood peaks were used in the plots. Generally, for rainfall equally distributed throughout a

basin, it is usual to see an increase in flood magnitudes

with increasing drainage area. The decrease in flood

magnitude between the Luraville and Branford stations on the Suwannee River is thought to be due in

part to peak attenuation along the channel as cross sectional area increases in a downstream direction. However, it is thought that a large part of the attenuation is

through transient stream losses to springs which, at

high stream levels, effectively become sinks.The same

phenomenon of peak attenuation occurs between Worthington Springs and High Springs on the Santa Fe

River.

Figure 2. Physiographic regions within the Suwannee River Water Management District (after SRWMD, 1994).

GULF COASTAL

LOWLANDS

NORTHERN HIGHLANDS

RIVER VALLEY LOWLANDS

SUWANNEE RIVER WATER

MANAGEMENT DISTRICT

BOUNDARY

EXPLANATION

Location of

Suwannee River

Water Management

District

0

0

20

20

40 MILES

40 KILOMETERS

84°00′ 83°30′ 83°00′ 82°30′ 82°00′

29°00′

30°30′

30°00′

29°30′

REGIONAL ANALYSIS

Bridges (1982) performed statewide multiple

regressions on 11 variables to develop relations

between flood-peak discharges and basin characteristics. The variables tested were drainage area, channel

slope, channel length, mean basin elevation, storage

area of lakes and ponds, storage area of lakes, ponds,

and swamps, forested area, maximum soil infiltration,

mean annual precipitation, and maximum 24-hour precipitation intensity. The assumed form of the regression equation used by Bridges was:

(2) QT CTX1

B1T

X2

B2T

.....XN

BNT

=
Regional Analysis 5

Figure 3. Relation of flood discharge to drainage area for selected frequencies on the Suwannee River. FLOOD DISCHARGE, IN CUBIC FEET PER SECOND

DRAINAGE AREA, IN SQUARE MILES

90,000

80,000

70,000

60,000

50,000

40,000

30,000

20,000

10,000

0

1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000 9,000 10,000

100-YEAR

50-YEAR

25-YEAR

10-YEAR

2-YEAR

5-YEAR

Suwannee River near Benton (02315000)

Suwannee River at White Springs (02315500)

Suwannee River at Suwannee Springs (02315550)

Suwannee River at Ellaville (02319500)

Suwannee River at Luraville (02320000)

Suwannee River at Branford (02320500)

Suwannee River near Bell (02323000)

Suwannee River near Wilson (02323500)

Figure 4. Relation of flood discharge to drainage area for selected frequencies on the Santa Fe River. FLOOD DISCHARGE, IN CUBIC FEET PER SECOND

DRAINAGE AREA, IN SQUARE MILES

18,000

20,000

22,000

24,000

26,000

16,000

14,000

12,000

10,000

8,000

6,000

4,000

2,000

0

0 100 200 300 400 500 600 700 800 900 1,000 1,100

100-YEAR

50-YEAR

25-YEAR

10-YEAR

2-YEAR

5-YEAR

Santa Fe River near Graham (02320700)

Santa Fe River near Fort White (02322500)

Santa Fe River near Worthington Springs (02321500)

Santa Fe River near High Springs (02322000)
6 Magnitude and Frequency of Floods in the Suwannee River Water Management District, Florida

where

QT is the flood peak discharge (dependent

variable) for recurrence interval of Tyears, in cubic feet per second;

X1 to XN are the basin characteristics (independent

variables);

CT is the regression constant for a given

recurrence interval; and

B1T to BNT are the regression coefficients for a given

recurrence interval.

The regression analysis used R2 (the square of

the multiple correlation coefficient) as a performance

criterion. R2 is a measure of the amount of variation in

the independent variable that can be accounted for by

the model. The one-variable model with the highest R2

was produced. Then, the variable that produced the

greatest increase in R2 was added to the equation. Each

variable in the two-variable model was compared to

each variable not in the model to determine if replacing one variable with another would improve the R2

coefficient. This procedure continued until the best

two-variable, three-variable, and so forth, model was

developed for each interval of T years. Bridges found

that, statewide, drainage area (DA) accounted for

62 percent of the variance of the dependent variable;

combining DA with lake area (LK) accounted for

80 percent; and, adding channel slope (SL) added only

3 percent. (For the SRWMD area, Bridges found that

adding channel slope to the regression equation produced insignificant improvement.) All three of these

parameters were determined from available USGS

topographic maps. Adding a fourth and fifth variable

did not contribute to significant improvement of either

R2 or the standard error of the regression equations in

any region of the State. The present (1996) study

adopted Bridges’ regression equation for Region B,

one of three high flow hydrologic regions Bridges

defined for Florida. Region B includes the entire

SRWMD and is nearly coincident with it. The regression equation for Region B includes only DA and LK

as variables. The equation is:

(3)

where

QT is the discharge for a recurrence interval of Tyears, in cubic feet per second;

CT is the regression constant for a recurrence

interval, T;

DA is the drainage area, in square miles;

LK is percentage of drainage area covered by

lakes (determined from USGS 7.5-minute

or 15-minute topographic maps); and

B1T and

B2T are exponents for various recurrence intervals.

The full suite of values for CT, B1T, and B2T is

given in table 1 along with R2 values and standard

errors for each of the regressions. The standard error

of estimate is the standard deviation of the distribution

of residuals about the regression line, meaning that

68 percent of the values are within one standard deviation of the regression line and 95 percent are within

two standard deviations.

QT CTDA

B1

T = ( ) LK + 0.6

B2

T

Table 1. Flood-frequency regression model for the Suwannee River Water Management District

Recurrence

interval,

 in years

(T)

Exceedence

probability

Regression

constant

(CT)

Exponents

R 2

Standard

error, in

percent

Accuracy,

in equivalent

years of record

(EY)

B1T B2T

2 0.5 44.2 0.658 -0.561 0.876 60.9 2

5 .2 113 .614 -.573 .869 59.7 3

10 .1 182 .592 -.580 .863 59.9 3

25 .04 298 .570 -.585 .853 60.9 5

50 .02 410 .556 -.589 .845 61.9 5

100 .01 584 .543 -.591 .836 63.1 6

200 .005 694 .533 -.593 .827 64.4 6

500 .002 936 .521 -.594 .815 66.3 6
Regional Analysis 7

Table 1 also gives the accuracy of the regional

relations in equivalent years of record, which is

defined as the number of years of actual streamflow

record required at a site to obtain an accuracy equal to

the standard error of prediction for the regression estimate (Hardison, 1971). Hardison showed that the

equivalent years of actual record required to produce

an accuracy for a T-year statistic equal to that of a

regional regression is given by:

(4)

where

EY is the accuracy of the T-year statistic at a site

not used in the regression, in equivalent

years of record;

CV is the coefficient of variation of annual events,

in this case, annual floods;

SEP is the standard error of prediction in percent.

Table 2 gives the probabilities that floods of a

given recurrence interval will be exceeded during indicated time periods. Note that probabilities are not multiplicative; that is, the probability of a flood of a given

recurrence interval occurring during a 5-year period is

not five times the probability of that magnitude flood

occurring in any one year. The probabilities are computed by the formula (Interagency Advisory Committee on Water Data, March, 1982):

PN=1-(1-1/T)

N (5)

where

PN is the probability of at least one exceedence

within the specified time interval;

N is the time period in years;

T is the recurrence interval in years.

EY

100CV

SEP

---------------     2

=

1

 Multiply probability values by 100 to obtain percent chance of exceedence.

2

Probability greater than 0.99 but less than 1.00.

Table 2. Probability that a flood of a given recurrence interval will be exceeded during indicated time period

Recurrence

interval, in

years

(T)

Period of time, in years

(N)

1 5 10 20 25 50 100 200 500

5 0.201 0.67 0.89 0 .99 (2) (2) (2) (2) (2)

10 .10 .41 .65 .89 0.93 (2) (2) (2) (2)

25 .04 .18 .34 .56 .64 0.87 0.98 (2) (2)

50 .02 .10 .18 .33 .40 .64 .87 0.98 (2)

100 .01 .05 .10 .18 .22 .40 .63 .87 (2)

Bridges (1982) used stations with drainage areas

ranging from 13.9 to 9,640 square miles in developing

the regression equations for Region B. Lake areas for

Region B ranged from 0 to 13.26 percent. The regression equations have not been validated outside these

limits. Also, the regressions are not considered valid

where anthropogehic changes have a significant effect

on flood runoff, such as regulation from dams, levees,

diversion canals, strip mining operation, and urban

development.

It is recommended that flood frequency estimates for ungaged sites on ungaged streams be determined by direct application of the regression equation

(3) alone in cases where there is no gage upstream or

downstream from the site of interest within a drainage

area range of greater than one-half and less than twice

that of the site of interest. The use of the regional

equation is illustrated a follows:

1) Determine the 50-year flood for station

02321446, Fivemile Creek near Dukes, Florida, at County Road 18A (fig. 1).

2) The DA is 11.80 mi2. The LK is 0.90 percent

of the drainage area.

3) Equation 3, as determined from table 1 for

the 50-year flood, becomes:

Q50=410 (11.80)0.556(0.90+0.6)-0.58

Q50=1,278 cubic feet per second

Appendix 1 shows calculated values of flood

magnitudes from equation 3 for recurrence intervals of

2, 5, 10, 25, 50, 100, 200, and 500 years at gaging stations. Although it is not recommended that regional

values derived from equation 3 be used alone where

estimates from gaged flows are also available, the folQT CTDA

B1

T = ( ) LK + 0.6

B2

T
8 Magnitude and Frequency of Floods in the Suwannee River Water Management District, Florida

lowing section shows how weighting of station and

regional values can be used to improve estimates of

flood magnitude and frequency at both gaged and

ungaged sites.

IMPROVED FREQUENCY ESTIMATES BY

USE OF WEIGHTING

Gaged Sites

Flood-frequency estimates based on station analysis and regional regression analysis tend to be independent of one another. Therefore, it is to be expected

that an estimate of frequency characteristics utilizing

both estimates is more likely to be closer to the “true”

frequency characteristic than either method by itself.

At gaged sites, a weighting procedure is recommended

which is contained in guidelines of the Interagency

Advisory Committee on Water Data (1982). In this procedure, station analysis is weighted by N, the number

of years of actual record, and the regression estimate is

weighted by EY, the equivalent years of record of the

regression analysis, according to the formula:

LogQwt=(NlogQg+EYlogQr)/(N+EY) (6)

where

Qwt is the weighted estimate of the T-year flood at

gaged site, in cubic feet per second;

Qg is the T-year flood estimate from log-Pearson

Type III frequency distribution of annual

peaks at gaged site, in cubic feet per second;

Qr is the regional flood estimate for gaged site,

computed from equation 3, in cubic feet per

second;

N is the the number of annual peaks used to

compute Qg in years;

EY is the accuracy of the regional flood estimate,

in equivalent years from table 1.

The weighting scheme of equation 6 is such that

at longer periods of record (N) the station record value

(Qg) carries more and more weight relative to the

regional value (Qr). Appendix 1 gives weighted estimates of the T-year floods for each of the 32 gaged

sites used in the regression analysis. At gaged sites,

the weighted estimate is considered to be better for

design or predictive purposes than either the log-Pearson Type III estimate alone or the regional estimate

alone.

Ungaged Sites on Gaged Streams

Estimates for ungaged sites near gaged sites on

the same stream can also be improved by weighting

techniques. One method (Hannum, 1976) uses a

weighted value of the ratio of the weighted and

regional estimate at the gaged site to adjust the

regional estimate at the ungaged site. This method is

suggested when the drainage area at the ungaged site

is more than half, but less than twice the drainage area

of the gaged site. Equation 7 or 8 can be used to adjust

the regional estimate at the ungaged site with the

weighted estimate from the gaged site depending on

the location of the ungaged site relative to the gaged

site:

Qu=Qru[((Qwt/Qr) -1)*((2Ag-Au)/Ag) +1] for site

downstream from gage (7)

Qu=Qru[((Qwt/Qr) -1)*((2Au-Ag)/Ag) +1] for site

upstream from gage (8)

where

Qu is the adjusted estimate for ungaged site, in

cubic feet per second;

Qru is the regional estimate for ungaged site, in

cubic feet per second;

Qwt is the weighted estimate of the T-year flood at

the gaged site, in cubic feet per second;

Qr is the regional estimate at gaged site, in cubic

feet per second;

Au is the drainage area for ungaged site, in square

miles;

Ag is the drainage area for gaged site, in square

miles.

The weighting schemes for equations 7 and 8 are such

that the weighted station value has full weight at the

station. At sites less than one-half and greater than

twice Ag, the regional value has full weight. An example of the use of this weighting technique is as follows:

1) It is desired to estimate the 100-year flood for

station 02320900, New River near Raiford

(fig. 1), an ungaged site on a gaged stream. Its

DA is 96.0 mi2

; the percent LK is 0.01.There

is a gaged site downstream, station 02321000,

New River at Lake Butler. Its drainage area is

191 mi2

; the percent LK is 0.03.

2) First, determine for the site of interest the

Regional estimate of the 100-year flood from

equation 3 and the appropriate values from

table 1:
Summary 9

Q100=584(96.0)0.543(0.03+0.6)-0.591

Q100=Qru=9,150 cubic feet per second.

 3) Since the site is upstream from a gage on the

same stream, equation 8 applies. Substituting

in equation 8 Qru obtained above for the

ungaged station and Qwt and Qr for the gaged

station as obtained from Appendix 1 for the

100-year flood for New River near Lake

Butler:

Qu=Qru[((Qwt/Qr) -1)*((2Au-Ag)/Ag) +1]

Qu=9,150[((18,300/13,300)-1)*((2(96)-191)/191)+1]

Qu=9,150[(.3759)*(0.0052)+1]

Qu=9,150[1.002]=9,170 cubic feet per second.

SUMMARY

Flood-frequency statistics were presented for

25 continuous-record and 7 high-flow partial-record

stations in the Suwannee River Water Management

District for recurrence intervals of 2, 5, 10, 25, 50,

100, 200, and 500 years, using three methods.The first,

utilizing station analysis, applied a log-Pearson Type

III function to the series of annual peaks. The second

method utilized regional relations based on regression

of values derived from station analysis with basin

parameters.Only two basin parameters, drainage area

and lake area, were significant enough to include in

final regression equations. Lastly, a weighting method,

utilizing both station and regional values, was used to

improve estimates of flood frequency statistics at

gaged sites. The weighted estimate is considered to be

more accurate than either the station estimate or the

regression estimate alone.

The regression equation can, of course, be used

to estimate flood-frequency characteristics at ungaged

as well as gaged sites. If the ungaged site is on a gaged

stream, then the estimate for the ungaged site can be

improved by transferring information from the gaged

site to the ungaged site and applying weighting procedures.

The karstic nature of much of the Suwannee

River Water Management District significantly attenuates flood peaks in some streams by providing

QT CTDA

B1

T = ( ) LK + 0.6

B2

T substantial subsurface storage when river stages are

high. Then, springs discharging into rivers may

reverse flow temporarily and become sinks.

REFERENCES

Bridges, W.C., 1982, Technique for estimating magnitude

and frequency of floods on natural- flow streams in

Florida: U.S. Geological Survey Water-Resources

Investigations Report 82-4012, 45 p.

Franklin, M.A., Giese, G.L., and Mixson, P.R., 1994,

Statistical summaries of surface-water hydrologic data

in the Suwannee River Water Management District,

Florida, 1906-93: U.S. Geological Survey Open-File

Report 94-709, 173 p.

Fenneman, N.M., 1938, Physiography of eastern United

States: New York, McGraw-Hill Book Company, Inc.,

714 p.

Hannum, C.H., 1976, Technique for estimating magnitude

and frequency of floods in Kentucky: U.S. Geological

Survey Water-Resources Investigations Report 76-62,

70 p.

Hardison, C.H., 1971, Prediction error of regression

estimates of streamflow characteristics at ungaged

sites: U.S. Geological Survey Professional Paper 750-

C, p. C228-C236.

Interagency Advisory Committee on Water Data, 1982,

Guidelines for determining flood-flow frequency:

Bulletin 17B of the Hydrology Subcommittee, Office

of Water Data Coordination, U.S. Geological Survey,

Reston, Va., 183 p.

Kirby, W. H., 1979, Annual flood-frequency analysis using

U.S. Water Resources Council guidelines (Program

J407): U.S. Geological Survey WATSTORE User’s

Guide, v. 4, chap. I-C.

Marella, R.L., 1995, Water-use data by category, county,

and water management district in Florida, 1950-90:

U.S. Geological Survey Open-File Report 94-521,

114 p.

Stamey, T.C., and Hess, G.W., 1993, Techniques for

estimating magnitude and frequency of floods in rural

basins of Georgia: U.S. Geological Survey WaterResources Investigations Report 93-4016, 75 p.

Suwannee River Water Management District, 1994, Water

Management Plan: Suwannee River Water

Management District, draft of August 8, 1994, 215 p.

U.S. Army Corps of Engineers, 1974, Suwannee River

floods, Florida and Georgia, December 1974: Special

Flood Hazard Information Report: U.S. Army Corps of

Engineers, Jacksonville District, Jacksonville, Florida,

17 p. and 41 plates.
10 Magnitude and Frequency of Floods in the Suwannee River Water Management District, Florida
APPENDIX
12 Hydrogeology of the Surficial and Intermediate Aquifer Systems in Sarasota and Adjacent Counties, Florida Appendix 1. Station, regional, and weighted T-year flood estimates for the Suwannee River Water Management District [Discharge-frequency relations for each station are presented as follows: Top line--log-Pearson Type III analysis for the indicated period of systematic record; Middle 

ine--weighted or best estimate of T-year flood; mi2, square miles; ft3/s, cubic feet per second] Station number Name

1Years of record Drainage

area

(mi2)

Lake area

(percent)

Discharge for recurrence interv

(ft3/s)

Systematic Historic 2 5 10 25 50 

202313400 Waccasassa

River nearBronson

23 -- 220 3.50 262

697283

613

1380

673

950

19601030

151028201690

203035802240

02314200 Tenmile Creek

nr LebanonStation

29 -- 26.0 1.00 607

290577

1250

640

1170

1840

954

1730

282014502540

373019003360

02315000 Suwannee

River nrBenton

19 -- 2090 0.24 6390

74606480

104001360010800

133001860013900

172002580018700

203003190022300

02315200 Deep Creek nr

SuwanneeValley

10 -- 88.6 0.50 617

800644

777

1680

928

874

24501110

989

36301530

107046901750

02315500 Suwannee

River at WhiteSprings

69 -- 2430 .31 7510

78707520

132001430013200

173001940017400

227002680023000

267003310027100

02315550 Suwannee

River atSuwanneeSprings

33 -- 2630 .37 7720

80007740

125001450012700

159001960016200

202002700021000

235003330024600

02317620 Alapaha River

nr Jennings

11 -- 1680 0 8390

64208050

148001190014100

198001620019000

269002260025500

327002800031200

202317630 Alapaha River

nr Jasper

27 47 1720 0 6920

79206990

123001470012500

165002010016800

222002800023000

269003480028000

02319000 Withlacoochee

River nr Pinetta

63 67 2120 .23 10400

7580

10300

207001390020300

298001890029200

439002620042300

563003240054100

02319500 Suwannee

River atEllaville

67 -- 6970 .27 19000

1620018900

332002800033000

441003720043800

595005010058800

721006100071300

02320000 Suwannee

River atLuraville

10 -- 7330 .27 19900

1670019300

349002890033400

465003830044500

631005160059000

767006270071700
Appendix 13

02320500 Suwannee

River atBranford

63 67 7880 .30 17000

1720017000

280002960028100

363003920036400

477005270048000

569006400057400

02320700 Santa Fe River

nr Graham

37 -- 94.9 13.26 466

202446

934410878

1310

587

1230

1830

858

1670

226011002070

02321000 New River nr

Lake Butler

25 -- 191 .03 2770

18202680

574037005480

834053307950

12300

7800

11400

15900

9980

14700

02321500 Santa Fe River

at WorthingtonSprings

63 -- 575 2.64 4440

15004290

889029008450

12500

3960

11900

17600

5600

16200

21800

7020

20100

202321600 Olustee Creek

nr Lulu

19 -- 49.1 .04 735

736735

148015901490

213023602160

311035603200

397046504100

202321700 Swift Creek nr

Lake Butler

25 -- 46.0 8.67 486

157447

790331720

1010

482

4770

1320

718

1190

1560

928

1430

202321800 Olustee Creek

nr Providence

12 -- 163 .88 2820

10102440

432020603730

538029604770

677043205930

784055307080

02322000 Sante Fe River

nr HighSprings

41 -- 868 1.90 3560

22703490

729042607030

10300

58709910

14600

8250

13700

181001030017000

02322016 Blues Creek nr

Gainesville

10 -- 5.12 .05 125

131126

221318240

297500335

404798507

492

1080

639

02322500 Santa Fe River

nr Fort White

64 -- 1017 1.73 3940

26203890

654048906460

851067208420

11300

9410

11200

135001170013400

02323000 Suwannee

River nr Bell

25 29 9390 0.49 16400

1730016500

275002960027800

369003890037100

514005210051500

645006300064300

Appendix 1. Station, regional, and weighted T-year flood estimates for the Suwannee River Water Management District --Continued

[Discharge-frequency relations for each station are presented as follows: Top line--log-Pearson Type III analysis for the indicated period of systematic record; Middle ine--weighted or best estimate of T-year flood; mi2, square miles; ft3/s, cubic feet per second]

Station

number Name

1Years of record Drainage

area

(mi2)

Lake area

(percent)

Discharge for recurrence interv

(ft3/s)

Systematic Historic 2 5 10 25 50 
14 Hydrogeology of the Surficial and Intermediate Aquifer Systems in Sarasota and Adjacent Counties, Florida

1Years of historic record include the period of systematic measurements as well as the intervening years to major flood events.2Peak-flow partial-record gaging station.

02323500 Suwannee

River nrWilcox

53 -- 9640 .54 20900

1720020800

317002930031600

396003850039500

502005150050300

586006230058900

02324000 Steinhatchee

River nr CrossCity

44 -- 350 .53 2030

19502030

394038403930

555054405540

799078207970

10100

9910

10100

02324400 Fenholloway

River nr Foley

39 -- 60.0 .04 368

840383

762

1800

810

114026601210

179039901960

241052002630

02324500 Fenholloway

River at Foley

46 -- 120 .37 570

1050

585

118021701220

176031501820

277046502910

374059803920

02325000 Fenholloway

River nr Perry

29 -- 160 .42 687

1230

713

109025201180

138036301510

178053102090

209068102490

02326000 Econfina River

nr Perry

44 -- 198 .85 643

1160

660

119023501240

160033601680

214048902330

254062302780

202326250 Aucilla River

nr Aucilla

11 -- 345 2.00 1980

12101840

299023602840

370033203620

464047604680

537060205560

202326300 Little Aucilla

River nr Greenville

14 -- 90.7 3.12 437

411434

796847805

109012201110

151018001580

186023201970

02326500 Aucilla River

at Lamont

38 -- 747 3.00 1990

16701970

502031504850

768043507370

11600

6120

10800

14800

7630

13700

02326512 Aucilla River

nr Scanlon

20 -- 805 3.00 2590

17602500

438033004220

575045505580

770063807420

929079609010

Appendix 1. Station, regional, and weighted T-year flood estimates for the Suwannee River Water Management District --Continued

[Discharge-frequency relations for each station are presented as follows: Top line--log-Pearson Type III analysis for the indicated period of systematic record; Middle ine--weighted or best estimate of T-year flood; mi2, square miles; ft3/s, cubic feet per second]

Station

number Name

1Years of record Drainage

area

(mi2)

Lake area

(percent)

Discharge for recurrence interv

(ft3/s)

Systematic Historic 2 5 10 25 50