Simulated Peak Flows and Water-Surface 

Profiles for Scott Creek near Sylva, 

North Carolina

By BENJAMIN F. POPE

U.S. GEOLOGICAL SURVEY 

Water-Resources Investigations Report 96-4226

Prepared in cooperation with Jackson County, North Carolina

Raleigh, North Carolina 

1996
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Gordon P. Eaton, Director

The use of firm, trade, and brand names in this report is for identification purposes 

only and does not constitute endorsement by the U.S. Geological Survey.

For additional information write to:

District Chief 

U.S. Geological Survey 

3916 Sunset Ridge Road 

Raleigh, NC 27607

Copies of this report can be purchased 

from:

U.S. Geological Survey 

Information Services 

Box 25286 

Federal Center 

Denver, CO 80225
CONTENTS

Abstract....................................................................^^ 1

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

Purpose and scope ....................................................................................................................................................... 3

Study area.................................................................................................................................................................... 3

Data collection............................................................................................................................................................. 3

Acknowledgments....................................................................................................................................................... 4

Peak flow simulation ............................................................................................................................................................. 5

Rainfall-runoff model description and calibration ...................................................................................................... 5

Antecedent conditions and synthetic rainfall data....................................................................................................... 8

Simulation results........................................................................................................................................................ 9

Water-surface profiles............................................................................................................................................................ 10

Application of results............................................................................................................................................................. 12

Summary....................................................................................................................................................^ 13

References ......................................................................................................................................................................^ 14

Appendix ............................................................................................................................................................................... 15

ILLUSTRATIONS

1. Map of Jackson County showing Scott Creek Basin divide and locations of streamflow gages,

rainfall gages, and climatic measuring stations ........................................................................................................ 2

2-8. Graphs showing:

2. Computed flood-frequency distribution for Scott Creek above Sylva, N.C..................................................... 4

3. Simulated and observed runoff volumes and peak flows for Scott Creek above Sylva, N.C........................... 7

4. Monthly means of total moisture for climatic years 1949-89........................................................................... 8

5. Simulated and observed peak flows for Scott Creek above Sylva, N.C.,

in response to selected 24-hour rainfall events................................................................................................. 10

6. Dimensionless input rainfall intensity and average dimensionless simulated flood hydrograph

for Scott Creek above Sylva, N.C..................................................................................................................... 11

7. Water-surface profiles for selected observed and simulated peak flows

along the reach of Scott Creek flowing through Sylva, N.C. ........................................................................... 12

8. Stage-discharge relations (rating curves) in effect in 1975 and 1995 for Scott Creek

above Sylva, N.C.............................................................................................................................................. 13

TABLES

1. Data-collection sites in Scott Creek Basin and surrounding areas.......................................................................... 3

2. Total storm rainfall, observed and simulated runoff volumes, and observed and simulated

peak flows for events used in calibration of the RRM............................................................................................ 6

3. Period and total rainfall for antecedent moisture conditions data used in peak-flow simulations.......................... 9

4. Simulated peak flows for selected 24-hour rainfall amounts and antecedent moisture conditions......................... 9

A-l. Units, definition, and calibrated value for the RRM parameters used in peak flow simulation

at Scott Creek above Sylva, N.C............................................................................................................................. 15

Contents
CONVERSION FACTORS, VERTICAL DATUM, AND DEFINITIONS

Multiply By To obtain

inch (in.)

foot (ft)

mile (mi)

acre

square mile (mi )

cubic foot per second (ftVs)

Length

25.4

0.3048

1.609

Area

4,047

0.4047

2.590

Flow

0.02832

millimeter

meter

kilometer

square meter

hectare

square kilometer

cubic meter per second

Sea level: In this report "sea level" refers to the National Geodetic Vertical Datum of 1929 

(NGVD) a geodetic datum derived from a general adjustment of the first-order level nets 

of both the United States and Canada, formerly called Sea Level Datum of 1929.

Definitions:

FEMA Federal Emergency Management Agency

HEC Hydraulic Engineering Center

NWIS National Water Information System

NWS National Weather Service

RRM Rainfall-runoff model

TVA Tennessee Valley Authority

USFS U.S. Forest Service

USGS U.S. Geological Survey

IV Contents
Simulated Peak Flows and Water-Surface Profiles for 

Scott Creek near Sylva, North Carolina

By Benjamin F. Pope

ABSTRACT

Peak flows were simulated for Scott Creek, 

just upstream from Sylva, in Jackson County, 

North Carolina, in order to provide Jackson 

County officials with information that can be used 

to improve preparation for and response to flash 

floods along the reach of Scott Creek that flows 

through Sylva. A U.S. Geological Survey rainfallrunoff model was calibrated using observed 

rainfall and streamflow data collected from March 

1994 through September 1995. Standard errors for 

calibration were 34 percent for runoff volumes and 

21 percent for peak flows. The calibrated model 

was used to simulate peak flows resulting from 

synthetic rainfall amounts of 1.0, 2.5, 5.0, and 

7.5 inches in 24-hour periods. For each rainfall 

amount, peak flows were simulated under low-, 

moderate-, and high-antecedent soil-moisture 

conditions, represented by selected 3-month 

periods of daily rainfall and evaporation record 

from nearby climatic-data measuring stations. 

Simulated peak flows ranged from 89 to 

10,100 cubic feet per second.

Profiles of water-surface elevations for 

selected observed and simulated peak flows were 

computed for the reach of Scott Creek that flows 

through Sylva, North Carolina. The profiles were 

computed using the U.S. Army Corps of Engineers 

HEC-2 Water Surface Profiles computer program 

and channel cross-section data collected by the 

Tennessee Valley Authority. The stage-discharge 

relation for Scott Creek at the simulation site has 

changed since the collection of the cross-section 

data. These changes, however, are such that the

water-surface profiles presented in this report 

likely overestimate the true water-surface elevations at the simulation site for a given peak flow.

INTRODUCTION

Scott Creek drains about 59 square miles (mi2 ) 

of northeast Jackson County, in the mountains of 

southwestern North Carolina. The creek flows 

westward across the county from its headwaters near 

Balsam Gap, through Sylva and Dillsboro, and into the 

Tuckasegee River (fig. 1). Large topographic relief in 

the drainage basin and steep stream slopes in the 

tributaries and main channel give rise to the potential 

for flash flooding. Flash flooding in the reach of Scott 

Creek that passes through Sylva is of special concern 

because of the proximity of businesses and residences 

to the stream.

Jackson County officials recognize the need to 

improve preparation for and response to flash floods 

along this reach of Scott Creek. In addition to the local 

needs, there also is need for better understanding of the 

relation between rainfall amounts and resulting peak 

flows in mountainous basins similar to the Scott Creek 

Basin. In response to these needs, the U.S. Geological 

Survey (USGS) in cooperation with Jackson County 

initiated an investigation in 1993 of the effects of 

various amounts of rainfall in the drainage basin on 

flows in Scott Creek in the Sylva area. The 

investigation included the application of a rainfallrunoff model to simulate peak flows in the creek and 

application of a step-backwater model to estimate flood 

elevations for the simulated peak flows. Results of the 

investigation can be used to qualitatively estimate peak 

flows and water-surface profiles that result from actual 

conditions.

Abstract
83°22'30" 83° 15' 83°07'30"

35°30' -

35°22'30" -

35°07'30" -

LOCATION OF STUDY AREA AND CLIMATIC

MEASURING STATIONS 5 AND 6

IN NORTH CAROLINA

Balsam 

Gap

HAYWOOD 

COUNTY

TRANSYLVANIA 

MACON COUNTY

COUNTY

EXPLANATION 

- - BASIN BOUNDARY 

A STREAMFLOW GAGE

NORTH_ _CAROLINA 

GEORGIA

35° 15' -

Figure 1 . Jackson County, showing Scott Creek Basin divide and locations of streamflow gages, rainfall gages, 

and climatic measuring stations.

2 Simulated Peak Flows and Water-Surface Profiles for Scott Creek near Sylva, North Carolina
Purpose and Scope Study Area

This report presents simulated peak flows for 

Scott Creek at site 1 (fig. 1; table 1) in response to 

synthetic rainfall events in the basin and estimated 

flood elevations associated with selected peak flows for 

the stream reach flowing through Sylva. Peak flows 

representing the response of the basin to 24-hour 

rainfall amounts ranging from 1.0 to 7.5 inches (in.) 

under three different antecedent soil-moisture 

conditions were simulated using the USGS rainfallrunoff model (RRM) (Dawdy and others, 1972). The 

RRM was calibrated by using data from one USGS 

stream gage and three USGS rainfall gages in the Scott 

Creek Basin, along with evaporation data obtained 

from nearby National Weather Service (NWS) and U.S. 

Forest Service (USFS) climatic stations.

The U.S. Army Corps of Engineers Hydraulic 

Engineering Center (HEC) Water Surface Profiles 

computer program (HEC-2) (U.S. Army Corps of 

Engineers Hydrologic Engineering Center, 1991) was 

used to compute profiles of water-surf ace elevations 

along the reach of Scott Creek that flows through 

Sylva. Channel cross sections and hydraulic properties 

required for the application of HEC-2 to the reach of 

Scott Creek that flows through Sylva were obtained 

from the Tennessee Valley Authority (TVA) 

(R. A. Milstead, Tennessee Valley Authority, written 

commun., June 28, 1996).

The Scott Creek Basin is contained entirely 

within Jackson County (fig. 1). Scott Creek rises near 

Balsam Gap at the eastern boundary of Jackson 

County, at an elevation of about 3,400 feet (ft), and 

flows westward, falling to an elevation of about 2,060 ft 

at the streamflow gage upstream from Sylva and then to 

about 2,000 ft where the stream flows into the 

Tuckasegee River. Scott Creek is flanked on the north 

by mountain peaks rising to more than 6,200 ft and on 

the south by peaks rising to more than 5,000 ft.

Upstream from Sylva, the basin, which is 

sparsely populated and lightly developed, is primarily 

forested with some small areas of cultivated fields and 

pasture. The areas downstream from the Sylva gage are 

relatively more developed; however, development is 

generally limited to streamside areas in this part of the 

basin.

Data Collection

Two streamflow gages (fig. 1; table 1) and three 

rainfall gages (fig. 1; table 1) were installed in the basin 

by the USGS for this investigation. The streamflow 

gages were operated from January 1993 through 

September 1995; discharge records for these two gages 

were published in U.S. Geological Survey annual 

water-data reports (Barker and others, 1994; Ragland

Table 1. Data-collection sites in Scott Creek Basin and surrounding areas

[mi , square mile; , not applicable]

Site 

number 

(fig.1)

I

2

3

4

5

USGS station 

number

03509000

03508910

352708083061445

352435083024245

Station name

Scott Creek above Sylva, N.C.

Scott Creek at Willits-Ochre Hill, N.C.

Woodfin Cascades Overlook

Grassy Ridge Mine Overlook

National Weather Service, Asheville

Data collected

discharge

discharge 1 , rainfall

rainfall

rainfall

rainfall

Drainage 

area 

(mi2)

51.0

22.4

Weather Station

U.S. Forest Service, Coweeta Experimental 

Station

evaporation

Data not used in model calibration.

Introduction
and others, 1995, 1996). The rainfall gages were 

operated from March 1994 through September 1995; 

rainfall records for these three gages were stored in the 

USGS National Water Information System (NWIS) 

data base. Streamflow data collected at site 1 and 

rainfall data collected at sites 2, 3, and 4 were used to 

calibrate the RRM. Daily evaporation and precipitation 

records from climatic sites located outside of the basin 

and operated by the NWS and USFS (fig. 1; table 1) 

also were used in the calibration and application of the 

RRM.

Prior to this study, 34 years of streamflow record 

were collected at site 1 during water years 1942-75 

(fig. 1; table 1). The highest peak flow of the total 37 

years of record occurred during this investigation. On 

March 27, 1994, flow at site 1 peaked at 4,440 cubic 

feet per second (ft3/s) in response to a 24-hour rainfall 

of 5.13 in. measured at site 2. A second smaller but

Water year is the period from October 

is identified by the year in which it ends.

to September 30 and

significant event occurred February 16, 1995, when 

flow at site 1 peaked at 1,450 ft3/s in response to a 

24-hour rainfall of 3.34 in. measured at site 2. These 

peaks occurred in late winter, typically the wettest part 

of the year. Antecedent 3-month total rainfall amounts 

were 14.05 in. for the March 1994 event and 13.21 in. 

for the February 1995 event. These two peak flows have 

recurrence intervals of greater than 200 years (March 

1994) and less than 2 years (February 1995), 

respectively, as determined by an estimate of the floodfrequency distribution based on the 37 years of record 

collected from 1942 to 1975 and 1993 to 1995 (fig. 2) 

(U.S. Water Resources Council, 1981).

Acknowledgments

This investigation was conducted in cooperation 

with Jackson County, North Carolina. The support of 

Mr. Donald Clawson, Jackson County Emergency 

Services Department; Mr. Bobby McMahon, Jackson 

County Land Records Office; and Mr. Tom Massie,

February 16, 1995

 COMPUTED FLOOD FREQUENCY DISTRIBUTION

* PLOTTING POSITION FOR ANNUAL PEAK FLOWS

A SIGNIFICANT PEAK FLOWS DURING STUDY PERIOD

1,000

1.25 1.43 1.67 2.50 3.33 5 10 20 

RECURRENCE INTERVAL, IN YEARS

50 100 500

Figure 2. Computed flood-frequency distribution for Scott Creek above Sylva, N.C.

Simulated Peak Flows and Water-Surface Profiles for Scott Creek near Sylva, North Carolina
formerly of the Jackson County Planning Department, 

is gratefully acknowledged. In addition, the advice and 

assistance of Mr. Chris Hughes and Mr. Roger A. 

Milstead, Tennessee Valley Authority, in providing the 

data used in computing the water-surface profiles and 

in applying the HEC-2 to those data are gratefully 

acknowledged.

PEAK FLOW SIMULATION

Peak flows were simulated at site 1 (fig. 1; 

table 1) using a calibrated version of the RRM to 

represent the response of the basin to selected hypothetical 24-hour rainfall events under high-, moderate-, 

and low-antecedent moisture conditions. This section 

describes the RRM and the calibration of the model for 

the Scott Creek Basin. Synthesis of the data set used in 

simulating the hypothetical events and applying the 

data set to the calibrated model are described. Results 

of the simulations are then presented.

Rainfall-Runoff Model Description and 

Calibration

The RRM, originally developed and documented 

by Dawdy and others (1972), simulates flow volumes 

and flow rates of surface runoff from small drainage 

basins. Its use is generally restricted to small basins so 

as not to violate the basic model assumptions: 

(1) ground-water contribution to the hydrograph is 

neglible, (2) rainfall is uniform across the basin and can 

be adequately represented by a single rainfall input, 

and (3) pertinent physical characteristics of the basin 

can be represented by basin-average values. The model 

has three primary components soil-moisture 

accounting; excess rainfall, or surface-runoff, 

computation; and surface-runoff routing. Eleven model 

parameters are used to simulate volume and flow rate of 

surface runoff generated by rainfall events. Several 

modifications have been made to the original model, 

most notably the inclusion of an automated routine for 

optimization of model parameters during model 

calibration based on comparison of simulated flow 

volumes and flow rates to observed values (E.J. Inman, 

U.S. Geological Survey, written commun., May 1994).

The RRM operates in two modes a daily mode 

and a storm mode. In the daily mode, daily rainfall and 

evaporation values are used to simulate the distribution 

of soil moisture in the soil column and evapotranspiration from the soil. In the storm mode, short-interval (15 

minutes for the Scott Creek Basin) rainfall data and 

output from the daily soil-moisture accounting are used 

to determine the amount of excess rainfall, or runoff 

volume, that results from each storm event. The runoff 

volume is then routed through the channel to create a 

flow hydrograph, which provides the time distribution 

of flows, including the peak flow, at the basin outlet. A 

more detailed description of the model algorithms is 

provided by Dawdy and others (1972).

Site 1 (fig. 1) has a drainage area of 51.0 mi2 , 

which is somewhat large for the application of the 

RRM. However, basin characteristics and results of 

model calibration suggest that the RRM can be 

successfully applied to the Scott Creek Basin.

Although the Scott Creek Basin is fairly large, 

land use and terrain are fairly uniform throughout, 

except for some small areas near the main stream. 

Because of this uniformity, it is reasonable to assume 

that average basinwide values can adequately describe 

the physical characteristics of the basin. Large groundwater contributions to the storm hydrograph usually 

occur in basins with thick, very transmissive layers of 

underlying soils. Because of the mountainous location 

of the Scott Creek Basin and the relatively shallow soils 

that are typical of such locations, the assumption of no 

significant ground-water contribution to the storm 

hydrograph can be made.

The assumption required for the application of 

the RRM that rainfall is uniform over the Scott Creek 

Basin is likely violated on occasion. Without a more 

dense network of raingages, however, it is difficult to 

determine true basinwide distribution of rainfall. A 

practical test of the assumption of uniform rainfall in 

the basin is the ability to adequately calibrate the model 

for a range of observed rainfall amounts and the 

resulting flows.

Model calibration is the process of adjusting 

model parameters such that simulated runoff volumes 

and flow rates agree with the corresponding observed 

runoff volumes and flow rates. For this investigation, 

calibration of the model was achieved using an 

automated parameter-optimization routine. This 

routine performs a systematic trial and error search for 

a set of model parameters that minimize selected 

combinations of five objective functions. Three of the 

objective functions measure error between simulated 

and observed values of runoff volume and peak flow for 

each rainfall-runoff event; the remaining two objective 

functions involve a combination of error and the slope

Peak Flow Simulation
of a regression line of simulated with observed values. 

Calibration procedures used in this study follow 

closely those described by Robbins and Pope (1996) in 

a study involving calibration of RRM for a number of 

small urban basins in North Carolina.

The quality of calibration is measured by the 

standard error of regression between simulated and 

observed values of respective runoff volume and peak 

flow, by the slopes of these regression lines, and by 

inspection of plots of simulated and observed values. A 

slope of 1.0 usually indicates no bias in the calibration, 

or that the tendency to over- or undersimulate observed 

values is not a function of the magnitude of the values. 

Inspection of the plots of simulated and observed 

values is used to confirm lack of bias.

Fourteen high-flow events from the discharge 

record at site 1 for the period March 1994 through 

September 1995 were selected for use in model 

calibration. Peak flows at site 1 (fig. 1; table 1) for these 

events were between 196 and 4,440 ft3/s, and total 

storm rainfall measured at site 2 (fig. 1; table 1) for 

these events ranged from 0.48 to 6.25 in. (table 2). 

Recorder malfunctions resulted in missing rainfall

record for 2 of the 14 events at site 3 (fig. 1; table 1) and 

5 of the 14 events at site 4 (fig. 1; table 1). Average 

absolute values of the percent differences between total 

storm rainfall measured at site 2 and total storm rainfall 

measured at sites 3 and 4 were 40 and 21 percent, 

respectively.

Separate RRM calibrations were performed for 

the selected high-flow events using rainfall data from 

each of the three rainfall gages within the basin (fig. 1; 

table 1). Calibrations using unit rainfall data from sites 

3 and 4 resulted in regression line slopes for volumes 

that statistically differed from 1.0 and large standard 

errors (greater than 58 percent for peaks and greater 

than 43 percent for volumes). However, for the 

calibration using rainfall data from site 2, the slopes of 

the regression lines for both runoff volumes and peak 

flows were not statistically different from 1.0, and 

inspection revealed no bias (fig. 3). In addition, using 

rainfall data from site 2, standard errors of regression 

for runoff volumes and peak flows were 34 percent and 

21 percent, respectively. These standard errors of 

regression are of similar magnitude to those achieved

Table 2. Total storm rainfall, observed and simulated runoff volumes, and observed and simulated peak flows for 

events used in calibration of the RRM

Total storm rainfall 

at site 2 

(inches)

0.47

.55

.63

.64

.73

1.06

1.13

1.21

1.26

1.40

1.74

2.84

4.03

6.25

Observed volume 

(inches)

0.02

.04

.04

.05

.05

.09

.21

.21

.09

.27

.33

.48

1.14

2.37

Simulated volume 

(inches)

0.02

.05

.03

.05

.05

.08

.08

.25

.16

.23

.35

.44

.96

2.49

Observed peak 

flows at site 1 

(ft3/s)

196

274

281

256

236

256

339

744

413

935

1,138

881

1,406

4,204

Simulated peak 

flows at site 1 

(ft3/s)

169

307

257

256

230

258

282

915

546

1,001

1,127

962

1829

4,403

Simulated Peak Flows and Water-Surface Profiles for Scott Creek near Sylva, North Carolina
SIMULATED PEAK FLOWS, IN CUBIC FEET PER SECOND

T] i

CO3' c '

033'

CD 

Q.

W 

Q.

O 

C7 

W 

CD

CD 

Q. CI

0_C3

CD

Q.

 D 

CD

03 oO

CD 

CD

C7 O

CD

 p>- en

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o o

~g o

o o

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ro co -p>- en ~g o

o "o o o "o o

o o o o o o

o o o o o o

SIMULATED R

go

I en

C ^mo

m 

co
by Robbins and Pope (1996) for other basins in North 

Carolina.

The RRM was calibrated for simulating peak 

flows at site 1 using streamflow recorded at site 1, 

rainfall measured at site 2, and evaporation measured at 

site 6 (fig. 1; table 1). Site 2 is near the center of the 

basin and probably more closely approximates 

basinwide rainfall than the other two rainfall gages. 

The standard errors of regression, along with a lack of 

bias, indicate that the RRM adequately simulates the 

Scott Creek Basin response to the range of rainfall 

amounts measured at site 2 and used in the calibration.

Antecedent Conditions and Synthetic 

Rainfall Data

The RRM requires daily rainfall and evaporation 

data for the period preceding a storm event in order to 

simulate the antecedent soil-moisture conditions. Soilmoisture conditions control the amount of infiltration 

and subsequently the volume of runoff during an event. 

Consequently, peak flow for a given rainfall will vary

with antecedent moisture conditions. Hence, it is 

reasonable to simulate the response of the Scott Creek 

Basin to rainfall for a range of antecedent conditions.

Concurrent daily records of rainfall from the 

NWS Asheville weather station and evaporation from 

the USFS Coweeta Experimental Station from 1949 to 

1989 were used to analyze moisture conditions. Daily 

evaporation was subtracted from daily rainfall; this 

difference was defined as the daily moisture. Monthly 

moistures are defined as the sum of the daily moisture 

amounts for each month. The values were grouped into 

climatic years (April through March), and the mean 

monthly moistures from 1949 to 1989 were computed 

and plotted. By examination, mean monthly moistures 

were grouped into 3-month periods representing low-, 

moderate-, and high-moisture conditions. May through 

July represents the low-moisture period, August 

through October represents the moderate-moisture 

period, and December through February represents the 

high-moisture period (fig. 4).

The individual monthly moistures for each 

3-month period were summed for each climatic year of

3.0 

2.5 

2.0 

1.5 

1.

0.5

lo o

ui -0.5

o

-1.0

-1.5

-2.0

-2.5

-3.0

APR. MAY JUNE JULY AUG. SEPT. OCT. NOV.

MONTH

DEC. JAN. FEB. MAR.

Figure 4. Monthly means of total moisture for climatic years 1949-89.

8 Simulated Peak Flows and Water-Surface Profiles for Scott Creek near Sylva, North Carolina
record, giving three annual series of moistures. The 

values in each series were ranked in ascending order, 

and the 1 Oth, 50th, and 90th percentiles of moisture 

were computed for each series. The observed 3-month 

sums that most closely matched the 90th percentile of 

the wet period, the 50th percentile of the moderate 

period, and the 10th percentile of the dry period were 

selected for use in the simulations (table 3). The 

original daily rainfall and evaporation records for the 

selected periods were used as input to the RRM.

Table 3. Period and total rainfall for antecedent moisture 

conditions data used in peak-flow simulations

Antecedent 

moisture 

condition

High

Period used

December 1986- 

February 1987

Total 

moisture 

(inches)

10.44

Total 

rainfall 

(inches)

10.44

Moderate August - October 1950 -1.12 6.73 

Low May 1977-July 1977 -5.56 6.35

During the high-moisture period, total moisture 

was a function of rainfall alone; no evaporation was 

recorded, and total rainfall was the same as total 

moisture. For the moderate- and low-moisture periods, 

however, evaporation governed moisture condition; 

rainfall amounts were nearly equal for the two periods, 

but total moistures differed by 4.44 in. The difference 

in moisture between high- and moderate-moisture 

periods is 11.56 in., about 2.5 times the difference 

between moderate- and low-moisture periods (table 3).

The range of rainfall events for which flows 

would be simulated were the selected 24-hour rainfall 

amounts of 1.0,2.5, 5.0, and 7.5 in. These amounts are 

approximately 20, 50, 100, and 150 percent of the 

24-hour rainfall of 5.13 in. that resulted in the peak 

flow of 4,440 ft3/s that occurred on March 27, 1994 

(fig. 2). This flow is the maximum recorded in 37 years 

of record and has an estimated recurrence interval of 

greater than 200 years. Therefore, it is reasonable to 

use the rainfall that resulted in this peak flow as the 

basis for developing a range of synthetic unit rainfall 

inputs.

Several possible time distributions of these total 

rainfall amounts were considered for use in simulating 

peak flows. A reasonable set of distributions were 

developed by Huff (1967) and are based on analysis of 

rainfall data collected from about 50 rainfall gages in 

Illinois from 1955 to 1966. A time distribution of total

storm rainfall that approximates the 50-percent 

probability distribution of second-quartile storms, as 

described by Huff (1967), was used to develop the 

synthetic unit rainfall distributions for this study. A 

second-quartile storm is one in which the heaviest 

rainfall occurs during the second quarter of the total 

storm period; the 50-percent probability distribution is 

the distribution that has a 50-percent chance of 

occurring. The time distribution of discharge is likely 

dependent on the time distributions of rainfall; 

however, in this study, only the second quartile 

distribution, the most frequently occurring storm type, 

was used (Huff, 1967).

Simulation Results

Peak flows were simulated by using selected 

daily rainfall and evaporation data, and synthetic storm 

rainfall distributions with the calibrated model. Three 

peak flows were simulated for each 24-hour rainfall 

amount, one each for the three sets of paired daily 

rainfall and evaporation data representing low-, 

moderate-, and high-antecedent moisture conditions 

(table 4).

Table 4. Simulated peak flows for selected 24-hour 

rainfall amounts and antecedent moisture conditions

[ft~ /s, cubic feet per second]

Simulated peak flow

24-hour 

rainfall 

(inches)

1.0

2.5

5.0

7.5

Lowantecedent 

moisture 

(ft3/s)

89.2

674

3,550

7,660

Moderateantecedent 

moisture 

(ft3/s)

102

814

4,070

8,240

Highantecedent 

moisture 

(ft3/s)

213

1,740

5,860

10,100

As expected, the range of 24-hour rainfall 

amounts resulted in a wide range of simulated peak 

flows from 89 to 10,100 ft3/s. The difference in flow 

because of antecedent moisture conditions is greater 

between moderate- and high-moisture conditions than 

between low- and moderate-moisture conditions 

(fig. 5). This is likely because of the relative differences 

in total moisture amounts among the three periods 

(table 3).

Simulation Results
20,000

18,000

16,000

14,000

12,000

^ 10,000

COZ)

O

-_ 8,000

O

6,000

4,000

2,000

- - SIMULATED PEAK FLOWS -- LOW ANTECEDENT MOISTURE

-A- SIMULATED PEAK FLOWS -- MODERATE ANTECEDENT MOISTURE

-9- SIMULATED PEAK FLOWS -- HIGH ANTECEDENT MOISTURE

X OBSERVED PEAK FLOWS

March 27, 1994

February 16, 1995

01234567

24-HOUR TOTAL RAINFALL, IN INCHES

Figure 5. Simulated and observed peak flows for Scott Creek above Sylva, N.C., in response to selected 24-hour 

rainfall events.

The RRM produces a simulated flood 

hydrograph for each storm event. Because each 

hydrograph is simulated using the same time to peak 

and recession slope developed in model calibration, 

hydrographs for the 18 simulated storms have only 

minor differences in shape. These hydrographs were 

normalized by dividing flow (Q) by the peak flow (Qp) 

and time (7) by the time to peak (Tp) and the minor 

differences averaged. In a similar fashion, plots of 

rainfall intensity were normalized by dividing intensity 

(/) by peak intensity (//?) and plotted against the 

normalized time of the hydrograph (fig. 6).

For this synthetic rainfall distribution, the peak 

simulated flow occurs before the end of rainfall, at 

about two-thirds of the total storm duration. The 

implication of this timing is that about 90 percent of the 

24-hour rainfall contributes to the peak flow, and the 

remainder contributes to flow after the peak. Therefore, 

for this rainfall distribution, there is only a short period 

of time between the occurrence of peak rainfall 

intensity and the occurrence of peak flow. For example,

for a 24-hour storm the peak flow occurs about 15 

hours after rainfall begins and about 6 hours after peak 

rainfall intensity for this synthetic rainfall distribution. 

Other rainfall distributions would result in different 

relations between time of maximum rainfall intensity 

and occurrence of peak flow.

WATER-SURFACE PROFILES

The peak flows simulated and presented in this 

report are for site 1 only (fig. 1). In general, peak flow 

increases with distance downstream and decreases with 

distance upstream. In addition, a peak flow is of little 

use in determining the extent of flooding without a 

means of associating the flow with a water-surface 

elevation. For these reasons, it is desirable to transfer 

flows at a single point to points upstream and 

downstream and compute a profile of water-surface 

elevations along the stream reach for selected flows of 

interest. This approach has been applied to Scott Creek

10 Simulated Peak Flows and Water-Surface Profiles for Scott Creek near Sylva, North Carolina
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2

TIME NORMALIZED BY TIME TO PEAK DISCHARGE (TfTP)

Figure 6. Dimensionless input rainfall intensity and average dimensionless simulated flood hydrograph for Scott 

Creek above Sylva, N.C.

in studies by the Federal Emergency Management 

Agency (FEMA), in which water-surface profiles 

for flows of selected recurrence intervals were 

developed and presented (Federal Emergency 

Management Agency, 1989, 1990).

Computation of the profiles for the FEMA 

studies was done by TVA personnel using the U.S. 

Army Corps of Engineers HEC-2 Water Surface 

Profiles computer program (R.A. Milstead, 

Tennessee Valley Authority, oral commun., July 

1996). Elevations, geometries, and hydraulic 

properties of cross sections located along the stream 

reach of interest are needed for the HEC-2 to 

compute a water-surface profile for specified flows. 

A detailed survey of cross sections and field 

determination of the hydraulic properties of the 

cross sections are required to develop the data 

necessary to apply HEC-2. The elevation, geometry, 

and hydraulic property data used by the TVA in 

their analyses (R.A. Milstead, Tennessee Valley

Authority, written commun., June 28, 1996) were 

obtained and used to compute water-surface profiles 

for flows simulated in this study.

In order to use HEC-2, the simulated flows at 

site 1 (fig. 1; table 1) were transferred upstream to 

the beginning of the reach, at a drainage area of 

49.1 mi2 , and downstream to the end of the reach, at 

a drainage area of 59.0 mi2 , using a drainage-area 

ratio. Flows for cross sections along the reach were 

determined by a linear interpolation of drainage 

area.

Water-surface profiles were determined using 

HEC-2 for the observed peak flow of 4,440 ft3/s and 

for the simulated peak flow of 8,240 ft3/s (table 4) 

from a point 1,500 ft upstream from the mouth of 

Scott Creek to a point 22,500 ft upstream from the 

mouth (fig. 7). Profiles for intermediate peak flows 

were not presented because they were so close in 

elevation that they were indistinguishable at the 

scale used. Water-surface profiles for higher peak

Water-Surface Profiles 11
2,150

2,140

PROFILE FOR PEAK FLOW OF 4,440 ft3/s, MARCH 27, 1994 

_._.. PROFILE FOR PEAK FLOW OF 8,240 ft3/s 

V LONGITUDINAL LOCATION OF BRIDGE OR FEATURE

1,960 * 

1,950

1,500 3,000 4,500 6,000 7,500 9,000 10,500 12,000 13,500 15,000 

DISTANCE FROM MOUTH, IN FEET

16,500 18,000 19,500 21,000 22,500

Figure 7. Water-surface profiles for selected observed and simulated peak flows along the reach of Scott Creek flowing 

through Sylva, N.C.

flows were not determined because of the very low 

probability of occurrence of such flows.

The water-surface profiles presented are 

representative of the conditions described by the cross 

section and hydraulic property data provided by the 

TVA, which were gathered during the execution of the 

FEMA study during 1988-89. A comparison of stagedischarge relations, or rating curves, for Scott Creek 

above Sylva from the earlier period of record (1942-75) 

and the study period (1993-95) indicates a considerable 

change in the hydraulic properties of the stream at the 

simulation site (fig. 8).

The relative change in the rating curves, 

however, is such that the stage associated with a given 

flow has decreased with time (fig. 8). A comparison of 

the observed peak stage for the 4,440 ft3/s flow, about 

2,063 ft above sea level, with the elevation taken from 

the computed profile at the gage location (site 1), which 

is about 2,067 ft above sea level, further illustrates this

change. Therefore, water-surface profiles presented in 

this report likely overestimate the true water-surface 

elevation for a given flow at site 1.

APPLICATION OF RESULTS

The simulated peak flows and associated watersurface profiles presented in this report represent the 

response of the Scott Creek Basin to several 

hypothetical combinations of synthesized antecedent 

moisture conditions and 24-hour rainfall amounts. 

Because these hypothetical conditions will likely not 

be exactly reproduced by natural events, the peak flows 

and elevations presented are intended to be used as a 

qualitative guide to the response of the basin to actual 

conditions. Errors in the rainfall-runoff model, 

differences between the hypothesized and actual 

antecedent moisture conditions, and differences in the 

distribution of rainfall between the synthesized and

12 Simulated Peak Flows and Water-Surface Profiles for Scott Creek near Sylva, North Carolina
2,070

2,068

2,066

2,064

2,062

2,060

~- 2,058 

O

> 2,056 LJJ _i

LJJ

2,054

2,052

2,050

 - STAGE-DISCHARGE RELATION, 1975

 STAGE-DISCHARGE RELATION, 1995

0 500 1,000 1,500 2,000 2,500 3,000 3,500

DISCHARGE, IN CUBIC FEET PER SECOND

4,000 4,500 5,000

Figure 8. Stage-discharge relations (rating curves) in effect in 1975 and 1995 for Scott Creek above Sylva, N.C.

actual storm events should be considered when using 

the results of this report.

As an example, consider a predicted 24-hour 

rainfall amount. Antecedent moisture conditions can be 

estimated by referring to published climatic records for 

the previous 3 months. Then, given the predicted 

rainfall total and estimated antecedent moisture 

conditions, the simulated peak flow for those 

conditions can be determined (fig. 6; table 4). This peak 

flow can then be compared to the peak flows associated 

for which computed water-surface profiles are 

presented (fig. 7), and an estimate of the potential 

water-surface elevation resulting from the predicted 

24-hour rainfall amount can be determined. Estimated 

peak flow and water-surface elevations resulting from 

such an analysis provide an approximation of the 

relative magnitude of the response of the Scott Creek 

Basin to actual 24-hour rainfall amount and antecedent 

moisture conditions.

SUMMARY

Peak flows were simulated for Scott Creek in 

Jackson County, North Carolina, using a calibrated 

version of the USGS rainfall-runoff model. Peak flows 

ranging from 89 to 10,100 ft3/s were simulated in 

response to synthetic unit rainfall distributions for 

24-hour storm rainfall amounts ranging from 1.0 to 

7.5 in. Each peak flow was simulated under low-, 

moderate-, and high-antecedent moisture conditions, 

represented by 3-month periods of daily rainfall and 

evaporation. These periods were selected on the basis 

of an analysis of 37 years of records from two nearby 

climatic stations.

The model was calibrated using observed flow 

and rainfall data for 14 storms collected from March 

1994 to September 1995. During the data-collection 

period, a peak flow of 4,440 ft3/s was observed on 

March 27, 1994. This flow has a recurrence interval 

greater than 200 years and is the maximum observed

Summary 13
flow for the periods of record at the gage, 1942-75 and 

1993-95.

Water-surface profiles were computed along the 

reach of Scott Creek flowing through Sylva, N.C., for 

selected simulated and observed flows. The watersurface profiles were computed using data obtained 

from the TVA and according to procedures generally 

equivalent to those used in previous FEMA studies of 

Scott Creek. As a result of changes in the stagedischarge relation at the simulation site since the data 

were collected by TVA, the profiles determined for this 

investigation likely overestimate true water-surface 

elevations for a given flow at the simulation site.

REFERENCES

Barker, R.G., George, E.D., Rinehardt, J.F., and Eddins, 

W.H., 1994, Water resources data, North Carolina, 

water year 1993: U.S. Geological Survey Water-Data 

Report NC-93-1,580 p.

Dawdy, D.R., Lichty, R.W., and Bergmann, J.M., 1972, A 

rainfall-runoff simulation model for estimation of flood 

peaks for small drainage basins: U.S. Geological 

Survey Professional Paper 506-B, 28 p.

Federal Emergency Management Agency, 1989, Flood 

insurance study, Jackson County, North Carolina, 

unincorporated areas: 15 p.

___revised 1990, Flood insurance study, Town of Sylva, 

North Carolina, Jackson County: 6 p.

Huff, FA., 1967, Time distribution of rainfall in heavy

storms: American Geophysical Union, Water Resources 

Research, v. 3, no. 4, p. 1007-1019.

Ragland, B.C., Smith, D.G., Barker, R.G., and Rinehardt, 

J.F, 1995, Water resources data, North Carolina, water 

year 1994: U.S. Geological Survey Water-Data Report 

NC-94-l,578p.

___1996, Water resources data, North Carolina, water year 

1995: U.S. Geological Water-Data Report NC95-1, 618 p.

Robbins, J.C., and Pope, B.F, 1996, Estimation of floodfrequency characteristics of small urban streams in 

North Carolina, 1995: U.S. Geological Survey WaterResources Investigation Report 96-4084, 21 p.

U.S. Army Corps of Engineers Hydrologic Engineering 

Center, 1991, HEC-2 Water surface profiles user's 

manual: U.S. Army Corps of Engineers CPD-2A, 

308 p.

U.S. Water Resources Council, 1981, Guidelines for 

determining flood flow frequency: U.S. Water 

Resources Council Bulletin 17B, 183 p.

14 Simulated Peak Flows and Water-Surface Profiles for Scott Creek near Sylva, North Carolina
APPENDIX

Table A-1. Units, definition, and calibrated value for the RRM parameters used in peak flow 

simulation at Scott Creek above Sylva, N.C.

[ , not applicable]

Parameter Units Definition Calibrated value

Antecedent soil-moisture accounting component

EVC

RR

BMSM

DRN

inches

inches per 

hour

Coefficient to convert pan evaporation to potential 

evapotranspi rati on

Proportion of daily rainfall that infiltrates the soil

Soil moisture storage volume at field capacity

The constant rate of drainage for redistribution of 

soil moisture

0.750

.500

14.36

1.00

Infiltration component

PSP

KSAT

inches

inches per 

hour

Minimum value of the combined action of 

capillary suction and soil moisture differential

Minimum saturated hydraulic conductivity used 

to determine soil infiltration rates

0.505

.021

RGF

EIA

Ratio of combined action of suction and potential 20.76 

at wilting point to that at field capacity

Effective ratio of impervious area to total basin .029 

area; a measure of the impervious area that is 

directly connected to the channel drainage 

system

Surface-runoff routing component

KSW hours

TC minutes

TP/TC 

Linear reservoir routing coefficient

Duration of the triangular translation hydrograph 

(time of concentration)

Ratio of time to peak to time of concentration

8.45

235.8

.500

Appendix 15