VOLUME 1 OF 2
SEBASTIAN COUNTY,
ARKANSAS
AND INCORPORATED AREAS
Community
Name
Community
Number
BARLING, CITY OF
050305
BONANZA, TOWN OF
050392
CENTRAL CITY, TOWN OF
050601
FORT SMITH, CITY OF
055013
GREENWOOD, CITY OF
050198
HACKETT, CITY OF
050199
HARTFORD, CITY OF
050200
HUNTINGTON, CITY OF
050334
LAVACA, CITY OF
050201
MANSFIELD, CITY OF
050202
MIDLAND, TOWN OF
050203
Sebastian County
SEBASTIAN COUNTY,
UNINCORPORATED AREAS
050462
REVISED:
PRELIMINARY
NOVEMBER 10, 2010
Federal Emergency Management Agency
FLOOD INSURANCE STUDY NUMBER
05131CV001B
NOTICE TO
FLOOD INSURANCE STUDY USERS
Communities participating in the National Flood Insurance Program have established repositories of flood
hazard data for floodplain management and flood insurance purposes. This Flood Insurance Study may
not contain all data available within the repository. It is advisable to contact the community repository for
any additional data.
Part or all of this Flood Insurance Study may be revised and republished at any time. In addition, part of
this Flood Insurance Study may be revised by the Letter of Map Revision process, which does not involve
republication or redistribution of the Flood Insurance Study. It is, therefore, the responsibility of the user
to consult with community officials and to check the community repository to obtain the most current
Flood Insurance Study components.
Initial Countywide FIS Effective Date: May 20, 2010
Revised Countywide FIS Date:
TABLE OF CONTENTS - Volume 1
1.0
INTRODUCTION .............................................................................................................................. 1
1.1
Purpose of Study........................................................................................................................... 1
1.2
Authority and Acknowledgments ................................................................................................. 1
1.3
Coordination ................................................................................................................................. 3
2.0
AREA STUDIED ............................................................................................................................ 5
2.1
Scope of Study .............................................................................................................................. 5
2.2
Community Description ............................................................................................................... 7
2.3
Principal Flood Problems ............................................................................................................. 9
2.4
Flood Protection Measures.... ....................................................................................................11
3.0
ENGINEERING METHODS ................................................................................................... ….11
3.1
Hydrologic Analyses .................................................................................................................. 12
3.2
Hydraulic Analyses..................................................................................................................... 20
3.3
Vertical Datum ........................................................................................................................... 24
4.0
FLOODPLAIN MANAGEMENT APPLICATIONS ................................................................... 24
4.1
Floodplain Boundaries ................................................................................................................ 25
4.2
Floodways................................................................................................................................... 26
5.0
INSURANCE APPLICATION ..................................................................................................... 54
6.0
FLOOD INSURANCE RATE MAP ............................................................................................. 54
7.0
OTHER STUDIES ......................................................................................................................... 56
8.0
LOCATION OF DATA ................................................................................................................. 56
9.0
BIBLIOGRAPHY AND REFERENCES.......................................................................................56
i
TABLE OF CONTENTS - Volume 1 - continued
FIGURES
Figure 1: Floodway Schematic ................................................................................................................... 27
TABLES
Table 1: Flooding Sources Studied by Detailed Methods ............................................................................. 5
Table 2: Flooding Sources Studied by Enhanced Approximate Methods .................................................... 5
Table 3: Flooding Sources with Redelineated Floodplains........................................................................... 6
Table 4: Scope of Revision ........................................................................................................................... 6
Table 5: Letters of Map Change ................................................................................................................... 7
Table 6: Stream Name Changes .................................................................................................................... 7
Table 7: Summary of Stillwater Elevations ................................................................................................ 14
Table 8: Summary of Discharges ....................................................................................................... ....15-20
Table 9: Summary of Manning's "n" Values...…….............................................................................. 22-23
Table 10: Floodway Data. ................................................................................................................ …..28-53
Table 11: Community Map History ............................................................................................................ 55
TABLE OF CONTENTS - Volume 2
EXHIBITS
Exhibit 1 – Flood Profiles
Adamson’s Creek
Arkansas River
Cox Creek
Hackett Creek
Heartsill Creek
Heartsill Creek Tributary 1
Hester Creek
Little Massard Creek
Little Vache Grasse Creek
Little Vache Grasse Creek Tributary 9
Main Branch
Massard Creek
May Branch
Mill Creek
Mill Creek Tributary
No Name Creek
Panels
Panels
Panel
Panel
Panels
Panel
Panels
Panels
Panels
Panels
Panel
Panels
Panels
Panels
Panels
Panels
ii
01P-03P
04P-05P
06P
07P
08P-11P
12P
13P-14P
15P-17P
18P-24P
25P-26P
27P
28P-29P
30P-32P
33P-35P
36P-37P
38P-42P
TABLE OF CONTENTS -Volume 2 – continued
EXHIBITS - continued
Exhibit 1 – Flood Profiles - continued
No Name Creek Tributary
No Name Creek Tributary B
Oak Park Tributary
Poteau River
South Branch
Spivey Creek
Sunnymede Creek
Vache Grasse Creek
West Creek
Panels
Panel
Panels
Panels
Panel
Panels
Panels
Panels
Panels
Exhibit 2 – Flood Insurance Rate Map Index
Flood Insurance Rate Map
iii
43P-44P
45P
46P-47P
48P-49P
50P
51P-53P
54P-56P
57P-60P
61P-62P
FLOOD INSURANCE STUDY
SEBASTIAN COUNTY, ARKANSAS AND INCORPORATED AREAS
1.0
INTRODUCTION
1.1
Purpose of Study
This Flood Insurance Study (FIS) revises and updates information on the existence and
severity of flood hazards in the geographic area of Sebastian County, including the Cities
of Barling, Fort Smith, Greenwood, Hackett, Hartford, Huntington, Lavaca, and
Mansfield; the Towns of Bonanza, Central City, and Midland; and the unincorporated
areas of Sebastian County (referred to collectively herein as Sebastian County), and aids
in the administration of the National Flood Insurance Act of 1968 and the Flood Disaster
Protection Act of 1973. This study has developed flood-risk data for various areas of the
community that will be used to establish actuarial flood insurance rates and to assist the
community in its efforts to promote sound floodplain management. Minimum floodplain
management requirements for participation in the National Flood Insurance Program
(NFIP) are set forth in the Code of Federal Regulations at 44 CFR, 60.3.
Please note that the City of Mansfield is geographically located in Scott and Sebastian
Counties. The flood-hazard information for the portion of the city located within
Sebastian County is shown on this FIS report and Flood Insurance Rate Map (FIRM). For
the portion of the city located within Scott County, see the separately published FIS
report and FIRM.
In some states or communities, floodplain management criteria or regulations may exist
that are more restrictive or comprehensive than the minimum Federal requirements. In
such cases, the more restrictive criteria take precedence, and the State (or other
jurisdictional agency) will be able to explain them.
1.2
Authority and Acknowledgments
The sources of authority for this FIS report are the National Flood Insurance Act of 1968
and the Flood Disaster Protection Act of 1973.
On May 20, 2010, this FIS was prepared in a countywide format to include incorporated
communities within and the unincorporated areas of Sebastian County. Information on
the authority and acknowledgements for each jurisdiction included in this countywide
FIS, as compiled from their previously published FIS reports, is shown below.
Unincorporated Areas
The hydrologic and hydraulic analyses for the May 7, 2001, study were performed by the
U.S. Army Corps of Engineers (USACE), Little Rock District, for FEMA, under Contract
No. EMW-95-E-4759, Project Order No. 4. This study was completed in September 1996
(Reference 1).
1
Town of Central City
The hydrologic and hydraulic analyses for the March 16, 1998, study were performed by
the USACE, Little Rock District, for FEMA, under Contract No. EMW-95-E-4759,
Project Order No. 4 (Reference 2).
City of Fort Smith
The hydrologic and hydraulic analyses in the original analyses were prepared by the
Little Rock District of the USACE for FEMA, under Inter-Agency Agreement No.
EMW-E-1153, Project Order No. 1, Amendment No. 4. In the revision dated July 16,
1991, an updated hydraulic analysis for the Arkansas River was prepared by the Little
Rock District of the USACE; and an updated hydraulic analysis for Mill Creek was
prepared by Mickle, Wagner, Coleman and Stodden, Inc. The work for the revised
analyses was completed in June 1989 and January 1990, respectively (Reference 3).
City of Greenwood
The hydrologic and hydraulic analyses for the October 15, 1998, study were performed
by the U.S. Geological Survey for the Federal Insurance Administration, under
Interagency Agreement No. IAA-H-9-77, Project Order No. 26, Amendment No. 1. This
study, which was completed in November 1978, covered all significant flooding sources
in the City of Greenwood. Approximate flood boundaries for Heartsill Creek were
determined in February 1976, by Michael Baker Jr., Inc., under contract to the Federal
Insurance Administration. The Arkansas State Highway and Transportation Department
provided base maps for the City of Greenwood (Reference 4).
City of Hackett
The hydrologic and hydraulic analyses for the September 15, 1981, study were performed
by Garver & Garver, Inc., for FEMA, under Contract No. EMW-C-0063. This study was
completed in September 1980 (Reference 5).
City of Hartford
The hydrologic and hydraulic analyses for the September 15, 1981, study were performed
by Garver & Garver, Inc., for FEMA, under Contract No. EMW-C-0063. This study was
completed in August 1980 (Reference 6).
City of Lavaca
The hydrologic and hydraulic analyses in the September 29, 1986, study represented a
revision of the original analyses prepared by Garver & Garver, Inc., for FEMA, under
Contract EMW-C-0063. The original work was completed in September 1980. The
revised hydrologic and hydraulic analyses were prepared by the Arkansas State Highway
and Transportation Department. The revised work was completed in January 1985
(Reference 7).
There were no previous FISs prior to the May 20, 2010, countywide FIS for the City of
Barling and the Towns of Huntington, Bonanza, and Central City; therefore the previous
2
authority and acknowledgment information for these communities in not included in this
FIS.
The hydrologic and hydraulic analyses for the May 20, 2010, countywide study were
performed by Watershed VI Alliance, for the Federal Emergency Management Agency
(FEMA), under Contract No. EMT-2002-CO-0048. This study was completed in March
2007.
The hydrologic and hydraulic analyses for this revision of No Name Creek, No Name
Creek Tributary, Massard Creek, Mill Creek, and Spivey Creek were performed by
RAMPP, for FEMA under Contract No. HSFEHQ-09-D-0369, Task Order HSFE06-09-J0002. This revision also updates the base map information provided by the City of Fort
Smith. This study was completed on August 31, 2010.
On selected FIRM panels, base map information was provided in digital format by the
City of Fort Smith. The base map for Sebastian County, Arkansas has been prepared in a
vector format at a scale of 1:12,000. Additional information may have been derived from
other sources.
The coordinate system used for the production of this FIRM is State Plane Arkansas
North Federal Information Processing Standards(FIPS) 0301 (feet) and North American
Datum of 1983 (NAD 83) horizontal datum.
1.3
Coordination
Consultation Coordination Officer’s (CCO) meetings may be held for each jurisdiction in
this countywide FIS. An initial CCO meeting is held typically with representatives of
FEMA, the community, and the study contractor to explain the nature and purpose of a FIS,
and to identify the streams to be studied by detailed methods. A final CCO meeting is held
typically with representatives of FEMA, the community, and the study contractor to review
the results of the study.
The dates of the initial and final CCO meetings held for Sebastian County and the
incorporated communities within its boundaries are shown below.
Unincorporated Areas
The results of the study were reviewed at the final CCO meeting held on April 21, 1997,
and attended by representatives of FEMA and Sebastian County. All problems raised at
that meeting have been addressed in this study.
Town of Central City
The results of the March 16, 1998 study were reviewed at the final CCO meeting held on
April 21, 1997, and attended by representatives of FEMA and the Town of Central City.
All problems raised at that meeting have been addressed in this study.
City of Fort Smith
On March 7, 1983, streams requiring detailed study were identified at an initial CCO
meeting attended by representatives of FEMA, the City of Fort Smith, and the Little Rock
3
District of the USACE (the study contractor). The City of Fort Smith and the Natural
Resources Conservation Service (NRCS) (formerly the Soil Conservation Service) were
contacted for information pertinent to this study.
On September 11, 1985, the results of the study were reviewed at a final CCO meeting
held with representatives of FEMA, the City of Fort Smith, and the study contractor. All
problems raised at that meeting have been addressed in this study.
City of Greenwood
Streams requiring detailed study were identified at a meeting attended by representatives
of the Geological Survey, Federal Insurance Administration (FIA), Arkansas Division of
Soil and Water Resources, City of Greenwood, and local businessmen in May 1977.
Results of the hydrologic analyses were coordinated with the NRCS.
On March 26, 1980, the results of the study were reviewed at the final meeting attended
by a representative of the study contractor, FIA, and community officials. The study was
acceptable to the community.
City of Hackett
Streams requiring detailed study were identified at a meeting attended by representatives
of the study contractor, FEMA, and representatives of the City of Hackett in May 1979.
On February 26, 1981, the results of the study were reviewed at the final meeting
attended by representatives of the study contractor, FEMA, and community officials. The
study was acceptable to the community.
City of Hartford
Streams requiring detailed study were identified at a meeting attended by representatives
of the study contractor, FEMA, and representatives of the City of Hartford, Arkansas in
May 1979.
On February 26, 1981, the results of the study were reviewed at the final meeting
attended by representatives of the study contractor, FEMA, and community officials. The
study was acceptable to the community.
City of Lavaca
On September 12, 1985, the results of this study were reviewed at a final CCO meeting
attended by representatives of FEMA, the City of Lavaca, and the Arkansas State
Highway and Transportation Department. The study was acceptable to the community.
There is not an effective flood insurance study for either the City of Mansfield or the
Town of Midland.
The initial CCO meeting for the May 20, 2010, countywide study was held on May 29,
2004, and attended by representatives of FEMA, community officials, and the study
contractor.
4
The results of the May 20, 2010, countywide study were reviewed at the final CCO
meeting held on June 14, 2007, and attended by representatives of the community,
FEMA, and the study contractor. All problems raised at that meeting have been
addressed in this study.
For this countywide revision, the initial CCO meeting was held on October 26, 2009.
This meeting was attended by representatives of the study contractor, the City of Fort
Smith, Sebastian County and FEMA.
2.0
AREA STUDIED
2.1
Scope of Study
This FIS report covers the geographic area of Sebastian County, Arkansas.
Table 1, “Flooding Sources Studied by Detailed Methods” shows the flooding sources
studied by detailed methods either as a part of the May 20, 2010, countywide revision or
as a part of previous FIS performed for individual jurisdictions listed in Section 1.1
Table 1: Flooding Sources Studied by Detailed Methods
Adamson’s Creek
Arkansas River
Cox Creek
Hackett Creek
Heartsill Creek
Heartsill Creek Tributary 1
Hester Creek
Little Massard Creek
Little Vache Grasse Creek
Little Vache Grasse Creek Tributary 9
Main Branch
Massard Creek
May Branch
Mill Creek
Mill Creek Tributary
No Name Creek
No Name Creek Tributary
Oak Park Tributary
Poteau River
South Branch
Spivey Creek
Sunnymede Creek
Vache Grasse Creek
West Creek
All or portions of numerous flooding sources were studied by approximate methods, as
presented in Table 2. Approximate analyses were used to study those areas having a low
development potential or minimal flood hazards. The scope and methods of study were
proposed to and agreed upon by FEMA and the Watershed VI Alliance.
Table 2: Flooding Sources Studied by Enhanced Approximate Methods
Cedar Creek (East)
Grayson Creek
Heartsill Creek Tributary 2
Main Branch
Massard Creek
South Branch
Spivey Creek
Spivey Creek Tributary 2
Vache Grasse Creek Tributary A
Vache Grasse Creek Tributary A Tributary
5
For the May 20, 2010, countywide revision, all or portions of the floodplain boundaries
of the flooding sources, listed in Table 3, that have been previously studied by detailed
methods were re-delineated based on more detailed and up-to-date topographic
information.
Table 3: Flooding Sources with Redelineated Floodplains
Arkansas River
Cox Creek
Hackett Creek
Little Massard Creek
Massard Creek
May Branch
Mill Creek
Mill Creek Tributary
No Name Creek
No Name Creek Tributary
Oak Park Tributary
Poteau River
Spivey Creek
Sunnymede Creek
Vache Grasse Creek
West Creek
The May 20, 2010, countywide FIS revision incorporated the effects of annexations or deannexations by the communities in Sebastian County.
As part of this countywide FIS, updated analyses were included for the flooding sources in
Table 4, “Scope of Revision.”
Table 4: Scope of Revision
Stream
Revised Reach
Massard Creek
From approximately 3,180 feet upstream of Rogers Avenue
to approximately 680 feet upstream of State Highway 255
Mill Creek
From approximately 850 feet upstream of Cavanaugh Road
to approximately 230 feet downstream of I-540
No Name Creek
From its confluence with Arkansas River to approximately
150 feet upstream of Duncan Road
No Name Creek
Tributary
From its confluence of No Name Creek to approximately
2,500 feet upstream of State Highway 22
Spivey Creek
From its confluence with Massard Creek to downstream of
Geren Road
The May 20, 2010, countywide FIS incorporated the determinations of Letters of Map Change (LOMC)
issued by FEMA for the projects listed in Table 5, “Letters of Map Change.”
6
Flooding Source
Table 5: Letters of Map Change
Project Identifier
Arkansas River
Little Massard Creek
Case
Number
06-91-176
03-06-847P
Little Massard Creek
May Branch
99-06-1574P
04-06-1185P
May Branch
01-06-1837P
May Branch
05-06-1454P
Mill Creek
Mill Creek
Channelization
Effective Date
8/7/1992
8/26/2004
96-06-313P
Ozark Oil and Gas Property
Little Massard Creek
Channelization
Phoenix Avenue Extension Project
Fort Smith Library Site and Old
Greenwood Road Improvements
May Branch Floodplain and Floodway
Revision
“P” Street Wastewater Treatment
Plant Expansion
Brandymill Phase I and II Subdivision
05-06-1081P
Mill Creek
3/8/2007
Sunnymede Creek
04-06-667P
Sunnymede Creek
6/28/2005
Main Branch, North
Branch, and South
Branch
02-06-1094P
Eastgate Estates Phases 1 through 4
11/25/2002
3/21/2000
1/13/2006
5/31/2002
12/30/2005
8/30/1996
Table 6, “Stream Name Changes” lists those streams whose name has been changed from
the previously published FIS reports for the unincorporated areas of Sebastian County
and the incorporated communities within prior to the May 20, 2010, countywide.
Table 6: Stream Name Changes
Community
Old Name
New Name
Unincorporated Areas
Massard Creek
Massard Creek (West)
Unincorporated Areas
and City of Fort Smith
Massard Creek
Cedar Creek (East)
2.2
Community Description
Sebastian County borders the western edge of Arkansas, between the Ozark and Ouachita
Mountains to the north and south, respectively. Sebastian County was founded by the Act
of January 6, 1851. It is located in west-central Arkansas, and was formed from territory
that previously comprised Crawford, Scott, and Polk Counties. Sebastian County was
named after William K. Sebastian, who was judge of the first circuit court for several
years after the State of Arkansas was admitted to the Union.
Sebastian County is bordered by Crawford County to the north; Franklin County to the
east, Logan County to the northeast; Scott County to the south; and Le Flore County,
Oklahoma, to the west.
7
According to the U.S. Bureau of the Census, the population of Sebastian County was
estimated to be 123,597 and the area was 536 square miles in 2009 (Reference 8).
Sebastian County has a warm, temperate, continental climate. Seasons are well defined,
and changes between seasons are gradual. The most variable weather occurs in spring
when local storms are severe and can bring large amounts of precipitation. In summer,
periods of hot weather are relieved by cool nights and occasional showers or
thunderstorms. Autumn weather is characterized by mild, sunny days interspersed with a
few days of moderate to heavy rains. Winters are generally sunny, with cold, blustery
weather persisting for a few days at a time, but generally followed by moderate weather
brought by southerly winds. The mean annual temperature for Sebastian County is
61.5°F. Monthly averages range from 40°F in January to 82°F in July. Daily maximum
temperatures in summer occasionally exceed 100°F (Reference 1).
The average annual precipitation for the area is approximately 38 inches (Reference 9).
Precipitation is fairly uniformly distributed throughout the year, with heavier and lesser
amounts occurring in the spring and summer, respectively. Intense thunderstorms,
however, do occur in the summer and may release large amounts of rain over a small area
in a short period of time. Snowfall is light within the area, amounting to approximately 5
inches annually.
The Arkansas River flows in an easterly direction across the State and empties into the
Mississippi River on the eastern edge of Arkansas. The Arkansas River extends from the
Ozark Dam west to the Rocky Mountains. Its basin contains 151,801 square miles above
the Ozark Dam. Due to the size and configuration of the Arkansas River basin, local
rainfall has very little effect on flooding. Rainfall that causes flooding on the Arkansas
River in the Sebastian County area occurs east of Hutchinson, Kansas, and is regional,
rather than local, in nature. In Sebastian County, heavy rainfall tends to occur in late
winter and early spring during the months of March, April, and May (Reference 1).
The more fertile soils in Sebastian County are on the narrow floodplains along the
Arkansas River, Vache Grasse and Big Creeks, and James Fork. They are used chiefly for
row crops. On the uplands, where the soils are less fertile, livestock raising and timber
production are the main enterprises (Reference 1).
Vache Grasse Creek has a total drainage area of 114 square miles at the downstream
study limit and an average stream slope of 2.33 feet per mile throughout the study reach.
It flows northwestward through the gently sloping plains to the confluence with Little
Vache Grasse Creek and empties into the Arkansas River. The drainage basin consists of
level to gently sloping valley fill and alluvial sediments that range from young
floodplains along the Arkansas River to old stream terraces in the broad valleys between
hills. The bottom land along the Arkansas River is intensively farmed. The other lands
are used mainly for forage crops and native or improved pasture.
The topography in Sebastian County is rolling to mountainous, with elevations generally
ranging from 370 feet North American Vertical Datum of 1988 (NAVD) near the
Arkansas River floodplain to 810 feet NAVD in the central area of the county.
8
The Town of Central City is located in north-central Sebastian County, near the south
bank of the Arkansas River, approximately 7 miles southeast and 150 miles northwest of
the Cities of Fort Smith and Little Rock, respectively. Rainfall runoff from the town
drains into the Arkansas River through Vache Grasse Creek (Reference 2).
The City of Fort Smith is bordered by the City of Van Buren, Arkansas, to the northeast;
the City of Barling, Arkansas, and Fort Chaffee, Arkansas, to the southeast; and the Town
of Arkoma, Oklahoma, to the west. The terrain in the City consists mainly of rolling
hills; however, some areas are within the floodplains of the Arkansas River and its
tributaries. Generally, the commercial and residential development is located along the
streams in the community (Reference 3).
The City of Greenwood is located in western Arkansas near the Arkansas-Oklahoma
State line in central Sebastian County. The city is located entirely within the Vache
Grasse drainage area. Most of the developed part of the City of Greenwood is drained by
Heartsill and Hester Creeks which join in the western part of the city and flow eastward
to a junction with Vache Grasse Creek. Topographic relief in the City ranges from
moderately hilly in the residential areas to flat or nearly flat in the Vache Grasse flood
plain (Reference 4).
The City of Hackett, Arkansas is located in central Sebastian County about one mile east
of the Arkansas-Oklahoma state line and eight miles south of Fort Smith, Arkansas.
Drainage from the community flows into southwest down Hackett Creek. Hackett Creek
flows into Oklahoma and empties into the James Fork River. The James Fork flows into
the Poteau River which flows northeast and empties into the Arkansas River at Fort
Smith, Arkansas. Most residential and commercial development in the City is located on
higher terrain around Hackett Creek (Reference 5).
The City of Hartford is located approximately four miles east of the Arkansas-Oklahoma
State Line and about twenty-five miles south of Fort Smith, Arkansas. It is located in a
valley between the Poteau and Sugar Loaf Mountains. All storm drainage from Hartford
enters West Creek and flows northeast. West Creek empties into James Fork northeast of
Hartford. James Fork flows west into Oklahoma and empties into the Poteau River. The
Poteau River flows northeast and empties into the Arkansas River at Fort Smith,
Arkansas. Most residential and commercial development in the City is located on higher
terrain north of West Creek. Some low density residential development is located in the
West Creek flood plain, but most of the floodplain is still in agricultural use
(Reference6).The City of Lavaca is located in the north-central portion of Sebastian
County, approximately 2 miles south of the Arkansas River and 8 miles east of Fort
Smith. Most residential and commercial development in the city is located on higher
terrain around Cox Creek (Reference 7).
2.3
Principal Flood Problems
The Arkansas River floods the low-lying areas adjacent to its channel and along many of
its tributaries during periods of high runoff from its basin. High water on the Arkansas
River also affects flood stages along Intercepting Drain because floodgates through the
levee that normally discharge directly into the river are closed. When the gates are closed,
this water is diverted into an overloaded storm sewer, which results in greater amounts of
flow along Intercepting Drain.
9
The flood of December 3, 1982, on the Arkansas River at Dardanelle, Arkansas, had a
discharge of 325,000 cubic feet per second (cfs). This flood has an approximate 15percent-annual-chance of occurring. The October 1986 flood on the Arkansas River in
Logan County had a discharge of approximately 345,000 cfs. This flood has an
approximate 25-percent-annual-chance of recurrence (Reference 1).
Continuous records of river stages on the Arkansas River at the City of Fort Smith gage
are available from April 1879 to December 1955. Fragmentary records and a number of
high-water elevations at this location are available from as far back as the 1830s.
Continuous records of river stages and discharges on the Arkansas River at Van Buren
are available since October 3, 1927, when the U.S. Geological Survey (USGS) installed a
chain gage on the U.S. Route 64-71 bridge. On July 5, 1934, the chain gage was replaced
by a recording gage. The crest readings of the City of Fort Smith and City of Van Buren
gages generally read within 1 foot of each other during the period when both were in
operation, except during extremely large floods when the City of Fort Smith reading was
considerably higher.
Discharges and elevations from the headwater and tailwater gages at Lock and Dam No.
13 are available for the period since December 1969 when the navigation pool was
raised. This station replaced the City of Van Buren gage for this portion of the Arkansas
River. The rain gage at this station was reactivated in September 1980.
Rainfall records for the City of Fort Smith area are available since 1882. Other available
data consist of a number of high-water marks of several past floods on the tributary
streams, newspaper accounts of floods, and interviews with local residents.
The upstream reaches of the watersheds of the tributary streams are rapidly developing.
This will require good floodplain management to avoid flood damages due to increased
runoff that will occur.
Stage readings from the headwater and tailwater gages at the Ozark Dam are also
available on the Arkansas River for the period since the McClellan-Kerr Arkansas River
Navigation System was completed.
The possibility of flooding along Hackett Creek within the City of Hackett is intensified
by the quick runoff occurring from the sloping terrain and tight soils. The sloping terrain
tends to confine runoff along the creek so that flooding rarely extends more than a few
hundred feet from the channel at any point within the City. Channel velocities tend to be
very high with average velocities reaching seven feet per second. Flooding occurs very
quickly but tends to be of short duration. The drainage basin of the creek is limited to a
small area north and east of the City and south of Backbone Mountain so that the heaviest
runoff occurs from local rainfall. The chances of flooding are greatest in the spring and
early summer when rainfall activity is more frequent, but flooding can occur at any time
of the year. The removal of vegetation and improvement of drainage for agricultural
purposes tends to increase runoff from the study area. Because of the high velocities in
the channel, siltation does not appear to be a problem. The limited capacity of the bridge
at Highway 10 restricts flood flows and causes water to cross over the roadway
(Reference 5).
10
The possibility of flooding along West Creek within the City of Hartford is intensified by
the quick runoff occurring from the sloping terrain and tight soils. The sloping terrain
tends to confine runoff along West Creek so that flooding rarely extends more then a few
hundred feet from the channel at any point within the City. Channel velocities tend to be
very high with average velocities reaching 10 feet per second. Flooding occurs very
quickly but tends to be of short duration. The drainage basin of West Creek is limited to
the area between the mountains surrounding the City so that the heaviest runoff occurs
from local rainfall. The chances of flooding are greatest in the spring and early summer
when rainfall activity is more frequent, but flooding can occur at any time of the year.
The removal of vegetation and improvement of drainage for agricultural purposes tends
to increase runoff from the study area. Because of the high velocities in the channel,
siltation does not appear to be a problem. The limited capacity of the bridges at Pine
Street and Highway 96 restrict flood flows and cause water to cross over the roadways
(Reference 6).
In the City of Lavaca, flooding occurs very quickly but tends to be of short duration. The
drainage basin of Cox Creek is limited to a small area around the City so that the heaviest
runoff occurs from local rainfall. The chances of flooding are greatest in the spring and
early summer when rainfall activity is more frequent, but flooding can occur at any time
of the year (Reference 7).
The eastern areas of the Town of Central City lie in the floodplain of Vache Grasse
Creek, which is affected by backwater from the Arkansas River (Reference 2).
2.4
Flood Protection Measures
There are 25 reservoir projects upstream from the City of Fort Smith that provide a total
of 10,710,000 acre-feet of flood-control storage on 1.5 inches of rainfall from a
controlled basin area of 142,317 square miles. This is approximately 94.6 percent of the
150,482 square miles of drainage area above the City of Van Buren gage (Reference 1).
Downstream from the City of Fort Smith are Lock and Dam No. 13, the Dardanelle Dam,
and the Ozark Lock and Dam. Lock and Dam No. 13 has no flood-control storage.
Dardanelle Dam is located on the Arkansas River just downstream of Franklin County.
Approximately 20 miles of the Arkansas River in Logan County are normally inundated
by the Dardanelle Reservoir. The Ozark Lock and Dam, which is approximately 1 mile
downstream from the City of Ozark, forms a long, narrow reservoir. Lock and Dam No.
13, the Dardanelle Dam, and the Ozark Lock and Dam are part of the McClellan-Kerr
Arkansas River Navigation System.
3.0
ENGINEERING METHODS
For the flooding sources studied by detailed methods in the community, standard hydrologic and
hydraulic study methods were used to determine the flood-hazard data required for this study.
Flood events of a magnitude that is expected to be equaled or exceeded once on the average
during any 10-, 50-, 100-, or 500-year period (recurrence interval) have been selected as having
special significance for floodplain management and for flood insurance rates. These events,
commonly termed the 10-, 50-, 100-, and 500-year floods, have a 10-, 2-, 1-, and
0.2-percent chance, respectively, of being equaled or exceeded during any year. Although the
recurrence interval represents the long-term, average period between floods of a specific
11
magnitude, rare floods could occur at short intervals or even within the same year. The risk of
experiencing a rare flood increases when periods greater than 1 year are considered. For example,
the risk of having a flood that equals or exceeds the 1-percent-annual-chance flood in any 50-year
period is approximately 40 percent (4 in 10); for any 90-year period, the risk increases to
approximately 60 percent (6 in 10). The analyses reported herein reflect flooding potentials
based on conditions existing in the community at the time of completion of this study. Maps and
flood elevations will be amended periodically to reflect future changes.
3.1
Hydrologic Analyses
Hydrologic analyses were carried out to establish peak discharge-frequency relationships
for each flooding source studied by detailed methods affecting the community.
Precountywide Analyses
Hydrologic data for the lower reach of Vache Grasse Creek near its confluence with the
Arkansas River were estimated and synthesized based on the Snyder's unit-hydrograph
method and by using actual basin characteristics.
A hypothetical-design storm with the 1-percent-annual-chance recurrence interval was
required and developed for this study. Weather Bureau Technical Paper Nos. 40 and 49,
"Rainfall Frequency Atlas of the United States," and National Weather Service Technical
Memorandum NWS Hydro-35, "Five- to 60-Minute Precipitation Frequency for the
Eastern and Central United States" (References 10 and 11, respectively), provided rainfall
depth-duration data for the 1-percent-annual-chance storm. Precipitation data from a
March 1992 reconnaissance report on a small flood-control project for May Branch, Fort
Smith, Arkansas, were also used for this study.
The USACE HEC-1 computer program (Reference 12) was used to develop synthetic
unit hydrographs and distribute rainfall excesses for the basins. Procedures presented in
Engineering Manual (EM) 1110-2-1405, "Flood Hydrograph Analyses and
Computations" (Reference 13), were used to develop the hydrographs. NRCS curvenumber methodology provided the loss rates used. Maps from the Soil Survey of
Sebastian County, Arkansas, published by the NRCS in March 1975 (Reference 14),
provided the soil-type classification for each basin. Soil coverage for each basin area was
determined by grid estimation. A wet antecedent soil moisture condition (AMC III) with
a short-duration storm was used as the starting condition to distribute the precipitation
excess.
The basin areas were determined by planimeteric outlined areas on USGS 30- by 60minute quadrangle mapping at a scale of 1 centimeter = 1 kilometer. Stream channel
lengths and lengths to centroids were measured from USGS 7.5-minute series
topographic maps at a scale of 1:24,000, enlarged to 1:6,000, with a contour interval of
10 feet (Reference 15). Weighted slopes were calculated using a LOTUS spreadsheet
following the procedure in EM 1110-2-1405. Using basin characteristic curves developed
by the USACE, Little Rock District, from known flood events at gaged streams in the
Little Rock District, Arkansas and Missouri, basins with similar drainage characteristics
were used to determine Snyder's coefficients Cp and Ct. Times to peak were calculated in
accordance with EM 1110-2-1405.
12
Drainage areas along Hackett Creek and West Creek were measured from topographic
maps at a scale of 1:24,000, with a contour interval of 20 feet (References 16 and 17,
respectively).
Synthetic storms were computed to define the discharge-frequency relationships for
Hackett Creek, West Creek, and Cox Creek. Rainfall distributions for the 10-, 2- and 1percent-annual-chance frequencies were computed from rainfall-frequency data
contained in the National Weather Service Technical Paper No. 40 (Reference 10).
Snyder's coefficients were used to compute hydrographs. For Cox Creek, these
coefficients were adjusted based on frequency-discharge data developed by the Little
Rock District of the USACE for streams in the study area. The hydrographs and rainfall
distributions were used to compute synthetic storms of the desired frequencies from
which the peak discharges were obtained. A log-probability relationship of the lower
frequency peak discharges was used to compute each of the 0.2-percent-annual-chance
peak discharges.
The hydrologic analyses for the Arkansas River were obtained from the detailed and
extensive engineering studies which have been previously made in connection with the
design, construction, and maintenance of the McClellan-Kerr Arkansas River Navigation
System. Continuous records of river stages on the Arkansas River at the Fort Smith gage
are available from 1879 to present. These flood data were investigated to determine the
discharge frequency used in the study, and they were adjusted as required for the effects
of the appropriate upstream storage reservoirs.
The Southwest District Watershed Runoff Model was used to determine the hydrology
for Little Massard Creek, Sunnymede Creek, No Name Creek, and Mill Creek (Reference
18).
Discharges for Massard Creek, Spivey Creek, No Name Tributary, Oak Park Tributary,
May Branch, Poteau River, and Mill Creek Tributary were determined using the
USACE’s HEC-1 computer program (Reference 12).
Discharges for Main Branch and South Branch were determined using the USACE’s
HEC-HMS computer program (Reference 19).
For the May Branch floodplain and floodway from the Fort Smith Levee and Floodwall
to immediately upstream of North 13th, North 18th and P Streets along May Branch, the
1-percent-annual-chance flood (base flood) elevation was determined by the HEC-IFH
model (Reference 20). The base floodplain area calculated in the interior pond analysis
for this portion of Fort Smith Levee and Floodwall interior flooding and ponding area is
approximately 4.5 million square feet (103.3 acres), and average floodwater storage
volume is 390 acre feet. The area of the floodway determined in the interior pond
analysis for this portion of Fort Smith Levee and Floodwall interior flooding and ponding
area is approximately 2.1 million square feet (48.2 acres) (Reference 21).
Table 7, "Summary of Stillwater Elevations" shows the stillwater elevation for the May
Branch ponding area.
13
Table 7: Summary of Stillwater Elevations
Flooding Source and
Location
MAY BRANCH
INTERIOR FLOODING
PONDING AREA
10-percent
410.0
Elevation (Feet NAVD)*
2-percent 1-percent
415.6
418.0
0.2-percent
420.2
* All elevations are backwater elevations from Arkansas River
The hydrology for other ponding areas was analyzed by applying the 1-percent-annualchance rainfall in 96 hours, subtracting the ponding losses, and applying the resulting
runoff.
May 20, 2010, Countywide Analyses
Rainfall-runoff models for Little Vache Grasse Creek and the upper reach of Vache
Grasse Creek watersheds were developed using the USACE’s HEC-HMS model
(Reference 22). Vache Grasse Creek watershed includes Vache Grasse Creek, Hester
Creek, Adamson’s Creek, Heartsill Creek, and Heartsill Creek Tributary 1 and Little
Vache Grasse Creek watershed includes Little Vache Grasse Creek and Little Vache
Grasse Creek Tributary 9. Peak discharges were calculated for the 10-, 2-, and 1-percentannual-chance flood events. The 0.2-percent annual chance discharge was determined
from a log-probability relationship. The Vache Grasse Creek watershed was divided into
18 sub watersheds ranging in size from 0.22 to 2.10 square miles, while the Little Vache
Grasse Creek watershed was divided into 10 sub watersheds ranging in size from 0.73 to
2.17 square miles. The NRCS unit hydrograph, NRCS curve numbers, and Modified
Plus flood routing method was used to develop the HEC-HMS model. Point rainfall used
to determine the discharge-frequency curve for all streams were taken from the U.S.
Weather Bureau Technical Paper No. 40 (Reference 10). A depth-area precipitation
reduction was used since some watersheds had large drainage areas.
Peak discharges for enhanced approximate study streams were estimated using regression
equations in the Magnitude and Frequency of Floods in Arkansas: USGS WaterResources Investigations Report 95-4224, which is a regional method based on regression
analysis (Reference 23). The method relates drainage area and channel slope to the peak
discharge by empirical equations.
14
Revised Countywide Analyses
Discharges for restudied detailed streams of No Name Creek, No Name Creek Tributary,
Mill Creek, Massard Creek and Spivey Creek were based on new hydrologic analyses.
The new hydrologic analyses of discharges were based on design storms computed using
HEC-HMS computer program (Reference 19). The HEC-HMS computer program
computes flood hydrographs using a unit hydrographic defined by the Soil Conservation
Service (SCS) method parameters. The estimated SCS Curve Number, the lag time (tl ag),
initial rainfall loss, the storm rainfall, and drainage areas were defined as input
parameters. The SCS Curve Number method, the SCS Unit Hydrograph method were
used to determine the loss-rate, transform rainfall excess into surface runoff. The
Mukingum-Cunge method was used to route the flow through the channel for steady-state
simulations. Rainfall data were developed using the "Rainfall Frequency Atlas of the
United States" (TP-40) or reports prepared by the National Weather Service.
Peak discharge-drainage area relationships for all the streams studied by detailed methods
are shown in Table 8, “Summary of Discharges”.
Table 8: Summary of Discharges
Flooding Source and Location
Drainage Area
(Square miles)
Peak Discharges (Cubic feet per second)
10-PERCENT
2-PERCENT
1-PERCENT
0.2-PERCENT
ADAMSON’S CREEK
At Mouth
7.32
3,100
4,750
5,600
8,180
At North Coker Street
6.75
3,010
4,310
5,410
7,810
Approximately 800 feet upstream
of Salesbee Road
5.81
2,670
3,970
4,620
6,550
151,801
290,000
415,000
480,000
600,000
ARKANSAS RIVER
At River Mile 256.77 (Ozark
Lock and Dam)
COX CREEK
At County Road
3.21
1,690
2,240
2,490
2,900
At State Highway 253
2.42
1,600
2,120
2,350
2,700
At North Davis Street
1.59
1,350
1,780
1,970
2,300
7.14
5,380
7,060
7,810
9,500
HACKETT CREEK
At State Highway 45
15
Table 8: Summary of Discharges - continued
Flooding Source and Location
Drainage Area
(Square miles)
Peak Discharges (Cubic feet per second)
10-PERCENT
2-PERCENT
1-PERCENT
0.2-PERCENT
HEARTSILL CREEK
At Mouth
9.38
3,950
5,850
6,940
10,280
At the confluence of Hester
Creek
4.09
2,540
3,820
4,500
6,540
At the confluence of Heartsill
Creek Tributary 1
2.29
1,800
2,810
3,300
4,790
At Westwood Avenue
0.6
720
1,100
1,280
1,780
At Mouth
1.23
1,100
1,700
1,970
2,750
Approximately 1,300 feet
upstream of Meadow Bridge
Drive
0.51
580
900
1,050
1,480
HEARTSILL CREEK
TRIBUTARY 1
HESTER CREEK
At Mouth
4.9
2,240
3,550
4,250
6,410
Approximately 900 feet
downstream of U.S Highway 71
3.76
2,370
3,620
4,200
5,930
4.1
2,652
3,901
3,916
5,352
LITTLE MASSARD CREEK
At its confluence with Massard
Creek
LITTLE VACHE GRASSE
CREEK
Approximately 3,650 feet above
mouth
Approximately 6,300 feet
downstream of the confluence of
Grayson Creek
17.73
4,250
5,910
6,730
9,100
16.44
4,240
5,790
6,680
9,320
16
Table 8: Summary of Discharges - continued
Flooding Source and Location
Drainage Area
(Square miles)
Peak Discharges (Cubic feet per second)
10-PERCENT
2-PERCENT
1-PERCENT
0.2-PERCENT
LITTLE VACHE GRASSE
CREEK (Continued)
At the confluence of
Grayson Creek
11.99
3,470
4,830
6,460
12,650
At the confluence of Little
Vache Grasse Creek
Tributary 11
8.62
2,620
4,130
5,100
8,275
Approximately 1,250 feet
upstream of Fort Smith
Boulevard
7.74
2,730
4,340
5,530
9,650
At the confluence of Little
Vache Grasse Creek
Tributary 9
5.29
2,380
3,970
4,970
8,320
At Custer Boulevard
4.34
2,260
3,850
4,700
7,460
2.17
1,740
2,640
3,090
4,430
1.48
610
940
1,110
1,600
Approximately 280 feet
downstream of State Highway 96
1.43
*
*
2,320
*
Approximately 660 feet upstream
of State Highway 96
1.34
*
*
2,310
*
Approximately 1,400 feet
upstream of State Highway 96
0.76
*
*
1,230
*
Approximately 5,600 feet
upstream of
Access Road
LITTLE VACHE GRASSE
CREEK TRIBUTARY 9
At Mouth
MAIN BRANCH
*Data Not Available
17
Table 8: Summary of Discharges - continued
Flooding Source and Location
Drainage Area
(Square miles)
Peak Discharges (Cubic feet per second)
10-PERCENT
2-PERCENT
1-PERCENT
0.2-PERCENT
MASSARD CREEK
At its confluence with Arkansas
River
17.02
6,756
9,440
10,868
14,307
At approximately 1,100 feet
upstream of State Highway 22
15.73
6,835
9,615
11,010
14,443
At approximately 1,300 feet
upstream of State Highway 22
11.74
4,318
6,200
7,172
9,532
At upstream of confluence with
Spivey Creek
8.26
2,161
3,099
3,584
4,780
At upstream of Fort Smith
Railroad
8.08
1,012
1,502
1,748
2,363
At approximately 200 feet
upstream of Fort Smith Railroad
6.76
2,048
2,913
3,383
4,512
5.83
1,880
2,310
3,523
6,737
2.02
1,419
1,982
2,261
2,944
0.67
1,050
1,300
1,430
1,720
At its confluence with Arkansas
River
7.51
5,378
7,411
8,401
10,847
At its confluence with
Sunnymede Creek
3.15
3,692
5,136
5,791
7,426
At its confluence with No Name
Creek Tributary
2.51
3,062
4,215
4,742
6,106
At approximately 1,500 feet
upstream of I-540
1.93
2,182
3,027
3,244
4,391
At approximately 250 feet
downstream of Gordon Lane
1.02
1,537
2,120
2,395
3,075
MAY BRANCH
At its confluence with the
Arkansas River
MILL CREEK
At approximately 1,500 feet
downstream of Cavanaugh Road
Railroad
MILL CREEK TRIBUTARY
At its confluence with Mill Creek
NO NAME CREEK
18
Table 8: Summary of Discharges - continued
Flooding Source and Location
Drainage Area
(Square miles)
Peak Discharges (Cubic feet per second)
10-PERCENT
2-PERCENT
1-PERCENT
0.2-PERCENT
0.22
400
560
630
800
1.32
2,180
2,840
3,150
3,850
1,898.50
21,514
44,443
50,156
53,378
0.43
*
*
760
*
At its confluence with Massard
Creek
2.67
2,349
3,260
3,684
4,778
At approximately 900 feet
downstream of Fort Smith
Railroad
1.99
531
744
545
1,097
At approximately 700 feet
downstream of Fort Smith
Railroad
1.44
1,441
1,985
2,241
2,879
7.15
6,795
8,235
8,862
10,261
114
*
*
15,200
*
105
*
*
14,400
*
NO NAME CREEK
TRIBUTARY
Just downstream of Waldron
Road
OAK PARK TRIBUTARY
At its confluence with the
Arkansas River
POTEAU RIVER
At its confluence with the
Arkansas River
SOUTH BRANCH
At Mouth
SPIVEY CREEK
SUNNYMEDE CREEK
At its confluence with the
Arkansas River
VACHE GRASSE CREEK
At Mouth
At Station 21,000
* Data Not Available
19
Table 8: Summary of Discharges - continued
Flooding Source and Location
Drainage Area
(Square miles)
Peak Discharges (Cubic feet per second)
110-PERCENT
2-PERCENT
1-PERCENT
0.2-PERCENT
WEST CREEK
At Chicago Rock Island & Pacific
Railroad
13.21
8,950
11,730
13,050
15,900
At City Park
12.39
8,700
11,300
12,800
15,400
At West Harford Road
3.68
5,250
6,480
7,470
8,710
3.2
Hydraulic Analyses
Analyses of the hydraulic characteristics of flooding from the sources studied were
carried out to provide estimates of the elevations of floods of the selected recurrence
intervals. Users should be aware that flood elevations shown on the FIRM represent
rounded whole-foot elevations and may not exactly reflect the elevations shown on the
Flood Profiles or in the Floodway Data tables in the FIS report. Flood elevations shown
on the FIRM are primarily intended for flood insurance rating purposes. For construction
and/or floodplain management purposes, users are cautioned to use the flood elevation
data presented in this FIS in conjunction with the data shown on the FIRM.
Precountywide Analyses
Cross sections for the backwater analyses of the lower reach of Vache Grasse Creek were
field surveyed. The structural geometry of bridges and culverts was obtained by field
surveys and from as-built plans. District field book 95FP-10 contains the survey data and
information for benchmarks that provided the vertical control for this project. Estimated
cross sections were also used and taken from USGS 7.5-minute series topographic maps
at a scale of 1:24,000, enlarged to 1:6,000, with a contour interval of 10 feet (Reference
15). The 10-foot contour map was used to estimate the overbank areas when extending
cross sections.
Below water sections of channels, bridges, and culverts for Hackett Creek, West Creek,
and Cox Creek, were obtained by field surveys. Overbank portions of the cross-sections
were obtained from an aerial photogrammetric survey. Additional information was
obtained from topographic maps of the study area.
Cross sections for the streams studied by detailed methods through the City of Fort Smith
and for Main Branch and North Branch were developed by field surveys. All bridges and
culverts were field surveyed to obtain elevation data and structural geometry.
Channel roughness factors (Manning's "n" values) for all streams studied by detailed
methods were obtained by engineering judgment and field investigations of the streams
20
and floodplain areas. The channel "n" and overbank "n" values are shown in Table 9
“Summary of Roughness Coefficients.”
Water surface profiles for Main Branch and South Branch were developed using the
USACE’s HEC-RAS computer program (Reference 24). Water surface profiles for all
other streams studied by detailed methods were developed for the floods of selected
recurrence intervals using the USACE’s HEC-2 computer program (Reference 25).
Starting water-surface elevations were determined by normal depth calculations.
May 20, 2010, Countywide Analyses
Cross Sections of Adamson’s Creek, Heartsill Creek, Heartsill Creek Tributary 1, Hester
Creek, and Vache Grasse Creek were developed from topographic maps with a contour
interval of 4 feet provided by the City of Greenwood (Reference 26). Cross Sections of
Little Vache Grasse Creek and Little Vache Grasse Creek Tributary 9 were developed
from topographic maps with a contour interval of 2 feet provided by the City of Fort
Smith (Reference 27) and the United States Army Corps of Engineers (Reference 28).
Cross sections for Adamson’s Creek, Hester Creek, and Little Vache Grasse Creek were
supplemented by field surveys.
Manning's "n" for all streams studied by detailed methods were obtained by engineering
judgment and field investigations of the streams and floodplain areas. The channel "n"
and overbank "n" values are shown in Table 9,“Manning's "n" Values.”
Water surface profiles for all streams studied by detailed methods were developed using
the USACE’s HEC-RAS computer program (Reference 31) for the 10-, 2-, 1-, and 0.2percent-annual-chance floods. Starting water-surface elevations were determined by
normal depth calculations.
Locations of selected cross sections used in the hydraulic analyses are shown on the
Flood Profiles (Exhibit 1). For stream segments for which a floodway was computed
(Section 4.2), selected cross-section locations are also shown on the FIRM.
The hydraulic analyses for this study were based on unobstructed flow. The flood
elevations shown on the Flood Profiles (Exhibit 1) are thus considered valid only if
hydraulic structures remain unobstructed, operate properly, and do not fail.
Revised Countywide Analyses
For the restudied detailed streams, water surface elevations for the 10-percent, 4-percent,
2-percent, 1-percent, and 0.2-percent-annual-chance floods were computed using the
USACE HEC-RAS Version 4.0.0 step-backwater computer program (Reference 21).
For these streams, below-water sections of channels and near overbanks of selected cross
sections, bridges, and culverts were field surveyed in detailed to obtain elevation data and
structure geometry. For each survey cross section, the field elevation were blended with
overbank topographic data obtained from the Triangulated Irregular Networks (TINs)
provided by City of Fort Smith.
21
For reaches of No Name Creek, Massard Creek and Spivey Creek, downstream boundary
conditions were estimated using a normal depth method. Downstream boundary
conditions for No Name Creek Tributary were set at water-surface elevation (WSEL)
at their junction with No Name Creek with the assumption of coincident peaks.
Downstream boundary conditions for Mill Creek was set at WSEL from a contiguous
effective FIS (Sebastian County, Arkansas and Incorporated Areas May 20, 2010)
immediately downstream of this study reach.
Channel roughness factors (Manning's "n") used in the hydraulic computations for all
streams studied in this project were chosen on the basis of engineering judgment, aerial
photos, and field observations. The channel and overbank Manning’s “n” values for the
new detailed streams are shown in Table 9, “Manning's "n" Values.”
Table 9: Summary of Manning's "n" Values
Flooding Source
Channel “n”
Overbank “n”
Adamson’s Creek
0.040-0.045
0.070-0.100
Arkansas River
0.018-0.030
0.075
Cox Creek
0.035-0.050
0.090-0.100
Hackett Creek
0.045
0.100-0.120
Heartsill Creek
0.040-0.060
0.060-0.100
Heartsill Creek Tributary 1
0.045-0.050
0.070-0.100
Hester Creek
0.040-0.100
0.070-0.150
Little Massard Creek
0.045-0.067
0.070-0.100
Little Vache Grasse Creek
0.050-0.065
0.080-0.100
Tributary 9
0.050-0.065
0.060-0.100
Main Branch
0.045
0.060
Massard Creek
0.030-0.040
0.030-0.120
May Branch
0.040-0.050
0.050-0.150
May Branch
0.040-0.050
0.050-0.150
Mill Creek
0.030-0.040
0.050-0.120
Mill Creek Tributary
0.040-0.060
0.100-0.120
No Name Creek
0.025-0.040
0.035-0.100
No Name Creek Tributary
0.013-0.040
0.035-0.040
Oak Park Tributary
0.040-0.080
0.100-0.150
Oak Park Tributary
0.040-0.080
0.100-0.150
Poteau River
0.040
0.100
Little Vache Grasse Creek
22
Table 9: Summary of Manning's "n" Values
Flooding Source
Channel “n”
Overbank “n”
South Branch
0.045
0.060
Spivey Creek
0.035-0.045
0.050-0.080
Sunnymede Creek
0.045
0.100
1
0.030-0.045
0.020-0.100
2
Vache Grasse Creek
0.030-0.060
0.050-0.100
West Creek
0.045
0.100-0.110
Vache Grasse Creek
1
From its confluence to approximately 24,600 feet upstream of its confluence with the
Arkansas River
2
From approximately 2,050 feet downstream of the confluence of Heartsill Creek to
approximately 5,000 feet upstream of Greenwood Dam
All elevations are referenced to the North American Vertical Datum of 1988 (NAVD88)
Qualifying bench marks within a given jurisdiction that are cataloged by the National
Geodetic Survey (NGS) and entered into the National Spatial Reference System (NSRS)
as First or Second Order Vertical and have a vertical stability classification of A, B, or C
are shown and labeled on the FIRM with their 6-character NSRS Permanent Identifier.
Bench marks cataloged by the NGS and entered into the NSRS vary widely in vertical
stability classification. NSRS vertical stability classifications are as follows:
Stability A: Monuments of the most reliable nature, expected to hold
position/elevation well (e.g., mounted in bedrock)
Stability B: Monuments which generally hold their position/elevation well (e.g.,
concrete bridge abutment)
Stability C: Monuments which may be affected by surface ground movements
(e.g., concrete monument below frost line)
Stability D: Mark of questionable or unknown vertical stability (e.g., concrete
monument above frost line, or steel witness post)
In addition to NSRS bench marks, the FIRM may also show vertical control monuments
established by a local jurisdiction; these monuments will be shown on the FIRM with the
appropriate designations. Local monuments will only be placed on the FIRM if the
community has requested that they be included, and if the monuments meet the
aforementioned inclusion criteria.
23
To obtain current elevation, description, and/or location information for bench marks
shown on the FIRM for this jurisdiction, please contact the Information Services Branch
of the NGS at (301) 713-3242, or visit their Web site at www.ngs.noaa.gov.
It is important to note that temporary vertical monuments are often established during the
preparation of a flood hazard analysis for the purpose of establishing local vertical
control. Although these monuments are not shown on the FIRM, they may be found in
the Technical Support Data Notebook associated with this FIS and FIRM. Interested
individuals may contact FEMA to access this data.
3.3
Vertical Datum
All FIS reports and FIRMs are referenced to a specific vertical datum. The vertical
datum provides a starting point against which flood, ground, and structure elevations can
be referenced and compared. Until recently, the standard vertical datum used for newly
created or revised FIS reports and FIRMs was the National Geodetic Vertical Datum of
1929 (NGVD). With the completion of the North American Vertical Datum of 1988
(NAVD), many FIS reports and FIRMs are now prepared using NAVD as the referenced
vertical datum.
Flood elevations shown in this FIS report and on the FIRM are referenced to the NAVD.
These flood elevations must be compared to structure and ground elevations referenced to
the same vertical datum. Some of the data used in this revision were taken from the prior
effective FIS reports and FIRMs and adjusted to NAVD. The datum conversion factor
from NGVD to NAVD in Sebastian County is +0.327 feet.
For information regarding conversion between the NGVD and NAVD, visit the National
Geodetic Survey website at www.ngs.noaa.gov, or contact the National Geodetic Survey
at the following address:
NGS Information Services
NOAA, N/NGS12
National Geodetic Survey
SSMC-3, #9202
1315 East-West Highway
Silver Spring, Maryland 20310-3282
(301) 713-3242
4.0
FLOODPLAIN MANAGEMENT APPLICATIONS
The NFIP encourages State and local governments to adopt sound floodplain management
programs. To assist in this endeavor, each FIS report provides 1-percent-annual-chance
floodplain data, which may include a combination of the following: 10-, 2-, 1-, and
0.2-percent-annual-chance flood elevations; delineations of the 1- and 0.2-percent-annual-chance
floodplains; and a 1-percent-annual-chance floodway. This information is presented on the FIRM
and in many components of the FIS report, including Flood Profiles, Floodway Data tables, and
24
Summary of Stillwater Elevation tables. Users should reference the data presented in the FIS
report as well as additional information that may be available at the local community map
repository before making flood elevation and/or floodplain boundary determinations.
4.1
Floodplain Boundaries
To provide a national standard without regional discrimination, the
1-percent-annual-chance flood has been adopted by FEMA as the base flood for
floodplain management purposes. The 0.2-percent-annual-chance flood is employed to
indicate additional areas of flood risk in the community. For each stream studied by
detailed methods, between cross sections, the boundaries were interpolated using
topographic data at a scale of 1:6000 with a contour interval of 2 feet.
For Hackett Creek and West Creek, between cross sections, the boundaries were
interpolated using topographic maps at a scale of 1:12,000 which were obtained from an
aerial photogrammetric survey.
For Cox Creek and the lower reach of Vache Grasse Creek, between cross sections, the
boundaries were interpolated using USGS topographic maps (References 32 and 15,
respectively).
For the streams studied by detailed methods and flowing through the City of Greenwood,
the floodplain boundaries were interpolated between cross sections using topographic
maps with 4 foot contour elevations and spot elevations provided by the City (Reference
26).
For Little Vache Grasse Creek, floodplain boundaries between cross sections were
interpolated using topographic mapping with 2 foot contour data (References 27 and 28).
For Little Vache Grasse Creek Tributary 9, the boundaries between cross sections were
interpolated using topographic mapping with 2 foot contour data provided by the City of
Fort Smith (Reference 27).
For Mill Creek, Spivey Creek, No Name Creek and No Name Creek Tributary, the 1- and
0.2- percent-annual-chance floodplain boundaries were delineated using the flood
elevations determined at each cross section. The floodplain boundaries between cross
sections were interpolated in the ESRI Arc View version 9.3 (Reference 29) environment
using the TIN topographic data provided by the City of Fort Smith (Reference 30).
The 1- and 0.2-percent-annual-chance floodplain boundaries are shown on the FIRM. On
this map, the 1-percent-annual-chance floodplain boundary corresponds to the boundary
of the areas of special flood hazards (Zones A and AE), and the
0.2-percent-annual-chance floodplain boundary corresponds to the boundary of areas of
moderate flood hazards. In cases where the 1- and 0.2-percent-annual-chance floodplain
boundaries are close together, only the 1-percent-annual-chance floodplain boundary has
been shown. Small areas within the floodplain boundaries may lie above the flood
elevations, but cannot be shown due to limitations of the map scale and/or lack of
detailed topographic data.
25
For the streams studied by approximate methods, only the 1-percent-annual-chance
floodplain boundary is shown on the FIRM.
4.2
Floodways
Encroachment on floodplains, such as structures and fill, reduces flood-carrying capacity,
increases flood heights and velocities, and increases flood hazards in areas beyond the
encroachment itself. One aspect of floodplain management involves balancing the
economic gain from floodplain development against the resulting increase in flood
hazard. For purposes of the NFIP, a floodway is used as a tool to assist local communities
in this aspect of floodplain management. Under this concept, the area of the
1-percent-annual-chance floodplain is divided into a floodway and a floodway fringe.
The floodway is the channel of a stream, plus any adjacent floodplain areas, that must be
kept free of encroachment so that the base flood can be carried without substantial
increases in flood heights. Minimum Federal standards limit such increases to 1 foot,
provided that hazardous velocities are not produced. The floodways in this study are
presented to local agencies as minimum standards that can be adopted directly or that can
be used as a basis for additional floodway studies.
The floodways presented in this study were computed for certain stream segments on the
basis of equal-conveyance reduction from each side of the floodplain. Floodway widths
were computed at cross sections. Between cross sections, the floodway boundaries were
interpolated. The results of the floodway computations are tabulated for selected cross
sections (Table 10). The computer floodways are shown on the FIRM (Exhibit 2). In
cases where the floodway and 1-percent-annual-chance floodplain boundaries are either
close together or collinear, only the floodway boundary is shown.
Portions of the floodway for Arkansas River extend beyond the county boundary.
No floodways were computed for Main Branch and South Branch.
The area between the floodway and 1-percent-annual-chance floodplain boundaries is
termed the floodway fringe. The floodway fringe encompasses the portion of the
floodplain that could be completely obstructed without increasing the WSEL of the base
flood more than 1 foot at any point. Typical relationships between the floodway and the
floodway fringe and their significance to floodplain development are shown in Figure 1.
26
Figure 1: Floodway Schematic
27
FLOODING SOURCE
CROSS SECTION
DISTANCE
1-PERCENT-ANNUAL-CHANCE-FLOOD
WATER SURFACE ELEVATION
FLOODWAY
1
WIDTH
(FEET)
SECTION
AREA
(SQUARE
FEET)
MEAN
VELOCITY
(FEET PER
SECOND)
583
407
336
176
184
289
193
163
154
205
271
240
211
3,406
2,378
1,638
892
1,019
1,294
1,045
1,058
960
1,225
2,014
1,424
1,118
1.6
2.3
3.3
5.2
4.5
3.6
4.4
4.4
4.8
3.6
2.2
3.1
3.9
REGULATORY
(FEET NAVD)
WITHOUT
FLOODWAY
WITH
FLOODWAY
(FEET NAVD)
(FEET NAVD)
495.7
496.6
499.3
502.3
505.0
508.6
511.2
515.0
519.1
522.6
529.2
530.2
532.5
496.5
497.6
500.3
503.0
505.9
509.6
512.1
516.0
520.0
523.6
529.4
530.9
533.3
INCREASE
(FEET)
ADAMSON’S
CREEK
A
B
C
D
E
F
G
H
I
J
K
L
M
2,720
3,571
4,530
5,703
6,898
8,256
9,277
10,543
11,685
12,782
13,924
15,089
16,106
495.7
496.6
499.3
502.3
505.0
508.6
511.2
515.0
519.1
522.6
529.2
530.2
532.5
1
Feet above confluence with Heartsill Creek
Table 10
FLOODWAY DATA
FEDERAL EMERGENCY MANAGEMENT AGENCY
SEBASTIAN COUNTY, AR
ADAMSON’S CREEK
AND INCORPORATED AREAS
28
0.8
1.0
1.0
0.7
0.9
1.0
0.9
1.0
0.9
1.0
0.2
0.7
0.8
FLOODING SOURCE
CROSS SECTION
DISTANCE
1-PERCENT-ANNUAL-CHANCE-FLOOD
WATER SURFACE ELEVATION
FLOODWAY
1
WIDTH
(FEET)
SECTION
AREA
(SQUARE
FEET)
MEAN
VELOCITY
(FEET PER
SECOND)
83,278
100,514
123,424
104,876
133,903
139,427
140,979
170,578
110,239
6.9
6.7
6.2
8.5
7.1
10.4
8.7
9.5
10.4
REGULATORY
(FEET NAVD)
WITHOUT
FLOODWAY
WITH
FLOODWAY
(FEET NAVD)
(FEET NAVD)
405.4
406.2
406.9
407.3
408.8
413.1
415.8
416.9
418.5
406.3
407.2
407.8
408.2
409.7
413.8
416.3
417.6
419.4
INCREASE
(FEET)
ARKANSAS
RIVER
A
B
C
D
E
F
G
H
I
293.23
294.05
294.96
296.26
297.60
301.92
304.18
305.88
308.19
2,3
3,400/1,343
2
4,350/2,793
2
3,784/1,345
2
4,700/719
2
8,000/6,303
2
3,500/2,201
2
5,000/3,414
2
4,500/2,816
2
2,797/628
1
Miles above confluence with Mississippi River
Width/width within Sebastian County
2
405.4
406.2
406.9
407.3
408.8
413.1
415.8
416.9
418.5
3
Combined Arkansas River/Massard Creek floodway
Table 10
FLOODWAY DATA
FEDERAL EMERGENCY MANAGEMENT AGENCY
SEBASTIAN COUNTY, AR
ARKANSAS RIVER
AND INCORPORATED AREAS
29
0.9
1.0
0.9
0.9
0.9
0.7
0.5
0.7
0.9
FLOODING SOURCE
CROSS SECTION
DISTANCE
1-PERCENT-ANNUAL-CHANCE-FLOOD
WATER SURFACE ELEVATION
FLOODWAY
1
WIDTH
(FEET)
SECTION
AREA
(SQUARE
FEET)
MEAN
VELOCITY
(FEET PER
SECOND)
240
350
1,053
904
2.2
2.2
REGULATORY
(FEET NAVD)
WITHOUT
FLOODWAY
WITH
FLOODWAY
(FEET NAVD)
(FEET NAVD)
402.9
405.5
403.1
406.1
INCREASE
(FEET)
COX CREEK
A
B
145
2,360
402.9
405.5
1
Feet above State Highway 253
Table 10
FLOODWAY DATA
FEDERAL EMERGENCY MANAGEMENT AGENCY
SEBASTIAN COUNTY, AR
COX CREEK
AND INCORPORATED AREAS
30
0.2
0.6
FLOODING SOURCE
CROSS SECTION
DISTANCE
1-PERCENT-ANNUAL-CHANCE-FLOOD
WATER SURFACE ELEVATION
FLOODWAY
1
WIDTH
(FEET)
SECTION
AREA
(SQUARE
FEET)
MEAN
VELOCITY
(FEET PER
SECOND)
205
300
200
250
250
200
213
97
1,202
1,362
1,476
1,637
2,093
1,164
1,170
800
6.5
5.7
5.3
4.8
3.7
6.7
6.7
9.8
REGULATORY
(FEET NAVD)
WITHOUT
FLOODWAY
WITH
FLOODWAY
(FEET NAVD)
(FEET NAVD)
500.2
509.8
512.3
514.8
516.2
519.1
521.7
523.4
501.2
510.5
513.2
515.0
516.8
519.1
521.9
523.5
INCREASE
(FEET)
HACKETT
CREEK
A
B
C
D
E
F
G
H
15.0
17.7
18.6
19.2
19.6
20.4
20.7
21.2
500.2
509.8
512.3
514.8
516.2
519.1
521.7
523.4
1
Thousands of feet above confluence with James Fork
Table 10
FLOODWAY DATA
FEDERAL EMERGENCY MANAGEMENT AGENCY
SEBASTIAN COUNTY, AR
HACKETT CREEK
AND INCORPORATED AREAS
31
1.0
0.7
0.9
0.2
0.6
0.0
0.2
0.1
FLOODING SOURCE
CROSS SECTION
DISTANCE
1-PERCENT-ANNUAL-CHANCE-FLOOD
WATER SURFACE ELEVATION
FLOODWAY
1
WIDTH
(FEET)
SECTION
AREA
(SQUARE
FEET)
MEAN
VELOCITY
(FEET PER
SECOND)
160
114
41
283
309
315
238
251
203
150
131
99
158
94
80
180
183
244
108
830
963
573
2,606
2,507
2,858
1,209
1,611
1,132
959
728
742
544
655
452
954
791
1,797
219
8.4
7.2
12.1
2.7
1.8
1.6
3.7
2.8
4.0
3.5
4.5
4.5
6.1
5.1
7.3
3.5
4.2
0.7
5.8
REGULATORY
(FEET NAVD)
WITHOUT
FLOODWAY
WITH
FLOODWAY
(FEET NAVD)
(FEET NAVD)
INCREASE
(FEET)
HEARTSILL
CREEK
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
6,810
7,869
8,592
9,579
10,602
11,380
12,780
14,187
15,231
16,170
17,016
17,787
18,780
19,803
20,968
21,820
22,824
24,124
24,733
491.4
499.8
503.7
509.4
510.4
511.3
513.5
518.8
522.0
526.7
530.0
536.1
542.2
548.1
552.1
556.4
560.7
573.0
574.6
2
491.1
499.8
503.7
509.4
510.4
511.3
513.5
518.8
522.0
526.7
530.0
536.1
542.2
548.1
552.1
556.4
560.7
573.0
574.6
1
Feet above confluence with Vache Grasse Creek
Elevations computed without consideration of backwater effects from Vache Grasse Creek
2
Table 10
FLOODWAY DATA
FEDERAL EMERGENCY MANAGEMENT AGENCY
SEBASTIAN COUNTY, AR
HEARTSILL CREEK
AND INCORPORATED AREAS
32
2
492.0
499.8
504.6
510.1
511.2
512.2
514.4
519.8
523.0
527.6
530.8
536.9
542.7
548.9
553.0
557.3
561.0
574.0
575.4
0.9
0.0
0.9
0.7
0.8
0.9
0.9
1.0
1.0
0.9
0.8
0.8
0.5
0.8
0.9
0.9
0.3
1.0
0.8
FLOODING SOURCE
CROSS SECTION
DISTANCE
1-PERCENT-ANNUAL-CHANCE-FLOOD
WATER SURFACE ELEVATION
FLOODWAY
1
WIDTH
(FEET)
SECTION
AREA
(SQUARE
FEET)
MEAN
VELOCITY
(FEET PER
SECOND)
68
49
74
70
186
117
352
311
326
431
648
472
5.6
6.3
6.0
4.6
3.0
4.2
REGULATORY
(FEET NAVD)
WITHOUT
FLOODWAY
WITH
FLOODWAY
(FEET NAVD)
(FEET NAVD)
525.0
527.3
534.3
538.7
541.4
547.9
525.7
528.2
535.1
538.9
542.2
548.9
INCREASE
(FEET)
HEARTSILL
CREEK TRIBUTARY 1
A
B
C
D
E
F
586
972
1,975
2,560
3,317
4,902
525.0
527.3
534.3
538.7
541.4
547.9
0.7
0.9
0.8
0.2
0.8
1.0
1
Feet above confluence with Heartsill Creek
Table 10
FLOODWAY DATA
FEDERAL EMERGENCY MANAGEMENT AGENCY
SEBASTIAN COUNTY, AR
AND INCORPORATED AREAS
HEARTSILL CREEK TRIBUTARY 1
33
FLOODING SOURCE
CROSS SECTION
DISTANCE
1-PERCENT-ANNUAL-CHANCE-FLOOD
WATER SURFACE ELEVATION
FLOODWAY
1
WIDTH
(FEET)
SECTION
AREA
(SQUARE
FEET)
MEAN
VELOCITY
(FEET PER
SECOND)
292
304
225
130
155
159
198
268
291
284
232
177
2,509
1,726
1,075
1,066
602
1,089
993
1,327
2,165
1,888
1,421
1,263
1.7
2.5
4.0
4.0
7.1
3.9
4.3
3.2
1.9
2.2
3.0
3.3
REGULATORY
(FEET NAVD)
WITHOUT
FLOODWAY
WITH
FLOODWAY
(FEET NAVD)
(FEET NAVD)
510.8
511.2
512.9
515.8
519.4
523.8
527.9
532.3
539.7
541.0
545.9
546.6
511.7
512.2
513.9
516.8
520.4
524.8
528.8
533.2
540.1
541.8
546.8
547.6
INCREASE
(FEET)
HESTER
CREEK
A
B
C
D
E
F
G
H
I
J
K
L
750
1,750
2,773
3,744
4,929
6,189
7,464
8,527
9,836
10,853
11,865
12,329
510.8
511.2
512.9
515.8
519.4
523.8
527.9
532.3
539.7
541.0
545.9
546.6
1
Feet above confluence with Heartsill Creek
Table 10
FLOODWAY DATA
FEDERAL EMERGENCY MANAGEMENT AGENCY
SEBASTIAN COUNTY, AR
HESTER CREEK
AND INCORPORATED AREAS
34
0.9
1.0
1.0
1.0
1.0
1.0
0.9
0.9
0.4
0.8
0.9
1.0
FLOODING SOURCE
CROSS SECTION
DISTANCE
1-PERCENT-ANNUAL-CHANCE-FLOOD
WATER SURFACE ELEVATION
FLOODWAY
1
WIDTH
(FEET)
SECTION
AREA
(SQUARE
FEET)
MEAN
VELOCITY
(FEET PER
SECOND)
167
100
150
90
647
924
1,119
467
6.0
4.0
5.1
3.6
REGULATORY
(FEET NAVD)
WITHOUT
FLOODWAY
WITH
FLOODWAY
(FEET NAVD)
(FEET NAVD)
408.9
418.0
419.8
429.1
409.1
418.2
420.6
429.5
INCREASE
(FEET)
LITTLE
MASSARD
CREEK
A
B
C
D
1.08
1.61
1.89
2.61
408.9
418.0
419.8
429.1
1
Miles above confluence with Massard Creek
Table 10
FLOODWAY DATA
FEDERAL EMERGENCY MANAGEMENT AGENCY
SEBASTIAN COUNTY, AR
LITTLE MASSARD CREEK
AND INCORPORATED AREAS
35
0.2
0.2
0.8
0.4
FLOODING SOURCE
CROSS SECTION
DISTANCE
1-PERCENT-ANNUAL-CHANCE-FLOOD
WATER SURFACE ELEVATION
FLOODWAY
1
WIDTH
(FEET)
SECTION
AREA
(SQUARE
FEET)
MEAN
VELOCITY
(FEET PER
SECOND)
REGULATORY
(FEET NAVD)
WITHOUT
FLOODWAY
WITH
FLOODWAY
(FEET NAVD)
(FEET NAVD)
390.22
394.52
397.02
398.22
399.32
401.62
407.9
410.3
411.4
415.4
417.8
422.6
426.2
428.6
439.1
449.0
451.3
455.5
459.8
390.32
395.42
398.02
399.02
400.12
402.52
408.8
411.1
412.4
416.3
418.7
423.5
427.2
429.5
439.8
450.0
452.2
456.3
460.8
INCREASE
(FEET)
LITTLE VACHE
GRASSE CREEK
10,866
245
2,043
3.3
A
16,369
466
3,903
1.7
B
18,842
470
2,614
2.6
C
20,983
778
6,065
1.1
D
22,664
547
3,940
1.6
E
24,888
198
1,632
4.0
F
27,126
450
3,874
1.3
G
28,887
331
2,778
1.8
H
30,139
265
2,376
2.2
I
32,148
271
2,108
2.4
J
33,406
186
1,464
3.8
K
34,660
341
2,034
2.7
L
36,137
330
2,462
2.2
M
37,362
263
1,918
2.9
N
40,061
131
846
5.9
O
41,798
415
3,533
1.3
P
43,515
298
1,631
2.9
Q
45,080
255
1,580
3.0
R
46,687
338
2,072
2.3
S
1
Feet above confluence with Vache Grasse Creek
2
Elevations computed without consideration of backwater effects from Arkansas River
402.0
402.0
402.0
402.0
402.0
402.0
407.9
410.3
411.4
415.4
417.8
422.6
426.2
428.6
439.1
449.0
451.3
455.5
459.8
Table 10
FLOODWAY DATA
FEDERAL EMERGENCY MANAGEMENT AGENCY
SEBASTIAN COUNTY, AR
LITTLE VACHE GRASSE CREEK
AND INCORPORATED AREAS
36
0.1
0.9
1.0
0.8
0.8
0.9
0.9
0.8
1.0
0.9
0.9
0.9
1.0
0.9
0.7
1.0
0.9
0.8
1.0
FLOODING SOURCE
CROSS SECTION
DISTANCE
1-PERCENT-ANNUAL-CHANCE-FLOOD
WATER SURFACE ELEVATION
FLOODWAY
1
WIDTH
(FEET)
SECTION
AREA
(SQUARE
FEET)
MEAN
VELOCITY
(FEET PER
SECOND)
277
289
201
232
161
1,873
1,387
995
1,268
771
2.5
3.4
3.1
2.4
4.0
REGULATORY
(FEET NAVD)
WITHOUT
FLOODWAY
WITH
FLOODWAY
(FEET NAVD)
(FEET NAVD)
465.9
470.4
474.3
480.5
487.5
466.8
471.4
475.3
481.4
488.3
INCREASE
(FEET)
LITTLE VACHE
GRASSE CREEK
(continued)
T
U
V
W
X
1
49,012
50,446
51,428
52,926
54,624
465.9
470.4
474.3
480.5
487.5
Feet above confluence with Vache Grasse Creek
Table 10
FLOODWAY DATA
FEDERAL EMERGENCY MANAGEMENT AGENCY
SEBASTIAN COUNTY, AR
LITTLE VACHE GRASSE CREEK
AND INCORPORATED AREAS
37
0.9
1.0
1.0
0.9
0.8
FLOODING SOURCE
CROSS SECTION
DISTANCE
1-PERCENT-ANNUAL-CHANCE-FLOOD
WATER SURFACE ELEVATION
FLOODWAY
1
WIDTH
(FEET)
SECTION
AREA
(SQUARE
FEET)
MEAN
VELOCITY
(FEET PER
SECOND)
49
182
94
55
325
118
126
70
256
1,085
490
277
857
424
398
282
6.3
1.5
3.3
5.8
1.3
2.6
2.8
3.9
REGULATORY
(FEET NAVD)
WITHOUT
FLOODWAY
WITH
FLOODWAY
(FEET NAVD)
(FEET NAVD)
438.8
446.7
448.0
453.0
458.3
464.6
471.4
478.3
439.7
447.7
448.9
453.5
458.6
465.5
472.4
479.2
INCREASE
(FEET)
LITTLE VACHE
GRASSE CREEK
TRIBUTARY 9
A
B
C
D
E
F
G
H
636
1,750
2,750
4,000
5,187
6,136
7,539
9,500
438.8
446.7
448.0
453.0
458.3
464.6
471.4
478.3
0.9
1.0
0.9
0.5
0.3
0.9
1.0
0.9
1
Feet above confluence with Little Vache Grasse Creek
Table 10
FLOODWAY DATA
FEDERAL EMERGENCY MANAGEMENT AGENCY
SEBASTIAN COUNTY, AR
AND INCORPORATED AREAS
LITTLE VACHE GRASSE CREEK TRIBUTARY 9
38
FLOODING SOURCE
CROSS SECTION
DISTANCE
1-PERCENT-ANNUAL-CHANCE-FLOOD
WATER SURFACE ELEVATION
FLOODWAY
1
WIDTH
(FEET)
SECTION
AREA
(SQUARE
FEET)
MEAN
VELOCITY
(FEET PER
SECOND)
734
332
139
300
478
516
244
133
333
141
117
113
110
143
263
5,522
1,989
1,314
1,958
2,133
2,239
868
689
873
601
582
625
731
674
1,005
1.3
3.6
5.5
3.7
3.4
1.6
4.2
5.2
4.1
6.0
5.8
5.4
4.6
5.0
3.4
REGULATORY
(FEET NAVD)
WITHOUT
FLOODWAY
WITH
FLOODWAY
(FEET NAVD)
(FEET NAVD)
405.8
407.1
408.6
409.9
410.2
411.6
411.7
413.0
413.8
413.8
414.7
415.5
418.8
419.4
420.1
406.5
407.7
409.4
410.3
410.6
412.1
412.3
413.5
413.8
414.4
415.3
415.9
419.6
419.7
421.1
INCREASE
(FEET)
MASSARD
CREEK
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
11,657
14,657
15,820
15,987
16,657
17,331
17,831
18,331
18,598
18,831
19,092
19,139
20,643
20,780
21,330
405.8
407.1
408.6
409.9
410.2
411.6
411.7
413.0
413.8
413.8
414.7
415.5
418.8
419.4
420.1
1
Feet above confluence with Arkansas River
Table 10
FLOODWAY DATA
FEDERAL EMERGENCY MANAGEMENT AGENCY
SEBASTIAN COUNTY, AR
MASSARD CREEK
AND INCORPORATED AREAS
39
0.7
0.6
0.8
0.4
0.4
0.5
0.6
0.5
0.0
0.6
0.6
0.4
0.8
0.3
1.0
FLOODING SOURCE
CROSS SECTION
DISTANCE
1-PERCENT-ANNUAL-CHANCE-FLOOD
WATER SURFACE ELEVATION
FLOODWAY
1
WIDTH
(FEET)
SECTION
AREA
(SQUARE
FEET)
MEAN
VELOCITY
(FEET PER
SECOND)
1,150
420
532
435
576
381
328
185
100
128
8,494
1,694
2,292
1,486
2,395
658
1,390
552
402
880
0.8
4.1
1.9
2.9
0.8
3.0
1.4
3.5
6.1
0.1
REGULATORY
(FEET NAVD)
WITHOUT
FLOODWAY
WITH
FLOODWAY
(FEET NAVD)
(FEET NAVD)
419.5
419.7
423.6
427.6
432.4
435.5
447.4
452.7
460.4
473.8
420.5
420.7
424.4
428.4
433.1
436.4
448.2
453.2
460.8
473.8
INCREASE
(FEET)
MAY BRANCH
A
B
C
D
E
F
G
H
I
J
0.95
1.12
1.57
1.81
2.10
2.35
2.60
2.77
2.90
3.27
420.7
420.7
423.6
427.6
432.4
435.5
447.4
452.7
460.4
473.8
1
Miles above confluence with Arkansas River
Table 10
FLOODWAY DATA
FEDERAL EMERGENCY MANAGEMENT AGENCY
SEBASTIAN COUNTY, AR
MAY BRANCH
AND INCORPORATED AREAS
40
1.0
1.0
0.8
0.8
0.7
0.9
0.8
0.5
0.4
0.0
FLOODING SOURCE
CROSS SECTION
DISTANCE
1-PERCENT-ANNUAL-CHANCE-FLOOD
WATER SURFACE ELEVATION
FLOODWAY
1
WIDTH
(FEET)
SECTION
AREA
(SQUARE
FEET)
MEAN
VELOCITY
(FEET PER
SECOND)
REGULATORY
(FEET NAVD)
WITHOUT
FLOODWAY
WITH
FLOODWAY
(FEET NAVD)
(FEET NAVD)
INCREASE
(FEET)
MILL CREEK
A
0.08
182/28³
3,062
3.0
B
0.56
188
2,340
4.0
C
0.89
160
1,929
4.8
D
1.27
124
1,282
7.2
E
1.48
114
1,121
8.2
F
1.80
123
1,047
8.8
G
2.14
106
955
8.7
H
2.48
240
1,568
5.3
I
2.78
270
1,669
4.3
J
3.03
110
1,233
5.8
K
3.38
103
840
8.5
L
3.62
145
919
7.7
M
3.72
100
642
11.3
N
3.97
101
961
7.6
O
4.30
85
753
7.5
P
4.80
388
1,782
3.2
Q
5.26
410
1,977
1.1
R
5.47
67
309
7.3
S
5.56
56
329
6.9
T
5.85
43
345
6.6
1
Miles above confluence with Poteau River
2
Elevations computed without consideration of backwater effects from Arkansas River
2
420.5
414.5
415.3
2
420.5
417.5
418.2
2
420.5
419.0
419.8
421.3
421.3
422.2
423.4
423.4
424.0
428.6
428.6
428.6
432.2
432.2
432.3
439.6
439.6
440.5
440.8
440.8
441.8
443.1
443.1
444.0
445.3
445.3
446.1
449.0
449.0
449.3
450.2
450.2
450.4
455.5
455.5
455.5
458.3
458.3
458.7
468.9
468.9
469.3
476.8
476.8
477.1
477.0
477.0
477.7
479.8
479.8
479.8
485.1
485.1
485.2
3
Width/width within corporate limits
Table 10
FLOODWAY DATA
FEDERAL EMERGENCY MANAGEMENT AGENCY
SEBASTIAN COUNTY, AR
MILL CREEK
AND INCORPORATED AREAS
41
0.8
0.7
0.8
0.9
0.6
0.0
0.1
0.8
1.0
0.9
0.8
0.3
0.2
0.0
0.4
0.4
0.3
0.7
0.0
0.1
FLOODING SOURCE
CROSS SECTION
DISTANCE
1-PERCENT-ANNUAL-CHANCE-FLOOD
WATER SURFACE ELEVATION
FLOODWAY
1
WIDTH
(FEET)
SECTION
AREA
(SQUARE
FEET)
MEAN
VELOCITY
(FEET PER
SECOND)
52
33
34
39
50
45
54
54
465
220
239
244
268
191
354
220
4.9
10.3
9.5
9.3
8.4
11.7
6.4
10.3
REGULATORY
(FEET NAVD)
WITHOUT
FLOODWAY
WITH
FLOODWAY
(FEET NAVD)
(FEET NAVD)
486.8
486.9
490.9
496.5
508.1
510.9
517.0
520.5
487.2
487.5
491.1
496.7
508.1
510.9
517.0
520.9
INCREASE
(FEET)
MILL CREEK
(continued)
U
V
W
X
Y
Z
AA
AB
5.86
5.94
6.04
6.28
6.60
6.70
6.85
6.89
486.8
486.9
490.9
496.5
508.1
510.9
517.0
520.5
1
Miles above confluence with Poteau River
Table 10
FLOODWAY DATA
FEDERAL EMERGENCY MANAGEMENT AGENCY
SEBASTIAN COUNTY, AR
MILL CREEK
AND INCORPORATED AREAS
42
0.4
0.6
0.2
0.2
0.0
0.0
0.0
0.4
FLOODING SOURCE
CROSS SECTION
DISTANCE
1-PERCENT-ANNUAL-CHANCE-FLOOD
WATER SURFACE ELEVATION
FLOODWAY
1
WIDTH
(FEET)
SECTION
AREA
(SQUARE
FEET)
MEAN
VELOCITY
(FEET PER
SECOND)
160
160
160
160
891
579
285
473
1.6
2.5
5.0
3.0
REGULATORY
(FEET NAVD)
WITHOUT
FLOODWAY
WITH
FLOODWAY
(FEET NAVD)
(FEET NAVD)
468.5
470.2
475.7
489.8
469.3
470.8
475.7
489.8
INCREASE
(FEET)
MILL CREEK
TRIBUTARY
A
B
C
D
0.24
0.37
0.52
0.81
468.5
470.2
475.7
489.8
1
Miles above confluence with Mill Creek
Table 10
FLOODWAY DATA
FEDERAL EMERGENCY MANAGEMENT AGENCY
SEBASTIAN COUNTY, AR
MILL CREEK TRIBUTARY
AND INCORPORATED AREAS
43
0.8
0.6
0.0
0.0
FLOODING SOURCE
CROSS SECTION
DISTANCE
1-PERCENT-ANNUAL-CHANCE-FLOOD
WATER SURFACE ELEVATION
FLOODWAY
1
WIDTH
(FEET)
SECTION
AREA
(SQUARE
FEET)
MEAN
VELOCITY
(FEET PER
SECOND)
368
405
190
101
553
113
59
57
266
85
74
87
81
145
180
109
3,376
2,964
929
476
7,691
657
347
421
1,763
339
280
634
409
283
633
386
1.7
2.0
6.2
12.2
0.8
4.9
9.4
7.7
1.8
9.6
11.6
5.1
7.9
5.9
2.6
5.4
REGULATORY
(FEET NAVD)
WITHOUT
FLOODWAY
WITH
FLOODWAY
(FEET NAVD)
(FEET NAVD)
416.1
419.9
419.9
423.4
436.4
436.6
440.1
443.0
445.7
446.2
450.6
455.5
455.5
467.4
475.0
506.8
416.1
420.0
420.0
423.4
437.1
437.5
440.6
443.0
445.8
446.4
450.6
456.2
456.4
467.9
475.9
507.2
INCREASE
(FEET)
NO NAME
CREEK
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
0.395
0.413
0.589
0.647
0.738
1.348
1.449
1.462
1.513
1.634
1.756
1.810
1.831
1.974
2.075
2.513
416.1
419.9
419.9
423.4
436.4
436.6
440.1
443.0
445.7
446.2
450.6
455.5
455.5
467.4
475.0
506.8
1
Miles above confluence with Sunnymede Creek
Table 10
FLOODWAY DATA
FEDERAL EMERGENCY MANAGEMENT AGENCY
SEBASTIAN COUNTY, AR
NO NAME CREEK
AND INCORPORATED AREAS
44
0.0
0.1
0.1
0.0
0.7
0.9
0.5
0.0
0.1
0.2
0.0
0.7
0.9
0.5
0.9
0.4
FLOODING SOURCE
CROSS SECTION
DISTANCE
1-PERCENT-ANNUAL-CHANCE-FLOOD
WATER SURFACE ELEVATION
FLOODWAY
1
WIDTH
(FEET)
SECTION
AREA
(SQUARE
FEET)
MEAN
VELOCITY
(FEET PER
SECOND)
22
70
26
15
103
100
53
67
66
96
380
205
133
78
241
217
186
346
244
145
2.7
4.9
7.6
13.0
4.2
4.6
5.4
2.9
4.1
7.0
REGULATORY
(FEET NAVD)
WITHOUT
FLOODWAY
WITH
FLOODWAY
(FEET NAVD)
(FEET NAVD)
458.1
482.3
482.4
487.7
490.9
491.9
499.0
504.3
504.5
517.7
458.1
482.6
483.3
487.7
491.7
492.2
499.7
505.0
505.2
517.7
INCREASE
(FEET)
NO NAME
CREEK TRIBUTARY
A
B
C
D
E
F
G
H
I
J
206
2,256
2,706
2,990
3,043
3,206
3,562
3,773
3,993
4,706
458.1
482.3
482.4
487.7
490.9
491.9
499.0
504.3
504.5
517.7
1
Feet above confluence with No Name Creek
Table 10
FLOODWAY DATA
FEDERAL EMERGENCY MANAGEMENT AGENCY
SEBASTIAN COUNTY, AR
NO NAME CREEK TRIBUTARY
AND INCORPORATED AREAS
45
0.0
0.3
0.9
0.0
0.8
0.3
0.7
0.7
0.7
0.0
FLOODING SOURCE
CROSS SECTION
DISTANCE
1-PERCENT-ANNUAL-CHANCE-FLOOD
WATER SURFACE ELEVATION
FLOODWAY
1
WIDTH
(FEET)
SECTION
AREA
(SQUARE
FEET)
MEAN
VELOCITY
(FEET PER
SECOND)
63
94
6.7
REGULATORY
(FEET NAVD)
WITHOUT
FLOODWAY
WITH
FLOODWAY
(FEET NAVD)
(FEET NAVD)
477.6
477.6
INCREASE
(FEET)
NO NAME
CREEK TRIBUTARY B
A
0.07
477.6
1
Miles above confluence with No Name Creek
Table 10
FLOODWAY DATA
FEDERAL EMERGENCY MANAGEMENT AGENCY
SEBASTIAN COUNTY, AR
NO NAME CREEK TRIBUTARY B
AND INCORPORATED AREAS
46
0.0
FLOODING SOURCE
CROSS SECTION
DISTANCE
1-PERCENT-ANNUAL-CHANCE-FLOOD
WATER SURFACE ELEVATION
FLOODWAY
1
WIDTH
(FEET)
SECTION
AREA
(SQUARE
FEET)
MEAN
VELOCITY
(FEET PER
SECOND)
189
195
133
1,472
1,326
622
2.1
2.4
2.2
REGULATORY
(FEET NAVD)
WITHOUT
FLOODWAY
WITH
FLOODWAY
(FEET NAVD)
(FEET NAVD)
415.7
416.8
423.9
416.6
417.7
424.8
INCREASE
(FEET)
OAK PARK
TRIBUTARY
A
B
C
0.526
0.717
0.962
415.7
416.8
423.9
1
Miles above confluence with Arkansas River
Table 10
FLOODWAY DATA
FEDERAL EMERGENCY MANAGEMENT AGENCY
SEBASTIAN COUNTY, AR
OAK PARK TRIBUTARY
AND INCORPORATED AREAS
47
0.9
0.9
0.9
FLOODING SOURCE
CROSS SECTION
DISTANCE
1-PERCENT-ANNUAL-CHANCE-FLOOD
WATER SURFACE ELEVATION
FLOODWAY
1
WIDTH
(FEET)
SECTION
AREA
(SQUARE
FEET)
MEAN
VELOCITY
(FEET PER
SECOND)
N/A
N/A
N/A
N/A
REGULATORY
(FEET NAVD)
WITHOUT
FLOODWAY
WITH
FLOODWAY
(FEET NAVD)
(FEET NAVD)
414.53
414.73
415.53
415.73
INCREASE
(FEET)
POTEAU RIVER
A
B
0.06
0.67
2
1,413/224
2
1,945/155
419.8
420.1
1
Miles above confluence with Arkansas River
3
Width/width within county boundary Elevations computed without consideration of backwater effects from Arkansas River
2
Table 10
FLOODWAY DATA
FEDERAL EMERGENCY MANAGEMENT AGENCY
SEBASTIAN COUNTY, AR
POTEAU RIVER
AND INCORPORATED AREAS
48
1.0
1.0
FLOODING SOURCE
CROSS SECTION
DISTANCE
1-PERCENT-ANNUAL-CHANCE-FLOOD
WATER SURFACE ELEVATION
FLOODWAY
1
WIDTH
(FEET)
SPIVEY CREEK
A
616
B
2,616
C
3,616
D
5,089
E
5,327
F
5,616
G
6,400
H
7,051
I
7,106
J
8,616
K
9,296
L
10,470
M
10,509
N
11,899
O
12,032
P
12,661
Q
12,730
R
13,116
S
13,307
T
14,013
1
Feet above confluence with Massard Creek
SECTION
AREA
(SQUARE
FEET)
MEAN
VELOCITY
(FEET PER
SECOND)
703
1,010
1,100
459
783
449
795
644
822
1,095
271
448
721
662
890
309
637
216
264
454
5.2
3.7
3.4
8.0
4.7
8.2
4.6
5.7
4.5
2.1
8.3
5.0
3.1
3.4
2.5
7.3
3.5
10.4
8.5
4.9
149
196
293
90
155
91
173
123
138
231
49
228
269
225
253
98
130
45
38
66
REGULATORY
(FEET NAVD)
411.7
417.0
418.8
422.6
424.2
424.6
429.7
431.1
432.2
436.3
436.6
441.1
441.2
444.2
446.1
446.1
447.8
447.9
450.1
452.7
WITHOUT
FLOODWAY
WITH
FLOODWAY
(FEET NAVD)
(FEET NAVD)
411.7
417.0
418.8
422.6
424.2
424.6
429.7
431.1
432.2
436.3
436.6
441.1
441.2
444.2
446.1
446.1
447.8
447.9
450.1
452.7
412.4
417.6
419.4
423.1
424.9
425.1
430.4
431.7
432.9
437.2
437.5
441.2
442.2
444.9
446.4
446.9
448.4
448.3
450.2
453.1
Table 10
FLOODWAY DATA
FEDERAL EMERGENCY MANAGEMENT AGENCY
SEBASTIAN COUNTY, AR
SPIVEY CREEK
AND INCORPORATED AREAS
49
INCREASE
(FEET)
0.7
0.6
0.6
0.5
0.7
0.5
0.7
0.6
0.7
0.9
0.9
0.1
1.0
0.7
0.3
0.8
0.6
0.4
0.1
0.4
FLOODING SOURCE
CROSS SECTION
DISTANCE
1-PERCENT-ANNUAL-CHANCE-FLOOD
WATER SURFACE ELEVATION
FLOODWAY
1
WIDTH
(FEET)
SECTION
AREA
(SQUARE
FEET)
MEAN
VELOCITY
(FEET PER
SECOND)
51
92
113
42
158
146
129
77
96
259
409
441
207
578
574
305
234
275
8.7
5.5
5.1
10.8
3.9
3.9
7.3
9.6
8.1
REGULATORY
(FEET NAVD)
WITHOUT
FLOODWAY
WITH
FLOODWAY
(FEET NAVD)
(FEET NAVD)
453.0
454.9
454.9
455.4
459.6
460.7
462.2
464.7
476.7
453.7
455.0
455.5
455.8
459.9
461.7
462.3
464.7
477.0
INCREASE
(FEET)
SPIVEY CREEK
(continued)
U
V
W
X
Y
Z
AA
AB
AC
14,501
14,691
14,791
15,024
15,782
15,872
16,210
16,385
18,115
453.0
454.9
454.9
455.4
459.6
460.7
462.2
464.7
476.7
1
Feet above confluence with Massard Creek
Table 10
FLOODWAY DATA
FEDERAL EMERGENCY MANAGEMENT AGENCY
SEBASTIAN COUNTY, AR
SPIVEY CREEK
AND INCORPORATED AREAS
50
0.7
0.1
0.6
0.4
0.3
1.0
0.1
0.0
0.3
FLOODING SOURCE
CROSS SECTION
DISTANCE
1-PERCENT-ANNUAL-CHANCE-FLOOD
WATER SURFACE ELEVATION
FLOODWAY
1
WIDTH
(FEET)
SECTION
AREA
(SQUARE
FEET)
MEAN
VELOCITY
(FEET PER
SECOND)
482
481
200
300
275
145
85
150
150
175
150
100
4,754
4,236
1,822
1,262
900
829
349
826
403
599
567
319
1.1
1.3
2.9
3.0
4.2
4.6
7.2
3.1
6.3
4.4
4.6
7.9
REGULATORY
(FEET NAVD)
WITHOUT
FLOODWAY
WITH
FLOODWAY
(FEET NAVD)
(FEET NAVD)
407.62
407.92
410.1
410.5
410.9
413.3
414.7
423.6
424.6
426.1
431.2
436.9
408.52
408.82
410.6
411.4
411.8
413.7
415.3
424.0
424.9
426.5
431.3
437.6
INCREASE
(FEET)
SUNNYMEDE
CREEK
A
B
C
D
E
F
G
H
I
J
K
L
0.50
0.95
1.51
1.90
1.93
2.07
2.29
2.60
2.81
2.94
3.16
3.39
408.0
408.3
410.1
410.5
410.9
413.3
414.7
423.6
424.6
426.1
431.2
436.9
1
Miles above confluence with Arkansas River
Elevations computed without consideration of backwater effects from Arkansas River
2
Table 10
FLOODWAY DATA
FEDERAL EMERGENCY MANAGEMENT AGENCY
SEBASTIAN COUNTY, AR
SUNNYMEDE CREEK
AND INCORPORATED AREAS
51
0.9
0.9
0.5
0.9
0.9
0.4
0.6
0.4
0.3
0.4
0.1
0.7
FLOODING SOURCE
CROSS SECTION
DISTANCE
1-PERCENT-ANNUAL-CHANCE-FLOOD
WATER SURFACE ELEVATION
FLOODWAY
1
WIDTH
(FEET)
SECTION
AREA
(SQUARE
FEET)
MEAN
VELOCITY
(FEET PER
SECOND)
1,400
1,400
1,400
685
1,175
1,374
1,700
1,484
1,789
2,153
1,319
578
116
95
377
654
254
1,218
12,987
13,422
13,139
8,904
11,691
7,787
9,578
11,275
14,025
10,812
4,457
3,307
1,664
1,273
8,407
7,639
2,642
10,417
1.2
1.1
1.2
1.7
1.3
2.7
2.2
3.4
1.5
1.9
4.8
3.8
7.5
9.8
1.8
2.0
5.8
1.5
REGULATORY
(FEET NAVD)
WITHOUT
FLOODWAY
WITH
FLOODWAY
(FEET NAVD)
(FEET NAVD)
INCREASE
(FEET)
VACHE GRASSE
CREEK
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
7,750
9,650
12,000
15,250
17,750
130,175
132,291
133,422
134,466
136,107
138,292
139,754
141,050
142,467
143,172
144,672
146,172
147,672
399.3
399.3
399.3
399.3
399.3
484.0
485.5
486.5
491.2
491.6
494.3
499.1
501.2
513.5
539.7
539.8
539.9
540.7
2
395.6
2
395.9
2
396.1
2
398.0
2
398.5
484.0
485.5
486.5
491.2
491.6
494.3
499.1
501.2
513.5
539.7
539.8
539.9
540.7
2
396.6
2
396.8
2
397.0
2
398.9
2
399.4
485.0
486.5
487.5
491.3
491.8
495.1
500.1
502.0
514.5
539.7
539.8
540.0
540.8
1
Feet above confluence with Arkansas River
Elevations computed without consideration of backwater effects from Arkansas River
2
Table 10
FLOODWAY DATA
FEDERAL EMERGENCY MANAGEMENT AGENCY
SEBASTIAN COUNTY, AR
VACHE GRASSE CREEK
AND INCORPORATED AREAS
52
1.0
0.9
0.9
0.9
0.9
1.0
1.0
1.0
0.1
0.2
0.8
1.0
0.8
1.0
0.0
0.0
0.1
0.1
FLOODING SOURCE
CROSS SECTION
DISTANCE
1-PERCENT-ANNUAL-CHANCE-FLOOD
WATER SURFACE ELEVATION
FLOODWAY
1
WIDTH
(FEET)
SECTION
AREA
(SQUARE
FEET)
MEAN
VELOCITY
(FEET PER
SECOND)
350
225
250
600
300
400
400
600
600
1,851
1,321
1,486
4,754
1,524
1,810
2,499
4,138
1,933
7.1
9.9
8.8
2.7
8.4
7.1
5.1
1.8
3.9
REGULATORY
(FEET NAVD)
WITHOUT
FLOODWAY
WITH
FLOODWAY
(FEET NAVD)
(FEET NAVD)
596.3
599.8
601.6
606.2
611.7
625.5
631.5
648.1
648.9
597.3
600.6
601.6
606.7
612.4
625.6
631.5
649.0
649.6
INCREASE
(FEET)
WEST CREEK
A
B
C
D
E
F
G
H
I
13.0
13.3
13.5
15.0
17.9
21.7
22.8
28.3
28.9
596.3
599.8
601.6
606.2
611.6
625.5
631.5
648.1
648.9
1
Thousands of feet above confluence with James Fork
Table 10
FLOODWAY DATA
FEDERAL EMERGENCY MANAGEMENT AGENCY
SEBASTIAN COUNTY, AR
WEST CREEK
AND INCORPORATED AREAS
53
1.0
0.8
0.0
0.5
0.7
0.1
0.0
0.9
0.7
5.0
INSURANCE APPLICATION
For flood insurance rating purposes, flood insurance zone designations are assigned to a
community based on the results of the engineering analyses. These zones are as follows:
Zone A
Zone A is the flood insurance rate zone that corresponds to the 1-percent-annual-chance
floodplains that are determined in the FIS report by approximate methods. Because detailed
hydraulic analyses are not performed for such areas, no base (1-percent-annual-chance) flood
elevations (BFEs) or depths are shown within this zone.
Zone AE
Zone AE is the flood insurance rate zone that corresponds to the 1-percent-annual-chance
floodplains that are determined in the FIS report by detailed methods. Whole-foot BFEs derived
from the detailed hydraulic analyses are shown at selected intervals within this zone.
Zone X
Zone X is the flood insurance rate zone that corresponds to areas outside the
0.2-percent-annual-chance floodplain, areas within the 0.2-percent-annual-chance floodplain,
areas of 1-percent-annual-chance flooding where average depths are less than 1 foot, areas of
1-percent-annual-chance flooding where the contributing drainage area is less than 1 square mile
(sq. mi.), and areas protected from the base flood by levees. No BFEs or depths are shown within
this zone.
6.0
FLOOD INSURANCE RATE MAP
The FIRM is designed for flood insurance and floodplain management applications.
For flood insurance applications, the map designates flood insurance rate zones as described in
Section 5.0 and, in the 1-percent-annual-chance floodplains that were studied by detailed
methods, shows selected whole-foot BFEs or average depths. Insurance agents use zones and
BFEs in conjunction with information on structures and their contents to assign premium rates for
flood insurance policies.
For floodplain management applications, the map shows by tints, screens, and symbols, the 1and 0.2-percent-annual-chance floodplains, floodways, and the locations of selected cross
sections used in the hydraulic analyses and floodway computations.
The current FIRM presents flooding information for the entire geographic area of Sebastian
County. Previously, separate Flood Hazard Boundary Maps and/or FIRMs were prepared for
each incorporated community with identified flood hazard areas and the unincorporated areas of
the county. Historical map dates relating to maps prepared for each community are presented in
Table 11, “Community Map History.”
54
INITIAL NFIP MAP
DATE
FLOOD HAZARD
BOUNDARY MAP
REVISIONS DATE
INITIAL FIRM
DATE
February 14, 1975
None
May 20, 2010
Bonanza, Town of
August 15, 1975
None
May 20, 2010
Central City, Town of
March 16, 1998
None
March 16, 1998
Fort Smith, City of
August 28, 1971
None
August 28, 1971
Greenwood, City of
June 14, 1974
February 20, 1976
April 15, 1981
Hackett, City of
October 18, 1974
December 5, 1975
March 15, 1982
Hartford, City of
March 8, 1974
November 7, 1975
March 15, 1982
Huntington, City of
May 7, 2001
None
May 7, 2001
Lavaca, City of
May 10, 1974
November 28, 1975
March 15, 1982
September 29, 1986
Mansfield, City of
March 15, 1974
October 17, 1975
June 18, 1987
May 7, 2001
Midland, Town of
August 16, 1974
January 9, 1976
June 1, 1987
May 31, 1977
None
April 1, 1988
COMMUNITY
NAME
Barling, City of
Sebastian County
(Unincorporated Areas)
FIRM
REVISIONS DATE
June 1, 1974
May 7, 1976
May 29, 1981
August 19, 1986
July 16, 1991
March 16, 1998
May 7, 2001
Table 11
FEDERAL EMERGENCY MANAGEMENT AGENCY
SEBASTIAN COUNTY, AR
COMMUNITY MAP HISTORY
AND INCORPORATED AREAS
55
7.0
OTHER STUDIES
This is a multi-volume FIS. Each volume may be revised separately, in which case it
supersedes the previously printed volume. Users should refer to the Table of Contents in
Volume 1 for the current effective date of each volume; volumes bearing these dates
contain the most up-to-date flood hazard data.
The USACE, Little Rock District, published the Arkansas River Land Impact Study (Reference
34). Water-surface elevations for the Arkansas River in this study were computed through the use
of the USACE LRD-1 computer program (Reference 35). This study is in agreement with that
study.
Flood Insurance Studies have been published for Crawford and Logan Counties; the City of Van
Buren; and Sequoyah County, Oklahoma (References 36 through 39, respectively). This study is
in agreement with those studies with regard to flood hazards on the Arkansas River.
This FIS report either supersedes or is compatible with all previous studies published on streams
studied in this report and should be considered authoritative for the purposes of the NFIP.
8.0
LOCATION OF DATA
Information concerning the pertinent data used in the preparation of this study can be obtained by
contacting Federal Insurance and Mitigation Division, FEMA Region VI, Federal Regional
Center, Room 206, 800 North Loop 288, Denton, Texas 76201-3698.
9.0
BIBLIOGRAPHY AND REFERENCES
1. Federal Emergency Management Agency, Flood Insurance Study, Sebastian County,
Arkansas (Unincorporated Areas), Washington, D.C., May 7, 2001.
2. Federal Emergency Management Agency, Flood Insurance Study, Town of Central City,
Sebastian County, Arkansas, Washington, D.C., March 16, 1998.
3. Federal Emergency Management Agency, Flood Insurance Study, City of Fort Smith,
Sebastian County, Arkansas, Washington, D.C., August 28, 1971, revised on July 16, 1991.
4. Federal Emergency Management Agency, Flood Insurance Study, City of Greenwood,
Sebastian County, Arkansas, Washington, D.C., October 15, 1980.
5. Federal Emergency Management Agency, Flood Insurance Study, City of Hackett, Sebastian
County, Arkansas, Washington, D.C., September 15, 1981.
6. Federal Emergency Management Agency, Flood Insurance Study, City of Hartford, Sebastian
County, Arkansas, Washington, D.C., September 15, 1981.
7. Federal Emergency Management Agency, Flood Insurance Study, City of Lavaca, Sebastian
County, Arkansas, Washington, D.C., September 29, 1986.
56
8. U.S. Department of Commerce, Bureau of the Census, “State & County Quick Facts –
Sebastian County, Arkansas”, [Online] Available
http://quickfacts.census.gov/qfd/states/05/05131.html, updated on August 17, 2010.
9. National Oceanic and Atmospheric Administration, National Climatic Data Center,
Climatography of the United States, No. 60, Climate of Arkansas, Asheville, North Carolina,
July 1978.
10. U.S. Department of Commerce, Weather Bureau, Technical Paper Nos. 40 and 49, Rainfall
Frequency Atlas of the United States, May 1961.
11. U.S. Department of Commerce, National Oceanic and Atmospheric Administration, National
Weather Service, Technical Memorandum NWS Hydro-35, Five- to 60-Minute Precipitation
Frequency for the Eastern and Central United States, Silver Spring, Maryland, June 1977.
12. U.S. Department of the Army, Corps of Engineers, Hydrologic Engineering Center, HEC-1
Flood Hydrograph Package, Generalized Computer Program, User’s Manual, Davis,
California, January 1976, Revised February 1981, March 1987, January 1990, September
1990, and June 1992.
13. U.S. Department of the Army, Corps of Engineers, Engineering Manual 1110-2-1405, Flood
Hydrograph Analyses and Computations, August 31, 1959.
14. U.S. Department of Agriculture, Soil Conservation Service, Soil Survey of Sebastian County,
Arkansas, March 1975.
15. U.S. Department of the Interior, Geological Survey, 7.5-Minute Series Topographic Maps,
Scale 1:24,000, enlarged to 1:6,000, Contour Interval 10 feet, Lavaca, Arkansas, 1987;
Barling, Arkansas, 1987.
16. U.S. Department of the Interior, Geological Survey, 7.5 Minute Series (Topographic) Maps,
Scale: 1:24,000, Contour Interval 20 feet: Hackett, Arkansas-Oklahoma (1948); Greenwood,
Arkansas (1951).
17. U.S. Department of the Interior, Geological Survey, 7.5 Minute Series (Topographic) Maps,
Scale: 1:24,000, Contour Interval 20 feet: Hartford, Arkansas (1947); Huntington, Arkansas
(1948).
18. U.S. Army Corps of Engineers, Southwest District, SWD Watershed Runoff Computer
Model for Historical and Hypothetical Storm Events, Dallas, Texas, March 1982.
19. U.S. Department of the Army, Corps of Engineers, Hydrologic Engineering Center, HECHMS Hydrologic Modeling System, version 3.4, Davis CA, August 2009.
20. U.S. Department of the Army, Corps of Engineers, Hydrologic Engineering Center, HECIFH Interior Flood Hydrology Package, version 2.01, December 1998.
21. Federal Emergency Management Agency, Letter of Map Revision, Case Number: 01-061837P, City of Fort Smith, Sebastian County, Arkansas, Washington, D.C., May 31, 2002.
22. U.S. Department of the Army, Corps of Engineers, Hydrologic Engineering Center, HECHMS Hydrologic Modeling System, User’s Manual, version 3.0.0., December 2005.
57
23. U.S. Department of the Interior, Geological Survey, Water Resources Investigations Report
95-4224, Magnitude and Frequency of Floods in Arkansas, Little Rock, Arkansas, 1995.
24. U.S. Department of the Army, Corps of Engineers, Hydrologic Engineering Center, HECRAS River Analysis System, version 3.0.1, March 2001.
25. U.S. Department of the Army, Corps of Engineers, Hydrologic Engineering Center, HEC-2
Water-Surface Profiles, Generalized Computer Program, Davis, California, April 1984 and
September 1988, Revised September 1982, September 1990, and May 1991.
26. City of Greenwood, Digital Elevation Model, 4 foot contours and spot elevations, June 2006.
27. City of Fort Smith, Digital Elevation Model, 2 foot contours and spot elevations, April 2003.
28. United States Army Corps of Engineers, Digital Elevation Model, 2 foot contours and spot
elevations, June 2004.
29. Environmental Systems Research Institute, ESRI Arc View version 9.3.1, Copyright 19992000 ESRI Inc.
30. City of Fort Smith, Digital Elevation Model – Triangulated Irregular Network, April 2010
31. U.S. Department of the Army, Corps of Engineers, Hydrologic Engineering Center, HECRAS River Analysis System, version 4.0.0, Davis CA, March 2008.
32. U. S. Department of the Interior, Geological Survey, 7.5-Minute Series Topographic Maps,
Scale 1:24,000, Contour Interval 20 feet: Lavaca, Arkansas, 1947.
33. U. S. Department of the Interior, Geological Survey, 7.5-Minute Series Topographic Maps,
Scale 1:24,000, Contour Interval 20 feet with intermittent 10 feet: Barling, Arkansas, Photo
Inspected 1975; Fort Smith, Arkansas-Oklahoma, Photorevised 1978; South Fort Smith,
Arkansas-Oklahoma, Photorevised 1978; Van Buren, Arkansas, Photorevised 1978.
34. U.S. Department of the Army, Corps of Engineers, Little Rock District, Arkansas River Land
Impact Study.
35. U.S. Department of the Army, Corps of Engineers, Little Rock District, LRD-1 Computer
Program.
36. Federal Emergency Management Agency, Flood Insurance Study, Crawford County,
Arkansas (Unincorporated Areas), Washington, D.C., May 20, 1996.
37. Federal Emergency Management Agency, Flood Insurance Study, Logan County, Arkansas,
March 4, 1991.
38. Federal Emergency Management Agency, Flood Insurance Study, City of Van Buren,
Crawford County, Arkansas, Washington, D.C., August 5, 1991.
39. Federal Emergency Management Agency, Flood Insurance Study, Sequoyah County and
Incorporated Areas, Oklahoma, Washington, D.C., May 4, 1992.
58