FIRM-SY940415FLOOL
INSUR
STUDY
CITY OF COPPELL,
TEXAS
DALLAS AND
DENTON COUNTIES
Wv, 1
REVISED:
APRIL 15, 1994
Federal Emergency Management Age ncy
COMMUNITY NUMBER - 480170
NOTICE TO
FLOOD INSURANCE STUDY USERS
Communities participating in the National Flood Insurance Program (NFIP) have
established repositories of flood hazard data for floodplain management and
flood insurance purposes. This Flood Insurance Study (FIS) 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 FIS may be revised and republished at any time. In
addition, part of this FIS may be revised by the Letter of Map Revision (LOMR)
process, which does not involve republication or redistribution of the FIS. It
is, therefore, the responsibility of the user to consult with community
officials and to check the community repository to obtain the most current FIS
components.
Initial FIS Effective Date: August 1, 1980
Revised FIS Date: February 15, 1984
October 16, 1991
April 15, 1994
TABLE OF CONTENTS
Page
1.0
INTRODUCTION
1
1.1 Purpose of Study
1
1.2 Authority and Acknowledgments
1
1.3 Coordination
2
2.0
AREA STUDIED
2
2.1 Scope of Study
2
2.2 Community Description
4
2.3 Principal Flood Problems
5
2.4 Flood Protection Measures
6
3.0
ENGINEERING METHODS
7
3.1 Hydrologic Analyses
7
3.2 Hydraulic Analyses
9
4.0
FLOODPLAIN MANAGEMENT APPLICATIONS
10
4.1 Floodplain Boundaries
11
4.2 Floodways
11
5.0
INSURANCE APPLICATIONS
15
6.0
FLOOD INSURANCE RATE MAP
17
7.0
OTHER STUDIES
17
8.0
LOCATION OF DATA
17
9.0
BIBLIOGRAPHY AND REFERENCES
17
i
TABLE OF CONTENTS - continued
Figure 1 - Vicinity Map
Figure 2 - Floodway Schematic
Table 1 - Summary of Discharges
Table 2 - Floodway Data
FIGURES
TABLES
EXHIBITS
Exhibit 1 - Flood Profiles
Elm Fork of Trinity River
Grapevine Creek
Tributary G -1 of Grapevine Creek
Denton Creek
Old Denton Creek
Cottonwood Branch
Panels 01P -02P
Panels 03P -05P
Panel 06P
Panels 07P -10P
Panel 11P
Panels 12P -15P
Exhibit 2 - Flood Insurance Rate Map and Street Index
ii
Page
3
15
8 -9
13 -14
FLOOD INSURANCE STUDY
CITY OF COPPELL, DALLAS AND DENTON COUNTIES, TEXAS
1.0 INTRODUCTION
1.1 Purpose of Study
This Flood Insurance Study revises and updates a previous Flood
Insurance Study /Flood Insurance Rate Map for the City of Coppell,
Dallas and Denton Counties, Texas. This information will be used by
the City of Coppell to update existing floodplain regulations as
part of the Regular Phase of the National Flood Insurance Program
(NFIP). The information will also be used by local and regional
planners to further promote sound land use and floodplain
development.
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 Flood Insurance Study are the
National Flood Insurance Act of 1968 and the Flood Disaster
Protection Act of 1973.
The hydrologic and hydraulic analyses in this study represent a
revision of the original analysis performed by the U. S. Army Corps
of Engineers (USACE), Fort Worth District, for the Federal Emergency
Management Agency (FEMA), under Inter - Agency Agreement No.
IAA- H -7 -76, Project Order No. 21 and Inter - Agency Agreement No. IAA -
H-10-77, Project Order No. 2. That work was completed in December
1978. Additional analysis for the February 15, 1984 revision was
performed by Carter and Burgess, Inc. and Albert H. Halff Associates
Inc., in November 1981, and reflects information on Denton Creek and
Cottonwood Branch. Levee improvements along the Elm Fork of Trinity
River, below Ledbetter Road, were incorporated at that time.
The hydrologic and hydraulic analyses for the October 16, 1991
revision were performed by the USACE, Fort Worth District, for the
Elm Fork of Trinity River. The work for these analyses was
completed in October 1986. As a result of these analyses, the
entire length of the Elm Fork of Trinity River, and portions of
Grapevine Creek and Denton Creek, both near their respective
confluences with the Elm Fork of Trinity River, were revised in the
City of Coppell. Also, Cottonwood Branch downstream of Sandy Lake
Road was revised to incorporate an updated hydraulic analysis,
performed by Weir & Associates, Incorporated, in February 1986. In
addition, Denton Creek downstream of Denton Tap Road was revised to
incorporate an updated hydraulic analyses, performed by Dannenbaum
Engineering Corporation in February 1985.
In this revision, updated hydraulic analyses for Elm Fork of Trinity
River, Grapevine Creek, Denton Creek, and Cottonwood Branch and new
hydraulic analyses for Old Denton Creek were prepared by Kimley -Horn
and Associates, Inc., and Morrison Hydrology /Engineering, Inc. This
work was completed in August 1991. Also, new hydrologic and
hydraulic analyses for Tributary G -1 of Grapevine Creek were
prepared by Goodwin and Marshall, Inc., and this work was completed
in August 1991.
Also in this revision, additional updated hydraulic analyses were
prepared for Denton Creek from the divergence of Old Denton Creek to
the upstream corporate limits to reflect the completed Lake Park
addition. This work was prepared by Dan M. Dowdey & Associates and
was completed in November 1991. In addition, an August 10, 1992,
Letter of Map Revision for a channelization and drop structure
project along Grapevine Creek has been incorporated.
1.3 Coordination
On July 12, 1976, an initial Consultation Coordination Officer's
(CCO) meeting was held with representatives of FEMA, the Community
Executive Officer (CEO), the Texas Department of Water Resources
(TDWR), and the USACE.
The Soil Conservation Service (SCS), the U. S. Geological Survey
(USGS), the State Department of Highways and Public Transportation,
and the TDWR were contacted for information related to the study.
For this revised study, final CCO meeting was held on January 28,
1992, and was attended by representatives of the Study Contractor,
City of Coppell, and FEMA.
2.0 AREA STUDIED
2.1 Scope of Study
This Flood Insurance Study covers the incorporated area of the City
of Coppell, Dallas and Denton Counties, Texas. The area of study is
shown on the Vicinity Map (Figure 1).
The Elm Fork of Trinity River was studied by detailed methods for
its entire length affecting the community. The following streams
were studied by detailed methods for their entire lengths within the
community: Grapevine Creek, Denton Creek, and Cottonwood Branch.
In the October 16, 1991 revision, the updated hydrologic and
hydraulic analyses incorporated the development along the Elm Fork
of Trinity River; and the revised hydraulic analysis for Denton
Creek and Cottonwood Branch annexations.
0A
In this revision, the following flooding sources were studied by
detailed methods: Elm Fork of Trinity River for its entire length
affecting the community; Grapevine Creek, Denton Creek, and
Cottonwood Branch for their entire lengths within the community; Old
Denton Creek from the confluence with Denton Creek to upstream
divergence from Denton Creek; and Tributary G -1 of Grapevine Creek
from the confluence with Grapevine Creek to approximately 0.6 mile
upstream of Bethel School Road.
Limits of detailed study are indicated on the Flood Profiles
(Exhibit 1) and on the Flood Insurance Rate Map (Exhibit 2). The
areas studied by detailed methods were selected with priority given
to all known flood hazard areas and areas of projected development
and proposed construction.
This study was prepared as part of a larger study entitled "Dallas
County Metro Area, Texas," covering all of Dallas County and
portions of Collin, Denton, Rockwall, and Tarrant Counties. In
order to facilitate the management, filing, and retrieval of the
large amount of data produced during the study and to insure
compatibility of data provided to adjoining communities, and to
simplify future changes due to changes in city boundaries, the
following plan of study was developed.
Topographic maps covering the entire area to be studied were
enlarged to a scale of 1:12,000. The upper and lower halves of the
enlarged maps were used to prepare master work maps for the entire
metro area study. As the engineering data, developed on a stream or
watershed basis, were completed, they were transferred to the
appropriate work map. Floodway data tables were also prepared as
the engineering on each stream was completed. When the master work
maps contained all the data necessary for the completion of this
study, copies were made and used to prepare the work maps for this
study. The flood profiles were prepared in a similar manner. Data
was extracted from the previously mentioned floodway data tables to
prepare similar tables for this study.
The remaining portion of Tributary G -1 of Grapevine Creek and
several unnamed tributaries were studied by approximate methods.
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
City of Coppell.
2.2 Community Description
The City of Coppell lies on the Dallas - Denton County boundaries in
north - central Texas. The city is bordered by the unincorporated
areas of Denton County to the north; the City of Carrollton and the
unincorporated areas of Dallas County to the east; the Cities of
Irving and Dallas to the south, and the City of Grapevine to the
west.
4
2.3
In 1980, Coppell had a population of 3,826 and in 1992, a population
of 19,250. Coppell is a sparsely developed residential community of
approximately 13.5 square miles. The city is drained by the Elm
Fork of Trinity River and its tributaries. The Elm Fork of the
Trinity River flows in a southern direction, while the tributaries
flow in an eastern direction.
The climate of the study area is warm, temperate, and humid.
Summers are hot, and winters are short and mild. The mean relative
humidity is 63 percent, and average temperature is 65.8 degrees
Fahrenheit ( °F). The record temperature extremes range from a
maximum of 111 °F in July 1954 to a minimum of 3 °F in January 1930.
Coppell has terrain characteristic of the Blackland Prairie. The
topography is gently rolling to almost level with narrow streams
being well incised. The soils in the area are mainly of the Houston
Black, Haiden, and Austin series. The native vegetation consists of
bunch and short grasses with scattered mesquite trees on the uplands
and hardwood, mainly elm, hackberry, and pecan occurring along the
streams.
Principal Flood Problems
Most of the flood producing storms that occur in Coppell are
experienced in the spring and fall. Most of the higher floods that
have occurred in the general geographical region have resulted from
prolonged or successive storms that produce heavy rainfall during
this period; however, severe flooding can be produced by intense
local thunderstorms at any time.
Coppell has experienced few flood problems in the past, due
primarily to the fact that little development exists on the
floodplains at this time.
There are two USGS stream gages in the study area. The gage on the
Elm Fork of Trinity River located at Sandy Lake Road was established
in November 1943. The gage on Denton Creek located at State Route
121 was established in October 1947 (Reference 1). Historical data
for ungaged watersheds are dependent on local resident observations,
news media records, published reports, and analysis of other nearby
watersheds with gage data and similar characteristics. Significant
flooding is known to have occurred within the vicinity in 1908, the
early 1930s, 1942, 1947, 1949, 1957, 1962, 1963, 1964, 1965, 1969,
1973, 1974, 1976, and 1977; however, little specific data is
available.
Grapevine Creek and Cottonwood Branch undoubtedly experienced
flooding during the periods listed above, but no details are
available.
5
According to local resident interviews, historical data for Denton
Creek begins in 1908 with a major flood, which is the maximum known
in the lower basin. However, no high -water marks or related stage
heights have been recorded. A flood in April 1942 reached 35.9
stage -feet (from high water marks) at the gage and is thought to be
the second largest. Grapevine Dam, completed in 1952, regulates
flows at the gage except those from a 10.3 square mile local area
between the dam and the gage.
The maximum flood of record on the Elm Fork of Trinity River near
Coppell prior to construction of the upstream reservoirs occurred in
May 1908. An estimated discharge of 145,000 cubic feet per second
(cfs) was experienced. The second largest discharge of record was
90,700 cfs, which occurred in April 1942. The existing upstream
reservoirs would have reduced the 1908 and 1942 floods near
Carrollton from 145,000 and 90,700 cfs to 26,000 and 24,100 cfs,
respectively, based upon reservoir regulation studies. In September
1964, a flow of 33,000 cfs was experienced at the Carrollton gage.
This flow was generated entirely from the uncontrolled area below
the reservoirs and approaches the magnitude of the 100 -year flood in
the Coppell area (Reference 1).
It should be noted that the frequencies assigned to the historical
floods should be viewed with caution since the estimates are made by
comparing high - -water marks or discharge estimates obtained from
historical records with determinations made in connection with this
or adjacent studies. It is seldom possible to determine the
conditions that: existed at the sites of the historical high -water
mark or discharge estimate. The hydrologic determinations for this
study are based on existing stream and watershed conditions that
existed at the time of the historical flood. Additionally, the
estimates may riot be valid except in the immediate area of the
estimate.
2.4 Flood Protection Measures
Grapevine Dam on Denton Creek and Lewisville Dam on the Elm Fork of
Trinity River are the only major flood control projects affecting
the study area. These reservoirs, providing flood control, water
supply, and recreation, were completed in 1952 and 1955,
respectively. Additionally, several short lengths of stream
straightening and /or enlarging have taken place in connection with
road construction. The City of Coppell regulates floodplain
development through a zoning ordinance.
the Gateway Development, along Denton Creek downstream of Denton Tap
Road in the northern part of the city, is protected from the 100 -
year flood by a. levee. This levee is NFIP- accredited since the
information for this area was taken directly from the Flood
Insurance Study and Flood Insurance Rate Map for Lewisville
(Reference 2).
6
For Tributary G -1 of Grapevine Creek, rainfall data was taken from
Reference 4 for storm durations from 2 to 12 hours. The point
rainfall from Reference 4 was converted from a partial to annual
series duration, and effective rainfall was adjusted based on
drainage area size by formulas in Reference 4. For storm durations
from five to 60 minutes, rainfall data were taken from the National
Oceanic and Atmospheric Administration's "five-to.-60-minute
precipitation frequency for eastern and central United States." A
12 -hour synthetic storm was developed from triangular depth- duration
distribution of rainfall data. It was assumed that the greatest
depth would occur at the mid -point of the storm, which would be hour
six. The watershed for Tributary G -1 of Grapevine Creek was divided
into five subareas, and a hydrologic model was prepared using the
USACE's HEC -1 Flood Hydrograph Computer Program (Reference 5).
Existing development was determined from aerial photographs and
field inspections. The curve number method was used to estimate
runoff in the HEC -1 model.
A summary of the drainage area -peak discharge relationships for the
stream(s) studied by detailed methods is shown in Table 1, "Summary
of Discharges."
TABLE 1 - SUMMARY OF DISCHARGES
FLOODING SOURCE DRAINAGE AREA PEAK DISCHARGES (cfs)
AND LOCATION (sq_ miles 10 -YEAR 50 -YEAR 100 -YEAR 500 -YEAR
ELM FORK OF TRINITY
RIVER
At USGS gage located
at Sandy Lake Road 104.011
GRAPEVINE CREEK
8,600
At confluence with
12,700
Elm Fork of Trinity
River 11.482
At South Fork
10.55
At Moore Road
9.65
Below Stream 7F1
8.14
Above Stream 7F1
7.17
At intersection with
15,100
extension of Cotton
9,400
Road
5.95
At Radio Road
3.68
24,400 39,900 48,600 60,400
5,900
8,600
9,700
12,700
6,900
9,700
11,800
15,200
7,500
10,500
11,800
15,100
7,600
10,600
11,900
15,100
6,700
9,400
10,500
13,400
7,300
10,000
11,200
14,200
5,100
7,000
7,900
9,900
TRIBUTARY G -1 OF GRAPE-
VINE CREEK
At confluence with
Grapevine Creek 1.09 1,860 2,735
'Drainage area below Grapevine Lake and Lewisville Lake
20utside corporate limits
E:?
3,140 4,210
3.0
Portions of the floodplains along Grapevine Creek and Elm Fork of
Trinity River are protected by the Irving Flood Control District
Section III levee, which is NFIP- accredited.
ENGINEERING METHODS
For the flooding sources studied in detail 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 which
are 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 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 which equals or exceeds the 100 -year flood (1 percent
chance of annual exceedence) in any 50 -year period is approximately 40
percent (4 in 10), and, 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 the peak
discharge- frequency relationships for each flooding source studied
in detail affecting the community.
The NUDALLAS computer program, which is developed by the USACE Fort
Worth District, was used in the hydrologic analysis to develop
discharge- frequency curves for the Elm Fork of Trinity River
(Reference 3).
Each of the remaining streams were divided into subareas, and
synthetic unit and flood hydrographs were developed at selected
locations. National Weather Service Technical Paper No. 40 was used
in developing the 10, 50, and 100 -year storms (Reference 4). The
500 -year storm was based on extrapolated data. Frequency peak
discharges at selected locations were computed. Additionally,
numerous; headwater areas of less than 1 square mile were modeled
using the rational method, where Q = CIA. The variables in this
method are defined as follows: Q is the peak discharge in cfs, C is
a runoff coefficient, I is the rainfall intensity in inches per hour
for the watershed time of concentration, and A is the drainage area
in acres.
VA
TABLE 1 - SUMMARY OF DISCHARGES - continued
FLOODING SOURCE
AND LOCATION
DENTON CREEK
At confluence with
Elm Fork of
Trinity River
Below Cottonwood
Branch
Above Cottonwood
Branch
At State Route 121
OLD DENTON CREEK
At confluence with
Denton Creek
COTTONWOOD BRANCH
At confluence with
Denton Creek
At Sandy Lake Road
At State Road
At Bethel Road
DRAINAGE AREA PEAK DISCHARGES (cfs)
(sq. miles) 10 -YEAR 50 -YEAR 100 -YEAR 500 -YEAR
24.231
10,600
15,500
17,800
36, 2M
19.451
13,300
18,900
21,300
36,2 002
12.521
9,400
13,200
14,900
36,2 002
10.301
10,000
13,800
15,600
36,2 002
*
*
*
5,000'
6.93
4,500
6,400
7,300
9,400
5.69
4,200
5,900
6,700
8,600
4.69
3,600
5,100
5,700
7,300
3.64
3,600
5,000
5,600
7,100
*Data not available
'Drainage area below Grapevine Lake
'Discharge for Denton Creek below Grapevine Dam controlled outflows from
Grapevine Lake
'100 -year discharge for Old Denton Creek used only in split floodway analysis.
For 10 -, 50 -, 100 -, and 500 -year natural conditions, Old Denton Creek was
modelled as part of Denton Creek
The decrease in peak discharge with an increase in drainage area for some
streams is due to watershed shape and /or overbank storage effects.
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.
Cross sections were obtained from Texas State Highway Department
bridge plans, field surveys, and studies prepared by Albert H. Halff
and Associates, Inc., URS Engineers, Nathan D. Maier and Associates,
Inc., Freeze and Nichols, and Caffey and Morrison, Inc.
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
9
4.0
cross - section locations are also shown on the Flood Insurance Rate
Map (Exhibit 2).
Water- surface elevations of floods of the selected recurrence
intervals were computed using the USACE HEC -2 step- backwater
computer program (Reference 6). Flood profiles were drawn showing
computed water- surface elevations for floods of the selected
recurrence intervals. Starting water - surface elevations for the Elm
Fork of Trinity River and Denton Creek were based on coincident
conditions. The remaining streams were based on slope /area
determinations.
Channel roughness factors (Manning's "n ") for these computations
were assigned on the basis of field inspections of floodplain areas
and on previous studies by the USACE. Channel and overbank "n"
values for the streams studied by detailed methods are shown in the
following tabulation:
Stream Name Channel "n"
Elm Fork of Trinity River 0.060 -0.100
Grapevine Creek 0.028 -0.055
Tributary G -1 of Grapevine
Creek 0.050 -0.065
Denton Creek 0.035 -0.045
Old Denton Creek 0.035 -0.045
Cottonwood Branch 0.020 -0.050
Overbank "n"
0.035 -0.150
0.045 -0.090
0.045 -0.050
0.035 -0.060
0.035 -0.060
0.020 -0.075
The hydraulic analyses for all flooding sources were computed using
the HEC -2 step- backwater program (Reference 6).
The hydraulic analyses for this study were based on unobstructed
flow. The flood elevations shown on the profiles are thus
considered valid only if hydraulic structures remain unobstructed,
operate properly, and do not fail.
All elevations are referenced to the National Geodetic Vertical
Datum of 1929 (NGVD). Elevation reference marks used in this study,
and their descriptions, are shown on the maps.
FLOODPLAIN MANAGEMENT APPLICATIONS
The NFIP encourages State and local governments to adopt sound floodplain
management programs. Therefore, each Flood Insurance Study provides
100 -year flood elevations and delineations of the 100- and 500 -year
floodplain boundaries and 100 -year floodway to assist in developing
floodplain management measures.
10
4.1 Floodplain Boundaries
To provide a national standard without regional discrimination, the
1 percent annual chance (100 -year) flood has been adopted by FEMA as
the base flood for floodplain management purposes. The 0.2 percent
annual chance (500 -year) flood is employed to indicate additional
areas of flood risk in the community. For the streams studied in
detail, the 100- and 500 -year floodplain boundaries have been
delineated using the flood elevations determined at each cross
section. For Denton Creek, Cottonwood Branch, Old Denton Creek, Elm
Fork of Trinity River, and the portion of Grapevine Creek studied in
detail in the October 16, 1991 revision, the floodplains were
delineated using topographic maps at a scale of 1" =400' with a
contour interval of 2 feet (Reference 7). For Tributary G -1 of
Grapevine Creek, floodplains were delineated using topographic maps
at a scale of 1" =100' with a contour interval of 2 feet (Reference
8). For the portions of Grapevine Creek studied in detail in this
revision, the floodplains were interpolated using topographic maps
at a scale of 1:12,000 with a contour interval of 10 feet (Reference
9).
The 100- and 500 -year floodplain boundaries are shown on the Flood
Insurance Rate Map (Exhibit 2). On this map, the 100 -year
floodplain boundary corresponds to the boundary of the areas of
special flood hazards (Zone A, Zone AE), and the 500 -year floodplain
boundary corresponds to the boundary of areas of moderate flood
hazards. In cases where the 100- and 500 -year floodplain boundaries
are close together, only the 100 -year 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.
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 National Flood Insurance Program,
a floodway is used as a tool to assist local communities in this
aspect of floodplain management. Under this concept, the area of
the 100 -year 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
100 -year flood can be carried without substantial increases in flood
heights. Minimum federal standards limit such increases to 1.0
11
foot, provided that hazardous velocities are not produced. The
floodways in this study are presented to local agencies as a minimum
standard 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 2). The computed
floodways are shown on the Flood Insurance Rate Map (Exhibit 2). In
cases where the floodway and 100 -year floodplain boundaries are
either close together or collinear, only the floodway boundary is
shown. Portions of the floodway widths for the Elm Fork of Trinity
River and Grapevine Creek extend beyond the corporate limits.
The area between the floodway and 100 -year floodplain boundaries is
termed the floodway fringe. The floodway fringe encompasses the
portion of the floodplain that could be completely obstructed
without increasing the water- surface elevation of the 100 -year flood
by more than 1.0 foot at any point. Typical relationships between
the floodway and the floodway fringe and their significance to
floodplain development are shown in Figure 2.
In the northern part of the city just south of the levee on Denton
Creek, Denton Creek has been channelized and straightened; however,
the Old Denton Creek Channel still exists just south of the
channelized portion. Old Denton Creek leaves the main channel
approximately 1.0 mile downstream of the confluence of Cottonwood
Branch and rejoins the main channel approximately 1.5 miles
downstream of Cottonwood Branch. For the unencroached hydraulic
analyses of the 10 -, 50 -, 100 -, and 500 -year floods, the main Denton
Creek channel and Old Denton Creek were considered a common
floodplain area; however, when determining the floodway for Denton
Creek by equal conveyance reduction, the floodway would not include
the Old Denton Creek Channel. To assure that Old Denton Creek is
kept free from encroachment, a split flow model through this reach
was prepared to determine separate floodways for Denton Creek;
therefore, there is separate information in the floodway data table
for Denton Creek and Old Denton Creek.
12
low
w
�i
FLOODING SOURCE
FLOODWAY
BASE FLOOD
WATER SURFACE ELEVATION
CROSS SECTION
DISTANCE
WIDTH
SECTION
AREA
MEAN
VELOCITY
WITHOUT WITH
REGULATORY FLOODWAY FLOODWAY INCREASE
(FEET)
(SQUARE
(FEET PER
FEET)
SECOND)
(FEET NGVD)
Elm Fork of
Trinity River
Al
83,4002
3,9545
26,766
1.7
439.0
439.0
439.1
0.1
B1
84,7502
4,1625
23,681
1.9
439.2
439.2
439.2
0.0
C
90,8802
6,9165
28,897
1.7
443.7
443.7
443.7
0.0
Grapevine Creek
A J.
n^
B
12,9003
940/4806
3,141
3.1
448.0
448.0
448.5
0.5
C
16,5433
638
4,624
2.4
461.5
461.5
462.3
0.8
D
20,7003
370
3,063
3.9.
471.5
471.5
472.3
0.8
E
22,7003
200
1,566
7.5
478.0
478.0
478.4
0.4
F
24,0003
349
2,498
4.7
482.5
482.5
482.9
0.4
G
26,9213
145
1,510
7.9
491.2
491.2
491.8
0.6
H
31,9163
119
1,301
8.1
506.4
506.4
506.9
0.5
Tributary G -1
of Grapevine
Creek
A
9454
137
777
4.0
493.7
493.7
493.7
0.0
B
3,1254
82
432
7.3
499.5
499.5
499.7
0.2
'Combined Grapevine Creek /Elm Fork of Trinity River floodway
2Feet above confluence with Trinity River
3Feet above confluence with Elm Fork of Trinity River 6Width /width within corporate limits
"Feet above confluence with Grapevine Creek *Located outside of community
FEDERAL EMERGENCY MANAGEMENT AGENCY
CITY OF COPPELL. TX
(DALLAS & DENTON COS.)
FLOODWAY DATA
ELM FORK OF TRINITY RIVER - GRAPEVINE CREEK -
TRIBUTARY G -1 OF GRAPEVINE CREEK
M
N
FLOODING SOURCE
FLOODWAY
BASE FLOOD
WATER SURFACE ELEVATION
CROSS SECTION
DISTANCE
WIDTH
(FEET)
SECTION
AREA
(SQUARE
MEAN
VELOCITY
WITHOUT WITH
REGULATORY FLOODWAY FLOODWAY
INCREASE
(FEET
(NGVD)
FE)
SECOND)
(FEET
Denton Creek
A
21,1401
329
5,316
3.9
452.4
452.4
453.4
1.0
B
23,3701
325
*
*
453.0
453.0
453.5
0.5
C
26,2201
571
3,677
5.8
454.7
454.7
455.1
0.4
D
29,2801
1,134
8,383
2.5
456.3
456.3
457.0
0.7
E
36,8661
931
7,271
2.1
466.7
466.7
467.4
0.7
F
39.2001
499
3,526
2.9
468.8
468.8
469.6
0.8
Old Denton Creek
A
1,1002
90
*
*
453.0
453.0
453.6
0.6
Cottonwood Branch
A
3,8502
356
1,780
4.1
463.7
463.7
464.5
0.8
B
6,7102
587
2,554
2.9
470.1
470.1
471.1
1.0
C
11,3352
270
2,156
3.1
487.4
487.4
488.1
0.7
D
14,7052
223
1,411
4.0
495.1
495.1
495.8
0.7
'Feet above confluence with Elm Fork of Trinity River
2Feet above confluence with Denton Creek
*Data not available
FEDERAL EMERGENCY MANAGEMENT AGENCY
CITY OF COPPELL, TX
(DALLAS & DENTON COS.)
FLOODWAY DATA
DENTON CREEK - OLD DENTON CREEK - COTTONWOOD BRANCH
FLOODWAY SCHEMATIC Figure 2
5.0 INSURANCE APPLICATIONS
For flood insurance rating purposes, flood insurance zone designations are
assigned to a community based on the results of the engineering analyses.
The zones are as follows:
Zone A
Zone A is the flood insurance rate zone that corresponds to the
100 -year floodplains that are determined in the Flood Insurance
Study by approximate methods. Because detailed hydraulic analyses
are not performed for such areas, no base flood elevations or depths
are shown within this zone.
Zone AE
Zone AE is the flood insurance rate zone that corresponds to the
100 -yeas- floodplains that are determined in the Flood Insurance
Study by detailed methods. In most instances, whole -foot base flood
15
elevations derived from the detailed hydraulic analyses are shown at
selected intervals within this zone.
Zone AH
Zone AH is the flood insurance rate zone that corresponds to the
areas of 100 -year shallow flooding (usually areas of ponding) where
average depths are between 1 and 3 feet. Whole -foot base flood
elevations derived from the detailed hydraulic analyses are shown at
selected intervals within this zone.
Zone AO
Zone AO is the flood insurance rate zone that corresponds to the
areas of 100 -year shallow flooding (usually sheet flow on sloping
terrain) where average depths are between 1 and 3 feet. Average
whole - depths derived from the detailed hydraulic analyses are shown
within this zone.
Zone A99
Zone A99 is the flood insurance rate zone that corresponds to areas
of the 100 -year floodplain that will be protected by a Federal flood
protection system where construction has reached specified statutory
milestones. No base flood elevations or depths are shown within
this zone.
Zone V
Zone V is the flood insurance rate zone that corresponds to the
100 -year coastal floodplains that have additional hazards associated
with storm waves. Because approximate hydraulic analyses are
performed for such areas, no base flood elevations are shown within
this zone.
Zone VE
Zone VE is the flood insurance rate zone that corresponds to the
100 -year coastal floodplains that have additional hazards associated
with storm waves. Whole -foot base flood elevations 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 500 -year floodplain, areas within the 500 -year
floodplain, and to areas of 100 -year flooding where average depths
are less than 1 foot, areas of 100 -year flooding where the
contributing drainage area is less than 1 square mile, and areas
protected from the 100 -year flood by levees. No base flood
elevations or depths are shown within this zone.
16
Zone D
Zone D is the flood insurance rate zone that corresponds to
unstudied areas where flood hazards are undetermined, but possible.
6.0 FLOOD INSURANCE RATE MAP
The Flood Insurance Rate Map 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 100 -year floodplains that
were studied by detailed methods, shows selected whole -foot base flood
elevations or average depths. Insurance agents use the zones and base
flood elevations 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 100- and 500 -year floodplains. Floodways and the
locations of selected cross sections used in the hydraulic analyses and
floodway computations are shown where applicable.
7.0 OTHER STUDIES
A report entitled Partial Hydrologic and Hydraulic Data for Floodplain
Delineation, Grapevine Creek. City of Coppell Texas, has been published
(Reference 10). That report covers a reach of Grapevine Creek located
between Moore Road and Denton Tap Road. The 100 -year frequency flood
elevations and discharges developed in that report are based on a fully
developed watershed and as would be expected are higher than those
developed for this study.
Flood Insurance Studies have been prepared for the Cities of Carrollton,
Grapevine, Irving, and Dallas, and the unincorporated areas of Dallas and
Denton Counties (References 11, 12, 13, 14, 15, and 16).
Because it is based on more up -to -date analyses, this Flood Insurance
Study supersedes the previously printed Flood Insurance Study for the City
of Coppell (Reference 17).
8.0 LOCATION OF DATA
Information concerning the pertinent data used in preparation of this
study can be obtained by contacting FEMA, the Natural and Technological
Hazards Division, Federal Regional Center, 800 North Loop 288, Denton,
Texas 76201 - 3698.
9.0 BIBLIOGRAPHY AND REFERENCES
1. U.S. Geological Survey, Stream Gaging Records.
17
2. Federal Emergency Management Agency, Flood Insurance Study, City of
Lewisville. Dallas and Denton Counties, Texas, Washington, D.C.,
October 18, 1988.
3. U.S. Army Corps of Engineers, Fort Worth District., Computer Program
NUDALLAS, Fort Worth, Texas, September 1986.
4. U.S. Department of Commerce, Weather Bureau, Technical Paper No. 40,
Rainfall Frequency Atlas of the United States, Washington, D. C.,
1961, Revised 1963.
5. U.S. Army Corps of Engineers, Hydrologic Engineering Center, HEC -1
Flood Hydrograph Package, Davis, California, February 1985.
6. U.S. Army Corps of Engineers, Hydrologic Engineering Center, HEC -2
Water Surface Profiles, Generalized Computer Program, Davis,
California, September 1982, updated May 1985.
7. Morrison Hydrology /Engineering, Incorporated, City of Coppell,
Texas, Topographic Maps, Scale 1" =4001; Contour Interval 2 Feet.
8. Tributary G -1 of Grapevine Creek, Copgell, Texas, Existing
Conditions, Topographic Maps, Goodwin and Marshall, Incorporated;
Scale 1" =100'; Contour Interval 2 Feet, August 1991.
9. U.S. Department of the Interior, Geological Survey, 7.5 Minute
Series Quadrangle Maps, Scale 1:12,000, Contour Interval 10 Feet:
Carrollton, Texas, 1959, Photorevised 1968 and 1973; Grapevine,
Texas, 1959, Photorevised 1968 and 1973.
10. Albert H. Halff Associates, Inc., Engineers, Partial Hydrologic and
Hydraulic Data for Floodplain Delineation, Grapevine Creek. City of
Coppell. Texas, for North Lake Woodlands Addition, Furguson and
Deere, Inc., November 18, 1978.
11. Federal Emergency Management Agency, Flood Insurance Study, City of
Carrollton, Dallas and Denton Counties, Texas, Washington, D.C.,
January 2, 1991.
12. Federal Emergency Management Agency, Flood Insurance Study, City of
Grapevine, Dallas and Tarrant Counties, Texas, Washington, D.C.,
August 15, 1989.
13. Federal Emergency Management Agency, Flood Insurance Study, City of
Irving, Dallas County, Texas, Washington, D.C., April 2, 1991.
14. Federal Emergency Management Agency, Flood Insurance Study, City of
Dallas, Dallas, Denton, Collin. Rockwall, and Kaufman Counties,
Texas, Washington, D.C., July 2, 1991.
15. Federal Emergency Management Agency, Flood Insurance Study,
Unincorporated Areas of Dallas County, Texas, Washington, D.C.,
July 19, 1982.
18
16. Federal Emergency Management Agency, Flood Insurance Study
Unincoryorated Areas of Denton County, Texas, Washington, D.C.,
November 20, 1991.
17. Federal Emergency Management Agency, Flood Insurance Stud_y, City of
Coppell, Dallas and Denton Counties, Texas, Washington, D.C.,
October 16, 1991.
Dan M. Dowdey & Associates, Lake Park Addition, Coppell, Texas, January
1991.
Dan M Dowdey & Associates, Construction Plans, Lake Park Addition, City of
Coppell, Dallas County, Texas, for Coppell Mud No. 1 District Engineer,
Pierce Lunsford Associates, Inc. April 2, 1990.
19