Freeport NIP(1)-SY 860310 Federal Emergency Management Agency
Washington, D.C. 20472
March 10, 1986 ·
RETURN RECEIPT REQUESTED
The Honorable Lou Duggan
Mayor of the City of Coppell
P.O. Box 478
Coppell, Texas 75019
Dear Mayor Duggan:
This is in reference to a letter dated December 9, 1985, from Mr. J. E.
Powell, P.E., City Engineer, to the Federal Emergency Management Agency
(F~4A). In his letter, Mr. Powell requested ~hat F~A issue a conditional
Letter of Map Revision (LOMR) for a proposed channelization project on a
portion of Cottonwood Branch, located west of State Road. With his letter,
Mr. Powell forwarded a flood plain study prepared by Caffey and Morrison, Inc.,
entitled Study Revision of Freeport North Addition Along Cottonwood Branch,
dated November 1985.
Based on our review of the submitted data, we believe that the channelization
project, if completed as proposed in the aforementioned flood plain study,
will be sufficient to reduce the. 100-year water-surface elevations, Special
Flood Hazard Area, and regulatory floodway from existing conditions within the
Cottonwood Branch flood plain in the vicinity of the proposed project. The
potential hydrologic impacts associated with the development of this area or
areas upstream have not been considered in this de%ermination. This coD~itional
LOMR is issued with respect to minimum standards of the National Flood Insurance
Program (NFIP). Because your community may enforce more restrictive flood
plain managemen~ criteria, final approval of the project rests with your
community.
The basis of this com~titional LOMR is a proposed channel modification project.
It should be noted that NFIP regulation.44 CFR 60.3(b)(7) req%lires that
communities "assure that the flood carrying capacity within the altered or
relocated portion of any watercourse is maintained." This provision is
incorporated into your community's existing flood plain management regulations.
Consequently, upon completion of this proposed project, the ultimate responsi-
bility for maintenance of the channel will rest with your community.
Upon completion of the proposed channelization, your communit~ may request a
revision to the effective Flood Insurance Study (FIS) and Flood Insurance Rate
Map (FIRM). The revision request must include the data listed below:
1. Documentation which indicates community endorsement of the revision
request.
2. "As-built" plans of the channelization, certified by a registered
engineer.
3. A written description of the methodology used to determine hydrologic
and/or hydraulic parameters, if different from the effective FIS and
FIRM.
4. Revised water-surface profiles of the 10-, 50-, 100-, and 500--year
floods reflecting "as-built" conditions, including a zone determina-
tion.
a. The methodology and starting parameters for the revised profiles
should be consistent with the present effective FIS, i.e., same
discharges add hydraulic model, "unless the parameters have been
superseded by more current and technically superior data and
analyses. (F~A approval should be obtained before deviating
from the effective FIS parameters. )
b. Since only a portion of the existing profiles is being revised,
the upstream and downstream portions of the revised profiles
should coincide exactly with the effective FIS profiles, i.e.,
hydraulic calculations should be continued for a great enough
distance upstream ~'nd downstream of the revised area until
water-surface elevations coincide with those in the effective
FIS.
5. Two floodway hydraulic backwater models should be submitted. The
first should be a duplication of the original baseline model used in
the effective FIS. This is required to ensure that the original data
has been duplicated correctly. The second model should incorporate
the completed channelization and include any other channel modifica-
tions or encroachment that may have occurred in the fiood plain since
the original floodway was delineated. If, however, additional cross
sections are used in the second model to provide a more detailed
analysis of the completed channel modification project and its
effects on flood hazards, an intermediate model should also be
submitted. This model must reflect-flood plain conditions as they
existed at the time that the original floodway was delineated and,
therefore, it is important that any cross sections added to the
original model describe those conditions. In addition, 'any improve-
ments to the original modeling technique may be inco~orated into
this intermediate model. This model will then become the new baseline
model and will be used to accurately measure the effe.cts of the
completed channel modification project.
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a. The methodology and parameters for the revised floodway should
be consistent with the effective FIS, i.e., equal conveyance
reduction to establish encroachment limits, unless changes as
specified in item 4a have been approved by FEMA.
b. Since only a portion of the floodway is being revised, it must
tie into the effective FIS floodway exactly by duplicating the
results of the original baseline model at cross sections upstream
and downstream of the completed project.
c. The revised floodway must carry the waters of the proposed base
(100-year) flood without increasing the water-surface elevations
of that flood by more than 1.0 foot over the original baseline
model at any point. If additional cross sections have been
incorporated, then revised floodway elevations also may not
exceed base flood elevations calculated in the new baseline
model by more than 1.0 foot.
6. Delineation of the 100- and 500-year flood boundaries, the 100-year
floodway boundary, and the location and alignment of cross sections
and flow line used in the hydraulic model.
a. This information should be shown on a map of suitable scale add
topographic definition to provide reasonable accuracy.
b. All items should be labeled for easy' cross-referencing to hy-
draulic model and summary data.
7. Source data and engineering documentation for the previously men-
tioned items, as Well as a bibliographic list of other sources of
information used.
Upon receipt and review by FEMA of items 1-7 above, and verification that the
design of the channelization project conforms to sound engineering practice
and meets all applicable NFIP standards, FEMA will take action to incorporate
the effects of the completed project into the effective FIS and FIRM for your
community.
We~nave enCl°Sed documents, entitled cOnditions and Criteria for Map Revisions
and Conditions and .Criteria. for Floodway Revisions, which fur~ther describe the
nature and extent of the material needed to support a request to revise an
effective FIS and FIRM. Compliance with the criteria outlined in these
documents will expedite F~4A's review process, thus allowing ~che effective FIS
and FIRM for your community to be revised as appropriate, in a timely manner.
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Should you have any questions regarding this matter, please do not hesitate to
contact the Chief, Natural and Technological Hazards Division of the Federal
Emergency Management Agency in Denton, Texas, at (817) 387-5811 or members of
our Headquarters staff in Washington, D.C., at (202) 646-2754.
Acting Chief, Risk Studies Division
Federal Insurance Administration
Enclosures
cc: Mr. J. E. Powell, P.E., City Engineer
City of Coppell, w/enclosures
Mr. Ronald W. Morrison, P.E., Vice-President,
Caffey and Morrison, Inc., w/enclosures
FEDERAL EMERGENCY MANAGEMENT AGENCY AUGUST, 19Ei4
Conditions and Criteria for
FLOODWAY REVISIONS
CONDITIONS AND CRITERIA FOR FLOODWAY REVISIONS
FOREWORD
Th~ National Flood Insurance Program makes flood insurance
available to property owners in communities that adopt and enforce
flood plain management measures to reduce future flood losses. The
Program provides' flood hazard maps and risk information on which
local flood plain management measures are based.
One aspect of a sound flood plain management program is the
maintenance of a floodway area to assure that the elevations of
future floods will not be increased significantly. The adoption of
a floodway by a community preserves th~ necessary conveyance area
for passage of the flood waters by resfricting actions within the
floodway which will result in any increase in flood elevation.
After a floodway is adopted, a community may encounter a compelling
need to change the configuration of their floodway and therefore
request that the floodway map prepared by the Federal Emergency
Management Agency (FEMA) be revised. The purpose of these
conditions and criteria is to set forth the nature and extent of
the material needed to support such a request. Compliance with the
criteria described herein will allow FEMA to review the material
and revise the floodway maps as appropriate in a timely manner.
Federal Insurance Administration
This document set~ forth FEHA policy concerning request~ for revising the
Flood ~u~a~y ~d Fl~wa~ ~p (~) prepared ~ ~ fo~ ~op~on by
co. unify ~ regulate fl~ways In a~ordan~ wi~ Title 44, Part 60.3(d) of
~ ~e of Federal
A. ~ ~le In Es~ablish~n~ FlOrals
The National FI~ Insurance Act of ~968, S~tion ~361 au~o~zed F~ to
car~ ou~ fl~ studies rela~ng ~ encFoac~n~ a~ ~struct~ons
stream ~annels ~d fl~ways. The ~r~se of ~e skud[es is ~ support
s~te ~d ~al effor~ ~:
(1) regulate ~e ~velo~nt sE ~nd ~ch ~s e~osed ~ fl~
whe re appropr ~a ks,
(2) guide pro~s~ ~ns~ruck~on away fro~ l~k~ons ~ch are ~eatened
by fl~ hazafd,
(3) assist in reducing ~ges ~us~ by ~s, ~
(4) otherwise ~prove ~e ~ng-~ange ~d ~age~nt ~d use
fl~pro~ areas.
~e section ~s e~l~c~t ~n ~t ~ sha~ ~rk closely wi~, ~d provide
necessa~ technic~ ass~s~an~ ~, s~e, ~nte~s~a~e, m~
gove~nt agencies ~ ~e~c efforts ~ard ~u~ fl~ plain ~agemen~.
~e ~ role ~nsis~ of establ~shing m~n~ standards ~ providing
~nfo~ti~ ~ skate ~d 1~1 regulat~g' agencies. Eskablished
cEiter~a are conta~n~ ~n Part 60.3 (d) o~ ~pter 1, T~tle 44 of
~e of F~eral bgulati~s. ~e re~ulat~ states ~t ~en F~ has
pcovid~ ~a fr~ whi~ a regulatory flyway shall ~ designated, ~e
~unity s~ll ~lect ~d ~opt a regularly flyway bas~ ~
pr~nciple ~at ~e area ~os~ f~ ~e reguhtoFy flyway ~st carry ~e
waters of ~e hse Ei~ (100-year) v~out ~ncreas~ng ~e water surface
elevatim of ~t ~1~ ~ce ~ 1.0 f~t at ~y ~nt. Hoover,
~un~ ~ ~opt a ~ce res~cickive ~l~vay ~ ~ich ~e
l~t ~ less ~ 1.0
~e ~ role, ~eref~e, Is ~ provide ~ fr~ ~ ~e ~un~ty
formulate ~ adopk a fLYway ~o~ regulatory pur~ses. FF~ a
· ~d~nt, ~ c~~tes v~ ~untt~ea during ~e~ ~
s~e ~ prepares ~ ~ a single fl~ay del~neak~ ~h~ reflec~
c~t~nues ~ au~rt ~ ~un~ty ~ ~ effor~ ~ enfor~ ~e
~ provides ~ies of ~ ~ps ~ users.
B. Definition and Purpose of Floodwa~
The floodway is defined as the channel of s stream and any ~djacent flood
plain areas that must be reserved in order to discharge the lO0-year
flood without increasing flood heights by s specified amount. The
purpose for establishing a floodway is to provide 'for the balancing of
~ cool,·ting uses of development against the resulting increase in flood
hazards. FEMA has set s minimum st·nd·rd, limiting the floodway width to
t/~at which will not result in an increase of the 100-year flood by ~ore
than 1.0 foot. Several states have adopted requirements which limit the
increases to less than the FEMA minimum standard. In cases where the
state has adopted ~ore stringent standards by legally enforceable statute
or regulation, FEMA computes fl.oodways based upon these standards. Once
the floodway has been adopted, encroachments, including fill, new
construction, substantial /mprovements, and other development within the
adopted floodway that would result in any increase in flood levels .within
the co~unity during the occurrence of the base flood discharge is
prohibited.
C. Floodway Revision - General Policies
Within the criteria established by FEMA, ~any floodway configurations may
be generated. However, once adopted by a community, a particular
floodway configuration becomes ~dministrstively established, much the
same ss other community regulations. An adopted floodway represents a
reasonable allocation of an ·rea for the passage of flood waters and does
not necessarily represent the minimu~ area required to meet the FF. mA
minimu~ standard. Once the .floedway is adopted, the flondw·y boundaries
· re intended to ten·in static.
During a restudy, the existing floodway-configuration should be checked
to assure that it meets acceptable surcharge limits under present
conditions. Where surchsrge l~uits ·re exceeded, the floodway should be
expanded.
A floodway may be reduced tn size if the discharges have been reduced as
· result of a physicsl change, such as a diversion of the flow, or the
construction of · dan o~ other major retention measures. A floodway nay
also be reduced as a result of stream modifications. Floodway changes
should not be based on physical changes, unless it has been established
that adequate naintenance bas been provided to assure continuation of the
structural effects.
a further basis for a floodway revision is the desire by s c°Snunity to
respond to · social o~ econOnic need for-developnent within · floodway.
Th~s need my be met by shifting the floodway boundaries, using
appropriate hydraulic analysis, while maintaining the flood carrying
capacity of the floodway.
After a floodway Is established, development may occur in the fringe area
(the area within the flood pla£n but outside the.floodway), hut should
not occur In the floodway unless it is shown that the development will
not reduce the conveyance of the floodway. If it becomes necessary to
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revise the £1oodway, the original hydraulic model ·mst be used
estsblis~ the base profile for the new encroacl~nent analysis. The model
would then be Bodified to include the in-place encroachnent for ~=he
revised floodway runs.
D. Conununity Floodvay Nap Revision Request
. The floodway designatlc~ is not '·ppealable" to FEP~A by Lndividuals,
since it is the con~unity that selecf~ end adopts the regulatory
floodway. Thus, en individual should appeal the floodway designation to,
or seek amendments from, the connunity. The cos~unity, in turn, may
· upport the amendment and request ~ to revise the orlg£nal, FF~dA-
produced Flood Boundary end Floodway Nap (FBFId). FF.~A will review the
request to ·scertain tJ~at the following conditions ·re Mt before
expending effort on the rev.~sion~
Condition Number 1
The connunity must demonstrate that it i~ prepared to ·dopt t~e modified
floodway ·nd that aL~ legal requirement~ will have been met before
floodway is revised. Before changes ·re made to connunity regulated
· teas, conmunities ·re required by the state to follow certain
· dministrative procedures. The request to ~ needs to include evidence
that ·ppropriate procedures have been followed. Typical information to
be submitted ·re ·s follows:
Copy of · public notice of the intent to modify the floodway.
A statement t~at the connunity has notified, and obtained ·pproval
£ron, any adversely l~pacted property owners o~ adjacent
Jurisdictions.
:1.3 In lieu of the above, · statement that the change will fesult in no
adverse imp·ct.
.~o4 & copy of the notification of the £11oodw·y change to the ~tate.
Condition #t~-~er 2
An engineering analysis has been performed to ·ubstanti·te that the
~evised floodway Becks FEIMA u~ninun standards as cell as any s~a~e
co~unt t~ £equ~fements.
Floodway rev:Lsions Bust be based o~ the hydraulic model use~ to develop
the floodway currently :~n effect. The community ·hould request, through-
Zhe F~A Regional O~fice, · copy of the input data used'in the computer
model £or :Lts effective Flood Insurance ~tudy. Mhere the ~nput data
representing the orig:Lnal hydraulic model :Ls unava£~able, on
&pprox~tnation should be developed. A new model should be estshlished
u·:Lng the original cross sect:Lon topographic :LnfoFmation, where possible,
and the d~scharges contained ~Ln the F~lood Insurance $~udy which
established the original floodway. The model Bust use the same effective
flow areas &s est&bi,shed in the original analysis and be ca~brated to
reproduce t. he original base flood elevations within O.1 fc~t. Where
reproduction of the original base flood elevations within 0.1 foot is not
possible, or results in unsound engineering practices, the con~nit¥
should contact the appropriate FE~ Regional Office for direction.
After the model has been checked and matches the original base /lood
slevations, the model can be Bodified for new floodway runs. The
analysis will be accomplished as specified for the following basis for
revisions
.Dischar~e Decreases
IPloodways may be revised when m base flood discharge decreases as a
result of structural improvements, such as the construction of a flood
control dam or other significant retention facilities. Floodways should
not be revised based o~ · computed discharge reduction which results
changed methodology or longer stress gage records, unless ~.he change is
statistically significant. The statistical significance criteria are
discussed in Section 2-6 'Hydrologic Analyses' of FE~A's Guidelines and
Specifications for Study Contractors (dated Septenber 1982). IA' cases
where the statistical significance test does not apply, a deterninat.ion
by a state or Federal agency that the change is significant may be
accepted as a basis for a floodway revision. If evident that the change
is statistically significant, the original hydraulic model should be
rerun, changing only the discharges. This will serve as the base run for
the subsequent floodway encroachment runs. The floodway model should
then be modified · to include current conditions, as ~ell as any
encroachment that may have occurred since the original model was
produced. The resulting floodway elevations must not exceed the base run
elevations by more than 1.0 foot, or as specified by the state
connunity.
.Discharge Increases
If watershed conditions have resulted in a significant increase in
discharge, the original hydraulic model needs to be run with the
increased discharges* The resulting base flood profile will serve as the
base for the floodway runs. The floodway runs must include any
encroachment that may have occurred since the delineation of the original
floodway.
Channel Modification
The original model will be modified to include the channel modification,
and any other encroachment occurring subsequent to the original floodway
delineation* The resulting base .flood profile will serve as the base for
the floodway runs. The floodway runs must include any e~ncroachment that
may have occurred since t. he delineation of the original floodway. The
resulting floodway slevations must not exceed the new base £1ood
elevations or the o~iginal base flood elevations by more than 1.0 foot,
o~ as specified by the state or community.
Social or Economic
Where it is desired to shift the floodway for ~ocial or economic reasons,
the base flood elevations iron the original model ~uld serve as the base
profile for the new floodway run. The model would t. hen be modified to
account for any encroachment in the floc~ plain since the original study.
The new floodway limits are to be set in a manner which will not result
' in an increase to the original base flood elevation in excess of ?.hat
permitted by FE~A or the state.
Condition Number 3'
Sub~nittal includes technical data that enables ~ ~o determine whether
the floodway revision ~eets the FEMA minimum standard. Submittal must
include the following:
3.1 & copy of the printout for the original hydraulic computer model
representing the 100-year floo~ profile run for conditions e~isting
at the time the currently effective floodway was developed. The
printout must include full input and output listings.
3.2 A copy of the printout for the original hydraulic computer ~odel
representing t~he floodway run for conditions under which the
currently effective floodway was developed. The printout
include full input and output listings.
3.3 A copy of the printout from the hydraulic computer model
repres, enting the new 100-year profile. The model should be the
same as that used in item 3.1, but modified to include any channel
auc~ification, fill or other encroachment that may have occurred in.
the flood plain since the original floodway was delineated.
3o 4 & copy of the pr intout from t~e hydraulic computer model
representing the floodway run for the proposed floodway, including
channel ~odifications and encroachments mince the original floodway
was established. The equal conveyance reduction method should be
"used to com~ute tJ~e revised floodway limits unless agreements have
been mede with affected property owners to utilize a different
method. The printout ~ust include full input and output listings
with all input changes from the original model highlighted. The
net effect of the channel ~odifications, ~he encroachments made
s~bsequent to the establishment of the original floodway, and the
new proposed floodway limits must not increase the water surface
elevation by norm than 1.0 foot above~at~ computed in item 3.1,
and al~o ~ust not' exceed l°0 foot above the new 100-year elevations
computed in item 3.3.
3oS A ~ of the revised Floodway Data Table represent'in9 data for the
proposed floodway configuration.
3.6 One copy of the cur:entry effective FBFK shoving the existing
floodway and one copy showing the proposed floodway configurations.
3.7 Certificatim from a registered professional engineer that the
physical parameters uaed in the proposed floodway delineation
represent actual coflditio~s and that Lhe standards contained in
these 'Conditions and Criteria" are Bet.
3.8 If the bas£a for the proposed floodway revision Is a channel
modification, ~he con~letion of a d&u, or any other structural
Be·sure, evidence Is required to demonstrate the design is
adequate, ·nd t~at maintenance ·nd operation provisions, where
applicable, have been Bede. The submittal must tmdicate what
entity has maintenance responsibility and how maintenance will be
accomplished. If maintenance is to be accon~Lished by agencies
other than t~e cosnnunity, · legal provision for community
monitoring and back-up assistance is required.
3.9 Documentation Of approval of the proposed floodway revision from
the appropriate state agenc~f for communities where the 'state
regulates the £1oodway.
3o10 Documentation of any variation frei Condition 2 and/or Condition'3.
This includes approval £rom the appropriate agencies and an
explanation of the reason £or variation.
..FKNA ltes~onse to Floodwa~ Revision Request
IF~4A will review a connunityts request fo: · floodway map revision. It
is recognized that while the floodway is adopted ~ a c~nunity, FLood
Insurance Study users other ,than the community My request copies of the
f~oodvay de~ineations. Therefore, in support of the eonnunit~ and other
users, FE]4A maintains · system fo: distributing floodway information and
revising delineations as the need arises. When the revision is
considered to be significant, F]2SA will - revise floodway maps and
distribute then to recognized, nsers. Significant floodway revisions &re
identified based on the size of ·rea, as yell as the number of interests
affected by the revision.
FZ~A will respond to · request froa the cc~unit~ for · floodway revision
bF one of the following Beans~
Reprinting the affected FB~N panels with corresponding map revised
dates. The panels will be accoa~anied b~ · revised index panel
shoving the BOer up-to-date flocclvay panels minta~ned at Fi24A.
Copies of the revised floodway panels will be sent to previous
recipients of the floodway maps. & revised Flood Insurance Study
report will accompany the floodway maps. Included Ln the report
will be · description of the revision and revised Floodway l~ata
TabLes. .'
(2) Send one ~ of the FKFH designating the approved floodway
revJJlon to the conauni~, and · letter of floodway revision
-stating that the review of the submitted mterial has resulted Ln
the determination that FEHA minLnun requirements have been met but
· reprinting of the FaFH is not warranted at this tine. The
mterial, ~nc~uding the ~ of the FBFP~ shoving ~e rev~s~
fl~wa~ Mp rev~s~. ~e a~rov~ ~ revision ~es
~ed. Since It ~s ~ssi~e ~at r~ ~s res~ w a
revisl~ r~ues~ ~ ~ls ~ne~, all F~ lnsur~ce S~ users
are e~ourag~ ~ ~n~c~ ~ ~nit~ ~fore prodding
p~ans fo~ ~ve~o~n~ v~n ~e f~ fringe ~eas ~ Mar
fl~vays.
(3} bad a let~e~ ~ ~e ~un~y otat~ng ~ ~e sumatra1
F. Subn~tnl of
Submi~ mte~al ~ ~e ~ bgi~al ~fXce. ~e Regional ~ee w~11
tev~ M~er~ for ~pleteness, ~u~ w~ ~e p~o~s~ c~ges,
a~rove ~y ~n~enance ordnances, and forward ~ ~cessary ~al
~e ~ Cen~al off~ce.
~ ~n~al e~ll teviee ~e hydrol~ic~ydrnulic back-up
eng~neerXn9 design, a~ ~y plus o~ ~erat~ ~ ~ntenance (~here
a~ropriate}, request ~diti~l revisits ~ ~rect~s as ~cessary,
~en tes~ ~ ~ ~nX~y by ~s of ~e appropriate act~ defined
u~e~ Sec~ E ~ve.
tev~s~s, ~unXt~es are encoutn9~ ~ ~n~act ~e app~opf~ake
~ql~l ~f~ce ~ ~ Central ~n ~ash~ng~n, D.C. at (202} 287-0230,