ST0701-SY110325 ;;p HALFF
March 25,2011
AVO 25816
FEMA LOMC Clearinghouse
7390 Coca Cola Drive, Suite 204
Hanover,MD 21076
Re: Southwestern Boulevard(LOMR)-Case No.: 11-06-0227P
Dear Ms.Namrata Batra
This letter is in response to your request for additional information for LOMR Case No. 11-06-0227P dated January
6,2011.Each issue that you listed is addressed below and the appropriate documentation is enclosed.
Item#1:
The typical contraction and expansion loss coefficients are equal to 0.3 and 0.5, respectively, at bridge and
culvert Sections 2, 3, and 4, and are equal to 0.1 and 0.3, respectively, at bridge and culvert Sections 1 and 5.
Please revise the submitted hydraulic model so that the contraction and expansion loss coefficients are equal to
0.3 and 0.5,respectively, at Cross-Section 31750, or provide an explanation why the contraction and expansion
loss coefficients used in the model were chosen.
Response to Item#1:
Contraction and expansion coefficients at cross-section 31750 were changed to 0.3 and 0.5,respectively and the
updated HEC-RAS model is included in the attached CD. Also attached is an updated copy of Table 2, 3 and 4
which reflect updated water surface elevation.
Item#2:
Typically, bridge and culvert Sections 1 and 4 are located sufficiently upstream and downstream from the
structure to correspond to where fully expanded flow occurs. Please revise Sections 1 and 4, which occur at
Cross-Section 30195 so that they are fully expanded,or provide an explanation.
Response to Item#2:
Information and location for the Southern Pacific Railroad Bridge(Section 30195)and bounding sections(1, 2,
3 and 4)comes from the effective models. Detailed information can be found in the report on page 2 and 3.
Item#3:
The topwidhs of the base (1-percent-annual-chance) floodplain computed in as-built conditions HECRAS
hydraulic model at the cross-sections listed in the table below do not match the approximate floodplain
topwidths shown on the topographic work map entitled "Southwestern Boulevard Bridge," prepared by Halff
Associates, dated September 2, 2010. Please submit a revised hydraulic analysis or revised work map as
appropriate.
Base Floodplain Topwidth(feet)
Cross-Section Model Map
30201 100 180
30130 235 330
HALFF ASSOCIATES,INC.
1201 N.BOWSER ROAD, TEL(214)346-6200 WWW.HALFF.COM
RICHARDSON,TX 75081 FAX(214)739-0095
::C HALFF
Response to Item#3:
Mapping in this area was adjusted to match model top widths. The revised "As-Built Conditions Floodplain
Workmap"is attached.
Item#4:
Please submit a copy of the public notice distributed by City of Coppell stating their intent to revise the
regulatory floodway, or provide a statement that city officials have notified all affected property owners, in
compliance with Paragraph 65.7 (b)( 1) of the National Flood Insurance Program regulations. These notices
must include the extent of revision, the changes to the regulatory floodway, and contact information for any
interested parties. An example of public notice can be found in Figure 3 of the MT-2 instructions, which may
be accessed at http://www.fema.gov/pdf/fhmlmt2 inst.pdf. Please submit draft version of notification for our
review.
Response to Item#4:
A copy of the public notification for floodway revision is attached.
Item#5:
Please submit documentation of the individual legal notices that were sent to property owners who are affected
by any increases in width and/or shifting of the base floodplain and increases in base flood elevations. Legal
notice may take the form of certified mailing receipts or certification that all property owners have been
notified, with an accompanying mailing list and a copy of letter sent. Example of individual notice can be
found on page 17 of the instructions for the MT-2 application forms. Please submit draft version of notification
for our review.
Response to Item#5:
Copies of the individual property owner notifications with the return receipts are attached.
Note:A pdf version of the revised report is included in the attached CD.
If you have any questions or need additional information,please do not hesitate to contact me at(214)217-6650
Sincerely,
HALFF ASSOC •TES,INC.
aria C Mart ez, '. , -- •
Water Resources Engineer
Cc:Ken Griffin,P.E.,CFM
Director of Engineering&Public Works
City of Coppell
Attachment:
Revised"As-Built Conditions Floodplain Workmap"
Revised"Annotated FIRM"
Revised Tables: 2,3 and 4
Copy of public notification for floodway revision
Copy of the individual property owner notifications with the return receipts.
CD :revised HEC-RAS model,GIS revised files and a pdf version of the revised report.
HALFF ASSOCIATES,INC.
1201 N.BOWSER ROAD, TEL(214)346-6200 WWW.HALFF.COM
RICHARDSON,TX 75081 FAX(214)739-0095
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Table 2-Computed Water Surface Elevation(CWSEL)Comparison Table-REVISED
(1%Annual Chance Storm Event)
Grapevine Creek
River Station(RS) CWSEL Feet
( ) Difference
1 8 g
g '.
Duplicate Corrected LOMR Descriptions Duplicate Effective d a Corrected Effective LOMR(AS-Built) LOMR(AS-Built)
Reach HEC-2:211 ProM.out Corrected Effective A LOMR(As-Built) -
Effective Effective (As-Built) 6 HEC-RAS.SW ph(p04) HEC-RAS.SW ph(p06) ' Less Less Less
HEC-RAS:7026.prj(08) @ E E Duplicate Effective Duplicate Effective Corrected Effective
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Table 3-Computed Water Surface Elevation(CWSEL)Comparison Table-REVISED
(0.2%Annual Chance Storm Event)
Grapevine Creek
River Station(RS) CINSEL(Feet)
Difference
•
Duplicate Corrected LOMR Descriptions Duplicate Effective $ $ $ Corrected Effective LOMR(AS-Built) LOMR(AS-Built)
Reach HEC-2:211 ProM.out Corrected Effective - LOMR(As-Built)
Effective Effective (As-Built) eg HEC-RAS:SW.prj(p04) HEC-RAS:SW.prj(p06) Lees Less Less
HEC-RAS'.7026.prj(p08) g Duplicate Effective Duplicate Effective Corrected Effective
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31750 31750 31750 Reach-1 Upstream Limit 509.6 511.8 510 2.2 0.4 -1.8
31675 31675 Reach-1 Added cross-section - 511.8 509.9 - - -1.9
31550 31550 Reach-1 -- 509.5 511.5 - 2.0
31500 Reach-1 -- 507.8 -
31490 31510 31522 Southwestern Boulevard Bridge • Bridge Bridge - - -
31471 31471 Reach-1 Added cross-section - 508.7 _ 508.5 - - -0.2
31468.25 31468.25 Reach-1 Utility crossing - Bridge Bridge - - -
31480 507.7 _ -
- - -
31451 31451 Reach-1 Added cross-section - 508.5 508.5 - - 0
31310 31310 31310 Reach-1 -- 507.7 • 508 508 0.3 0.3 0
30250 30250 30250 Reach-1 -- 505.8 5064 506.4 0.6 as 0
30201 30201 30201 Reach-1 -- 504.2 504.8 504.8 0.6 0.6 0
30195 30195 30195 Reach-1 Southwestern Pacific Railway Bridge Bridge Bridge - - -
30189 30189 30189 Reach-1 -- 503.7 503.7 5031 0.0 0.0 0
30130 30130 30130 Reach-1 Downstream Limit 503.7 503.7 _ 503.7 0.0 0.0 0
Note:Duplicate effective computed water surface elevations from cross-section 31750 to cross-section 30189 come from HEC-2 model provided by FEMA
Duplicate effective computed water surface elevation for cross-section 30130 comes from HEC-RAS model provided by Nathan D.Maier Engineers.
I,\25000sh258i6\f-4It\LOMR\report\Revised LOMR _documents_2011\Table 3_LOMR_CWSEL Comp_500yr_rev_015 Table 3
3/23/2011 6:27 PM CWSEL-0.2%Annual Chance Storm Event Page 7
Table 4-Floodway Data Comparisons-REVISED
With 1%Annual Chance Computed Water Surface Elevation(CWSEL)
Grapevine Creek
Duplicate Effective Corrected Effective As-Built LOMR)
River Station(RS) HEC-2:211PROFW.out Difference Difference ( Difference
HEC-RAS:7026.prj(p11) HEC-RAS.SW.prj(p03) HEC-RAS:SW.prj(p01)
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Duplicate Corrected LOMR CWSEL a CWSEL with g a° CWSEL with a° a° CWSEL with a°
Reach n 1 Delta CWSEL CWSEL - a Delta CWSEL CWSEL A �, Delta CWSEL
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31750 31750 31750 Reach-1 Upstream limit 507.8 508.3 0.5 509.47 509.83 0.36 508.3 508.61 0.31
31675 31675 Reach-1 Added cross-section - - 509.43 509.79 0.36 508.24 508.55 0.31
31550 31550 Reach-1 -- 507.5 507.9 0.4 509.03 509.46 0.43 510.51 510.66 0.15
31500 Reach-1 -- 506.6 507.1 0.5 510.51 • 510.66 * 0.15
31490 31510 31522 Southwestern Boulevard Bridge Bridge - Bridge Bridge - Bridge Bridge -
- - - - - 510.51 510.66 0.15
31471 31471 Reach-1 Added cross-section - - - 507.16 507.54 0.38 507.06 507.52 0.46
31468.25 31468.25 Reach-1 Utility crossing - - - Bridge Bridge - Bridge Bridge
31480 506.4 506.9 0.5 - - - 510.51 510.66 0.15
31451 31451 Reach-1 Added cross-section - 507.11 507.53 0.42 507.06 507.52 0.46
31310 31310 31310 Reach-1 - 506.2 • 506.6 • 0.4 506.38 506.45 0.07 506.38 506.45 0.07
30250 30250 30250 Reach-1 - 504.0 - 504.4 0.4 504.06 504.05 -0.01 504.06 504.05 -0.01
30201 30201 30201 Reach-1 -- 503.0 503.6 0.6 503.06 503.04 -0.02 503.06 503.04 -0.02
30195 30195 30195 Reach-1 Southwestern Pacific Railway Bridge Bridge - Bridge Bridge - Bridge Bridge
30189 30189 30189 Reach-1 - 502.1 502.9 0.8 502.24 502.21 -0.03 502.24 502.21 -0.03
30130 - 30130 30130 Reach-1 Downstream Limit 502.1 502.1 0 502.05 502.07 0.02 502.05 502.07 0.02
Note:Duplicate effective computed water surface elevations from cross-section 31750 to cross-section 30189 come from HEC-2 model provided by FEMA
Duplicate effective computed water surface elevation for cross-section 30130 comes from HEC-RAS model provided by Nathan D.Maier Engineers.
•
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I:\25000s\25816\HH\LOMR\report\Revised_documents_2011\Table 4_Floodway_Table_rev_2decimals_.xls Table 4
3/23/2011 6:28 PM Floodway-1%Annual Chance Flood Event Page 8
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j LEGAL NOTICE
PUBUC NOTIFICATION FOR Pacific Railroad. As a result of the
FLOODWAY REVISION floodway revision, the floodway
It, r t shall narrow with a maximum nar-
.7 The City of Coppell Engineering rowing of approximately 260 feet at 1
5 Department, in accordance with a point approximately 180 feet north
National Flood Insurance Program of Southwestern Boulevard. There
i '" egulation 65.7(b)(1). herby gives is no widening of the existing flood- a
notice of the property owners along way due to this revision.
L.: the the Grapevine Creek intent to re-
' - 1_ vise the floodway,generally located Maps and detailed analysis of the
between Southwestern Boulevard floodway revision can be reviewed
" and Southern Pacific Railroad. at the City of Coppell Engineering
#1 -- Specifically, the floodway shall be Department. Interested persons
" -_, '{ revised from a.point of appro xi. riaay call Mr.Ken Griffin,RE_at 972-
mately 250 feet south of°nuthwest- 304-3686 for additional inform a- i
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Map Document: (I: \25000s\ 25816\ Flhi\ LOMR\ ExhibitslExlribit _4_Floodplain_Workmap_rev.mxd)
ILegend
Cross Sections
Grapevine Creek Centerline
A As -Built Floodway
As -Built 100 -YR Floodplain
As -Built 500 -YR Floodplain
Floodway - Revised Preliminary DFIRM
100 -YR Floodplain - Revised Preliminary DFIRM
} 500 -YR Floodplain - Revised Preliminary DFIRM
FIRM 100 -YR Floodplain
FIRM 500 -YR Floodplain
FIRM Floodway
NCTCOG 2001 Contours
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Map Document: (I: \25000s\ 25816\ Flhi\ LOMR\ ExhibitslExlribit _4_Floodplain_Workmap_rev.mxd)
ILegend
Cross Sections
Grapevine Creek Centerline
A As -Built Floodway
As -Built 100 -YR Floodplain
As -Built 500 -YR Floodplain
Floodway - Revised Preliminary DFIRM
100 -YR Floodplain - Revised Preliminary DFIRM
} 500 -YR Floodplain - Revised Preliminary DFIRM
FIRM 100 -YR Floodplain
FIRM 500 -YR Floodplain
FIRM Floodway
NCTCOG 2001 Contours
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