Item 6A - Floodplain Storage Modificationskimley-horn.com 6160 Warren Parkway, Suite 210, Frisco, TX 75034 972 335 3580
March 22nd, 2021
Sandra Endy or Kevin Ferrer, P.E., CFM
City of Southlake
1400 Main Street, Suite 320
Southlake, Texas 76092
RE:Methodist Southlake Hospital
421 E State Hwy 114 – Floodplain Storage Modifications
Southlake, Tarrant County, Texas
To Whom it May Concern,
This letter summarizes the Floodplain Storage Modifications proposed by Kimley-Horn and Associates,
Inc. (“Kimley-Horn”) for Methodist Southlake Hospital improvements. Methodist Southlake Hospital
(“subject property”) is an approximately 13-acre parcel with existing healthcare facilities on the property.
The existing underground detention is proposed to be replaced with an above ground detention pond.
The detention pond was sized using modified rational method using the information from the record
drawings. The detention pond outfall was designed to account for the tailwater from Dove Creek using
a coincidental peak based on watershed size. Detention calculations are included with the construction
plans.
The project proposes cut and fill within the FEMA Effective Zone AE Floodplain at the proposed
detention pond. The site outfalls into Dove Creek and is identified on FEMA’s Flood Insurance Rate
Map Panel 48439C0085K and has a Zone AE floodplain designation assigned. See Exhibit 1 below for
location of the effective floodplain relative to the subject property.
Exhibit 1: FEMA Effective Floodplain
Page 2
kimley-horn.com 6160 Warren Parkway, Suite 210, Frisco, TX 75034 972 335 3580
The proposed grading for the detention pond will be located at the upstream face of the State Hwy 114
frontage road within the ineffective flow area associated with the culvert crossing of Dove Creek to the
east. Floodplain storage within the Dove Creek 100-YR FEMA floodplain was evaluated to ensure
floodplain storage would not be decreased as a result of the detention pond improvements. A floodplain
storage surface comparison was performed in AutoCAD Civil 3D using the 100-Year FEMA water
surface elevation. The total amount of pre-project floodplain storage on the subject property (0.61 ac-
ft) and post-project floodplain storage (1.98 ac-ft) was calculated in AutoCAD and used to determine
the net change in floodplain storage (+1.37 ac-ft) due to proposed grading. Proposed grading for the
detention pond can be seen on the attached Floodplain Storage Workmap.
Table 1: Floodplain Storage Comparison (ac-ft)
Volume (ac-ft)
Pre-Project 0.61
Post-Project 1.98
Net Floodplain Storage Gain 1.37
Table 1 above shows that there is not a decrease in 100-Year floodplain storage with the proposed
improvements within the subject property. If you have any questions or comments on the contents of
this report, please contact me at (972) 770-1399 or carolyn.cox@kimley-horn.com.
Sincerely,
Carolyn Cox, P.E., CFM
Cc: Caitlin Flaherty, P.E, CFM. – Kimley-Horn and Associates, Inc.
Eric Mueller, EIT – Kimley-Horn and Associates, Inc.
Attachments:1 – Floodplain Storage Workmap
2 – FEMA Firmette
3 – FEMA FIS Excerpts
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Not for construction or permit purposes.
FOR REVIEW ONLY
Engineer
P.E. No.Date FEB 2020122243
CAROLYN COX
FL
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NORTH
PRE-PROJECT POST-PROJECT
EXH
LEGEND
PROPERTY BOUNDARY
1-FOOT ON GROUND TOPOGRAPHY (ADAMS
SURVEYING COMPANY, 2019 & AJ BEDFORD
GROUP, 2011)
1-FOOT PROPOSED CONTOURS
(KIMLEY-HORN, 2021)
EXISTING ON-SITE 100-YEAR FEMA
FLOODPLAIN
PROPOSED ON-SITE 100-YEAR FEMA
FLOODPLAIN
555
555
National Flood Hazard Layer FIRMette
0 500 1,000 1,500 2,000250
Feet
Ü
SEE FIS REPORT FOR DETAILED LEGEND AND INDEX MAP FOR FIRM PANEL LAYOUT
SPECIAL FLOOD
HAZARD AREAS
Without Base Flood Elevation (BFE)
Zone A, V, A99
With BFE or DepthZone AE, AO, AH, VE, AR
Regulatory Floodway
0.2% Annual Chance Flood Hazard, Areas
of 1% annual chance flood with average
depth less than one foot or with drainage
areas of less than one square mileZone X
Future Conditions 1% Annual
Chance Flood HazardZone X
Area with Reduced Flood Risk due to
Levee. See Notes.Zone X
Area with Flood Risk due to LeveeZone D
NO SCREEN Area of Minimal Flood Hazard Zone X
Area of Undetermined Flood HazardZone D
Channel, Culvert, or Storm Sewer
Levee, Dike, or Floodwall
Cross Sections with 1% Annual Chance
17.5 Water Surface Elevation
Coastal Transect
Coastal Transect Baseline
Profile Baseline
Hydrographic Feature
Base Flood Elevation Line (BFE)
Effective LOMRs
Limit of Study
Jurisdiction Boundary
Digital Data Available
No Digital Data Available
Unmapped
This map complies with FEMA's standards for the use of
digital flood maps if it is not void as described below.
The basemap shown complies with FEMA's basemap
accuracy standards
The flood hazard information is derived directly from the
authoritative NFHL web services provided by FEMA. This map
was exported on 2/28/2021 at 3:31 PM and does not
reflect changes or amendments subsequent to this date and
time. The NFHL and effective information may change or
become superseded by new data over time.
This map image is void if the one or more of the following map
elements do not appear: basemap imagery, flood zone labels,
legend, scale bar, map creation date, community identifiers,
FIRM panel number, and FIRM effective date. Map images for
unmapped and unmodernized areas cannot be used for
regulatory purposes.
Legend
OTHER AREAS OF
FLOOD HAZARD
OTHER AREAS
GENERAL
STRUCTURES
OTHER
FEATURES
MAP PANELS
8
B 20.2
The pin displayed on the map is an approximate
point selected by the user and does not represent
an authoritative property location.
1:6,000
97°8'57"W 32°57'42"N
97°8'20"W 32°57'12"N
Basemap: USGS National Map: Orthoimagery: Data refreshed October, 2020
F L O O D P R O F I L E S
A N D I N C O R P O R A T E D A R E A S
F E D E R A L E M E R G E N C Y M A N A G E M E N T A G E N C Y
LEGEND
0.2% ANNUAL CHANCE FLOOD
CROSS SECTION LOCATION
STREAM BED
T A R R A N T C O U N T Y , T X
179PSTREAM DISTANCE IN FEET FROM ESTIMATED FLOW LINE OF DENTON CREEK-GRAPEVINE LAKE
D O V E C R E E K
615
610
605
600
595
230002250022000215002100020500
E L E V A T I O N I N F E E T (N A V D 8 8 )
1% ANNUAL CHANCE FLOOD
2% ANNUAL CHANCE FLOOD
10% ANNUAL CHANCE FLOOD
F G
615
610
605
600
CULVERT
CITY OF SOUTHLAKE
L I M I T O F D E T A I L E D S T U D Y
W E S T H I G H L A N D S T R E E T
S T A T E H I G H W A Y 1 1 4 S O U T H
B O U N D (N O R T H W E S T P A R K W A Y )
S T A T E H I G H W A Y 1 1 4 N O R T H
B O U N D (N O R T H W E S T P A R K W A Y )
S T A T E H I G H W A Y 1 1 4
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79
TABLE 4 – SUMMARY OF DISCHARGES (continued)
PEAK DISCHARGES (cfs)
FLOODING SOURCE
AND LOCATION
DRAINAG
E AREA
(sq. mile)
10%
Annual
Chance
2%
Annual
Chance
1%
Annual
Chance
0.2%
Annual
Chance
Redelineation Detailed Study Streams
NORTH FORK OF DEER CREEK
Just upstream of Burlington Northern and
Santa Fe Railroad 0.65 ** ** 700 **
SOUTH FORK OF DEER CREEK
At confluence with Deer Creek 4.32 5,100 7,000 7,800 9,900
Downstream of confluence of North
Tributary of South Fork of
Deer Creek
4.19 5,400 7,400 8,300 10,500
Upstream of confluence of North
Tributary of South Fork of
Deer Creek
3.61 4,600 6,300 7,100 9,000
SOUTH FORK OF NORTH BRANCH OF DEER CREEK
At confluence with North Branch of
Deer Creek 2.82 3,750 5,100 5,700 7,200
DENTON CREEK
At Tarrant County boundary4 3.96 6,000 8,300 9,400 36,200
Below Tributary D-l4 3.615 5,900 8,000 9,400 36,200
Above Tributary D-14 2.425 4,050 5,500 9,400 36,200
Above Tributary D-24 1.60 2,550 3,500 3,900 4,900
Above Grapevine Lake outlet channel 1.19 2,050 2,800 3,100 3,900
DOVE CREEK
At Grapevine Lake 3.22 2,650 3,750 4,250 5,500
At Dove Street 2.14 2,150 3,000 3,400 4,350
At Carroll Avenue 1.51 1,850 2,600 2,900 3,700
DUTCH BRANCH
At Benbrook Lake 7.30 6,900 9,500 10,900 13,800
Approximately 1.52 miles upstream of
Benbrook Lake 4.90 5,700 7,900 9,000 11,300
TRIBUTARY DB-3
Approximately 480 feet upstream of
the confluence with Dutch Branch 0.52 917 1,366 1,479 2,059
EDGECLIFF BRANCH
Above its confluence with
Sycamore Creek 6.52 5,800 8,250 9,300 11,400
Downstream of left bank tributary located
east of Chelsea Drive 5.92 5,750 8,250 9,300 11,850
Upstream of left bank tributary located
east of Chelsea Drive 5.04 5,050 7,250 8,150 10,400