Item 6C - Floodplain Storage ModificationsKimley>»Horn
March 22" d, 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 48439CO085K 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
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Kimley>»Horn
Page 2
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, RE, 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
♦ ♦ i
GRAPHIC SCALE IN FEET
0 10 20 40
PRE -PROJECT
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PROPERTY BOUNDARY
555 1-FOOT ON GROUND TOPOGRAPHY (ADAMS
SURVEYING COMPANY, 2019 & AJ BEDFORD
GROUP, 2011)
555 1-FOOT PROPOSED CONTOURS
(KIMLEY-HORN, 2021)
EXISTING ON -SITE 100-YEAR FEMA
FLOODPLAIN
PROPOSED ON -SITE 100-YEAR FEMA
FLOODPLAIN
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FOR REVIEW ONLY
Not for construction or permit purposes.
Kimleyl>Horn
Engineer CAROLYN COX
P.E. No. 122243 Date FEB 2020
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SHEET NUMBER
EXH
National Flood Hazard Layer FIRMette .., FEMA
Legend
97o8'57"W 32o57'42"N
0 250 500 1,000 1,500 2,000
Basemap: USGS National Map: Orthoimagery. Data refreshed October, 2020
SEE FIS REPORT FOR DETAILED LEGEND AND INDEX MAP FOR FIRM PANEL LAYOUT
Without Base Flood Elevation (BFE)
Zone A, V, A99
SPECIAL FLOOD With BFE or Depth Zone AE, AO, AH, VE, AR
HAZARD AREAS 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 mile zone x
_Future Conditions 1%Annual
Chance Flood Hazard zone x
Area with Reduced Flood Risk due to
OTHER AREAS OF Levee. See Notes. zone X
FLOOD HAZARD ��� Area with Flood Risk due to Leveezone D
NO SCREEN Area of Minimal Flood Hazard zonex
Q Effective LOMRs
OTHER AREAS Area of Undetermined Flood Hazard zone
GENERAL -- - - Channel, Culvert, or Storm Sewer
STRUCTURES IIIIIII Levee, Dike, or Floodwall
V Cross Sections with 1% Annual Chance
17•5 Water Surface Elevation
a - - - Coastal Transect
—si3 - Base Flood Elevation Line (BFE)
Limit of Study
Jurisdiction Boundary
— --- Coastal Transect Baseline
OTHER
_ Profile Baseline
FEATURES
Hydrographic Feature
Digital Data Available N
No Digital Data Available
MAP PANELS
Unmapped
QThe
pin displayed on the map is an approximate
point selected by the user and does not represent
an authoritative property location.
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.
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STREAM DISTANCE IN FEET FROM ESTIMATED FLOW LINE OF DENTON CREEK -GRAPEVINE LAKE
179P
TABLE 4 - SUMMARY OF DISCHARGES (continued)
PEAK DISCHARGES (cfs)
FLOODING SOURCE DRAINAG 10% 2% 1% 0.2%
AND LOCATION E AREA Annual Annual Annual Annual
(sq. mile Chance Chance Chance Chance
Redelineation Detailed Study Streams
NORTH FORK OF DEER CREEK
Just upstream of Burlington Northern and
Santa Fe Railroad
0.65
SOUTH FORK OF DEER CREEK
At confluence with Deer Creek
4.32
Downstream of confluence of North
Tributary of South Fork of
4.19
Deer Creek
Upstream of confluence of North
Tributary of South Fork of
3.61
Deer Creek
SOUTH FORK OF NORTH BRANCH OF DEER
CREEK
At confluence with North Branch of
2.82
Deer Creek
DENTON CREEK
At Tarrant County boundary'
3.96
Below Tributary D-1'
3.615
Above Tributary D-14
2.425
Above Tributary D-24
1.60
Above Grapevine Lake outlet channel
1.19
DOVE CREEK
At Grapevine Lake
3.22
At Dove Street
2.14
At Carroll Avenue
1.51
DUTCH BRANCH
At Benbrook Lake
7.30
Approximately 1.52 miles upstream of
Benbrook Lake
4.90
TRIBUTARY DB-3
Approximately 480 feet upstream of
the confluence with Dutch Branch
0.52
EDGECLIFF BRANCH
Above its confluence with
Sycamore Creek
6.52
Downstream of left bank tributary located
east of Chelsea Drive
5.92
Upstream of left bank tributary located
east of Chelsea Drive
5.04
700
5,100
7,000
7,800
9,900
5,400
7,400
8,300
10,500
4,600
6,300
7,100
9,000
3,750
5,100
5,700
7,200
6,000
8,300
9,400
36,200
5,900
8,000
9,400
36,200
4,050
5,500
9,400
36,200
2,550
3,500
3,900
4,900
2,050
2,800
3,100
3,900
2,650
3,750
4,250
5,500
2,150
3,000
3,400
4,350
1,850
2,600
2,900
3,700
6,900
9,500
10,900
13,800
5,700
7,900
9,000
11,300
917 1,366 1,479 2,059
5,800
8,250
9,300
11,400
5,750
8,250
9,300
11,850
5,050
7,250
8,150
10,400
79