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Item 6 - Floodplain Storage Modifications (3) Kimley>>> Horn March 22", 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 :+FEA:_F f11NI1aAL aYI HA -.C: f1 rt-u.0 fOC�71.• ' 6160 Warren - 210, Frisco, TX 75034 :� Kim ey>>> 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(a-)-kimley-horn.com. Sincerely, C&74 W 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 kimley-horn.com6160 Warren Parkway, Suite 210, Frisco,TX 75034 • :1 Page is too large to OCR. National Flood Hazard Layer FI RMette = FEMA Legend 97°8'57"W 32°57'42"N SEE FIS REPORT FOR DETAILED LEGEND AND INDEX MAP FOR FIRM PANEL LAYOUT r Without Base Flood Elevation(BFE) # PF V Zone A,V,A99 SPECIAL FLOOD With BFE or Depth Zone AE,AO,AH.VE.AR P 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 Zonex FutureChance Conditions Annual nceFl odHazardZoeX Area with Reduced Flood Risk due to OTHER AREAS OF Levee.See Notes.zone x FLOOD HAZARD �j Area with Flood Risk due to Leveezone D If NO SCREEN Area of Minimal Flood Hazard Zonex LOM el 7V6_4776 o . f7$ � Effective LOMRs P OTHER AREAS Area of Undetermined Flood Hazard Zone AREA DF N11NIm FL'ODD H_ A7JkRD _ _ GENERAL - -- Channel,Culvert,or Storm Sewer Q0O' w w, STRUCTURES I I I I I I I Levee,Dike,or Floodwall LLI LL i + ` * to Cross Sections with 1%Annual Chance 17.5 Water Surface Elevation 1'1"Y F, OUTHT.A E; a- - - Coastal Transact ' ..«..51j— Base Flood Elevation Line(BFE) 80 12 AO. � Limit of Study Jurisdiction Boundary �{ —--- Coastal Transact Baseline OTHER►, r Zone 4 _ FEATURES Profile Baseline Hydrographic Feature FOWDII4NA`f } r f Z-O Digital Data Available N • No Digital Data Available MAP PANELS Unmapped 14 7 The 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 Zone 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, 97°8'20"W 32°57'12"N FIRM panel number,and FIRM effective date.Map images for 25� 5�� ,�� ,5�� 2,��et 1-6 000 unmapped and unmodernized areas cannot be used for regulatory purposes. Basemap:USGS National Map:Orthoimagery.Data refreshed October,2020 a x w J Y LL LLJ Q W � U Lu 0 � J LL co co 0 z 615 615 z Lu w J- z z 0 a 610 610 JI Bill W V 605 605 Lu c-0 a x z cn Lu a Lu a 600 600 cD LEGEND z = o 0.2%ANNUAL CHANCE FLOOD Q cm Q U o ------- 1%ANNUAL CHANCE FLOOD U —595 2%ANNUAL CHANCE FLOOD z z Lu Q z ————————- 10%ANNUAL CHANCE FLOOD C_ OC Lu z STREAM BED Q Q Lu G CROSS SECTION LOCATION Q w 20500 21000 21500 22000 22500 23000 Lu 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 101%, 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 0.65 ** ** 700 ** Santa Fe Railroad 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 4.19 5,400 7,400 8,300 10,500 Deer Creek Upstream of confluence of North Tributary of South Fork of 3.61 4,600 6,300 7,100 9,000 Deer Creek SOUTH FORK OF NORTH BRANCH OF DEER CREEK At confluence with North Branch of 2.82 3,750 5,100 5,700 7,200 Deer Creek DENTON CREEK At Tarrant County boundary' 3.96 6,000 8,300 9,400 36,200 Below Tributary D-l' 3.61' 5,900 8,000 9,400 36,200 Above Tributary D-1' 2.42' 4,050 5,500 9,400 36,200 Above Tributary D-2' 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 1,850 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 4.90 5,700 7,900 9,000 11,300 Benbrook Lake TRIBUTARY DB-3 Approximately 480 feet upstream of 0.52 917 1,366 1,479 2,059 the confluence with Dutch Branch EDGECLIFF BRANCH Above its confluence with 6.52 5,800 8,250 9,300 11,400 Sycamore Creek Downstream of left bank tributary located 5.92 5,750 8,250 9,300 11,850 east of Chelsea Drive Upstream of left bank tributary located 5.04 5,050 7,250 8,150 10,400 east of Chelsea Drive 79