Item 6C - Applicant's presentationrmg
Methodist Southlake
Hospital
Detention Basin Replacement Project
City Council Presentation
ZA21-0016
June 15, 2021
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Methodist
SOUTHLAKE HOSPITAL
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Objective & History
■ Seeking approval of Revised Site Plan to undertake replacement of the existing
underground detention system installed in 2013.
■ Originally planned and approved circa 2009 as a surface detention basin.
■ Developer chose to construct underground detention system circa 2011 to address
additional parking structure to be constructed in Phase 1.
■ Underground drainage system proposed by Contractor and Civil Engineer and
presented to City of Southlake by ADS StormTech (mfg.) as a viable underground
detention system.
■ Use of system approved by City of Southlake and formally accepted in January 2014
as an alternative to standard above ground detention.
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Methodist
SOLITHLAKE HOSPITAL
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Methodist
5OUTHLAKE HOSPITAL
Previously Approved Site Plan — Surface Detention
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SOUTHLAKE HOSPITAL
Existing Underground Detention — Plan
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Problems & Investigations
■ Existing underground detention system has suffered a failure.
Bed 3 and Bed 4 of the 4-bed system affected.
■ Damage includes:
Settlement and cracking of Fire Lane and ground above.
Buckled, wracked and breached chambers.
Trapped water beneath impermeable layer and underdrain displacement.
■ Investigations determined the following:
Complete removal required. Replacement with similar system not feasible.
Future development adversely impacted.
Replacement with surface detention system best solution possible. Presents
least possibility of future issues.
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SOUTHLAKE HOSPITAL
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rmg Solution
■ Proposed solution includes:
Removal of existing system, design and installation of new system.
Installation of new pipe, gravity stone retaining wall and safety fence.
Installation of new landscape per buffer yard requirements.
Installation of enhanced landscape features for screening.
Repair of affected fire lanes, landscape and irrigation.
■ Proposed solution conforms to approved Concept Plan and is permitted by current
zoning SP2.
■ Proposed solution improves existing area, enhances views from Highway and
screens views into basin structure.
■ City requires
detention but City's Retention &
Detention policy
allows the
developer to
choose the type of detention or
retention system
implemented.
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Methodist
SOUTHLAKE HOSPITAL
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VICINITY MAP
for •
F"LY
SINGLE FAMILY ISF, a1
.V.D.:
MEDIUM DENSRY
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Methodist po
SOUTHLAKE HOSPITAL
Approved Concept Plan
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SOUTHLAKE HOSPITAL
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Concept Plan — Existing Conditions
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SOUTHLAKE HOSPITAL
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Concept Plan — Master Plan Conditions
New Roedway Conllgurat:on j
E HIGHLAND STp
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Sr"�re
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5OUTHLAKE HOSPITAL
Concept Plan - Enlarged
�amoo
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\/ - PONDA -
PRO POBED DETENTION POND '
/ 100-YEAR WSEL�1625
PROP. RETAINING WALL.
STRUCTURAL PLANS
FOR DETAILS
IOU YEAR WATER SURFACE
2' FREEBOARD ELEVATION = 616.25
PROP. OUTFALL
STRUCTURE,
REF. DETAIL -THIS
SHEET —
AFL = 606.60
SECTION D-B
STA. O-W TO STA. 2+49
m
POND OUTFALL JUNCTION BOX DETAILS
W8E4wrrt-615.]2 �` 81525
CPEUIUG ax � .814.OD
--_--_-. WSEL,_-613.611 I I
WSFLN.e612.850
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6'f- CITY OF S W THI.AKE
L,J�_ i MANHDLE q
60650
FRONT VIEW TOPVIEW
---' WEIR CALCULATIONS: ORIFICE CALCULATIONS:
O-C.A.I2.amfe
C=B.D C=0.6
g=32.2 FTN
OPENING 01 OPENING p2
A=3.33 FN A-0.72 FT'
INVERT ELEVATION-606.50 INVERTELEVATION=614.00
H=WSEL DMNSTREAMHGL H=WSEL-CENTRDID
>rma MEL— =612E5 DOWNSTREAM HGL -612,83
WSEL -51360 DOWNSTREAMHGLi-612.9D
WSEL -61S.72 DOWNSTREAM HGL =51320
M.
NOTE: IF THE DOWNSTREAM HGL 15 GREATER THAN THE CENTROIDTHEN THE
COWNSTREAM HGL WAS USED IN THE ORIFICE CALCULAnCN.
C=3.0
8.0
EMERGENCY SPILLWAY DETAIL
NDFE: IF THE PRIMARY SPI LLWAY BECOM ES OBSTRUCTED, EMERGENCY OVERFLOW
WILL BEGIN AT ELEVATION 61 S.50, USING THE WEIR EQUATION AND THE PARAMETERS
LI5TED BELOW, AN EMERGENCY OVERFLOW LENGTH OF 18.0 FEET IS NECESSARY TO
CONVEYTHE UNDETAINE , 1I PEAK FLOW. _
20%y Jp3y5P�'�
r 6.0'
PILOT CHANNEL
VARIABLE ®'PILOT VARIABLE
PROP. RETAINING WIDTH C IN L WIDTH PROP. RETAINING
WALL VP1L
100-YEAR LASE:
616.25
VARIABLE SLOPE *VARIABLE SLGPE
5EE PILOT CHANNEL DETAIL
secn. N A A
C-103
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SOUTHLAKE HOSPITAL
Concept Plan - Landscape
PLANT MtEGLLE
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Grasses :
2' — 8' Heights
2' — 8' Widths
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SOUTHLAKE HOSPITAL
Landscape Material
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Shrubbery
2' — 5' Heights
.:' 3' —10' Widths
,v�=. 8' — 50' Canopies
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SOUTHLAKE HOSPITAL
Retaining Wall Material — Stacked Stone
Sandstone - Random Ashlar Pattern
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SOUTHLAKE HOSPITAL
Detention Basin Elevation/Section Study
620
A03' from Basin
ST�TF
r14 o sls
252from Basin
197' trom Basin
` 614 %;N
226' from Basin
613
115'irom Basin •
EXIST, GRADF-- -
614.2 9
Z, -? I i 512
i ` 70P OF BANK: 193' from Basin
6
I # TOM OFF
BOTBASIN EXIST- GRADE:
'�, ♦ 657 - 612
4 EXIST. GRADE: 70P OF BANK:
6152 - 617.25
.anti W
] TOP DF BANK' M'�
// t 617,25 MFt
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SOUTHLAKE HOSPITAL
Detention Basin Section A -A
1 114
DISTANCE FROM FRONTAGE RD,
Exienng Plantirgs
—E State Highway 114 Pedeslhan Sight Line
Existing Trail Vehicular Sight Line
Properly Line y„
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Land—pe
Plantings
Section A -A
MSH - Southlake Detention Basin GRAPHIC SCALE IN FEET
Methodist Southlake Hospital, 421 E. State Hwy 114 0 5 10 20
Southlake, TX 76092
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Key Map
xi9ting Tree oft Property
Proposed Retaining Wall
Pond
Guard Rail
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5OUTHLAKE HOSPITAL
Detention Basin Section B-B
Section b-i
MSH - Southlake Detention Basin GRAPHIC SCALE IN FEET
Methodist Southlake Hospital, 421 E. State Hwy 114 0 10 Z0 40
Southlake, TX 76092
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5OUTHLAKE HOSPITAL
Detention Basin Section C-C
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] EKisling Trail- 320' from Basin Rim
e s
4 Property Line
Existing Plamings
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Pedestdan Sight Line
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9 Highland Street n�ewvo s�reeei _
Landscape Screen _
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Key Map
Vehicu€er Sight Line Proposed Retaining Wall
Guard Rail
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Section C-C
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MSH - Southlake Detention Basin FEET
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Methodist Southlake Hospital, 421 E. State Hwy 114 ° ]o zo
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Southlake, TX 76092
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rMg Detention Basin Rendering —View South, Frontage Rd.
Sidewalk
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n w Y. 11-r
View
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SOUTHLAKE HOSPITAL
Rendering, After Improvements
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Detention Basin
Options Study
ZA21-0016
June 10, 2021
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Methodist
SOUTHLAKE HOSPITAL
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Objective / Criteria for Study
■ Address the concerns of Council and Staff by studying City cited examples of
Detention/Retention basins in the Southlake area.
■ Provide a summary of features, elements and/or conditions observed at the cited
locations.
■ Compare and contrast observed features, elements or conditions to proposed
basin.
■ Investigate alternatives to the proposed basin and provide pros and cons of each.
■ Utilizing supportive data, provide recommendation as to the best solution that
complies with applicable codes and ordinances.
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SOLITHLAKE HOSPITAL
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History
■ January 14, 2021: Initial meeting conducted with City Staff to discuss underground
detention system failure and planned solution. Revised Site Plan approval required
by city.
■ March 30, 2021: Meeting with Corridor Planning Committee (CPC) to present and
discuss detention basin solution and revised Site Plan. CPC stated concerns over
views toward detention basin and type of proposed basin.
■ April 12, 2021: Revised Site Plan documents submitted to City addressing CPC view
issues. Original surface basin solution with enhanced landscaping submitted.
■ April 22, 2021: City of Southlake Plannin & Zonin unanimously approves Site Plan
revision incorporating surface detention asin ancl enhanced landscape screening.
■ May 4, 2021: City Council expresses intent to den applicants request for a revised
Site Plan. Applicant advised to work with City Sta f and investigate alternatives to
the proposed basin.
■ May 10, 2021: Applicant and City Staff discuss proposed solution, alternative
systems and existing detention & retention structures to provide guidance on a
solution.
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Methodist
SOUTHLAKE HOSPITAL
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rmg Existing Basins Studied
■ Central Market Detention Basin.
■ Nolan Drive Retention Basin.
■ Southlake Meadows Retention Basin.
■ Southridge Lakes Retention Basin.
■ FM 1709 & State Hwy. 114 Detention Basin.
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SOUTHLAKE HOSPITAL
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SOUTHLAKE HOSPITAL
Central Market Detention Basin
Observed Features, Elements and Conditions
■ Surrounded by streets and parking.
■ Visible from sidewalks, not from street.
■ Stacked Stone Walls approx. 10' high.
■ Concrete drainage structures and pilot
channel.
■ Wrought iron fencing.
■ Landscape obscuring basin from street and
surrounding area.
■ Trees located within basin perimeter.
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Methodist
SOUTHLAKE HOSPITAL
Central Market Detention Basin — Storm Event
Observed Conditions
■ Not visible from street.
■ Water detained with measured release as
designed.
■ Landscape obscuring basin from street and
surrounding area.
■ Trees located within basin perimeter both
water and drought tolerant.
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SOUTHLAKE HOSPITAL
Nolan Drive Retention Basin
Observed Features, Elements and Conditions
■ Surrounded by streets and parking.
■ Pond visible from drive lanes.
■ Stone wall edge with sloped grass freeboard.
■ Concrete drainage and overflow structures.
■ Landscape surrounding basin.
■ Trees located in freeboard area.
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SOUTHLAKE HOSPITAL
Southlake Meadows Drive Retention Basin
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Observed Features, Elements and Conditions
■ Surrounded by streets and sidewalks.
■ Visible from streets and sidewalks.
■ Stacked Cobblestone walls with grass
freeboard area.
■ Concrete drainage and overflow structures.
■ Stone water feature.
■ Wrought iron fencing.
■ Landscape surrounding basin.
■ Trees located in freeboard area.
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SOUTHLAKE HOSPITAL
Southridge Lakes Retention Basins
Observed Features, Elements and Conditions
■ Surrounded by streets and sidewalks.
■ Visible from streets and sidewalks.
■ Natural grass edge with grass freeboard
area.
■ Concrete drainage and overflow structures.
■ Multiple fountain features.
■ Structures built over and in basin.
■ Landscape surrounding basin.
■ Trees located in freeboard area.
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SOUTHLAKE HOSPITAL
FM 1709 & State Hwy. 114 Detention Basin
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Observed Features, Elements and Conditions
■ Surrounded by streets.
■ Visible from streets.
■ Natural grass freeboard area.
■ Concrete drainage and overflow structures.
■ Landscape islands surrounding basin.
■ Shrubbery located in freeboard area.
■ Center island/tree in basin area.
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Summary Table: Conditions, Elements & Features
Visible from Road
or Sidewalk
Stacked Stone
Walls or Edging
Sloped Grass
Freeboard
Fountain or Water
Feature
Wrought Iron
Fencing
Landscape
Surrounding Basin
Trees Located in
Basin
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Methodist
5OUTHLAKE HOSPITAL
Sidewalk
,�nly
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■ Retention Basin.
Alternative Designs Considered
■ Two Basin Option (North and South).
■ Sloped Grass and Shallow Bottom Option.
■ Raised Basin /Rim Option.
■ Underground Option.
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SOUTHLAKE HOSPITAL
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rmg Retention Pond with Water Feature
■ Pros:
— None.
■ Cons or Items of Note:
More costs — deeper excavation, wet pond liner, etc.
Safety concerns — drowning hazard.
Cannot see the water below — adds no value.
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SOUTHLAKE HOSPITAL
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5OUTHLAKE HOSPITAL
Retention Pond Section A -A
�.,•�P �ry �
a
V
i DISTANCE FROM 1 114
103'Y.
DISTANCE FROM FRONTAGE RD..,_
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i
p�y�rypp
n,e�vseo sikee. � -_
Existing Plantings \s�i�� -
Key Map
Slate Highway 114 PedesVian Sight one
Landscape Screen
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9
yyy Exksiing Trai{ VehicWar Sight Line
Enhanced Plentirtgs
Lxishng Tree oh Property
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R e t Y
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Props Line 1..
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Proposed Retaining Wall
etention Pond
aard Rail
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Section A -A
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MSH - Southlake Retention Basin
GRAPHI° SCALE IN FEET
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Methodist Southlake Hospital, 421 E. State Hwy 114
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Southlake, TX 76092
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5OUTHLAKE HOSPITAL
Retention Pond Section B-B
Section B-B
MSH - Southlake Retention Basin IN FEET
Methodist Southlake Hospital, 421 E. State Hwy 114 0 10 zo 40
Southlake, TX 76092
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Two Basin Option (North and South)
■ Pros:
Northern pond wall heights reduced by 2'-3'.
■ Cons or Items of Note:
7.5' high walls on the southern pond and northern pond. Stone walls
easier to see due to height reduction of basin rim.
Bottom of the ponds are held by the existing inlet pipes and outlet pipe
to S H 114.
Top of the pond is held by the 100-year water surface elevation in the
existing system, plus the 1' of freeboard per City requirements.
Additional grading needed northwest of the pond as well as the east to
get the top of the pond above the 100-year water surface elevation.
Cannot extend further north due to existing utilities in an easement.
Utilities cannot be in detention ponds.
Cannot extend further west due to future buildout.
Will experience additional material and labor costs due to second basin.
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SOLITHLAKE HOSPITAL
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Methodist
SOUTHLAKE HOSPITAL
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Two Basin Option (North and South).
HIGHLAND ST
Iml
o—W, below.
Call Wom"uft.
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DEVELOPMENT
rmg Sloped Grass and Shallow Bottom Option
■ Pros:
Softer sloped walls.
■ Cons or Items of Note:
Reroute existing water and sewer to expand the pond.
Eliminates most, if not all, of the future building.
Larger pond footprint.
Additional grading needed to tie the pond into the existing grade.
Because the pond is on a sloped grade — the west side of the pond needs
to grade at a 4:1 slope from the bottom of the pond to the existing grade
for about 80' horizontally.
Cannot extend further north due to existing water and sewer lines in an
easement — water and sewer lines cannot be in detention ponds.
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SOUTHLAKE HOSPITAL
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SOUTHLAKE HOSPITAL
Sloped Grass and Shallow Bottom Option
�` � �� 'l �.� e,� rgre Future Building and Frelane woRrn
" -.. / �.� H/ Negatively ImpeCted_
Additional Grading To Tie
Into Existing Grades-
40' Sloped Grass Freeboard
Area at 41 Slope
d
f / Basin No Longer Conlined ^ sc
i I to Flood Plain. .�-.
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`- � /�-� � ♦/ Ef�J6TING WR ER AN66PN TEIR�Y -. � t' I Kitt�eyr110iIt
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ii�s C-101
DEVELOPMENT
rmg Raised Basin / Rim Option
■ Pros:
Raises basin rim to further obstruct views.
■ Cons or Items of Note:
Additional fill in the floodplain could lead to CLOMR/LOMR process. Additional
9-12 months. Current solution is a net cut or increase in capacity.
Cannot grade further east because of the buffer yard requirement, trees and
plantings cannot be in 4:1 slopes.
Additional fill on top of the utilities will cause the utilities to be unnecessarily
deep.
Sanitary sewer in area approx. 9' deep. Additional fill on top of the sanitary
sewer line will cause the lineto be more than 10feet deep — the pipe will no
longer meet city requirements with the additional fill.
Can not berm up the west side of the basin similar to the east and north sides
because it will prevent water from flowing into the basin.
Any additional height in the pond wall is not needed from an engineering
standpoint. Additional height will force the pond to shift west so there is room
to tie that additional 1' into the existing sur ace.
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SOUTHLAKE HOSPITAL
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rmg Underground Option
■ Pros:
Basin structure buried underground.
■ Cons or Items of Note:
Extensive additional engineering required due to initial failure.
Subject to same failure conditions as existing basin.
Cost to replace estimated to be 3X cost of proposed surface basin.
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SOUTHLAKE HOSPITAL
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rmg City of Southlake Retention & Detention Policy
■ City of Southlake drainage criteria requires that retention or detention of
stormwater shall be provided in this circumstance;
■ The City's Retention and Detention Policy (Div. 8, Sec. 9.5-162(b), (c) and Sec.
9.5-163 (b), (c)) allows the developer to choose whether they provide
retention or detention;
■ Per the City's Retention and
Detention Policy (Sec. 9.5-163
(b),
(c)), a
detention
pond
in the
form
of
a grass
lined
depression
is
listed
as an
approved option;
■ The City has no specific requirements for landscaping surrounding a
detention pond.
0
Methodist
SOUTHLAKE HOSPITAL
4
L ANDPLAN
DEVELOPMENT
rmg
Conclusions
■ Proposed surface detention basin contains the same types of elements and
features provided at the Central Market location.
■ Alternate solutions not feasible due to existing topography/grades, existing
utilities and easements, existing drainage tie-in elevations, lack of visibility,
negative impact on future development and exorbitant costs.
■ Use of surface detention not prohibited by SP2 Zoning or Zoning Ordinance
No. 480-527.
■ Concept in keeping with the original approved Concept Plan.
■ The proposed location of the basin is consistent with the approved Concept
Plan.
■ Solution complies with City of Southlake Retention and Detention Policy.
0
Methodist L ANDPLAN
SOLITHLAKE HOSPITAL
DEVELOPMENT
rmg
Conclusions
■ Detention basin wall structure not visible from Hwy. 114 or Frontage Road.
■ Existing area will be improved and enhanced from its original Concept Plan.
■ Enhanced landscape will improve views from the Hwy. 114 and Frontage
Road.
■ Addresses the concerns of the Corridor Planning Committee, Staff and
Council.
■ Proposed solution remains best solution available to the Methodist
Southlake Hospital.
■ The proposed detention basin solution when combined with the substantial
amount of proposed landscaping, exceeds the City's requirements and,
therefore, should be approved as submitted.
0
Methodist
SOUTHLAKE HOSPITAL
4
L ANDPLAN
DEVELOPMENT
rmg
Questions
ZA21-0016
0
Methodist
SOUTHLAKE HOSPITAL
4
L ANDPLAN
DEVELOPMENT