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Item 6A - Applicant MSH Detention Basin-P&Z Presentation-Final 4-22-21Methodist Southlake Hospital Detention Basin Replacement Project Planning & Zoning Commission Presentation ZA21-0016 Objective & History ▪Seeking approval to undertake replacement of the existing underground detention system installed in 2013. ▪Originally planned and approved circa 2009 as a surface detention basin. ▪Developer proposed 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. Previously Approved Site Plan –Surface Detention Existing Underground Detention –Plan 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. 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 does not mandate type of detention (above ground vs underground). Approved Concept Plan Concept Plan –Existing Conditions Concept Plan –Master Plan Conditions Concept Plan -Enlarged Concept Plan -Landscape Landscape Material Grasses Shrubbery Trees 2’–8’ Heights 2’–8’ Widths 2’–5’ Heights 3’ –10’Widths 10’ –60’ Heights 8’ –50’ Canopies Retaining Wall Material –Stacked Stone Sandstone -Random Ashlar Pattern Detention Basin Elevation/Section Study Detention Basin Section A-A Detention Basin Section B-B Detention Basin Section C-C Detention Basin Rendering –View South, Frontage Rd. Before After Conclusion ▪Concept in keeping with the original approved Concept Plan. ▪Use of surface detention not prohibited by SP2 Zoning or Zoning Ordinance No. 480-527. ▪Detention basin 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. Questions ZA21-0016