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Item 8 - Preliminary Drainage Study KIRKMAN ENGINEERING :: 5200 State Highway 121, Colleyville, TX 76034 Ph: 817-488-4960 :: TBPE Firm #15874 Page 1 of 4 Timberlake Office – Preliminary Drainage Study City of Southlake, Tarrant County, Texas January 31, 2020 Prepared for: Department of Planning & Development Services City of Southlake 1400 Main Street, Suite 310 Southlake, Texas 76092 Prepared by: Kirkman Engineering 5200 State Highway 121 Colleyville, TX 76034 817.488.4960 KIRKMAN ENGINEERING :: 4821 Merlot Avenue, Suite 210, Grapevine, TX 76051 Ph: 817-488-4960 :: TBPE Firm #15874 Page 2 of 4 1.0 INTRODUCTION The Timberlake Office is a proposed office building located along Southlake Blvd, approximately 0.25 miles east of Shady Oaks Dr. The office is proposed on a 0.859-acre site, which was recently cleared of the residential structures. The current effective FEMA Flood Insurance Rate Map (FIRM) for this area of Tarrant County, panel 85, dated September 25, 2009, shows that the full extents of the property is located within Zone X, which is an area determined to be outside the 0.2% annual chance floodplain. The owner of this site is proposing to rezone the property for office use. The purpose of this report is to analyze any drainage impacts that changing the use for this property would have on-site or on adjacent properties. Existing Residence Zoned “AG” KIRKMAN ENGINEERING :: 4821 Merlot Avenue, Suite 210, Grapevine, TX 76051 Ph: 817-488-4960 :: TBPE Firm #15874 Page 3 of 4 2.0 HYDRAULICS Kirkman Engineering has completed a drainage analysis for the existing site to determine the total runoff currently being discharged to adjacent properties along with a post-developed drainage analysis to determine the storm water effects of the site development. The rational method was used to compute the total runoff values. The runoff coefficients are based on the City of Southlake drainage criteria, and the time of concentration calculations are based on the iSWM Technical Manual: Hydrology. 2.1 EXISTING CONDITIONS HYDRAULICS The site has been cleared of the existing improvements within the property boundary. There are currently no drainage structures on-site and the existing topography directs flows universally away from the original residence on the property. The land is currently zoned AG. The existing use of the property was single family residential, therefore a runoff coefficient of 0.51 was used per the City of Southlake’s drainage criteria. Using the rational method, this results in a total runoff value of 4.05 cfs. The runoff discharges directly to adjacent properties to the south and east. 2.2 PROPOSED CONDITIONS HYDRAULICS The purpose of this rezoning is to allow for office us. Per the proposed improvements associated with this development, the total runoff from the overall site will increase to 6.99 cfs. However, the proposed flow draining to the adjacent properties will decrease from existing conditions due to the site is being designed to allow for most of the discharge to flow north to FM 1709. Once developed, the amount of flow from this site to the residential property to the south and east will be reduced to 0.45 cfs. The remainder of the on-site flow will drain to the north and onto FM 1709, and ultimately to an existing curb inlet to the west. This inlet and associated storm pipe were designed to accept our on-site developed flow. Per the TxDOT drainage infrastructure plans, the design flow for this existing inlet is 9.6 cfs and the pipe capacity is 38.4 cfs. After development of our site, the total flow draining to the existing inlet will be 8.65 cfs, which is less than the design flow. Existing Residence 0.86 0.51 0.44 10.0 6.01 2.63 9.24 4.05 Drains to Adjacent Properties Q5 (c.f.s.) I100 (in./hr.) Q100 (c.f.s.)Acres CA Drainage Area Runoff Coeff. Tc (min.) DRAINAGE AREA CALCULATIONS (EXISTING) CommentsI5 (in./hr.) Proposed Office 0.86 0.88 0.76 10.0 6.01 4.54 9.24 6.99 Total Developed Flow Q100 (c.f.s.)Comments DRAINAGE AREA CALCULATIONS (PROPOSED) Drainage Area Acres Runoff Coeff.CA Tc (min.) I5 (in./hr.) Q5 (c.f.s.) I100 (in./hr.) KIRKMAN ENGINEERING :: 4821 Merlot Avenue, Suite 210, Grapevine, TX 76051 Ph: 817-488-4960 :: TBPE Firm #15874 Page 4 of 4 3.0 CONCLUSION This report includes preliminary drainage computations pertaining to the existing conditions and proposed conditions for the Timberlake Office Zoning case. The post-development flow will see an increase in runoff, but this increase flow will be redirected to the north and into the existing TxDOT drainage system. Once developed, the flow to the adjacent properties will be reduced from existing conditions. A more detailed study of the site, along with the downstream components of the existing drainage infrastructure will be completed during the full civil design phase of the project.