October 7, City of Thornton 9500 Civic Center Drive Thornton, CO (303) RE: Maverik Thornton, CO - Drainage Report

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October 7, 2016 City of Thornton 9500 Civic Center Drive Thornton, CO 80229 (303) 538-7295 RE: Maverik Thornton, CO - Drainage Report As per your request, we are submitting to you the drainage report

2 October 7, 2016 City of Thornton 9500 Civic Center Drive Thornton, CO (303) RE: Maverik Thornton, CO - Drainage Report As per your request, we are submitting to you the drainage report and calculations of the drainage design for the Maverik located at 88 th Avenue and Pecos Street in Thornton, CO. The stormwater drainage report meets the criteria and specifications per Section 100 of the City of Thornton design manual. The Rational Method was used to calculate the runoff for historic and post-development per the UDFCD manual and imperviousness table. Using the UD-Detention spreadsheet, the detention pond/outlet structures were sized per COT and UDFCD details. The majority of storm water generated onsite will sheet flow to newly installed inlet boxes. The last inlet box prior to the detention basin is equipped with a sump and snout that will filter the storm water from hydrocarbons and other pollutants. After passing through the filtration process, the storm water will be directed to an above ground open detention basin that has been sized for the WQCV, EURV, and 100-year volume per USDCM and City of Thornton criteria. Once the storm water is in the detention system, it will be gravity fed towards a control box that is equipped with orifice plates and overflow structures that have been sized to control the outflow at rates for WQCV, EURV, and to below pre-historic flows for the 5-yr and 100-yr storms. The storm water hydraulic and hydrology calculations including the basin sizing and orifice plate are discussed in this report. If you have any questions, or we can be of further assistance, please let us know. Sincerely, Nate Reeve, P.E. Principal Engineer Reeve & Associates, Inc. nreeve@reeve-assoc.com Anna Newman, E.I.T. Civil Engineer Reeve & Associates, Inc. anewman@reeve-assoc.com Solutions You Can Build On Civil Engineering / Land Planning / Structural Engineering / Landscape Architecture / Land Surveying 5160 South 1500 West Riverdale, Utah Tel: Fax: ogden@reeve-assoc.com Website:

4 Reeve & Associates, Inc. Table of Contents MAVERIK, INC. 88th Avenue and Pecos Street Thornton, CO Item Section Introduction 1 Drainage System and Description 2 Drainage Analysis and Design Criteria 3 Summary and Conclusion 4 Appendix 5

5 Stormwater Drainage Design Report 1.0 Introduction 1.1 Purpose The purpose of this drainage report is to reflect proposed changes to the current drainage patterns associated with the proposed Maverik and subdivision. Proposed changes to the property include construction of a 5,518 sq. ft. Maverik convenience store with fuel pumps and associated utility service connections. The detention basin and associated facilities have been designed for a full build-out scenario. 1.2 General Property Description At the request of Thornton City, Reeve & Associates has prepared the following Storm Drainage Design Report for the proposed Maverik located at 88 th Avenue and Pecos Street in Thornton, Colorado. The site is located in the southeast quarter of section 21, township 2 south, range 68 west. See Figure 1 for a general location map that shows the proposed location and surrounding properties. A private storm sewer system will collect stormwater runoff from the proposed development. The storm sewer system will discharge into a private detention pond located on the eastern side of the property. An outlet pipe from the detention pond will outlet to the public right of way located along the north side of W. 88 th Avenue. The proposed development includes an access road and vacant land for future development. The detention basin and outlet structures are sized for a full buildout scenario for all future developments. 1.3 Pre-developed Drainage Characteristics The existing site is currently undeveloped and consists mostly of dry weeds and sandy loam with the soil type being C/D. The existing runoff from this property is divided into two sub-basins (EX-1 and EX-2) but sheet flows primarily from the north to the south and into the curb and gutter on 88 th Avenue as outline in this section and depicted on the predevelopment drainage plan located in the Appendix; with design point Z being the outfall of these existing basins. There is a third drainage basin (EX-3) on the north east corner of the site that sheet flows to the north east and into the Orchard Hill Senior Apartment Complex with design point Y being the outfall of this basin. 3

6 Stormwater Drainage Design Report The property is bordered to the north by residential property to the west by Pecos Street and Water World Amusement Park, to the south by 88 th Ave, and to the east by Orchard Hill Senior Apartments. The pre-developed stormwater runoff leaves the site without the use of engineered quality or detention facilities. 1.4 Post-developed Drainage Characteristics The proposed site will operate as a Maverik Convenience Store with vehicle fueling stations. The entire parcel sits on 6.35 acres. The site will contain 197,154 s.f. of undeveloped area, 54,546 s.f. of hard surface area, 19,880 s.f. of landscaped area and 5,518 s.f. of building area. Note that the detention basin has been sized to accommodate future developments on the site to tie into the detention basin. The proposed grading will divide the site into twelve drainage basins and seven design points as outlined in this section and depicted on the developed drainage plan located in the Appendix. Basin R-1 encompasses 0.43 acres of sidewalk and landscape between the onsite curb and gutter and Pecos Street and W. 88 th Avenue right of way. Stormwater generated in this basin will sheet flow undetained into the curb and gutter within the right-of-way and into the southeast corner of the lot. Basin R-2 encompasses 0.23 acres of sidewalk and landscape between the onsite curb and gutter and W. 88 th Avenue right of way. Stormwater generated in this basin will sheet flow undetained into the curb and gutter within the right-of-way and into the southeast corner of the lot. Basin R-3 encompasses 0.02 acres of asphalt paving on the southeast corner of the lot. Stormwater generated in this basin will sheet flow undetained into the curb and gutter within the right-of-way in W. 88th. Basin R-4 encompasses 0.02 acres of asphalt paving on the northwest corner of the lot. Stormwater generated in this basin will sheet flow undetained into the curb and gutter within the right-of-way on Pecos and follow the curb and gutter to the southeast corner of the lot. Basin B-1 encompasses 0.63 acres of parking and drive aisles of the proposed convenience store. Stormwater generated in this basin will sheet flow to a low point on the curb with an inlet box titled A. The water will be conveyed by a private storm sewer system to the private detention pond. Basin B-2 encompasses 0.02 acres of parking and drive aisles of the proposed convenience store. Stormwater generated in this basin will sheet flow to a low 4

7 Stormwater Drainage Design Report point on the curb with an inlet box titled d B. The water will be conveyed by a private storm sewer system to the private detention pond. Basin B-3 encompasses 0.14 acres of parking and drive aisles of the proposed convenience store. Stormwater generated in this basin will sheet flow to a low point on curb with an inlet box titled C. The water will be conveyed by a private storm sewer system to the private detention pond. Basin B-4 encompasses 0.57 acres of parking and drive aisles, the building and canopy for the proposed convenience store. Stormwater generated in this basin will sheet flow to a low point on curb with an inlet box titled D. The water will be conveyed by a private storm sewer system to the private detention pond. Basin B-5 encompasses 0.40 acres of parking and drive aisles, the building and canopy for the proposed convenience store. Stormwater generated in this basin will sheet flow to a low point on curb with an inlet box titled E. The water will be conveyed by a private storm sewer system to the private detention pond. Basin B-6 encompasses 0.35 acres of parking and drive aisles, the building and canopy for the proposed convenience store. Stormwater generated in this basin will sheet flow to a low point on curb with an inlet box titled F and G. The water will be conveyed by a private storm sewer system to the private detention pond. Basin B-7 encompasses 0.82 acres of detention pond. This has been sized to detain the 100-year storm event per COT code with an outlet structure and orifice plates for the WQCV, EURV and 100-year storm event. It will be landscaped per COT code. Basin F-1 encompasses 2.73 acres and will be developed at a later date. However, the detention basin design has included this area in a full build-out scenario. Future developments will convey their storm water to the proposed private detention pond. Pipe P1 conveys the stormwater from Basin B-2 from the inlet box B. Pipe P2 conveys the stormwater from Basin B-1 from the inlet box A. This pipe also carries the water from Basin B-2. Pipe P3 conveys the stormwater from Basin B-3 from the inlet box C. Pipe P4 conveys the stormwater from Basin B-4 from the inlet box D. This pipe also carries the water from Basin B-2 and B-1. Pipe P5 conveys the stormwater from B-5 from the inlet box E. This pipe also carries the water from Basin B-4, B-3, B-2 and B-1. 5

8 Stormwater Drainage Design Report Pipe P6 conveys the stormwater from B-6 from the inlet box F. This pipe also carries the water from Basin B-5, B-4, B-3, B-2 and B-1. Pipe P7 also conveys the stormwater from B-6 from the inlet box G. This pipe also carries the water from Basin B-5, B-4, B-3, B-2 and B-1. Ultimately, all storm water runoff from this site is conveyed to the South Platte River. In a fully developed scenario, 1% or cfs and 0.17 cfs will exit the site at each access untreated during the 5-yr and 100-yr storms respectively. The remaining storm water from hard surface areas will be directed to a private detention pond that is sized for the WQCV, EURV and 100-yr volumes. It will release 0.5 cfs and 8.60 cfs into the 88 th Ave right-of-way during the 5-year and 100-year storm events respectively. 1.5 Water Quality A private water quality facility will be provided for the full build-out at this time. Post-development stormwater runoff will be collected via curb and gutter and private combination curb inlets throughout the property. A private storm sewer system will convey storm water flows to the private detention pond facility located along the northeast property line. The detention pond will provide storage for the water quality capture volume of acre-feet or 6,926 c.f. with a water surface elevation of and the EURV storage has also been provided with a volume of acre-feet or 13,765 c.f. with an elevation of An outlet structure within the detention pond will provide water quality and staged release of stored stormwater that meets the City of Thornton Storm Drainage Design requirements. Because the 5-yr, and 100-yr design storm event developed flows are larger than the pre-developed site runoff totals, the detention pond outlet structure also reduces the release rates of the post-developed flows to the pre-developed rates. The pond will provide storage for the 5-yr design storm event and the water quality volume of acre-feet or 21,867 c.f. with a water surface elevation of The pond will also provide storage for the 100-yr, design storm event and the water quality volume of acre-feet or 35,240 c.f. with a water surface elevation of During the 100-yr design storm event, the pond outlet structure will provide a release rate of 8.6 cfs which is less than the predeveloped outflow of 9.7 cfs provided the UD-Detention analysis of the detention pond volume. A private 24 RCP pipe will connect to the outlet structure and release into a sidewalk chase drain and into the curbline along the north side of W. 88 th Avenue. 6

9 Stormwater Drainage Design Report See Hydraulic Calculations located in the Appendix for more details regarding detention pond design. 1.6 Hydrology Calculations The City of Thornton Storm Drainage Design, Grading, and Water Quality Technical Criteria within the City of Thornton Standards and Specifications was used to perform hydrology calculations. The Rational Method has been used to compute storm water runoff, as preferred for drainage areas less than 100-acres. Design storms with the 5-yr, and 100-yr, occurrence intervals were used. Weighted or composite C values for the site were calculated using the following formula. 5-year, and 100-yr, flows for the proposed site are summarized in the Appendix. Table 1 shows a comparison between pre-development discharges and postdevelopment discharges from the project site. Tablie 1: Peak Discharge Comparison Pre-Development Discharge (cfs) Post-Development Discharge (cfs) Discharge Difference (cfs) Q5 Pecos Q100 Pecos Q5 88th Q100 88th NE Corner NE Corner Pipe and inlet capacity calculations were performed at each design point as shown on the drainage plan. Storm sewer for this project was designed to convey the required 100-yr storm even with a surcharge hydraulic grade line not less than one foot below finished grade. Hydrologic and hydraulic calculations are located in the Appendix. All assumptions shall be field verified by the contractor prior to commencing construction, and discrepancies shall be reported to the engineer immediately for review. 7

10 Stormwater Drainage Design Report 1.7 Floodplain Statement According to the Federal Emergency Management Agency (FEMA), as depictured on Flood Insurance Rate Map (FIRM) 08001C0582H with an effective date of March 5, 2007, the site lies within areas determined to be outside the 500- year floodplain. 1.8 Erosion Control It is respectfully requested that an erosion control plan be submitted in conjunction with the final grading plan and not be required to be submitted with this drainage report. 1.9 Summary Developed runoff generated from the project site during the 100-yr. design storm event are anticipated to be less than or equal to the pre-developed runoff rates. Prior to being released off-site, runoff will be treated via a private detention pond and water quality facility, while a small percentage of the site s runoff will discharge (un-detained) to the adjacent right-of-way or adjacent properties. Downstream of the detention pond, a private 24 RCP pipe and sidewalk chase drain will convey treated runoff into the W. 88 th Avenue right of way. A series of private combination inlets will collect runoff in multiple locations on site and convey the runoff to the proposed private detention pond east of the Maverik building. The storm sewer has been designed to handle developed flows in the required 100-yr. design storm event condition within minimal surcharge. Since the site s anticipated developed runoff rate is less than the allowed predevelopment runoff rate, development on this site as described in the enclosed drainage plan is not anticipated to adversely impact downstream stormwater infrastructure. Upon the completion of construction, if conditions or outfall locations differ from the assumptions stated within this drainage study, an amendment to this report must be completed. Maverik Inc. will be responsible for any maintenance on the drainage facilities. 8

11 Stormwater Drainage Design Report APPENDIX 9

12 Stormwater Drainage Design Report Figure 1 Vicinity Map 88 th Avenue and Pecos Street Project Site The project site is located at 88 th Avenue and Pecos Street in Thornton, CO. 10

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25 Hydraulic Grade Line - 5-yr Maverik, Inc 88th Ave & Pecos - Thornton, CO 8/29/2016 AHN Pipe Size (in) Slope (%) Length (ft) Flow (cfs) Invert Elev Invert Elev Up HGL Down (ft) (ft) Down HGL Up P P P P P P P P * All pipes analyzed with a Manning's roughness coefficient of Hydraulic Grade Line yr Maverik, Inc 88th Ave & Pecos - Thornton, CO 8/29/2016 AHN Pipe Size (in) Slope (%) Length (ft) Flow (cfs) Invert Elev Invert Elev Up HGL Down (ft) (ft) Down HGL Up P P P P P P P P * All pipes analyzed with a Manning's roughness coefficient of

26 Storm Runoff Calculations Maverik, Inc 88th Ave & Pecos - Thornton, CO 6/3/2016 TJH Basin Designation EX-1 EX-2 EX-3 Composite 'C' Factors - Pre & Post-Development GRASS ROOFS PAVEMENT GRAVEL COMPOSITE C FACTOR Grass Roofs Paved Gravel Total 5-YR 10-YR 100-YR 5-YR 10-YR 100-YR 5-YR 10-YR 100-YR 5-YR 10-YR 100-YR 5-YR 10-YR 100-YR % IMPERVIOUS R-1 R-2 R-3 R-4 B-1 B-2 B-3 B-4 B-5 B-6 B-7 F-1 * EX = existing * R = undetained * B = detained * F = future Basin Designation Time of Concentration C5 Acre Length (ft) Slope % Ti (min) Length (ft) Slope (ft/ft) Vel. (fps) Tt (min.) Total EX-1 EX-2 EX R-1 R-2 R-3 R-4 B-1 B-2 B-3 B-4 B-5 B-6 B-7 F-1 * EX = existing * R = undetained * B = detained * F = future Design Point Z Y * EX = existing * R = undetained * B = detained * F = future min. assumed min. assumed min. assumed min. assumed min. assumed min. assumed min. assumed min. assumed min. assumed assumed Design Storm: 5-Year Runoff Basin Area (ac) Coeff. (c) Tc C*A I (in./hr.) Q (cfs) Sum Area Max (Tc) Sum C*A Total Q (cfs) EX EX EX R-1 R-2 R-3 R-4 B-1 B-2 B-3 B-4 B-5 B-6 B-7 F

27 Design Storm: 10-Year Runoff Design Point Basin Area (ac) Coeff. (c) Tc C*A I (in./hr.) Q (cfs) Sum Area Max (Tc) Sum C*A Total Q (cfs) Z EX EX Y EX * EX = existing * R = undetained * B = detained * F = future R-1 R-2 R-3 R-4 B-1 B-2 B-3 B-4 B-5 B-6 B-7 F Design Point Z Y Basin EX-1 EX-2 EX-3 R-1 R-2 R-3 R-4 B-1 B-2 B-3 B-4 B-5 B-6 B-7 F-1 Design Storm: 100-Year Runoff Area (ac) Coeff. (c) Tc C*A I (in./hr.) Q (cfs) Sum Area Max (Tc) Sum C*A Total Q (cfs) Sum Area Max (Tc) Sum C*A Total Q (cfs) A B C D E F G FOR PIPE CAPACITY CALCULATIONS P1 P2 P3 P4 P5 P6 P7 Sum Area (Tc) Sum C*A I (in./hr) Total Q (cfs) P8 See UD-Detention Spreadsheet 8.90 Z post Y post See UD-Detention Spreadsheet

PHASE III DRAINAGE REPORT

PHASE III DRAINAGE REPORT FOR Eastlake Assisted Living & Memory Care April 20, 2016 June 3, 2016 August 5, 2016 Prepared for: 3301 E 120 th Ave, LLC. 8200 E. Maplewood Ave., Suite 150 Greenwood Village