PRELIMINARY DRAINAGE REPORT Sunlight Peak Solar Adams County, Colorado JUNE, 2020 PREPARED FOR: PREPARED BY: Namaste Solar Electric Inc 888 Federal Blvd. Denver, CO 80204 1| Confidential and Proprietary. TBPLS Firm #10074302 Preliminary Drainage Report Sunlight Peak Solar Adams County, Colorado Prepared For: Prepared By: Namaste Solar Electric Inc Westwood Professional Services 888 Federal Blvd. 10170 Church Ranch Way, Suite 200 Denver, CO 80204 Westminster, CO 80021 (720) 531-8350 Project Number: R0026622.00 Date: June, 2020 2| Confidential and Proprietary. TBPLS Firm #10074302 Master Drainage Plan | Sunlight Peak Solar June 10, 2020 Signature Page for Report on Master Drainage Plan Sunlight Peak Solar Prepared For Namaste Solar Electric Inc 888 Federal Blvd. Denver, CO 80204 Reviewed and Approved By: Brendan Miller, P.E. Civil Project Manager Westwood Professional Services 3 | Confidential and Proprietary. TBPLS Firm #10074302 Master Drainage Plan | Sunlight Peak Solar June 10, 2020 Table of Contents A. Introduction ...................................................................................................... 5 1. Location .................................................................................................................................... 5 2. Proposed Development ............................................................................................................. 5 B. Historic Drainage............................................................................................... 5 1. Description of Property .............................................................................................................. 5 2. Overall Basin Description ........................................................................................................... 5 C. Design Criteria .................................................................................................. 6 1. List References ........................................................................................................................... 6 2. Hydrologic Criteria ..................................................................................................................... 6 Hydraulic Criteria ..........................................................................................................................10 D. Drainage Plan ................................................................................................. 10 1. General Concept .......................................................................................................................10 2. Specific Details .........................................................................................................................10 E. Conclusions...................................................................................................... 11 1. Compliance with Standards ..................................................................................................... 11 2. Summary of Concept................................................................................................................ 11 F. List of References ............................................................................................. 12 Data Sources .................................................................................................................................. 12 Exhibits Exhibit 1: Vicinity Map Appendices Appendix A: Soils Information Appendix B: UD-WORKBOOK-UD-RATIONAL-2.00.xlsm 4 | Confidential and Proprietary. TBPLS Firm #10074302 Master Drainage Plan | Sunlight Peak Solar June 10, 2020 A. Introduction 1. Location a) The proposed Sunlight Solar Garden Project is a proposed solar project with two phases for a total of a 4.0 MW-AC solar array located on a 21.1 acre site. The project is located southwest of the intersection of East 26th Avenue and Monaghan Road, on the south side of East 26th Avenue, approximately 3 miles west of Watkins, CO. b) The proposed project is located in East ½ of Section 33, T3S, R65W, and County of Adams, Colorado. c) The location can be seen on Exhibit 1: Vicinity Map. 2. Proposed Development a) The proposed project will consist of a 4.0 MW-AC solar array built in two phases located on a 21.1 acre site. The solar farm will consist of the solar array along with associated electrical equipment (0.02 acres) and a gravel access road (1.09 acres) as the only impervious areas. The solar panels will be mounted on posts and stand above ground. The ground surface below the panels and throughout the site will be planted with low- maintenance grass seed mix to create a pervious surface across the reminder of project area (19.99 acres). B. Historic Drainage 1. Description of Property a) A review of the 21.1 acre site’s existing and historic conditions shows that the project area currently is farmed with a row crop and aerial imagery also shows the historic land use as farmed row crop. Per the USDA Web Soil Survey (Appendix A) is Hydrologic Soils Group (HSG) C soils, which have a relatively low infiltration rate. b) The site drains overland to the east and west with slopes of ~4%. There is no offsite runoff entering the site. The existing drainage can be seen on Sheet 2 of the Preliminary Drainage Plan. 2. Overall Basin Description a) The project area is located along a Hydrologic Unit Code 12 (HUC 12) boundary and has no offsite watershed flowing to the project area. b) The closest FEMA zone is a Zone AE located approximately 1 mile to the west of the project area on an unnamed stream. c) There are no major ditches, canals or other irrigation facilities on or within 100’ of the project boundary that will influence or be influenced by the local drainage. d) The project area is located on FEMA panels 08005C0068L and 08005C0206L and show no FEMA zones onsite. e) The existing drainage patterns through the site are sheet flow and shallow concentrated overland flow through the existing row crops. No existing impervious areas are within the project boundary. f) The outfalls downstream of the property are more agricultural fields continuing that continue at a similar slope until the water converges in concentrated drainage ways. 5 | Confidential and Proprietary. TBPLS Firm #10074302 Master Drainage Plan | Sunlight Peak Solar June 10, 2020 C. Design Criteria 1. List References a) An online search for adjacent existing drainage report for surrounding properties did not yield any results, however since the project has no offsite watershed no offsite water will be affecting the site. b) To meet local requirements the City of Aurora Regulations the Urban Storm Drainage Criteria Manual (USDCM) was referenced along with the City of Aurora Storm Drainage Design and Technical Criteria Manual. c) The City Master Plan for the project area lists it as an “Industry hub” zone which can cover commercial, industrial, or agricultural activities such as the commercial agriculture that the project is currently used for. 2. Hydrologic Criteria a) The UD-WORKBOOK-UD-RATIONAL-2.00.xlsm workbook was used to calculate the runoff from the project area using the Rational Method, and was edited to properly meet the City of Aurora Drainage Manual requirements. However, this workbook and design manual were created for a more urban land conversion than the project site is currently proposed for and does not have the proper conversion to a solar project with a proposed low-maintenance grass mix, rather the workbook just assumes the imperviousness of an area rather than the actual land cover that will be in place. This distinction is important because an agricultural field may have little imperviousness however the runoff quantity will be higher and the water quality lower when compared to the same area planted with a low-maintenance grass mix which has a consistent year round cover, deeper root structure, more dense cover and minimal addition of any pesticides or fertilizers. Based upon this the site was assumed to keep the same runoff coefficients in both existing and proposed conditions. Additionally other calculation methods that are not included in this drainage plan show the site having a reduction in flows from existing to proposed conditions. Westwood also has experience across the United States using this methodology and it has been accepted on hundreds of solar projects with positive post- construction results. The access road which is the additional impervious onsite will be graded to flow through the proposed vegetation onsite in order to treat and spread the water back out into a more sheet flow condition. The UDFCD Grass Buffer calculator was used to determine how much buffer was needed for the 2-year flow rate, the required width is W=Q2/0.05. This gives a required grass buffer of 33’ for DA-1 and 26’ for DA-2. The grass buffer widths are ~500’ and 1,100’ respectively which is more than adequate to meet this calculation. b) Based upon the proposed land use of the site a reduced runoff rate will occur based on other methods calculation and using the Rational Methods shows the flows rates the same or less in proposed conditions as seen in Table 1 and Appendix B and due to the runoff reduction no detention is required. c) Calculations for the 2, 5, 10, 100 -year events were looked at. The rainfall used in the spreadsheet is based on the closest location (D.I.A.). 6 | Confidential and Proprietary. TBPLS Firm #10074302 Master Drainage Plan | Sunlight Peak Solar June 10, 2020 Table 1 Peak Runoff Flowrates – Existing Conditions