Sunlight Peak Solar Adams County, Colorado

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PRELIMINARY DRAINAGE REPORT

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

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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

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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

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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.

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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.).

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Master Drainage Plan | Sunlight Peak Solar June 10, 2020

Table 1 Peak Runoff Flowrates – Existing Conditions vs. Proposed Conditions

Sub-Catchment Area Peak Flow, Q (cfs) (Acres) 2-yr 5-yr 10-yr 100-yr DA-1 Existing 1.70 0.5 0.8 0.9 1.7 DA-1 Proposed 1.70 0.5 0.7 0.9 1.6 DA-2 Existing 19.40 5.34 8.0 10.0 18.0 DA-2 Proposed 19.40 4.49 6.7 8.4 15.1

As seen in Table 1 the conversion of the project area from an intermittent land cover of row crop to a year round established low-maintenance grass mix will lower the runoff from the project area. This reduction in runoff rates shows that no detention basin is needed. Example pictures of what the site will look like after initial grass establishment are below in Images 1, 2, & 3. A rendering of an example project is shown in Image 4. Image 5 shows an example of mature growth after full establishment of the grasses. Vegetation under the panels will be of the same or greater density than the vegetation between the rows. Anecdotal evidence of this shows the vegetation under the panels growing faster and thicker due to the minor shading that allows for less evaporation and increased moisture retention. The tracker array will most likely be mounted on 6” I beam pile driven into the ground. These minor impervious additions will have a de minimus effect on the drainage. The total area covered by panels is and the solar panels are ~9’ wide and have a spacing of 14’ between the rows. The array layout will allow for growth underneath the panels and as a tracker array will assist in having only a minor dripline due to the changing slope of the panels at different times of the day along the with site having C soils which are not expected to erode once vegetation is established. Driplines have not risen as an issue on other projects once vegetation has been established. Image 1

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Master Drainage Plan | Sunlight Peak Solar June 10, 2020 Image 2

Image 3

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Master Drainage Plan | Sunlight Peak Solar June 10, 2020 Image 4

Image 5

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Master Drainage Plan | Sunlight Peak Solar June 10, 2020 Hydraulic Criteria NA-no proposed drainage structures onsite. D. Drainage Plan

1. General Concept a) The site will only have minimal grading to install the access road, the remainder of the project will have no grading and all drainage patterns will be maintained the same as in existing conditions with a reduction in flows due to the improved land cover. No variances are being requested. b) Due to the project having no offsite flows or major drainage ways no conveyance through the site is necessary. c) NA d) NA e) NA f) As discussed in the previous Hydrologic Criteria Section of this report the site will be converted from a row crop to a proposed 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. This conversion will improve water quality and decrease runoff to downstream properties. 2. Specific Details a) The project should not encounter any drainage problems due to the lack of offsite flows and minimal onsite flows. The only issues encountered on the design side were obtaining the proper C values for proposed conditions which has been discussed under the Hydrologic Criteria section. b) There are no existing drainage ways for creeks however returning the landcover to a more natural low-maintenance grass mix will assist in improving water quality for downstream drainage ways. c) NA – project runoff is being reduced. d) NA e) NA f) NA g) NA h) The project will improve the water quality and decrease the peak flow to downstream properties. i) NA j) NA k) NA l) The water quality of the project is being enhanced through the conversion of row crop to a low maintenance grass mix. This will reduce the fertilizers and pesticides that run off of agricultural field along with increased infiltration due to an improved root structure and increased density of cover.

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Master Drainage Plan | Sunlight Peak Solar June 10, 2020 E. Conclusions

1. Compliance with Standards This project complies with all applicable regulations from the Urban Storm Drainage Criteria Manual (USDCM) along with the City of Aurora Storm Drainage Design and Technical Criteria Manual. 2. Summary of Concept a) The proposed project will protect the existing runoff conditions and will protect and improve the integrity of onsite infiltration conditions and increase water quality. b) The project will maintain all existing on-site drainage patterns and due to the conversion of row crop to a low-maintenance grass mix the stormwater quality will increase. c) The effect of the proposed development will have no effect on upstream properties as there are none, and all adjacent and downstream properties will see reduced runoff and improved water quality.

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Master Drainage Plan | Sunlight Peak Solar June 10, 2020 F. List of References

Data Sources

TABLE 2: DATA SOURCES

Task Format Source Use 2013 South Platte Watershed Elevation 1-meter LiDAR River Flood Area 1 Delineation Lidar Landcover Aerial Imagery Google Earth Landcover USGS Web Soil Soils PDF Curve Numbers Survey Runoff XLSM UDFCD Design Storms Calculations Runoff PDF City of Aurora Technical Criteria Calculations

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Exhibits

OK AZ NM TX

Jackson Logan Larimer ^_- Project Location Weld Morgan

Grand Boulder

Broomfield Adams Gilpin Washington Denver Clear ^_ Creek Arapahoe Summit Arapahoe Jefferson

Douglas Elbert Park Lincoln

Teller El Paso

Adams Arapahoe M P

8 5 : 4 4 : 1

0 2 0 2 / 2 1 / 5

d x m . p a M n o i t a c o L 1 x E _ e r u t n e V _ 2 1 - 5 0 - 0 2 0 2 \ s t i b i h x E o r Data Sources: Westwood (2020); Esri WMS d y Basemap Imagery (Accessed 2020); USGS H \ (2020); FEMA (2020); USDA (2020) S I G \ Sunlight Peak Solar Project 0

0 Legend . Adams County, Colorado 2 2 6 6 2 0 0 \ :

N Project Boundary Miles

: t n e 0 0.5 Exhibit 1: Location Map m u c ± o D

p County Boundary a May 12, 2020 M

Appendix A Soils Information

Hydrologic Soil Group—Adams County Area, Parts of Adams and Denver Counties, Colorado (2020-05-12ProjectBoundary) 104° 40' 10'' W 104° 39' 48'' W

528310 528360 528410 528460 528510 528560 528610 528660 528710 528760 528810 39° 45' 12'' N 39° 45' 12'' N 4400420 4400420 4400370 4400370 4400320 4400320 4400270 4400270 4400220 4400220 4400170 4400170

Soil Map may not be valid at this scale. 4400120 4400120

39° 45' 1'' N 39° 45' 1'' N 528310 528360 528410 528460 528510 528560 528610 528660 528710 528760 528810

Map Scale: 1:2,390 if printed on A landscape (11" x 8.5") sheet. Meters N 0 35 70 140 210 104° 40' 10'' W 104° 39' 48'' W Feet 0 100 200 400 600 Map projection: Web Mercator Corner coordinates: WGS84 Edge tics: UTM Zone 13N WGS84

Natural Resources Web Soil Survey 5/12/2020 Conservation Service National Cooperative Soil Survey Page 1 of 4 Hydrologic Soil Group—Adams County Area, Parts of Adams and Denver Counties, Colorado (2020-05-12ProjectBoundary)

MAP LEGEND MAP INFORMATION

Area of Interest (AOI) C The soil surveys that comprise your AOI were mapped at Area of Interest (AOI) 1:20,000. C/D Soils D Warning: Soil Map may not be valid at this scale. Soil Rating Polygons Enlargement of maps beyond the scale of mapping can cause A Not rated or not available misunderstanding of the detail of mapping and accuracy of soil A/D Water Features line placement. The maps do not show the small areas of Streams and Canals contrasting soils that could have been shown at a more detailed B scale. Transportation B/D Rails Please rely on the bar scale on each map sheet for map C Interstate Highways measurements. C/D US Routes Source of Map: Natural Resources Conservation Service Web Soil Survey URL: D Major Roads Coordinate System: Web Mercator (EPSG:3857) Not rated or not available Local Roads Maps from the Web Soil Survey are based on the Web Mercator Soil Rating Lines projection, which preserves direction and shape but distorts Background A distance and area. A projection that preserves area, such as the Aerial Photography Albers equal-area conic projection, should be used if more A/D accurate calculations of distance or area are required. B This product is generated from the USDA-NRCS certified data as B/D of the version date(s) listed below.

C Soil Survey Area: Adams County Area, Parts of Adams and Denver Counties, Colorado C/D Survey Area Data: Version 16, Sep 12, 2019

D Soil map units are labeled (as space allows) for map scales 1:50,000 or larger. Not rated or not available Date(s) aerial images were photographed: Jul 17, 2015—Oct 2, Soil Rating Points 2017 A The orthophoto or other base map on which the soil lines were A/D compiled and digitized probably differs from the background B imagery displayed on these maps. As a result, some minor shifting of map unit boundaries may be evident. B/D

Natural Resources Web Soil Survey 5/12/2020 Conservation Service National Cooperative Soil Survey Page 2 of 4 Hydrologic Soil Group—Adams County Area, Parts of Adams and Denver Counties, Colorado 2020-05-12ProjectBoundary

Hydrologic Soil Group

Map unit symbol Map unit name Rating Acres in AOI Percent of AOI

AcC Adena-Colby C 12.3 58.2% association, gently sloping AcD Adena-Colby C 7.2 34.0% association, moderately sloping PlC Platner loam, 3 to 5 C 0.2 0.9% percent slopes WmB Weld loam, 1 to 3 C 1.5 7.0% percent slopes Totals for Area of Interest 21.1 100.0%

Natural Resources Web Soil Survey 5/12/2020 Conservation Service National Cooperative Soil Survey Page 3 of 4 Hydrologic Soil Group—Adams County Area, Parts of Adams and Denver Counties, Colorado 2020-05-12ProjectBoundary

Description

Hydrologic soil groups are based on estimates of runoff potential. Soils are assigned to one of four groups according to the rate of water infiltration when the soils are not protected by vegetation, are thoroughly wet, and receive precipitation from long-duration storms.

The soils in the United States are assigned to four groups (A, B, C, and D) and three dual classes (A/D, B/D, and C/D). The groups are defined as follows:

Group A. Soils having a high infiltration rate (low runoff potential) when thoroughly wet. These consist mainly of deep, well drained to excessively drained sands or gravelly sands. These soils have a high rate of water transmission.

Group B. Soils having a moderate infiltration rate when thoroughly wet. These consist chiefly of moderately deep or deep, moderately well drained or well drained soils that have moderately fine texture to moderately coarse texture. These soils have a moderate rate of water transmission.

Group C. Soils having a slow infiltration rate when thoroughly wet. These consist chiefly of soils having a layer that impedes the downward movement of water or soils of moderately fine texture or fine texture. These soils have a slow rate of water transmission.

Group D. Soils having a very slow infiltration rate (high runoff potential) when thoroughly wet. These consist chiefly of clays that have a high shrink-swell potential, soils that have a high water table, soils that have a claypan or clay layer at or near the surface, and soils that are shallow over nearly impervious material. These soils have a very slow rate of water transmission.

If a soil is assigned to a dual hydrologic group (A/D, B/D, or C/D), the first letter is for drained areas and the second is for undrained areas. Only the soils that in their natural condition are in group D are assigned to dual classes. Rating Options

Aggregation Method: Dominant Condition Component Percent Cutoff: None Specified Tie-break Rule: Higher

Natural Resources Web Soil Survey 5/12/2020 Conservation Service National Cooperative Soil Survey Page 4 of 4

Appendix B UD-Workbook-UD-Rational-2.00.xlsm

Note: The red strike throughs have bene ignored as they do not meet the COA manual calculations. Purple and grey cells were used for calculation purposes.

Calculation of Peak Runoff using Rational Method Designer: Matthew Hildreth Version 2.00 released May 2017 Select UDFCD location for NOAA Atlas 14 Rainfall Depths from the pulldown list OR enter your own depths obtained from the NOAA website (click this link) 0.395 1.1 − C L t = 5 (urban) Company: Westwood Professional Services 5 i minimum 2-yr 5-yr 10-yr 25-yr 50-yr 100-yr 500-yr ti = Computed tc = ti + tt 0.33 tminimum= 10 (non-urban) Date: 6/4/2020 Cells of this color are for required user-input Si 1-hour rainfall depth, P1 (in) = 0.98 1.39 1.65 2.00 2.35 2.70 3.55 Project: Sunlight Peak Solar Project Cells of this color are for optional override values L L L a b c a ∗ P t = t = t Regional t = 26 − 17i + t 1 Location: 39.751190, -104.666853 Cells of this color are for calculated results based on overrides t c Selected tc = max tminimum , min Computed tc , Regional tc Rainfall Intensity Equation Coefficients = 28.50 10.00 0.786 I 𝑖푛/ℎ푟 = c Q 푐푓푠 = CIA 60K St 60Vt 60 14i + 9 St b + tc Runoff Coefficient, C Overland (Initial) Flow Time Channelized (Travel) Flow Time Time of Concentration Rainfall Intensity, I (in/hr) Peak Flow, Q (cfs) NRCS Subcatchment Area Percent Overland U/S Elevation D/S Elevation Overland Overland Channelized U/S Elevation D/S Elevation Channelized NRCS Channelized Channelized Hydrologic Computed Regional Selected Name (ac) Imperviousness 2-yr 5-yr 10-yr 25-yr 50-yr 100-yr 500-yr Flow Length (ft) (ft) Flow Slope Flow Time Flow Length (ft) (ft) Flow Slope Conveyance Flow Velocity Flow Time 2-yr 5-yr 10-yr 25-yr 50-yr 100-yr 500-yr 2-yr 5-yr 10-yr 25-yr 50-yr 100-yr 500-yr Soil Group tc (min) tc (min) tc (min) Li (ft) (Optional) (Optional) Si (ft/ft) ti (min) Lt (ft) (Optional) (Optional) St (ft/ft) Factor K Vt (ft/sec) tt (min) 0.01 0.05 0.15 0.33 0.40 0.49 0.59 28.44 30.94 27.01 1.63 2.32 2.75 3.34 3.92 4.50 5.92 0.03 0.20 0.69 1.87 2.68 3.77 5.98 DA-1 Existing 1.70 C 2.0 300.00 0.015 150.00 0.040 5 1.00 2.50 27.01 0.18 0.19 0.20 0.22 24.68 27.18 27.18 1.63 2.31 2.74 3.32 3.91 4.49 5.90 0.50 0.75 0.93 1.87 2.67 1.68 5.96 0.19 0.26 0.33 0.47 0.53 0.59 0.67 23.11 28.11 22.39 1.82 2.57 3.06 3.70 4.35 5.00 6.58 0.59 1.12 1.72 2.96 3.89 5.05 7.52 DA-1 Proposed 1.70 C 27.0 300.00 0.015 150.00 0.040 2.5 0.50 5.00 22.39 0.18 0.19 0.20 0.22 24.89 29.89 29.89 1.54 2.19 2.59 3.14 3.70 4.25 5.58 0.47 0.71 0.88 2.51 3.31 1.59 6.39 0.01 0.05 0.15 0.33 0.40 0.49 0.59 23.40 33.40 31.05 1.51 2.14 2.54 3.07 3.61 4.15 5.46 0.30 2.13 7.22 19.70 28.24 39.64 62.93 DA-2 Existing 19.40 C 2.0 300.00 0.028 600.00 0.040 5 1.00 10.00 31.05 0.18 0.19 0.20 0.22 20.31 30.31 30.31 1.53 2.17 2.57 3.12 3.66 4.21 5.54 5.34 7.99 9.98 19.98 28.64 17.97 63.83 0.02 0.06 0.15 0.34 0.41 0.50 0.60 23.17 43.17 30.80 1.51 2.15 2.55 3.09 3.63 4.17 5.48 0.48 2.48 7.62 20.13 28.72 40.16 63.56 DA-2 Proposed 19.40 C 3.0 300.00 0.028 600.00 0.040 2.5 0.50 20.00 30.80 0.18 0.19 0.20 0.22 20.26 40.26 40.26 1.29 1.82 2.16 2.62 3.08 3.54 4.65 4.49 6.72 8.39 17.08 24.38 15.11 53.95 Rainfall Chapter 5

Project Location 0.98

Figure 5-1. Rainfall depth-duration-frequency: 2-year, 1-hour rainfall

5-10 Urban Drainage and Flood Control District January 2016 Urban Storm Drainage Criteria Manual Volume 1 Chapter 5 Rainfall

Project Location 1.39

Figure 5-2. Rainfall depth-duration-frequency: 5-year, 1-hour rainfall

January 2016 Urban Drainage and Flood Control District 5-11 Urban Storm Drainage Criteria Manual Volume 1 Rainfall Chapter 5

Project Location 1.65

Figure 5-3. Rainfall depth-duration-frequency: 10-year, 1-hour rainfall

5-12 Urban Drainage and Flood Control District January 2016 Urban Storm Drainage Criteria Manual Volume 1 Chapter 5 Rainfall

Project Location 2.0

Figure 5-4. Rainfall depth-duration-frequency: 25-Year, 1-hour rainfall

January 2016 Urban Drainage and Flood Control District 5-13 Urban Storm Drainage Criteria Manual Volume 1 Rainfall Chapter 5

Project Location 2.35

Figure 5-5. Rainfall depth-duration-frequency: 50-year, 1-hour rainfall

5-14 Urban Drainage and Flood Control District January 2016 Urban Storm Drainage Criteria Manual Volume 1 Chapter 5 Rainfall

Project Location 2.7

Figure 5-6. Rainfall depth-duration-frequency: 100-year, 1-hour rainfall

January 2016 Urban Drainage and Flood Control District 5-15 Urban Storm Drainage Criteria Manual Volume 1