HCI ENGINEERING. March 27, City of Thornton, Development Engineering 9500 Civic Center Drive Thornton, CO 80229

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Blake Kelly
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1 HCI ENGINEERING A division of HABERER CARPENTRY INC. March 27, 2014 Attn: Heath Klein, P.E. City of Thornton, Development Engineering 9500 Civic Center Drive Thornton, CO Re: Drainage Letter for T Mobile Site at Plazas Las Americas Dear Mr. Klein, This letter serves to inform you that the proposed development for the T Mobile Store Site at 88th Avenue and Washington Street complies with the Final Drainage Report for Plaza Las Americas, by S.A. Miro, Inc., with a final revision date of October 11, 2007 (hereafter referred to as the Master Report ). The intent of this letter is to discuss drainage patterns and design with respect to the above mentioned site as a substitute to a required drainage report. The following letter is written on behalf of Washington Box LLC, Denver CO Project Description: The proposed T Mobile Store Site is located on Lot 5A4, City View Heights Subdivision, Filing No. 1, Amendment No. 3, in the northwest quarter of Section 26, Township 2 South, Range 68 West, of the Sixth Principal Meridian, City of Thornton, County of Adams, State of Colorado. This site will be part of the existing Plaza Las Americas commercial development consisting of a grocery store and other retail buildings and pad sites. The property currently consists of approximately 0.7 acres of previously graded, partially vegetated, undeveloped land. The site has approximately 10 feet of grade change from the high point nearest 88th Avenue to the low point at the southwest corner. The existing grade across the middle of the site is approximately 2.5%. The site slopes from north to south at about 2.5% (the exception is the landscape lining the west boundary this landscaping forms a drop off to Washington Street which is not considered as part of the overall slope. The proposed development will include vertical construction of approximately 2,520 square feet of building for the purpose of cell phone sales/service center and parking areas with associated drive lanes, parking stalls, trash/recycling enclosure, utilities and landscaping. Refer to the following site map for location and details: 621 SOUTHPARK DR., SUITE #1600 LITTLETON, CO (720) FAX (303) COLEH@HABERERGROUP.COM Page 1 of 3

SITE MAP Hydrologic Compliance: The project area consists of a portion of the Master Report drainage basins A2, and A3. The Master Report assumed a composite imperviousness for these two basins of 78.

2 SITE MAP Hydrologic Compliance: The project area consists of a portion of the Master Report drainage basins A2, and A3. The Master Report assumed a composite imperviousness for these two basins of 78.2%. The Master Report designed a regional detention pond at the southeast corner of the Plaza Las Americas development. This pond provides water quality and detention for the T Mobile Store Site. The proposed site composite percent imperviousness is 48.41%, thus remaining in compliance with the Master Report. Please reference the enclosed Site Land Usage Exhibit for composite imperviousness calculations. The proposed development will consist of a commercial building that is approximately 2,520 square feet. 8.2%, (2,520 sf), of the site will consist of building, 49.1%, (15,110 sf), will consist of landscape and 42.7%, (13,136 sf) will consist of paving and walks. Per Master Report, the approved percent imperviousness for the pad site is 80%. The proposed development has an imperviousness value of 48.41%. Please reference the enclosed drainage exhibit for composite imperviousness calculations and comparison exhibits (Figures C thru F). In addition, the following, illustrates that the Proposed Condition Runoff Coefficients are less than the approved Commercial values for this site: EVENT EXISTING C VALUE PROPOSED C VALUES APPROVED C VALUES 2 YEAR 5 YEAR N/A YEAR SOUTHPARK DR., SUITE #1600 LITTLETON, CO (720) FAX (303) COLEH@HABERERGROUP.COM Page 2 of 3

The existing drainage patterns will be maintained and on site flows will be conveyed southeast via private curb and gutter, and drainage pans.

Phase III Drainage Report Compliance Les Schwab Tire Center 88 th & Washington M/M Project No. 12.

Stormwater runoff from the existing site drains offsite via overland sheet flow to the southeast access drive.

3 The existing drainage patterns will be maintained and on site flows will be conveyed southeast via private curb and gutter, and drainage pans. These flows will enter a proposed curb and gutter at the driveway near the site access point, and will be conveyed along existing curb and gutter to an existing sump inlet at Design Point 8 of the Phase III Drainage Report Compliance Les Schwab Tire Center 88 th & Washington M/M Project No report prepared by Martin/Martin and dated Feb 4 th, 2013, hereafter referred to as the Les Schwab Tire Center Report. Stormwater runoff from the existing site drains offsite via overland sheet flow to the southeast access drive. Runoff is directed by curb and gutter to an existing Type R inlet southeast of the site near the Master Report design point 3. An existing 24 storm sewer directs runoff southeast to the regional detention pond. The existing 24 storm sewer was designed and installed per the Master Report to serve the T Mobile project area. The storm sewer was designed to accept a routed 100 yr flow of 19.1 CFS from Basins A1 A4 at design point 4. The attached Rational Method calculation shows that that the proposed development will contribute a 100 yr flow of 4.4 CFS at this location. The 2 year Minor Storm will contribute less than 1.0 CFS to the existing inlet discussed in the following paragraph. As assumed by the Master Report, the proposed design will serve the entire project site. Developed Sub Basins D2 and D3 will contribute some direct flow to an existing inlet at design point MR4 of the Les Schwab Tire Center Report. Please refer to Figures E & F of the Appendix for basin comparisons. Surface runoff at the west boundary of the site (0.2 acres) will be conveyed via sheet flow condition across landscaped areas and into the North Washington Street Corridor. Incidental water quality is provided in addition to conveying the runoff from this site. Based upon the above observations the proposed development complies with the master drainage study for this site. In summary, it is the opinion of HCI Engineering that the proposed site runoff and drainage patterns are in compliance with the above referenced drainage reports. As mentioned above both detention and water quality are already provided per the master report for this site. HCI Engineering does not foresee adverse impacts to downstream or adjacent properties due to the development of this site. Please refer to the attached calculations and feel free to contact me at any time. Sincerely, Cole C Haberer, P.E. #43259 HCI Engineering Attachments: FIGURE A, Vicinity Map FIGURE B, NRSC Soil Survey (9 Pages) FIGURE C, Percent Impervious, Time of Concentration and Runoff Flows FIGURE D, Land usage Exhibit T Mobile Store Site Only FIGURE E, Drainage Map ( Master Report Basin Comparison) FIGURE F, Drainage Map ( Les Schwab Tire Center Basin Comparison) 621 SOUTHPARK DR., SUITE #1600 LITTLETON, CO (720) FAX (303) COLEH@HABERERGROUP.COM Page 3 of 3

104 58' 39'' W Soil Map Adams County Area, Parts of Adams and Denver Counties, Colorado 104 58' 37'' W 39 51' 22'' N 39 51' 19'' N 501903 501912 501921 501930 501939 501948 501957 4411722 4411731

5 104 58' 39'' W Soil Map Adams County Area, Parts of Adams and Denver Counties, Colorado ' 37'' W 39 51' 22'' N 39 51' 19'' N ' 22'' N 39 51' 19'' N ' 39'' W N Map Scale: 1:359 if printed on A portrait (8.5" x 11") sheet. Meters Feet Map projection: Web Mercator Corner coordinates: WGS84 Edge tics: UTM Zone 13N WGS ' 37'' W Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey FIG B1 3/27/2014 Page 1 of 3

6 Soil Map Adams County Area, Parts of Adams and Denver Counties, Colorado MAP LEGEND MAP INFORMATION Area of Interest (AOI) Area of Interest (AOI) Soils Soil Map Unit Polygons Soil Map Unit Lines Soil Map Unit Points Special Point Features Blowout Borrow Pit Clay Spot Closed Depression Gravel Pit Gravelly Spot Landfill Lava Flow Marsh or swamp Mine or Quarry Miscellaneous Water Perennial Water Rock Outcrop Saline Spot Sandy Spot Severely Eroded Spot Sinkhole Slide or Slip Sodic Spot Spoil Area Stony Spot Very Stony Spot Wet Spot Other Special Line Features Water Features Streams and Canals Transportation Rails Interstate Highways US Routes Major Roads Local Roads Background Aerial Photography The soil surveys that comprise your AOI were mapped at 1:20,000. Warning: Soil Map may not be valid at this scale. Enlargement of maps beyond the scale of mapping can cause misunderstanding of the detail of mapping and accuracy of soil line placement. The maps do not show the small areas of contrasting soils that could have been shown at a more detailed scale. Please rely on the bar scale on each map sheet for map measurements. Source of Map: Natural Resources Conservation Service Web Soil Survey URL: Coordinate System: Web Mercator (EPSG:3857) Maps from the Web Soil Survey are based on the Web Mercator projection, which preserves direction and shape but distorts distance and area. A projection that preserves area, such as the Albers equal-area conic projection, should be used if more accurate calculations of distance or area are required. This product is generated from the USDA-NRCS certified data as of the version date(s) listed below. Soil Survey Area: Adams County Area, Parts of Adams and Denver Counties, Colorado Survey Area Data: Version 10, Dec 24, 2013 Soil map units are labeled (as space allows) for map scales 1:50,000 or larger. Date(s) aerial images were photographed: Apr 29, 2011 Apr 13, 2012 The orthophoto or other base map on which the soil lines were compiled and digitized probably differs from the background imagery displayed on these maps. As a result, some minor shifting of map unit boundaries may be evident. Natural Resources Conservation Service FIG B2 Web Soil Survey National Cooperative Soil Survey 3/27/2014 Page 2 of 3

7 Soil Map Adams County Area, Parts of Adams and Denver Counties, Colorado Map Unit Legend PlB ReD Adams County Area, Parts of Adams and Denver Counties, Colorado (CO001) Map Unit Symbol Map Unit Name Acres in AOI Percent of AOI Platner loam, 0 to 3 percent slopes Renohill loam, 3 to 9 percent slopes % % UlC Ulm loam, 3 to 5 percent slopes % Totals for Area of Interest % Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 3/27/2014 Page 3 of 3 FIG B3

8 Map Unit Description: Ulm loam, 3 to 5 percent slopes---adams County Area, Parts of Adams and Denver Counties, Colorado Adams County Area, Parts of Adams and Denver Counties, Colorado UlC Ulm loam, 3 to 5 percent slopes Map Unit Setting Elevation: 4,000 to 5,600 feet Mean annual precipitation: 12 to 14 inches Mean annual air temperature: 48 to 52 degrees F Frost-free period: 125 to 155 days Map Unit Composition Ulm and similar soils: 80 percent Minor components: 20 percent Description of Ulm Setting Landform: Plains Landform position (three-dimensional): Talf Down-slope shape: Linear Across-slope shape: Linear Parent material: Residuum weathered from sandstone and shale Properties and qualities Slope: 3 to 5 percent Depth to restrictive feature: 40 to 60 inches to paralithic bedrock Drainage class: Well drained Capacity of the most limiting layer to transmit water (Ksat): Moderately low to moderately high (0.06 to 0.20 in/hr) Depth to water table: More than 80 inches Frequency of flooding: None Frequency of ponding: None Calcium carbonate, maximum content: 15 percent Maximum salinity: Nonsaline (0.0 to 2.0 mmhos/cm) Available water capacity: Moderate (about 8.2 inches) Interpretive groups Farmland classification: Prime farmland if irrigated Land capability classification (irrigated): 3e Land capability (nonirrigated): 4e Hydrologic Soil Group: C Ecological site: Loamy Plains (R067BY002CO) Typical profile 0 to 7 inches: Loam 7 to 13 inches: Silty clay 13 to 30 inches: Clay 30 to 48 inches: Clay loam 48 to 52 inches: Unweathered bedrock Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 3/27/2014 Page 1 of 2 FIG B4

9 Map Unit Description: Ulm loam, 3 to 5 percent slopes---adams County Area, Parts of Adams and Denver Counties, Colorado Minor Components Renohill Percent of map unit: 13 percent Shingle Percent of map unit: 5 percent Apishapa Percent of map unit: 2 percent Landform: Swales Data Source Information Soil Survey Area: Adams County Area, Parts of Adams and Denver Counties, Colorado Survey Area Data: Version 10, Dec 24, 2013 Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 3/27/2014 Page 2 of 2 FIG B5

10 Map Unit Description: Renohill loam, 3 to 9 percent slopes---adams County Area, Parts of Adams and Denver Counties, Colorado Adams County Area, Parts of Adams and Denver Counties, Colorado ReD Renohill loam, 3 to 9 percent slopes Map Unit Setting Elevation: 4,000 to 5,600 feet Mean annual precipitation: 12 to 14 inches Mean annual air temperature: 48 to 52 degrees F Frost-free period: 125 to 155 days Map Unit Composition Renohill and similar soils: 85 percent Minor components: 15 percent Description of Renohill Setting Landform: Plains Landform position (three-dimensional): Talf Down-slope shape: Linear Across-slope shape: Linear Parent material: Residuum weathered from shale Properties and qualities Slope: 3 to 9 percent Depth to restrictive feature: 20 to 40 inches to paralithic bedrock Drainage class: Well drained Capacity of the most limiting layer to transmit water (Ksat): Moderately low to moderately high (0.06 to 0.20 in/hr) Depth to water table: More than 80 inches Frequency of flooding: None Frequency of ponding: None Calcium carbonate, maximum content: 15 percent Maximum salinity: Nonsaline (0.0 to 2.0 mmhos/cm) Available water capacity: Low (about 4.8 inches) Interpretive groups Farmland classification: Not prime farmland Land capability classification (irrigated): 4e Land capability (nonirrigated): 6e Hydrologic Soil Group: C Ecological site: Loamy Plains (R067BY002CO) Typical profile 0 to 4 inches: Loam 4 to 23 inches: Clay 23 to 28 inches: Clay loam 28 to 32 inches: Unweathered bedrock Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 3/27/2014 Page 1 of 2 FIG B6

11 Map Unit Description: Renohill loam, 3 to 9 percent slopes---adams County Area, Parts of Adams and Denver Counties, Colorado Minor Components Terry Percent of map unit: 5 percent Shingle Percent of map unit: 5 percent Samsil Percent of map unit: 5 percent Data Source Information Soil Survey Area: Adams County Area, Parts of Adams and Denver Counties, Colorado Survey Area Data: Version 10, Dec 24, 2013 Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 3/27/2014 Page 2 of 2 FIG B7

12 Map Unit Description: Platner loam, 0 to 3 percent slopes---adams County Area, Parts of Adams and Denver Counties, Colorado Adams County Area, Parts of Adams and Denver Counties, Colorado PlB Platner loam, 0 to 3 percent slopes Map Unit Setting Elevation: 4,200 to 5,500 feet Mean annual precipitation: 13 to 15 inches Mean annual air temperature: 48 to 52 degrees F Frost-free period: 135 to 155 days Map Unit Composition Platner and similar soils: 80 percent Minor components: 20 percent Description of Platner Setting Landform: Plains Landform position (three-dimensional): Talf Down-slope shape: Linear Across-slope shape: Linear Parent material: Alluvium derived from mixed Properties and qualities Slope: 0 to 3 percent Depth to restrictive feature: More than 80 inches Drainage class: Well drained Capacity of the most limiting layer to transmit water (Ksat): Moderately low to moderately high (0.06 to 0.20 in/hr) Depth to water table: More than 80 inches Frequency of flooding: None Frequency of ponding: None Calcium carbonate, maximum content: 15 percent Available water capacity: High (about 10.2 inches) Interpretive groups Farmland classification: Prime farmland if irrigated Land capability classification (irrigated): 2e Land capability (nonirrigated): 3c Hydrologic Soil Group: C Ecological site: Loamy Plains (R067BY002CO) Typical profile 0 to 9 inches: Loam 9 to 18 inches: Clay 18 to 49 inches: Clay loam 49 to 60 inches: Sandy loam Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 3/27/2014 Page 1 of 2 FIG B8

13 Map Unit Description: Platner loam, 0 to 3 percent slopes---adams County Area, Parts of Adams and Denver Counties, Colorado Minor Components Weld Percent of map unit: 10 percent Ulm Percent of map unit: 10 percent Data Source Information Soil Survey Area: Adams County Area, Parts of Adams and Denver Counties, Colorado Survey Area Data: Version 10, Dec 24, 2013 Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 3/27/2014 Page 2 of 2 FIG B9

14 HCI ENGINEERING A division of HABERER CARPENTRY INC. Surface % Imp. C 2 Value C 5 Value C 10 Value C 100 Value Asphalt Roof Landscape Concrete Values per Figures RO 3 & RO 5 in Runoff Chapter of USDCM for typ C soils Sub basin I.D. Formula: [(Area)(C Value)+(Area)(C Value)+(Area)(C Value)]/(Total Area) Basin I.D. Surface Type Area (S.F.) Asphalt Roof Concrete Landscape Total Area (S.F.) % Impervious C 2 C 5 C10 C 100 PROPOSED % 8.19% 16.60% 49.11% FIGURE C1 Impervious Cover

15 TIME OF CONCENTRATION Basin Data Initial / Overland Time (t i ) Travel Time (t t ) t c Check (Urbanized Basins) Final t c AREA Length SLOPE Length SLOPE Vel. Total tc = (L/180)+10 Basin I.D. C 5 t i t t COMP t c Min (Ac) (ft) (%) (ft) (%) (FPS) Length Ft Min PROPOSED FIGURE C2 Time of Concentration

16 P P POINT VALUES from Figures RA 2 & RA 6, UDFCD (V.1) DESIGN POINT AREA DESIGN AREA (AC) RUNOFF COEFF tc (min) C x A (AC) I IN/HR Q (CFS) AREA DESIGN AREA (AC) RUNOFF COEFF tc (min) C x A (AC) I IN/HR Q (CFS) AT DESIGN POINTS SUM OF 100 YR FLOWS (CFS) HCI ENGINEERING A division of HABERER CARPENTRY INC. DIRECT RUNOFF 5 YR STORM DIRECT RUNOFF 100 YR STORM PROPOSED PROPOSED FIGURE C3A Runoff Flows

17 HCI ENGINEERING A division of HABERER CARPENTRY INC. P POINT VALUE From Figure RA 1, UDFCD (V.1) DIRECT RUNOFF 2 YR STORM DESIGN POINT AREA DESIGN AREA (AC) RUNOFF COEFF t c (min) C x A (AC) I IN/HR Q (CFS) PROPOSED FIGURE C3B Runoff Flows

18 ASPHALT: SF / CONC: 5106 SF / AC ROOF: SF / AC LS: SF / AC FIGURE - D LAND USE EXHIBIT DATE: SHEET 1 OF 1 SUBMITTAL:

A2 0.37 0.54 0.69 A3 1.25 D3 0.63 0.74 0.

22 AC D1 - SHEET FLOWS TO STREET CORRIDOR (NO

19 A A D AC D1 ON-SITE SUB-BASINS 0.22 AC D1 - SHEET FLOWS TO STREET CORRIDOR (NO DETENTION PROVIDED) D2 - MAIN SITE AREA - PAN AND GUTTER FLOWS TO EXISTING INLET D3 - EXISTING PARKING - FLOWS SOUTH TO EXISTING INLET D AC DESIGN POINT 3 DESIGN POINT 4 FIGURE - E DATE: SHEET 1 OF 1 BASIN COMPARISON - MASTER REPORT SUBMITTAL:

EXISTING INLET D1 0.22 AC D2 0.46 AC D3 0.29 AC DESIGN POINT 8 EX.

20 ON-SITE SUB-BASINS D1 - SHEET FLOWS TO STREET CORRIDOR (NO DETENTION PROVIDED) D2 - MAIN SITE AREA - PAN AND GUTTER FLOWS TO EXISTING INLET D3 - EXISTING PARKING - FLOWS SOUTH TO EXISTING INLET D AC D AC D AC DESIGN POINT 8 EX. INLET (SUMP) FIGURE - F DATE: SHEET 1 OF 1 BASIN COMPARISON - LES SCHWAB TIRE CENTER REPORT SUBMITTAL:

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