Astra Engineering LLC 7887 E. Belleview Avenue, Suite 1100, Englewood, Colorado 80111 Ph. 303-475-5691 DRAINAGE CONFORMANCE LETTER Lot 1E, Skylake Ranch, Amendment #5. City of Thornton, Adams County, State of Colorado July 27, 2016 Jason O’Shea Development Director City of Thornton City Development Department 9500 Civic Center Drive Thornton, CO 80229-4326 303-538-7295 Re: Skylake Ranch Daycare Dear Mr. O’Shea: This letter is written to state that the drainage patterns and provisions for Skylake Ranch Daycare, Lot 1E, Skylake Ranch Amendment #5, now named “The Learning Experience – Skylake Ranch Plaza” conforms to the “Preliminary Drainage Study for Skylake Ranch,” prepared by CLC Associates, Inc., Greenwood Village, Colorado, August, 2012, (”Preliminary Report”,) and also to the “Final Drainage Report for Skylake Ranch,” Point Consulting LLC, Littleton, CO, June 29, 2016 (“Final Report”. ) The 1.16-acre site is located in the southwest corner of the 11.7-acre overall site at the northwest corner of 119 th Avenue and Holly Street. It slopes to the southeast at an average slope of 2.7%, and is covered with native grasses and weeds. Soils are loamy, and of Hydrologic soil type “C.” Most of the site is a rectangular area for the daycare center and parking, with a small dog-leg north to tie the lot into a partially-completed internal drive, via a shared access. Runoff for the site will be carried by storm sewer and as sheet flow towards a regional detention pond to be provided by the owner of the overall development (Skylake Ranch) adjacent to the southwest corner of the site. Peripheral landscape areas facing Holly Street and 119 th Avenue drain undetained to the streets at near or less than existing flows, as most of the area will be diverted to the pond. The preliminary report assumed 95% imperviousness for this commercial shopping area. No detention sizing was made to the southern end of the site, and the proposed imperviousness for the TLE site is 57%.
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DRAINAGE CONFORMANCE LETTER - City of Thornton · DRAINAGE CONFORMANCE LETTER Lot 1E, Skylake Ranch, Amendment #5. City of Thornton, Adams County, State of Colorado July 27, 2016
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Astra Engineering LLC
7887 E. Belleview Avenue, Suite 1100, Englewood, Colorado 80111 Ph. 303-475-5691
DRAINAGE CONFORMANCE LETTER
Lot 1E, Skylake Ranch, Amendment #5. City of Thornton, Adams County, State of Colorado
July 27, 2016 Jason O’Shea Development Director City of Thornton City Development Department 9500 Civic Center Drive Thornton, CO 80229-4326 303-538-7295 Re: Skylake Ranch Daycare Dear Mr. O’Shea: This letter is written to state that the drainage patterns and provisions for Skylake Ranch Daycare, Lot 1E, Skylake Ranch Amendment #5, now named “The Learning Experience – Skylake Ranch Plaza” conforms to the “Preliminary Drainage Study for Skylake Ranch,” prepared by CLC Associates, Inc., Greenwood Village, Colorado, August, 2012, (”Preliminary Report”,) and also to the “Final Drainage Report for Skylake Ranch,” Point Consulting LLC, Littleton, CO, June 29, 2016 (“Final Report”. ) The 1.16-acre site is located in the southwest corner of the 11.7-acre overall site at the northwest corner of 119th Avenue and Holly Street. It slopes to the southeast at an average slope of 2.7%, and is covered with native grasses and weeds. Soils are loamy, and of Hydrologic soil type “C.” Most of the site is a rectangular area for the daycare center and parking, with a small dog-leg north to tie the lot into a partially-completed internal drive, via a shared access. Runoff for the site will be carried by storm sewer and as sheet flow towards a regional detention pond to be provided by the owner of the overall development (Skylake Ranch) adjacent to the southwest corner of the site. Peripheral landscape areas facing Holly Street and 119th Avenue drain undetained to the streets at near or less than existing flows, as most of the area will be diverted to the pond. The preliminary report assumed 95% imperviousness for this commercial shopping area. No detention sizing was made to the southern end of the site, and the proposed imperviousness for the TLE site is 57%.
The site was divided into 8 small basins as well as one offsite basin to the northeast, which will be tributary to the site at the north access drive. This will allow that area to drain to the regional detention pond, as planned by the previous master drainage studies. Flowrates for the small basins are shown on the drainage plan and in the calculations in the Appendix of the report. The runoff coefficient for the offsite basin (O1, 0.60 acres) was taken from the values assumed in the Final Report. The onsite sub-basins were labeled starting with “B1” instead of the usual “A” names to avoid confusion with the Final Report, the plan for which is reproduced on this site’s drainage plan According to the Final Report the overall developer for the larger site will construct a private storm sewer system beginning approximately 125’ northeast of the TLE site within the existing access drive for the overall site. The system will continue southwest, turning south just beyond the north corner of the TLE site, with sump inlets on both sides of the access road again. From there it drains south then west along the north edge of the regional pond to an outfall into the pond near the northwest corner of it. Private storm sewer (12” PVC) will extend west to tie into the regional storm at two locations, one from a low point in the shared access drive, and one from a low point in TLE’s parking lot. Sub-basin B1 (0.4 acres of playground, landscape and parking lot drains to a sump inlet near the southeast corner of the parking lot. Sub-basins B2 and B3 consist of half the roof each (0.11 acres each) which will drain north in 6” PVC pipes to tie into the 12” PVC. A 5-foot curb opening inlet will have the capacity for both the 5-year and 100-year storm. (See capacity calculations in the Appendix.) Offsite sub-basin O1 drains east, joining runoff from sub-basin B4 (0.18 acres of parking and landscaping) at the low point in the access drive. A 5-foot curb opening inlet will pick up the 5-year and approximately half of the 100-year storm. Sub-basin B5 (0.08 acres of parking lot and landscape) will drain to the west side of the access drive. A 5-foot curb opening inlet will pick up the 5-year runoff and the 100-year storm, plus potentially the excess 100-year runoff from the east side of the street. Sub-basin C1 (0.07 acres of landscape and sidewalk) along the west side of the site will sheet flow off into the regional pond. Sub-basins D1 (0.18 acres of landscape and sidewalk) will sheet flow off to the south (119th Avenue), and Sub-basin D2 (0.2 acres of landscape) will sheet flow off to Holly Street. The hydraulic grade line analysis was taken from the Final Report, which includes the laterals extending into the site, in order to show how the entire system will work. The Final Report analysis shows the 5-year storm will be contained within the storm system, and there may be some ponding and overflow towards the regional pond,
during the 100-year storm. Complete analysis of the ponding and over flow will have to be completed when the design of the offsite storm system by others is finalized Conclusion The regional pond west of the proposed site will provide detention and water quality improvement for the impervious areas of the site. It is being designed concurrently by Point Consulting, LLC, Littleton, Colorado. We believe the drainage provisions for the proposed development of the “The Learning Experience – Skylake Ranch Plaza” (Skylake Ranch Daycare,) Lot 1E, Skylake Ranch Amendment #5, is in substantial conformance with the approved preliminary drainage study, and the Final Drainage Report, currently under review. Sincerely, Connie Ellefson, P.E. Astra Engineering, LLC 303-523-3664
DND Properties, LLC hereby certifies that the drainage facilities for The Learning Experience, Skylake Ranch, will be constructed according to the design presented in this report. I understand that the City of Thornton does not and shall not assume liability for the drainage facilities designed and/or certified by my engineer. I understand that the City of Thornton reviews drainage plans but cannot, on behalf of DND Properties, LLC, guarantee that final drainage design review will absolve DND Properties, LLC and/or their successors and/or assigns of future liability for improper design. I further understand that approval of the Plat and/or Development Permit does not imply approval of my engineer's drainage design. Attest: ______________________________ DND Properties, LLC ___________________________ ______________________________ Notary Public Authorized Signature "I hereby certify that this report (plan) for the final drainage design of The Learning Experience, Skylake Ranch was prepared by me (or under my direct supervision) in accordance with the provisions of the City of Thornton Standards and Specifications for the Design and Construction of Public and Private Improvements for the Responsible Parties thereof. I understand that the City of Thornton does not and shall not assume liability for drainage facilities designed by others _________________________________________ Registered Professional Engineer State of Colorado No. 23371
Astra Engineering Inc.The Learning Center
Thornton, CO 7/26/2016
Proposed Composite Coefficients of Runoff
Site 1.11 acres 48400
Proposed Composite Coefficients of Runoff
Cover type Area (sf) Imperviousness
Landscape 17,479 2%
Roof 10,000 90%
Playground 5,206 75%Concrete 15,715 90%
48,400
Proposed Imperviousness I= 56.6%
B1 0.40 acres 17211Proposed Composite Coefficients of Runoff
Cover type Area (sf) Imperviousness
Landscape 10,830 2%Roof 0 90%
Playground 5,206 75%Concrete 1,175 90%
17,211
Proposed Imperviousness I= 30.1%
c5 = 0.30
c100= 0.57
B2 and B3 are all roof I = 90%
c5 = 0.75
c100= 0.83
B4 0.18 acres 7710Proposed Composite Coefficients of Runoff
Cover type Area (sf) Imperviousness
Landscape 460 2%Roof 0 90%
Playground 0 75%
Concrete 7,250 90%
7,710
Proposed Imperviousness I= 84.7%
c5 = 0.68
c100= 0.79
B5 0.08 acres 3320Proposed Composite Coefficients of Runoff
Cover type Area (sf) Imperviousness
Landscape 400 2%
Roof 0 90%
Playground 0 75%
Concrete 2,920 90%
3,320
Proposed Imperviousness
I= 79.4%
c5 = 0.63
c100= 0.74
C1 0.07 acres 3156Proposed Composite Coefficients of Runoff
Cover type Area (sf) Imperviousness
Landscape 1,846 2%
Roof 0 90%
Playground 0 75%
Concrete 1,310 90%
3,156
Proposed Imperviousness I= 38.5%
c5 = 0.34
c100= 0.58
D1 0.18 acres 7642
Proposed Composite Coefficients of Runoff
Cover type Area (sf) Imperviousness
Landscape 6,882 2%Roof 0 90%
Playground 0 75%Concrete 760 90%
7,642
Proposed Imperviousness
I= 10.8%
c5 = 0.22
c100= 0.53
STANDARD FORM SF - 1TIME OF CONCENTRATION
PROJECT: Thornton TLEDATE: 7/26/2016 tc = ti + tt
INITIAL / OVERLAND TRAVEL TIME tc CHECK FINAL REMARKS
TIME (ti) (tt) (URBANIZED BASINS) tc
BASIN AREA C5 LENGTH SLOPE ti LENGTH SLOPE VEL. tt COMP.TOTAL
LENGTHtc=(l/180)
+10DESIGN. Ac Ft % Min Ft % FPS Min tc (Ft) Min Min
1. G.C. MUST PROVIDE & INSTALL ALL PRODUCTSPER PLANS. ONLY SUBSTITUTED PRODUCTS NEEDTO BE SUBMITTED TO THE ARCHITECT FORAPPROVAL. UNAPPROVED SUBSTITUTIONS WILLBE REPLACED AT THE EXPENSE OF THE G.C.
2. VERBAL REPRESENTATION HAS NO VALUE ANDALL REQUESTS TO CHANGE ANY PRODUCTS ORSPECIFICATIONS PER PLANS, MUST BESUBMITTED IN WRITING TO THE ARCHITECT &T.L.E. FOR APPROVAL.
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DATE: ISSUE:TLE REVIEW03.02.201695% TLE REVIEW SET04.08.2016
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100% FINAL REVIEW SET04.26.2015CONSTR. REV. #107.15.2016 1
THE LEARNING EXPERIENCE
CONSTRUCTION PLANS
FROM: "FINAL DRAINAGE REPORT FOR SKYLAKERANCH," POINT CONSULTING LLC," LITTLETON, COJUNE 29, 2016
1 OF 1
DRAINAGE PLAN
FINAL DRAINAGE PLAN
SCALE: 1" = 40'
SCALE: 1" = 70'
WORK SHALL BE CONSTRUCTED TO CITY OF THORNTONSTANDARDS AND SPECIFICATIONS. THIS APPROVAL ISFOR CONFORMANCE TO THESE STANDARDS ANDSPECIFICATIONS AND OTHER CITY REQUIREMENTS. THEDESIGN AND CONCEPT REMAINS THE RESPONSIBILITY OFTHE PROFESSIONAL ENGINEER OR LANDSCAPEPROFESSIONAL.
SUMMARY RUNOFF TABLE
DESIGN POINT CONTRIBUTING CONTRIBUTING 5-YR 100-YR BASIN(S) AREA RUNOFF RUNOFF