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Home My WebLink AboutFIRST STOP PLAZA - BASIC DEVELOPMENT REVIEW - BDR180031 - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE RELATED DOCUMENTFORT COLLINS: 301 North Howes Street, Suite 100, 80521 | 970.221.4158 GREELEY: 820 8 th Street, 80631 | 970.395.9880 | WEB: www.northernengineering.com September 12, 2018 Mr. Shane Boyle City of Fort Collins 700 Wood Street Fort Collins, Colorado 80522 RE: Preliminary Drainage Memorandum for 1st Stop Plaza Fort Collins, Colorado Dear Shane: Northern Engineering Services is pleased to submit this Preliminary Drainage Memorandum for 1st Stop Plaza. As a result of the recent design improvements to the intersection of Suniga Drive and College Avenue, the driveway access for Lots 1 and 2 of 1st Stop Plaza will be relocated. The existing driveway is a full-service access from College Avenue. In conjunction with the improvements to Suniga Drive, the access on College Avenue will become right turn in and out only and relocated 60 feet north of the current location. A full- service access will be provided from Suniga Drive 390 feet east of College Avenue. A shared driveway will be constructed to allow vehicular movement between the two new access locations. As part of the improvements for Suniga Drive and College Avenue intersection, Interwest Consulting Group prepared a drainage memorandum to North East College Corridor Outfall (NECCO) Design Report dated August 13, 2018. Lots 1 and 2 of 1st Stop Plaza coincide with Basin B of the Interwest Drainage Memorandum. Interwest assumed an impervious area of 44.4% and a runoff coefficient of 0.53 for Lots 1 and 2 of 1st Stop Plaza. The impervious area with the proposed driveway improvements is 35% and the runoff coefficient is 0.47. The proposed driveway improvements comply with assumptions used in the Interwest Drainage Memorandum for Suniga Drive. Detention and water quality for the site is part of the North East College Corridor Outfall (NECCO), but the site will need to meet the City’s LID requirements. Permeable pavers will be provided in the southeast corner of the site, and 92% of new impervious area (and 86% of the entire parking lot) will drain towards the permeable pavers. We feel the current proposed drainage plan for 1st Stop Plaza is in compliance with the concepts shown in the NECCO Design Report and the Interwest Drainage Memorandum for Suniga Drive and College Avenue. The proposed project will satisfy the City’s LID requirements. If you have any questions, please feel free to contact me at your earliest convenience. NORTHERN ENGINEERING SERVICES, INC. Frederick S. Wegert, PE Project Engineer 1st Stop Plaza CHARACTER OF SURFACE: Runoff Coefficient 1 Percentage Impervious 2 Project: Streets, Parking Lots, Roofs, Alleys, and Drives: Calculations By: Asphalt ………………… ................... .......................................................... 0.95 100% Date: Concrete ………………. ................... .......................................................... 0.95 96% Gravel (packed) ………. ................... .......................................................... 0.50 40% Roofs………………….. ................... .......................................................... 0.95 90% Pavers…………………. ................... .......................................................... 0.40 40% Lawns and Landscaping Sandy Soil Flat <2% ………….. ................... .......................................................... 0.10 0% Average 2% to 7% ................... .......................................................... 0.15 0% Steep >7% ………… ................... .......................................................... 0.20 0% Clayey Soil Flat <2% ………….. ................... .......................................................... 0.20 0% Average 2% to 7% ................... .......................................................... 0.25 0% Steep >7% ………… ................... .......................................................... 0.35 0% 2-year C f = 1.00 10-year C f = 1.00 USDA SOIL TYPE: C Sub-Basin ID Sub-Basin Area (sq. ft.) Sub-Basin Area (ac.) Area of Asphalt & Concrete (ac.) Area of Roofs (ac.) Area of Gravel (ac.) Area of Landscaping 2% to 7% (ac) 2-year Composite Runoff Coefficient 10-year Composite Runoff Coefficient 100-year Composite Runoff New Impervious Area 0.187 Ac. Paver Area 0.048 Ac. Traditional Pavement Area 0.139 Ac. Other Impervious Surfaces (Roofs, Concrete Walks, etc.) 0.000 Ac. Required Minimum Impervious Area to be Treated 0.094 Ac. Paver LID Treatment Paver area 0.048 Ac. Run-on area for Pavers (parking area) 0.125 Ac. Run-on area for Pavers (other - concrete walks, etc.) 0.000 Ac. Impervious Area Treated by LID Pavers 0.173 Ac. Rain Garden Treatment Rain Garden Area 0.000 Ac. Run-on area for Rain Garden 0.000 Ac. Impervious Area Treated by Rain Gardens 0.000 Ac. Total Impervious Area Treated 0.173 Ac. Percent of Impervious Area Treated 92.38 % New Pavement Area 8,165 Sq. Ft. Required Minimum Area of Permeable Pavement 2,041 Sq. Ft. Total Permeable Pavement Area 2,099 Sq. Ft. Actual % of Permeable Pavement Provided 25.7 % 50% On-Site Treatment by LID Requirement 25% Permeable Pavement Requirement Interwest Consulting Group RUNOFF COEFFICIENTS & % IMPERVIOUS LOCATION: SUNIGA PROJECT NO: 1326-069-00 COMPUTATIONS BY: ES DATE: 7/18/2018 Recommended Runoff Coefficients from Table RO-11 of City of Fort Collins Stormwater Code, Volume I Recommended % Impervious from Table RO-3 Urban Storm Drainage Criteria Manual, Volume I Runoff % coefficient Impervious C Streets, parking lots (asphalt): 0.95 100 Sidewalks (concrete): 0.95 96 Roofs: 0.95 90 Gravel or Pavers: 0.50 40 Landscape Areas (Flat, heavy) : 0.20 0 Landscape Areas (Steep, heavy) : 0.35 0 SUBBASIN TOTAL TOTAL ROOF PAVED PAVERS SIDEWALK LANDSCAPE RUNOFF % DESIGNATION AREA AREA AREA AREA AREA AREA AREA COEFF. Impervious REMARKS (ac.) (sq.ft) (sq.ft) (sq.ft) (sq.ft) (sq.ft) (sq.ft) (C) A 0.98 42,631 0 42,631 0 0 0 0.95 100 NORTH HALF SUNIGA B 3.22 140,134 0 62,219 0 0 77,914 0.53 44.4 C 0.98 42,720 0 42,720 0 0 0 0.95 100 SOUTH HALF SUNIGA Equations - Calculated C coefficients & % Impervious are area weighted C = Σ (Ci Ai) / At Ci = runoff coefficient for specific area, Ai Ai = areas of surface with runoff coefficient of Ci n = number of different surfaces to consider At = total area over which C is applicable; the sum of all Ai's 7-18-18 FC Flow Suniga.xls Coefficient Composite % Imperv. B 139,891 3.21 0.94 0.21 0.00 2.07 0.47 0.47 0.58 35% 2) Percent impervious are taken from Suniga Road Design, College Avenue to Blondel Street Drainage Memorandum prepare by Interwest Consulting Group. 100-year C f = 1.25 1) Runoff coefficients are taken from the Fort Collins Stormwater Criteria Manaual, Table 3.2.1 and Table 3.2.2. Composite Runoff Coefficient with Adjustment PROPOSED COMPOSITE % IMPERVIOUSNESS AND RUNOFF COEFFICIENT CALCULATIONS 1st Stop Plaza F. Wegert September 12, 2018 9/12/2018 2:30 PM D:\Projects\1564-001\Drainage\Hydrology\1564-001_Rational Fort Collins.xlsx\Composite C