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Home My WebLink AboutFOOTHILLS MALL MULTI-FAMILY - MAJOR AMENDMENT - MJA150002 - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORT5285 McWhinney Blvd., Suite 160 TEL 303.237.2072 Loveland, CO 80538 FAX 303.237.2659 www.olssonassociates.com April 24, 2015 Stormwater Engineering City of Fort Collins 281 North College Avenue Fort Collins, CO 80524 RE: Drainage Analysis for the Major Amendment to a Portion of Foothills Mall Redevelopment Planning Area 3 (Lots 3, 4, and 5 of Foothills Mall Redevelopment) Dear Sir or Madam: Olsson has completed an analysis of the drainage design for Lot 3, 4, and 5 of Foothills Mall Redevelopment to determine the changes from the original to the proposed design. The original design is defined within the following drainage report: Storm Water Management Plan for Foothills Redevelopment, prepared by R.A. Smith National, dated Revised December 5, 2013 (existing drainage report). The lots fall within Basin 25 of the Existing Drainage Report. The original basin’s imperviousness was compared to the imperviousness of the proposed design to determine if further analysis would be required. The original design’s overall composite percent imperviousness was determined to be 68 percent. The percent imperviousness of the proposed design was calculated to be 63 percent. Based on the calculations, a reduction of the developed runoff would be anticipated. See attached calculations and supporting material. Since the intended drainage patterns will remain the same, detailed analysis will not be required. In addition, the runoff for original design was intended to drain to sand filters and rain gardens within the Foothills Mall Redevelopment: Sand Filters SF-2.0 and SF-4.0; and Rain Gardens RG-29 and RG-31. These drainage systems have been designed per the existing drainage report and are being constructed as part of the Foothills Redevelopment Planning Area 2 Final Plans. The rain garden and sand filter design’s assumed the contributing area was 90 percent imperviousness. Based on that assumption, additional analyses will not be required. Since rain garden RG-33 was being constructed as part of Foothills Redevelopment Planning Area 3 Final Plans, analysis of the rain garden was completed for the proposed design. Based on the analysis a rain garden will be provided in the northeast corner of Lot 3 meeting the original intent. The following calculations and supporting material are attached for your reference: • Exhibit A – Vicinity Map • Exhibit B – Existing Drainage Basins • Exhibit C – Percent Imperviousness Drainage Analysis for the Major Amendment to a Portion of Foothills Mall Redevelopment Planning Area 3 (Lots 3, 4, and 5 of Foothills Mall Redevelopment) Page 2 April 24, 2015 • Exhibit D – Lot Map (Rain Garden Locations) • Exhibit E – Revised Rain Garden, RG-33 • Exhibit F – Revised Rain Garden, RG33, Calculations If you have any questions or need additional information, please contact me. Sincerely, Olsson Associates Daniel Hull, PE Senior Project Engineer Enclosures TEL 970.431.7733 Loveland, CO 80538 Suite 160 5285 McWhinney Boulevard Foothills Mall Multi-Family R Exhibit A Vicinity Map Map taken from Storm Water Management Plan for Foothills Redevelopment, prepared by R.A. Smith National, dated Revised December 5, 2013 Lot 3, 4 and 5 TEL 970.431.7733 Loveland, CO 80538 Suite 160 5285 McWhinney Boulevard Foothills Mall Multi-Family Exhibit B R Existing Drainage Basins Lot 3 , 4, and 5 Map taken from Storm Water Management Plan for Foothills Redevelopment, prepared by R.A. Smith National, dated Revised December 5, 2013 TEL 970.431.7733 Loveland, CO 80538 Suite 160 5285 McWhinney Boulevard Foothills Mall Multi-Family Exhibit C R Lot Map LOT 3 LOT 4 LOT 5 SAND FILTER 2.0 SAND FILTER 4.0 RAIN GARDEN 29.0 RAIN GARDEN 31.0 RAIN GARDEN 33.0 TEL 970.431.7733 Loveland, CO 80538 Suite 160 5285 McWhinney Boulevard Foothills Mall Multi-Famliy Exhibit D R Percent Imperviousness Lot Area (SF) AREA (AC) Landscape Paved Roof Walks Percent Impervious 0% 100% 90% 90% NA 3 170,929 3.92 47,519 47,007 58,191 18,212 68% 4 112,588 2.58 36,461 1,162 66,796 8,169 61% 5 101,716 2.34 15,199 1,782 68,600 16,135 77% Total 385,233 8.84 99,179 49,951 193,587 42,516 68% Imperviousness Existing Design, Planning Area 3 Lot Area (SF) AREA (AC) Landscape Paved Roof Walks Percent Impervious 0% 100% 90% 90% NA 3 170,929 3.92 50,093 59,087 59,234 2,515 67% 4 112,588 2.58 39,573 32,060 32,372 8,583 61% 5 101,716 2.34 38,113 25,111 34,096 4,396 59% Total 385,233 8.84 127,778 116,258 125,702 15,494 63% Imperviousness Proposed Design TEL 970.431.7733 Loveland, CO 80538 Suite 160 5285 McWhinney Boulevard R T T Foothills Mall Multi-Family Exhibit E Revised Rain Garden RG-33 Contributing Area 1.56 acres Rain Garden RG-33.0 Sheet 1 of 2 Designer: Company: Date: Project: Location: 1. Basin Storage Volume A) Effective Imperviousness of Tributary Area, Ia Ia = 68.0 % (100% if all paved and roofed areas upstream of rain garden) B) Tributary Area's Imperviousness Ratio (i = Ia/100) i = 0.680 C) Water Quality Capture Volume (WQCV) for a 12-hour Drain Time WQCV = 0.21 watershed inches (WQCV= 0.8 * (0.91* i3 - 1.19 * i2 + 0.78 * i) D) Contributing Watershed Area (including rain garden area) Area = 68,348 sq ft E) Water Quality Capture Volume (WQCV) Design Volume VWQCV = 1,213 cu ft Vol = (WQCV / 12) * Area F) For Watersheds Outside of the Denver Region, Depth of d6 = 0.00 in Average Runoff Producing Storm G) For Watersheds Outside of the Denver Region, VWQCV OTHER = 0.0 cu ft Water Quality Capture Volume (WQCV) Design Volume H) User Input of Water Quality Capture Volume (WQCV) Design Volume VWQCV USER = cu ft (Only if a different WQCV Design Volume is desired) 2. Basin Geometry A) WQCV Depth (12-inch maximum) DWQCV = 12 in B) Rain Garden Side Slopes (Z = 4 min., horiz. dist per unit vertical) Z = 4.00 ft / ft (Use "0" if rain garden has vertical walls) C) Mimimum Flat Surface Area AMin = 809 sq ft D) Actual Flat Surface Area AActual = 993 sq ft E) Area at Design Depth (Top Surface Area) ATop = 1692 sq ft F) Rain Garden Total Volume VT= 1,343 cu ft (VT= ((ATop + AActual) / 2) * Depth) 3. Growing Media 4. Underdrain System A) Are underdrains provided? B) Underdrain system orifice diameter for 12 hour drain time i) Distance From Lowest Elevation of the Storage y = 60.0 ft Volume to the Center of the Orifice ii) Volume to Drain in 12 Hours Vol12 = 1,213 cu ft iii) Orifice Diameter, 3/8" Minimum DO = 0.40 in Design Procedure Form: Rain Garden (RG) Daniel Hull Olsson Associates April 24, 2015 Major Amendment to a Portion of Foothills Mall Redevelopment Planning Area 3 Modified Rain Garden 33 Choose One Choose One 18" Rain Garden Growing Media Other (Explain): YES NO Rain Garden 33.0 Calcs.xlsm, RG 4/24/2015, 1:57 PM Exhibit F Modified Rain Garden, RG33 1of 2 Sheet 2 of 2 Designer: Company: Date: Project: Location: 5. Impermeable Geomembrane Liner and Geotextile Separator Fabric A) Is an impermeable liner provided due to proximity of structures or groundwater contamination? 6. Inlet / Outlet Control A) Inlet Control 7. Vegetation 8. Irrigation A) Will the rain garden be irrigated? Notes: Design Procedure Form: Rain Garden (RG) Daniel Hull Olsson Associates April 24, 2015 Major Amendment to a Portion of Foothills Mall Redevelopment Planning Area 3 Modified Rain Garden 33 Choose One Choose One Choose One Sheet Flow- No Energy Dissipation Required Concentrated Flow- Energy Dissipation Provided Plantings Seed (Plan for frequent weed control) Sand Grown or Other High Infiltration Sod Choose One YES NO YES NO Rain Garden 33.0 Calcs.xlsm, RG 4/24/2015, 1:57 PM Exhibit F Modified Rain Garden, RG33 1of 2