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Home My WebLink AboutDrainage Reports - 04/01/20165285 McWhinney Blvd., Suite 160 TEL 970.461.7733 Loveland, CO 80538 FAX 970.635.3709 www.olssonassociates.com March 25, 2016 Dan Mogen, EI, CFM Development Review Engineer Stormwater Engineering City of Fort Collins 281 North College Avenue Fort Collins, CO 80524 RE: Addendum 5 Low Impact Design Analysis for the Major Amendment to a Portion of Foothills Mall Redevelopment Planning Area 3 (Lots 3, 4, 5, and 6 of Foothills Mall Redevelopment) Dear Mr. Mogen: Per the City of Fort Collins Ordinance #152: No less than seventy five percent (75%) of any newly added impervious area must be treated using one or a combination of low impact design (LID) techniques. In order to determine if the intent of the ordinance for Lot 3, 4, 5 and 6 (a portion of the Foothills Redevelopment Planning Area 3) was being met: • We reviewed the original design as 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), • We analyzed the conditions of the mall that existed prior to the ordinance, and • We analyzed the proposed design submitted as part of the major amendment. Based on the review and evaluation, the following conclusions were made: Per the ordinance, no less than seventy five percent (75%) of any newly added impervious area must be treated using one or a combination of LID techniques. Based on our analysis of the existing drainage report, a series of rain gardens and sand filters (SF-1.0, SF-2.0, SF-4.0, RG– 29, RG–31, and RG-33) were designed to capture and treat on-site runoff. See Exhibit B for original drainage basin map and Exhibit C for location of the lots. As part of the analysis, it was noted in the original report, the overall average tributary imperviousness was assumed to be equivalent to 90% for the calculations. This was a conservative approach. In addition, the rain gardens were designed to treat 100 percent of the contributing area. Based on the analysis, the proposed site layout and grading will be consistent with the intent of the LID techniques. In addition, a reduction in the overall percent imperviousness from the planned percent imperviousness will reduce the impact to the rain gardens and sand filters. See Exhibit D. Since Rain Garden 33 falls within the proposed development, the rain garden was updated to fit within Addendum 5 Low Impact Design Analysis for the Major Amendment to a Portion of Foothills Mall Redevelopment Planning Area 3 (Lots 3, 4, 5 and 6 of Foothills Mall Redevelopment) Page 2 the proposed site plan design. Calculations were completed to verify the updated rain garden would meet the intent of the ordinance. The rain garden is designed to treat 37 percent of the contributing area. See Exhibit E and F for those calculations. Since there was a reduction in the area treated by Rain Garden 33, the overall area treated has been reduced to 98 percent. Based on the analysis, the LID techniques provided exceed the intent of the ordinance. The following supporting materials are attached for your reference: • Exhibit A – Vicinity Map • Exhibit B – Original Drainage Basin Map • Exhibit C – Lot Map • Exhibit D – Percent Imperviousness • Exhibit E – Revised Rain Garden RG-33 Treatment Area • Exhibit F – Revised Rain Garden RG33 Calculations • Exhibit G – Percent of Area Treated In conclusion, the stormwater quality as a result of the proposed major amendment will be an improvement to the intent of the original approved design as well as an improvement to the existing condition prior to the ordinance. Overall the proposed design will have a significant improvement to stormwater quality. 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, 5 and 6 R TEL 970.431.7733 Loveland, CO 80538 Suite 160 Foothills Mall 5285 McWhinney Boulevard Multi-Family Exhibit B Original Drainage Basin Map Map taken from Foothills Redevelopment Planning PA2 Final Plans, dated 04/25/2014, prepared by R.A. Smith National Sand Filter 2 Basin 11.47 acres Rain Garden 29 Basin 6.22 acres Sand Filter 4 Basin 4.38 acres Rain Garden 31 Basin 1.36 acres Rain Garden 33 Basin 1.76 acres Sand Filter 1 Basin 22.72 acres 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 LOT 6 SAND FILTER 1.0 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 Porous Paving Percent Impervious 0% 100% 90% 90% 90% 3 170,929 3.92 47,519 47,007 58,191 18,212 0 68% 4 112,588 2.58 36,461 1,162 66,796 8,169 0 61% 5 101,716 2.34 15,199 1,782 68,600 16,135 0 77% 6 134,248 3.08 33,514 2,263 89,864 8,607 0 68% Total 519,481 8.84 132,693 52,214 283,451 51,123 0 68% Lot Area (SF) AREA (AC) Landscape Paved Roof Walks Porous Paving Percent Impervious 0% 100% 90% 90% 90% 3 170,929 3.92 60,271 44,931 42,897 3,831 18,999 61% 4 112,588 2.58 44,439 25,197 29,278 5,663 8,011 57% 5 101,716 2.34 26,054 29,121 32,548 4,580 9,413 70% 6 134,248 3.08 30,846 47,255 38,542 5,166 12,439 73% Total 519,481 8.84 161,610 146,504 143,265 19,240 48,862 65% Existing Design Proposed Design Imperviousness Imperviousness 12/12/2015 TEL 970.431.7733 Loveland, CO 80538 Suite 160 5285 McWhinney Boulevard R TULANE BUILDING 5 TYPE D3 BUILDING 6 TYPE C3 BUILDING 7 TYPE E2 BUILDING 8 TYPE D2 Foothills Mall Multi-Family Exhibit E Revised Rain Garden RG-33 Contributing Area 1.76 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 = 100.0 % (100% if all paved and roofed areas upstream of rain garden) B) Tributary Area's Imperviousness Ratio (i = Ia/100) i = 1.000 C) Water Quality Capture Volume (WQCV) for a 12-hour Drain Time WQCV = 0.40 watershed inches (WQCV= 0.8 * (0.91* i3 - 1.19 * i2 + 0.78 * i) D) Contributing Watershed Area (including rain garden area) Area = 28,665 sq ft E) Water Quality Capture Volume (WQCV) Design Volume VWQCV = 956 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 = 637 sq ft D) Actual Flat Surface Area AActual = 637 sq ft E) Area at Design Depth (Top Surface Area) ATop = 1692 sq ft F) Rain Garden Total Volume VT= 1,165 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 = 0.0 ft Volume to the Center of the Orifice ii) Volume to Drain in 12 Hours Vol12 = N/A cu ft iii) Orifice Diameter, 3/8" Minimum DO = N/A in Design Procedure Form: Rain Garden (RG) Daniel Hull Olsson Associates March 25, 2016 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 50% Imper treated, RG 3/25/2016, 10:23 AM 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 March 25, 2016 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 50% Imper treated, RG 3/25/2016, 10:23 AM Exhibit F Modified Rain Garden, RG33 1of 2 TEL 970.431.7733 Loveland, CO 80538 Suite 160 5285 McWhinney Boulevard Foothills Mall Multi-Family Exhibit G R Percent of Area Treated Treatment Structure Basin Area (acres) Percent of Area Treated Area Treated (acres) Sand Filter 1 22.72 100% 22.72 Sand Filter 2 11.47 100% 11.47 Sand Filter 4 4.38 100% 4.38 Rain Garden 29 6.22 100% 6.22 Rain Garden 31 1.36 100% 1.36 Rain Garden 33 1.76 37% 0.65 47.91 98% 46.80