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Home My WebLink AboutFOOTHILLS MALL MULTI-FAMILY - MAJOR AMENDMENT - MJA150002 - SUBMITTAL DOCUMENTS - ROUND 1 - STORMWATER-RELATED DOCUMENTS (3)5285 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: Low Impact Design 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: Per the City of Fort Collins Ordinance #152: No less than fifty percent (50%) of any newly added impervious area must be treated using one or a combination of low impact design (LID) techniques; and no less than twenty five percent (25%) of any newly added pavement areas must be treated using permeable pavement. In order to determine if the intent of the ordinance for Lot 3, 4, and 5 (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 fifty percent (50%) 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-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. However, 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 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 100 Low Impact Design 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 percent of the contributing area. See Exhibit E and F for those calculations. Based on the analysis, the LID techniques provided exceed the intent of the ordinance. Per the ordinance, no less than twenty five percent (25%) of any newly added pavement areas must be treated using permeable pavement technology. The existing pavement area prior to the ordinance was calculated to be 290,321 square feet. The proposed pavement area is 116,258 square feet. See Exhibit D for pavement area calculations. Based on our analysis of the existing and proposed pavement, there is no newly added pavement. No permeable pavement is required. See Exhibit H and I for a graphical representation of the pavement areas. 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 (updated), Calculations • Exhibit G – Existing Pavement • Exhibit H – Proposed Pavement 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 and 5 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 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 16,186 112,523 32,645 9,575 88% 4 112,588 2.58 16,266 96,322 0 0 86% 5 101,716 2.34 16,673 81,476 0 3,567 83% Total 385,233 8.84 49,125 290,321 32,645 13,142 86% Imperviousness Existing Condition Prior to Redevelopment 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 R TEL 970.431.7733 Loveland, CO 80538 Suite 160 5285 McWhinney Boulevard Foothills Mall Multi-Family Exhibit G Existing Pavement -Pavement Lot 3 Lot 4 Lot 5 R TEL 970.431.7733 Loveland, CO 80538 Suite 160 5285 McWhinney Boulevard T T T T T T T T Foothills Mall Multi-Family Exhibit H Proposed Pavement -Pavement Lot 3 Lot 4 Lot 5