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HomeMy WebLinkAboutDrainage Reports - 02/02/2015City'of Ft. Collins prowed Plans ;Approved By�. -NM NORTHERN ENGINEERING Drainage Memorandum Date: January 20, 2015 Project: Prairie Village Project No; 1013-001 Project Development Plan/Final Plan ' Fort Collins, Colorado Attn: Mr. Wes Lamarque City of Fort Collins Stormwater Utility 700 Wood Street Fort Collins, Colorado 80521 Dear Wes: This letter serves to address the stormwater impacts of the Prairie Village project. The existing site consists of vacant land, situated southwest of the Seneca Center Development. The site is bounded by an existing parking lot to the northeast, Fromme Prairie Way on the south, and Overlook at Woodbridge Subdivision lots on the west. The proposed development will consist of 8 townhome units, sidewalks, paver driveways, and utility services. The project is expected to disturb approximately 0.73 acres of land. This project site was originally a part of the Overlook/Seneca Center P.U.D. As such, drainage facilities were planned for this area and were built with the Seneca Center Development. This letter is intended to show compliance with the original drainage plan "Final Drainage and Erosion Control Report — Overlook/Seneca Center P.U.D." dated April 17, 2000 by North Star Design and the Fort Collins Amendments to the Urban Drainage and Flood Control District Criteria Manual. The site is located within the Mail Creek Stormwater Basin. Existing site drainage overland flows to Fromme Prairie Way. A web soil surrey indicates a soils rated as Hydrologic Soil Group B, which has a moderate infiltration rate. The proposed site has a percent impervious of 59%, which is lower than the previous Overlook/Seneca Center P.U.D. at 64% impervious. As such, detention will not be required for this site. Both the previous study and this study used the most recent Fort Collins Intensity -Du ration - Frequency in their calculations. Rational method calculations were used to determine the proposed runoff for sizing storm facilities. These calculations are attached to this letter, as is a drainage exhibit. There are no regulatory floodplains associated with the project. Although stormwater quantity detention is not required, stormwater quality mitigation will be addressed by both temporary and permanent Best Management Practices (BMPs). During 301 N. Howes Street, Suite 100, Fort Collins, CO 80521 1 970.221.4158 1 www.northernengineering.com NORTHERN ENGINEERING construction, the Contractor will follow the appropriate and applicable City of Fort Collins standards for erosion and sediment control. Since the approximately 0.73 acre area of disturbance for the project is less than one acre, a comprehensive Stormwater Management Plan will not be prepared for this project. Post construction water quality and erosion control will be achieved by a fully established and stabilized site. All areas disturbed during construction will receive permanent hardscape, landscape, or building structure. There are no existing stormwater BMP's currently on -site. In order to bring the site into compliance with the Low Impact Development Standards adopted by the city, bio-retention/rain gardens and cobble swales will be provided on -site. Water quality treatment will occur through a variety of methods. The first method is the use of a bio-retention area along the north side of the proposed buildings that intercepts runoff from Basin A. This area will have an engineered section designed to remove particulates and contaminants from the runoff. While most minor events are expected to infiltrate into the ground, a subdrain is provided to drain the bio-retention area should it become overburdened. The underdrain will outfall directly into an existing storm sewer. The runoff from the south side (Basin B) will be routed through a cobble swale. This cobble swale has a 1% grade. This lower grade will allow sediment to settle and additional infiltration. This cobble swale will convey flows to a proposed inlet, outfalling into the existing storm sewer. Stormwater flows from Basin C will continue as historic, sheet flowing into the existing parking lot and being collected in downstream inlets. The City LID requirements specify that no less than 25% of all new paving shall be treated using permeable paving technique. The City LID requirements also specify that no less than 50% of any new impervious areas must be treated using LID techniques. Unfortunately, the site configuration and drainage patterns for this project do not lend themselves to the use of permeable pavers. In lieu of permeable pavers, 90% of the new impermeable areas will be treated for water quality. The combination of treatment provided by the bio-retention area and cobble swale far exceeds the amount required for LID treatment, and provides water quality treatment equal to or better than meeting the requirement for 25% permeable pavements. To summarize, the proposed grading concept closely is in compliance with previous drainage studies. On -site detention is proven to be unnecessary. Stormwater quality has been provided, and exceeds the city requirements for Low Impact Development treatment. Therefore, it is my professional opinion that the Prairie Village project satisfies all applicable stormwater criteria. Please do not hesitate to contact me if you have questions or require additional information. Sincerely, W0 -4im,711MI Stephanie Thomas, PE Project Engineer enc. AlAJILt, kllAd- las W. 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Basin Storage Volume A) Effective Imperviousness of Tributary Area, I, I, = 63.0 % (100% if all paved and roofed areas upstream of rain garden) B) Tributary Area's Imperviousness Ratio (i = I,/100) 1 = 0:830! C) Water Quality Capture Volume (WQCV) for a 12-hour Drain Time WQCV = 0:20� watershed inches (WQCV=0.8"10.91'1'-1.19-i'-0.78-i) D) Conaibu6ng Watershed Area (including rain garden area) Area = 14,202 sq ft E) Water Quality Capture Volume (WOCV) Design Volume Vwocv = 234 cu ft Vol = (WQCV / 12)' Area F) For Watersheds Outside of the Denver Region, Depth of d„ = In Average Runoff Producing Storm G) For Watersheds outside of the Denver Region, VwGcv maee = 0.0 cu ft Water Quality Capture Volume (WQCV) Design Volume H) User Input of Water Quality Capture Volume (WOCV) Design Volume VWOCV uses = cu ft (Oily if a different WQCV Design Volume is desired) 2. Basin Geometry A) WQCV Depth (12-inch maximum) Dwocv = 9 in 8) Rain Garden Side Slopes (Z = 4 min., hortz. dirt per unit vertical) Z = 4.00 ft / ft (Use "0" R rain garden has vertical walls) C) Mlmimum Flat Surface Area AM,,, = t66 sq ft D) Actual Flat Surface Area A, = 460 sq ft E) Area at Design Depth (Top Surface Area) AT. = 600 sq ft F) Rain Garden Total Volume VT= 398 cu ft VT= ((AT. - A,) 12) * Depth) 3. Growing Media Choose One 8 18" Rain Garden Growing Media O Other (Explain): 4. Underdrain System A) Are underdrains providetl? Choose One O YES ONO B) Underdrain system orifice diameter for 12 hour drain time i) Distance From Lowest Elevation of the Storage y= 1.8 ft Volume to the Center of the Orifice III Volume to Drain in 12 Hours V0112= 234cu ft III) Orifice Diameter, 318" Minimum Do - 0.38. in MINIMUM DIAMETER = 71E" 1013-001_UD-BMP_v3.02.x1s, RG 8/27/2014, 12:56 PM Design Procedure Form: Rain Garden (RG) Sheet 2 01 2 Designer: Stephanie Thomas Company: Northern Engineering Date: August 27, 2014 Project: Prairie Village Location: Harmony and Seneca 5. Impermeable Geomembrane Liner and Geotextile Separator Fabric Choose One O YES A) Is an impermeable liner provided due to proximity NO of structures or groundwater contamination? 6- Inlet / Outlet Control Choose One 0 Sheet Flow- No Energy Dissipation Required A) Inlet Control O Concentrated Flow- Energy Dissipation Provided Choose One 7. Vegetation O Seed (Plan for frequent weed control) 0 Plantings Q Sand Grown or Other High Inflltratlon Sod 8. Irrigation Choose One 0 YES A) Will the rain garden be irrigated? 0 NO Notes: 1013-001_UD-BMP_0.02.x1s, RG 8/27/2014. 12:56 PM / }� KRk I small I: I Isms . ��: Area Inlet Performance Curve: Prarie Village - Design Point A Governing Equations: At low flow depths, the inlet will act like a weir governed by the following equation: ' where P = 2(L + W) Q = 3 .0 P H • where H corresponds to the depth of water above the flowline At higher flow depths, the inlet will act like an orifice governed by the following equation: ' where A equals the open area of the inlet grate Q = 0.67 A (2 gH 0 .5 • where H corresponds to the depth of water above the cenlroid of the cross -sectional area (A) The exact depth at which the inlet ceases to act like a weir, and begins to act like an orifice is unknown. However, what is known, is that the stage -discharge curves of the weir equation and the orifice equation will cross at a certain flow depth. The two curves can be found below: r 4� 9.00 8.00 „ 7.00 6.00 � 5.00 A U 4.00 0 3.00 2.00 1.00 0.00 0.00 Stage - Discharge Curves 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 Stage (ft) If H > 1.792 (A/P), then the grate operates like an orifice: otherwise it operates like a weir. Input Parameters: Type of Grate: Neenah R-3560 Shape Rectangular Length of Grate (ft): 2.1 Width of Grate (ft): 2.1 Open Area of Grate (ft): 1.50 Flowline Elevation (ft): 5145.450 Allowable Capacity: 50% vs. Flow: Shallow Orifice Actual Elevation Weir Flow Flow Flow Depth Above Inlet (ft) (ft) (cfs) (cfs) (cfs) 0.00 5145.45 0.00 0.00 0.00 0.09 5145.54 0.34 1.21 0.34 0.18 5145.63 0.96 1.71 0.96 F Q2 0.27 5145.72 1.77 2.09 1.77 0.36 5145.81 2.72 2.42 2.42 0.45 5145.900 3.80 2.70 2.70 0.54 5145.99 5.00 2.96 2.96� Q100 0.63 5146.08 6.30 3.20 3.20 0.72 5146.17 7.70 3.42 3.42 0.81 5146.26 9.19 3.63 3.63 0.90 5146.350 10.76 3.82 3.82 Inlet at Design Point A is designed to intercept the full 100-yr of 2.99 at 0.55 ft above the inlet Area Inlet Performance Curve: Prairie Village - Design Point B Governing Equations: At low flow depths, the inlet will act like a weir governed by the following equation: ' where P=2(L+W) Q = 3 .0 PH ' where H corresponds to the depth of water above the flowline At higher flow depths, the inlet will act like an orifice governed by the following equation: ' where A equals the open area of the inlet grate g l 0.5 = 0.67 A 2 H / ' where H corresponds to the depth of water above the centroid o/ the cross -sectional area (A) The exact depth at which the inlet ceases to act like a weir, and begins to act like an orifice is unknown. However, what is known, is that the stage -discharge curves of the weir equation and the orifice equation will cross at a certain flow depth. The two curves can be found below. Stage - Discharge Curves 1.60 1.40-wl,- 1.20 - w u 1.00 - m � 0.80 - q u 0.60 - - _N 0.40 'I 0.20 0.00 - - - 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 Stage (ft) If H > 1 792 (A/P), then the grate operates like an orifice: otherwise it operates like a weir. Input Parameters: Type of Grate: Nyloplast 12" H-20 Shape Circular Length of Grate (ft): 1 Width of Grate (ft): 1 Open Area of Grate (ft): 0.39 Flowline Elevation (ft): 5142.230 Allowable Capacity: 50% Depth vs. Flow: Shallow Orifice Actual Elevation Weir Flow Flow Flow Depth Above Inlet (ft) Ift) (cfs) (cfs) (cfs) 0.00 5142.23 0.00 0.00 0.00 0.05 5142.28 0.05 0.24 0.05 0.10 5142.33 0.15 0.33 0.15 0.15 5142.38 0.27 0.41 0.27 0.20 5142.43 0.42 0.47 0.47 0.25 5142.480 0.59 0.53 0.53 0.30 5142.53 0.77 0.58 0.58 Q2 E- 0.35 5142.58 0.98 0.62 0.62 0.40 5142.63 1.19 0.67 0.67 0.45 5142.68 1.42 0.71 0.71 0.50 5142.730 1.67 0.75 0.75 Inlet at Design Point B is designed to intercept the full 2- r of 0.60 cfs at 0.33 feet above the inlet <o N O O N n N d a :3 O 0 LJD N N C O d a E Z c+� v 0 n E of c U) U) ii ii d 0 a` R d IL it O/� � ifs E CO L v/ E S. O ca c O c m C O. 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M feel Weet of CIIiOpercaNa Or., on imaele bar 0e pooh end mM>fenEe (Bou01wm1 ¢^ler rn / q Regan Perk Sub). �\ N4VD89=5119.I9 BENCHMARKA2 cry of Fort Glina Berpinnrt tow " Al' " Weet Sib of South Tan HB Ed 1 W feel None aue old We9I Harmony Rd. intereemlan, M too 0"Dead end of lb Sould 0000fele ,mamma wan fora natural Sae OPEmw Ann. Nok o-513078 OVEPo.00Y! °U°91H FOR DRAINAGE REVIEW ONLY NOT FOR CONSTRUCTION _-t — FROMME PRAIRIE WAY (5P ROVV) CAUanLNYNOTFI T(OolCENTER Gf OCCRADO l4aW Enerhalrnv. Call Befdey9Ealg City of Fort Collins, Colorado UTILITY PLAN APPROVAL IED: try Engineer :D BY RR erAE g mil— m BY: � a Y m By" Parke RRecreetlon ED B � — g0 eRaroomww etmner zCD IS Z aq LLJ = w f—Z O" Zw 8ry +B qF J REM H CO 2 x W W C7 Q Z_ Q n Sheet C700 OF 15 Sheets ROMWGEISTII