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HomeMy WebLinkAboutDrainage Reports - 06/05/2017City of Fort Col ' Approv la Approved by: Date: 6 s -toy A This Drainage Report is consciously provided as a PDF. Please consider the environment before printing this document in its entirety. When a hard copy is absolutely necessary, we recommend double -sided printing. June 5, 2017 DRAINAGE ANALYSIS LETTER FOR 1212 Riverside Fort Collins, Colorado Prepared for: City of Fort Collins Prepared by: ■� NORTHERN ENGINEERING 301 N. Howes Street, Suite 100 Fort Collins, Colorado 80521 Phone: 970.221.4158 Fax: 970.221.4159 www.northernengineering.com Project Number: 100-021 e NorthernEnnineerino.corn // 970.221.4758 1 1 NA June 5, 2017 NORTHERN ENGINEERING Mr. Shane Boyle City of Fort Collins Stormwater 700 Wood Street Fort Collins, Colorado 80524 RE: 1212 Riverside Avenue, Drainage Analysis Fort Collins, Colorado Dear Shane: The site improvements for 1212 Riverside Avenue consists of renovating the existing building and parking lot into performing arts school and commercial space. The subject site is currently composed of a large masonry building and hardscape (asphalt and concrete) parking lot. The project consists of the eastern half of the existing building, eastern truck dock, and eastern parking lot. The site was divided into three drainage basins: ' 1. Basin A: The western driveway of the eastern parking lot up to the covered entrance along Riverside Avenue. The basin includes roof drainage from the two downspouts immediately west of the building entrance and surrounding landscaping. ' 2. Basin B: The eastern driveway, parking lot, and roof drainage immediately west of the covered entrance. Basin B does not include any drainage at the foot of the eastern truck dock. 1 3. Basin C: The immediate area around the eastern truck dock collected by an existing trench drain and discharged through the building towards the north. ' Basin A: ' The existing and proposed drainage patterns for Basin A remain unchanged. Drainage from the roof and covered entrance sheet flows south towards the curb and gutter along the southern edge of the parking lot. Drainage will then flow along this existing curb and gutter towards the western ' driveway and into the existing curb and gutter on Riverside Avenue. The proposed improvements to Basin A include realigning the western driveway, a concrete plaza area at the western end of the proposed improvements, and replacing the sidewalk along the front ' of the building. The improvements to Basin A will increase the impervious area (see Table 1), but the proposed improvements in the adjacent basins will result in a decrease in the area and flowrate for Basin A (see Table 1). The proposed 100-year flowrate for Basin A is 3.46 cfs (see Table 1). ' 301 N. Howes Street, Suite 100, Fort Collins, CO 80521 I 970.221.4158 I www.northernengineering.com 1 1 1 NEE 1 1 1 ' Basin B: NORTHERN ENGINEERING Table 1: Basin A Drainage Characteristics Existing I Proposed Basin Area (sq. ft.) 19,508 17,454 Impervious Area (sq. ft.) 17,442 16,272 Percent Impervious 88.2% 89.2% 2-Year Flowrate (cfs) 0.87 0.79 10-Year Flowrate (cfs) 1.48 1.35 100-Year Flowrate (cfs) 3.78 3.46 ' The existing drainage pattern for Basin B begins with drainage from the roof discharging from four downspouts: two in front east of the covered entrance and two near the truck dock behind an existing transformer. The stormwater will then sheet flow across the existing parking lot towards the ' southeast at the southeast corner of the property. Curb and gutter along the south of the parking lot, and a retaining wall along the east property line, collects and directs the drainage towards the existing curb and gutter in Riverside Avenue via the eastern most driveway. 1 1 The proposed drainage patterns from the front two existing downspouts and parking lot will only vary slightly from the existing drainage pattern. Two proposed sidewalk culverts, one with as swale and the other with a concrete chase, will direct the drainage away from the eastern driveway and into the existing curb and gutter along Riverside Avenue. However, the two rear downspouts near the existing transformer will discharge into the existing trench drain in Basin C. The proposed improvements in Basin B will result in a decrease in the impervious area, basin area, and flowrate in Basin B (see Table 2). Table 2: Basin B Drainage Characteristics Existing Proposed Basin Area (sq. ft.) 27,899 20,367 Impervious Area (sq. ft.) 26,623 18,361 Percent Impervious 92.9% 86.6% 2-Year Flowrate (cfs) 1.35 0.86 10-Year Flowrate (cfs) 2.31 1.47 100-Year Flowrate (cfs) 5.91 3.76 Basin C: The existing drainage pattern for Basin C consists of sheet flow across the existing concrete towards a trench drain at the base of the existing eastern truck dock. The trench drain discharges, under the existing building, through a 6 inch PVC pipe into the existing Great Western Railway right-of- way along the northern property line. The overflow for Basin C is through a slot drain in the existing retaining wall along the east property line, across an existing asphalt parking lot, and into the Great Western Railway near the discharge of the 6 inch PVC pipe. 301 N. Howes Street, Suite 100, Fort Collins, CO 80521 1 970.221.4158 1 www.northernengineering.com rJ 1 11 NORTHERN ENGINEERING Due to the proposed raised plaza, drainage area for Basin C is increased to include the discharge from two existing roof drains. The two roof drains behind an existing transformer discharges into Basin B under pre -developed conditions, and they will discharge into the trench drain in Basin C under post -developed conditions. The area within the immediate vicinity of the truck dock will continue to discharge into the trench drain in Basin C. The overflow for Basin C remains at the existing slot drain in the retaining wall along the east property line. The proposed improvements result in an increase in the basin area and flowrate (see Table 3). The impervious area for Basin C remains the same (see Table 3). Table 3: Basin C Drainage Characteristics Existing I Proposed Basin Area (sq. ft.) 1,360 10,947 Impervious Area (sq. ft.) 1,360 10,947 Percent Impervious 95.0% 95.0% 2-Year Flowrate (cfs) 0.07 0.57 10-Year Flowrate (cfs) 0.12 0.97 100-Year Flowrate (cfs) 0.31 2.47 Total Impervious Area: Table 4: Total Impervious Area Existing Proposed Basin A (sq. ft.) 17,442 16,272 Basin B (sq. ft.) 26,623 18,361 Basin C (sq. ft.) 1,360 10,947 Total Impervious Area (sq. ft.) 45,425 45,580 ' For the entire project, there is a net increase of 155 square feet of impervious area (see Table 4). City of Fort Collins Stormwater Requirements does not apply since the change in impermeable area ' is less than the allowable increase of 350 square feet. Impacts to Adjacent Streets and Properties: ' The stormwater impact on the adjacent streets and properties along Riverside Avenue from this project is negligible. The proposed improvements will result in a decrease of the 100-Year Storm of 2.47 cfs within Riverside Avenue. There will be no adverse impact from the proposed improvements to the existing infrastructure along Riverside Avenue. The stormwater impact the Great Western Railway right-of-way is a net increase of 2.16 cfs for the 100-Year Storm.• The Great Western Railway right-of-way consists of an existing swale that ultimately discharges into the Cache la Poudre River. The additional 2.16 cfs will discharge into this existing swale. The additional discharge will not have a significant impact on the Great ' Western Railway right-of-way. ' 301 N. Howes Street, Suite 100, Fort Collins, CO 80521 I 970.221.4158 I www.northernengineering.com NORTHERN ENGINEERING NORTHERN ENGINEERING SERVICES, INC. Frederick S. Wegert, PE Project Engineer . Pp 0 L /CF ry � 'scoir' �iS 45018 ^7 S3/ONA� ECG\ 301 N. 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TO ME SURROUNDON ISURFMESry WHEN WINDS MEOF `BOWNG oBERATED. CONDITION 6TMEaRIVENWAMC5NOR BARE c«L, AMID DUST MI CH sS L/L/L/q OOGB MR GRAVEL ROCKS FOR PAVEMENT ETC CPIrvIM.Is xPUROOUS TO THE PUBLIC HEum uaO wELGME. G. CCNmocrORM RESPOx511u srPBx¢MGN1 SORES, PNNCULRLVTMFE CONSTRAINTS (EG.EMPIRE SUMMi AND WINTER MONTIM) Lfpi STRG. MP Ts, PRRUAX6CATIO xMIONALLEGENDMCED B OR SEROspX TEMPORARY IMPLICATION,ANDBE WITH UL HTAT OR HER MATERIALS WEL COME BY CHEAPEN AND STARTUP THE SLOPE NECESSARYroESTABLISH VEGETATIVE cwMMosTMUIrRMEs.ORE. � COMMULCHADDMULCHERER NO Ory HE ALUMENCENAL WATERTER SILT CLEANUP. OR OTHER MEASURES WAY BE NECESSARLY N?OT PLOT UNALL seo MENTBE ALLOWED m CROPS EE PUR ETs wAcALIES TABLE OF CONSTRUCTION SEQUENCE AND BMP APPLICATION I eY e LDO • e rm�,,EE ®� I g�N�iaa 1-01�1�_1s� DESIGN I I ARfd I I I ]-YRTC 1100i MERE II p T p O Cx FEET) LEGEND: APPROXIMATE EMUS OF CONSTRUCTION SIEMENS MEN EMPIRE EwrnxG RIGHT-OF-WAY/PROPERTY LUXE "SING MAYOR SCHEME — — --A15— EWmNG MINOR CONTOUR — — —SEND— DISTRICT RAILROAD RAILROAD TRACKS I I ICI I SAWCUi UPS , 1 ' I— ENRTNG CURB h CL11ER EXISTING CONCRETE PROPOSED CONCRETE PAREMENT PRINCEED &BEHALF PAMLPR ENSURE MINES L y EXIST" RIDGE MAIN \! PROPOSED STIXM AMP PROPOSING SFPNM INLET PROPOSED CONTOURS O 04 POINT Q FOR ARROW'S 4111110 M MEN DRAINAGE SAM! LABEL 2 NEW TAW Hs DRAINAGE BASIN BOUNDARY MEMO PROPOSED STORM SEAR PROPOSED UNDERMINE! 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