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HomeMy WebLinkAboutFile Documents.210 S 1st St.0281.2017 (23).ARBK p•-i . MOUNTAIN CROSS .' ENGINEERING, INC. / Civil and Environmental Consulting and Design September 28, 2018 Ms. Gretchen Greenwood Gretchen Greenwood&Associates, Inc. 210 South Galena Street, Suite 30 Aspen, CO 81611 RE: Urban Runoff Management Plan—210 South First Street Dear Ms. Greenwood: The purpose of this correspondence is to evaluate the construction of a residence at 210 South First Street from a storm water perspective,provide design recommendations pertaining to urban runoff management for incorporation into the site design, and incorporate comments provided from the City of Aspen review. This letter is intended to replace the previous letter dated August 15, 2017. These recommendations were generated from the site plans that were submitted to our office and a site visit performed on May 22, 2017. Attached are plan sheets that show the design recommendations and details. Also attached are the drainage calculations that were generated. The project is at an elevation of approximately 7,900 feet above sea level. An existing Victorian residence sits on the north half of the lot and has an address of 135 West Hopkins Avenue. Ultimately, runoff from the site will travel to the Roaring Fork River through a series of roadside conveyances and storm drain appurtenances. Runoff from snowmelt is typically very large in volume but because it happens over a much longer time period, the peak flows are usually less than a rainfall event. Also, the Urban Runoff Management Plan states that they are not necessary at this time. Therefore storm water is defined in this study to be surface water that is a direct result from a rainfall event. No major irrigation structures cross the site. The project is located greater than 100' from any mapped floodplain boundaries. Two other drainage reports have been prepared for the site: 1) "Drainage Report for 135 West Hopkins Avenue"dated May 2008,revised August 2008,prepared by WRC Engineering,Inc, and 2)"Storm Water Detention System for 135 West Hopkins Avenue" date November 11,2009,prepared by Timberline Engineering. This office was only able to obtain a copy of the second report and it is attached. Storm water from the site is evaluated in accordance with the standards of applicable sections of the City of Aspen, Urban Runoff Management Plan (Code). All flows are listed in units of cubic feet per second(cfs)unless otherwise noted. Methodology The design of a drainage plan begins with a calculation of the flowrate of water that will be produced from a rainfall event. Since the watershed basins are small (less than 90 acres IVED ,., 10/09/18 8261/2 Grand Avenue, Glenwood Springs,CO 81601 P:970.945.5544 F: 970.945.5558 www.mountaincross-eng.com ASPEN BUILDING DEPARTMENT 210 South First St. September, 2018 Page 2 of 5 Code, the Rational method was used for estimating the amount of runoff that will occur. This method calculates runoff(Q) in cubic feet per second (cfs) from basin area (A) in acres, runoff coefficient(C), and rainfall intensity (I) in inches per hour: Q =C * I * A When acres and inches per hour are used as the units, the conversion into cubic feet per second is 1.008 but is usually ignored and it has been here. The runoff coefficient is a dimensionless coefficient. Basin Area Drainage basins have the characteristic that any precipitation falling within that area will drain to the same point of discharge. The project basin was delineated from project topography,project site plan, and building architecture. Runoff Coefficient A runoff coefficient is assigned to each basin that gives a relationship between the amount of precipitation that becomes surface water and the amount of water that is lost to infiltration, evaporation, or transpiration. The runoff coefficient is a function of drainage basin soil types, surface area, and/or land-use. Because the land-use and the surface cover often vary through the project, a composite coefficient is often assigned to each drainage basin, based on the weight of the areas and their respective coefficients. Rainfall Intensity Rainfall intensity is determined from intensity duration frequency curves, or IDF curves. IDF curves are graphs of, more or less, parallel frequency curves that yield rainfall intensities based on storm durations. Frequency: The return frequency of a rainfall storm is the statistical probability that a given storm event will occur on average in a given period.For instance a 100- year storm has the statistical probability of occurring once in a 100 year span or it has a 1% chance of occurring in any given year. It is important to emphasize that it is based on probability statistics and therefore does not reflect actual storm frequency. Storms of a 100-year magnitude can occur in sequential years, even in the same year. The return frequency of design is chosen and then referred to as the design storm. Duration: The duration of a storm is chosen to coincide with the time of concentration. Per the Code,the FAA Overland flow equation was used to estimate the time of concentration. The parameters needed to determine the flow elements include length, slope, and the Rational runoff coefficient. The theory states that if the duration is equal to the time of concentration,the length of time will be adequate for the entire basin to contribute flow. RECEIVED Mountain Cross Engineering, Inc. Civil and Environmental Consulting and Design 10/0 9/18 8261/2 Grand Avenue, Glenwood Springs,CO 81601 P:970.945.5544 F: 970.945.5558 www.mountaincross-eng.com ASPEN BUILDING DEPARTMENT 210 South First St. September, 2018 Page 3 of 5 Analysis Project basins were delineated based on a review of existing site conditions, aerial photography, roof lines, site grading, and project topography. The backyard of the Victorian has been graded and drains to an existing storm structure; the backyard is its own basin. The alley is the boundary to the south. The north boundary is the existing residence that is fully guttered with the southern half of the roof-downspouts piped into the storm structure. South First Street is the western boundary. The neighboring property to the east drains into its own system or into the alley. According to the report by Timberline Engineering,the existing storm structure was designed as a combination detention and water quality structure. The outlet is metered by an orifice to restrict the 2-year storm flows to 0.05 cfs. The 100-year storm event was calculated as 0.56 cfs. A direct connection from the structure was installed to the storm sewer in West Hopkins Avenue. However, since the project is within the Aspen Mountain Drainage Basin, detention is no longer required. In the event of a storm larger than the 100-year,water would overtop the grate and flow to the west into South First Street. The proposed drainage pattern is intended to remain. All proposed storm water is captured and conveyed by swales, trench drains, and downspouts and directed into the existing storm structure. A concrete gutter is proposed to direct any flows from the alley into South First Street and avoid trespass onto 210 South First Street. Runoff coefficients were determined based on site soils and percentage imperviousness, per the appropriate figures in the Code. The hydrologic soil group was determined from Figure 3.1 and based on the project location is, "B". Based on the corresponding Figure 3.3 the applicable coefficients were determined and are shown in the attached calculations. The parameters for calculating the time of concentration were determined from the site, slope, length, and land uses. The time of concentrations for the site basin that were calculated are between 3.2 and 8.5 minutes; 5 minutes was the fastest time used. The calculations are attached. The rainfall intensities were determined by using the appropriate storm curve, duration to match the time of concentration, and the IDF curve provided in the Code. The basin flow rates were calculated based on the Rational Method described in the Code for the existing (pre-project flows) and also for the proposed conditions. The calculations are attached. Basin Area Coefficient Flow Description (acres) (cfs) Existing - 100 yr 0.081 0.35 0.14 Proposed- 100 yr 0.083 0.65 0.34 Trench drains and swales capture flows and direct water into 6" PVC storm pipes at a minimum of 2% slope. 100 year flows would fill the pipe a little over halfway. These calculi E IvE LEI Mountain Cross Engineering, Inc. Civil and Environmental Consulting and Design 10/0 9/18 8261/2 Grand Avenue, Glenwood Springs,CO 81601 P:970.945.5544 F: 970.945.5558 www.mountaincross-eng.com ASPEN BUILDING DEPARTMENT 210 South First St. September, 2018 Page 4 of 5 slightly less than the previous study. This is attributable to changes in the Code. Grates for the vault inlet and the trench drains were analyzed with 50% clogging factor. The grates are able to pass the 100-year storm even at 50% clogget. Previous report and calculations are attached in the Appendix. Water Quality Water quality capture volume for the north portion of the site was installed in a previous construction project. The project installed two detention structures with metered release rates. One was installed beneath the northeast patio. The second was installed to the south west of the existing residence. This is the structure that the proposed project will use. The water quality capture volume for the proposed project is calculated to be 34.2 cubic feet, or 255.8 gallons. Calculations are attached. The plans show that the existing structure is a 1,000 gallon concrete tank with an orifice restricting flows to the two-year rate. It is proposed to continue using this existing structure for water quality. As constructed, the volume is larger than necessary since detention is no longer required. Explaii structi, Temporary structures are intended to be used during construction activities. Permanent structures does tl are intended to be used continually after the construction activities have been completed. when Temporary erosion control measures that are to be employed during construction have been : the str designed to contain sediment on the site and to mitigate erosion from construction activities. Silt ; type 01 fencing will be placed around the downhill limits of disturbance. Permanent erosion control ' provide measures are revegetation of disturbed areas and design of conveyances to prevent erosion. The water quality structure will capture any transported sediment. The site will disturb less than one acre and therefore will not require a permit from CDPHE. Maintenance Maintenance will be required periodically for the drainage system. At a minimum the following should be done bi-annually in the spring and fall: swales and the water quality structure should be cleaned and cleared of mud and debris. Temporarily, the general notes require the maintenance and frequent inspection of the silt fence. Attached in the Appendix is a Maintenance Plan that should be used by the Owner. Low Impact Development Principle 1: At the outset of the drainage design, WQCV was anticipated. The site grading was designed to convey runoff to a WQCV. Principle 2: The design used the entire site by minimizing and eliminating impervious areas. Were possible, flows are in permeable swales. Principle 3: The impervious areas have been minimized as much as practical and still meet the requirements of the Owner. RECEIVED Mountain Cross Engineering, Inc. Civil and Environmental Consulting and Design 10/0 9/18 8261/2 Grand Avenue, Glenwood Springs,CO 81601 P:970.945.5544 F: 970.945.5558 www.mountaincross-eng.com ASPEN BUILDING DEPARTMENT 210 South First St. September,2018 Page 5 of 5 Principle 4: Similar to Principal 2,the impervious areas are minimized and swales are natural lined conveyances to allow for infiltration. Principle 5: Only WQCV is proposed since the project is in the Aspen Mountain Drainage Basin. Principle 6: There is an existing WQCV facility,treating water to enhance the community. Principle 7: The site has a treatment train approach in that drainage is conveyed in natural swales to the WQCV where it is treated prior to conveyance to the storm system of the City. Principle 8: All features are accessible to make maintenance as easy as possible. Principle 9: N/A. None of the facilities are located in areas accessible to the general public. Results The proposed drainage pattern is intended to remain. All proposed storm water is captured and conveyed by swales,trench drains, and downspouts and directed into the existing storm structure. A concrete gutter is proposed to direct any flows from the alley into South First Street and avoid trespass onto 210 South First Street. The proposed site was anticipated in a previous design. Since then, the Code requirements have changed. The proposed site drainage calculations are congruent with the previous design and the existing storm structure as constructed will be used to provide WQCV. All swales, trench drains, and downspouts will be directed into the structure through a system of 6" PVC pipes. From the structure,flows are metered by an orifice into the City storm drain. The structure provides WQCV volume even though it was intended for detention. Detnetion is no longer required. There is a certain amount of uncertainty in hydrologic calculations. However, when constructed in accordance with this report and the drainage plan, it is our opinion that the design will safely convey the runoff flows and volume of the 10-year design storm event and the WQCV required for this site and will not cause flooding damage to this or adjacent sites per the requirements of the City of Aspen. Thank you for the opportunity to provide this report. Feel free to call if you have any questions, concerns, or comments. Sincerely, Mountain Cross Engine rin , Inc. ,, -0. O , /r .,. l•1' �<� 35�54 r D y (elir is Hale,PE �` . l/Z l8 tY f" Mountain Cross Engineering, Inc. RECEIVED Civil and Environmental Consulting and Design 10/0 9/18 826 1/2 Grand Avenue, Glenwood Springs,CO 81601 P:970.945.5544 F:970.945.5558 www.mountaincross-eng.com ASPEN BUILDING DEPARTMENT VI 1 ••••••• --°'1Fr....„ • .1 r 1 t,.vi. ,l-1 1:_ii • • 1111111711 ." • ,! '- • ' t qv.. - 4 1 `� • NIL . y • 'm' • r:lirr r g 1, . + !A. fk . r ' -,..„,s , 1' . •--.111".tr • ehoolOili • ... • • 4 •r t,- -- ,! -ji, . YY7i ' • : joall. ''• '...a.A ,./. __ _. _ . , • ,i r. 4 - n `..� - . - -- •` N ' e r • .101 r�• _ e • -4.424:_iNIArt , 'N., 441Z4li'*'' '' 1 ilf 4 1 •. At. I 40. . , •1• r •, 1' r` ,� • ' •` tad �� • r t I# , ,.. . 1 •'.,s, 44.410 .. , • Ire ,1 Mg- ----------/— i . .. L[ . it: . e , ... 1 .. ,, i oL f _ .....7114 ' `.4, 1c , .a. ,t + p ' . 4. •A° IF.-e• S\\.. 4,1 .141. ..._" / ',N..'A ._-_,_ ' r'"11.„A - • .,_•,, . ire „ • .44 ki•,\ 1 . t:),7 . . '. ' . ...... iiriiiiikiLik-.• . - I .4r, : ,.. :;t DATE 08'15'17 FIGURE 1 SCALE 1•=50' MOUNT/MN R", §E ) ENGINEERChris Hole 210 South First Street Tw ENGINEERING, 'P DWG FILE - / Civil and Environmental Consultin1O5(0djiAi $ 210sFirstDesign / 826 1/2 Grand Avenue Glenwood Springs,CO 81601 JOB Gretchen Greenwood & Assc. \ /// 556-012 ph 970.945.5544 fx 970.945.5558 www.mountaincrgs,-pAg.com NJJI `` T � c o Existing ICI V o � c w IIu os„ 20 10 0 20SCALE: 1" - /O' i �STM r W U axN vUI l_i /44 ,"„ _...... 044 -------„rl,I ,ilt ftlr fi iftelrir I A } I air (0.... ,i i r ,r w to, • w • • iss ig IMIL c° //rill • 98 0 i I ,i4_, 1 LEGEND CD o cti PATH FOR TIME OF ` 100-yr FLOWRATE (CFS)`• _ 40 Z CONCENTRATION AREA (ACRES) Q LL w • RUNOFF COEFFICIENT Awil 13 • E • • o • r0^ I �90 V/ coV DRAINAGE of O w BASIN c • • N • • 02 LLEy BL0�II ,\ ° n 60co a R t L o W 10/ _®98 F U' '0 m BUILDING DEPARTMENT 1lri CJ 1: / - O N Proposed d O . w ; c c cd Li a -a �I�I U o 3 a M w N N 20 .26 10 o T T o > ~ � a u�i M yYESI IVOPIctivs�q P SCALE: 1" — 11 . C 3 R N • o ENU rISA � � � 4 /14-11 A lill rii rw __........ 11 1 1 1 1 1 i. rn - N _______ ' A���� I , ' tr, it fti ft"erinvo.,•.„, ''411111111rh , 4111 4 &, 1 tr-‘ a , . I . , I &, 1*' , I � � • i ° ° ° °� lifift' ° ° i LEGEND z PATH F R TIME Failli ` Y OO / �� , 100-yr FLOWRATE (CFS)`_ La CONCENTRATION �., o; ;/�`�� AREA (ACRES) Q w I aft . RUNOFF COEFFICIENT 1M A ._,iiir / iiriptic 0 IN/1-- 4-. w a iilf 0,11ti Z co BASIN \i� I (f) .. I'if �� o . ) 411111111114,411111 !-, * '90 I It .£Z Bl c _C1Y d „ v01 B0 . N 2 •( OI II L 0 E 8 0 — � U ia 10/6 98 1 kS4E ! 4 BUILDING DEPARTMENT V _ Road; `` HSG - 40 I o >‘- lb - , -'''''''I'llIllHllihrriry I Aw• Spruce St AllAk `�• r j ice; >_ ` Site 1 _ l:�! r.i .E- Location e1/4 O r Figure 3.1 Natural Resource Conservation Service(NRCS)Soil Map for Aspen 1 00 0 90 080 0 70 ,X 1 U 060 -+-100-yr Area -a-50-y r E g 0 50 —A—25.yr U -x-5-yr u ,r- yr 0 40 x -x�x -+-2-yr 0 30 'x-+ 020fx 0 10 000 0/ 10% 7,1, 30 io 40% 50% 60`%, 70% 80% 90".t 100% Watershed Percentage Imperviousness Figure 3.2- Runoff Coefficients for NRCS Hydrologic Soil Group B RECEIVED 10/09/18 ASPEN BUILDING DEPARTMENT 1 00 • 0 90 080 060 m w —a-50-yr Area 0 o so —.—zs-yr P cl ar I -x-10-yr t 5-yr 0 40 x —41--2-yr 0 30 �c Ix� 020 0 10 000 -- - - --- -- - - _- ------ --- -- 0% 10% 20% 30% 4095 50% 600- 70% 80% 90% Watershed Percentage Imperviousness Figure 3.2—Runoff Coefficients for NRCS Hydrologic Soil Group B RECEIVED 10/09/18 ASPEN BUILDING DEPARTMENT DRAINAGE.CALCULATIONS fior = M®CIN'I'IAIN CROSS 210 S First Street ENGINEERING, INC '7/28/20172 Civil and Environmental Consulting and Design ,_ 826 1R Grand Avenue Glenwood Springs,CO 01601 pn 970,9415544 Ix 970,945.5558vmmw.mounts Encroee-engcom Drainage Area : Existing/Historic Surface description: Unimproved PROPERTY GENERAL SURFACE DATA Total Drainage Area: 3,520 sq. ft. 0.081 ac. RUNOFF COEFFICIENT imperviousness = 0% HSG = B from Fig 3.2 C100 = 0.35 C10 = 0.15 C5 = 0.08 C2 = 0.03 100% 0.081 TOTAL BASIN ACERAGE 0.081 TIME OF CONCENTRATION Overland Flow Time Length Slope C5 Tc Upstream Elevation 7901.50 66 0.056 0.08 8.47 Dnstream Elevation 7897.80 Channel Flow Time none Length Slope K Tt Upstream Elevation 7897.80 0.0 1.100 7.000 0.00 Dnstream Elevation 7897.80 Total 8.47 min RUNOFF Existing/Historic Intensity Flowrate (in/hr) (cfs) 2-yr Storm 1.6 0.00 10-yr Storm 3.0 0.04 100-yr Storm 5.1 0.14 NOTES: - Soils are hydrologic soil group B from Figure 3.1 - K 7.0 based on lawn RECEIVED lu/U9/18 ASPEN BUILDING DEPARTMENT DRAINAGE CALCULATIONS ' t� OUNThlIN CROSS 21•0 S First Street ENGINEERING, INC 7/28/201.7 civil aria Environmental ConsoII g and o g $2&72°rand Avenue Alenwood Springs.c°a76u7 -" ph9T0.ndi5544 h9TO.945.5558 vNNnnuntalnnona. g -' Drainage Area : Proposed Surface description: Turf, landscaping, roof area, and hardscape PROPERTY GENERAL SURFACE DATA Total Drainage Area: 3,600 sq. ft. 0.083 ac. RUNOFF COEFFICIENT sq. ft. or Land Use est. % acres imperviousness = 74% Building Roof 2265 0.052 HSG = B from Fig 3.2 Ponds 0 0.000 C100 = 0.65 Hardscape and Paving 390 0.009 C10 = 0.57 Landscaped turf and planters 945 0.022 C5 = 0.53 Pine and Aspen 0% 0.000 C2 = 0.49 Sage and Grasses 0% 0.000 TOTAL BASIN ACERAGE 0.083 TIME OF CONCENTRATION Overland Flow Time Length Slope C5 Tc Upstream Elevation 7901.50 36 0.083 0.53 3.07 Dnstream Elevation 7898.50 Channel Flow Time none Length Slope K Tt Upstream Elevation 7898.50 42.0 0.083 20.000 0.12 Dnstream Elevation 7895.00 Total 3.19 min use 5 min Tc RUNOFF Proposed Intensity Flowrate (in/hr) (cfs) 2-yr Storm 2.1 0.085 10-yr Storm 3.7 0.174 100-yr Storm 6.3 0.338 NOTES: - Soils are hydrologic soil group B from Figure 3.1 - K 20 based on hard channel RECEIVED lu/u /18 ASPEN BUILDING DEPARTMENT Rainfall IDF for Aspen, Colorado 7 , 6 4 u c N C c 3 2 1 0 0 5 10 15 20 25 30 35 40 45 50 SS 60 Duration in Minutes 2-yr t 5-yr --= 10-yr t 25-yr —0—50-yr —100-yr RECEIVED 10/09/18 ASPEN BUILDING DEPARTMENT - pas- WQC Volume 13 I `.,,'/ Level 1: Over turf to capture t I Le.d� Tot Imp = 74 % 4 �� I L'I�W t from Fig. 8.14 yields r. L._te N. 1'ir... Eff Imp = 72 % it " . ~, � ;.j \ from Fig. 8.13 yields ` r' WQCV= 0.145 watershed-in SC • Area = 0.065 acres s Req'd Volume= 34.2 cubic feet i : = %' ' - - r V 1: .0 aV 4: OU OU i. W 'IV .. lu441 luipvt:ivu A,us%(141 L.'ni) Figure 914 Imperviousness Adjustments for Level 1 and 2 MCA(UDFCD 1999) 0 30 - - . >000000, 3 -- u.:: g 0.05 0.00 1 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 Effective Imperviousness of Tit butaryArea to BMP(percent) Figure 8.13 Aspen Water Quality Capture Volume RECEIVED 10/09/18 ASPEN BUILDING DEPARTMENT Channel Report Hydraflow Express Extension for Autodesk®AutoCAD®Civil 3D®by Autodesk,Inc. Sunday,Jul 30 2017 6 inch - 100 year Circular Highlighted Diameter (ft) = 0.50 Depth (ft) = 0.29 Q (cfs) = 0.340 Area (sqft) = 0.12 Invert Elev (ft) = 1.00 Velocity (ft/s) = 2.87 Slope (%) = 1.00 Wetted Perim (ft) = 0.87 N-Value = 0.013 Crit Depth, Yc (ft) = 0.30 Top Width (ft) = 0.49 Calculations EGL (ft) = 0.42 Compute by: Known Q Known Q (cfs) = 0.34 Elev (ft) Section 2.00 1.75 1.50 1.25 1.00 \\\ „,_/'/// 0.75 0 1 RECEIVED 10/09/18 Reach (ft) ASPEN BUILDING DEPARTMENT Inlet Report Hydraflow Express Extension for Autodesk®AutoCAD®Civil 3D®by Autodesk, Inc. Tuesday,Aug 7 2018 Trench - 50% Clogged Drop Grate Inlet Calculations Location = Sag Compute by: Known Q Curb Length (ft) = -0- Q (cfs) = 0.34 Throat Height (in) = -0- Grate Area (sgft) = 1.49 Highlighted Grate Width (ft) = 0.33 Q Total (cfs) = 0.34 Grate Length (ft) = 4.50 Q Capt (cfs) = 0.34 Q Bypass (cfs) = -0- Gutter Depth at Inlet (in) = 0.62 Slope, Sw (ft/ft) = 0.065 Efficiency (%) = 100 Slope, Sx (ft/ft) = 0.065 Gutter Spread (ft) = 1.92 Local Depr (in) = -0- Gutter Vel (ft/s) = 1.30 Gutter Width (ft) = 0.33 Bypass Spread (ft) = -0- Gutter Slope (%) = -0- Bypass Depth (in) = -0- Gutter n-value = -0- All dimensions in feet ♦ ♦ ♦ RECEIVED 10/09/18 ASPEN BUILDING DEPARTMENT Inlet Report Hydraflow Express Extension for Autodesk®AutoCAD®Civil 3D®by Autodesk, Inc. Thursday,Aug 9 2018 Grate - 50% Clogged Drop Grate Inlet Calculations Location = Sag Compute by: Known Q Curb Length (ft) = -0- Q (cfs) = 0.34 Throat Height (in) = -0- Grate Area (sqft) = 0.56 Highlighted Grate Width (ft) = 0.75 Q Total (cfs) = 0.34 Grate Length (ft) = 0.75 Q Capt (cfs) = 0.34 Q Bypass (cfs) = -0- Gutter Depth at Inlet (in) = 1.35 Slope, Sw (ft/ft) = 0.020 Efficiency (%) = 100 Slope, Sx (ft/ft) = 0.020 Gutter Spread (ft) = 13.25 Local Depr (in) = -0- Gutter Vel (ft/s) = -0- Gutter Width (ft) = 2.00 Bypass Spread (ft) = -0- Gutter Slope (%) = -0- Bypass Depth (in) = -0- Gutter n-value = -0- All dimensions in feet 0 r-1 .11 RECEIVED 10/09/18 ASPEN BUILDING DEPARTMENT _......, i�� MOUNTAIN CROSS yy ENGINEERING, INC. / Civil and Environmental Consulting and Design Maintenance Plan The following is a checklist to aid the Property Owner of ongoing drainage system maintenance for 210 South First Street in Aspen CO. The site will require more frequent cleaning of sediment initially. As the site matures, maintenance should be performed after large magnitude rainfall events and at the changing of the seasons. Damaged, cracked, or missing items should be repaired and replaced as discovered. This is not intended to be an all-inclusive list: Drainage Swales: • Verify positive drainage and grade any ponding areas to drain • Check for exposed and/or damage to PVC liner and repair • Clean out swales from deposited sediment and debris Concrete Pans, and Curb and Gutter: • Clean off deposited sediment and debris • Check for and fill any cracks • Repair and/or replace broken and damaged items Pipes and Trench Drain: • Clear inlet grates of debris • Clear piping of any obstructions • Clean sumps and piping of deposited mud and debris • Check offsite end is clear • Check exposed PVC piping for UV degradation and cracking Heat Tape and Snowmelt: • Check for correct operation in pipes, roof gutters, and downspouts and repair as necessary • Check for correct operation in trench drains and drywell and repair as necessary • Check for correct operation in snowmelt area and repair as necessary • Turn on heat tape for cold seasons and turn off for warm seasons Water Quality Capture Volume Storm Structures • Remove sediment, debris, and litter from structure • Clear mud and debris from inlets and outlets • Drain the structure via pumping and clean by jetting and pumping to remove sediment Overall: • Check site for differential settlement creating any areas of ponding and grade to drain • Check site for locations where runoff may breach and bypass the drainage system and correct • Keep roof gutters and downspouts clear of debris RECEIVED 826 1/2 Grand Avenue,Glenwood Springs,CO 81601 10/0 9/18 P:970.945.5544 F:970.945.5558 www.mountaincross-eng.com ASPEN BUILDING DEPARTMENT TIMBERLINE ENGINEERING CIVIL/STRUCTURAL/CERTIFIED ENERGY DESIGN PROFESSIONALS November 11,2009 City of Aspen Engineering Department 130 South Galena St. Aspen,CO 81611 RE: Storm Water Detention System for 135 West Hopkins Avenue Summary This report is intended to serve as an addendum to the WRC Engineering,Inc. "Drainage Report for 135 West Hopkins Avenue"dated May 2008, Revised August 2008. This addendum presents detention tanks as an alternative to the use of the drywells specified in the WRC report. The detention tanks will be in the same locations as the drywells in the WRC report. Calculations used for the detention tanks sizing were based on the following information: 1. Storm water runoff from the 5 year event developed site, as presented in the WRC report,was used as the inflow into the tanks. 2. The storm water runoff from the 5 year event is released out of the tanks at a rate equal to or less than the historic 2 year event for the undeveloped property. 3. Accumulative runoff and release calculations at the above rates using the methodology presented in section 3.8.1.1.1 of the City of Aspen Design and Construction Standards, June 2005. Each detention tank is proposed to be to a pre-cast, single-compartment, 1000-gallon concrete tank. Discharge from the detention tanks is coursed to the City of Aspen Storm Sewer System. A manhole will be constructed in Hopkins Ave. at the location of the tie-in to the existing 24" storm sewer. Attached are runoff and detention calculations, a discharge pipe sizing calculation and Sheet C.100 which shows the proposed plan and associated details. General Concept Runoff from the site is designed to be intercepted by two detention tanks. The westerly detention tank, located along First Avenue,is located in the WRC Basin 2 and will intercept all runoff from the southern most structure and a portion of the north structure as shown in the basin delineation in the WRC report and Sheet C.100 attached to this report. The northern detention tank, located along West Hopkins Avenue,is located in the WRC Basin 1 and will intercept runoff from a portion 10/09/18 P.O. BOX 631 CARBONDALE, CO. 81623 P. 970 963 9869 timberlines �iCn BUILDING DEPAR I MENT November 11,2009,Storm Water Detention System for 135 West Hopkins,page 2 most structure, as shown in the basin delineation in the WRC report and Sheet C.100 attached to this report. All roof drains and downspouts will be directed to the tanks per the WRC report. Surface runoff from roof drains, grass lined swales, overland flow, and trench drains, will be coursed to both tanks as shown in the WRC Report Specific Details Tank Sizing Calculations Two detention tanks are designed for the site. The required detention tank volume was calculated based on the methodology presented in Section 3.8.1.1.1 of the City of Aspen Design and Construction Standards, June 2005. Tank inflows and volumes were from the post development, 60-minute rainfall duration, 5-year storm event as calculated in the WRC report. Tank outflows were restricted to the historic undeveloped, 60-minute rainfall duration, 2-yr storm discharge rate per the attached calculations. Based on the Section 3.8.1.1.1 calculations,the northerly tank, located along West Hopkins Avenue, requires 477 gallons of capacity and the westerly tank, located along First Avenue, requires 842 gallons of capacity. Sheet C.100 specifies both detention tanks as 1,000 gallon tanks with an effective storage volume of 850 gallons. Tank discharge orifice calculations An orifice in the tank discharge pipe controls the release rate. The orifices were sized in accordance with the City of Aspen Design and Construction Standards. The calculations are attached. The westerly detention tank, located along First Avenue, will require a 7/8" orifice and its maximum discharge rate is .05 cfs. The northern detention tank, located along West Hopkins Avenue,will require a 1/2 " orifice and its maximum discharge rate is .01 cfs. The combined total discharge is 0.06 cfs which is less than the historic undeveloped, 60-minute rainfall duration,2-yr storm discharge rate of 0.07 cfs. The orifice inlets will require a debris screen to prevent clogging. The debris screen specification and detail is shown on the attached Sheet C.100 Tank Details The tank locations and details are shown on the attached Sheet C.100, Surface Drainage&Detention Plan. Both detention tanks are to be a pre-cast, single-compartment, 1000-gallon concrete tank.. The detention tanks will be fitted with an inlet grate for surface runoff. The inlet grate will also serve as an access point to the manhole for maintenance. The inlet grate is a Neenah R-2510-2 Type G, as specified in the WRC report. RECEIVED 10/09/18 ASPEN BUILDING DEPARTMENT November 11,2009,Storm Water Detention System for 135 West Hopkins,page 3 Discharge Pipe Per the WRC report, the developed site storm water runoff flow rate for the 100 year storm event is 0.30 cfs for Basin 1 and 0.56 for Basin 2 for a total site discharge of 0.86 cfs. The full flow capacity of the 6" PVC discharge pipe at a minimum 2%grade into the storm sewer is 1.03 cfs. Operation and Maintenance Operation and maintenance of the storm water facilities will be the responsibility of the owner. The detention tanks should be inspected and cleaned once a year,preferably during the spring months. All other facilities including grass-lined swales and trench drains should be inspected on a regular basis and maintained on an as-needed basis. Please call us at 970-963-9869 if you have any questions. Sincerely, TIMBERLINE ENGINEERING David A. Powell,PE RECEIVED 10/09/18 ASPEN BUILDING DEPARTMENT 135 W. HOPKINS DRAINAGE CALCULATIONS TIMBERLINE ENGINEERING 10/30/2009 Drainage Calculations for 135 West Hopkins Historic Basin Rational Method: Q=CfCIA Cf 5— 1.00 Cf too= 1.25 Historic Basin 1 Area A (sf) (acre) Historic Basin 5270.76 0.121 Runoff Coefficient C= 0.25 TRAVEL TIME: Tc=Tov+Tt Time of overland flow=Tov: Elev top= 106 ft Elev bot= 101 ft Elev Diff= 5 ft Overland Flow Length D(ft)= 113 ft (500'max for undeveloped area,200'max for developed area) Slope S= 4.4% Tov= 1.87(1.1 -CCf)D112 S'a Tov-2yr= 10.29 mins Tov-looyr= 9.54 mins Time of Concentrated Flow=Tt: Tt= 0 Time of Concentration=Tc: Tc-min= 10 min.for non-urban TC 2 =Tov+Tt= 10.29 use=> 10.29 min. Tc too=Tov+Tt= 9.54 use=> 10.00 min. I2= 2.3 in/hr too= 7.4 in/hr Q2 = CfCIA= 0.07 cfs Q100=CfCIA= 0.28 cfs RECEIVED 10/09/18 ASPEN BUILDING DEPARTMENT 135 W.HOPKINS DRAINAGE CALCULATIONS TIMBERLINE ENGINEERING 10/30/2009 Drainage Calculations for 135 West Hopkins DEVELOPED BASIN 1 ORIFICE AND DETENTION TANK SIZING ORIFICE CALCULATIONS: for orifice outlet: Q= C0A(2gh)^1/2 Orifice diameter' 0.5 I inch Area= 0.001ugF38 sf head Q g= 32.20 ft/sec ft cfs Co= 0.65 ft 0 0.00 BELOW HISTORIC 0.5 0.01 BELOW HISTORIC 1 0.01 BELOW HISTORIC 1.5 0.01 BELOW HISTORIC 2 0.01 BELOW HISTORIC 1.5 0.01 BELOW HISTORIC 2 0.01 BELOW HISTORIC 2.5 0.01 BELOW HISTORIC 3 0.01 BELOW HISTORIC 3.5 0.01 BELOW HISTORIC DETENTION TANK SIZING Developed Basin 1 Area= 0.040 acre Developed Basin 1 Runoff Coefficient=C= 0.81 Historic Rainfall Intensity(12)= 2.30 in/hr Q2-Hat= 0.02 cfs CI,proposed w/orifice= 0.01 cis HISTORIC ACCUMULATIVE RUNOFF VOLUME PROPOSED ACCUMULATIVE RUNOFF VOLUME WITH ORIFICE CONTROL TIME Time ACC.Runoff TIME Time ACC.Runoff Q2-Hist x T Q5-Hst x T min. sec. ft3 min. sec. ft 5 300 7 5 300 4 10 600 14 10 600 7 15 900 21 15 900 11 20 1200 28 20 1200 15 30 1800 42 30 1800 22 40 2400 56 40 2400 30 50 3000 70 50 3000 37 60 3600 83 60 3600 44 DEVELOPED ACCUMULATIVE 5-YR RUNOFF VOLUME Q3..Ho4/I=CxA= 0.03 Acc.Runoff TIME Time Rainfall Vol. Intensity(5yr) (CxA)(l) min. sec. in/hr ft3 5 300 5.65 54 10 600 3.7 71 15 900 2.55 74 20 1200 1.9 73 30 1800 1.3 75 40 2400 1.04 80 50 3000 0.92 88 60 3600 0.85 98 DETENTION POND SIZING BASED ON ACCUMULATIVE DIFFERENCE TIME Acc.Dev. Acc.Praposed. Acc.Diff Runoff Vol. Rel.Rate Vol. in Runoff min. ft3 ft3 5 54 4 51 10 71 7 64 MAXIMUM 15 74 11 62 20 73 15 58 30 75 22 53 40 80 30 50 50 88 37 51 60 98 44 54 MAXIMUM Required Storage Volume For Developed Basin 1 = 64 cf = 0.001 ac-ft = 477 GALLONS Max Release Rate= 0.01 cfs RECEIVED 10/09/18 ASPEN BUILDING DEPARTMENT 135 W.HOPKINS DRAINAGE CALCULATIONS TIMBERLINE ENGINEERING 10/30/2009 Drainage Calculations for 135 West Hopkins DEVELOPED BASIN 2 ORIFICE AND DETENTION TANK SIZING ORIFICE CALCULATIONS: for orifice outlet Q= CoA(2gh)"1/2 Orifice diameter) 0.88 I inch Area= 0.004175837 sf head Q g= 32.20 ft/sec ft cfs Co= 0.65 ft 0 0.00 BELOW HISTORIC 0.5 0.02 BELOW HISTORIC 1 0.02 BELOW HISTORIC 1.5 0.03 BELOW HISTORIC 2 0.03 BELOW HISTORIC 2.5 0.03 BELOW HISTORIC I 3 0.04 BELOW HISTORIC I 3.5 0.04 BELOW HISTORIC 4 0.04 BELOW HISTORIC 4.5 0.05 BELOW HISTORIC 5 0.05 EXCEEDS HISTORIC DETENTION TANK SIZING Developed Basin 2 Area= 0.081 acre Developed Basin 2 Runoff Coefficient=C= 0.75 Historic Rainfall Intensity(12)= 2.30 in/hr Q2-Hst= 0.05 cfs Q5 proposed w/orifice= 0.04 cfs HISTORIC ACCUMULATIVE RUNOFF VOLUME PROPOSED ACCUMULATIVE RUNOFF VOLUME WITH ORIFICE CONTROL TIME Time ACC.Runoff TIME Time ACC.Runoff Q2+tst x T Q54st x T min. sec. ft3 min. sec. ft3 5 300 14 5 300 11 10 600 28 10 600 23 15 900 42 15 900 34 20 1200 56 20 1200 45 30 1800 83 30 1800 68 40 2400 111 40 2400 91 50 3000 139 50 3000 113 60 3600 167 60 3600 136 DEVELOPED ACCUMULATIVE 5-YR RUNOFF VOLUME Q2_Ht/I=CxA= 0.06 Acc.Runoff TIME lime Rainfall Vol. Intensity(5yr) (CxA)(I) min. sec. in/hr ft3 5 300 5.65 103 10 600 3.7 135 15 900 2.55 140 20 1200 1.9 139 30 1800 1.3 143 40 2400 1.04 152 50 3000 0.92 168 60 3600 0.85 186 DETENTION POND SIZING BASED ON ACCUMULATIVE DIFFERENCE TIME Acc.Dev. Acc.Proposed. Acc.Diff Runoff Vol. Rel.Rate Vol. in Runoff min. ft3 ft3 ft3 5 103 11 92 10 135 23 113 MAXIMUM 15 140 34 106 20 139 45 94 30 143 68 75 40 152 91 61 50 168 113 55 60 186 136 51 Required Storage Volume For Developed Basin 2 = 113 cf = 0.003 ac-ft = 842 GALLONS Max Release Rate= 0.04 cfs RECEIVED 10/09/18 ASPEN BUILDING DEPARTMENT Worksheet for 135 West Hopkins Discharge Pipe capacity Project Description Friction Method Manning Formula Solve For Full Flow Capacity Input Data Roughness Coefficient 0.010 Channel Slope 0.02000 ft/ft Normal Depth 0.50 ft Diameter 0.50 ft Discharge 1.03 ft3/s Results Discharge 1.03 ft3/s Normal Depth 0.50 ft Flow Area 0.20 ft2 Wetted Perimeter 1.57 ft Hydraulic Radius 0.13 ft Top Width 0.00 ft Critical Depth 0,48 ft Percent Full 100.0 % Critical Slope 0.01733 ft/ft Velocity 5.25 ft/s Velocity Head 0.43 ft Specific Energy 0.93 ft Froude Number 0.00 Maximum Discharge 1.11 fN/s Discharge Full 1.03 ft'/s Slope Full 0.02000 fUft Flow Type SubCritical GVF Input Data Downstream Depth 0.00 ft Length 0.00 ft Number Of Steps 0 GVF Output Data Upstream Depth 0.00 ft Profile Description Profile Headloss 0.00 ft Average End Depth Over Rise 0.00 Bentley Systems,Inc. Haestad Methods Solution Center Bentley FlowMaster [08.11.0003] ■■■■ 11/11/2009 12:27:13 PM 27 Siemons Company Drive Suite 200 W Watertown,CT 06795 USA +1-203-755-1666 Pag 1 f0E IVE D 10/09/18 ASPEN BUILDING DEPARTMENT LEGEND _ 9 0` ALL ROOF DRAINS AND DOWNSPOUTS 8001 -PROPOSED FINISHED GRADE 1"CONTOURS Ir. 0 BE DIRECTED TO DRAIN INLETS. \ ��--� 0005 -PROPOSED FINISHED GRADE 5'CONTOURS \ � - --E _ -t_ _ -8001 -EXISTING GRADE 1"CONTOURS -� E-� )naint. 3 INLET 510 =1E.5 - - -800r - - -EXISTING GRADE 5'CONTOURS NEENAH R-2510 TYPE G DL GRATE 103.5 � � GRATE 105.5/ -4--- -DIRECTION OF SURFACE DRAINAGE PER SPECS NV-102.5 = NV.=104.5 Garage (8RC) - - I O \O IWRCJ SOLID LID BELOW GRAD o T 0 slab PAVED DRIVE \ +102. \ 103.5 SLOPE=5X Flat _ COPYRIGHT (WRC) _ / W tool)GALLON CONC. T NK. DRIVE OOf = SINGLE COMPARTM T. zi _ TOP OF TANK= 10 �' TRENCH - BOTTOM OF TANK 98.83, \ RAIN PER = �2-YR ORIFICE=Yz"D P I ��WRC SPECS. o s _ / __ r -► `/ RELEASE RATE 0.02 CF = !I N GRATE 103.5 - _ 2 W EXISTING STORM SEWER. Q E '�NV. 96.5 y J�VC)02.2 SCALE ft.ch= 5 105\ OCATION AND ELEVATION TO BE ,� I VERIFIED IN FIELD M LW 114 = I I W O + M o '='. Flat Flat zw s'PVC I 02.5 -, -. o Roof I Roof •PVC ��r�•, a + + \ - TRENCH DRAIN,GRATE V A� 1y� 1' � INVERT ELEVATIONS Garage • PER WRC SPECS. W Q g as 2 kki \nt.ex 13 Porch T O slab W o 105.5 - "°-, O / ,2 maint.ex 106 -- Q m G5. Residence �l w.... W _ llvv�yl' + W FFE 104 LA LW �� Roof � Poo 10 i � � I 1 o - � Waa I c 1 4 © \ _ 0 N Q. Ci� \ Residence = I GRATE=105.5 DRAWN BY T _ NV.=103.8 LW FFE 105 (WRc) DP J + + 102 12.44 + 103.94 Ft t 1 \� 101+/- \ R f LW _ �� �\ / 1000 GALLON CONC. TANK co + TOR INV 00.0 T/1 N�'�7 SINGLE COMPARTMENT `/1 STORM DRAIN MANHOLE PER CITY OF ( i 102 \ TOP OF TANK=101.0 maint.ex 105 W ASPEN STANDARDS. \ PORCH INLET GRATE=101.5 _ BOTTOM OF TANK 95.83 w LACE MANHOLE OVER EXISTING \ NEENAH R-2510-2 TYPE G W O STORM SEWER. ACTUAL LOCATION TO I \ PER WRC SPECS ,7 Q BE VERIFIED IN FIELD. \ \SOLID LID BELOW GRADE NV=100.0 �� 103 Fr+i1 Q' - RELEASE DRAT- 7/8"DIA NEW NV=101.0 v V] \,:,, I RELEASE RATE0.05 CFS • _ \ POI ]� �» maint.ex 10�� Z Z 0 (�'L_- '1 �--2R PIPE GRADE + + 104.94 PVC W a 8 INV.=94.85 MAX OR LOWER TO "PVC 2x PIP GRADE.,MIN. M - �- -- • -_ - MATCH STORM SEWER INVERT. - - PVC NV.=96.5 I O U \ InsvE1.55_. - x f COMBO FOR TIE ALL ROOF DRAINS AND DOWNSPOUTS W ---- - TO BE DIRECTED TO DRAIN INLETS. Fa+11 IN I + 103.44 O/ to maint.ex 1 Q \ I ` OW 103.44 03.44 + 102.44+/- W .. \ pim4 // DATE � � 11/11/09 _ REVISIONS SUMMARY RUNOFF TABLE: NOTES:1. ALL PVC PtPJ TO BE SI9t140 PVC. xx ..4_' 2. ALL DRAINAGE PIPE TO PRESSVRE TESTED PRIOR TO BACKFILL. Q PRE-DEVELOPMENT 3. USE DOUBLE 45° BENDS AT 90° TURNS. Q2 = 0.07 cfs Q5 = 0.17 ifs Q,00 = 0.28 cfs SOUTH FIRST S TREF,T 4. FOUNDATION DRAINAGE SYSTEMS BY OTHERS. c2 = 0.25 C5 = 0.25 Goo = 0.25 Cr = 1.0 c. = 1.0 Cr = 1.25 5. FOUNDATION DRAINAGE SYSTEMS CANNOT BE TIED TO STORM DRAINAGE SYSTEM. J A = 0.121 ac A = 0.121 ac A = 0.121 ac T. = 5.0 min. T. = 5.0 min. T. = 5.0 min. DEBRIS SCREEN HANGERS: INLET GRATE SIZING PER WRC 6" OVERFLOW SCALE 12 = 2.3 in/hr Is = 5.6 in/hr hoo = 7.4 in/hr TO BE%z" DIA STAINLESS STEEL. / J SCREWED CAP CLEANING ACCESS AS SHOWN 3" DEEP DRILLED HOLE IN CONC TANK. TOP OF TANK INSTALL WITH APPROPRIATE EPDXY. ®.. -I POST-DEVELOPMENT " ° T �' DRAWING DESIGN POINT 1 OUTLET PIPE: f' 1 ,,C 1000 GALLON SINGLE-COMPARTMENT CONC. TANK. USE NEXT LARGER STANDARD DIAMETER TO T Q v 6" SANITARY TEE Qs = 0.18 cfs Owe = 0.30 cfs Q 1 f_ STORM Cs = 0.81 Goo = 0.81 ORIFICE DIAMETER PVC WASTEPIPE (E.G.7/8" 1 / DIMENSIONS BASED ON TANKS BY COPELAND 1• SCHED.40 PIPE TYP. DRAINAGE Cr = 1.0 Cr = 1.25 ORIFICE, USE LARGER WASTEPIPE) CONNECTED CONCRETE. ENGINEER TO VERIFY TANK v a /OO DIMENSIONS AND CAPACITY IF ANOTHER J STRUCTURE To &DETENTION A = 0.040 ac A = 0.040 ac TO OVERFLOW PIPE WITH REDUCER BUSHINGS. MANUFACTURER IS CHOSEN. T° = 5.0 min. T. = 5.0 min. ") " PLAN Is = 5.6 in/hr hoo = 7.4 in/hr ....14 Ib-°' 6" STREET 45 DEBRIS SCREEN: ph O J i---, MESH TO MOUNTED ON STEEL FRAME. MESH �'.....gu AND FRAME TO BE GALVANIZED OR STAINLESS IA..1'' Y 6" WYE STEEL. SCREEN TO BE REMOVABLE FOR Oa°i= h 2 YR ORIFICE SHEET POST-DEVELOPMENT ORIFICE PLA TE: �� CLEANING. MESH SIZE TO BE NO MORE THAN .t3S; I V SEE PLAN FOR ORIFICE SIZE TO STORM SEWER DESIGN POINT 2 PVC END CAP, THREADED ON 70%OF THE ORIFICE DIAMETER. I'v;;' Qs = 0.34 cfs No = 0.56 cfs AND DRILLED TO CORRECT a • 1 Cs = 0.75 Goo = 0.75 ORIFICE DIAMETER. J DEBRIS SCREEN BOTTOM OF TANK EPDXY MORTAR SEAL I . 1 1 00 Cr = 1.0 Cr = 1.25 NOTES: A = 0.081 ac A = 0.081 ac 1. FLOW CALCULATIONS BASED ON WRC T. = 5.0 min. T. = 5.0 min. ENGINEERING MAY 2008 DRAINAGE ORIFICE DETAIL DEBRIS SCREEN DETAIL DETENTION TANK DETAIL C IVED Is = 5.6 in/hr hoo = 7.4 in/hr REPORT REVISED AUGUST 2008. :/1 9/18 ASPEN BUILDING DEPARTMENT