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Home My WebLink AboutChange Order.1015 Waters Ave.0204.2019 (13).ARBK MOUNTAIN CROSS ENGINEERING, INC. Civil and Environmental Consulting and Design October 10, 2019 Ms. Gretchen Greenwood Gretchen Greenwood&Associates, Inc. 210 South Galena Street, Suite 30 Aspen, CO 81611 RE: Urban Runoff Management Plan— 1015 Waters Avenue Dear Ms. Greenwood: The purpose of this correspondence is to evaluate the construction of a residence at 1015 Waters Avenue from a storm water perspective and provide design recommendations pertaining to urban runoff management for incorporation into the site design. These recommendations were generated from the site plans that were submitted to our office and a site visit performed on March 27, 2018. 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 7,960 feet above sea level. An existing residence sits on the lot and will be razed prior to construction of the proposed residence. 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. There is an irrigation ditch that cuts across the south west corner of the lot. The project is located greater than 100' from any mapped floodplain boundaries. No other drainage studies are known to have been performed specific to this lot. 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), per the 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: RECEIVED 10/10/2019 8261/2 Grand Avenue, Glenwood Springs,CO 81601 P:970.945.5544 F: 970.945.5558 www.mountaincross-eng.com ASPEN BUILDING DEPARTMENT 1015 Waters Ave October, 2019 Page 2 of 5 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. Analysis Project basins were delineated based on a review of existing site conditions, aerial photography, roof lines, site grading, and project topography. The lot drains from south to north intt r E IvE D Mountain Cross Engineering, Inc. �(� Civil and Environmental Consulting and Design 10/10/2 019 8261/2 Grand Avenue, Glenwood Springs,CO 81601 P:970.945.5544 F:970.945.5558 www.mountaincross-eng.com ASPEN BUILDING DEPARTMENT 1015 Waters Ave October, 2019 Page 3 of 5 Avenue. The south boundary is the existing irrigation ditch. There is a site wall on the neighboring property line that acts as the eastern boundary. The existing conditions may have some trespass of runoff from the subject property onto the neighboring property to the west; this was difficult to verify due to the still significant snow coverage. If this is the case, the proposed plan will remedy this. Other than maybe trespass to the west, the proposed drainage pattern is intended to remain. All proposed storm water is captured and conveyed by swales and/or piping to the northwest corner where a bio-retention pond provides the required water quality capture volume. A scupper/opening in the proposed site wall drains water above the capture volume into Waters Avenue. Since the project is within the Aspen Mountain Drainage Basin,detention is not required. 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 concentration for the site basin is less than 5 minutes so 5 minutes was 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 historic (pre-project flows) and also for the proposed conditions. The calculations are attached. Basin Area Coefficient Flow Description (acres) (cfs) Historic - 100 yr 0.148 0.35 0.32 Proposed- 100 yr 0.148 0.52 0.48 Inlets and swales capture flows and direct water into 8" PVC storm pipes at a minimum of 2% slope. 100 year flows would fill the 8"pipe halfway. Calculations are attached in the Appendix. Water Quality Water quality capture volume was calculated to be 44.1 cubic feet. Calculations are attached. The plans call for a bio-retention pond in the northwest corner. Since the project has type B soils, infiltration is proposed. Flows above the 44.1 cubic feet will be released into Waters Avenue. Temporary structures are intended to be used during construction activities. Permanent structures are intended to be used continually after the construction activities have been completed. Temporary erosion control measures that are to be employed during construction have been designed to contain sediment on the site and to mitigate erosion from construction activities. Silt fencing will be placed around the downhill limits of disturbance. Permanent erosion control measures are revegetation of disturbed areas and design of conveyances to prevent erosi ft he water quality structure will capture any transported sediment. G E R/E D Mountain Cross Engineering, Inc. Civil and Environmental Consulting and Design 10/10/2 019 8261/2 Grand Avenue, Glenwood Springs,CO 81601 P:970.945.5544 F: 970.945.5558 www.mountaincross-eng.com ASPEN BUILDING DEPARTMENT 1015 Waters Ave October, 2019 Page 4 of 5 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. Permeable pavers are used instead of impervious pavement. Where possible, flows are in permeable swales. Principle 3: Permeable pavers are used. The impervious areas have been minimized as much as practical and still meet the requirements of the Owner. Principle 4: Similar to Principal 2, permeable pavers are used and the impervious areas are minimized. 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 a proposed 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 leaving the site. 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 and pipes into the bio-retention pond. An 8"PVC pipe will release water over a curb into Waters Avenue. The proposed bio-retention provides the required WQC volume of 44.1 cubic feet. Detention is not 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. RECEIVED Mountain Cross Engineering, Inc. Civil and Environmental Consulting and Design 10/10/2 019 8261/2 Grand Avenue, Glenwood Springs,CO 81601 P:970.945.5544 F:970.945.5558 www.mountaincross-eng.com ASPEN BUILDING DEPARTMENT 1015 Waters Ave October, 2019 Page 5 of 5 Thank you for the opportunity to provide this report. Feel free to call if you have any questions, concerns, or comments. Sincerely, Mountaincross Engineerif g, Inc. ;;;_,; ._ N( 0 S. s.1) w • (j'l , . , .„ T. 61 -( . i:'! 35^6 i ,< Chris Hale, PE • '-.i t i i� 0 4 kt)?'t 114 e i eQ lif(:1 5 *517 cli*P \19 ` /Y's Mountain Cross Engineering. Inc. �.Q �'- �� Civil and Environmental Consulting and Design _\� 8261 Grand Avenue,Glenwood Springs,CO 81601 O\\� P:970.945.5544 F:970.945.5558 www.mountaincross-eng.com \ r • A..... .-. _. - ' . ...• ... , ..... . . . • ,. ,,, . , . , . viti. 1}. 41,: .-0 ' -• • •I r i4 li h.. . ' • 'v. •-a '4 Tr;14 _ . - # . _ y la 1 'LI' (lb\ .7_... 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Ir, - *- , " ' 4 4 DATE SCALE °4'°sob FIGURE 1 MOUNTAIN V e -. .;, cis.)E ENGINEERChris Hale 1015 Waters Avenue 'i" ENGI1NEERING, • DWG FILE - / Civil and Environmental ConsultijiaaeAeQ 19 WatersDesign / 826 1/2 Grand Avenue Glenwood Springs,CO 81601 JOB Gretchen Greenwood & Assc. - /// cA ' ph 970.945.5544 fx 970.945.5558 www.mountaincr 556-014 BUILDING DEPARTMENT � Si E Iw T� y � o� rye zi3 g — 2 O N • Proposed = N ° Historic & Existing 0 10 20 RS t, w �c r C 'C I T N 'a E ~4 U 1.1Mc w N N (, N irWd C • . n 0 • k PATH FOR TIME OF � � � � " � Wc4 0 I CONCENTRATION I O N a? 0 Uallo a PATH FOR TIME OF s , $ I , \ CONCENTRATION .,.. - • �� , ,$ j 1 �1 \ ah,..._ ems, \\ `� • -•�\►� \`•\\\\ -} \\ i., � \ \ � Lir \lI �` W • ` \�` " _ . s my DRAINAGE BASIN • E 0.32 • \ II i • �� 0.350.15 �� \\\ \ cgs` * Iii /64/6 P ii i e,...„ tIi.)i,,•,•'•• •I 0.3ft •ll so • • I_:r, ---0.E.0.t --Ai.1 lIi,,,i I.r/ • , • 59 • • 1 ♦ j1 ' ♦♦ / 1 �i a.uiwz__)), ♦'•♦ //' DRAINAGE •, W C = BASIN Q •� 1 / S:t sse • as. ~ C " i ip-$tens LEGEND �<4venue <1(, 0 ,\ 100—yr FLOWRATE (CFS)�� A ,pier 0 p: AREA (ACRES) O` N = o 0 MilI RUNOFF COEFFICIENT A o .II L o , i O W b J ` _Roads \`�; HSG _e _o Nli lib Bav«31 J il _.... . •' ` . _. �t" Alk a': � r« Site Location 41W...„„,„...r. 1 ikark1 n-. r (. fr o r. 7 Figure 11.1 Rotund Rwoura.Conwwatlon Service(NRCS)Bog Hap for Aspen 1 00 1 090 080 1 0 70 U 0 60 t 100-yr Area m —ta—50-yr c0 50 --+—25-yr E v —x—10-yr c —u--5-yr G 40 x ---x i_2-yr is 0 30 --Y-� 0 �x 20 . a 0 10 0 00 -- — -- - 0"b; 10% 2J-s; 50`.., 60% 70% 90% 90% 100% Watershed Percentage Imperviousness Figure 3.2- Runoff Coefficients for NRCS Hydrologic Soil Group B RECEIVED 10/10/2019 ASPEN BUILDING DEPARTMENT 1.00 -- 090 0.80 070 • 060 • 100-yr 50-yr Area 0 0.50 • T a -yr—x-10-yr—x—5-yr 2-yr 030 0.20 ,.a 0.10 0% 10`r: 20% 30% :0-:S ;C':4. 03 70% 80% 90% 10D% Watershed Percentage Imperviousness Figure 3.2- Runoff Coefficients for NRCS Hydrologic Soil Group B RECEIVED 10/10/2019 ASPEN BUILDING DEPARTMENT DRAINAGE CALCULATIONS for h> MOUNTAIN CROSS 1015 Waters Avenue 416,j ENGINEERING, INC. 12/15/2018 Civil and Environmental Consulting and Design 626112 Grand Avenue Glenwood Springs,CG 81601 ---- ph9]0.845.SSM fz 810.90.5.5568 waw.mounlaincrosseng.com Drainage Area : Existing/Historic Surface description: Unimproved PROPERTY GENERAL SURFACE DATA Total Drainage Area: 6,445 sq. ft. 0.148 ac. RUNOFF COEFFICIENT imperviousness = 0°/0 HSG = B from Fig 3.2 C100 = 0.35 C10 = 0.15 C5= 0.08 C2 = 0.03 100% 0.148 TOTAL BASIN ACERAGE 0.148 TIME OF CONCENTRATION Overland Flow Time Length Slope C5 Tc Upstream Elevation 7971.00 30 0.333 0.08 3.17 Dnstream Elevation 7961.00 Channel Flow Time none Length Slope K Tt Upstream Elevation 7961.00 130.0 0.023 7.000 2.04 Dnstream Elevation 7958.00 Total 5.21 min RUNOFF Existing/Historic Intensity Flowrate (in/hr) (cfs) 2-yr Storm 2.0 0.01 10-yr Storm 3.6 0.08 100-yr Storm 6.2 0.32 NOTES: - Soils are hydrologic soil group B from Figure 3.1 - K= 7.0 based on lawn i_ I_ f li ri / �' i9 r lU/lUP1019 ASPEN BUILDING DEPARTMENT DRAINAGE CALCULATIONS for > MOUNTAIN CROSS 1015 Waters Avenue 416,j ENGINEERING, INC. 1 0/1 0/2019 Civil and Environmental Consulting and Design 626112 Grand Avenue Glenwood Springs,CG 81601 ---- ph9]0.845.SSM fz 810.90.5.5568 waw.mounlaincrosseng.com Drainage Area : Proposed Surface description: Turf, landscaping, roof area, and hardscape PROPERTY GENERAL SURFACE DATA Total Drainage Area: 6,445 sq. ft. 0.148 ac. RUNOFF COEFFICIENT sq.ft. or Land Use est. °/G acres imperviousness = 48°/G Building Roof 2450 0.056 HSG = B from Fig 3.2 Permeable Pavers 595 0.014 C100 = 0.52 Hardscape and Paving 615 0.014 C10 = 0.39 Landscaped turf and planters 2,785 0.064 C5 = 0.34 Pine and Aspen 0% 0.000 C2 = 0.28 Sage and Grasses 0% 0.000 TOTAL BASIN ACERAGE 0.148 TIME OF CONCENTRATION Overland Flow Time Length Slope C5 Tc Upstream Elevation 7970.50 20 0.488 0.34 1.70 Dnstream Elevation 7960.75 Channel Flow Time none Length Slope K Tt Upstream Elevation 7960.75 65.0 0.058 7.000 0.64 Dnstream Elevation 7957.00 Total 2.35 min use 5 min Tc RUNOFF Proposed Intensity Flowrate (in/hr) (cfs) 2-yr Storm 2.1 0.09 10-yr Storm 3.7 0.21 100-yr Storm 6.3 0.48 NOTES: - Soils are hydrologic soil group B from Figure 3.1 - K= 7 based on gravel channel ill., lU/lui-2019 ASPEN BUILDING DEPARTMENT Rainfall IDF for Aspen, Colorado 7 , 6 I 5 4 t u .- a N C Ql C 3 2 1 0 0 5 10 15 20 25 30 35 40 45 50 55 60 Duration in Minutes -1111-2-yr t5-yr -0-10-yr -111-25-yr -0-50-yr t100-yr RECEIVED 10/10/2019 ASPEN BUILDING DEPARTMENT 100- WQC Volume Level 1: Over turf to capture x 1P, llnf:a Tot Imp = 48 % • ;,`, from Fig. 8.14 yields Ler,3 i, C ''.1Tr.i,» ;A Eff Imp = Ceelwue. ;,- it Cr.*` ' ' from Fig. 8.13 yields 2 r:,' WQCV= 0.082 watershed-in 7 50 kl Area = 0.148 acres s Req'd Volume = 44.1 cubic feet s /*. 0 10 20 30 4: 60 40 .. C0 w0 .. 1u1e1 huprrViouxw!sq.wuenl. I Figure&14 Imperviousness Adjusbns1ts for Leval 1 and 2 MDCIA(UDFCD 1999) 0.30 025 i 0 20 $ . i 0.15 2 - - - • - - . - - - 010 1 0.05 0.00 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 Effective Imperviousness of Tributary Area to BMP (percent) Figure&13 Aspen Water Quality Capture Volume RECEIVED 10/10/2019 ASPEN BUILDING DEPARTMENT Channel Report Hydraflow Express Extension for Autodesk®AutoCAD®Civil 3D®by Autodesk, Inc. Monday,Apr 2 2018 100- yr, 8 inch Circular Highlighted Diameter (ft) = 0.67 Depth (ft) = 0.30 Q (cfs) = 0.490 Area (sqft) = 0.15 Invert Elev (ft) = 1.00 Velocity (ft/s) = 3.19 Slope (%) = 1.00 Wetted Perim (ft) = 0.98 N-Value = 0.013 Crit Depth, Yc (ft) = 0.33 Top Width (ft) = 0.67 Calculations EGL (ft) = 0.46 Compute by: Known Q Known Q (cfs) = 0.49 Elev (ft) Section 2.00 1.75 1.50 1.25 1.00 r rr 0.75 0 1 10/10/2019 Reach (ft)'Dl G DEPARTMENT Channel Report Hydraflow Express Extension for Autodesk®AutoCAD®Civil 3D®by Autodesk, Inc. Monday,Apr 2 2018 Swale Triangular Highlighted Side Slopes (z:1) = 2.00, 2.00 Depth (ft) = 0.33 Total Depth (ft) = 0.50 Q (cfs) = 0.490 Area (sqft) = 0.22 Invert Elev (ft) = 1.00 Velocity (ft/s) = 2.25 Slope (%) = 2.00 Wetted Perim (ft) = 1.48 N-Value = 0.025 Crit Depth, Yc (ft) = 0.33 Top Width (ft) = 1.32 Calculations EGL (ft) = 0.41 Compute by: Known Q Known Q (cfs) = 0.49 Elev (ft) Section Depth (ft) 2.00 1.00 1.75 0.75 1.50 - 0.50 v 1.25 0.25 1.00 0.00 0.75 Cl0r1) 0 5 1 1.5 2 2.5 10/i0/2019 Reach (ft) ASPEN BUILDING DEPARTMENT (---- ---N MOUNTAIN CROSS l'i 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 1015 Waters Avenue 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 1 • Clean out swales from deposited sediment and debris Reviewed by Engineering 10/30/2019 11:26:55 AM Concrete: "It should be known that this review shall not • Clean off deposited sediment and debris relieve the applicant of their responsibility to comply with the requirements of the City of • Check for and fill any cracks Aspen.The review and approval by the City is • Repair and/or replace broken and damaged items offered only to assist the applicant's understanding of the applicable Engineering requirements."The issuance of a permit based Pipes and Inlets: on construction documents and other data shall not prevent the City of Aspen from requiring the • Clear inlet grates of debris correction of errors in the construction • Clear piping of any obstructions documents and other data. • 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 Storm Water Quality Structure • Remove sediment, debris, and litter from structure • Clear mud and debris from inlets and outlets Overall: 1 ,4::5 • Check site for differential settlement creating any areas of ponding and t a drai • Check site for locations where runoff may breach and bypass the dr q . tystent\correct '' • Keep roof gutters and downspouts clear of debris Qes. `�, G '` "Yi 'il' SV 826'Vz Grand Avenue,Glenwood Springs,CO 81601 \\V P:970.945.5544 F:970.945.5558 www.mountaincross-eng.com ,`�Q