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HomeMy WebLinkAboutFile Documents.535 E Cooper Ave.0004.2019 (13).ACBK RECEIVED 1/7/2019 ASPEN BUILDING DEPARTMENT Drainage Report STEIN BUILDING 531 EAST COOPER AVE ASPEN, CO 81611 December 12,2018 Prepared by Richard Goulding, P.E. Roaring Fork Engineering 592 Highway 133 Carbondale, CO 81623 ROARING FORK ENGINEERING 1 RECEIVED 1/7/2019 ASPEN BUILDING DEPARTMENT Drainage Report STEIN BUILDING 531 EAST COOPER AVE ASPEN, CO 81611 I HEREBY AFFIRM THAT THIS REPORT FOR THE IMPROVEMENTS AT 531 EAST COOPER AVE WAS PREPARED BY ME FOR THE OWNERS THEREOF IN ACCORDANCE WITH THE PROVISIONS OF THE CITY OF ASPEN URBAN RUNOFF MANAGEMENT PLAN AND APPROVED VARIANCES AND EXCEPTIONS LISTED THERETO. I UNDERSTAND THAT IT IS THE POLICY OF THE CITY OF ASPEN THAT THE CITY OF ASPEN DOES NOT AND WILL NOT ASSUME LIABILITY FOR DRAINAGE FACILITIES DESIGNED BY OTHERS. rove ei` p00 L/0 4 � QQ;.B G� V RICHARD GOULDING,P.E. >,GQ'• ,% , • . • RFE Project#2018-33 =9'+�a J • y1 WeiVALNY 2 RECEIVED 1/7/2019 ASPSI of Contents BUILDING DEPAfF PN7 1.0 General 4 1.1 Existing Site 4 1.2 Proposed Conditions 4 1.3 Previous Drainage Studies 5 1.4 Offsite Drainage &Constraints 5 2.0 Drainage Basins and Sub-basins 5 2.1 Drainage Basins 6 2.2 Peak Discharge Calculations 6 3.0 Low Impact Site Design 7 3.1 Principles 7 4.0 Hydrological Criteria 9 4.1 Storm Recurrence and Rainfall 9 4.2 Peak Runoff and Storage Volumes Methodology 9 5.0 Hydraulic Criteria 9 5.1 Inlets 9 5.2 Pipes 10 6.0 Proposed Facilities 10 6.1 Proposed structures 10 6.2 Infiltration 11 6.2.1 Drywells and Screened Rock Bed 11 7.0 Operation and Maintenance 11 7.1 Drywell 11 8.0 Appendices 12 Drawings 11 x 17 12 3 RECEIVED 1/7/2019 ASPENneral BUILDING DEPARTNIIE 1.1 Existing Site The Stein building located at 531 East Cooper Avenue in Aspen, CO, is an existing two-story mixed-use building constructed in 1888. This property is a 0.149 Acre Lot and is located within the Aspen Mountain Drainage Basin,with commercial space on the main level and residential units on the second floor. This historic structure is located southwest of the intersection of Hunter Avenue and Cooper Avenue,with the alleyway to the south of the property and another commercial building to the West. The sidewalk along the East side of the building is 8' wide with a strip of pavers between the sidewalk and the curb and gutter following Hunter Avenue. The sidewalk following Cooper Avenue is 5' wide with a similar paver buffer.Trees are interspersed within the paver buffer along both streets. The curb and gutter slopes north along Hunter Ave to the intersection,then follows the flow line to the west along Cooper Ave. An existing bidirectional curb ramp is located at the intersection, and an alley ramp is located southeast of the property. All runoff from the street and the right of way flows into the curb and gutter, and is collected by an inlet on Hunter Ave just south of the bidirectional curb. Any runoff that is not collected by this inlet then continues to flow down the curb and gutter to another inlet on Cooper Ave further to the west. The alley behind the building is asphalt and is relatively flat. There is an existing landscaped area between the alleyway and the Stein building, including trees, shrubs,patios,walkways, fences, and a concrete parking space. Many doors into the existing structure surround the south, east, and north side of the structure. The shallow utilities are all located in the alley to the south of the site, along with the sanitary sewer. A transformer and communications pedestals are located between the Stein Building and the neighboring structure. The water service runs to the north of the site and ties into the water main located in Cooper Avenue. A Geotechnical Report has been performed by H-P Kumar for this project and was produced on September 24,2018. The borings logged on September 12,2018 were used for all calculations and percolation test information included in this document. The borings were performed in the concrete parking located on the south portion of the site. The subsoils encountered consist of about 4-5 feet of sandy and gravely fill,overlying sand and gravel slightly silty with cobbles. The percolation test results confirmed a rate of 5 minutes per inch,or 12 inches per hour. 1.2 Proposed Conditions This project is classified as a `Major Project under 25%of the site' as per Table 1.1 of the URMP. The proposed development is over 1000 square feet and disturbs an area of approximately 1600 square feet., just under 25%of the site. The intent of this report is to demonstrate compliance with the requirements of the Urban Runoff Management Plan(URMP). The Low Impact Design(LID)Principles in the introduction of the URMP were used as a guide throughout the design process. The proposed project onsite consists of a remodel of the interior of the existing structure,an addition to the structure between it and the alley, a new stairway access from the alleyway, a trash enclosure and a 4 RECEIVED 1/7/2019 ASPEN patio. The majority of the civil design and disturbance is occurring in the right of way for BUILDING DEPAgrgerywayright of way improvements. To the east of the project,the alley ramp, sidewalk, curb and gutter, and half of the street asphalt is being regraded to better meet the City of Aspen Engineering Standards. The existing pavers are being removed and a planting area is to be utilized within this area instead. The road is being adjusted to reduce the cross slope, and the existing inlet and vault are being replaced. The curb ramps at the intersection have been redesigned as well to match the to-be constructed curb ramps across the street that will be constructed in the spring, including a bulbout into Cooper Avenue. To the north of the structure,a new,wider 8' sidewalk is proposed, along with a planting buffer and new curb and gutter. The curb and gutter along Cooper Avenue has been moved further into the street to allow for more pedestrian space, as discussed previously with The City of Aspen Engineering Department. The proposed design in the right of way does not affect the historical drainage patterns, and the planting buffers decrease the impervious area. Because of this,no hydrological calculations were performed for offsite analysis. Onsite,the runoff from the addition will be collected through a system of roof drains, gutters, and downspouts and routed to a drywell located to the south of the structure. The patios and stairways are sloped to inlets,where runoff will be conveyed to the drywell as well. The drainage infrastructure from the existing roof will be maintained, as this is not being disturbed. The utilities on the site require updates given the remodel. The existing transformer does not have a vault underneath or an easement, so it is proposed to develop an easement, install a vault and relocate the transformer so that all setbacks are met, and relocate the service to the building. The existing communications pedestals will be utilized in the remodel;however,the service to the building will be relocated. The gas line will be upgraded as necessary and relocated to the southeast portion of the structure. The existing sanitary service will be utilized pending inspection by the contractor. If necessary, the service can be replaced.A new larger water service is proposed to tap into the water line north of the site. This 4-inch line will replace the existing 2-inch line to meet the capacity requirements for the proposed fire sprinkler system. 1.3 Previous Drainage Studies The City completed a Drainage Master Plan in November 2001 using the consultant WRC, and is part of the basin that feeds into system 3. Figure 7.1 of the URMP defines the mudflow zones for the City of Aspen. This property is not impacted by mudflow. 1.4 Offsite Drainage & Constraints There is no offsite drainage that impacts the property. The proposed development will not impact any irrigation ditches or waterways. 2.0 Drainage Basins and Sub-basins The disturbed area was analyzed as a single drainage basin,which was then subdivided into smaller sub- basins. Basin and Sub-basin delineations are shown on Sheets C5 of the permit drawings. These sheets 5 RECEIVED 1/7/2019 ASPP -rvious areas,runoff coefficients,peak flows, and the required volume of runoff to be detained. BUILDING DEPA u Basins were created to calculate the maximum flow entering each inlet of the proposed storm systems. The sub-basin peak flows were then used to size pipes and inlet capacities. 2.1 Drainage Basins Basin 1 is 1537.31 square feet and is 100%impervious. The majority of the basin is the roof of the addition on the southernmost side of the residence. The downspouts can tie directly into the proposed drywell,which has capacity for full detention of this basin for a 100-year storm. Small portions of patios and stairwells are conveyed to the drywell as well. 2.2 Peak Discharge Calculations The peak flows were calculated for each Major Basin for 10 and 100 year storm events. Rainfall intensity was calculated using a Time of Concentration(Td)of 5 minutes. Actual Time of Concentration on the site is significantly less than 5 minutes,but according to the City of Aspen URMP, equations used to calculate rainfall intensity are only valid for a Time of Concentration of greater than 5 minutes so the smallest valid Time of Concentration value was used. The 1 hour Rainfall depth(Pi), given in Table 2.2 as 0.73 inches for a 10-year event and 1.23 inches for a 100-year event. Equation 2.1 was referenced when solving for the Rainfall Intensity(I). I=88.8P1/(10+Td)1.052 (Equation 2.1) Runoff Coefficients(C),a function of the Soil Group(in this case B)and the percentage of impervious area within each sub basin were developed using Figure 3.2.The Runoff Coefficient(C)was then multiplied by the Rainfall Intensity(I)and the acreage of each Major Basin(A)to determine the peak discharge for each Major Basin. Qp =CIA Q, =Peak Discharge(cfs) A =Area(Acres) I =Rainfall intensity(inches per hour) C =Runoff Coefficient These peak flow values were used to calculate the size of the proposed detention and conveyance structures, such as drywells, inlets and pipes. The tables below contain the peak flows for developed and undeveloped conditions for 10-and 100-year storm events. 6 RECEIVED 1/7/2019 ASPEN PA BUILDING DEPAMF1T"Peak Discharge Developed Calculations 1 Hour(P1) 0.77 Return Period 10 Basin ID Total Area Imp.Area Impervious C Value Time of C Intensity Q Max See(D1) (ft2) (ft) (%) From Table (Td) 1=88 8P1/(10+Td)1.°52 (ft3/sec) 1 1537.31 1537.31 100.00% 0.916 5 3.96 0.13 10 Year Peak Discharge Pre Development Calculations 1 Hour(P1) 0.77 Return Period 10 Basin ID Total Area Imp.Area Impervious C Value Time of C Intensity Q Max See(D1) (ft2) (ft) (%) From Table (Td) I=88.8P1/(10+Td)l.o52 (ft3/sec) 1 1537.31 0.00 0.00% 0.150 5 3.96 0.02 100 Year Peak Discharge Developed Calculations 1 Hour(P1) 1.23 Return Period 100 Basin ID Total Area Imp.Area Impervious C Value Time of C Intensity Q Max See(D1) (ft2) (ft2) (%) From Table (Td) 1=88.8P1/(10+Td)l.o52 (ft3/sec) 1 1537.31 1537.31 100.00% 0.950 5 6.33 0.21 100 Year Peak Discharge Pre Development Calculations 1 Hour(P1) 1.23 Return Period 100 Basin ID Total Area Imp.Area Impervious C Value Time of C Intensity Q Max See(D1) (ft2) (ft) (%) From Table (Td) I=88.8P1/(10+Td)l.o52 (ft3/sec) 1 1537.31 0.00 0.00% 0.350 5 6.33 0.08 3.0 Low Impact Site Design 3.1 Principles Principle 1: Consider storm water quality needs early in the design process. The Grading and Drainage design was coordinated between the architect,landscape architect and civil engineering teams throughout the design process. Multiple site visits ensured proper understanding of existing conflicts and opportunities to improve existing onsite drainage patterns. Principle 2: Use the entire site when planning for storm water quality treatment. Storm water quality was considered in the design of every part of the site that is being affected by the proposed construction. Gutters and downspouts will be added to collect runoff from the roof and routed to either to a drywell or screened rock. Principle 3: Avoid unnecessary impervious area. 7 RECEIVED 1/7/2019 ASPP the building is the majority of the property. Therefore,the potential for pervious space is BUILDING DEPA till nonexistent. However,planting areas were added to the right of way design, so there are improvements offsite that allow for additional pervious space. Principle 4: Reduce runoff rates and volumes to more closely match natural conditions. All runoff from proposed surfaces on the property is collected and routed to a drywell. Because the drywell is sized to fully detain a 100-year storm event,runoff rates from the property will be greatly reduced. Principle 5: Integrate storm water quality management and flood control. The use of the drywells to collect runoff from all impervious surfaces simultaneously aids in storm water quality and flood control on the site. Principle 6: Develop storm water quality facilities that enhance the site,the community and the environment. The use of the two chambered drywell for water quality reduces the runoff of sediment and contaminants from the site. This reduces the site's effect on the Roaring Fork River and the community. Significant right of way work is proposed with this design, improving pedestrian sidewalks,bidirectional curb ramps, and replacing an existing vault and inlet.None of the surfaces are for vehicular use, so no sand-oil interceptor is necessary. Principle 7: Use treatment train approach. Given the layout of this project, it was difficult to apply the treatment train approach. The use of sumps along the pipe networks captures sediment at each inlet instead of in the drywell.Also,the proposed sidewalks slope into the planting area before overflowing into the curb and gutter and allowing for treatment. Principle 8: Design sustainable facilities that can be safely maintained. Inlets and piping will be vacuumed or flushed periodically to maintain adequate flow. Proper grading reduces dangerous slopes and proper drainage to reduce ice buildup. Sidewalks are snowmelted to ensure that the sidewalks are properly maintained. The drywell is located in an easily accessible location. Principle 9: Design and maintain facilities with public safety in mind. All right of way updates were designed to ensure the public safety. The curb ramps, sidewalks, and entryways are designed to minimize steep slopes.The sidewalk is snowmelted,reducing risk of ice and dangerous conditions in the public right of way. 8 RECEIVED 1/7/2019 ASPEN,, drolo ical Criteria BUILDING DEPA>�'fIC�13 g 4.1 Storm Recurrence and Rainfall The property is located outside of the commercial core but is served by a storm drains so this property classifies as a"Sub-urban area served by public storm sewer". The total site is required to meet either the Water Quality Capture Volume (WQCV) and release into the right of way, or have detention for a 100-year historical storm events. The 1 hour Rainfall depth(Pi) is given in Table 2.2 as 0.73 inches for the 10-year event and 1.23 inches for the 100-year event. The Intensity in inches per hour for different storm duration(I) is calculated using the Equation 2.1 from the Aspen URMP. 4.2 Peak Runoff and Storage Volumes Methodology Using the peak flows for each Major Basin established in section 2.2, storage requirements were calculated for full detention as there was not a feasible option to tie into the existing storm system. Drywell A was sized for full detention of a 100 year storm event. The table below shows storage volume requirements for the proposed detention systems. Full Detention Storage Basin Total Area Impervious Area Impervious Full Detention Depth Factor of Safety Required Storage BMP (ft2) (ft2) (%) (in) F.O.S. (ft3) 1 1537.31 1537.31 100.00% 1.23 1 158 Dyw lllA 5.0 Hydraulic Criteria Sub-basins were delineated per the design points of concentrations created by roof drains and inlets. Pipes connect the sub-basins and convey the flows to the overall point of concentration for the basin.The 100-year peak flow for each sub-basin was calculated. 100 Year Sub Basin Peak Discharge Developed Calculations 1 Hour(P1) 1.23 Return Period 100 Sub Basin Total Area Imp.Area Impervious C Value Time of C Intensity Sub Basin Flow Rate (Name) A,(ft2) A;(ft2) A;JAt(%) From Table (Td) I=88.8P1/(10+Td)ol.o52 CSub(ft3/sec) 1.1 297.31 297.31 100.00% 0.950 5 6.33 0.04 1.2 219.87 219.87 100.00% 0.950 5 6.33 0.03 1.3 761.34 761.34 100.00% 0.950 5 6.33 0.11 1.4 106.35 106.35 100.00% 0.950 5 6.33 0.01 1.5 61.18 61.18 100.00% 0.950 5 6.33 0.01 1.6 91.26 91.26 100.00% 0.950 5 6.33 0.01 5.1 Inlets Basin 1 was divided into sub-basins according to what inlet or downspout they discharged into. The peak flows for the 100-year event in each sub-basin were used as the flowrate to size the proposed inlet drains 9 RECEIVED 1/7/2019 ASPEN„,,,, Ilrains. Equations 4.17 through 4.20 from the URMP were used in these calculations. The BUILDING DEPARTIE I equations incorporate a 50% clogging factor and assume a 40%opening in the grates. A water depth of 0.5 inches was assumed and all the inlets were treated as sumps, as they will be set a minimum of 0.5 inches below flow lines. Sub Basin and Circular Inlet Calculations 1 Hour(P,) 1.23 m=40% Y,=.04(Depress inlet by 0.04') Return Period 100 Cr=50% Co.0.65 Inlet ID Basin ID Total Area Imp.Area Impervious C Value Concentration Intensity Q Max Inlet Type Diameter Area(EQ.4-20) Inlet Capacity(EQ4-19) Has Capacity See(D1) (ft'l (ft5) (%) From Table (TO 1=88.8P5/(10+Td)i'° ft3/sec Wo(inches) AQ(1-Cs)mA Q=CoAoV2gY, (Yes/No) INLET-Al 1.5 61.18 61.18 100.00% 0.950 5 6.33 0.008 6"CIRCULAR 6 0.039 0.046 Yes INLET-A2 1.6 91.26 91.26 100.00% 0.950 5 6.33 0.013 6"CIRCULAR 6 0.039 0.046 Yes 5.2 Pipes Pipes used will be PVC SDR-35 with a Manning's coefficient(n)of 0.01. The pipes were sized to accommodate peak flows for a 100 year event from all contributing sub-basins. Calculated pipe sizes were tested for hydraulic capacity at 80%of their full flowrate. Calculations of depth of flow for each pipe were calculated. Design charts giving Qdesial/Q fun were downloaded from Federal Highway Administration(FHWA)and the equations in Section 4.8.4 were used as the basis for these calculations. Calculated pipe sizes and depth of flow for onsite pipes are shown below. Pipe Sizing K=0.462 Design Flow Proposed Manning Required Pipe Diameter Required Pipe Proposed Pipe Pipe Rate Slope Coefficient Equation 4-31 Diameter Diameter Claes(ft3/sec) S(%) n d(ft)={nQde,/KVS)34 D,e5(in) Dwo(in) Al 0.01 2.00% 0.01 0.08 0.99 4.0 A2 0.01 2.00% 0.01 0.10 1.15 4.0 Hydraulic Grade Line and Pipe Capacity Design Flow Proposed Pipe 80%of Proposed Manning Full Pipe Cross Q Design/ Hydraulic Grade Line Depth of Flow Less Than Pipe Slope Full Pipe Flow Rate d/D Rate Diameter Pipe Diameter Coefficient Sectional Area Q Full (Depth of Flow) 80%of Pipe Diameter Qd,.(ft3/sec) De,o(in) 5(%) Dp,o•.8(in) n A(ft)=n(De,o/2)5 Qrw(ft3/s)=A(1.49/n)KDoof48)V3)5ia QeoJ4un (from Chart) d(in)_(d/D)'Dpro (Yes/No) Al 0.01 4.0 2.00% 3.2 0.01 0.087 0.351 0.02 0.08 0.32 Yes A2 0.01 4.0 2.00% 3.2 0.01 0.087 0.351 0.04 0.12 0.48 Yes Exit Velocities Pipe Design Flow Proposed Pipe Slope d/D Manning Rh/D Hydraulic Radius Exit Velocity Rate Diameter Coefficient (ID) Q, (ft3/sec) Dpro(in) (%) (from Chart) n (from Chart) Rh(ft)=(Rh/D)Dpro V(ft/sec)=[1.49/n]Ref'V5 Al 0.008 4.0 2.00% 0.08 0.01 0.05 0.05 2.91 A2 0.013 4.0 2.00% 0.12 0.01 0.08 0.08 3.76 6.0 Proposed Facilities 6.1 Proposed structures A Drywell is being utilized to meet URMP requirements for storm water management. Detention volumes were calculated using equations 5-1 through 5-4 from the City of Aspen URMP. 10 RECEIVED 1/7/2019 ASPS 1 A collects runoff from Basin 1, and is a 10 feet deep 4 feet diameter precast drywell that meets BUILDING DEPAt e requirements of the City Of Aspen with a storage capacity of 194 cubic feet,The total capacity for the drywells exceeds the required detention volume of 158 cubic feet for Basin 1 to meet full detention. Drywell Storage Drywell Basins Diameter Storage Depth Perforated Depth Internal Volume External(18"of Screened Rock)Volume Total Capacity Required Capacity (Name) (8) D(ft) H(ft) P(ft) n*H*(D/2)2)(ft3) 0.3*1t*P*((D/2)+1.5)2-(D/2)2)(ft3) (ft3) (ft3) Drywell A 1 5 8 4 157 , 37 194 158 I\ Diameter is called out differently within report. 6.2 Infiltration Please verify diameter and adjust report. 6.2.1 Drywells and Screened KOCK Bea Part of the Analysis is to ensure that the drainage structures can completely drain within 24 hours. Below is a calculation showing that there is enough perforation area for the drywell to drain within 24 hours using the percolation rate 24 inches per hour for the entire site. Section 8.5.4.2 was referenced for these calculations. Irhis calculation does not appear to confirm that the detention volume drains within 24 hrs as required. Please provide Drywell Infiitralvolume drainage time within report. Name Diameter Perforation Height Perforated Area Total Capacity Infiltration Rate Infiltration Time Volume Infiltrated in 24 Hours (Name) D(ft) H(ft) A(ft2)=3.14*D*H V(ft3) I(in/hr) T(hr)=V/(A*I/12) V,ou,i(ft3)=V*T Drywell A 5 4 62.83 36.76 12 0.585 21.50 7.0 Operation and Maintenance 7.1 Drywell Drywells must be inspected and maintained quarterly to remove sediment and debris that has washed into them. Minimum inspection and maintenance requirements include the following: • Inspect drywells at least four times a year and after every storm exceeding 0.5 inches. • Dispose of sediment, debris/trash, and any other waste material removed from a drywell at suitable disposal sites and in compliance with local, State, and Federal waste regulations. • Routinely evaluate the drain-down time of the drywell to ensure the maximum time of 24 hours is not being exceeded.If drain-down times are exceeding the maximum,drain the drywell via pumping and clean out the percolation area (the percolation barrel may be jetted to remove sediment accumulated in perforations. If slow drainage persists,the system may need to be replaced. 11 RECEIVED 1/7/2019 ASPS BUILDING DEPA VIE4WPendices Drawings 11x17 12