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Home My WebLink AboutFile Documents.332 W Main St.0031-2023-BCOM (17) Am) billiddimul Minor Grading and Drainage Report 332 W Main Street Unit#101 Aspen, Colorado 81611 April 24th, 2023 Revised August 21st, 2023 Existing Conditions The property described as 332 W Main Street Unit #101 is a commercial property located at the northeastern corner of the Main Street and 3rd Street intersection in downtown Aspen, CO. The site is designated as Property ID #273512441012 and is the unit closest to Main Street. The commercial unit had been previously used as a doctor's office space, but is currently unoccupied. The surrounding units are mixed use units (commercial and high density residential)located within the same structure, with neighboring commercial and residential properties in close proximity. Existing building setbacks include a ten foot front yard setback to the south, a five foot side setback to the east, a five foot rear setback to the north, and a three inch side setback to the west. u'°B \ � RED MT,y 4/�} Rp► • ter._.. N�fp.41N� c� M SN/D DR 1 J \ p' � _ \\ 4 • / • • i cHer may p4 _ r •4. i' Q. X CaV A. 1 • 0 - _ LLLESPIES. 1Nke / o, �.® HWyB/ WN' _'❑ Mf�^mil ;' �p V�FST V tl� V I f�.il�fh3T. e y � qy gT~ 1KE ST'y , 2 - \1 \ 4,,, t r� `a ��) }� ICI( /i 7 f Qw �N3I, �_ STO y 'E A U Yr . t. g/ O gtiSH wN❑� ' �FQ ST 41 }1t 1 [[ i a� �2 8 `\SiteW$o o c,.ORsc, 9' 43PEH\---\!..-_,,,,,,_,,„,e, I (\ ,/�� ASPEN 40.FA1. n''� ,, �(� `ram''�' "l\`P1 _� ' Shown above is an image of the Pitkin County GIS Map displaying the location of 332 W Main Street Unit#101 There is no direct access point or designated parking area for the property under evaluation. Tenants and clients of Unit #101 use surrounding street parking for access. There is plenty of adjacent street parking near the unit. The attached neighboring units to the north utilize t\ t r;'. i , . Crystal River Civil LLC 970.510.5312 Page8f/25/2023 ASPEN BUILDING DEPARTMENT [18'If LI I,i I'I I1IU'I I I alley for access. The parcel includes a lawn area to the front with mature deciduous trees surrounding the perimeter, wooden fence line. Concrete walkways and steps are used for pedestrian access to the front porch of the unit. A side access composed of flagstone pavers is also located to the west edge of the front porch. The unit is accessed via the covered, front porch. The topography of the site generally slopes from the northwest corner of the shared parcel to the southeast corner. Slopes are very flat and do not exceed ten percent anywhere. It appears that drainage is routed into the lawn space to the southeast of the residence and east of the walkways. There are no signs of improper drainage or erosion on the parcel. The concrete walkways are not snowmelted so there is no ice damming occurring at the edge of concrete. The walkway connects to the Main Street concrete sidewalk to the south and the side access connects to the 3rd Street concrete sidewalk system to the west. All right-of-way runoff drains into the street and does not effect the parcel under evaluation. Both Main Street and 3rd Street include a curb and gutter conveyance system. 3rd Street Right-Of-Way includes a grassy ditch between the edge of pavement and concrete sidewalk as an additional method of offsite stormwater conveyance that is routed to the north of the property. Water and sewer service are assumed to service the residence from Main Street right-of-way. Domestic water is provided by Aspen Water Department, with a shutoff located just south or the southeastern property corner. Sanitary sewer service is provided by Aspen Consolidated Sanitation District. Electric meters are located on the eastern edge of the existing structure, near the AC units. Gas meters are shown on the northern and western edge of the structure. Electric and telecom pedestals are located outside of the northern property line, near the alley and are protected by bollards. All underground utility locates will be completed prior to any construction. Proposed Conditions The proposed scope of the project includes the installation of a concrete walkway branching from the existing concrete walkway that is used as the main access point to Unit#101. The purpose of the proposed walkway is to provide an ADA accessible route to the front entrance at the covered porch. There are no other changes being made to the residence, outside of fixing the existing porch boards and steps. Disturbance in the front and side yard will be kept to a minimum at an approximate area of 260 square feet. The entire project will remain under 1,000 square feet to be considered a minor project as defined in Table 1.1 in the City of Aspen Urban Runoff Management Plan (URMP). The utilities and grades surrounding the rest of the structure will remain as shown on the existing conditions survey provided with this submittal package. The proposed, three foot wide, concrete ADA accessible walkway leading up to the front porch entry from the southern end of the existing concrete sidewalk does not exceed five percent centerline slope. The ADA walkway will include a two percent cross slope after the landing at the tie in to the porch, from the west towards the east, for proper drainage off the walkway. Both the tie in at the porch and the connection at the existing walkway will include ADA accessible turning areas. The slopes from the ADA walkway follow the existing topography and drainage pattern of the existing conditions. The concrete, ADA walkway complies with the up to date version of the U.S. Access Board's "Americans with Disabilities Act Accessibility Standards". Chapter 4 of the standards was used as the design reference for the ADA walkway. RECEIVED Crystal River Civil LLC 970.510.5312 Pagee8f/625/2023 ASPEN BUILDING DEPARTMENT Am) mum A section of the ADA walkway encroaches under the dripline of existing trees. A floating sidewalk is to be installed in this area to mitigate conflicts with the existing root systems of existing trees and to minimize disturbance. A floating, concrete sidewalk section detail is included within this submission. All areas of the ADA walkway that are not located under the drip line of an existing tree will use the typical, concrete sidewalk section detail, which is also included within the drawings. The ADA walkway is not snowmelted, which will ensure no ice damming at the edge of concrete interface. All drainage is routed towards green space that is located to the southeast of the front porch, following the existing drainage pattern of the site. There are no offsite drainage basins for the site due to existing Right-Of-Way drainage infrastructure in place at Main Street and 3rd Street. All existing drainage infrastructure onsite will remain the same, including the roof drainage system. Drainage Analysis This drainage analysis uses procedures that have been standardized by the Mile High Flood District's Urban Storm Drainage Criteria Manual, or the USDCM. Peak flows of the site have been calculated using the Rational Method to determine the impacts the proposed project has on the existing conditions, along with verifying the capacity of the proposed infrastructure. This method uses the analyzed basin's area, soil type (assumed soil type B), percentage of imperviousness, time of concentration, and the historic storm data from NOAA Atlas 14 to determine the maximum flow rate from a specified storm event. The Rational Method uses the equation below to determine the Maximum Flow Rate: Q=CIA Where, - Q is the peak stormwater runoff rate, in cubic feet per second - C is a runoff coefficient of the drainage area - I is the storm intensity, in inches per hour - A is the area of the drainage basin, in acres The Runoff Coefficient, C, is determined using table 6-5 from the USDCM. To determine the storm intensity, I, Equation 2.1 from the City of Aspen's Urban Runoff Management Plan is being utilized, as it is a neighboring jurisdiction that has more similar historical data in comparison to the USDCM. For smaller basins with a time of concentration of less than five minutes, a time of concentration of five minutes is assumed —when smaller times of concentration are used in the Rational Method, the results tend to be inaccurate, as per the USDCM. The rainfall events used for this analysis are for one hour, 10-year and 100-year storm events. Below is a table summarizing the calculations to determine the predeveloped and developed conditions for both events. RECEIVED Crystal River Civil LLC 970.510.5312 Paged8f/25/2023 ASPEN BUILDING DEPARTMENT P6-diljb RI IAIRI IR(I IL 10-Year 1-Hour Onsite Peak Discharge Calculations 86-6P, Rainfall depth,P,(in) 0.771 Soil Class B Intensity(in/hr) I—(10+Td)'c- Discharge(ft3/Sec) Q=CIA, Note:For basins with a flow length of less than 500 feet,a Time of Concentration is assumed at 5 minutes.These calculations are assuming a NRCS Hydrologic Soil Class B. Rainfall depth values derived from NOAA Atlas 14 data.Intensity equation has been derived from the Aspen area NOAA Atlas 14 IDF Curve. Predeveloped Conditions Basin Total Area ImperJer Areffilious IIIMIP Time of Concentratio Intensity Peak Discharge (Name) A,(ft2) A;(ft2) A;/A,(%) To(min) I(in/hr) Q„(ft2/sec) 1 260 0 0.00% 0.040 5 3.96 0.00 Developed Conditions Basin Total Area Impervious Area Percent Impervious C Value Time of Concentration Intensity Peak Discharge (Name) A,(ft2) A;(ft2) A;/A,(%) Tc(min) I(in/hr) Qd(ft3/sec) 1 260.00 106.00 40.77% 0.380 5 3.96 0.01 100-Year 1-Hour Onsite Peak Discharge Calculations 'Ilki Rainfall depth,Pt(in) 1.23 Soil Class B Intensity(in/hr) I= 88.8Pi (lo+Td)'osz Discharge(ft3/Sec) Q=CIA, Note:For basins with a flow length of less than 500 feet,a Time of Concentration is assumed at 5 minutes.These calculations are assuming a NRCS Hydrologic Soil Class B. Rainfall depth values derived from NOAA Atlas 14 data.Intensity equation has been derived from the Aspen area NOAA Atlas 14 IDF Curve. Predeveloped Conditions Basin Total Area Impervious Area Percent Impervious C Value T Time of Concentration Intensity Peak Discharge (Name) A,(ft2) A;(ft) A;/At(%) To(min) I(in/hr) Qo(ft3/sec) 1 260 0 0.00% 0.430 5 6.33 0.02 Developed Conditions Basin Total Area Impervious Area Percent Impervious C Value Time of Concentration Intensity Peak Discharge (Name) A,(ft) A;(ft2) A/At(%) Tc(min) I(in/hr) Qd(ft3/sec) 1 260.00 106.00 40.77% 0.610 5 6.33 0.02 The URMP requires that a minor project shall either route runoff from the disturbed area to green space or store the Water Quality Capture Volume (WQCV) using an approved BMP. For this design, the proposed walkway will route runoff to the green space to the east of the existing, concrete walkway. There is no concern about the small amount of additional runoff being routed to the existing green space to the east. The 10-year, 1-hour storm event peak flows show a 0.01 CFS increase, while the 100-year, 1-hour event shows no increase in flows due to the 106 sq. ft. of concrete walkway being added to the site. For reference, the water quality capture volume (WQCV) for the area of disturbance is shown below, with only 2.6 cubic feet of WQCV required. Water Quality Capture Volume Storage Calculation Inches Water Quality Capture Volume(ft3) WQCV=WQCV in Watershed 12 x Area x FOS FOS Drywells=1.5,FOS Other BMP's=1.0 Basin Point Of Concentration Total Area Impervious Percentage Water Quality Depth Factor Of Safety Water Quality Capture Volume (Name) (Name) At(ft2) A;/A1(%) (in) (FOS) WQCV(ft3) 1 Green Space 260.00 41% 0.08 1.5 2.60 RECEIVED Crystal River Civil LLC 970.510.5312 Pagee}gf/25/2023 ASPEN BUILDING DEPARTMENT 41 (R1SIAIRIVER(I11II The project is routing all runoff to the existing green space located to the east of the area of disturbance, with no proposed BMP. Below is a photo of the existing grass area being drained to, adjacent (photo right) to the existing concrete walkway. t wxlM — `r n .; k....1 _ . ,. - - _ _ CCSMEI.0 MEDIC NI 4 111 I IHHIIlH : :.: IFIII1 ' HlH, . . I I ill 11_ ,L, I Ilititi147 .11.,1,1,,L1 1 1 .. If you have any questions on this report or analysis, please contact me directly. Vince Tomczak p0 Li ce Principal, P.E. 47& .•:, ''j;.I0... %re Crystal River Civil, LLC ..g v : , ?;.. O Vince@CrystalRiverCivil.com .�, 56615 `�. • (970) 510 —5312 �`' 4 ,4*(*::-/.1..... ..--.,,y() S/ONAI RECEIVED Crystal River Civil LLC 970.510.5312 Paged 8725/2023 ASPEN BUILDING DEPARTMENT