Loading...
HomeMy WebLinkAboutFile Documents.315 E Hyman Ave.0011-2023-BCOM (41) DRAINAGE REPORT For GRAVITY HAUS - ASPEN Parcel Number 2737-18217-801 Gravity Haus 315 E. Hyman Ave. Aspen, Colorado 81611 Prepared By: Hans E. Brucker, PE Pinnacle Design Consulting Group, Inc. 0805 Buckpoint Road Carbondale, Colorado 81623 March 20, 2023 RECEIVED 04/10/2023 ASPEN BUILDING DEPARTMENT GRAVITY HAUS - ASPEN Drainage Report 315 E. HYMAN AVE. March 20, 2023 TABLE OF CONTENTS Cover Sheet 1 Table of Contents 2 Introduction/General Description ..3 Project Description .. 3 Existing Drainage Conditions .3-4 Proposed Drainage System Design .4 URMP Design Principles .4 Water Quality .4-5 Peak Flow Rates On-Site-Rational Method 5-6 Pipe Capacities 6 Detention 7-8 Outlet Works .8 Erosion Control 8 Maintenance 8-9 Nearest Flood Zone 9 Mudflow Area 9 Conclusions .9 Recommendations .....10 References 10 Engineering Certificate . 10 Appendix: Reduced Civil Drawings(Sheets Cl through C4) Soils Report Computer Printouts RECEIVED 2 04/10/2023 ASPEN BUILDING DEPARTMENT GRAVITY HAUS - ASPEN Drainage Report 315 E. HYMAN AVE. March 20, 2023 Introduction/General Description This drainage report has been prepared for the redevelopment of a commercial building located at 315 E. Hyman Avenue in Aspen, Colorado. The building lies just south of Hyman Avenue, and is bordered by Wagner Park to the south and a commercial building to the east and the Prospector Lodge to the west. As part of the remodel, a majority of the existing structure will be disturbed, including the garden level heated slab. In addition, most of the all of the hardscape in the garden level will be replaced to accommodate a new snow melt system. A site visit was performed as part of this evaluation to verify, to the extent possible, the existing flow regimes and to provide the necessary input data required to model storm water runoff quantities, which in this instance,will be routed to a detention/water quality tank under the existing garden level slab. Project Description The drainage areas analyzed in this report consist of the entire Lot area. The aforementioned impervious areas, analyzed for water quality and detention, consist of approximately 8,805 square feet. To complete the drainage analysis, we have analyzed the developed conditions on the site as primarily (1) one drainage basin. Based on an analysis of the Lot and a site visit, we have determined that little, if any of the storm water from the Lot flows offsite. This is largely due to building roof drains and patio drains conveying stormwater to the lower garden level. See Sheet C2 for the analyzed basin areas. As part of the drainage design, and per our discussions with City of Aspen Engineering staff, we have concluded that redevelopment of the site, due to its low-lying nature and minimal green space area, requires 100% detention. Furthermore, in this particular case, there is not sufficient area on the site, except for under the garden level, to satisfy the detention volume requirement. As part of our final design calculations, we will a provide a detention tank which will accommodate both the 100-year storm, 1-hour precipitation depth. Existing Drainage Conditions The Aspen Townsite Lots are considered flat with grades generally between zero (0) and two (2)percent. The Lots slope from southwest to northeast and storm water is ultimately tributary to the Roaring Fork River Basin via infiltration. The gross Lot area is 9,007 square feet or .207 acres. There is no vegetation on the Lots or major defined drainage basins on the Lots or immediately up- gradient which pose a flooding concern in the study area. A soils letter was provided at the time of this report. The underlying soils consist of mostly sands and coarse gravels. Our experience suggests that larger cobbles and boulders may also be present. RECEIVED 3 04/10/2023 ASPEN BUILDING DEPARTMENT GRAVITY HAUS - ASPEN Drainage Report 315 E. HYMAN AVE. March 20, 2023 According to the City of Aspen Urban Runoff Management Plan (URMP) the project is classified as a Major since more than 1,000 square feet of impervious area will be added or disturbed and more than 50% of the existing structure will be disturbed. To the best of our knowledge, a comprehensive drainage study has not been previously performed for these Aspen Townsite Lots. Proposed Drainage System Design: Per the City of Aspen URMP, latest addition, mitigation of on-site storm water requires providing water quality treatment and detention for all of the impervious areas. As part of the drainage analysis, we have provided calculations for the Major peak flow rates, water quality control volumes, and detention volume requirements. In addition, we have utilized the 100-year storm precipitation in order to size the required 100-year detention tank. The proposed design contemplates capturing the runoff from the commercial impervious areas and routing it to a single centralized detention tank and providing centralized detention. To the best of our knowledge, items such as streets, utilities, and existing structures will not be adversely impacted by the proposed drainage design, except for items that have been specifically noted on the proposed Civil Plans for the Project. URMP Design Principles: The project, to the extent possible considering all of the input parameters, has been designed in concert with the principles of the URMP. We considered water quality and detention early in the design process and have used the only feasible location on the site for incorporating detention and water quality mitigation measures for the project. The impervious areas have been minimized, to the extent possible, given the ultimate design goals by the design team and the owner. We have maintained, to the extent possible,the flat area on the site for water quality benefits. The drainage system has been designed to allow for maintenance via the use of manhole access lids. Water Quality: As part of the site drainage calculations using the WQCV methodology, we have analyzed the site primarily as a single, fully developed drainage basin. This drainage basin is shown on sheet C2. The Water Quality Control Volume (WQCV) calculations for the main drainage basin are based on effective values determined by applying Level 0 MDCIA. The analysis includes the application of Figures 8.14 and 8.16. Below is a summary table of the required water quality control volume for the overall drainage basin, as well as, application of the following formula: The WQCV(ft3)=(watershed in)(Basin Area ft2)(1 ft/ 12 in) RECEIVED 4 04/10/2023 ASPEN BUILDING DEPARTMENT GRAVITY HAUS - ASPEN Drainage Report 315 E. HYMAN AVE. March 20, 2023 315 E.Hyman Ave Water Quality Control Volume Requirement Level 0 Basin Total % MDCIA or Area Impervious Effective (Watershed-in) WQCV Sub-Basin sf % in cf Imperviousness "B" 8,805 100% 100 0.255 187.1 Peak Flow Rates On-Site—Rational Method: Based on the URMP,we will calculate the peak flow rates for each Sub-Basin and return period by using the Rational Method as follows: Rational Method: Given: Q=CIA Where: Q=Peak Discharge, cfs for the return period C=Runoff Coefficient I =Rainfall Intensity,inches/hour A=Drainage Area,acres Based on the time-intensity-frequency curve for the Aspen area and for a 60-minute duration storm, the intensity for a 5-minute time-of-concentration (the minimum rate in the URMP and recommended for small City of Aspen Lots) is approximately as follows: 100-year intensity: Iloo =6.32 inches per hour The historic 100-year runoff coefficient is as follows for NRCS Hydrologic Soils Type`B": Ctoohi5t =0.35 The developed 100-year runoff coefficients, assuming 100% impervious area, are as follows for NRCS Hydrologic Soils Type"B": C100dev =0.96 RECEIVED 5 04/10/2023 ASPEN BUILDING DEPARTMENT GRAVITY HAUS - ASPEN Drainage Report 315 E. HYMAN AVE. March 20, 2023 Since we are not flood routing in this case with the 100-year and 5-year storms, we only calculated the historic and developed 100-year peak flow rates for each Basin. The historic 100-year Sub-Basin Peak Runoff Rates are as follows: 315 E.Hyman Ave. Historic 100-Year Peak Flow Rate Major Tc=5- Basin Runoff % Total Area 100-year min. 100-year Classification Impervious Area acres Runoff 100-year Peak Flow sf Coefficient Intensity cfs in/hr "A" Pervious 0 9,000 0.207 0.35 6.32 0.46 The developed 100-year Basin Peak Runoff Rate is as follows: 315 E.Hyman Ave Developed 100-year Peak Flow Rate Basin % Area Area 100-year Tc= 5- Major min. or Impervious sf Acres Runoff 100-year 100-year Sub- Coefficient Intensity Peak Flow Basin in/hr cfs "B" 100% 8,805 0.202 0.95 6.32 1.21 Pipe Capacities: Pipes from roof areas have already been routed to the drywell/detention area as part of the original project and will be connected to the new drywell. RECEIVED 6 04/10/2023 ASPEN BUILDING DEPARTMENT GRAVITY HAUS - ASPEN Drainage Report 315 E. HYMAN AVE. March 20, 2023 Detention: In order to provide detention for the proposed improvements, the only feasible solution is a cast in place, concrete detention tank. Based on a 100-year precipitation rate of 1.23 inches and a total impervious tributary area of 8,805 ft2,the total detention requirement is (8,805 ft3 x 1.23 in/(12 in/ft)=902.5 ft3). The tank should be sized as follows: W=6.5 ft L= 12ft H= 10.5 ft The total usable tank volume is therefore (6.5 ft x 12 ft x 10.5 ft=819.0 ft3). If we assume that there will be 12" if washed rock around the perimeter of the tank, it will provide an additional volume of((2 x 6.5+2 x 12)x 10.5 x 1)x .30= 116.6 ft2. So,the total tank volume with surrounding washed rock is 925.6 ft2. The tank,therefore,has been designed with sufficient capacity. We have located the detention tank in the only feasible position on the site. In order to maximize the setback distances from the adjacent footings and foundation, the tank will be cast in place concrete and has been designed in concert with the structural plans for the new remodel. The percolation area within the perimeter of the detention tank must also be verified by the following, based on the drywell side wall area: CTL Thompson has estimated that the percolation rate of the site will be around 20 min./in. Based upon our experience working in the downtown core we have found the percolation is almost always greater than 8 min./inch given the sandy nature of the soils. If we use a percolation rate of 8 min/in, the hydraulic conductivity of the soil will be 15 ft/day or 1.74 x 10-4 ft/sec. The minimum percolation area can be calculated based on the URMP using the following equation: Ap=(Vr)/(K)(43,200) Where: Ap= Total area of the sides of the percolation area, square feet Vr=Runoff volume for 100-year event,cubic feet K=Hydraulic conductivity of soil,feet/second. AP=(902.5 ft3)/[(1.74 x 10-4)(43,200)] = 120 ft2 RECEIVED 04/10/2023 7 ASPEN BUILDING DEPARTMENT GRAVITY HAUS - ASPEN Drainage Report 315 E. HYMAN AVE. March 20, 2023 The bottom 48"of the detention tank area being 148 ft2, is greater than 120 ft2, so this requirement will be satisfied. The drain time of the detention tank is a function of the total storage height. For calculation purposes we will assume that the total height is 10.5 feet. Therefore, if we use a conservative percolation rate of 8 min./inch,the total drain time will be as follows: (10.5 ft x 12 in/ft x 8 min./in)x(1 hr/60 min)= 16.8 hours Therefore, a conservative estimate of the drain time will be 16.8 hours. If the percolation rate is found to be slower than 8 min./in the detention tank can be evacuated via pumping. Outlet Works: The outlet works for the project will be facilitated via infiltration from the detention/water quality tank. We have also provided a secondary simplex pump, backup system which will pump to an exterior area drain (release location) adjacent to the Wagner Park. Given the side of the detention/water quality tank, coupled with the screened rock under the basement slab, we do not anticipate that this system will ever initiate pumping. Erosion Control: Given the size of the project, disturbance to the site will be minimal. There is not sufficient area on the site for a vehicle tracking pad, however, the site will likely be hand excavated, so mud tracking will not be an issue. All excavations should be planned to minimize, to the extent possible, damage to surrounding vegetation. Maintenance: Piping shown on the plan should be checked for debris buildup on an annual basis. Any encountered debris should be removed as required to ensure that facilities are working properly. The drainage collection system should also be checked after construction to ensure that it is free of debris and silt. If debris and silt are encountered, it should be removed via pumping or other mechanical means. Per the City of Aspen URMP, drywells must be inspected and maintained yearly to remove sediment and debris that may be encountered. Maintenance should be in accordance with a submitted maintenance plan to the City describing the maintenance schedule that will be facilitated by the owners of the development. Minimum inspection and maintenance for the drywell should include the following: - Inspect drywell annually and after every storm that exceeds 0.5 inches of precipitation. - Remove sediment, debris and trash or other waste material from the drywell with a trash pump or by hand and dispose of at suitable disposal site per local, state and federal waste regulati¢�.�CE I E D 8 04/10/2023 ASPEN BUILDING DEPARTMENT GRAVITY HAUS - ASPEN Drainage Report 315 E. HYMAN AVE. March 20, 2023 - Evaluate the drain-down time of the drywell to ensure that the maximum 24-hour maximum is not being exceeded. If the drain-down time is being exceeded then the drywell percolation area should be further cleaned via pumping or jetting. Additional percolation perforations in the sidewalls could also be pursued by additional drilling methods. The following checklist should be administered for the pump vault biannually: - Check the pump vault twice annually. Before the Spring snowmelt is preferable. - Refer to manufacturer's instructions for specific pump maintenance. - Check pump electrical circuit and Ground Fault Circuit Interrupter(GFCI)to assure proper function. - Remove the pump vault cover and check for silt or debris.—Remove as may be required. - Check that the float level controls including the alarm float are functioning properly. - Check that the weep hole in the discharge pipe is free of obstructions. - Exercise pump and any valves on the system piping. - Check for any leaks around discharge pipe. - Make sure that the pump body is submersed in water so that the seals do not dry out. - Check that the exterior discharge location is free of obstructions and that it will drain back to the pump vault accordingly. - Verify that alarm(activated when the tank is full)is functioning. Nearest Flood Zone: The subject property is not located within a major FIRM area. Mudflow Area: Based on URMP Figure 7.1c,the Lot is not in a mudflow Zone. Conclusions: The proposed design utilizes a treatment train approach, whereby the runoff will be collected once it has passed through drain pipe with disconnected impervious areas (used as much as possible in this case) via the use of area drains and roof drains. The drainage will then be conveyed to the detention tank/water quality tank from where it will infiltrate into the ground. To the extent possible, we have avoided unnecessary impervious areas and have designed the proposed plan in concert with the URMP and the desires of the client. The proposed design will improve the existing drainage conditions by increasing water quality treatment in areas not previously mitigated as such. RECEIVED 9 04/10/2023 ASPEN BUILDING DEPARTMENT GRAVITY HAUS - ASPEN Drainage Report 315 E. HYMAN AVE. March 20, 2023 Recommendations: 1. Positive drainage should be constructed away from the building foundation in accordance with the grading and drainage plan(if proposed grades are shown). 2. A separate concrete, detention/water quality tank should be installed to provide detention and water quality for the development. 3. Erosion control measures recommended above should be strictly followed. 4. Maintenance for drainage facilities should be in accordance with the aforementioned maintenance section. References: City of Aspen URMP(latest edition) USDCM Vols. 1-3 and Spreadsheets Engineering Certificate: I hereby affirm that this report and accompanying plans, for the proposed remodel located at 315 E. Hyman Ave, were 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. Hans E. Brucker License No. 35088 Licensed Professional Engineer, State of Colorado Poo L/C-` k cP II := 35088• 3): I2"J� ��`'i 1kt\/oN`L E Ar • RECEIVED to 04/10/2023 ASPEN BUILDING DEPARTMENT APPENDIX RECEIVED 04/10/2023 ASPEN BUILDING DEPARTMENT a = $h ��'2 O C 4�U C y 2 3 iSF �N W N Wm : o y 'pk22pF iG y F J i 3 4i, Y$r�-�yy p I NQ2ld y' U FS1Sj ;Mo`io a k 1 ;rig J771y S R I 4 in I l U ax^a v0 / ' �g� / r # �B a/ ter ,- \^, 1' - / / / `/1' �_ ., / / I A� Y,rJ:1 Pi 0 /41;II.1 T.::".0(4)7 il INAz :,..,,�8M�/y°allo�bxroa! �r sb a" 3��ym» e / 1::171:1/ / � / AY741::"6,11/0/11..... ..,--'1-7'1'' - - 3g m ��!/ll s a g / Wig / k/l�l jilAl! �I/l�ll�/' ` \�� a 2 a a i �i ht yljl/j/pI 0 a�2�>az d � 1 ;r _ • 7/ �I me v 0 ,ffi s as // // nn I/I J 5 4 l I / ti ab lllli '� o ysz �e a, 1 Il////I / /I/lll Ili►: �� � a �lll[/�llllll / cc o �l//lll 7� ���wy g s /l[/ 11l \x I//llll/ C �O Po �. �4 •,a?eff i ilk l o �x ll/ ll \ �S sN ld l //l l z a`� iGll/I/l \� Im W • • /0 i lraj////Ij/ h n z • ///j� /// / 0 • ���/ljlj/lll��l/ll,l�/�ik�s o o: y �� ���j�/j��l//// �� //11 I/l / O j R� a O g a %�j j//ll�l / o 7l � x 0 l�/47111/I °/WI /'/��/you I/l�/ill/I/'/ 4 �a a sa,N\ AW Y sti a` I/ll 111/Ijll l r / ry n nrNo..," np 1 a Est %��I�l�l/l�l q e ^A/aj i la ttt, o+ko�yoi, o3 � ....� as ll, '/ll/// '11 l� v . / i I. cL i e s / / K o / /// / / / / / REIVED 1a 04/10/2023 tW W (ASPEN BUILD NG DEPARTMEN1" 1 N� m N M o ¢� o 0 J t Q Q le a W ct r s 0 e be - Q 3 a IO € I- j 4■ 1R Q. 3 �jg �0Q0 I 2= S� i , - i ez Q E• e r.}c ; I Q i1. g 6s a p ., a m m >LL O inm m I e 1 M z e co o 1 - W t� 11 s , ' o , ur p 1. i j k' , , a. ' 8 , W W' , a �c ti i Q F i i, Z i a a W , Q I i N, of / ,L•i$ 3 U P =,. d 2 0 O / Q c 1 , on , _ $: i i O ,E:f." ! i / V ^1 a i REIVED 04 10/2023 m m 1A E'EN BUILD1NGEF5ARTMENT in v h Q h 1 !� it a!i 1"! 0 Lim : . w V 1U ,� e m = Q o Wes` Li h QWO V U IJW �S W ce Q 41 Q fig'0 Ng W 1 ~ ss! 0 y~ , _ AR s LIjQ -isva2 o R Z Q °j Q CO ~ �s : • ° b y r) h 114 4 Aug c W _ --J U k g8 U II c. $- CDg J. �' 0J k J h O hh h 2 h Q 3i h. W $ . u i Z Ngl inT Q __ z =_ ! a g g � . 1 �' o d z , W OF O O k. Q �1_I__�l .11 o W z -1 ' obi .a 8 o g ass Q V� w -e z ., we Mh il REIVE! o 04 /10/2023 m WW ,sEN BUILDNGf I RTMENT . § / '4 o 3* . / , �V \ \§\ , ci .\ \ .- !IX. :A ;< eie, �/ 4 \\ m : _ a .!!! // r:f !!e - b; !#I % / /( )§b > . z . / / o ) / J - { !/° / / /- , !! / 0 \\ ( ! _. ::rz :/: : , : \( \/ \ !_.r, , . - !- / \( \\ \ ;:,.8 Q# ( ( |/ Ll (q h r- \ \ ( / m\ ( / b /_ ) u 17. P. 0wz ) ) � d!, , )/ /� c g _ '§ � m_ \} § j( `/ R » \§ e % ;, • E\ k.3 (! § \] :! \ \� , REQ1VED ., 04/1O/2023 ) ) . : S§§N B$L&G]EAR MEN CTLTHOMPSON March 16, 2023 Gravity Haus 315 E. Hyman Avenue Aspen, CO 81611 Attention: Grady Huff Subject: Preliminary Design Parameters for Water Quality Vault AspenHaus 315 E. Hyman Avenue Aspen, Colorado Project No. GS06759.000-125 CTLIThompson, Inc. (CTLIT) has been retained to perform a geotechnical engineering investigation for the AspenHaus project located at 315 E. Hyman Av- enue in Aspen, Colorado. The scope of our services was generally described in our Proposal No. GS 23-0009 which includes drilling one exploratory boring south of the existing building. The site is currently not accessible to drilling due to per- mitting and access restrictions. Prior to our subsurface investigation, we are providing preliminary recommendations for design of the water quality vault and anticipated infiltration rates of the subsoils based on information from nearby sites. Prior to construction of the vault, we will need to verify the assumptions in this letter by drilling an exploratory boring at the site. A new water quality vault is planned in the courtyard area in the center of the building. Plans for the new vault were not available at the time of this letter. We understand the vault will be approximately 8 feet wide by 10 feet long. The bottom of the new vault will be about 5 to 6 feet below the top of the basement slab elevation. This would likely be at least 6 feet below the elevation of the court- yard area. We should be provided with plans for the vault when they are devel- oped so that we can revise our recommendations, if necessary. In the opinion of CTLIT, the primary geotechnical concern with the water quality vault construction is undermining and/or destabilizing the existing building foundation during excavation. Our subsurface information and experience from nearby sites indicate the subsoils at the site will classify as Type C, pursuant to OSHA standards governing excavations. Unbraced excavation slopes in Type C soils should be no steeper than 1.5 to 1.0 (horizontal to vertical). For a 6-foot- deep excavation, this would result in a slope that extends 9 feet laterally from the vault perimeter. We suggest that the contractor consider pre-shoring of the vault excava- tion. An appropriate technique would be to install micropiles around the perimeter of the vault. This would significantly reduce the volume of excavated soils, as well CTLIThompson, Inc. RECEIVED Denver, Fort Collins, Colorado Springs, Glenwood Springs, Pueblo, Summit County—Colo,[a l 4/ 10/2023 Cheyenne, Wyoming and Bozeman, Montana U ASPEN BUILDING DEPARTMENT as subsequent backfill. Typically, micropiles for this type of application are de- signed by specialty design-build contractors. For preliminary design, we anticipate micropile lengths of about 15 feet. This should be re-evaluated once we are able to drill the exploratory boring at the subject site. Our subsurface and laboratory data from nearby sites, and our experience in the Aspen Core area, indicate the natural soils contain less than 20 percent silt and clay size material. CTLIT judges that a hydraulic conductivity rate (k) of 0.034 to 0.0034 ft/sec for the silty gravel soils anticipated at the site is appropriate for preliminary design. We judge this hydraulic conductivity (k) correlates to a vertical percolation rate faster than 3 inches per hour (i.e., 20 mins/inch). We are available to discuss the contents of this letter. Please contact us if you have questions or need additional information. Respectfully submitted, CTLITHOMPSON, INC. PDO LiCF��11�� O,,�Ft. Bq,Q�ti� 11 qz„, V. Ryan R. Barbone, P.E. ;w 61683 in: 13 Division Manager, Glenwood ".fi s% 3 vizoz `41 rbarbone@ctlthompson.com 111�‘‘vioNA L,-�G RRB:GWB/abr Via email: grady(a�gravityhaus.com RECEIVED ASPENHAUS c /10/2023 315 E.HYMAN AVENUE PROJECT NO.GS06759.000-125 ASPEN BUILDING DEPARTMENT