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HomeMy WebLinkAbout006 - Appendix E - Engineering Report• • www.seaeng.com Engineers and Land Surveyors 851 Bridger Drive, Suite 1, Bozeman, MT 59715 | phone: 406-522-8594 | fax: 406-522-9528 Bozeman Yards North Site, Water, Sewer, Storm Water Engineering Report May 31, 2024, Site Plan Prepared for: Jackadoo LLC Prepared by: Stahly Engineering and Associates Engineer of Record: Cordell D. Pool, PE Quality Control Reviewer: Zach Lowe, PE Introduction The Bozeman Yards North is the second building proposed within the overall Bozeman Yards (Block 104) infill redevelopment project. The Bozeman Yards South “aka Block 104 South Building” (located south of the alley) has been submitted previously, is currently in review and is envisioned to be the first building constructed. The Bozeman Yards North is located north of the alley on Lots 7- 16, Block 104. The existing lot size is 29,491 square feet, but 10’ of additional right-of-way will be granted by easement to allow Front Street to have a 60’ right-of-way. The net lot area after street dedication is 27,700 sf. As part of development, the lots will be aggregated so the building is contained on a single lot. Figure 1 – The Bozeman Yards (Block 104) Proposed Development Page | 2 Existing conditions on the Bozeman Yards North site consist primarily of a gravel parking lot, with some minimal landscaping and an existing one-story building. The Bozeman Yards North site will be used for construction staging area for the Bozeman Yards South building. The Bozeman Yards North is currently envisioned to be constructed after the south building is largely completed and the staging area is no longer required. The Bozeman Yards North building consists of a 6-story main building, with a 2-story portion along Front Street with a total of 26 residential condominium units. Both portions of the building will be on a common foundation and basement and served by single utility services. The basement contains underground parking accessed from the alley, and there are 3 on-street parking spaces along the Tamarack Street frontage. The proposed building and site improvements are shown on Figure 2. Civil Specifications and Design Standards The civil specifications for the project are the Montana Public Works Standard Specifications (MPWSS) and the City of Bozeman Modifications to MPWSS (COB Mods). Construction plans are developed in accordance with the City of Bozeman Design Standards. Off-site Street and Improvements Adjacent to Site Off-site infrastructure improvements are part of a separate infrastructure project to be constructed concurrently with Bozeman Yards South (aka Block 104 South Building). Off-site improvements consist of new water and sewer mains in Aspen Street, new street and storm drainage improvements on Aspen Street and Front Street, conversion of Ida Avenue between Aspen and Front streets to open space/park, Triangle Park improvements, sidewalks, and lighting. The franchise utilities currently located within adjacent streets will be relocated underground in new utility easements fronting the Block 104 streets. The alley running through Block 104 will be completed with the Bozeman Yards South site improvements. The off-site project will bring needed street and utility improvements to the area as part of a long- term vision for improving pedestrian and vehicular circulation to the neighborhood. Site Demolition and Construction Facilities The existing building, parking lot, and utility services on the Bozeman Yards North site will be demolished after the site is no longer needed to support the Bozeman Yards South construction. Construction staging for Bozeman Yards North is anticipated to be along the Tamarack and Front Street frontages. The edges of the adjacent streets will be enclosed by fencing and traffic control for construction offices and staging areas, leaving a 24-ft wide drive aisle outside of construction fencing. Site Improvements Site improvements consist of site hardscape improvements, franchise utility services, building water and sewer services. Stormwater mitigation will be provided by a shared system in the alley serving both buildings, constructed with the Bozeman Yards South site improvements. On-site parking is provided underground within the basement of the building and accessed from the alley. Basement parking provides 26 parking stalls, with an additional 3 on-street spaces available. Two condominium units along Front Street have individual garages with driveways from Front Street. The alley will be improved and to accommodate building access concrete bands are added to the Page | 3 alley to provide 24’ of travel width and/or pedestrian pathways. Trash is collected and stored within the building and rolled out to the alley for loading. The trash access and basement ramp are heated. Site hardscape is predominantly located on the roof deck of the basement garage, providing private patios and a common courtyard on the ground level. The main building entrance and courtyard are accessed by steps and an ADA ramp. The units along Front Street have individual front doors from the sidewalk. Residents are provided with interior bike storage and exterior bike racks are provided for visitors. Figure 2 – Bozeman Yards North Site Layout Gas, communication, and power building services are proposed to be run from Tamarack Street along the west property line to the south in a proposed 10’ utility easement on the north and south properties during the Bozeman Yards South (Block 104 South Building) Project. Utility meters are proposed along the west wall of the building, and a backup generator will be housed in the southwest corner of the building. Water and Sewer An existing 12” ductile iron City of Bozeman water main runs along Tamarack Street. Domestic water and fire service will be provided by connection to this existing main. The City of Bozeman 2017 Water Facility Plan Update did not identify any fire protection limitation in this area of town. A fire hydrant is located within 75 feet of the building. Furthermore, as part of the 2017 Water Facility Plan Update, a fire flow test was performed on a hydrant approximately 900’ away at the northwest corner of Ida Avenue and Peach Street intersection. The results for this test (test number 45) are included with this report. Based on hydrant flow test, the water pressure is estimated to be approximately 145 psi. This test also shows a fire flow available in this area of town to be over 3000 gpm, indicating adequate water supply for fire protection. Services are sized in accordance with plumbing and fire Page | 4 code and fixture counts. Water fixture counts are preliminarily estimated to be approximately 688 units requiring a 4” water service and 3” meter. The fire service is assumed to be 6”. The riser room is proposed in the northwest corner of the basement, with direct access from Tamarack Street. Irrigation water supply will be provided through the domestic water service. Preliminary irrigation demands were provided by the landscape architect. Irrigation is anticipated to occur over an approximately 100-day season resulting in a total annual water use of 110,727 gallons, or 0.34 ac-ft. The average daily irrigation use is 1,107 gpd. The estimated water use calculations for Bozeman Yards North are based on a usage rate of 89 gpd/capita (same as wastewater) with an estimated 2.17 persons per unit for residential units, plus the irrigation water use. The estimated annual water use for the entire building is anticipated to be approximately 6,128 gallons per day (gpd), with a peak hour demand of 18.3 gallons per minute (gpm), as shown in Table 1. The Bozeman Yards North anticipates water use that is lower than typical city design flow estimates for residential single household lots. There is not currently an established methodology for estimating water use in this kind of urban, high-density residential use where per capita landscape irrigation is much less than typical residential single household units. Table 1 – Bozeman Yards North Annual Domestic Water Use Cash-in-lieu-of water rights will be used to satisfy the project’s water adequacy requirement. An existing 24” PVC sewer main is located in Tamarack Street, which serves the existing building on the site. This existing 4” service will continue to be used for the 2-story portion of the building and a new 6” sewer service is proposed to be connected to the 24” main in Tamarack for the main portion of the building. Daily wastewater generation was determined utilizing the same residential unit estimate described in the water use estimate, but with infiltration and inflow and without the irrigation demand. Applying these values results in an average daily flow for the Bozeman Yards North of 5,123 gpd, and a peak hour flow of 15.3 gpm as shown in Table 2. Bozeman Yards North Estimated Water Use Residential Use Units Capita/Unit Population GPD/Capita Water Use (gpd) Apartment Units 26 2.17 56 89 5,021 Total 26 56 5,021 Site Area - Lot (ac)0.68 Average Day Water Use [Equal to Wastewater Flow](gpd) 5,021 Irrigation (gpd)1,107 Max Day Water Use [Average Day Use + Irrigation] (gpd)6,128 ;P = Pop. ÷ 1,000 Peaking Factor (PF)4.30 Water Use Peak Hour Flow (gpm)18.3 Page | 5 Table 2 – Bozeman Yards North Estimated Sewer Use and Demands Storm Water The existing site conditions consist of a building and a gravel parking lot with some minor landscaping areas without any stormwater mitigation. Currently, runoff from the site is directed north or east, towards Tamarack Street or Front Street, eventually running into the City storm sewer in Tamarack Street. The development is required to provide on-site stormwater mitigation to manage stormwater originating from the lot development. The original concept for Bozeman Yards (aka Block 104) had independent subsurface stormwater systems located on each lot, and a separate permeable paver system for the alley. The developer now prefers to have a single shared stormwater sub-surface system serving both building sites and the alley located within the alley right-of-way. This is more cost efficient to build and maintain compared to separate systems. A utility occupancy permit will be required to allow the use of City- owned alley right-of-way for stormwater mitigation. Because existing and proposed franchise utilities run along street frontages, there are no existing or proposed utilities within the alley to conflict with the subsurface stormwater system. The system is designed to accommodate typical alley traffic over the top of it. Existing and post development stormwater runoff volumes were determined by the Rational Method with a weighted coefficient determined for the building’s rooftop, pavement area, gravel parking area, sidewalks, patios, and landscaped areas. Areas were tabulated for the Bozeman Yards South (aka Block 104 South Building), Bozeman Yards North and alley sub-areas, and the total combined site of approximately 1.5 acres as shown on Exhibit 1 at the end of this report. Storm runoff calculations for the entire combined site are shown in Table 3. The post development runoff coefficient is higher than the existing conditions, producing approximately 60% more runoff than the existing conditions. Runoff flow rates are provided for the highest intensity (short duration) events for a range of storm frequencies. Runoff volumes are provided for the different storm frequencies and precipitation depths. Note runoff flow rate and volume are not related. Peak runoff flow rate only depends on rainfall intensity at the time of concentration and the frequency of the event. Runoff volume depends only on the storm duration and precipitation depth. For example, the 10-year, 2hr and 10-year, 24hr storms have the same peak flow, but different runoff volumes due to the different precipitation depths. The Bozeman Yards (Block 104) development is proposed to have a shared subsurface retention system located in the alley between the buildings. The retention system consists of a gravel bed with one row of Stormtech chambers beginning and ending with inlet/access manholes. The site runoff Bozeman Yards North Estimated Wastewater Flow Residential Use Units Capita/Unit Population GPD/Capita Wastewater Flow (gpd) Apartment Units 26 2.17 56 89 5,021 Total 26 56 5,021 Site Area (ac)0.68 Inflow & Infiltration (gal/ac/day)150 Inflow & Infiltration (gpd)102 Wastewater Flow + Inflow & Infiltration (gpd)5,123 ;P = Pop. ÷ 1,000 Peaking Factor (PF)4.30 Wastewater Peak Hour Flow (gpm)15.3 Page | 6 (mostly rooftop) from each building will be directed through building storm drain outlets and into the chambers via connection to the infiltration chamber. The alley will drain in an asphalt swale towards any of three slotted drain inlets and drop into the retention system. A groundwater monitoring well was installed near the stormwater retention system and groundwater levels were monitored through the summer of 2023. The highest groundwater elevation is approximately 8’ below finished grade, and greater than 3 feet below the infiltration chambers. The retention system is sized to contain runoff from the 10-year, 2-hour storm event which is a 0.82” storm event. This storm event represents the 96th percentile storm event in Bozeman and produces approximately 3,476 cubic feet (cf) of storm runoff for the combined sites and alley. To reduce post- development runoff leaving the site from larger storm events to below existing flows, a retention volume of 4,386 cubic feet is proposed. Table 3 shows that stormwater from the design event (10- year, 2-hour storm) is completely retained and the estimated reduction of runoff from larger storm events. Table 3 – Bozeman Yards (Block 104) Combined System Stormwater Calculations Land Classification C Rooftops 0.9 Pavement 0.9 Gravel Alley 0.5 Sidewalk 0.9 Gravel Parking 0.5 Landscape 0.2 Total Weighted Runoff Coeff. (C ) Design Storm 10y 2hr 10-Year 25-Year 50-Year 100-Year Drainage Area (acres)1.543 1.543 1.543 1.543 1.543 Drainage Area (sf)67,196 67,196 67,196 67,196 67,196 Slope (%)2 2 2 2 2 Time of Concentration (min)5.0 5.0 5.0 5.0 5.0 24 Hour Precipitation Volumes (in)0.82 1.84 2.16 2.42 2.67 Design Storm 10y 2hr 10-Year 25-Year 50-Year 100-Year Intensity at Tc (Figure I-2 pg. 29) (in/hr)0.41 3.22 3.83 4.74 5.34 Peak Runoff Rate at Tc (Q = CIA) (cfs)0.30 2.32 2.75 3.41 3.84 Runoff Volume (cf)2,142 4,807 5,643 6,323 6,976 Design Storm 10y 2hr 10-Year 25-Year 50-Year 100-Year Intensity at Tc (Figure I-2 pg. 29) (in/hr)0.41 3.22 3.83 4.74 5.34 Peak Runoff Rate at Tc (Q = CIA) (cfs)0.48 3.76 4.47 5.54 6.23 Runoff Volume (cf)3,476 7,800 9,156 10,258 11,318 Design Storm 10y 2hr 10-Year 25-Year 50-Year 100-Year Runoff Volume Increase (cf)1,334 2,992 3,513 3,936 4,342 Retention Volume (cf)4,386 4,386 4,386 4,386 4,386 Net Runoff Volume Post Mitigation (cf)-910 3414 4770 5872 6932 % Decrease in Runoff from Existing 100%29%15%7%1% Event Retained 10-y,2-hr 10y,4.5-h 25-y, 2.25-h 50-y,1.41-h 100-y,1.08-h Duration Retained (min)N/A 240.0 135.0 85.0 65.0 Intensity at Time of Overflow (in/hr)N/A 0.3 0.5 0.7 1.0 Peak Post Retention Overflow Rate (cfs)N/A 0.3 0.5 0.9 1.1 % Decrease in Peak Flow from Existing N/A 87%80%75%71% Post Dev Peak Flow Calculations Mitigation Calculations 67,196 67,196 0.47 0.76 Design Storm Information Existing Peak Flow Calculations 22,577 0 26,990 13,727 3,000 0 Site Statistics Existing Area (sf)Post Dev Area (sf) 12,369 43,121 0 4,022 2,260 6,326 Page | 7 The combined retention system storage calculations are shown in Table 4, accounting for both the chamber and gravel storage. The proposed storage volume exceeds the runoff generated from the 10-year, 2-hour design storm event. The retention system is sized without consideration of infiltration, which provides significant additional capacity and a comfortable margin of safety. With infiltration considered, the retention system is capable of fully containing the 100-yr, 24hr storm runoff. Table 4 – Bozeman Yards (Block 104) Combined Retention System Calculations Similar calculations were done for each sub-area to determine the peak flow rate and conveyance capacity of the piping. For the purposes of conveyance of building and site runoff, a 100-year storm was used. Table 5 – Bozeman Yards (Block 104) Sub-Area Peak Flow Calculations Subsurface Chamber System Chambers Length (ft)140 Number of chambers 20 Storage per chamber 45.9 Chambers Storage (cf)918 Gravel Bed Length (ft)155 Gravel Bed Width (ft)23 Gravel Bed Depth (ft)3.5 Cube Volume (cf)12477.5 Net Gravel Volume (cf)11559.5 Gravel porosity 0.30 Total Gravel Storage (cf)3468 Total Storage 4,386 Sub-area Sub Area 1 Sub Area 2 Sub Area 3 Land Classification C Boz Yards South Boz Yards North Alley Rooftops 0.9 24,052 19,069 0 Pavement 0.9 0 0 4,022 Gravel Alley 0.5 0 0 0 Sidewalk 0.9 2,282 2,034 2,010 Gravel Parking 0.5 0 0 0 Landscape 0.2 8,109 5,618 0 Total 34,443 26,721 6,032 Weighted Runoff Coeff. (C )0.74 0.75 0.90 Design Storm 100-Year 100-Year 100-Year Drainage Area (acres)0.791 0.613 0.138 Drainage Area (sf)34,443 26,721 6,032 Slope (%)2 2 2 Time of Concentration (min)5.0 5.0 5.0 24 Hour Precipitation Volumes (in)2.67 2.67 2.67 Design Storm 100-Year 100-Year 100-Year Intensity at Tc (Figure I-2 pg. 29) (in/hr)5.34 5.34 5.34 Peak Runoff Rate at Tc (Q = CIA) (cfs)3.10 2.47 0.54 Site Statistics Design Storm Post Dev Peak Flow Calculations Page | 8 Each building has multiple storm drain pipes running into the system, but no less than one 10” and one 8” pipe run at a 2% slope. The combined capacity of these pipes is 4.39 cfs, which exceeds the 100-year peak flow from either building. The alley has a peak flow of 0.54 cfs, which equates to a flow depth of approximately 1”. However, there are 3 inlets in the alley so flow depths will be much less. Storm Water Maintenance The Bozeman Yards (Block 104) shared stormwater system will be maintained by the Bozeman Yards Property Owner’s Association (POA). It is the POA’s responsibility to monitor and maintain the combined stormwater mitigation system, and alleys are required to be maintained by adjacent property owners. The combined stormwater retention system is designed to capture sediment and debris and requires regular monitoring and periodic maintenance to remove sediments and debris. If this sediment and debris is not periodically removed, it can cause undesired ponding and clogging. The stormwater retention system consists of manhole structures and underground chambers. These components need to be inspected quarterly, except in winter, and cleaned if required. • Inspect for sediment or debris in the manhole/inlet structure and remove if present. • Inspect infiltration chambers through inspection ports for sediment accumulation. Sediment depth less than 3” is acceptable. Sediment depth exceeding 3” requires removal by a cleaning service. • Check for damage, repair as needed. • Maintain records of quarterly inspections and cleaning and provide records to the City upon request. BLOCK 1 0 4 COMM COMM COMM COMM COMM COMM COMM COM M COM M COM M COM M COM M COM M COMMCOMMCOMMCOMMGAS GAS GAS GAS GAS GAS GAS GAS GAS GAS GAS GAS GAS GASGASGASGASGASUGE UGE UGE UGE UGE UGE UGE UGE UGE UGE UGE UGE UGE UGEUGEUGEUGEUGEB LO C K 1 0 2 BLOCK 103 E. ASPEN STREETIDA AVENUE FRONT STREETALLEYTAMARACK STREET[][][][][]OHEOHEOHE OHETF UGCWWWWWWWWWWWWWWWWWWWWWWWWWWSSOHEOHEWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSS SSSSSD SDSDSDSDSDSDSDSDSDSDSDSDGASIRIRIRSSSSSSSSSSSSSSSSSSSSWUGEUGEUGEGASGASGASGASGASGASGASGASUGEUGEUGEUGEUGEUGE SSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSLOTS 7-16, BLOCK 104[PLAT C-23-A49]LOTS 17-26, BLOCK 104[PLAT C-23-A49]UGE UGE UGE UGEUGESSSDSD>>>>>>>>>>>>>>>>>>>>>>>>>>>>COMM COMM COMM COMM COMM COMM COMMMECH/TRASHFITNESS LOBBYPACKAGE ROOMCOMMERCIAL RESTROOMCIRCULATION RESTROOMS MECH/ELEC BIKE STORAGERISERSPA MECHUNIT B-1108MECHMECHGENERATOR ROOM ELECTRICA L 1 BED / 1 BATH + DENUNIT A-11061 BED / 1 BATHUNIT A-11041 BED / 1 BATHUNIT C-61022 BED / 2 BATHUNIT C-21012 BED / 2 BATHUNIT C-11032 BED / 2 BATHUNIT C-4 TYPE A1052 BED / 2 BATHUNIT D-21072 BED / 2 BATH + DENSTAIR 1ELEVATOR RES. TRASHSTAIR 2JANITORMAIL VESTIBULEVESTIBULE STORAGE ROOM WW SD STAIR 1LOBBY PACKAGE ROOMCIRCULA TIONMECH/TRASH MECH ELEVATOR 1MECH STAIR 2CIRCULATIONUNIT 1032 BED / 2.5 BATH + DEN UNIT 1022 BED / 2 BA TH + D ENUNIT 1011 BED / 1 BA TH UNIT 12 BED / 2 .5 BA TH UNIT 23 BE D / 3 .5 BA TH + DE N UNIT 32 BED / 2 .5 BA TH + D EN & G ARAG E UNIT 42 BED / 2 .5 B ATH + DEN & GAR AGE MECHGENERATOR ROOMUNIT 1041 BED / 1 BATH BIK E & GE AR R OOM 1 B IKE PARK INGSPACE 1 BIK E PARKINGSPACEMAIL BOXESCRAWL SPACENORTH BUILDINGSOUTH BUILDINGSS SS SS SS SS SS GASUGEUGEGASSUB-AREA 1SUB-AREA 2SUB-AREA 31 5/3/2024 CONCEPT PLAN CDP 2 5/31/2024 SITE PLAN SUBMITTAL CDP ZWL PROFESSIONALENGINEERS &SURVEYORSBOZEMAN YARDS NORTH622 E TAMARACK ST.STAHLYENGINEERING& ASSOCIATESJACKADOO LLCBOZEMAN, MT 0020'40'0020'40'EXHIBIT 1STORMWATERDRAINAGEEXHIBIT Bozeman Water DistributionSystem Model CalibrationField Test Data Sheet Test Number: __________ Test Date: ______________ Start Time: ___________ End Time: ___________ Test By: _________ Zone: ___________________ 45 9 757 964RESIDUAL HYDRANT Hydrant No. ______ HPR No. ________ Static: __________ Residual: ________ FLOW HYDRANT #1 Hydrant No. ______ HPR No. ________ Flow: ______________ FLOW HYDRANT #2 Hydrant No. ______ HPR No. ________ Flow: ______________ 45GFGFGFGFGFGFGFGFGFGFGFGFGFGFGFGFGFGFGFGFGFGF GFGFGFG!! 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