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HomeMy WebLinkAbout19 - Design Report - AC Hotel - Water, Sewer, Storm• • www.seaeng.com Engineers and Land Surveyors 851 Bridger Drive, Suite 1, Bozeman, MT 59715 | phone: 406-522-8594 | fax: 406-522-9528 AC Hotel Bozeman Site, Water, Sewer, Storm Water Engineering Report February 19, 2019, Site Plan Submittal – Updated May 18, 2019 Prepared for: HomeBase Montana Prepared by: Stahly Engineering and Associates Engineer of Record: Cordell D. Pool, PE Quality Control Reviewer: Zach Lowe, PE Introduction The AC Hotel Bozeman building is an infill redevelopment of the property defined as Lots 1-5 of Block G Amended Plat C-1-F located at the northeast intersection of Mendenhall Street and Tracy Ave. The site is 0.469 acres (19,977 sf) and currently occupied by 4 separate one-story buildings—5 and 15 East Mendenhall St, and 106 and 112 North Tracy Ave. Two of the structures are part of an automotive repair business and the other two structures are single-family homes. The existing buildings are currently served by four (4) 3/4” water services—two from the main in Mendenhall and two from the main in South Black Avenue—and one 4” sanitary service from Mendenhall. The existing buildings will be demolished and replaced with a hotel building. The proposed AC Hotel building will be 6 stories. The building occupies most of the property with a ground level footprint of approximately 15,400 sf and a roof area of 17,165 sf. The ground level floor will be hotel lobby, lounge, and hotel support facilities. The second through sixth levels will have 143 hotel rooms. The sixth level also has an indoor/outdoor lounge area. Civil Specifications and Design Standards The civil specifications for the project will be the Montana Public Works Standard Specifications (MPWSS) and the City of Bozeman Modifications to MPWSS (COB Mods). Construction plans will be developed in accordance with the City of Bozeman Design Standards. Site improvements The AC Hotel building occupies most of the property, so site improvements are not extensive. Parking is provided by leased spaces in the Bridger Park garage located directly across Mendenhall, or by other off-site leased property. Pedestrian access to the building will be on the Mendenhall and Tracy frontages and from the alley. The existing sidewalks and curbs adjacent to the building will be replaced after building construction. With this project Tracy Avenue will be widened from 31’ back of curb to 33’ back of curb by moving the east curb along this property back 2’. The widening will Page | 2 transition back to existing width north of the alley. Utility construction in the alley will require replacement of the alley adjacent to the building. Site vision triangles are shown for the intersection of Mendenhall Street and Tracy Avenue. A non-typical solution was required due to unique intersection conditions. Mendenhall is a two-lane one- way street with stop-controlled access from Tracy Street. Two separate site vision triangles are proposed, one for traffic accessing Tracy Avenue, and one for traffic accessing or crossing Mendenhall Street. For traffic accessing Tracy Avenue, a City standard 40’ local street vision triangle is proposed. This is because the 25MPH speed limit on Mendenhall, does not require typical arterial vision. For traffic accessing Mendenhall Street, a City standard vision triangle was not the most appropriate. For this vision triangle, MDT guidelines were used that are more applicable to stop-controlled accesses. This vision triangle requires more site distance along Mendenhall, so stopped vehicles have time to enter the intersection. Franchise utilities will be provided by existing utilities located in the alley. Service lines to the existing buildings will be removed, along with an existing Northwestern Energy pole supporting these services. The removal of the pole will require some existing overhead utility lines to be relocated underground. Also, there is an existing easement for communication facilities on the property adjacent to the alley; however, no utilities exist in this easement. As part of the relocation of existing utility lines, this easement is proposed to be relocated 10’ to the west which allows relocated communication facilities to be outside of the alley right-of way and within the easement. Water City water mains exist in North Tracy Avenue and Mendenhall Street, both are 8” diameter. The existing four water services will be capped at the main, and new fire and water services will be provided from the water main in Tracy Ave. The new water service size is based on the plumbing fixture counts in the building and the Uniform Plumbing Code flow rate for this number of fixture counts. Estimated water fixture counts at this time are 800 water fixture units, equating to a maximum flow rate of 210 gpm. Based on this flow rate, a 4” water service line is proposed with a 3” water meter. The fire service line size has not been determined yet, but a 6” service is anticipated. The water and fire service lines will be designed by a PE and submitted for review as required by the building permit. The City of Bozeman 2017 Water Facility Plan Update did not identify any fire protection limitation in this area of the town. Four (4) fire hydrants are located on the adjacent streets within 300 feet of the building. Furthermore, as part of the 2017 Update, a fire flow test was performed on a hydrant approximately two blocks away at the corner of Villard and North Tracy Ave, and the test results are included with this report. The static water pressure is approximately 137 psi. Two adjacent hydrants were opened simultaneously at a 2.5” diameter nozzle, one flowed at 1601 gpm and the other at 1,744 gpm, for a total flow of 3,345 gpm. These flows resulted in a 25 psi drop at the residual test hydrant, which had a residual pressure of 112 psi. This test indicates that reasonable urban fire flows can be met in this area, even though some water mains are only 6” diameter. Specifically, each of the 4 nearby hydrants could be expected to provide similar flows, resulting in approximately 4,000-6,000 gpm of fire flow available to the AC Hotel building. Additionally, this test indicates that the water system network in this area can provide the anticipated peak domestic demand of the AC Hotel building with very little impact on the system pressures. Page | 3 Estimated water use for the AC Hotel building is provided in Table 1. Estimated water use is based on City water use data for similar Hotels and, specifically, water meter data obtained from the Element Hotel. The Element has a swimming pool, so its water use is likely higher than the AC Hotel. Element Hotel water meter data shows that the highest water use occurs during summer and is equivalent to 93 gpd/room. Average annual water use is 67 gpd/room. Estimated water use for the AC Hotel is 91 gpd/room, since there is not a swimming pool proposed. With this estimate the full occupancy total domestic use is estimated to be approximately 13,000 gallons per day (gpd). Annual water use is less than this value, due to seasonal occupancy variance. Average annual domestic water use is anticipated to be 65 gpd/room, for an average day flow of approximately 9,300 gpd. Due to the urban design, irrigation water use is minimal, only 0.03 acre feet/year. The total annual water use is anticipated to be approximately 10.44 acre feet per year (AF/year). This does not account for the existing water use present at the site, which will be discontinued. Table 1. AC Hotel Estimated Water Use Design Flow Average Annual Flow Hotel Use # Units Gpd/Unit Gpd Gpd/Unit Gpd AF/year Unit 143 91 13,013 65 9,295 10.41 Irrigation Use Gpd (Gal/year) AF/year Total Irrigation 96 9612 0.03 Total Water Use 13,109 10.44 A 6” clay sewer exists in Mendenhall Street. This main is undersized and not in a favorable location and thus will not be used to serve the AC Hotel building. Instead, a new 8” sewer main is proposed to be extended from the existing 10” sewer main at the intersection of Lamme and Tracy to serve the AC Hotel building. The sewer main is proposed to be extended from this intersection, south in Tracy Ave to the alley north of the building and further extended in the alley to serve the building. The main building sewer service size is based on the drainage fixture counts in the building. Estimated drainage fixture counts at this time are 679 drainage fixture units. This requires an 8” service at 1% slope. Additionally, to accommodate hotel food preparation a 1,000-gallon grease interceptor is provided between the building and the alley. Daily wastewater generation is anticipated to be equal to the domestic use of 13,013 gallons per day. Using a peaking factor of 8 (the entire day’s use in 3 hours), the peak hour flow discharging to the sewer is estimated to be 72 gallons per minute. Storm Water The site is currently developed and has a substantial impervious area. Currently site storm water is directed by surface drainage towards the adjacent streets and alleys. Runoff is then collected in existing curb inlets and transported to a storm drain main in Beall Street. Though antiquated, the existing storm drainage system appears to be functioning adequately. The existing curbs on Tracy Ave adjacent to the property are badly deteriorated and will be replaced with this project. Storm water mitigation is based on the redevelopment low impact design requirement in the City Design Standards to “infiltrate, evapotranspire, or capture for reuse the runoff generated from the first 0.5” of rainfall”. Additionally, to reduce impacts to the existing storm drainage infrastructure, the Page | 4 stormwater mitigation will reduce peak flows from larger storm events to below existing values. Due to the existing impervious areas, the most limiting criteria is the capture of the first 0.5” of rainfall. The proposed stormwater mitigation system will capture, retain, and infiltrate the building rooftop runoff and adjacent sidewalk areas for storms up to and exceeding 0.5” of rainfall. The infiltration system will capture rooftop runoff and pipe it to a subsurface chamber/gravel infiltration system within the building footprint. All stormwater piping has been sized to handle the 25-year design event and all sidewalk chases have been sized to handle the 100-year design event. Permeable pavers will be installed along the sidewalk to further reduce stormwater runoff generated from the improved sidewalk areas and mitigate the loss of the grass boulevard. A summary of the stormwater calculations showing the mitigation of the increased stormwater runoff has been provided below in Table 2. Table 2. AC Hotel Storm Water Calculations Site Statistics Land Classification C Existing Area (sf) Post Dev Area (sf) Rooftops 0.9 5,692 17,262 Pavement/Concrete 0.9 6,305 1,093 Gravel 0.75 3,490 0 Permeable Pavers 0.3 0 1,386 Landscape 0.2 4,490 236 Total 19,977 19,977 Weighted Runoff Coeff. (C) 0.72 0.85 Design Storm Information Design Storm 0.5-Inch 10-Year 25-Year 50-Year 100-Year Drainage Area (acres) 0.459 0.459 0.459 0.459 0.459 Drainage Area (sf) 19977 19977 19977 19977 19977 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) 0.50 1.84 2.16 2.42 2.67 Existing Peak Flow Calculations Design Storm 0.5 Inch 10-Year 25-Year 50-Year 100-Year Intensity at Tc (Figure I-2 pg. 29) (in/hr) NA 3.22 3.83 4.74 5.34 Peak Runoff Rate at Tc (Q = CIA) (cfs) NA 1.06 1.26 1.56 1.75 Runoff Volume (cf) 596 2195 2576 2886 3185 Post Dev Peak Flow Calculations Design Storm 0.5 Inch 10-Year 25-Year 50-Year 100-Year Intensity at Tc (Figure I-2 pg. 29) (in/hr) NA 3.22 3.83 4.74 5.34 Peak Runoff Rate at Tc (Q = CIA) (cfs) NA 1.25 1.49 1.85 2.08 Runoff Volume (cf) 708 2604 3057 3425 3779 Mitigation Calculations Design Storm 0.5 Inch 10-Year 25-Year 50-Year 100-Year Runoff Volume Increase (cf) 0 409 481 538 594 Mitigation Volume (cf) 2138 2138 2138 2138 2138 Net Runoff Volume Post Mitigation (cf) 0 466 919 1287 1641 % Decrease in Runoff from Existing >100% 79% 64% 55% 48% Page | 5 The proposed chamber system configuration provides a net storage volume of approximately 2,138 cf which exceeds the required storage volume for the 0.5” event of 708 cf. The proposed infiltration system will completely retain runoff from storms up to the 1.51-inch event. In addition to containing the volume of the first 0.5”, the proposed storage also contains the runoff volume increase from pre- existing to post-development in the 10, 25, 50, and 100-year 24-hour events. Since the 0.5” runoff and the runoff volume increase are contained within the retention facility, the post-development runoff is significantly reduced below the pre-development runoff. Any flows generated in storms that exceed the storage volume will be conveyed by concrete chases into the curb and gutter on Tracy Avenue and Mendenhall Street then into the existing storm collection system. The storage volume of this system does not account for expected infiltration during the design storm event. Conveyance capacity of the stormwater system is analyzed with respect to the post development stormwater flows. Rooftop runoff is collected in one of two 8” pipes and then conveyed into the infiltration system. An 8” PVC pipe has a full-flow capacity of 1.58 cfs at the specified 1.5% slope, exceeding the 25-year storm run-off flow rate. For larger storms the remaining storm runoff will be directed to the curb and gutter through two sidewalk chases. The sidewalk chases each have a capacity of 1.39 cfs at the dimensions and grades shown on the plans. The combined capacity of the two sidewalk chases exceeds the runoff flow rate from the 100-year event. Storm Water Maintenance: General Information The proposed storm water conveyance and infiltration facilities will be operated and maintained by the property manager. Storm Water Facilities Maintenance Schedule 1. Site Housekeeping. (Continuously as needed) The main cause of storm water facility damage is poor site housekeeping. Sediment tracked onto pavement can be washed into storm water appurtenances and damage these facilities. Trash can clog conveyance structures, potentially causing property damage. • Keep sidewalk, permeable pavers, and parking areas clean. • Pick up trash. • Restore damaged landscaping in order to prevent sediment runoff. 2. Curb, Sidewalk Chase, and Infiltration System Maintenance. (Quarterly) All storm water conveyance structures can acquire sediment and debris buildup. If this sediment and debris is not periodically removed, it can cause undesired ponding and clogging. These conveyance structures need to be inspected and cleaned if required. • Inspect for sediment or debris in the structures and remove if present. • Inspect infiltration system through inspection ports for sediment accumulation. Sediment depth less than 3” is acceptable. • Check for damage, repair as needed. Page | 6 3. Curb, Sidewalk Chase, and Infiltration System Maintenance. (Long-term) If regular housekeeping and maintenance is not performed adequately, sediment and debris can accumulate in the storm water conveyance structures and infiltration system and clog them beyond repair. • If greater than 3” of sediment is present in infiltration system, hire a contractor with a Jet-Vac chamber cleaning system to remove the sediment from the infiltration system. • If original system performance can be achieved through maintenance, hire a contractor to repair and return conveyance structures and infiltration system to the initial design condition found on City engineering plans. 4. System Monitoring. (Quarterly, except in winter) The storm water facilities shall be inspected quarterly to quickly identify small issues before expensive damage can occur. In addition to regular monitoring, the best time to inspect the performance of storm water facilities is during runoff events. • Observe system during runoff. Look for ponding on permeable pavers or inlet structures. This can indicate a clogged paver infiltration and/or clogged conveyance structure. • Open infiltration system inspection ports within 24-hours of a storm event and look for ponded water in the infiltration system. This can indicate clogged infiltration system. If clogged hire a contractor with a Jet-Vac chamber cleaning system to remove the sediment from the infiltration system. 5. PERMEABLE INTERLOCKING CONCRETE PAVEMENT (PICP) Inspection & Maintenance Guidelines. Service inspection and maintenance shall include the following activities: • Winter Maintenance: o Ensure snow is not stockpiled on permeable pavement surface. o Ensure only joint aggregate stone (typically # 8, #89 or #9 washed chip stone) is used for traction as needed. Sand should not be used for winter traction. • Normal Maintenance: o Inspect surface for ponding after large rain events. If ponding is observed, identify areas with severe sediment loading and vacuum to remove and replace with new washed joint aggregate (typically # 8, #89, or # 9 washed chip stone). o Note any sediment laden run-off from adjacent areas onto permeable pavement. If needed, correct with erosion control measures. • Annual inspection and maintenance shall include the following activities: o Replenish paver joints with additional aggregate if level is more than ½ in. below chamfer bottoms. o Inspect vegetation around PICP perimeter for cover & soil stability, repair/replant as needed. o Inspect and repair all paver surface deformations (depressions/settlement) exceeding 1/2 in. Page | 7 o Repair paver heights offset by more than 1/4 in. above or below adjacent units or offset by more than 1/8” lippage from paver-to-paver. o Replace cracked paver units impairing surface structural integrity. o Check drains and outfalls (if existing) for free flow of water. Remove any obstructions. o Check observation wells (if existing) to confirm reservoir is draining (based on size of last rain event). o Vacuum surface (typically spring), adjust vacuuming schedule per sediment loading. Once a year sweeping is normal unless excessive silts and fines are present in joints. o Test surface infiltration rate using ASTM C1781. If pavement infiltration rate is < 100 in/hr. employ remedial maintenance procedure utilizing a vacuum sweeper/method to extract affected clogged joints/voids and replace joint/void areas with #8, #89 or #9 washed chip aggregates and retest infiltration rate to confirm reinstated areas exceed 100 in/hr. flow rate. Repeat remedial process as needed to exceed the 100 in/hr. criteria. • Additional Normal Maintenance Notes: o A dry mechanical or regenerative air-type sweeper may be used during dry periods to remove encrusted sediment, leaves, grass clippings, etc. Vacuum or sweeper settings may require adjustments to prevent uptake of aggregate from the paver voids or joints. Leaf blowers or other standard onsite manual methods that are used for standard pavement maintenance may be employed to remove this surface debris. o It is not recommended to utilize a pressure washer to clean joints. o Remove snow with standard plow/snow blowing equipment. o Deicing salt may be used on permeable pavers (proper application and appropriate salt type) but consult property owner or project engineer before usage. In some regions deicing salt use is restricted. Salt use can affect water quality and have environmental impact.