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HomeMy WebLinkAbout19 - Design Report - The Merin - 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 The Merin Water, Sewer, Storm Water Engineering Report May 14, 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 Merin building is an infill redevelopment of the properties located at 116 and 122 North Bozeman Avenue in the City of Bozeman Montana. The site is currently occupied by 1 existing residential building and one vacant lot which previously held a residential building on it until its demolition in 2018. The total lot area is 16,907 sf consisting of 8 individual parcels that will be aggregated as part of this project. The Merin is a 5-story, mixed-use building with retail, building support, interior parking, and a residential unit on the ground floor. 28 residential units are located on floors 2-5. The total building gross floor area is 70,985 sf. The proposed building and site improvements are shown on the Civil plans provided with the Site Plan submittal. This Engineering Report supports the design of the water and fire building service lines, sewer service, and storm water mitigation. 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. Water City water mains exist in North Bozeman Avenue and East Lamme Street. The water main in North Bozeman Avenue and East Lamme Street are both 8” Ductile Iron and were installed in 2007 and 2008. Two new services, fire and domestic, are proposed to connect to the existing 8” water main in East Lamme Street. The City of Bozeman 2017 Water Facility Plan Update did not identify any fire protection limitation in this area of the town. Three (3) 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 four blocks away the proposed building at the corner of North Montana Ave and Page | 2 Peach Street. One of the flow hydrants used in the test is located in the northwest corner of North Bozeman Avenue and Lamme Street, which is directly adjacent to the proposed building location. The test results for the test (test number 35) are included with this report. The static water pressure is approximately 142.7 psi. Two adjacent hydrants were opened simultaneously at a 2.5” diameter nozzle, the hydrant nearest the proposed building location flowed at 1,796 gpm and the other at 1,890 gpm, for a total flow of 3,686 gpm. These flows resulted in an 8.9 psi drop at the residual test hydrant, which had a residual pressure of 133.8 psi. This test indicates that reasonable urban fire flows can be met in this area. 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 Merin building. A new fire hydrant will be installed on the southeast corner of East Lamme St. and North Bozeman Avenue as required by the City of Bozeman Fire Department. 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 new water service was sized by the plumbing fixture counts in the building and the Uniform Plumbing Code flow rate for this number of fixture counts. The preliminary water fixture unit count is 740 fixture units, resulting in a peak flow of 166 gpm. This flow is within the capacity of a 3” meter at a velocity less than 10 ft/sec. Therefore, a 4” service line to the building is proposed with 3” meter inside the building. Irrigation water supply will be provided through the domestic water service. Irrigation demands were provided by the landscape architect. Irrigation is anticipated to occur over 18 weeks of the year, resulting in a total annual water use of 9,045 gallons, or 0.028 ac-ft. The resulting daily irrigation flow is 71.8 gpd. Water use for The Merin building has been estimated based on City of Bozeman engineering standards and separate irrigation estimates. The City of Bozeman provided a multi-family average daily usage rate of 105.3 gallons/day/unit. The estimated water use is provided below in Table 1. Table 1. The Merin Annual Domestic Water Use Annual Domestic Water Use Residential #Units Gpd/Unit Gallons/day Apartment Unit 29 105.3 3054 Non-Residential Area (sf) Gpd/1000sf Gallons/day Non-Residential Space 3,469 30.0 104 Total Domestic Use 3,158 Domestic Annual Use (gallons) 1,155,586 Irrigation Annual Use (gallons) 9,045 Total Annual Use (gallons) 1,152,586 Total Annual Use (ac ft) 3.57 Less Historical Use (ac ft) *TBD Adjusted Annual Use (ac ft) 3.57 The total new annual domestic and irrigation water use from The Merin building is 3.57 ac ft. The total CILWR is $21,420. Page | 3 Sewer An existing 10” Vitrified Clay sewer main is in both the North Bozeman Avenue and East Lamme Street. One new sewer service is proposed to be connected to the 10” sewer main in East Lamme Street. The new sewer service was sized by the plumbing fixture counts in the building and the Uniform Plumbing Code flow rate for this number of fixture counts. The preliminary drainage fixture unit count is 660 fixture units. For this number of fixture units, the plumbing code requires a 6” sewer service line at a 2 % slope. Daily wastewater generation was determined utilizing city-reported meter readings of 105.3 gpd for multi-family residential units. Applying this value results in an average daily flow for the project of 2.19 gpm. Using a peaking factor of 12 (the entire day’s use in 2 hours), the peak hour flow is estimated at 26.3 gpm. These values and demands are summarized and tabulated below. Table 2. The Merin Estimated Sewer Use and Demands Estimated Sewer Use and Demands Residential #Units Gpd/Unit Gallons/day Apartment Unit 29 105.3 3,054 Non-Residential Area (sf) Gpd/1000sf Gallons/day Non-Residential Space 3,469 30.0 104 Total Domestic Use 3,158 Average Day Demand (gpm) 2.19 Peaking Factor 12.0 Peak Hour (gpm) 26.3 Storm Water The existing site conditions can be classified as a low-to-medium density residential development with primarily disconnected impervious areas. Currently, site storm water is directed towards the adjacent street curb and gutter on the south side of East Lamme Street where it is conveyed east to an existing curb inlet and discharged into Bozeman Creek. Existing curbs adjacent to the subject property will be replaced with this project. Storm water mitigation is based on the redevelopment project low impact design (LID) 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 stormwater mitigation will reduce peak flows from larger storm events to below existing values. The most limiting stormwater mitigation criteria for this site is to provide mitigation of the larger storm events; ensuring that the post developed runoff is equal to or less than existing condition runoff values. The proposed stormwater mitigation system will capture, retain, and infiltrate the building rooftop runoff 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. Page | 4 A summary of the stormwater calculations showing the mitigation of the increased stormwater runoff has been provided below in Table 3. The required storage volume for the 0.5” event is 634 cf and the required storage volume to mitigate peak flows is 1858 cf. The proposed net retention storage volume is 2,010 cf. The proposed infiltration system will completely retain runoff from storms up to the 1.59-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 East Lamme Street and then into the existing storm collection (curb and gutter) system. The storage volume of this system does not account for expected infiltration during the design storm event. Table 3. The Merin Storm Water Calculation Site Statistics Land Classification C Existing Area (sf) Post Dev Area (sf) Rooftops 0.9 4,694 15,709 Gravel Parking 0.85 514 0 Sidewalk 0.9 685 1,198 Permeable Pavers 0.3 0 0 Landscape 0.2 11,014 0 Total 16,907 16,907 Weighted Runoff Coeff. (C) 0.44 0.90 Design Storm Information Design Storm 0.5-Inch 10-Year 25-Year 50-Year 100-Year Drainage Area (acres) 0.388 0.388 0.388 0.388 0.388 Drainage Area (sf) 16,907 16,907 16,907 16,907 16,907 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 1.95 2.45 2.71 2.88 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 0.55 0.66 0.81 0.92 Runoff Volume (cf) 311.70 1,213.44 1,526.64 1,689.76 1,797.04 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 1.12 1.34 1.66 1.86 Runoff Volume (cf) 634.01 2,468.21 3,105.26 3,437.06 3,655.27 Mitigation Calculations Design Storm 0.5 Inch 10-Year 25-Year 50-Year 100-Year Runoff Volume Increase (cf) 322.31 1,254.76 1,578.63 1,747.30 1,858.23 Retention Volume (cf) 2010 2010 2010 2010 2010 Net Runoff Volume Post Mitigation (cf) 0.00 458.39 1,095.44 1,427.24 1,645.45 % Decrease in Runoff from Existing 100.0 61.2 28.2 15.5 8.4 Page | 5 Conveyance capacity of the stormwater system is analyzed with respect to the post development stormwater flows. Rooftop runoff is collected in two 8” pipes joining to a single 8” pipe to the infiltration system. An 8” PVC pipe has a full-flow capacity of 1.82 cfs at the specified 2% 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 1.89 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. 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. Page | 6 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. 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. Page | 7 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.