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HomeMy WebLinkAbout012.1 - Water Waiver Request Memo1. CLUBHOUSE WATER SERVICE PRESSURE CALCULATIONS 1.1. OVERVIEW AND SUMMARY OF REQUIREMENTS The calculations included in this memorandum have been prepared to support the design of the proposed 2-inch water service line that serves the clubhouse (Building 2 as shown in Sheet C4.00 attached to this memorandum). This memorandum seeks to provide an exemption from BMC 38.410.070.B – “The length of water service lines from the main to the structure may not exceed 150 feet in length, unless approved in writing by the city” by providing calculations that verify adequate pressure can be maintained at the clubhouse. The proposed water service line connects to the proposed 8-inch diameter water main in South 17th Ave and exceeds the maximum allowable service line length of 150 feet per Chapter 4.2.9 (Services), Item C (Service Length) of the City of Bozeman Design and Construction Standards dated October 2024 (2024 City Design Standards). As such, a deviation is requested to eliminate the requirement for a water main extension to serve the clubhouse since it is the only service connection that exceeds the allowable service line length. To establish that the deviation is acceptable, this memorandum includes calculations to demonstrate that required minimum water system pressures will be met at the clubhouse water service meter without a water main extension. 1.2. AVAILABLE WATER SYSTEM PRESSURE Per Chapter 4.2.2 (Main Size) of the 2024 City Design Standards, water mains must maintain the following minimum pressures: • 20 psi under maximum day demand (MDD) plus Fire Flow demand conditions, and • 50 psi under peak hour demand (PHD). For the purposes of this deviation request, it is assumed the minimum water pressures for water mains must be met at the clubhouse water service meter for the proposed extended water service connection. The following values for MDD and PHD for the project area are established in Table 3.1 of the Alder Ridge & Sage Peak Apartments Water Design Report prepared by Kimley-Horn dated June 2025 (2025 Water Design Report): • MDD for Total Project Area: 215 gpm • PHD for Total Project Area: 274 gpm The following value for Fire Flow demand requirement is per Table 4.2.4 of the 2024 City Design Standards for medium and high-density residential land use: • Fire Flow Demand for Project Area: 3,000 gpm Therefore, the MDD plus Fire Flow condition for the project area has a demand flow rate of 3,215 gpm. The available static pressure measured at Hydrant 2870 is used to establish the available system pressure at the water main service connection under the required system conditions. The static pressure available at Hydrant 2870 is 54.4 psi based on a reading taken on December 13th, 2023 documented in Appendix B of the 2025 Water Design Report. Hydrant 2870 is at an elevation that is approximately same as the proposed water service connection being evaluated, so no major differences in system operating pressures are anticipated. Figure 1 is from the 2025 Water Design Report and shows the calculated hydrant curve for Hydrant 2870. Residual pressures for the PHD condition and MDD plus Fire Flow conditions are identified in Figure 1. Figure 1. Hydrant 2870 Curve with Calculated Residual Pressures at Different Operating Conditions During a flow condition of 400 gpm, which is slightly higher than the PHD operating condition, the residual system pressure is anticipated to be approximately 53.6 psi. During a flow condition of 3,400 gpm, which is slightly higher than the MDD plus Fire Flow operating condition, the residual system pressure is anticipated to be approximately 41.3 psi. 1.3. WATER SERVICE LINE HEAD LOSS CALCULATION The total length of the proposed water service line is 510 linear feet (LF) as shown in Sheet C4.00 attached to this memorandum. The clubhouse will have laundry facilities, fitness equipment and offices. There is no fire suppression system for the clubhouse. To be conservative, a maximum water demand of 20 gallons per minute (gpm) is used for the head loss calculations. This flow rate assumes the following number of water fixtures to be served at the clubhouse, their operating flow rates, and that all fixtures would be in use simultaneously, which is very conservative and an unlikely operating condition: • 1 service sink = 1.5 gpm assumed per fixture x 1 fixture = 1.5 gpm • 1 drinking fountain = 0.75 gpm assumed per fixture x 1 fixture = 0.75 gpm • 2 water closets = 1.28 gallons per flush (assumed 1 flush per minute) x 2 fixtures = 2.6 gpm • 2 public lavatories = 0.5 gpm per fixture x 2 fixtures = 1 gpm • 5 laundry washing machines = 2 gpm per fixture x 5 fixtures = 10 gpm • Total Demand = 15.9 gpm rounded up to 20 gpm Head loss due to friction in the water service line from the point of connection at the water main to the proposed internal water meter at the clubhouse is calculated with the Hazen-Williams equation. The Hazen- Williams equation is defined as: H= 0.2083 x (100/C)1.852 x Q1.852 ÷ D4.865 x (L/100) Where: H = friction head loss (ft) C = Hazen-Williams coefficient (no units) Q = flow rate (gpm) D = internal pipe diameter (inches) L = length of pipe (ft) Parameters used to calculate friction loss are summarized in Table 1 along with the resulting calculated head loss presented in both feet of head loss and psi of pressure loss. Table 1. Parameters for Hazen-Williams Calculation and Total Calculated Friction Loss Parameter Value Pipe Type Type K Copper Tubing Pipe Nominal Size 2-inch diameter Pipe Internal Diameter (D) 1.985 inches Pipe Length (L) 510 ft Flow Rate (Q) 20 gpm (0.045 cfs) Hazen-Williams Coefficient (C) 140 Calculated Friction Head Loss (H) 5.2 ft (2.25 psi) H= 0.2083 x (100/140)1.852 x (201.852) ÷ (1.9854.865) x (510/100) = 5.2 ft ÷ 2.31 = 2.25 psi Minor losses are calculated with the following equation: H= K x V2 ÷ 2 x g Where: H = head loss (ft) K = minor loss coefficient (no units) V = velocity (feet per second, fps) g = acceleration due to gravity (32.2 ft/s2) The velocity of flow in the water service line is calculated as follows: V= Q ÷ A Where: V = velocity (feet per second, fps) Q = flow rate (cubic feet per second, cfs) A = area of pipe (square feet) calculated as Pi (π) x Pipe Radius squared (r2) V= 0.045 cfs ÷ π x (1.985 in. diam ÷ 12 inches ÷2)2 = 2.07 fps Parameters used to calculate minor losses are summarized in Table 2 along with the resulting calculated head loss presented in both feet of head loss and psi of pressure loss. Table 2. Parameters for Minor Losses Calculation and Total Calculated Minor Losses Type of Fitting (assumed) K Value Minor Losses Calculated Per Fitting (ft) (H=K*V2/2g) No. of Fittings (assumed) Calculated Minor Losses (ft) Tee, Branch Flow (Connection to Water Main) 0.8 0.053 1 0.053 Ball Valve, Fully Open (Isolation Valves) 0.05 0.003 2 0.007 90-Degree Bends 0.3 0.020 6 0.120 Check Valve 2 0.134 1 0.134 Water Meter 7 0.467 1 0.467 Total Minor Losses 0.78 ft (0.34 psi) Total head loss in the water service line is the sum of the friction loss (5.2 ft or 2.25 psi) and minor losses (0.78 ft or 0.34 psi), which is 5.98 feet or 2.59 psi. 1.4. RESULTS During the PHD operating condition, the residual system pressure is anticipated to be approximately 53.6 psi at the water service connection point and a total of 2.59 psi of pressure loss will occur in the water service line. Therefore, the available water pressure at the clubhouse water service meter is anticipated to be 51 psi during the PHD operating condition. This exceeds the required minimum pressure of 50 psi during the PHD operating condition. During the MDD plus Fire Flow operating condition, the residual system pressure is anticipated to be approximately 41.3 psi at the water service connection point and a total of 2.59 psi of pressure loss will occur in the water service line. Therefore, the available water pressure at the clubhouse water service meter is anticipated to be 38.7 psi during the MDD plus Fire Flow operating condition. This exceeds the required minimum pressure of 20 psi during the MDD plus Fire Flow operating condition. Overall, the calculations demonstrate that the required minimum service pressures will be met at the clubhouse water meter, without the need for a water main extension. Taylor Bellefeuille, PE D D D D D D D D S S SSS S S S HH HH HHHH HH HHDYH DYH DYH DYH TOP NUT EL=4975.33 W W W W W W W W W WW W W W WWWWWWWWWW WWWWWWW W W SSSSSSSSSSSSSSSDSDSDSD SD SD SD SDSDSDSDSDSDSDSDSDSDSD SD SD SD SD SD UGE UGEUGE UGE UGEUG E UGE UGEUGE UGEU G E UGE UGE UGE UGE UGE UGE UGE UGE UGE UGE UGE UGE UGE TOP NUT EL=4974.11 TOP NUT EL=4973.16 S S S D D T T TE EE SANITARY MH RIM EL=4980.71 FULL OF WATER SANITARY MH RIM EL=4979.93 8" INV W=4974.07 8" INV S=4974.28WATER WATER WATER WATER WATER WATER WATER WATERMECHWATER / RISER ROOM BUILDING 1 BUILDING 3 BUILDING 7BUILDING 8 BUILDING 4BUILDING 5BUILDING 6BUILDING 10BUILDING 9BUILDING 2 CLUBHOUSE STAUDAHER ST SOUTH 15TH AVEWEST GRAF STREET 10.0' BUILDING SETBACK 10.0' BUILDING SETBACK 10.0' BUILDING SETBACK 10.0' BUILDING SETBACK NWE 10.0' UTILITY EASEMENT SOUTH 17TH AVENORTH ©ALDER RIDGE & SAGE PEAKAPARTMENTSPREPARED FORGRAF STREETAPARTMENTS, LLC.® BEFORE YOU DIG, CALL 1-800-424-5555 C4.00OVERALL UTILITYPLANUTILITY LEGEND PROPERTY LINE SETBACK LINE PROPOSED LOT LINE PROPOSED EASEMENT PROPOSED 100' HYDRANT OFFSET PROPOSED PVC SANITARY SEWER PIPE @ 1.05% MIN SLOPE PROPOSED WATER PIPE PROPOSED STORM DRAIN PROPOSED SANITARY MANHOLE (PER DETAIL NO 02720-3) SANITARY SEWER CLEANOUT (PER DETAIL NO 02730-4) PROPOSED STORM DRAIN MANHOLE (PER DETAIL NO 02720-3) PROPOSED SQUARE STORM DRAIN INLET (PER DETAIL NO 02720-1A) PROPOSED STANDARD RISER INLET (PER DETAIL NO 02720-2) PROPOSED CURB STOP AND BOX PROPOSED POST INDICATOR VALVE PROPOSED FIRE DEPARTMENT CONNECTION PROPOSED FIRE HYDRANT ASSEMBLY (PER DETAIL NO 02660-4) PROPOSED BUILDING INTERNAL 3" WATER METER PIPE FLOW DIRECTION PROPOSED LIGHT POLE BOZEMAN FACILITIES MAP