A hydraulic computer model is a digital representation of physical features and characteristics of the water system, including sources, pipes, valves, storage tanks, and pumps. Key physical components of a water system are represented by a set of user-defined parameters that represent the characteristics of the system. The computer model utilizes the digital representation of physical system characteristics to mathematically simulate operating conditions of a water distribution system. Computer model output includes pressures at each node, flow rate for each pipe in the water system, and water surface levels in storage tanks.
The City maintains a water model of its system and has used this model for previous master planning. A copy of this model was provided to BC&A for evaluation of the existing and future water system. The City's existing model was developed as an EPANET model using the InfoWater software developed by Innovyze.
To update the model for this study, recently constructed pipes were added to the model and existing pipe sizes were checked. Demand scenarios were updated based on the analysis contained in this report and the model was calibrated. The City was previously unable to export raw data from their SCADA system, but screenshots of tank levels and system pressures were used to check the calibration of the model and found the model to be reasonably calibrated.
The City's hydraulic model is setup to run extended period simulations. The model results that are most useful for evaluating the distribution system performance include operating conditions for several conditions: peak day demands with fire flow, peak hour demands, and buildout peak demands with improvements. Model results for the following scenarios have been documented to aid in evaluating system performance.
• Peak Day Demand - This scenario represents the average daily demands on the system during the peak usage day of the year. This scenario is primarily used to simulate fire flows to identify areas that do not meet fire flow requirements. It can also be used to identify source deficiencies within tank service areas to determine if sufficient production and conveyance capacity exists to fill and drain tanks properly during peak demands.
• Peak Hour Demands - The purpose of this scenario is to identify existing pressure deficiencies under peak hour demand conditions. For the culinary water system, a peak hour to peak day peaking factor of 1.45 was used for indoor demands and 2.27 for outdoor demands based on the City's historical diurnal patterns. As an extended period model simulation, these patterns are applied to the average demand for the peak day and minimum pressures throughout the simulation are used for evaluation.
• Buildout Peak Demands with Improvements - This scenario includes an extend period model that captures both peak day and peak hour demands. It includes additional piping and infrastructure to accommodate growth of demands in currently undeveloped areas. Originally the improvements that were proposed as part of the 2016 Master Plan were included and then evaluated and adjusted as needed. Modeling results from this scenario will be detailed in the following chapter.
The performance of the system was evaluated using the following criteria:
• Pressure within the system during peak demands - The state of Utah requires that a public water system maintain a minimum pressure standard of 30 psi during peak hour demands and 40 psi during peak day demands. This is the minimum design standard the City maintains. However, the City tries to maintain pressures above 50 psi for both peak day and peak hour demands in most of the distribution system. It only makes exceptions for areas with topography challenges that would require excessive additional pressure zones to otherwise resolve.
• Pressure within the system during peak day demands with fire flow - The State of Utah requires that a public water system be capable of conveying required fire flow with a residual pressure of 20 psi. Any node in a residential area incapable of supplying 1,000 gpm with a 20 psi residual was identified as deficient. It should be noted that 1,000 gpm was the typical residential fire requirement up until the 1990s, when fire code and increasing home sizes increased the recommended fire flow to 1,500 gpm. Large portions of the City were constructed prior to this change and are satisfactorily served by available fire flow of 1,000 gpm. For this reason, 1,000 gpm has been used for evaluation generally, but new construction and larger residential properties will need to meet the 1,500 gpm requirement. Commercial areas were evaluated using a specific list of requirements which ranged between 2,000 gpm and 4,000 gpm with a 20 psi residual.
• Maximum pipe velocities - High instantaneous velocities in a pipeline are not generally as much of a concern to the system as low pressures. However, they can help indicate areas where additional conveyance improvements will have the most benefit. Pipelines with velocities above 7 ft/sec have been identified to indicate areas where additional conveyance improvements would be beneficial. However, they have not specifically been identified as a deficiency unless they have a maximum velocity greater than 10 ft/sec during peak hour demands. In this case, the extreme velocity can cause damage to pipe linings, increase potential for hydraulic surges, and potentially lead to pipe failure.
The hydraulic computer model was used to simulate system conditions for existing demands. The model was set up as an extended period peak day model to capture both peak day and peak hour demand periods. Peak day demands with fire flow were also modeled. Model results for these scenarios are included in the following figures:
1. Figure 7-1 shows minimum pressures for the existing water system with peak day demands.
a. Zone 1 has minimum pressures greater than 120 psi along the eastern edge near the Jordan River. This area typically ranges between 120 psi and 150 psi. While the City prefers pressures to remain under 120 psi, it is not a requirement. All service lines in the area should be equipped with a functioning service line PRV.
b. Along the western edge of Zone 1, west of 2700 W, minimum pressures drop below 50 psi. While pressures are lower than the desired range, they are not in violation of State requirements.
c. The southwest corner of Zone 1 also has pressures that drop below 50 psi. This area can range between 43 psi and 55 psi, primarily due to its elevation along the western boundary of the zone.
d. In Zone 4, pressures are above 120 psi north of 7800 S, along the eastern pressure zone boundary. This area ranges between 120 psi and 140 psi. While the City prefers pressures to remain under 120 psi, it is not a requirement. All service lines in the area should be equipped with a functioning service line PRV.
e. The northern half of Zone 5 along the western pressure zone boundary have pressures that drop below 50 psi. This area can drop to 45 psi during the peak of the day.
2. Figure 7-2 shows maximum pipe velocities for the existing water system with peak day demands.
a. A large majority of the pipes in the City's water system have velocities under 5 ft/second.
3. Figure 7-3 shows available fire flows during the peak day of demand with a residual pressure of 20 psi.
a. There are a few areas of the distribution system that do not meet fire flow requirements. In general, most fire flow deficiencies are caused by the following concerns:
i. High Elevation - Junctions near the upper end of pressure zones will have difficulty meeting fire flow requirements without large supply pipes and looping.
ii. Dead-Ends - Dead end connections often have fire flow deficiencies because high velocities through a single pipe cause higher pressure losses. Dead-end connections frequently require oversized pipes to meet fire flow requirements unless the connection can be looped another way.
iii. 6-inch and Smaller Pipes - The City has a few areas that cannot meet fire flow demands due to small pipes. Many cul-de-sacs are fed by a 6-inch, dead end pipe. If the small pipe is in a long street, or a neighborhood completely fed by small pipes, fire flows at the furthest junction may not be able to meet the 1,000 gpm requirement.
b. One non-residential location, West Jordan Elementary School, is not able to meet the specified 3,250 gpm fire flow requirement. The model shows the current available fire flow at just over 1,600 gpm.
c. Residential areas that are not able to meet the fire flow requirement include:
i. 9240 S at Edenbrooke Way
ii. Executive Drive, from 7000 S to 7265 S
iii. Mcgregor Lane at Dunlop Drive
iv. Wood Cove Drive at 2940 West
v. Beverly Glen Avenue at 2470 West
vi. 2980 West at 7140 South
