3. WATER SYSTEM ANALYSIS

3.1 WATER SYSTEM DESCRIPTION

The City’s water distribution system is divided into four major pressure zones and includes approximately 75 linear miles of pipeline. The water system is supplied by five turnouts from the San Francisco Public Utilities Commission’s (SFPUC) Regional Water System. The Specific Plan Area is located in the City’s Pressure Zone (Zone) 4, which serves the lower elevation areas by the San Francisco Bay. Zone 4 is supplied by four SFPUC turnouts, located on Murchison Drive, El Camino Real at Victoria Avenue, Magnolia Avenue and Green Hills Drive. The turnout at El Camino Real and Victoria Avenue has the highest capacity and serves the Specific Plan Area from the south.

Figure 3-1 shows the existing potable water pipelines within the City’s system. The Specific Plan Area is served by pipelines that range from 6-inch diameter to 12-inch diameter, with the majority of the area served by 8-inch diameter and 10-inch diameter pipelines.

3.2 HYDRAULIC MODEL UPDATES

As part of the 2015 WMP, the hydraulic model of the City’s water system was updated and validated. The existing demands are based on 2012 water consumption data and the 2035 buildout demands are based on future demand projections as part of the City’s 2010 Urban Water Management Plan. The existing demands in the hydraulic model were updated to reflect the pipeline projects described in Section 2.2. The pipeline projects are near-term developments that are to be assumed as part of the existing system conditions for the purposes of this evaluation. The estimated buildout demands in the hydraulic model were updated to reflect the Specific Plan intensification by 2035. Due to the uncertainty of how water demands were allocated during the 2015 WMP for buildout conditions, a conservative assumption was made to replace previously allocated water demands that were located fully within the Specific Plan Area with the new water demand estimates listed in Table 2-3. For proposed buildings that would most likely be served from pipelines along the boundary of the Specific Plan, previously allocated demands were reduced by half and then the demands for the Specific Plan were added for the proposed building on the Specific Plan boundary. This is to account for demands from nearby buildings that are outside of the Specific Plan Area with no current plans of redevelopment but may be served by the same pipeline. The original 2035 maximum day demand within the Specific Plan Area was approximately 1.2 mgd based on the 2015 WMP demand allocation. After using the demand update methodology, the net increase to the 2035 maximum day demand with the Specific Plan land use intensification is estimated to be 0.6 mgd.

The same assumption was used for updating the existing demands with the pipeline projects. If water demands were already allocated to a junction in the hydraulic model, the demand was reduced by half first and then the pipeline project demand was added. For junctions with no existing demand, the demand for the pipeline project was added. The demands for the Pipeline Projects were allocated to junctions that either had no demand or a very small demand. A few of the Pipeline Projects are located outside of the Specific Plan boundary and therefore, the net increase to the systemwide existing maximum day demands was estimated to be 0.6 mgd.

FIGURE 3-1. MILLBRAE WATER SYSTEM OVERVIEW

Sym bo ogy Water System Fac ties ¼Ð Ú Pu mp St at o n

Press u re Red u cing Va v e s ting P pel ne s, Zone

3.3 METHODOLOGY AND ANALYSIS CRITERIA

The City’s InfoWater hydraulic model was used to summarize the existing system conditions within the Specific Plan Area and to evaluate the ability of the existing system to meet the Specific Plan demands at buildout conditions while meeting the City’s water performance criteria.

Table 3-1 summarizes the water performance criteria obtained from the 2015 WMP and discussions with the City’s Fire Marshal.

TABLE 3-1. SUMMARY OF WATER SYSTEM SERVICE AND PERFORMANCE CRITERIA (C)

Component Criteria

(A) City of Millbrae. 2015 Water Master Plan. Chapter 5 Evaluation Criteria, except for fire flow requirements, see footnotes (b) and (c)

(B) Existing non-sprinklered single family residential, commercial, and multi-family residential minimum flow requirement.

(C) Minimum fire flow requirement for new development, selected in consultation with the City based on recent new development requirements in the Specific Plan Area.

3.3.1 HYDRAULIC MODELING METHODOLOGY

To evaluate distribution system performance, steady state hydraulic modeling evaluations were conducted to identify projected future capacity deficiencies. The following scenarios were evaluated using the City’s hydraulic model under both Maximum Day Demand plus Fire Flow and Peak Hour Demand conditions:

• Scenario 1 – Existing System. This scenario presents results with existing water demands (including demands for pipeline projects identified in Chapter 2) and existing infrastructure and assumes that the Specific Plan has not been constructed. This scenario represents existing baseline conditions for the Specific Plan Environmental Impact Report analysis.

• Scenario 2 – Buildout of the Specific Plan. This scenario assumes anticipated development within the Specific Plan with 2035 demand conditions elsewhere in the system and existing infrastructure.

• Scenario 3 – Buildout of the Specific Plan with Proposed Improvements. This scenario assumes anticipated development within the Specific Plan with 2035 demand conditions elsewhere in the system and proposed infrastructure to address capacity deficiencies.

3.4 ANALYSIS RESULTS

The following sections discuss the hydraulic model results and distribution system deficiencies identified in each scenario.

3.4.1 EXISTING SYSTEM ANALYSIS (SCENARIO 1)

Figure 3-2 shows the available fire flow for the existing system at each tested junction while maintaining a minimum residual pressure of 20 psi, and a maximum pipeline velocity of 10 fps within Zone 4. Results show available fire flow ranging from less than 1,500 gpm to greater than 3,000 gpm within the Specific Plan Area. The available fire flow at various locations is constrained by the velocity criterion of 10 fps along the smaller diameter pipelines. Results for the existing system analysis are reflective of the fact that much of the City’s water system is older, designed to earlier fire standards. Fire flows also meet minimum requirements for existing residential and commercial development, per the 2019 Fire Code.

Figure 3-3 shows pressures and pipeline velocities for the existing system during peak hour demands. Hydraulic modeling results indicate that the peak hour demand pressures within the Specific Plan Area are greater than 80 psi. Pipelines within the Specific Plan Area have velocities that are less than 7 fps during peak hour demand analysis.

3.4.2 SPECIFIC PLAN BUILDOUT SYSTEM ANALYSIS (SCENARIO 2)

A hydraulic performance evaluation was conducted to assess the existing distribution system’s capacity and ability to convey flows, while meeting operational and performance criteria for maximum day plus fire flow and peak hour demand scenarios to meet the demands under the Specific Plan Buildout conditions.

Figure 3-4 presents the available fire flow at each tested junction while maintaining a minimum residual pressure of 20 psi, and a maximum pipeline velocity of 10 fps within Zone 4. As shown on Figure 3-4, results indicate that significant deficiencies exist throughout the Specific Plan Area due to the increased fire flow demand requirement for new development. Review of the hydraulic modeling results indicate that the deficiencies are generally caused by the inability to meet the velocity criterion in the 6-inch diameter and 8-inch diameter pipelines located in El Camino Real and adjoining streets, and pressure criteria at high elevation demand nodes in northern and western side of Zone 4.

Figure 3-5 presents pressures and velocities for the existing distribution system while meeting peak hour demands at buildout within the Specific Plan Area. Hydraulic modeling results indicate that the peak hour demand pressures within the Specific Plan Area are greater than 80 psi and meet the minimum pressure criterion of 40 psi for normal operating conditions. All pipelines within the Specific Plan Area have velocities that are less than 7 fps.

|| Downtown and El Camino Real Specific Plan | City of Millbrae

FIGURE 3-2. MILLBRAE WATER SYSTEM EXISTING MAX DAY DEMAND AVAILABLE FLOW FIRE FLOW

FIGURE 3-3. MILLBRAE WATER SYSTEM EXISTING PEAK HOUR DEMAND PRESSURES AND VELOCITIES

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FIGURE 3-4. MILLBRAE WATER SYSTEM BUILDOUT MAX DAY DEMAND AVAILABLE FLOW FIRE FLOW

FIGURE 3-5. MILLBRAE WATER SYSTEM BUILDOUT PEAK HOUR DEMAND PRESSURES AND VELOCITIES