Memorandum - Beaverton, Oregon

20-2755 of 7 Cooper Mountain Utility Plan January 2021 City of Beaverton

Memorandum

Date: January 26, 2021

Project: Cooper Mountain Utility Plan (CMUP)

To: Sheila Sahu, PE, LEED AP BD+C City of Beaverton

From: Kevin Cook, PE Murraysmith

Reviewed By: Heidi Springer, PE Murraysmith

Re: Stormwater Alternatives Executive Summary

Executive Summary

The Cooper Mountain Utility Plan (CMUP) team developed utility alternatives to support planning consistent with the Cooper Mountain Community Plan, which is being developed in parallel. These parallel efforts help ensure that the CMUP supports the goals and objectives for existing natural features and future land use identified by the Community Plan.

Background Data

The Cooper Mountain project area has several constraints such as steep slopes, landslide hazard areas, water bodies, wetlands, floodplains, Metro Title 3 Land, Title 13 riparian habitats and wildlife corridors, and, upon annexation to the Clean Water Services (CWS), Vegetated Corridors as outlined in the CWS Design and Construction Standards. This information was summarized as part of the Community Plan Natural Resources Inventory and used to identify developable land within the Community Plan Draft Buildable Lands Inventory (BLI) (City of Beaverton, 2020). The 2014 Land Use Framework was used in conjunction with the draft BLI to estimate changes in land use and impervious area (City of Beaverton, 2014).

The Natural Resources Inventory was also used to evaluate existing stream corridor conditions to determine the suitability of incorporating stream corridors as part of overall stormwater management within Cooper Mountain. Existing stream conditions in some areas are already degraded or in degrading states. Enhancing stream conditions and incorporating streams as part

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of stormwater alternatives were evaluated and identified as potential resilient stream corridors (RSCs).

Regulatory Standards

Development within the project area requires compliance with applicable City, CWS, State, and national regulatory stormwater standards. These standards are related to managing water quality and quantity due to hydromodification, required stormwater controls, relevant design criteria, floodplain mapping, land use ordinances, and protection of wetlands and habitats in waterways. Permitting considerations are discussed in more detail in the Cooper Mountain Utility Plan Stormwater Alternatives Development Technical Memorandum (Murraysmith, 2020).

Alternatives Development

Development criteria and regulatory standards and discussions with the Community Plan team, City staff, and CWS staff recommended a regional stormwater strategy (RSS) approach for Cooper Mountain. Several conventional stormwater management techniques were considered as part of the RSS as well as non-traditional stormwater management approach utilizing RSCs.

▪ Low Impact Development Approach (LIDA) – LIDA facilities provide an opportunity for reduced impervious area and water quality and quantity benefits, through either retention and infiltration or detention. Several techniques were considered including green roofs, porous pavement, flow-through planters, infiltration planters/rain gardens, vegetated filter strips, and swales. Challenges to incorporating LIDA facilities include space constraints in dense urban environments and maintenance in both private and public settings.

▪ Detention Facilities – Detention facilities provide storage of stormwater runoff before discharging via flow control outlets to receiving conveyance or downstream water bodies. These non-, or low-infiltrating facilities provide peak flow attenuation and water quality benefits. Detention can be provided aboveground as a pond or underground in piping. Water quality treatment must be provided upstream or as part of the detention facility. These facilities can be used in both project-specific and RSS approaches.

▪ Piped Conveyance – Piped conveyance provides a backbone stormwater management system to convey flows to and from existing and proposed stormwater infrastructure and receiving waters by using existing topography and natural flow paths.

▪ Resilient Stream Corridors – RSCs are topographic changes to the receiving streams that provide natural processes to dissipate stream flow energy through channel and floodplain roughening and by maximizing floodplain engagement. This technique aids in maintaining and restoring functional stream corridors, while also acting as conduits that provide community infrastructure resources, such as trails and sewer. RSCs are tailored to specific stream reaches, depending on existing channel conditions, slope and topography, as well


as upland development, and require water quality treatment prior to discharge to RSCs. More information regarding RSCs can be found in the Resilient Stream Corridor Handout.

Proposed Alternatives

Conceptual planning based on review of proposed development and constraints within Cooper Mountain offers two approaches to be explored in future analyses. Both offer flexibility for implementation but also may have unique site-by-site applications. The two approaches outlined below provide an overview for comparative purposes.

Regional Stormwater Facility with Detention and LIDA

Strategic locations for detention facilities were identified to minimize the number of individual facilities in Cooper Mountain and were based on existing topographic conditions to maximize upstream contributing area. Facility size was dependent on upland conditions and contributing area and ease of connection to existing stormwater infrastructure and receiving streams. Large regional detention ponds are unlikely to be able to manage all stormwater runoff given topographic limitations as well as practical application for isolated areas of development. Areas not served by large regional detention ponds would be served by smaller on-site facilities. Figure 1 shows conceptual locations of detention facilities as well as conceptual LIDA locations in the right-of-way. LIDA could also be placed in on private property as part of land use planning and development.

Regional Stormwater Facility with Resilient Stream Corridors and LIDA

With the exception of the Cooper Mountain Nature Park, RSCs were conceptually proposed within the major creek systems in Cooper Mountain. Numerous treatments were identified to engage the full potential of a floodplain based on slope and streamflow. Figure 2 shows a conceptual overview of LIDA in the right-of-way and RSCs. LIDA could also be placed in on private property as part of land use planning.

Climate Change

Climate change resiliency was accounted for with respect to impact on proposed stormwater management alternatives. Changes in climate can lead to increased frequency of higher intensity and larger volume storms and have potential to make existing design standards for peak flow and flow duration standards less effective due to changes in precipitation and the resulting stormwater flows. Sizing of traditional stormwater facilities (LIDA, detention, piped conveyance) provide a first step in assessing feasibility of stormwater management.

RSCs provide a stormwater management approach that offer benefits to create a more resilient RSS by improving conditions of an area that is already at risk due to existing precipitation. This process builds upon existing design standards for traditional stormwater approaches and provides a proactive mechanism and strategy to manage effects on stormwater due to climate change.


Next Steps

RSCs have been discussed as an option for managing stormwater in Cooper Mountain. As further analysis on a land development framework is established, incorporating upland stormwater management techniques and quantifying impacts to receiving streams can be updated to ensure that facilities will be sized appropriately. Detailed connectivity of stormwater facilities will also be completed to ensure that developed areas are sufficiently served and that upland water quality treatment requirements are met. Further analysis is recommended for the following:

▪ Analysis of impacts to receiving water downstream of proposed developed areas within Cooper Mountain. Changes made via RSCs that could negatively impact downstream receiving waters and areas outside of the project area may be required to be included as part of an RSS.

▪ Field investigation of sensitive areas within stream corridors, and evaluation of upstream and/or downstream impacts, based on site observations and modeling results to assess if additional upland or in-stream treatments are required.

▪ Detailed stormwater facility layout for entire Cooper Mountain area based on adopted? conceptual land development. This layout should consider property acquisition, offsite tributary areas, and include contingencies to address changes to development and standards or as additional information is received.

▪ Financial analysis of proposed stormwater techniques to further quantify conventional versus non-conventional stormwater management techniques. The financial analysis would include capital, development and long-term operational costs.

▪ A phased approach for implementing an RSS given the unique topographic challenges, development timing, and variance and location of developable areas within Cooper Mountain.


Figure 1: Regional Stormwater Facility – LIDA and Detention


Figure 2: Regional Stormwater Facility – LIDA and Resilient Stream Corridors


References

City of Beaverton. (2014, December). South Cooper Mountain Concept Plan.

City of Beaverton. (2020, June). Draft Buildable Lands Inventory.

Murraysmith. (2020, December). Cooper Mountain Utility Plan Stormwater Alternatives Development (Draft). Prepared for the City of Beaverton.

20-2755 Page 1 of 8 CMUP – Preliminary Sewer Alternatives January 2021 City of Beaverton

Memorandum


Project: Cooper Mountain Utility Plan (CMUP)

To: Sheila Sahu, P.E. City of Beaverton

From: Andrew Henson, P.E. Murraysmith

Reviewed by: Heidi Springer, PE Murraysmith

Re: Sewer Alternatives Executive Summary

Introduction

This memorandum summarizes the development of the preferred and alternative sanitary sewer alignments for the Cooper Mountain Utility Plan (CMUP). These alignments are being developed in coordination with Cooper Mountain Community Plan (currently being developed in parallel) to ensure the CMUP is consistent with the goals and objectives identified in the Community Plan. More detailed discussion of these alignments can be found in the preliminary sewer alignments technical memorandum (Murraysmith, 2020a). The following are described at a high level (i.e. in generalities) in the sections below:

• Planning criteria, design criteria, and assumptions.

• Proposed connection points to existing or planned sanitary sewer systems.

• The preferred and alternative alignments.

• Next steps.

The preferred and alternative sewer alignments are intended to provide sewer service to new and existing developments within Cooper Mountain. This area is not currently served by a public sewer system. However, areas to the east and northeast of Cooper Mountain are currently served by public sewer systems and sewer service has now been planned for the South Cooper Mountain area. The preferred and alternative alignments rely on gravity conveyance and will connect to existing sewers and planned sewers, including the Tile Flat Pump Station (planned to be built by Clean Water Services (CWS) in 2024) that will be located southwest of Cooper Mountain. Both

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sewer alignments consider potential inflow from existing and future developments in the North Cooper Mountain (NCM) area.

Assumptions

Assumptions for the development of sanitary sewer alignments were documented in the Vision Statement and Guiding Principles memorandum (Murraysmith, 2020b). These assumptions include planning-level requirements related to placement of alignments, average daily flow rates based on development type, methods and assumptions for calculating rainfall-derived infiltration and inflow (RDII), and integration with systems in surrounding areas.

The Buildable Lands Inventory (BLI) dataset categorized the project area by percent constrained and development status and were used to determine the amount of developable land in each parcel (City of Beaverton, 2020). The percent constrained describes the percentage of area in each parcel that is physically constrained and considered unbuildable and the development status describes the level of existing or planned development in each parcel. The results of the BLI analysis were used as the basis for computing average daily flows and rainfall derived infiltration and inflow (RDII). The distribution of developable land also informed the location of the alignments.

Average daily sewer flows were based on estimates of development densities and the associated average daily sewer flows used in the South Cooper Mountain Sanitary Sewer Master Plan (David Evans and Associates, 2015). RDII for CMUP project area and the NCM area were computed as described in the South Cooper Mountain System Analysis Technical Memorandum (HDR, 2019b).

Design Criteria

The proposed alignments were designed based on the CWS design criteria (Clean Water Services, 2017) and the 2019 City of Beaverton Sewer Master Plan Update (HDR, 2019a). Table 1 provides a summary of design criteria used in the development of the proposed alignments.


Table 1: CWS and City of Beaverton Design Criteria

Design Criteria Value

Minimum Pipe Velocity (ft/s)1 2.0

Maximum depth/diameter (d/D) 0.8

Minimum Pipe Cover – Paved Areas (in)2 48

Minimum Pipe Cover – Un-Paved Areas (in)2 36

Manning’s “n” 0.013

Minimum Pipe Diameter (in) 8

Minimum Slope3 0.4%

Maximum Slope4 20%

Downstream Pipe Offset (ft) 0.2

1 – When flowing full or half full. 2 – The “other” material category was used for selecting minimum cover to allow for the use of seismically resilient materials. 3 – Minimum slope is for the minimum pipe diameter of 8 inches. All pipes, regardless of diameter, were designed to the minimum slope. 4 – Maximum slope without the use of anchor walls or metal pipe slope anchors.

Proposed Alignments

Two preliminary sewer alignments, preferred and alternative, were developed to provide sewer service to Cooper Mountain. The following sections describe these alignments.

Preferred Alignment

The preferred alignment is a combination of alignments located in resilient stream corridors (RSCs), where possible, and alignments along roadways. The location of alignments is primarily driven by the feasibility of RSCs and the BLI and were sited within RSCs identified by the stormwater team. Alignments along roadways were sited based on existing and planned roads and the available road profiles provided by the Cooper Mountain Community Plan (CMCP). Placing the sewer alignment in the RSCs allows the collection system to be at lower elevations than developable land, thereby facilitating gravity flow. The preferred alignment also provides more options for crossing McKernan Creek but creates unconventional maintenance and access for pipes located within the RSCs.

The preferred alignment is subject to refinement as the road corridors and stormwater management strategy for the study area continue to evolve. The preferred alignment includes an


extension of the existing sewer along Summer Creek which faces physical challenges due to topography as well as hydraulic constraints in the downstream system. If an extension of the existing sewer in this area is deemed infeasible, then serving this portion of Cooper Mountain by gravity conveyance would pose additional challenges. Alignments within RSCs comprise a significant portion of the preferred alternative. If RSCs are determined to be infeasible, the alternative alignment, described in the following section, would become the primary alignment.

The preferred alignment conveys flow to previously planned connection points in the existing or planned systems on the eastern and southern boundaries of the Cooper Mountain project area and to the planned Tile Flat Pump Station located in the southwest corner of the project area. Future analysis will evaluate the estimated average daily sewer flows and RDII from Cooper Mountain to ensure future flows align with the previously planned flow at the proposed connection points. The preferred alignment, developable lands, and the proposed connection points are shown in Figure 1.


Figure 1: Preferred Sewer Alignment with Developable Lands


Alternative Alignment

The alternative alignment is primarily sited along roadways, with an additional alignment along Summer Creek. The location of the alignments prioritizes roadways where possible; the alignment along Summer Creek is necessary to serve this portion of the project area via gravity conveyance. Alignments along roadways were sited based on existing and planned roads and the available road profiles provided by the CMCP. The alternative alignment provides conventional sewer alignments and ease of maintenance. However, crossing McKernan Creek using gravity conveyance creates challenges, with some stretches of the alignment sited relatively deep to alignments elsewhere in the project area.

The alternative alignment is subject to refinement as the road corridors and stormwater management strategy for the study area continue to evolve. The alternative alignment also includes an extension of the existing sewer along Summer Creek which is challenging as noted in the previous section. If an extension of the existing sewer in this area is deemed infeasible, serving this portion of Cooper Mountain via gravity conveyance would be difficult.

The alternative alignment conveys flow to previously planned connection points in the existing or planned systems on the eastern and southern boundaries of the Cooper Mountain project area and to the planned Tile Flat Pump Station located in the southwest corner of the project area. Future analysis will evaluate the estimated average daily sewer flows and RDII from Cooper Mountain to ensure future flows align with the previously planned flow at the proposed connection points. The alternative alignment, developable lands, and the existing connection points are shown in Figure 2.


Figure 2: Alternative Sewer Alignment with Developable Lands


Next Steps

The next steps for the proposed sewer alignments include:

• Coordination with the Cooper Mountain Community Plan to incorporate revisions to planned roadways and trails.

• Continued study of the proposed gravity sewer along Summer Creek.

• Coordination with the stormwater team for the evaluation and selection of RSCs which impact the proposed alignments.

• Coordination with the City of Beaverton to incorporate revisions to the BLI and development plans to the estimated sewer flows.

References

City of Beaverton. (2020). Draft Buildable Lands Inventory.

Clean Water Services. (2017). Design and Construction Standards For Sanitary Sewer and Surface Water Management.

David Evans and Associates. (2015). City of Beaverton South Cooper Mountain Phase 1 - Sanitary Sewer Master Plan.

HDR. (2019a). City of Beaverton Sewer Master Plan Update.

HDR. (2019b). South Cooper Mountain and Rural Area Sewer System Analysis.

Murraysmith. (2020a). Sewer Alternatives Development.

Murraysmith. (2020b). Subtask 2.3, 3.3, and 4.3: Vision Statement and Guiding Principles – FINAL.

20-2755 Page 1 of 4 CMUP Water Executive Summary January 2021 City of Beaverton

Memorandum


Project: Beaverton Cooper Mountain Utility Plan (20-2755)

To: Sheila Sahu, PE City of Beaverton

From: Claire DeVoe, PE Murraysmith


Re: Potable Water Alternatives Executive Summary

Executive Summary

This memorandum summarizes the development of the proposed potable water service alternatives for the Cooper Mountain Utility Plan (CMUP). The CMUP work is in conjunction with the Cooper Mountain Community Plan (CMCP, Angelo Planning Group). The planning and design criteria, proposed alternative, and next steps are described in this memorandum at a high level. More detailed discussion of utility alternatives can be found in the preliminary Potable and Non-Potable Water Alignments Technical Memorandum (Murraysmith, 2021).

Design Criteria

Design criteria used to develop a potable water service alternative are consistent with the 2019 Beaverton Water System Master Plan (Murraysmith). Proposed pressure zones in the CMUP project area are extensions of existing Beaverton water service area zones and aligned with existing and planned infrastructure, such as Cooper Mountain Reservoir 1 and 2. Potable water infrastructure alternatives are based on distribution service pressures of 50 to 95 pounds per square inch (PSI) under peak day demand and a minimum of 20 PSI under fire flow conditions during peak day demand.

Level of Detail

This effort focused on developing a plan for major water main alignments, a “backbone”, to provide water service to the CMUP project area. Proposed pipelines are aligned with available

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roadway information. The complete water distribution network will be refined as the land use framework is finalized and individual properties are developed. This planning effort follows stormwater and sewer planning, which can have more direct impacts on the community planning process, including considerations for neighborhood greenspace along stream corridors, siting detention basins, and topography considerations for gravity collection.

Proposed Water Alternative

Figure 1 illustrates the proposed backbone alignments for the CMUP area, in addition to relevant existing infrastructure. The water infrastructure to serve the CMUP area includes the following:

▪ Proposed major transmission mains along preliminary CMUP area roadways.

▪ A proposed 550 Zone finished water storage reservoir, also known as Cooper Mountain Reservoir #3 (CMR 3, CIP 4058B) and the existing 794 Zone Cooper Mountain Reservoirs 1 and 2 (CMR 1 and 2, CIP 4058A).

▪ A proposed Tile Flat Pump Station (CIP 4142) to supply the future CMR 3 from the Willamette Water Supply System (WWSS) connection at Tile Flat Road.

▪ A pump station located at the proposed CMR 3 site and associated transmission to the existing 24-inch 794 Zone transmission main in 175th Avenue, which supplies the existing CMR 1 and 2.

▪ A pump station located at the existing CMR 1 and 2, the Upper Zone Booster Pump Station, to supply the future 920 and 860 Zones.

Next Steps

To provide a complete water alignment alternative, the following next steps are recommended:

▪ Update the water supply alternative to be consistent with any new refinements in the land use framework from the Community Plan.

▪ The following preliminary alignments or sites should be selected:

o The transmission route from the proposed 550 Zone Pump Station to the 175th Avenue transmission main.

o The site for the proposed Tile Flat Pump Station for future supply from the WWSS.

o The site for the proposed CMR 3.

▪ Discussion with Tualatin Valley Water District (TVWD) about potential acquisition of existing facilities east of 175th Avenue.


▪ Continued coordination with storm, sewer, and non-potable utility planning and the CMCP.

References

Murraysmith. (2021). Draft Potable and Non-Potable Water Alternatives Development.

Murraysmith. (2019). Water System Master Plan.

20-2755 Page 1 of 5 CMUP Non-Potable Executive Summary January 2021 City of Beaverton

Memorandum


Project: Beaverton Cooper Mountain Utility Plan (20-2755)

To: Sheila Sahu, PE City of Beaverton

From: Claire DeVoe, PE Murraysmith


Re: Non-Potable Water Alternatives Executive Summary

Executive Summary

This memorandum summarizes the development of the proposed non-potable water alternative for the Cooper Mountain Utility Plan (CMUP). The CMUP work is in conjunction with the Cooper Mountain Community Plan (CMCP, Angelo Planning Group). The planning and design criteria, proposed alternatives, and next steps are described in this memorandum at a high level. More detailed discussion of these alignments can be found in the preliminary Potable and Non-Potable Water Alignments Technical Memorandum (Murraysmith, 2021).

Design Criteria

Design criteria are consistent with the on-going South Cooper Mountain (SCM) Non-Potable Water Planning project, led by Murraysmith. Non-potable pressure zones in the CMUP area are aligned with existing and proposed infrastructure serving the SCM Area. Proposed zone boundaries are selected to provide non-potable distribution service pressures of approximately 45 to 85 pounds per square inch (PSI). Proposed non-potable mains are sized to meet peak hour irrigation demands.

Level of Detail

This effort focused on developing major non-potable “backbone” pipe alignments to provide non-potable service to the CMUP area. Proposed pipelines are aligned with available roadway information. The complete non-potable distribution network will be developed as the land use

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framework is refined. This planning effort follows stormwater and sewer planning, which can have more direct impacts on the community planning process, including considerations for neighborhood greenspace along stream corridors, siting detention basins, and topography considerations for gravity collection.

Proposed Alternatives

Two alternatives were developed, illustrated in Figures 1 and 2. Alternative 1 (Figure 1) relies on a reservoir to supply upper zones, while Alternative 2 (Figure 2) relies on pumping to supply upper zones.

Proposed Alignment 1 - Reservoir

Alignment 1 includes a reservoir with a 750-foot overflow located east of Cooper Mountain Nature Park to serve upper zone non-potable customers. Additional aquifer storage and recovery (ASR) wells will supplement the existing ASR 3, located east of SCM, to supply the SCM and CMUP areas with non-potable water. These ASR wells are proposed at the potable 550 Reservoir site (Cooper Mountain Reservoir 3 (CMR 3)), the proposed 750 Reservoir site, and within the 605 Zone.

Well construction should occur as development warrants, with lower zones (507 and 438) tying into the SCM non-potable system and upper zones primarily served through new ASR wells. The proposed 750 Reservoir will be supplied by both a pump station located at CMR 3 and a proposed ASR well located at the 750 Reservoir site. A booster pump station located at the 750 Reservoir site will supply higher non-potable zones. Pressure reducing valves (PRVs) will connect and supply zones below 750 feet, like the potable system.

Proposed Alignment 2 - Pumping

Alignment 2 uses booster pumping and hydropneumatic tanks to serve non-potable customers. As in Alternative 1, ASR 3 will supply the lower three non-potable zones by pumping or PRV (507, 438, and 410 Zones), and additional ASR wells will supplement supply. Upper zones will be supplied by pump station(s) located at CMR 3, as development warrants. A pump station to serve the immediate surrounding area and upper reaches of SCM should be constructed, along with a hydropneumatic tank to mitigate demand peaking. A second pump station and hydropneumatic tank up to the 882 Zone could be built once development occurs above the 605 Zone. Zones between the 882 and 605 Zones would be served by PRVs.


Next Steps

To provide a complete non-potable water alternative, the following next steps are recommended:

▪ Update the non-potable supply alternatives to be consistent with any new refinements in the land use framework from the CMCP.

▪ Address stormwater injection requirements for ASR use as non-potable supply.

▪ Continue coordination with the non-potable system in South Cooper Mountain.

▪ Discuss siting for additional non-potable and ASR facilities.

References

Murraysmith. (2021). Draft Potable and Non-Potable Water Alternatives Development.

Memorandum - Beaverton, Oregon

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