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148th Avenue Southeast at Larsen Lake Flood Mitigation: Online Open House

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Thank you for your interest in the 148th Avenue Southeast Online Open House. The consultation period for this online open house has concluded.

Welcome to the 148th Avenue Southeast at Larsen Lake Flood Mitigation online open house! This project aims to reduce the frequency of flooding on 148th Avenue Southeast between Southeast Eighth Street and Main Street. The project team is performing engineering studies and evaluating alternatives to develop a design that reduces or eliminates roadway flooding in the project area.


The City of Bellevue and King County Flood Control District are working together to develop and implement a future design to provide healthier stormwater drainage between Larsen Lake/Lake Hills Greenbelt and Upper Kelsey Creek.

Use this online open house to learn about the project and design alternatives we are considering. Please provide your feedback by Friday, September 15, by 5 p.m.

Background

Following moderate to heavy rain or long rain events, 148th Avenue Southeast has been fully or partially closed by the city due to high water levels on numerous occasions between Southeast Eighth Street and Main Street.

The 148th Avenue corridor is the only major north-south arterial roadway in east Bellevue, a federally designated truck route, and a critical commuter and emergency services access route between Interstate 90 (I-90) and State Route 520 (SR 520) with an average daily traffic over 33,000 vehicles.

During closures, adjacent corridors, including residential areas with limited vehicle capacity, have been impacted. Flooding also threatens nearby parklands, blueberry fields, businesses and residences.

The city will reduce negative impacts in the project area through the planned mitigation measures by reducing the recurrence of flooding and preventing further roadway sinkage and culverts from warping.

Evaluating the Alternatives

Previous 2019 studies of the issues along this arterial included a Flood Investigation Analysis and an Alternatives Screen Technical Memorandum prepared by the City of Bellevue Utilities Department. The latter evaluated several concepts for addressing flood recurrences:

  • Alternative 1: No Action
  • Alternative 2: Operational Improvements
  • Alternative 3: Local Conveyance Improvements
  • Alternative 4: Downstream Improvements
  • Alternative 5: Floodwall
  • Alternative 6: Raise Roadway

Following the memorandum, and for the safety of the public, long-term solutions (Alternatives 4, 5 and 6) were selected to carry forward for further analysis by the City of Bellevue Transportation Department. Modifications to Alternative 4 were deemed too impactful to the downstream conveyance systems and not a practical solution. As a result, the project team analyzed optimization of storage in the wetland complex around Larsen Lake. The project team further analyzed the following alternatives in 2023:

  • Optimize Storage in Larsen Lake Complex
  • Install Levee or Floodwalls
  • Raise Roadway

Currently, we are evaluating the alternatives based on the following criteria:

  • Technical Evaluation: Technical feasibility to implement and construct a given alternative.
  • Long Term Feasibility: Ability to eliminate persistent roadway settlement and flooding. Alternatives with short-term project effectiveness will be assessed negatively.
  • Operations and Maintenance: Resources needed to maintain the operation of the proposed design and prevent ongoing settlement and flooding of 148th Avenue Southeast.
  • Environmental Impacts: Ability to avoid or minimize impacts to critical areas and buffers. Addressing fish passage barriers and the addition of a new fish-passable crossing will be viewed positively.
  • Permitting/Regulatory: The ability and level of effort required to obtain permits.
  • Community Impacts During Construction: Impacts to private property or character of the neighborhood and the level of impact to traffic and the neighborhood during construction.
  • Equity, Social Justice and Sustainability: Considers changes to travel patterns, impacts on social and public services, traffic and pedestrian safety, and project benefits to the community.
  • Construction Cost: Projects costing less than $3 million and those exceeding the minimum criteria will be considered positively. Projects over $10 million would fail to meet expectations.

The evaluation of each alternative is based on the ability to meet or exceed the defined performance criteria and is not based on a comparison between alternatives. However, some factors, such as construction cost, are based on total estimated values.

For more details, a Draft Alternatives Analysis is available using the link on the right toolbar.

Welcome to the 148th Avenue Southeast at Larsen Lake Flood Mitigation online open house! This project aims to reduce the frequency of flooding on 148th Avenue Southeast between Southeast Eighth Street and Main Street. The project team is performing engineering studies and evaluating alternatives to develop a design that reduces or eliminates roadway flooding in the project area.


The City of Bellevue and King County Flood Control District are working together to develop and implement a future design to provide healthier stormwater drainage between Larsen Lake/Lake Hills Greenbelt and Upper Kelsey Creek.

Use this online open house to learn about the project and design alternatives we are considering. Please provide your feedback by Friday, September 15, by 5 p.m.

Background

Following moderate to heavy rain or long rain events, 148th Avenue Southeast has been fully or partially closed by the city due to high water levels on numerous occasions between Southeast Eighth Street and Main Street.

The 148th Avenue corridor is the only major north-south arterial roadway in east Bellevue, a federally designated truck route, and a critical commuter and emergency services access route between Interstate 90 (I-90) and State Route 520 (SR 520) with an average daily traffic over 33,000 vehicles.

During closures, adjacent corridors, including residential areas with limited vehicle capacity, have been impacted. Flooding also threatens nearby parklands, blueberry fields, businesses and residences.

The city will reduce negative impacts in the project area through the planned mitigation measures by reducing the recurrence of flooding and preventing further roadway sinkage and culverts from warping.

Evaluating the Alternatives

Previous 2019 studies of the issues along this arterial included a Flood Investigation Analysis and an Alternatives Screen Technical Memorandum prepared by the City of Bellevue Utilities Department. The latter evaluated several concepts for addressing flood recurrences:

  • Alternative 1: No Action
  • Alternative 2: Operational Improvements
  • Alternative 3: Local Conveyance Improvements
  • Alternative 4: Downstream Improvements
  • Alternative 5: Floodwall
  • Alternative 6: Raise Roadway

Following the memorandum, and for the safety of the public, long-term solutions (Alternatives 4, 5 and 6) were selected to carry forward for further analysis by the City of Bellevue Transportation Department. Modifications to Alternative 4 were deemed too impactful to the downstream conveyance systems and not a practical solution. As a result, the project team analyzed optimization of storage in the wetland complex around Larsen Lake. The project team further analyzed the following alternatives in 2023:

  • Optimize Storage in Larsen Lake Complex
  • Install Levee or Floodwalls
  • Raise Roadway

Currently, we are evaluating the alternatives based on the following criteria:

  • Technical Evaluation: Technical feasibility to implement and construct a given alternative.
  • Long Term Feasibility: Ability to eliminate persistent roadway settlement and flooding. Alternatives with short-term project effectiveness will be assessed negatively.
  • Operations and Maintenance: Resources needed to maintain the operation of the proposed design and prevent ongoing settlement and flooding of 148th Avenue Southeast.
  • Environmental Impacts: Ability to avoid or minimize impacts to critical areas and buffers. Addressing fish passage barriers and the addition of a new fish-passable crossing will be viewed positively.
  • Permitting/Regulatory: The ability and level of effort required to obtain permits.
  • Community Impacts During Construction: Impacts to private property or character of the neighborhood and the level of impact to traffic and the neighborhood during construction.
  • Equity, Social Justice and Sustainability: Considers changes to travel patterns, impacts on social and public services, traffic and pedestrian safety, and project benefits to the community.
  • Construction Cost: Projects costing less than $3 million and those exceeding the minimum criteria will be considered positively. Projects over $10 million would fail to meet expectations.

The evaluation of each alternative is based on the ability to meet or exceed the defined performance criteria and is not based on a comparison between alternatives. However, some factors, such as construction cost, are based on total estimated values.

For more details, a Draft Alternatives Analysis is available using the link on the right toolbar.

Thank you for your interest in the 148th Avenue Southeast Online Open House. The consultation period for this online open house has concluded.

  • Alternative 1: Optimize Storage in Larsen Lake Complex

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    CLOSED: This discussion has concluded.

    Alternative 1 would modify the surrounding drainage courses and/or wetland areas to provide additional flood storage. This could be accomplished through a stream and wetland restoration alternative that would focus on removing non-native vegetation and realigning the existing drainage channels providing more natural meandering through the wetland areas to better engage them hydraulically during storm events. Woody material could be installed or provide grade controls to plug channels and create overland flow. This alternative would provide additional flood storage in the wetland complex around Larsen Lake and/or the wetlands on the west side of 148th Avenue Southeast. Additionally, one of the three existing fish barrier culverts would be replaced with a fish-passable culvert on the north end of 148th Avenue Southeast.

    Construction impacts

    Alternative 1 would benefit from minimal construction impacts to 148th Avenue Southeast since most work would occur in the Larsen Lake complex, also known as Lake Hills Greenbelt. A portion of 148th Avenue Southeast would need to be raised at the proposed culvert crossing, which would impact traffic during installation.

    Potential results

    Alternative 1 would provide minimal relief from flooding since 148th Avenue Southeast is continuing to settle. Only so much improvement to flood storage can be created in the Larsen Lake complex. In addition to not being a feasible long-term solution to chronic flooding, this alternative would have significant environmental impacts and require a more extensive permitting process.

  • Alternative 2: Install Levee or Floodwalls

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    CLOSED: This discussion has concluded.

    Alternative 2 will regulate the water level within the roadway by implementing one of the following options.

    • Earthen Embankment Levee: This option would incorporate an elongated earthen embankment spanning the east and west sides of 148th Avenue Southeast. The existing pedestrian path will be raised to serve as an earthen embankment/dam.
    • Flood Wall: Similar to the option above but with low-height retaining walls to support the raised embankment to minimize wetland impacts. This flood wall will span the east and west sides of 148th Avenue Southeast, with the pedestrian path located on top.

    Alternative 2 would also include replacement of the culvert at the north end of 148th Avenue Southeast with a fish-passable culvert.

    Construction impacts

    The levee or floodwall alternative would temporarily impact traffic on 148th Avenue Southeast. Construction would occur in sequence on the northbound or southbound sidewalks to minimize traffic impacts. During construction of the raised paths, pedestrian traffic would be rerouted to the opposite side of the street. A single lane of traffic would need to be closed during construction. During the installation of the culvert, the roadway would be reduced to a single lane in each direction as the culvert is installed in phases.

    Potential results

    Alternative 2 would provide relief to flooding; however, due to the nature of the existing median and roadway layout, it is likely that pump systems would need to be installed in the median to remove any excess water. This alternative would have higher maintenance costs.

  • Alternative 3: Raise Roadway

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    CLOSED: This discussion has concluded.

    Alternative 3 will elevate the roadway above the Base Flood Elevation of 258.7 feet with a minimum freeboard of 3 feet. The three existing fish barrier culvert crossings will be replaced with fish-passable structures, and 10’x10’ precast concrete culverts are proposed. The dimensions of the culvert are based on the width of the existing stream and anticipated depths of flow to allow for a 100-year flood event.

    Three options for implementing this alternative have been evaluated:

    • Pin Pile-supported Slab: This option consists of constructing a concrete slab supported by piles driven into existing ground, eliminating any future settlement.
    • Lightweight Fill: This option would excavate some of the existing roadway and place low density material to raise the roadway, which would result in a net reduction in weight of the roadway, thereby stopping or reducing settlement.
    • Overbuild and Long-term Settlement: This option would raise the road with excess material, allow time to settle, and then open to traffic.
    • Ground Improvements: This option would install concrete walls or columns in existing ground to reduce settlement.

    Construction impacts

    Raising the roadway would have the most substantial up-front construction cost and traffic impacts. Northbound and southbound construction would happen sequentially, with traffic reduced to a single lane in each direction during construction.

    Potential results

    Raising the roadway profile will offer the best long-term solution relative to the other alternatives. Raising the roadway above the existing water surface elevation and supporting the extra weight with one of the options listed above will reduce or eliminate settlement of the roadway. In addition, three fish-passable culverts will be installed, which will benefit fish and allow flood waters to pass under the roadway.

  • Evaluation Summary and Recommendations

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    CLOSED: This discussion has concluded.

    Recommendation

    The project team recommends the Raise Roadway alternative (Alternative 3) as it provides the most effective solution for reducing flooding of 148th Avenue Southeast, addresses roadway settlement issues, and improves the environment by removing three existing fish barrier structures. Although the initial project cost for Alternative 3 is higher than the other alternatives, the project team developed innovative solutions to minimize the construction costs.