What strategies are being considered for this project?

    The City Council recently adopted a Flood Risk Reduction Strategy identifying four areas of work:

    • Infrastructure
    • Regulation
    • Outreach and Engagement
    • Emergency Services

    This project will explore actions under the Infrastructure and Outreach and Engagement areas of work. Exploring regulatory changes to address flooding is not part of this project.

    How does flooding occur?

    Direct flooding over the land surface happens when water is higher than the foundation of a house and it saturates the wood framed portions of the house.

    Indirect flooding happens when there are prolonged wet periods or standing water that saturates the ground, putting pressure on the foundation of a house, and resulting in water leaks through cracks and joints in the foundation or a concrete floor.

    Sanitary backflow flooding happens when flood water flows into the sanitary sewer system and exceeds the capacity of the sewer system, causing sewage to back up in the pipe and spill into buildings.

    What is flood risk?

    Risk is measured by the probability of flooding influenced by a variety of conditions including topography, soil conditions, elevation of buildings, age of infrastructure, rainfall, and other factors.

    The predicted amount of flooding is based on historic rainfalls and measured as rainfall events that are likely to occur every 1 year, 5 years, 10 years, 50 years, or 100 years.

    Climate, exposure, and vulnerability define flood risk.

    For more in depth information about the development of the Flood Risk Reduction Strategy and resident Task Force work, visit the Water Resources Library.

    How much water?

    A 1% annual chance event (often referred to as a 100-year storm) is 7.5 inches in a 24 hour period. In the Morningside Neighborhood, after accounting for infiltration, the runoff volume is 159-acre feet, or 51,810,300 gallons of water. This is enough runoff to fill the Southdale water tower 103 times.

    It would take about 942,000 rain barrels (55 gallon size) to hold that volume of stormwater runoff.

    Due to climate change, flood events are happening more frequently than in the past.

    What infrastructure options are available?

    Infrastructure options that will be evaluated for flood reduction effectiveness and cost may include, but are not limited to:

    • Increasing the size of storm sewer pipes
    • Creating more areas for water storage through excavation, berms and/or flood walls
    • Providing additional flood storage with underground water storage tanks
    • Installing predicting pumping systems, which would pump water to different locations based on anticipated rain events
    • Various combinations of the above strategies

    What is within the scope of the project, what is out of scope, and what decisions have already been made?

    Things that will be considered in this project include:

    • concept-level flood infrastructure public improvements
    • when, where, and how much flood water is stored in streets
    • when, where, and how much flood water is stored in parks

    Things that will not be considered in this project include:

    • Roadway reconstruction elements such as street width, pedestrian facilities, and sanitary sewer and water improvements
    • Ordinance changes for impervious cover limits

    Decisions already made include:

    • Roadway reconstruction schedule
    • Renewal of sanitary sewer, water, and storm sewer utilities upon roadway reconstruction in 2022 and 2023
    • A dedicated bicycle facility along the east side of Grimes Avenue will be installed during construction