10.1 SR 10-03-2022 - Laserfiche WebLink

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A public hearing on the proposed improvements is held. Estimated date: November 21, 2022 After the City Council considers the presentation of the plans and specifications, it may pass a Resolution Approving Plans and Specifications and Ordering an Advertisement for Bids. Estimated date: January 16, 2023 The advertisement for bids should be published in the official local newspaper and trade publication such as the "Construction Bulletin" at least three weeks prior to the bid opening date. Estimated bid opening date: February 8, 2023 If the City Council wishes to continue with the improvements after consideration of bids, the City Council may pass a Resolution Accepting Bid. This resolution would award the project to the lowest responsible bidder and construction may begin. Estimated date: February 20, 2023 Following completion of the proposed improvements, a public hearing is held to set the final assessment amounts. Estimated date: November 20, 2023 III. Current Drainage Conditions The Morton Ave subwatershed is primarily single-family residential land use. The soil types are Hydraulic Soil Group A, which are sandy and have a high potential for infiltration. The area is served by City storm sewer. Refer to Figure 1 in Appendix B for a subwatershed area map. Approximately 11.3 acres drains to the Morton Ave Low point, with approximately 2.33 acres of direct drainage contribution to the Morton Ave catch basins. The low point can pond to approximately 0.8 feet (9.6 inches) before overtopping into the front yard and driveway of 319 Morton Ave and into the low area behind (west of) the home. Exhibit 1: 319 Morton Avenue low point According to City staff, it was observed that the flooding is largely a function of high intensity storms. That is, when it rains 1" to 2" in an hour or less, the low area will flood from the storm sewer pipe backing up through catch basin grates, limited inlet capacity at the catch basin grates, or a combination of both. According to Atlas 14 rain fall data in Elk River, a 2" storm that occurs in 30 minutes has an equivalent return interval of 25 to 50-years. Therefore, a good estimate of total potential runoff flood volume is to model a 25 to 50-year, 24-hour storm (5.25" to 6.10" in 24-hours). Hydraulic Modeling and Assessment of Flooding The source of the flooding at Morton was confirmed through hydraulic modeling. Autodesk Storm and Sanitary Analysis (SSA) was chosen for its comprehensive and full hydrodynamic ability. The low area was modeled as a storage area to best understand the depth of water in the intersection, how the pipes may surcharge and add to flooding, the bypass flow from upstream catch basins during large events, and the capacity of the pipe system through the rain fall event. From this assessment, it is evident that multiple factors contribute to flooding. Prepared by: Bolton & Menk, Inc. Morton Avenue Drainage Study I OR11.127809 Current Drainage Conditions Page 2