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South Boulder Creek Flood Mitigation Open Space Land DisposalFrom:Gordon McCurry To:OSBT-Web Subject:South Boulder Creek Flood Mitigation Open Space Land Disposal Date:Thursday, February 22, 2024 3:23:10 PM External Sender Notice This email was sent by an external sender. Hello OSBT members, I am writing in regards to your deliberations to dispose of Open Space lands associated with the construction of the floodwall that is part of the South Boulder Creek Phase 1 Flood Mitigation project. I am the Chair of the Water Resources Advisory Committee but these comments are my own, as a professional hydrologist and interested member of the public. I strongly encourage the OSBT to abide by your Resolution to City Council, dated June 9, 2021, which stated “OSBT will not officially consider the approval and recommendation of a disposal of any of the Floodwall Acres for construction of the Floodwall unless and until all the following conditions have been first fulfilled or addressed to the satisfaction of the OSBT”. Those recommendations include: obtaining all necessary permits and approvals, preparing an Environmental Impact Statement and a Site-Specific Biological Assessment, preparing an Environment Mitigation Plan and having it approved by OSMP, OSBT and Council, there being sufficient planning to design a groundwater conveyance system through the proposed floodwall that maintains existing groundwater conditions at the site, and transferring title of 119 acres of land from CU to the City. Obtaining reviews and approvals by the regulatory agencies (CDOT, CO-DWR, FEMA, COE, etc) is especially important since the proposed groundwater conveyance system through the floodwall appears to be the first of its kind, and therefore untested. The City Utilities Department has been asked for at least 4 years by members of the public to provide examples of a similar conveyance system – a conveyance system that would be used for a flood control structure that is dry except during flood events and has not done so – presumably because no other such conveyance system exists. As you are probably aware, the lands on either side of the proposed floodwall are high-value wet meadows that are designated State Natural Area lands and is the habitat for several threatened and endangered species. The shallow groundwater level in those wet meadows is critical to sustaining those meadows and the species living within them, so the groundwater conveyance system must be able to maintain existing groundwater levels in perpetuity. The groundwater conveyance system must also be able to withstand the significant water pressure that will exist during flooding events when the floodwall is full of water. The current design requires closing down a series of approximately nine connector pipes that go through the floodwall to reduce flow. Previous flood modeling showed that the peak flow will reach the floodwall less than 5 hours after the rainstorm that produces the flood. That is very little time to adjust the valves to reduce flow in the connector pipes, being in the midst of a flood, and assumes that staff can access the connector system and that all of the valves remain in good working order so that they can be adjusted. Failure to reduce the flow in each of the individual conveyance pipes could lead to damage of the system, or localized failure. The review of this groundwater conveyance system by regulatory experts will provide a necessary third-party assessment and must be undertaken, as stated in the OSBT’s June 2021 Resolution to Council, before the OSBT agrees to Dispose of the Open Space lands associated with the project. Thank you for considering my thoughts on this matter. Thanks, Gordon Gordon McCurry, Ph.D., P.G. Principal Hydrologist McCurry Hydrology, LLC 303-520-1349 gmccurry@mccurryhydro.com www.mccurryhydro.com Confidentiality Notice: This Email is covered by the provisions of the U.S. Electronic Communications Privacy Act and may contain confidential information. If it has been sent to you in error please reply that you received it and then delete the message. Any distribution or copying of this message or attachments contained herein other than by its intended recipient is strictly prohibited.