Could you please provide more information about the possible alternatives under consideration for the implementation of "UGC Reach 6"? Specifically, I’d like to understand why is the city recommending improved conveyance of Goose Creek instead of piping flood flows under the roadway? The City of Boulder prioritizes a nature-based approach to flood mitigation citywide as it restores natural stream functionality, increases resilience to flooding events, enhances urban ecosystems and supports the city’s climate goals. Such an approach is in alignment with national trends, industry standards, and the 2022 Comprehensive Flood and Stormwater Master Plan (see excerpt below). The current recommendation for Upper Goose Creek Reach 6 proposes improving channel conveyance and restoring the stream channel using this nature-based design approach.
Previous investigation of a piped alternative under Edgewood indicated that such an approach was not viable. However, given community interest, the team will take another look at a more detailed design to see what it would take to route flood flows under the road and share that report with the community when it has become available.
Is there still an alternative to convey Twomile Canyon Creek flow down Floral Drive to reduce flows along Goose Creek? Yes. The Floral Drive alternative has been included in the flood mitigation plan and is currently being evaluated in more detail. City council communicated a preference for the Floral Drive alternative and additional design details will be provided to the community when we have more information available.
Will there be public access along Goose Creek Reach 6 as part of the project? No. Public access is not included in the Goose Creek flood mitigation plan for Reach 6. There is also no reference to future public access in the Transportation Master Plan or Greenways Master Plan, which all generally guide future city projects that have been approved by council.
What are the details in the mitigation plan for the size of the culverts, and the width and configuration of the proposed channels, and what will the channels look like? The design of the details will not occur until after the mitigation plan is approved by City Council, and as that process moves forward, the community will be informed of the design as it is developed. More detail will be provided for each property owner once plans are finalized as we work through the design and continue to receive feedback through engagement in the development of the plans.
Was any consideration given to utilizing additional street conveyance down Iris, Edgewood, and other streets rather than the alternatives shown? The streets collect water in a storm sewer system that is not sized for a major storm event, so streets are currently conveying excess stormwater. Increasing the depth of water even more could result in turning them into high hazard zones, jeopardizing life safety and emergency access.
How were the alternatives scored to make the decisions presented in the mitigation plan? After the results of the community outreach to create alternatives for the various reaches of the UGT creek system, the community was encouraged to vote on preferences. This information was combined with the community values described in the CFS, and the results were tabulated and given scores to determine the priorities and preferences in a general sense for the mitigation plan.
Do the mitigation plans include things like debris and blockages? Debris, damage, blockages and clogging are always considerations for the design of storm sewers, channels, box culverts, and all the elements of flood conveyance. The hydraulic model includes blockages ranging from 10% to 100%, depending on conditions and the storm event.
Will the revised floodplains be constructed with concrete-lined channels? No. There are no plans to construct concrete-lined channels, as this is contrary to the current thinking, guidelines, wetland regulations, water quality considerations and desires of both the city and the community. The intent is to construct sustainable, attractive, re-vegetated stream corridors that encourage natural riparian habitats and wildlife ecosystems.
Why is flow from Twomile Canyon Creek being directed toward Goose Creek along Edgewood, with modifications to Goose Creek on that reach, and won’t that increase the flood risk to those areas, causing those homes to be flooded? Historically, water flows overland from Twomile Canyon Creek toward Goose Creek, and it will continue to do so, regardless of improvements upstream. The city will not revise any floodplain to put any homes at further risk from additional flooding than before, and reconfiguration is intended to lower the water surface elevation in a flood event, removing homes from the floodplain where possible, decreasing the overall risk for homes. Even though this portion of the creek did not flood significantly in 2013, flooding could occur in the future from the more typical flood event.
If Goose Creek is widened between 19th and 24th St. are the engineers considering the difficulties and costs to stabilize the existing steep slope and protect the homes on the south side of the creek? Yes. Portions of the existing slope are already experiencing severe erosion and deteriorating trees that can lose the root structure and put the area at risk of further slope failures. A detailed and thorough geotechnical investigation to design appropriate slope stability is a key element of the improvements to this reach of Goose Creek.
Why are multiple paths shown on the mitigation plan for Twomile Canyon Creek south of Linden Ave.? The multiple paths shown on the mitigation plan represent the problem. The loss over time of the original creek channel has caused the flow to deviate from the main channel of Twomile Canyon Creek into other pathways, meandering through local streets and neighborhoods south of Linden both east and west of Broadway until it re-enters Goose Creek, and is joined by flow from Elmer’s Twomile Creek, just east of Valmont and 28th. This results in substantial flooding of those streets and residential areas.
If there is not enough room for the “100-year” channel, why doesn’t the city use imminent domain law to acquire necessary land for public good between Linden and Broadway instead of designing the channel for less than “100 -year” flood conveyance? Respecting the wishes of the community and the residents of the neighborhood is extremely important. Forcibly taking over property is very rare. The city will negotiate with the private property owners to acquire drainage easements prior to any final plans or construction. In some instances, private property owners are willing to vacate their property and sell it to the city, and when appropriate, the city will consider purchasing the property.
Will Twomile Canyon Creek mitigation east of Broadway handle the “100-year” flood? The proposed mitigation plan will convey the “100-year” flood in this reach to 19th Street, with a combination of an open channel along Iris Avenue and storm sewer improvements along 13th Street, Hawthorn Avenue, 16th Street, and Grape Avenue. The exact alignment and combination will be determined in the next phase of design development.