South Boulder Creek Flood Mitigation Frequently Asked Questions

The South Boulder Creek flood mitigation project is the city’s top priority flood mitigation project. This project will provide protection from major floods and enhance safety for vulnerable community members in southeast Boulder. The construction approach will include both installation of flood infrastructure and floodplain restoration and protection, while also preserving access along US36 during a flood emergency and minimizing impact on open space.

The city has collaborated with the community, City of Boulder Open Space and Mountain Parks, University of Colorado, Boulder, Mile High Flood District and the Colorado Department of Transportation to develop flood mitigation solutions for this area, which benefits 2,300 community members, 260 structures and crucial infrastructure including US36 and Foothills Parkway. 

The Front Range Flood in September 2013 resulted in damaging storm runoff citywide, reinforcing the need for this project. The flood magnitude, as measured throughout various parts of Boulder, ranged between a 25- and 50-year event for many watersheds. Portions of lower South Boulder Creek, including the West Valley, experienced flows on the order of a 100-year flood, and water overtopped US36 which presents a safety issue and can prevent emergency services from accessing parts of the city during a flood event.

The CU Boulder South annexation agreement, approved in 2021, brought the 308-acre site into Boulder city limits and established the framework for the university to transfer approximately 155 acres to the city, including 119 acres that will become new permanently protected open space. The agreement has also been voted on by the community several times since the early 2000s and each time voters have supported advancement of the flood project. This land supports both flood mitigation infrastructure and future open space use, while also enabling potential future university development, including recreational opportunities for the community. 

The flood detention dam will hold back floodwaters during a flood event allowing it to safely pass downstream rather than overtopping and flooding US36 as it did in 2013. This will help prevent water from overtopping the highway and blocking vital entry and exit points during an emergency. This work also aims to protect wildlife and vegetation, including the federally protected Preble’s meadow jumping mouse and the Ute Ladies’ Tresses Orchid, and to restore and preserve historic South Boulder Creek floodplain.

In addition to enhancing community safety, this project aligns with the city’s climate resilience goals. The focus on flood mitigation at CU Boulder South reflects a core principle of Boulder’s Climate Action Plan: climate action is not only about reducing emissions, but also about adapting to the impacts we are already experiencing. Alongside aggressive mitigation efforts, Boulder must plan for increasing risks such as flooding, extreme heat, wildfire and drought by integrating adaptation into land use, infrastructure, nature-based solutions and emergency preparedness. In that context, CU Boulder South represents a meaningful climate resilience partnership. By restoring floodplain function, preserving open space and incorporating nature-based approaches to reduce risk, the site supports both ecological health and community safety.

This project will impact approximately two acres of city-owned open space. Through negotiations with the university, however, the university donated 119 acres of land to the city next to the flood mitigation footprint that will be restored as natural habitat and managed as new city open space. The project also allows for future development of the CU Boulder South site by the university, which will include recreational opportunities available to the community.

In the mid-1990s, Flatiron Companies sold the former sand and gravel mining property south of US36 to the University of Colorado. Around the same time, Boulder City Council adopted a resolution directing the city to work with Boulder County to ensure that any development of the site would follow the goals and policies of the Boulder Valley Comprehensive Plan.

During the 2000 Boulder Valley Comprehensive Plan Major Update, the university requested a public land-use designation for CU Boulder South to allow potential housing, research, academic and recreational uses. The city decided to delay any land-use changes until it could complete a South Boulder Creek Floodplain Study.

In 2001, City Council also stated its willingness to purchase the property for open space or flood control purposes if it became available from a willing seller. Over the next several years, the city and the university continued discussions about possible future land uses for the site.

In 2015, City Council approved the South Boulder Creek Flood Mitigation Plan. The plan included regional detention of floodwater upstream of US36 on the CU Boulder South campus and along Colorado Department of Transportation right-of-way. With the plan adopted, the city updated the site’s land-use designation and adopted CU Boulder South Guiding Principles as part of the comprehensive plan major update.

In 2019, the University of Colorado Boulder submitted an annexation application for CU Boulder South. In 2020, City Council voted unanimously to move forward with the 100-year South Boulder Creek Flood Mitigation Project while continuing to evaluate additional upstream options. The city conducted community engagement and negotiated an annexation agreement with the university, which City Council approved in 2021. The agreement has also been voted on by the community several times since the early 2000s and each time voters have supported advancement of the flood project.

As part of the annexation agreement, the university agreed to a land swap with the city for the flood mitigation project and future open space, in exchange for being able to annex the CU Boulder South site, and for future development which include recreational amenities that will be open to the community. Through negotiations with the university and the Open Space Board of Trustees the university donated 119 acres of land to the city alongside the flood mitigation footprint that will be restored as natural habitat and will be managed as city open space.

The project will include:

• An earthen flood detention dam along the north and west edges of the CU Boulder South campus to store flood waters
• A concrete spillway along US36
• An outlet tunnel beneath US36
• A groundwater management system
• Demolition of existing structures, including a warehouse and tennis courts
• Reestablishment of South Loop Drive for future access after construction
• Vegetation clearing and ecological restoration

The city evaluated 58 alternatives, including extensively studying an upstream alternative that would utilize the existing mined gravel area, and arrived at the current project configuration through over a decade of planning. Alternatives have been thoroughly evaluated, and this design was approved by City Council, considering cost and environmental resources.

The design and technical analyses were reviewed by multiple independent third-party consultants. Throughout the permitting process there have also been multiple reviews and approvals, including:

• The State Engineers Office, Dam Safety Branch
• Colorado Department of Transportation
• Federal Highway Administration

Floodplain changes were reviewed by the Federal Emergency Management Agency (FEMA) and its contract consultant. All permits have been approved and received.

One of the key design criteria and requirement for FEMA approval of the project was to ensure that the project did not cause negative impacts to upstream or downstream properties. City staff and consultants have conducted extensive technical analysis demonstrating that the project will improve downstream flood conditions. That analysis has been independently reviewed by FEMA and its contract consultants. If you would like to know how the floodplain would change as a result of the project, please visit the proposed floodplain map.

Concrete Express, Incorporated (CEI): In fall 2025, CEI was selected through a competitive bid process.

The project has all permits and approvals to begin construction including approvals from the following agencies:

• U.S. Army Corps of Engineers
• U.S Fish and Wildlife Service
• Federal Emergency Management Agency
• Colorado Department of Transportation
• Colorado State Engineer’s Office
• City of Boulder

The total estimated flood project cost is $76.5 million to be paid by the City of Boulder Storm and Flood Utility in partnership with the Mile High Flood District.

The city will be using City Council-appropriated stormwater revenues to fund the project. The city originally planned to fund the project through bond sales, which would have allowed the city to complete the project in three years.

Before addressing the specifics of the 2015 benefit cost analysis, it's worth noting that the city has moved away from traditional cost-benefit analysis as the basis for the city’s evaluation of capital flood projects. Benefit-cost ratios based on avoided property damage direct capital investment to wealthier areas because higher property values produce larger calculated benefits. This creates a built-in inequity that a purely financial ratio does not address. Instead, the city now uses the project prioritization framework established in the 2022 Comprehensive Stormwater and Flood Plan, which weighs project cost alongside other factors such as life-safety risk, number of structures and residents affected, and equity considerations, among others. Applying that framework across the roughly 35 flood mitigation projects the city has identified, the South Boulder Creek Flood Mitigation Project ranks at the top of the list.

That said, the underlying figures in the 2015 benefit cost analysis are also worth addressing directly, because the 2015 calculation compared to a 2026 update, if it were completed, is not a like-for-like comparison. The 2021 annexation agreement with the University of Colorado restructured both sides of the ledger, including what the city pays, what the city receives, and what the community gets in return. Escalating a 2015 benefit estimate into current dollars while holding the project definition constant does not capture that change and comparing it against today's expanded cost figures exaggerates any perceived shortfall.

On the cost side, a substantial share of consultant expenditures incurred before 2021 was attributable to the broader public process surrounding the CU Boulder South site flood protection and annexation, which includes various analyses, extensive community engagement, negotiation support, and policy work. These were legitimate public expenditures, but they are not typical project delivery costs and including them in a benefit-cost ratio departs from how such ratios are conventionally constructed.

On the benefit side, several significant elements fall outside the 2015 framework entirely. For example individual property investments at Frasier Meadows should not be netted out of the project's benefit. While private floodproofing protects specific structures at specific elevations, it does not protect the surrounding roadways, utility connections, or emergency access routes residents depend on during and after a flood event. Regional flood protection continues to reduce the depth and duration of flooding around the property, preserve access for emergency responders, and lower risk to the systems that Frasier Meadows relies on. Site-level and regional protection are complements, not substitutes.

The annexation agreement also produces value the original scenario never contemplated. The city projects receipt of approximately $25 million in Plant Investment Fees associated with development on the annexed property, which is a direct, quantifiable offset. The city further acquired more than 180 acres of university property within city limits, carrying future value as open space, recreation, public safety infrastructure, and transportation connectivity for the Boulder community, in addition to flood protection. A ratio built solely on avoided structural damages assigns no value to such benefits.

Also, the 2015 figure monetizes property damage. It does not monetize loss of life. South Boulder Creek's flood risk is a life-safety risk, and the 2013 event is a reminder of how quickly conditions change for people in harm's way.

Updating the 2015 analysis to reflect current conditions and the full scope of the annexation agreement would produce a materially broader set of benefits alongside the broader costs. But because the project's priority was evaluated through the 2022 framework rather than a B/C ratio, a recalculated ratio would not change where this project stands.

Lastly, the possibility of purchasing the roughly 260 structures and associated properties has been considered as an alternative to building the flood mitigation project, but given the Boulder real estate market, the cost of doing so would far exceed the costs associated with the planned capital project.

No, there will not be a sudden spike in utility costs when the project starts. The costs involved with this project have been planned for and included in the city’s annual storm and flood rate change proposals since the project was approved in 2015. The storm and flood rate changes include all storm and flood projects across the city, including the South Boulder Creek flood mitigation project, and are focused on funding projects that improve safety citywide. The Water Resources Advisory Board and Boulder City Council review utility rates annually to ensure the city can support the increasing costs to maintain and improve Boulder’s aging infrastructure, provide clean drinking water, keep our wastewater facility running, maintain water mains, sewers, the stormwater system and complete flood mitigation projects. Any changes in your utility bill will be related to rate changes, approved by City Council, and will be implemented at the beginning of the year.

Boulder’s Stormwater and Flood Utility prefers a “conveyance” approach – keeping floodwater in the creek channel – to flood management where conditions allow. In situations such as South Boulder Creek south of US36 where conveyance of flood waters would increase flooding above existing conditions, detention is used as an alternative flood mitigation tool. The project will integrate environmental and aesthetic features into the overall project design as has been the city’s practice on flood utility projects.

The spill wall, or flood wall, paralleling US36 will be 6 to 18 ft in height. The concrete will be stamped and colored to limit visual impacts. Fill and revegetation will also be used to hide the overall wall structure facing US36. The rendering below shows what this will look like from US36, headed west into Boulder.

This work will require closure of the CU Boulder South site, including University of Colorado, Boulder managed trails, to prioritize the safety of community members and their pets and to minimize impacts on open space during construction. As construction progresses, the city will continue to assess site conditions and construction needs and evaluate opportunities to refine the closure area should conditions allow.

The site will close fully beginning Oct. 5, 2026. Community members are asked to obey signage and stay out of fenced and other construction areas. Updated information on road and other closures can be found on Cone Zones, including closures and detours of the US36 Regional Bikeway.

The site will be closed to protect the safety of community members, pets and construction crews. Based on the planned construction activities, keeping the site open during construction is not feasible while maintaining safe construction operations. Construction vehicles and equipment will also need to move throughout the site, making it difficult to safely separate public access from active construction areas. As construction progresses, the city will continue to assess site conditions and construction needs and evaluate opportunities to refine the closure area should conditions allow.

Community members are asked to follow signage and stay out of fenced and other construction areas.

Work includes:

• Environmental enhancement work is planned at the south end of the property.
• Flood mitigation construction will occur at the north end.
• Active construction areas, limited access through the property and contractor staging and equipment movement between the north and south areas do not leave sufficient space or separation to safely accommodate continued public access.

The CU Boulder South site will be closed, including University of Colorado, Boulder managed trails, to prioritize the safety of community members and their pets and to minimize impacts on open space, which will be undergoing restoration during construction. As construction progresses, the city will continue to assess site conditions and construction needs and evaluate opportunities to refine the closure area should conditions allow.

The city understands the popularity of existing visitor use at CU Boulder South, specifically opportunities involving recreation with dogs on and off leash. There are many areas around Boulder managed by the city that allow dogs, including on-leash trails, off-leash trails managed through the Voice and Sight Tag program and dog park locations. A full list of recreation opportunities in Boulder can be found at Recreation in Boulder | City of Boulder.

Please use caution and remain aware of possible construction equipment and materials being moved through the area, follow signage and be cautious when on the site.

The annexation agreement with the University of Colorado and City of Boulder allows for future development of the site by the university. Future plans will include recreation amenities managed by the university open to community members. The newly restored 119-acres of open space will be subject to a future planning effort. No decision has been made about possible visitor uses yet.

In addition to enhancing community safety, this project aligns with the city’s climate resilience goals. The focus on flood mitigation at CU Boulder South reflects a core principle of Boulder’s Climate Action Plan: climate action is not only about reducing emissions, but also about adapting to the impacts we are already experiencing. Alongside aggressive mitigation efforts, Boulder must plan for increasing risks such as flooding, extreme heat, wildfire and drought by integrating adaptation into land use, infrastructure, nature-based solutions and emergency preparedness. In that context, CU Boulder South represents a meaningful climate resilience partnership. By restoring floodplain function, preserving open space and incorporating nature-based approaches to reduce risk, the site supports both ecological health and community safety.

The CU Boulder South property has had different land uses and has experienced ecological changes over time. The necessary planning studies, assessments and discussions between CU and the city considered the ecology of the existing land and informed the 119 acres that will be restored and managed in the future as city open space. These 119 acres provide an opportunity for wetland and riparian preservation and restoration while providing necessary and required ecological mitigation opportunities to offset impacts of the full project. A future site planning process will be completed for the 119 acres to inform the long-term goals of this area for open space uses, including ecological preservation and any visitor use.

An estimated 260 properties will be removed from the flood plain, resulting in both lower risk and possible lower insurance premiums. To see if your property will be impacted, search your address in the map below.

Properties in the floodplain have been notified by mail.

• View the proposed floodplain map

An estimated 260 properties will be removed from the flood plain. To see if your property will be impacted, search your address in the map below.

Properties in the floodplain have been notified by mail.

• View the proposed floodplain map

For information on the university’s development of the site, please visit the University of Colorado website.

The South Boulder Creek Flood Mitigation Project is the city's highest-priority flood mitigation project because it addresses one of the community's most significant flood risks. Advancing this project is increasingly important as climate change contributes to more frequent and intense storm events, underscoring the need to improve community resilience and reduce flood risk. 

The City of Boulder has an important role to play in welcoming, supporting and serving people of diverse backgrounds in our community and in government processes. While we have done valuable equity work in the past, including the creation of a diversity policy two decades ago, an inclusivity assessment conducted in 2017 showed us that our impact has been limited. The city is currently incorporating the use of a Racial Equity Assessment Tool into project approach and implementation. While still relatively new, the hope is that applying the racial equity lens to projects such as this will create more equitable delivery of services.