South Boulder Creek Flood Mitigation Project
The South Boulder Creek Flood Mitigation project is part of the City of Boulder's efforts to minimize the damaging effects of a major flood. Alongside the University of Colorado-Boulder, Open Space and Mountain Parks, the Mile High Flood District and Colorado Department of Transportation (CDOT), the project team has worked to identify the best flood mitigation option.
The City of Boulder is committed to its core value of community safety and is proactively taking steps to limit the spread of COVID-19. To accommodate social distancing protocols, meetings will be held remotely. Updates will be posted to this webpage.
The CU South Annexation Project Team will be hosting a handful of office hours during the first half of December 2020. Visit the CU South Annexation Project Page for dates and times.
Mid-November. – February 2021: Community Engagement Window. Visit Be Heard Boulder to engage on this project
City Council Process Subcommittee Meeting will be held from 11 a.m. to 12 p.m. (meeting information on the Visit CU South Annexation Project Page).
Feb. 2, 2021: City Council Meeting - rescheduled from Jan. 5 agenda item. Jan. 5 Council Memo
Jan. 5, 2021: City Council Meeting - note: council delayed discussion of this item to Feb. 2 due to late hour
- Read the Jan. 5 City Council Memo
Dec. 16, 2020: OSBT Meeting
- Read the Dec. 16 OSBT Memo
Dec. 11, 2020: City Council Process Subcommittee Meeting
Dec. 3, 2020: Planning Board Meeting - update under Matters
Nov. 18, 2020: Open Space Board of Trustees Meeting and Public Hearing.
CU South Process Subcommittee: The purpose of this group is to explore options to respond to the second part of City Council’s June 16, 2020 motion.
South Boulder Creek Advisory Group: The purpose of this group is to advise staff in advance of the Sept.25 Process Subcommittee meeting on development of a process through which staff can respond to the second part of City Council’s June 16, 2020 motion.
There are an estimated 600 structures and 3,500 people in the South Boulder Creek floodplain within city limits. Flood mitigation studies have looked at several locations along the creek and through extensive public feedback and analysis, found the best location for stormwater detention was near US-36 on property owned by the University of Colorado-Boulder.
The purpose of flood mitigation, or lessening the damaging effects of a flood, is to provide protection for community members and major utilities and transportation infrastructure including US-36 and Foothills Parkway, while being the least impactful to open space. Since City Council approved the plan in 2015, city staff has been working collaboratively with CU-Boulder and the Colorado Department of Transportation (CDOT) in developing flood mitigation options for the area.
Over the last 80 years, South Boulder Creek has flooded significantly six times, with overtopping of US-36 happening in 1969 and 2013. As a result, the city has been working to mitigate future flood impacts over the last decade. Learn about major milestones in the South Boulder Creek Flood Mitigation project. You can review past milestones from the project. Major Milestones .
Collaboration with University of Colorado - Boulder: For some time, the city has been working with CU-Boulder to find a flood mitigation plan that will work best for everyone. For more information about annexation view the South Project Page
Review past project milestones Milestones.
Mile High Flood District is supporting the South Boulder Creek Flood Mitigation project with funding and based on the MHFD Five Year Capital Improvement Plan, learn more MHFD
The South Boulder Creek Flood Mitigation project is applying the city’s Engagement Strategic Framework. Extensive community feedback was provided during the assessment and selection of project concept alternatives through the South Boulder Creek Master Plan process and Phase I concept development phase to inform City Council decisions on the project. Staff plans to provide regular updates to inform the public, WRAB, OSBT, and Council as the project progresses through preliminary design.
The flood mitigation study examined multiple locations along South Boulder Creek and through extensive public feedback and the analysis process, found that the best location for stormwater detention was near US36 on the University of Colorado Boulder’s property. The recommended plan was accepted by City Council in 2015. You can review this final Flood Mitigation Study
There are currently approximately 600 structures and over 3,500 people in the South Boulder Creek Floodplain within city limits. A regional stormwater detention structure at U.S. 36 designed for a 100-year event would provide protection to an estimated 260 structures and 2,300 people. Mitigation would provide protection for vulnerable populations as well as major transportation and utilities infrastructure including US36 and Foothills Parkway.
As of July 2018, there are three different concept variations that are being considered. The Master Plan, Variant 1 and Variant 2. Each concept has sub-variations including options for a 100 and 500-year design storm and with and without the CU levee. City staff has received feedback from the community and input from the Open Space Board of Trustees, Planning Board and the Water Resources Advisory Board. The concepts will be presented to City Council on August 7 for their consideration.
The South Boulder Creek regional detention concepts were evaluated for both a 100 and 500-year design storm. The 100-year storm has a 1 percent change of occurring in any given year, the 500-year a .2 percent chance in any given year. In August and September 2018 City Council directed staff to proceed with a concept designed to mitigate a 500-year design storm. The proposed mitigation concepts would capture flows from South Boulder Creek to prevent overtopping of US36 and further flooding into the West Valley. Floodplain maps are available on the city's website showing the current 100-year and 500-year floodplains.
Due to the size of the proposed concept, it would be considered a “high hazard potential dam” under the oversight of the State Engineers Office. High hazard dams include an emergency spillway that routes events larger than the detention design storm up to the Probable Maximum Flood (PMF) without compromising the integrity of the dam. Activation of the emergency spillway would result in flows spilling, as they do today, but these flows would be the incremental difference between the design detention event and the actual event.
The City of Boulder continues to work closely with the Mile High Flood District (MHFD) to understand and plan for the potential impacts of climate change on flood risk. There's currently no regulatory standard for incorporating climate change impacts into flood mitigation designs. However, the city is looking at ways the flood mitigation concept could better adapt to a changing climate.
Potential groundwater impacts will be evaluated during preliminary design. Currently, groundwater data will be collected to document existing conditions. Groundwater modeling will then be performed to determine what likely impacts the dam cutoff system will have on normal ground water movement. The design will then include mitigation measures such as filtered drain systems to ensure pre-construction groundwater conditions.
This project, similar to all of the city’s flood mitigation projects, cannot negatively impact existing floodplains or ditch flows. The specific methods to accomplish this will be developed during preliminary design. The South Boulder Creek Master Plan Phase 1 recommendation included separating Viele Channel from the pond using piping or other means. Ditch flows would be maintained in a similar manner. The Phase I concepts selected by council in Aug./Sept. 2018 to move forward into preliminary design shows the detention area being drained to the north side of US36. Discharge of this water would be carefully considered during preliminary design to ensure that flows do not negatively impact downstream features including Viele Channel or Dry Creek No. 2 Ditch. The attached PDF figure shows the existing drainage and ditch systems in and around CU south parcel.
It is anticipated that once the design is completed and all necessary permits are secured, construction of the regional stormwater detention structure will last approximately two years. Construction would include the use of large, heavy equipment to construct the earthen embankment, create an outlet and emergency spillway and complete any necessary excavation.
A significant portion of the land identified for Phase 1 flood mitigation is owned by the University of Colorado. The proposed city mitigation project would provide protection to neighborhoods downstream of U.S. 36 by flooding portions of the CU property during significant rainfall events. The city will need to acquire land and/or easement rights from the university prior to proceeding with mitigation. CU is seeking an agreement with the city to address the overall future uses of the property prior to conveying land for flood mitigation. Visit the city's CU South webpage to learn more.
The existing levee on the CU property is recognized on the existing FEMA Flood Insurance Rate Map as providing some level of flood protection to an area on the CU South parcel. The levee does not provide downstream benefits. It has been determined that the existing CU levee can be removed and not affect the Phase I regional detention concepts. CU has not proposed specific uses for the area protected by the levee at this time. Information about the current land use suitability analysis occurring through the Boulder Valley Comprehensive Plan major update is available at the city's CU South webpage.
In August and September 2018, City Council directed staff to move forward with design on a modification of the Variant 1, 500-year concept. The project team is currently evaluating the option selected by City Council on Sept. 20, including opportunities to address council’s stated preference to not include excavation on the OS-O area of CU South and CU’s interest in future use of the detention area for recreational fields. Staff will provide a project update to council on Feb. 5, 2019. Project design could take approximately two years or more to complete. The timeframe for design could be extended depending how long it takes to secure the necessary project permits, approval by the State Engineer’s Office and rights to construct the project from property owners. Following design, permitting and property agreements, construction could begin. It is anticipated that construction would take approximately two years to complete.
Like any structure such as a bridge or a building, a dam can fail. However, that risk is weighed against the consequences of not having the structure in the first place. In the case of the proposed South Boulder Creek mitigation, the consequence of not building the structure is the continued significant risk of future loss of life and property damage during a major precipitation event. South Boulder Creek has experienced flooding multiple times during the city’s relatively short history, including the most recent flood in 2013. There is little doubt that the area will experience flooding again in the future. By comparison, failure of the proposed flood mitigation dam is extremely unlikely, and the risk of failure can largely be mitigated through proper design, which starts with the classification of hazard potential.
Every dam has a hazard potential rating that is based on the consequences of failure. A dam with a “high hazard” potential classification is not an indication that a current or proposed dam is, or is expected, to be in poor condition. While the proposed dam would be relatively small and would only impound a significant amount of water during major storm events, it would be designed based on the assumption that failure could result in the loss of human life. This classification results in a structural design and spillway capacity based on the most theoretically extreme conditions, a “probable maximum” event. This event is significantly greater than the FEMA 100-year or 500-year storm events. Proceeding with proposed mitigation assumes that reducing the risk of loss of life and property damage during storms that can be anticipated to occur regularly outweighs the incremental difference in impact during an event with an extremely low probability of occurrence.
During more routine storm events, South Boulder Creek stays within its banks and a limited amount of water would flow through the new detention area and drain back to South Boulder Creek. During storms large enough to exceed the flow capacity of the primary outlet structure, water would begin to back up behind the dam while continuing to be released through the outlet structure in a controlled manner. The dam would be designed to store and release up to the design storm event (either a 100 or 500-year storm event) through the primary outlet structure. Storms exceeding the design storm event would completely fill the detention area, and excess water would be released through a controlled spillway. This would avoid uncontrolled overtopping of the dam and appropriately direct flows downstream. The specific location and nature of the spillway would be determined through the engineering design process, but would be along U.S. 36.
The 2015 flood mitigation plan recommends implementing flood mitigation for South Boulder Creek in three phases. All three phases can be done separately with the highest priority phase the U.S. 36 detention. The second phase includes improvements downstream in the West Valley and the third phase includes detention storage in Flatirons Golf Course. These phases are not currently included in the city’s six-year capital improvements program due to funding constraints. Future phases would go through additional public process prior to design.
For more information, including past meeting materials, additional background and past community engagement, visit Additional Documents
Brandon Coleman, Engineering Project Manager, Utilities
Video Tour! View the area upstream of the proposed flood mitigation project to understand flood design consideration and the environmental resources and mitigation opportunities in the area. View the Video
|Inside Boulder News - CU South and BVCP
From City of Boulder
|Inside Boulder News - South Boulder Creek Flood Mitigation
From City of Boulder