Show/Hide

The City of Boulder welcomes your feedback. Use our Inquire Boulder customer service tool to tell us what’s on your mind.

  • Assistance
  • Rebuild and Restore
  • Recovery and Resilience
  • Creek Projects
  • Maps of Floodplains
  • Floodplain Development
  • Prepare for Floods
  • Protect Your Property
  • Respond to Floods

South Boulder Creek Flood Mitigation Project

Read the Flood Mitigation Study pdf  ●  Visit CU South Project Page  ●  Learn About Flood Mitigation  

What's Happening Now

The city and the Colorado Department of Transportation (CDOT) continue to discuss how they can work together on the South Boulder Creek flood mitigation project. You can read the City of Boulder and CDOT's Statement of Collaboration. pdf

On Tuesday, March 5, City staff provided an update to City Council on the flood mitigation project, including recent discussions with CDOT, anticipated project schedule, and next steps. You can watch the video of the presentation.

City staff met with CDOT Region 4 staff on Feb. 12 to discuss the South Boulder Creek flood mitigation project. As a first step, city and CDOT staff verbally agreed to evaluate the engineering and legal details of the project for mutual benefit opportunities between the city and CDOT.  City and CDOT staff are working on a joint written statement to document the conversation, which will be posted on this website once approved. 

On Tuesday, Feb. 5 City Council directed staff to move forward with preliminary design for the original Variant 1 -500 concept (see concept image and summary of meeting outcomes below) for South Boulder Creek flood mitigation. Council also asked staff to keep them apprised of conversations with the Colorado Department of Transportation (CDOT) regarding the use of a portion of CDOT’s right-of-way for the project.

 

Update from Feb. 5 Council Meeting

As a follow-up to the Feb. 5 council agenda item 6A “Update on the South Boulder Creek Flood Mitigation Project and Current Status of the CU South Annexation Process,” below is staff’s understanding of the direction provided by council. Shortly after the meeting, the project team started pursuing next steps in response to this direction. 

  • Continuing discussions with CDOT: Council directed staff to pursue written documentation from CDOT staff confirming their commitment to work collaboratively with the city through the engineering and legal details of constructing a floodwall in the US36 right-of-way as part of the South Boulder Creek flood mitigation project.
  • Flood Mitigation Project Design Concept: Council provided the following direction to staff regarding next steps of design for the project:
    • Move forward with preliminary engineering for the original Variant 1, 500-year concept.
    • The Variant 1, 500-year concept footprint should be considered a fixed point in future annexation-related discussions about other future uses on the Public land use area of CU South. 
    • The next steps of project design should focus on the engineering needs of the project, with public safety being the primary driver.
    • The design should not focus on accommodating future sports fields in the detention area.
    • In moving forward with the design, if the project engineering team identifies opportunities to reduce the project’s impacts on the Public land use area, the team has the discretion to evaluate such opportunities only if they would not affect public safety, increase the costs of the project, significantly increase excavation, or delay the project design schedule. Staff noted at the meeting that it is unlikely that significant reductions in the project’s extent onto the Public land use area would occur given these parameters.
  • Annexation Process Timeline: Council members agreed to continue the annexation process discussed with council at the Oct. 9, 2018 study session. The goal would be to be to align both the preliminary engineering and the annexation application processes so that one process does not delay the other.

Council concluded the agenda item by recommending that staff use emails and verbal council briefings to provide updates to council about the flood mitigation project and CU South annexation process. Staff is working on scheduling a council briefing for March 5 to provide an update on the next steps for preliminary design of the project. Staff will also use the council email and Heads Ups to provide updates on key milestones or any significant changes to the project or schedule.

 

 

Background

Over the past 80 years, South Boulder Creek (SBC) has flooded significantly six times, with overtopping of US36 occurring in 1969 and 2013. As a result, the city has been working to mitigate future flooding impacts over the last decade. Below are the major milestones in the SBC flood mitigation project.

South Boulder Creek Master Plan – On Aug. 4, 2015 Boulder City Council accepted the Final South Boulder Creek Major Drainage Plan, with recommendations for three phases of flood mitigation. Regional detention at US36 to prevent overtopping of US36 during large storm events was selected as Phase 1 due to the large downstream benefits. You can read the memo or watch the video (1:05:15) of that decision.

BVCP Update - In September 2017 City Council and Boulder County approved the updated Boulder Valley Comprehensive Plan (BVCP), including Guiding Principles for future uses on the University of Colorado’s Boulder South property (CU South). Use of the property for city flood mitigation would require City Council approval of an annexation agreement between the city and the university. Visit the CU South project page for more information about the annexation process.

Phase 1 Project Concept Development - the city selected the RJH Consultant team (RJH) to provide engineering services for preliminary design of the Phase 1 South Boulder Creek flood mitigation project. In 2017 and 2018 the project team engaged the community to review and gather feedback on design concepts based on Phase 1 project goals and the CU South Guiding Principles. The goal was to identify a community preferred alternative to help move forward into preliminary design.

Community Engagement

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.

The preliminary design phase of the project will be focused primarily on meeting engineering design best practices and federal, state, and local regulatory requirements, and thus will be largely driven by discussions and review by the State Engineer’s Office, environmental agencies, and agencies involved in floodplain review.

The project team will be utilizing the “inform” level of community engagement during preliminary design. Staff plans to provide regular updates to inform the public, WRAB, OSBT, and Council as the project progresses through preliminary design. 

Past Community Engagement Activities

November 2018:

  • Meeting with member of the South Boulder Creek Action Group

October 2018:

  • Meeting with members of the Southeast Boulder Neighborhoods Association
  • Meeting with members of the Frasier Meadows Retirement Community, South Boulder Creek Action Group

June 2018:

May 2018:

  • Meeting with ad-hoc group of interested community members

April 2018:

March 2018:

  • Meeting with interested community members and city consultants
  • City staff reviewed projects related to the CU South property as a part of a CU Boulder class
  • Meeting with ad-hoc group of interested community members

January 2018:

  • Meeting with Frasier Meadows Retirement Community, South Boulder Creek Action Group, Council members Jill Adler and Bob Yates.

 

 

City Council and Board Meetings

 

 

 

 

 

 

 

 

Key Information

  • Read the Frequently Asks Questions below to learn more about the background, approval process and next steps for the project.
  • View an annotated  simulation of a 100-year flood and what occurs without flood mitigation in the area.
  • See illustrations of what the site may look like after flood mitigation. These are based on the SBC Master Plan Option D. Updated renderings based on the current concept will be added soon. 
  • View an episode of Channel 8's  Inside Boulder , featuring information about the study.

 

Frequently Asked Questions

Use the follow drop-down menu items to learn more about this project.

Why was detention near US36 chosen?

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 pdf 

How many people are in the floodplain now and what are the benefits of flood mitigation?

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.

What alternatives are being evaluated and considered as part of the preliminary design phase?

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. 

What type of storm would the proposed Phase 1 mitigation protect against?

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.

Has climate change been considered?

The City of Boulder continues to work closely with the Urban Drainage and Flood Control District (UDFCD) to understand and plan for the potential impacts of climate change on flood risk. UDFCD and FEMA are not recommending changes to the hydrologic assumptions used to map floodplains and develop corresponding flood mitigation measures for the Front Range, including the Boulder area. The city is looking at ways the flood mitigation concept could better adapt to a changing climate. 

How does the design of the dam account for groundwater flow?

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.

Will this project impact existing floodplains or disrupt irrigation ditch flows?

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.

What will be the impact of project construction, and how long will it last?

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. 

How is this project connected to development of CU South?

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.

What is the status of the existing levee on the CU property?

The existing levee on the CU property is recognized on the existing FEMA Flood Insurance Rate Map pdf 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.  

Can the dam fail?

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. 

Are there further mitigation measures proposed for South Boulder Creek?

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.

Where can I find additional documents on the progress of the project?

Follow this link to review project team meeting minutes and monthly progress reports. 

 

Contact

Molly Scarbrough, Senior Project Manager

303-441-4939

[email protected]