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South Boulder Creek Flood Mitigation Study

Read the Study pdf  ●  Visit CU South Project Page  ●  Learn About Flood Mitigation  ●  Sign Up for Alerts

The city has conducted a South Boulder Creek Flood Mitigation Study pdf to develop and evaluate alternatives that may reduce flooding along South Boulder Creek. Boulder City Council accepted the Flood Mitigation Plan including recommendations on Aug. 4, 2015. You can read the memo or watch the video (1:05:15) of that decision.

This is webpage provides additional information about the development, public process and approval processes related to the project.

Key Information

Link to renderings of SBC flood mitigation.

  • Read the Frequently Asks Questions below to learn more about the background, approval process and next steps for the project.
  • See illustrations of what the site may look like after flood mitigation (Option D). 
  • View a recent episode of Channel 8's  Inside Boulder , featuring information about the study.
  • Learn more about the public process pdf for this project.
  • Review the project timeline pdf

Frequently Asked Questions

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

Where can I review the South Boulder Creek Flood Mitigation Study?

The South Boulder Creek Flood Mitigation Study pdf is available online. You may also wish to review the following attachments to the plan:

What is the history of the mitigation study?

In 1996, the University of Colorado commissioned a flood mapping study as part of its due diligence to purchase the 315-acre site currently referred to as CU-Boulder South Campus. The multi-year study effort resulted in FEMA replacing the 1986 flood insurance rate maps for the area with new mapping, in 2009 pdf.

A key finding of the new mapping study was that during a major storm event significant flows would leave the main stem of South Boulder Creek prior to reaching the existing culvert under U.S. 36.  These flows would travel west before overtopping the highway and flowing into the Frasier Meadows neighborhood. The 2013 flood confirmed the existence of this “West Valley Overflow.”

A flood mitigation study was initiated in late 2009 and evaluated numerous mitigation options prior to Council acceptance on Aug. 4, 2015. For project milestone, including information about the public process and options considered, please review the project timeline.

The plan accepted by council included three phases of mitigation:

  • A regional stormwater detention facility at U.S. 36
  • West Valley improvements, including a stormwater detention facility at or near Manhattan Middle School, a small stormwater detention storage area at the intersection of Foothills Parkway and Baseline Road, and enlarging the capacity of Dry Creek No. 2 Ditch
  • A stormwater detention facility located at Flatirons Golf Course

Regional stormwater detention at U.S. 36 was identified as the top funding priority for the study and is designated as Phase 1.

Who approved the flood mitigation study, and how was the public included in the process?

This planning effort was vetted through an extensive public process including 15 public meetings. You can review detailed information about the full public process pdf for this project.

The following provides a summary of the final stages of the approval process:

  • The recommended plan was presented to the Open Space Board of Trustees in 2015. The Board motioned 4-0 to support staff’s recommendation for City Council to accept the South Boulder Creek Major Drainageway Flood Mitigation Plan, specifically Option D (single berm using Colorado Department of Transportation Right of Way - and requiring no disposal of City of Boulder Open Space and Mountain Parks lands), which significantly lessens environmental impacts to Open Space Lands for Regional Detention at U.S. 36. This was conditioned in the motion that staff return to the Open Space Board of Trustees in the event staff determines construction will involve non-trivial impacts to Open Space.
  • The recommended plan was also presented to the Water Resources Advisory Board in 2015. The Board voted 3-0 (two board member were absent) to recommend that City Council accept the South Boulder Creek Major Drainageway Flood Mitigation Plan including Option D (single berm using CDOT Right of Way) for ‘Regional Detention at US 36’ along with the downstream improvements as the recommended comprehensive alternative to mitigate flood risks associated with South Boulder Creek.

Why is the detention upstream at U.S. 36 the top priority, and what are the benefits?

There are currently 515 structures and over 1,838 dwelling units in the South Boulder Creek Floodplain within city limits. A regional stormwater detention structure at U.S. 36 would provide protection to an estimated 199 structures and 1,273 dwelling units. Mitigation would provide protection for vulnerable populations as well as major transportation and utilities infrastructure.

What alternatives were evaluated and considered as part of this process?

The city initiated the flood mitigation study in 2010. This included a wide range of flood mitigation measures. These were screened based on input received at public and board meetings, hydraulic modeling and field visits. The screening results were used to formulate 15 alternative plans.

Concept-level sizing, configurations and costs were developed for each of the 15 plans, along with an estimate of likely benefits and environmental and social impacts. This work reduced the list to nine “Best Alternative Plans," which were presented at a public meeting and to the Water Resources Advisory Board (WRAB) in 2010. The WRAB recommended moving forward in evaluating the following four alternatives:

  • Maintaining the status quo
  • High-hazard zone flood mitigation and critical facility protection
  • Regional detention at U.S. 36 with downstream improvements
  • Distributed regional flood detention

Based on feedback from the 2014 public process, six new conceptual options for stormwater detention at U.S. 36 were developed. The options included variations of single- and dual-berm detention systems, and breaching the existing CU South levee:

  • Single berm with excavation
  • Single berm with excavation and fill
  • Single berm CDOT ROW
  • Dual berm no excavation
  • Dual berm with excavation
  • Dual berm with excavation and fill

The WRAB and City Council accepted the master plan in 2015, including the alternative to create a single berm regional detention with downstream improvements (called "Option D"). Downstream improvements include proposed facilities in the West Valley to capture, and convey locally generated storm flows and stormwater detention at Flatirons Golf Course. Each phase could be constructed separately with regional detention at US36 providing the most flood mitigation. 

A comprehensive summary of all alternatives considered since analysis began in 2010 is available as part of the project timeline, located at https://bouldercolorado.gov/links/fetch/33711 pdf.

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

The South Boulder Creek mitigation study is based on the runoff resulting from a storm event having a 1 percent probability of occurrence in any given year. This storm is commonly referred to as a “100-year storm.” The proposed mitigation would capture flows into the west valley up to and including that magnitude of event and release them downstream over several days in a controlled manner. Use of a 100-year design storm for floodplain management is standard practice both in the region and nationally. 

Due to the location of the proposed facility in relation to occupied dwellings and structures, the facility is required to be designed as a “high hazard 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 back to the stream, but these flows would be the incremental difference between the design detention event and the actual event.

The city and FEMA map areas with a 0.2 percent chance of flooding any given year (the 500-year floodplain). A significant portion of the city is in the 500-year floodplain, and the city currently regulates only “critical facilities” in those areas. Floodplain maps are available on the city’s website, including a map of 100-Year and 500-Year Floodplains pdf.

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. This is summarized in the following excerpt from a recently published paper from Wright Water Engineers and UDFCD:

While global and regional climate models are generally consistent in projections of future increases in average temperatures, hydrologic effects of climate change are far less certain and range from decreases to increases in annual and seasonal precipitation. The natural variability of hydrology and the short period of record of available data make it very difficult to detect trends (if any) in long-term precipitation due to changes in climate. In addition, urban flood events and infrastructure design are usually governed by short-duration rainfall events rather than season or annual averages. At this point in time, there are insufficient data to reliably forecast changes in intensity-duration-frequency estimates used to define design storms, especially for less-frequently occurring events that are of most concern for flooding. Read the full paper .

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

Potential groundwater impacts will be evaluated during preliminary design. First, 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.

The below image illustrates a partial cut-off trench example that would potentially be used with the Alternative D Preliminary Design Concept, which includes chimney and horizontal drains to control seepage through the flood control dam when it is impounding water. This approach would allow groundwater to flow normally when in the typical empty condition. This water seepage control option would be modeled to determine if pore pressure relief wells, toe drain and drainage channel systems would potentially also be required at maximum flood storage levels. Dam safety requirements would be met on any of the dam alternative designs and groundwater impacts would be fully investigated/modeled to confirm that the existing conditions would not be adversely impacted.

This is a link to the SBC Option D Dam Illustration.

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. Option D included separating Viele Channel from the pond using piping or other means. Ditch flows would be maintained in a similar manner.

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. A conceptual rendering of the dam is shown below.

Image of conceptual rendering of SBC project.

What will the area look like when the project (Option D) is completed?

Final design for the flood mitigation is dependent on current and ongoing work. However, the following renderings illustrate what flood mitigation will look like in concept.

Image of conceptual rendering of SBC project.

Image of conceptual rendering of SBC project.

Image of conceptual rendering of SBC project.

Image of conceptual rendering of SBC project.

What is the estimated cost, and how will it be funded?

As conceived, the estimated cost to construct the regional stormwater detention structure at U.S. 36 is $22 million. The cost estimate was developed based on the UDFCD master planning cost methodology handbook and associated spreadsheets. It includes a 50 percent contingency, as is common for concept-level planning efforts. The U.S. 36 regional stormwater detention structure would be funded by the stormwater and flood management utility enterprise fund through issuance of bonds.

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. Future uses are currently being evaluated as part of the Boulder Valley Comprehensive Plan major update . 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 immediately downstream. While the existing levee is shown as remaining in place in the proposed “Option D” flood mitigation scenario, it is not necessarily for downstream flood mitigation. 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.

Can the city remove the existing levee on CU South and allow "natural" flood mitigation across that property?

The existing levee on CU South provides flood risk reduction for a limited portion of that property. FEMA does not consider the levee to be able to provide protection to the West Valley Area north of U.S. 36. The area that FEMA does currently consider to be protected is depicted on the adopted FEMA Flood Insurance Rate Map. As depicted on the FEMA map, flood flows travel around the northern end of the levee and into the West Valley under current conditions. If the levee was removed, flood waters would still flow toward the same low point near the Table Mesa Park-n-Ride and into the Frasier Meadows neighborhood. 

Engineering analysis conducted to date does not support the theory that the area inside the existing levee has the ability to fully absorb, detain or otherwise substantially mitigate the impacts of a 100-year storm event on downstream properties without structural improvements. The final set of Phase 1 mitigation alternatives considered by City Council included three options (E., F., and G.) that incorporated the concept of breaching the existing CU levee. The engineering consultant, CH2M, concluded that a downstream structure adjacent to U.S. 36 would still be needed to mitigate flood risk under all three of the scenarios tested.

Prior to the mitigation study, the impact of the levee was evaluated by HDR Engineering, Inc. during the development of the hydraulic flood model for South Boulder Creek. Hydraulic model runs with the levee removed resulted in greater impacts to the West Valley. The  hydraulic modeling report pdf is also available for review.

Finally, historical documents indicate that the West Valley overflow previously occurred during a May 1969 flood event. The existing CU levee was constructed by the prior owners of the property in 1981, more than a decade later.

Thunderbird Drive, May 1969
Thunderbird Drive, May 1969

South Boulder Creek area during 1969 flood
South Boulder Creek area during 1969 flood

What is the status of the design and construction of the mitigation project?

The selected mitigation alternative, “Option D,” is highly conceptual and intended to provide a general sense of the scope and extent of the mitigation project. The city has selected an engineering consultant team to prepare preliminary design of the U.S. 36 regional detention facility and an initial scope of work has been negotiated. Work on preliminary design will begin once there is additional clarity regarding the city’s ability to acquire necessary property. Specific design details and permitting would be addressed as part of that process.

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 beyond predicted 100-year or 500-year storm event. 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, a primary outlet structure, and then a culvert under U.S. 36 eventually returning 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 a 100-year storm event through the primary outlet structure. Storms exceeding a 100-year 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. Depending on the final configuration of the detention pond, the existing culvert that conveys the main stem of South Boulder Creek under U.S. 36 could potentially serve as the spillway.

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.

 

Link to South Boulder Creek Flood Mitigation project timeline

Contact

Kurt Bauer , Engineering Project Manager

303-441-4232

bauerk@bouldercolorado.gov

Sign up for emails about the South Boulder Creek Flood Mitigation Planning Study.

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