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South Boulder Creek Flood Mitigation Project

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

Background

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

In September 2017 City Council and Boulder County approved the updated Boulder Valley Comprehensive Plan (BVCP). Within the BVCP, Guiding Principles for the South Boulder Creek flood mitigation efforts were also approved. The principles provide a framework for assessing flood mitigation options and for discussions between the city and the university about future uses of the CU Boulder South property. Use of the property for city flood mitigation would require City Council approval of an annexation agreement between the city and the university.

What's Happening Now

On Aug. 21 City Council directed staff to proceed with the next steps of preliminary design and landowner negotiations for the Variant 1, 500-year concept, without CU levee, and, within that concept, evaluate ways to minimize flood detention on the Public (PUB) land use on CU South. On Sept. 20, 2018 staff presented council with several options that would reduce the impact to the Public land use on the site and City Council confirmed which option best met their Aug. 21 direction (see figure below). Additionally, on Sept. 20 council stated a preference for the following considerations regarding design of the project:

  • Council prefers the design not include excavation on the area designated Open Space – Other on the CU South site; when moving the excavation area, staff should avoid impacting the existing tennis courts on the Public land use area.
  • Throughout preliminary design, council has a preference that staff explore and implement to the maximum extent possible the July 16 Open Space Board of Trustees recommendations while retaining some flexibility in project location;
  • Council prefers realigning Dry Creek Ditch #2 south of US36 and securing water rights for irrigation.

On Oct. 9, 2018, there will be a council study session to discuss the future annexation process for CU South with City Council and University of Colorado Boulder staff.  You can review the study session memo. This meeting will not include a public hearing.

On Monday, Oct. 1, the City of Boulder received a letter from the University of Colorado pdf outlining their requirements for the site and the proposed community benefit they will offer. This letter will help guide the Study Session discussion on Oct. 9.

Community Engagement

Thank you to everyone who attended the June 7 open house. You can review the staff presentation pdf and questionnaire results. pdf related to the concept evaluations. 

Thank you to everyone who submitted an Evaluation Criteria questionnaire. You can view a summary of the results here. pdf For those that were unable to attend the April 23 open house, you can review the staff presentation here  pdf along with the open house summary. pdf

The South Boulder Creek Flood Mitigation project is applying the city’s Engagement Strategic Framework. Community members can expect opportunities to review and provide feedback on proposed project configurations and the project evaluation criteria before any recommendations are made to City Council.

All community feedback will be considered equally and will be compiled, made publicly available on the city website and given to Boards and City Council for consideration when staff provide a recommended project configuration this summer. You can review additional community, CU and consultant meeting notes here.

Past Community Engagement Activities

June 2018:

  • Open House
  • Online Questionnaire
  • Meeting with community members on Dry Creek Ditch No. 2

May 2018:

  • Meeting with ad-hoc group of interested community members
  • Open House
  • Online Questionnaire

April 2018:

  • Presentation to Fairview High School's Net Zero Club
  • Meeting with ad-hoc group of interested community members
  • Presentation to CU Boulder class
  • Public open house
  • Citywide open house- What's Up Boulder

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 (Option D). 
  • 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 are being 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. 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 concepts, all 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 flood mitigation concepts that could better adapt to a changing climate. At this time Variant 2 could likely be the best option for future adaptation measures. 

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 Master Plan concept included separating Viele Channel from the pond using piping or other means. Ditch flows would be maintained in a similar manner. Two of the Phase I concepts show the detention area being drained to the north side of US36 (Master Plana and Variant 1 concepts). 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.

City Council will be presented with an evaluation of the Phase 1 concepts on August 7. If council selects one concept variation to move forward, preliminary design will begin. It is anticipated that design could take approximately two years to complete. Securing the necessary project permits would occur concurrently. Rights to construct the project would also need to be secured 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

Kurt Bauer , Engineering Project Manager

303-441-4232

[email protected]

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