South Boulder Creek Flood Mitigation/CU South

The proposed flood mitigation project has been found to have the least environmental impacts of the alternatives considered, including those to wetlands. Project design criteria includes replication of existing groundwater flow patterns to prevent upstream groundwater mounding, drying up downstream wetlands and/or other potential adverse impacts. The design has not been determined to effect wildlife passage under US36, although enhanced wildlife passage under US36 could be considered as possible environmental mitigation during the preliminary design phase of the project. No project impacts are anticipated to the existing South Boulder Creek trail and the US36 regional bike trail will be maintained in approximately its existing alignment. Environmental considerations associated with the existing levee, including its relationship to the floodplain, will be discussed at the June 3 Open Space Board of Trustees meeting.   

The table below provides the design detention volumes for each option: 

It is important to note that flood mitigation results from both detention and conveyance. The detention volume is one component of the design along with the conveyance capacity of the outlet works. The current configuration of the CU South property does not provide substantial storage without additional modification (i.e., construction of a berm/embankment) to detain water. 

Protecting City of Boulder and Boulder County residents from future flooding events is a primary driver of activity in this area. The city Utilities Department does not own the land or have utility easements for the proposed flood project. A portion of the land needed for flood mitigation is owned by the university. The land needed for flood mitigation is only available if and when the entire CU South property is annexed. There are currently no options to proceed with the flood mitigation dam without annexing CU South. Unfortunately, the timing of the city’s flood mitigation project and planning for future development on this site do not align. In light of the desire to move forward with flood control (accompanied by annexation) even in the absence of development plans from CU Boulder, the city and county adopted the CU South Guiding Principles in 2017 as part of an update to the Boulder Valley Comprehensive Plan. The guiding principles include concepts for controlling development like building height limits, an emphasis on residential uses, public access to trails and ways to address off-site transportation impacts (e.g., the number of trips per day, transit service). Further refinement of annexation details are in part dependent upon a flood design. Therefore, more in depth annexation negotiations and community engagement opportunities are anticipated after a flood design is chosen in June 2020.  

1. No taxpayer money is going to CU for the flood mitigation project. The project will be paid for by Stormwater and Flood Management Utility rate payers.   

Follow-up #1. No. Project costs are paid for through utility bills, not through taxes.  

Follow-up #2: The $15 million is an estimate of existing structures or features that will be impacted by the flood mitigation project and is unrelated to the 80 acres of land the university has committed to convey to the city for construction of a flood mitigation project or to be used for open space mitigation related to the project. If CU did not convey the 80 acres, the city would have to purchase that as an additional project cost. 

2. The flood mitigation project will address flooding in the West Valley overflow and will not address flooding in surrounding neighborhoods. Impacts of CU South annexation to surrounding neighborhoods will be considered as part of the annexation process. Annexation agreement negotiations will be guided by the CU South Guiding Principles and will provide opportunities to influence the annexation terms through city boards and the City Council meetings. The schedule and process for these meetings will be further refined once a South Boulder Creek flood mitigation design level is chosen, which is anticipated in June 2020.  

Additionally, a multi-modal transportation and access study will be completed prior to annexation to determine necessary improvements, appropriate points of access, performance standards (e.g., trip budget) and other related topics. City staff will coordinate with CDOT during and after the study and each impacted neighborhood will be consulted throughout that process. The study can be completed once a flood mitigation concept is chosen and more is known about the related impacts to the site.  

3. Follow-up #1: The city Utilities Department does not own the land or have utility easements for the proposed flood project. The university owns a portion of the property. Page 3, paragraph 1.b. of the Boulder Valley Comprehensive Plan CU South Guiding Principles states “Specific real property ownership, easements, and/or agreements will be established during annexation for the area necessary for floodwater improvements and other uses...” The related figures depict the 129 acres of developable land required by the university in any acceptable flood alternative.  

Follow-up #2: Boulder City Council representatives will ultimately vote on whether to accept the negotiated annexation agreement on behalf of the City of Boulder. 

Work subsequent to 2001 has informed the current project approach. Specifically, floodplain mapping accepted by FEMA in 2010 describes the West Valley flood risk discussed in the South Boulder Creek Major Drainageway Plan that was accepted by council in 2015. In 2017 the city and county incorporated these floodplain considerations in the Boulder Valley Comprehensive Plan CU South Guiding Principles that allow for flood mitigation on and guide the development of the CU South property.  

The flood mitigation project’s design criteria require no adverse impacts to downstream property owners, including the US highway 36 bridge. This means a successful design must match existing hydraulic conditions through the bridge. The 500-yr event flows and depths increase through the US36 bridge and therefore do not meet these criteria.   

1. Two separate sets of regulatory standards apply to the project, one for the floodplain and one for dam safety and design. Regulatory standards for floodplain mapping and flood mitigation do not require an allowance for climate change. However, city staff are evaluating if and how a voluntary climate change allowance might be incorporated into the design. The state engineer’s office has regulatory jurisdiction over the dam structure and does require a climate change allowance for design of high hazard dams, and such an allowance is being incorporated into the South Boulder Creek flood mitigation design. 

2. The best way to identify structures currently located in a floodplain is through the City’s flood hazards map. Proposed projects currently identified in each drainage are included on the city’s flood information webpage. Attachment C to the April 20, 2020 Water Resources Advisory Board materials (page 209 of 242) provides cost estimates related to the broader Stormwater and Flood Management Utility. Each drainageway is evaluated through a “lifecycle” of mapping, mitigation planning, design and construction and each drainageway is at a different point in the lifecycle. Therefore, existing mitigation cost estimates for all drainages in the city are a work in progress and potentially understate the overall actual need. Updated cost estimates for the various drainageways are continuously being developed and will be a topic included in the Comprehensive Flood and Stormwater Master Plan update taking place in 2020 and 2021.  

3. The city has held preliminary discussions with CDOT about the project but won’t be able to obtain more definitive determinations until staff presents the alternative council chooses and enough associated design work, including hydraulic analysis at the US36 bridge, for their input and review. As with any project, the city would need to reevaluate the proposed flood mitigation approach if required permits or approvals were denied. 

4. Maintenance of Viele channel is included in the ongoing Stormwater and Flood Management Utility as one of the 16 major drainage ways in Boulder and is planned for irrespective of the flood design project. Post construction, the Viele channel would be maintained as required to provide adequate capacity for the flood mitigation outlet works. 

5. Current environmental impacts were estimated by overlaying existing Geographic Information Systems (GIS) data, including environmental information and environmental surveys of the project area, with the proposed project footprint(s). The table below provides a summary of the resulting environmental impacts, including the impacts to Threatened and Endangered species habitat. The work done to date is adequate for comparison purposes at the conceptual design level and will be further refined as design proceeds. 

Potential Environmental Impacts 

Note: Figures are rounded to the nearest 1/10th of an acre 

* Placement of fill is considered an impact in USACE 404 permitting, whereas inundation with water is not considered an impact. Because fill placement is unique to each of the alternatives based on the non-inundated property available, the 200-yr storm has more land available for fill, including more wetlands, thus the larger area of impact.    

6. US 36 is not an approved flood control structure. The FEMA letter associated with the highway widening documents changes to the SBC floodplain resulting from the US36 widening project. These changes do not prevent US36 from overtopping during a 100-yr flood and do not provide flood benefits for the West Valley area that the proposed flood mitigation project is designed to protect.  

Asking permitting agencies to speculate in writing about specifics of what they might or might not approve before such detail is available to do so would be a dramatic departure from normal project process and would not be conducive to efficient agency approvals. If such out-of-process documentation will be a requirement of the project going forward, staff would not recommend proceeding. Conditions that will be fundamental to approval for any alternative presented to CDOT involving the US36 bridge cannot be met for the 200- and 500-year flood levels.  

The project costs include costs for construction of Phase 1 from the South Boulder Creek Master Plan. This includes the direct costs of constructing the flood mitigation structure (detention facility), the earth fill cost to provide 129 acres of PUB land use area, and estimated cost of impacts to existing university facilities. These line items in the table below are delineated because earth fill is not required for the flood mitigation protection but needed to offset land use impacts from creation of flood detention. 

Opinion of Probable Construction Cost 

Note: Figures have been rounded to the nearest million dollars 

(1) Includes impacts to CU Tennis Courts, South Loop Drive, Warehouse, Utilities Connections, Recreation Fields. Estimated costs/impacts will be refined as the flood mitigation project progresses through the design process and finalized through the annexation process  

Long term operation and maintenance would typically be estimated at 0.5% to 1% per year of initial construction cost of dam features and is not included in cost tables but is within the margin of accuracy of the estimates. All alternatives would have similar features and would require similar maintenance. 

Environmental impacts are based on the project footprint (area underneath a design element) over ecologically sensitive areas (i.e., threatened and endangered species habitat, wetland, etc.). The disturbance area used for the project footprint includes an allowance for construction access and staging. Any additional construction access roads and staging areas, if needed, would be determined once a design concept is selected and construction drawings and specifications are developed. 

A multi-modal transportation and access study will be completed prior to annexation to determine necessary improvements, appropriate points of access, performance standards (e.g., trip budget) and other related topics. City staff will coordinate with CDOT during and after the study and each impacted neighborhood will be consulted throughout that process. We anticipate the transportation study being completed after the flood mitigation project is selected in June 2020, given that the different concepts impact the site differently (e.g., perimeter access points).