By Mary Dolores Young
One major community concern regarding South Boulder Creek (SBC) flood mitigation is that, given climate change, flood mitigation would be best at the 500-year flood level. I would like to provide background and history that informed Council’s decision to select a 100-year protection level instead. A mitigation project implemented for a 100-year flood event (1% chance of occurring in any year) will detain all of the local flood waters. A 500-year level project would detain all of the local waters from a 500-year flood event (0.2% chance each year).
In the last 80 years, SBC has flooded significantly six times, overtopping U.S. 36 twice, in 1969 and 2013. The first flood mitigation planning for SBC was undertaken in 1973, but was never formally adopted.
The potential for significant SBC floods overtopping U.S. 36 and battering east Boulder was first identified in 1996 through a flood study commissioned by the University of Colorado (CU) as part of its due-diligence for purchasing the 308-acre site now known as CU South. The study pinpointed inaccuracies in the 1986 Federal Emergency Management Agency (FEMA) flood maps. Those maps were corrected and officially revised in 2010.
Boulder’s Water Resources Advisory Board (WRAB) spent 2010 considering alternative approaches to SBC flood mitigation, narrowing the list of concepts to nine specific candidates. Studies continued, and by 2014 the WRAB, the Open Space Board of Trustees and Council landed on the approach with the most advantages: Regional Detention at US-36. However, both boards and City Council voiced significant concern over potential environmental impacts to Open Space and Mountain Parks (OSMP) lands. Staff was, therefore, directed to explore options that would reduce impacts to sensitive habitat, including use of a larger portion of the CU South property.
The next refinement was Option D: detention structures located within existing Colorado Department of Transportation (CDOT) right of way, affecting OSMP lands only when stormwater is detained. While CU preferred the 2014 plan, they indicated a willingness to consider using their land to facilitate Option D. The design would require permitting by six agencies: Army Corps of Engineers, US Fish and Wildlife, Federal Emergency Management Agency, CDOT, State Engineer’s Office, and City wetlands and floodplain permits.
In August of 2015, City Council accepted the South Boulder Creek Major Drainageway Flood Mitigation Plan including Option D.
At this point in discussions, CU conditioned use of the property for City flood mitigation on approval of an annexation agreement between the parties, thus launching a separate process.
In September 2017, the City and County approved the updated Boulder
Valley Comprehensive Plan, including Guiding Principles for future uses and annexation of CU South.
In 2017 and 2018 the City engaged the community to review and gather feedback on Option D design concepts and the CU South Guiding Principles. The four concepts were: Variant 1, 100-year and 500-year; Variant 2, 100-year and 500-year. Over the strong objection of CU, Council eliminated both Variant 2 options and directed staff to begin preliminary design for the Variant 1, 500-year option.
Engineering problems always come with constraints and criteria that must be met. In the case of Option D, one of the main constraints was the bridge at US-36 under which South Boulder Creek flows. That bridge is under the jurisdiction of CDOT. Any flood mitigation project is not allowed to make flooding conditions worse than the status quo.
In 2019, through the preliminary design, engineering determined that the Variant 1, 500-year option was infeasible. Not only could the U.S. 36 bridge constraint not be overcome, additionally, the 500-year design would have a much larger environmental impact – recall that this was a significant concern to WRAB, OSBT and Council in 2014.
As a proponent of 500-year flood mitigation and a skeptic of its purported infeasibility, in early 2020 I walked the site with engineering staff as they pointed out, answered questions and explained the challenges associated with Variant 1, 500-year. That field trip made it abundantly clear to me that mitigating to the 500-year flood level was, beyond a doubt, infeasible.
Also, in 2020, staff explored 200-year flood level mitigation as well as an upstream option advocated for by the community. Both were deemed to have higher environmental impacts, questionable feasibility and a likelihood of jeopardizing agency permitting.
While it is true that a 500-year flood mitigation would provide greater protection in the face of climate change, this project’s context, constraints and criteria make that level of mitigation functionally untenable.
Project engineering has been methodical. Community ideas and concerns have been duly considered and woven into the process. We have developed a solution that meets the constraints and criteria: Variant 1, 100-year. The 100-year project will not only fully detain local waters from a 100-year flood event, it will detain over half of the local waters arising from a 500-year event. During a 500-year flood on South Boulder Creek, the proposed 100-year project will keep US-36 open for much longer than current conditions and will provide more time for warning and evacuation downstream.
Flood mitigation design was approached as what it is – an engineering problem, not a negotiable annexation issue. While the 100-year project is not perfect, it is very good, and as Confucius said, “Better a diamond with a flaw than a pebble without.”
Mary Young has served our community since 2009 with service on the City’s Planning Board and, beginning in 2013, as a member of the Boulder City Council. She holds BS and MS Degrees in Mechanical Engineering and has called Boulder home since 1982.
