[BoulderCouncilHotline] Questions Ahead of 7/16 Meeting

[Young, Mary]

Dear Staff,
At the September 20, 2018 Council meeting, Item 6a (Next Steps for South Boulder Creek Flood Mitigation) three potential modifications to the selected design were presented (screen shot below). Option C included upstream gravel-pit storage, i.e. area north of the berm. This option had no berms, no inlet channel, and relied only on existing topography. The presenter stated that the upstream storage area would detain about 20% of the flow. Option C also had the most favorable rating except for OS-O enhancement opportunities and so was not retained.
Testimony at Tuesday’s (6/18) Open Comment raised a good point. Recent consideration of new parameters to the approach to preliminary design of South Boulder Creek mitigation bear some reconsideration of previously disregarded features that could resolve some of the newly identified challenges.
Questions:
1. Regardless of final design, removal of the berm provides a measure of protection for downstream residents. What would be necessary to expedite removal of the berm in order to provide an immediate and interim measure of flood mitigation?
2.  How could additional flood water storage using the area north of the berm (as described above: no berms, no inlet channel, existing topography) be included in the current preliminary design phase to address some of the more recent challenges that have arisen with respect to the permanent flood wall (i. e. potential Open Space disposal, manner and use of CDOT right of way, effects to the State Natural Area)?
3. As I currently understand it, the US 36 flood wall will include an underground component to maintain the integrity of wetlands through normal conditions and of the flood wall during periods of adjacent pooling. I would like to understand the reliability and cost of this component and how depth and height of the flood wall affect both reliability and cost.
4. How could the current mitigation concept be enhanced using the low lying area just north of the CU berm such that the flood wall depth and height variables along US 36 might be minimized and with an eye toward having no cut-off wall and/or related underflow system? In other words, how could varying flood water storage volumes (using variables such as berms and/or inlet channels) in the low lying area just north of the CU berm increase/decrease cost and/or reliability of the flood wall?
Below I have also attached a screenshot of the 100 year flood simulation (at about 43 seconds) from the city’s website (link below, scroll all the way down) that illustrates graphically as well as in narrative how removal of the berm would provide flood mitigation. Please note the widest band of water heading directly toward the berm.
https://bouldercolorado.gov/flood/south-boulder-creek-flood-mitigation-planning-study
[image2.png]

[image3.png]

Thank you.

Mary Dolores Young
Boulder City Council
303-501-2439

“We are caught in an inescapable network of mutuality, tied in a single garment of destiny. Whatever affects one directly, affects all indirectly.” ~Martin Luther King Jr.

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