[BoulderCouncilHotline] FW: DRAFT - Re: Questions Ahead of 7/16 Meeting

[Arthur, Jeff]

Dear Council Members,

Responses to the Hotline questions regarding South Boulder Creek flood mitigation are provided in bold below.  The text of the original Hotline is included in italics.  Additional information is provided in the memorandum for the July 16 agenda item.
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.
Upstream options could be further evaluated based on the latest information from CDOT and the University of Colorado.  The July 16 memorandum includes additional information about the upstream options that have been previously evaluated.
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?
Staff is not aware of a hydraulic analysis that indicates that removal of the existing CU levee (the berm) would provide protection for downstream residents.  This question was investigated by HDR Engineering in 2008, including hydraulic modeling with and without the levee in place.  The study concluded that removing the levee had no material impacts to the floodplain, except for in the immediate vicinity of the CU levee.  In general, some portion of the floodwaters that currently flow around the north end of the levee may instead take a more direct route across the CU South property.   An excerpt from the study is attached.
The flood mitigation concept currently proceeding in preliminary design was also hydraulically modeled with and without the CU levee in place.  This modelling did not identify the CU levee as a having an impact on the flood mitigation alternatives.

  1.  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)?
The referenced hydraulic models with the CU levee removed did not include modifications such as berms, inlet channels, or other modifications to existing topography.  The conclusion that removal of the levee had no material impacts to the floodplain, except for in the immediate vicinity of the CU levee, reflects flows travelling over the existing topography.

  1.  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.



The US 36 floodwall would be designed to convey groundwater through the subsurface portion of the wall in a manner that replicates existing flow patterns.  The current groundwater system concept involves a collection system upstream of the wall's foundation (similar to a French drain), a distribution system downstream of the foundation (also similar to a French drain), and a set of pipes through the foundation to connect the upstream and downstream systems.



The depth of the floodwall is largely fixed by the structural design and dam safety requirements.  Variations in the height of the wall above grade are expected to have minimal impact on the reliability or cost of the groundwater system.  It is estimated that the groundwater conveyance system would be approximately 5% of the total project construction cost.  The intent is to design the system to function with gravity flow to maximize reliability and minimize long term maintenance and operations costs.



  1.  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?
The Option C Concept presented on Sep. 20, 2018 reduced inundation in the Public BVCP land use designation, but did not eliminate the need for a floodwall/groundwater conveyance system along US36 (drawing attached).  Staff could develop additional upstream options with the project goal of optimizing the floodwall along US36 utilizing additional upstream storage.  The use of the upstream storage would create more disturbance on OS-O.  It is also anticipated that increasing the upstream storage area would require more substantial construction and environmental impacts than the Option C concept.
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
The attached map depicts the topography on the CU South site using different colors to depict different elevations.  While removal of the berm would result in flows at the western edge of the floodplain flowing onto the CU South site, that area has a gradual downhill slope to the north and delivers flows to the West Valley.  As noted above, hydraulic modeling does not indicate that the existing topography of this area would materially mitigate downstream flooding.    Water flowing across this area would have a potentially shorter flow path than under current conditions where it flows around the CU levee and then northwest along US36.
Staff will plan to review this information as part of the July 16 presentation.
Best,
Jeff
Jeff Arthur
Director of Public Works for Utilities
303-441-4418
arthurj at bouldercolorado.gov<mailto:arthurj at bouldercolorado.gov>


[PublicWorks_lockup_COBLogo]
1739 Broadway
P.O. Box 791
Boulder, CO 80306-0791
Bouldercolorado.gov<http://www.bouldercolorado.gov/>

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