[BoulderCouncilHotline] South Boulder Creek flood mitigation

[Brockett, Aaron]

Dear colleagues and Hotline followers,

First off, apologies to those of you who aren't deep into the details of the South Boulder Creek flood mitigation project! This is a technical email about that subject.

I'm the city's representative to the Urban Drainage and Flood Control District, which helps design and manage flood mitigation projects across the Denver region. UDFCD is assisting the City of Boulder with our plans for South Boulder Creek flood mitigation. Since there has been a lot of concern voiced recently about the safety of the flow control restriction at the South Boulder Creek underpass at US 36 that is proposed as part of the Variant 2 (V2) flood control option, I reached out to my contacts there to see if I could learn more from the engineering professionals involved. The response I received as I understand it shows that the risk from such a flow constriction is very low. Please see below:

Summary from Rodney W. Eisenbraun, senior dam engineer from RJH Consultants, Inc.:

  *   It does not appear that blockage is very likely at the existing US36 Bridge.


  *   Opening width is a key variable when predicting blockage potential and since the opening width of the V2 restrictor is the same as the current US36 Bridge opening width the blockage potential for the V2 restrictor is expected to be similar to that of the existing bridge (i.e. low).



  *   Blockage potential is low yet any of the options could experience blockage.



  *   Adequate freeboard currently exists with the V2 500-year option to handle the 100-year flood event plus significant blockage.



  *   If during preliminary design it is determined that significant blockage is likely, additional measures can be likely be employed to address blockage.


Full details below:
Email from Jim Watt, UDFCD:
Thank you for reaching out to discuss Phase I of the South Boulder Creek flood mitigation project. To better respond to some of your concerns, I reached out to RJH Consultants for more information on the perceived debris clogging risks associated with the proposed improvements. RJH’s response and general project information is provided in the email found below. It is important to note that debris has the potential to impact all variants that are being considered, and is not unique to Variant 2. At this point in the design process, I believe that the design team has appropriately considered all the risks associated with the various alternatives. The alternatives that have been presented to council for consideration have various pros and cons associated with them, but the design team does not currently see any fatal flaws. As mentioned by Mr. Eisenbraun, there are multiple methods which can be used to manage or mitigate debris blockage risks. The appropriate application of these methods will be fully considered during the future design phases. Ultimately, RJH Consultants will need to design and stamp (by a Professional Engineer) a project that is capable of safely meeting the project goals. By stamping the project, RJH will be accepting liability for the project improvements. In addition, the project would need State Engineer’s approval, acceptance by FEMA, review by UDFCD, and permitted by regulating agencies.

My main take away from RJH’s email is as follows:

Freeboard is an engineering approach that is used on most drainage projects to account for unknowns such as debris blockage. As you know, one of the alternatives that is being considered for advancement is designing the flood mitigation facility to the 500-year storm event. If this alternative is selected, then the 500-year facility would automatically have built-in freeboard (safety factor) for the 100-year design storm.

I found it interesting that the current US36 Bridge did not experience significant debris blockage issues during the 2013 flood. The center opening of the existing bridge, which is essentially the same width of the opening being considered for Variant 2, safely passed any 2013 flood debris through the structure. This seems to validate the studies which indicate that minimal blockage should be anticipated.

Please let me know if additional discussion can be provided.
Sincerely,

Jim Watt, P.E., CFM
Project Manager | Watershed Services
URBAN DRAINAGE AND FLOOD CONTROL DISTRICT
Protecting people, property, and the environment
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Email from Rodney W. Eisenbraun, senior engineer from RJH Consultants, Inc.:
Based on the studies we have reviewed and FEMA design guidance, opening width and log size are the key parameters in predicting the likelihood and severity of blockage at culverts and bridges.  Studies show that if log size is less than opening width blockage is not likely to occur.  Log size is a function of several factors including the maximum length of sturdy logs and channel width upstream of the structure.

The current US36 Bridge has an opening of 107 feet long, however the distance between the two sets of piers is 47 feet. This 47 foot opening in the center of the bridge passes the bulk of the flood flows for the US36 Bridge.  Due to this, the effective opening width (47 feet) is essentially the same for Variant 1 (V1) and Variant 2 (V2) from a debris clogging standpoint.   A photo of the US36 Bridge following the 2013 event shows that there was no blockage at the bridge.  There was considerable blockage at other structures during the 2013 event (i.e. the pedestrian bridge upstream of South Boulder Road had to be removed due to the amount of debris blockage that it experienced) so, despite the presence of considerable debris, blockage did not occur at the US36 Bridge.  The likely reason that blockage did not occur at the US36 Bridge is because it’s opening width exceeded the length of logs in the creek.  Since the opening width of the V2 restrictor is the same as the US36 Bridge opening width, it seems reasonable to expect that the blockage potential of the V2 restrictor is also low.

While the likelihood for blockage appears low, partial blockage could occur.  However, should a tree large enough to partially block the V2 opening find its way to the bridge it seems reasonable to expect that this tree would also partially block the existing US36 Bridge and as such blockage needs to be considered for all project options.

One common method to address blockage is to include adequate freeboard (excess storage) to prevent overtopping should blockage occur.  As an example of how freeboard can address blockage concerns, we ran a simplified calculation.  The results show that if the V2 500-year project was built and was subjected to a 100-year event the freeboard would be sufficient to prevent the floodwall from overtopping should the restrictor (or Bridge) became 30% blocked about 1 hour before the flow peaked and remained 30% blocked for the remainder of the event.

During preliminary/final design we intend to take a close look at blockage potential and severity.  If we learn that significant blockage is likely to occur several measures can be employed to minimize the risk including:

-          Employ a debris trap upstream (boulders strategically placed in SBC)
-          Use a different type of flow control structure (i.e. a baffle placed in SBC just upstream of the Bridge)
-          Provide more freeboard
-          Combinations of above

In summary:

It does not appear that blockage is very likely at the existing US36 Bridge.

Opening width is a key variable when predicting blockage potential and since the opening width of the V2 restrictor is the same as the current US36 Bridge opening width the blockage potential for the V2 restrictor is expected to be similar to that of the existing bridge (i.e. low).

Blockage potential is low yet any of the options could experience blockage.

Adequate freeboard currently exists with the V2 500-year option to handle the 100-year flood event plus significant blockage.

If during preliminary design it is determined that significant blockage is likely, additional measures can be likely be employed to address blockage.

Please do not hesitate to contact me if you have any questions.

Regards,

Rod

Rodney W. Eisenbraun
Principal

RJH Consultants, Inc.



Aaron Brockett

Mayor Pro Tem, City of Boulder

brocketta at bouldercolorado.gov

(720) 984-1863
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