[bouldercouncilhotline] Hotline: Sanitary Sewer - Questions related to resilience - Design goal of sanitary sewer system

[cmosupport at bouldercolorado.gov]

Sender: Cowles, Macon

Dear Jeff and Maureen,


Good afternoon. I am putting this on the Hotline because I would like to see a discussion during the CIP, budget and rate making hearings a consideration of the following question: what is the design capacity of our sanitary sewer system to withstand a
 100 year rain event, and is the current and proposed budget for maintenance and upgrades to the system adequate in light of what we learned from the rains and flooding of September 2013?


Sewer backups, and Sewer Design Criteria. Prompted by several dozen contacts I have received since last September from people who had a backup of the sanitary sewer into their homes during the heavy rains, I looked at the Wastewater Utility Master
 Plan on our website [available here https://www-static.bouldercolorado.gov/docs/boulder-wastewater-utility-master-plan-1-201406101038.pdf]
  to find out what we have adopted as the design and maintenance criteria for the sanitary sewer. Our storm water master plan has the 100 year event as the design goal. There is apparently no design event [20 year, 100 year, 500 year, 1,000 year] for the sanitary
 sewer--at least none that I could find. Nor could I find such in Boulder's Design and Construction Standards. (BDCS).


Bob Harberg commented at 1:48 of the April 21, 2014 WRAB meeting that the wastewater design event is not as well defined as it is for the storm water system. Bob said that there is an expectation of some backups in the sanitary sewer system if there is
 an event that is greater than the 20 year event. If interested, the recording of the WRAB meeting of April 21 is here:

https://documents.bouldercolorado.gov/whttps://documents.bouldercolorado.gov/weblink8/0/doc/125236/Electronic.aspxeblink8/0/doc/125236/Electronic.aspx



The BDCS on wastewater <available here https://www-static.bouldercolorado.gov/docs/ch06-1-201306271135.pdf> require that sanitary sewers be designed so that the pipes can
 carry the peak flow, §6.04(A)(1) and only be half full.§6.06(A)(2) Pressurized flow is prohibited. BDCS, §6.06(A)(3). In determining the peak flow capacity of the system, the BDCS require that the inflow and infiltration rate (i.e., surface water  or groundwater
 entering the sanitary sewer, as it did during the September 2013 rains) be included in designing sewer pipes to carry peak flow. The heavy rains caused a fair part of the sanitary sewer to become pressurized. This raises a question as to whether the inflow
 and infiltration rate in the BDCS is adequate.? (200 gals per inch diameter mile, see BDCS Table 6-1). 


Our sanitary sewer, designed for a 20 year event, was overwhelmed by inflow and infiltration caused by the rain. The rain caused surface water to run into manholes and subsurface water to enter sewer pipes through cracks and joints. The rain was a 1,000
 year event. Even though it was a 1,000 year rain that caused the sewer backup in people's homes, the policy question for Council becomes this: do we need to upgrade the design standard and upgrade the system so that sewer backups will be less frequent in the
 future?



The reason I am asking this question is that in presenting to Council the CIP in July and the issue of utility rates this summer, it would be helpful for staff to present to Council the cost, the constraints and the benefits of upgrading our sanitary sewer
 in the near term (perhaps within a six year CIP cycle) so that it can handle flow without backups up to the 100 year event. The precipitation projections done a few years ago by Lee Rozaklis group to model changing precipitation patterns anticipate less snow
 and more rain, such that we could be having 20 year events much more frequently then every 20 years. I think our residents would want us to have a vigorous and well informed discussion--post flood--about whether designing only to the 20 year event is
 adequate.


I am attaching the three page Collection System Analysis from the 2009 Wastewater Utility Master Plan, which identifies Recommended System Improvements. The recent survey by camera of our sewer system has probably also identified other important repairs
 that need to be made. It seems to me at the very least we should implement the Tier 1 projects and any Tier 2 projects where the risk of "lateral backup" (which I take to mean will cause some people's basements to flood) is "high."


Thanks very much.









Macon Cowles
Boulder City Council Member





1726 Mapleton Ave.





Boulder, Colorado 80304
CowlesM at bouldercolorado.gov
(303) 447-3062
(303) 638-6884


PS. Lay people can learn some interesting facts about our sanitary sewer system from the attached three pages. For example, I did not know that sewer pipes are classified almost the same was as roads. Sewer pipes are local, collectors and interceptors;
 roads are classified as local, collectors or arterials.!!
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