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Flow Components Report

Posted by Robert Czachorski on 17 Oct, 2014
Category: No category

Folks,

Posting the Flow Components Report for your information. This report provides the breakdown in the system flow between the various source components, to help guide future decisions about how to manage the system and where to target flow removals in the system, should they be required. The report defines several flow components, and provides a separate breakdown for the system flow during dry weather conditions and wet weather conditions. A summary of the breakdown for the system as a whole is as follows:

Dry weather:

62%   Domestic, Commercial and Industrial flow (normal sewage)
38%   Ground Water Infiltration (including dry weather flow from footing drains)
------
100% Total

* Note that is it not uncommon for sanitary sewer systems to have 25-50% of their dry weather flow from ground water infiltration. 

Wet weather:

20%   Domestic, Commercial and Industrial flow (normal sewage)
13%   Ground Water Infiltration
19%   Footing drain flows
48%   Other source of inflow and infiltration (non-footing drain)
------
100% Total

The report also provides a breakdown of dry and wet weather flows for each sub-district that was metered in 2013.  Below is a summary of the significant findings and conclusions from the report:

  1. Although City-wide, only 19% of peak flows are from footing drains, the value in each district varies tremendously from 1% to 85% depending on the density of potentially connected footing drains present and the production rate of those footing drains.
  2. The analyses performed estimated the peak production rate from footing drains for a 25-year frequency flow for each meter district. This analysis showed that the average production rates of the remaining footing drains is 1.7 gpm per footing drain. Although the City-wide average is only 1.7 gpm, the value in each district varies tremendously from 0.1 gpm to 6.3 gpm.
  3. Given that several of the priority district production rates were in the range of 4-6 gpm, this shows that the City did a good job of selecting the priority districts for footing drain disconnection. This conclusion is not surprising, given the high incidence of sanitary basement backup in these areas prior to footing drain disconnection.
  4. Neighborhoods within large meter districts may have higher footing drain production rates than estimated due to being lumped together with multiple neighborhoods. If sewer system backups are occurring, discrete metering is recommended to more accurately determine the footing drain flows and whether disconnection would provide the desired relief. This may be the case for the Pittsfield Valley area (project area D).
  5. The City should consider the footing drain production rates within each district when evaluating alternatives or considering the impact of footing drain disconnections for developer offset mitigation.

As always please feel free to contact me or make a positing here if you have any questions.

Take care,
Robert

Comments

Patricia Marten on 17 Oct, 2014:

Wet weather:
20%   Domestic, Commercial and Industrial flow (normal sewage)
13%   Ground Water Infiltration
19%   Footing drain flows
48%   Other source of inflow and infiltration (non-footing drain)
------

Can you list some of the 48% --other sources?
Thanks

Robert Czachorski on 18 Oct, 2014:

Thank you for your question, Pat.

Some of the other sources of inflow and infiltration into the sanitary sewer include:

  • Leakage into manholes, through pick holes in the cover, around the lid and frame, and through cracks and defects in the manhole itself.
  • Leakage into cracks in the pipes. Often the pipe trenches are backfilled with sand under the pavement, and this can create a conduit for stormwater to quickly reach pipe defects.
  • Leakage into service leads, i.e. the pipes that convey sanitary sewage from the house to the sanitary sewer main in the street.
  • Directly connected stormwater sources such as improperly connected storm drains, yard drains, or driveway drains, broken clean-out covers, cross connections between the storm system and the sanitary system, or roof downspouts that are improperly connected to the sanitary sewer system.
  • River or creek inflow where they are crossed by sanitary sewer pipes. 
  • Migration from the storm sewer to the sanitary sewer.  Because the sanitary sewer is typically the deepest pipe in the street, a deteriorated storm pipes can result in stormwater exfiltrating the storm pipe and discharging to the sanitary pipe, with the greatest risk where they cross each other.

Determining the magnitude of these sources and their breakdown requires a field investigation that is called a Sanitary System Evaluation Survey (SSES), which includes many field investigation techniques to find and quantify these sources. 

It is important to keep in mind that stormwater flows are usually much, much higher than sanitary sewer flow, perhaps by as much as 10x to 100x, depending on the area. So a relatively small amount of stormwater can result in a relatively high amount of flow in the sanitary pipes. In our assessment, the City's sanitary sewer system is not unusual in the rates of inflow and infiltration that it experiences.  But there are some districts that are higher than others, and when those cause problems, it is necessary to investigate them and correct them.  That may be the case with the Glen Leven district and with the Pittsfield Valley area, as described in the project sheets for those two areas.

Hope this helps!

Take care,
Robert

Patricia Marten on 7 Nov, 2014:

Thanks !
I hope that we can include this info in our final report.  Many FDDs were done with good results, in most areas.  BUT the big problem seems to be the remaining 48%.  Hopefully the Stormwater report will point out some things to be fixed.  Certainly the Upper Mallets Creek had some good plans, that need to be implemented.  Before considering more FDDs, the biggest problem area, should be addressed.