Frank Burdick Log Number: 43

Submitted by: Log

Category (1,2,3): 1

Status: Closed

Estimated date to address: N/A

A.43. The purpose of the FDD project was to reduce the incidents of basement backups in the five neighborhoods that had experienced 50% of all reported basement backups in the City during wet weather. The next phase of the SSWWE project will investigate the system’s hydraulic performance and will determine whether infiltration is a concern. That information will inform the recommendations made to City Council.

In addition, Evan Pratt, PE, Water Resources Commissioner, Washtenaw County, provided this information in response to a question posed by CAC member, Frank Burdick:

“Frank

 

Thank you for taking the lead on this item, and thank you to the CAC members for volunteering the time.

 

The list looks pretty comprehensive, and although I have not reviewed in detail since the sewers are not our area, it looked like Question #43 warrants response from this office, and subsequent clarification, as the numbers being questioned are unrelated.  In summary, the smaller number is about flow in a large storm pipe, and the other is most likely about clean water entering a much smaller sewage pipe.  Although I do not know the exact size of the sewer pipes in this neighborhood, they are much smaller than stormwater pipes, as detailed below. It is a point of fact that a flow component that eats up 36% of a 1’ pipe will only eat up 1% of a 6’ pipe.  This is not an obvious point to folks who do not deal with these things on a daily basis, so the confusion is understood as I have clarified similar questions many times.  Many of the other questions look similar to what people in other communities have raised when they are going through the process of learning the cause and potential solution to wet weather issues in sewage systems.

 

I would also like to clarify that my number was estimated to ensure conceptual cost estimates would not need to be revisited regardless of how the sewer study turns out.   I was taught long ago to keep the initial concepts conservative, as spending less money on final design and installation is not usually an obstacle.  Since the sewer study is not complete, we are providing a conceptual recommendation that will achieve and slightly exceed the stated goals the County’s Upper Mallett’s Creek Study to address flooding in the neighborhood, regardless of the outcome of the sewer study. 

 

So there is no purpose to comparing capacities of different sized pipes designed for totally different purposes in determining the best strategies for sanitary sewer.  As appropriately identified in the last sentence of Question #43, the important issue is to determine where the clean water that gets into the sanitary sewer is coming from.  I presume that the City’s study will at some point include discussion on the breakdown between footing drain water, groundwater, and rainwater entering the sewer system through cracks or otherwise.  While “leakage” may not be quite the right term, it sounds like folks have incorrectly interpreted the numbers in Question #43 as evidence that the majority of clean water entering the sewers is coming in through cracks and openings in pipes and manholes.  I do not know the facts on this breakdown, so I will leave it to the City to present those facts. 

 

I also offer the following detailed clarification in case there are CAC members who might like to see more back-up for the statements I have made above:

 

1.    These numbers are unrelated in three ways.  First, the numbers (2-7%) attributed to me are not a dilution rate for clean water in a sewage pipe, they reflect a very conservative estimate of how much capacity of the stormwaterpipes only in Upper Mallett’s might be utilized if all 1700 homes in our study area were disconnected and discharged into a storm pipe in this neighborhood.  The concern raised at the first public meeting for the stormwater study was that during a flood situation, full pipes could not handle any more water, a very reasonable concern in my opinion, so I promised we would account for that concern in any recommendation.   I do not know what the 40% specifically references, but from prior discussion with Mr. Bill Higgins, I infer that the 40% may be a number in a sanitary sewer report identifying what percent of flows in the sanitary sewer system are attributed to rainwater, possibly on a City-wide basis, but it is also possibly a representation of plant capacity vs average pipe capacity – again, I don’t know the source of the number so will leave it to the City to clarify.  I presume that this 40% is also based on some actual field data collection during storms, and is a measure of the capacity used up for one particular type of storm (like the 25 year, 24 hour or the 10 year, 2 hour, etc) – typically, I understand this percent of the pipe or plant consumed by rain will vary depending on the amount of rainfall in a given storm, and whether or not the pipes are filled to capacity. 

2.    Secondly, while this may not be obvious to the layperson, it sounds like the response may need to explain that capacity design for sewage pipes and stormwater pipes (or other aspects of the respective systems) are unrelated.  Sewage flows are predictable and much smaller than the massive amounts of water resulting from the wide range of rain events we see in the Midwest.   The 10 States’ standard design manual for sanitary sewers is used throughout the Midwest because sewer flows are generally the same throughout.  But just in Michigan alone, there are 10 different zones of varying rainfall, and the amount of rain for all recurrence frequencies (10-year, 100-year, etc.) varies within each zone.  The net result is that a sewage pipe is much smaller, so the same volume of water will take up a greater percentage of pipe capacity in a smaller sewage pipe than the much larger stormwater pipe.  The example I used in talking to Mr. Higgins is the County storm pipes in Upper Malletts are mostly from 4’ to 6’ diameter.  I don’t know the size of the sanitary sewers, but 1’ to 1.5’ diameter might be typical.  Due to the exponential relationship between diameter and cross-sectional area, along with the direct relationship between area and capacity, that the 4’ pipe can carry 16 times as much water as a 1’ pipe, although the diameter is 4 times larger.  And the 6’ pipe can carry 36 times as much water as a 1’ pipe, 16 times as much as a 1.5’ pipe.  Obviously the same multiplier goes with the percentages of capacity – a flow component that eats up 36% of a 1’ pipe will eat up 1% of a 6’ pipe. 

 

3.    Third, the numbers attributed to me are rough conservative estimates to ensure there is space at each site for the worst case scenario flowing to our proposed solutions, so our calculated FDD volume is intended to be greater than what is calculated from real field data when the City’s sewer study is complete.  When the sewer study is complete, I would expect that regardless of the recommendations of that study, the size of the basins could be reduced a bit to account for only the groundwater component of footing drain flows in this neighborhood, because our stormwater calculations and modeling already account for rain that hits rooftops or other areas that may route rain into footing drains.  Using more accurate numbers from the sewer study would allow us to still meet the stated goals of full protection for an event like 3/15/12.  As noted above, I was taught long ago to keep the initial concepts conservative, as spending less money on final design and installation is not usually an obstacle.  Since the sewer study is not complete, we are providing a conceptual recommendation that will achieve and slightly exceed the stated goals to address flooding in the neighborhood regardless of the outcome of the sewer study. 

The main point of this email is to clarify that the numbers compared in Question #43 are unrelated, and what really matters is how the clean water is getting into the sewer so the best strategies can be compared. 

 

If I understand the last question of #43, I would suggest that #43 could be restated to say that folks are looking to understand where the clean water is coming from so they can understand and debate the most effective strategies to get water out of the sewer.  The detailed point that I felt warranted a response from this office is that our purpose and method of arriving at an estimated FDD volume is different from the City’s, and should not be used for the purpose of sanitary sewer evaluation, because I directed our consultant to over-estimate the impact on the stormwater volumes from FDDs.  Again, the purpose was to ensure conceptual feasibility for recommendations to address flooding in the neighborhood, specifically with respect to the area and preliminary cost estimates needed for storage basins regardless of the outcome of the sewer study.

 

I trust this is helpful, if a bit lengthy, understanding that these are not issues that folks deal with on a daily basis.  You all are asking good questions that frequently come up in communities with wet weather issues in their sewer and stormwater systems, and I appreciate the time volunteers are putting in to help the decision making process, since I live in the City and will no doubt help fund whatever the solution is through my utility bill!” (12/5/13)