Published on March 14th, 2016 |1
BV checks DWM Commissioner’s claim raising manholes will solve problem
The Peachtree Creek Watershed Action Plan presented March 9 in the City Council Utilities Committee Work Session includes Short Term (3-9 month) improvements and process changes costing approximately $400,000. The plan includes “raising five (5) manholes within Memorial Park and Bobby Jones Golf Course areas.” Department of Watershed Management Commissioner Jo Ann Macrina told the committee: “We do raise manholes in a floodplain so that we don’t have a major inflow problem.”
decided to evaluate Macrina’s statement that raising manholes (two feet according to the DWM plan) does in fact prove to be an effective way to remove inflow problems into the sewer system and thus would reduce sewage spills and pollution of Memorial Park.
During the Utilities Committee work session, Council members asked many questions about this and other features of the plan. In response to Council Member Mary Norwood asking if in fact raising manholes was bad policy, Commissioner Macrina responded: “I’m not saying that it is a good policy. What I’m saying it’s an option for us to look at.”
Consider the second part of Macrina’s statement first in the context of Memorial Park sewage overflows.
Is the Peachtree Creek trunk sewer surcharged (under pressure) because of too much demand or is the creek flowing into the sewer in Memorial Park?
The neighbors have provided eye witness accounts indicating sewage is flowing out of the manholes rather than the creek flowing in. Photos and video shared by news organizations such as WSB and FOX5 also indicate sewage spewing out with the Creek level substantially below the top of the manhole.
Macrina often refers to hydraulic modeling of the system which indicates sewage overflows are a result of insufficient capacity with too much demand. The DWM report completed in September 2008 titled Draft Peachtree Creek Basin SSO Remedial Measures Plan shows significant segments of the basin sewer system surcharging with sewer overflows.
The hydraulic modeling completed as part of the Consent Decree requirements shows results for conditions before and after planned remedial measures (shown in red in an excerpt from Figure 2-3) surcharging with overflows. The report also includes alternatives requiring significant capital investment that would alleviate the surcharging.
While the full picture is incomplete, we do know that the city completed the Liddell storage facility with a great deal of media coverage which has 10 million gallons of storage capacity. However, the alternatives analysis in this report indicate substantially more storage capacity was required to alleviate surcharging and overflows. The alternatives presented included tunneling and surface storage. Further that storage required to eliminate surcharging and overflow was more than double what was constructed based on the growth projections at the time.
Given the observation of neighbors and the city report, it is clear that the system is in fact surcharging based on too much demand with sewage spewing out of the manholes. It is not a result of Peachtree Creek flowing into these manholes.
Could Macrina actually deliver on this proposal to the community?
According to Justin Wiedeman, a professional engineer who was invited to speak to the Utilities Committee, “The First Amended Consent Decree required the city to show that all significant sewers in the system, based on hydraulic modeling, could pass peak flows without surcharging. The requirements are based on forecasted flow, not just the current flows, and certainly Midtown continues to grow.
Remedial measures are required for sewers that do not meet the criteria, so a proposal that would result in greater surcharging is contrary to the Consent Decree requirements. The Consent Decree includes provisions for Capacity Certification that apply for sewer basins that do not meet the requirements. The Federal Court (in the Consent Decree) had good reasons for telling the city not to do what the commissioner is proposing.”
It’s unclear if Macrina understands the requirements of the Consent Decree, but the language in the Consent Decree is very clear.
Is it a good idea to raise manholes that are surcharging to reduce overflows?
There apparently are many reasons why engineers rarely design gravity sewers under pressure and why the state of Georgia is inclined not to approve such a design including:
Structural Integrity Issues
Generally large gravity sewers are concrete. Concrete has very little tensile strength (without steel reinforcing) so putting pressure on the walls of the pipe can cause damage. This is particularly true for older pipes. The DWM commissioner discussed the prospect of a structural lining to be installed in the older Peachtree Trunk ,but this would be many years in the making.
Wiedeman explained on the surge issue: “I don’t see much potential for surge (water-hammer), but you don’t need that type of pressure to destroy a sealed manhole. If you surcharge the sewer to the extent that you are seeing geysers from the raised and vented manholes then the pressure would be significant on the sealed manholes which are closer to ground level. Older cast-in-place manholes were typically not designed for surcharge (pressure). It’s reasonable to assume that surcharging caused the catastrophic failure of the manhole in April 2014 that dumped approximately 10 million gallons of sewage into Memorial Park and Peachtree Creek, according to city estimates submitted to USEPA/GAEPD.”
“Infiltration and exfiltration are both functions of height of the groundwater table in the vicinity of the sewer, type and tightness of sewer joints, and soil type. Exfiltration is undesirable since it may tend to pollute local ground-waters, while infiltration has the effect of reducing the capacity of the sewer for conveying the waste flows for which it was designed.”
If the pipe is near the creek, then exfiltration of sewage may find a path to discharge directly to the creek. Increasing the hydraulic grade and corresponding pressure would further increase the rate of exfiltration. If pressure is great enough, sewage could seep to the surface. This would be a function of friction losses through the soil and the extent of pressure.
As the wet event subsides, the process reverses. The points of structural defect become channels for infiltration. The sewage temporarily stored outside of the pipe as well as dirt can then enter the pipe, creating voids as well as causing problems at the treatment plant.
Neighborhood Sewers and Basement Sewage Flooding
Grease becomes an even bigger problem when sewage is stagnant. If flow from neighborhood sewers cannot get into the sewer trunk because of increased (back) pressure, then overflows further up into the neighborhood may result. This could also result in sewage backups in basements if the system hydraulics are not thoroughly understood by DWM planners as they determine the extent to which the manholes will be raised.
inflow problem.”) to be generally suspect in engineering theory. Further, DWM reports indicate that sewer overflows in Memorial Park are in fact a result of surcharging of the sewer based on demands, rather than inflow from Peachtree Creek.
We also believe Macrina’s response to Councilwoman Norwood (“I’m not saying that it is a good policy. What I’m saying it’s an option for us to look at”) further questions the validity of this being a substantive component of the short-term solutions to the problems at Memorial Park. We question if Macrina can actually deliver on this given the Consent Decree requirements.
Council Member Norwood’s assertion that raising the manholes is generally bad policy in this case generally appears to be TRUE. Further, statements as to why the manholes would be justifiably raised have nothing to do with sewage overflows in Memorial Park and appear UNRELATED to the topic at hand.