Sewer Purgatory
Sewer Purgatory: Keeping Away from Enforcement Actions
As published in Underground Infrastructure Management Magazine, July/August 2005
Applications:
- Proactive Collection System Management/Performance
- Flow Monitoring/Data Analysis
- I/I Reductions/Collection System Capacity
- Scattergraph Analysis of Flow Data
By Patrick L. Stevens, P.E.
We regularly see some agencies become the focus of state or federal enforcement action for collection system problems while others do not. Being subject to an enforcement action is similar to being in purgatory — “a state after death in which the soul destined for heaven is purified.”
Once being subject to an enforcement action, it is difficult to become free of its scrutiny. Is there a common characteristic among cities and agencies that keep out of trouble? Conventional wisdom would say that proactive management is the key to remaining in compliance. As a general rule, it appears that cities in trouble have operated in more of a reactionary mode and cities that have avoided problems have been more proactive.
Being in a reactionary’s mode in collection system management means reacting to the symptoms that indicate something is wrong — sanitary sewer overflows (SSOs), basement flooding or perhaps street collapse. These incidents all can be considered “failures” because the system is failing to operate as designed.
Managing a collection system is a difficult task because everything about a collection system, except failure, is invisible. Absent these visible failures, the reactive managers will assume everything seems “OK” and will react only when a visible failure occurs.
On the other hand, proactive managers somehow figure out early on what pipes must be repaired, what basins need to be rehabilitated or what trunk line must be enlarged or replaced. They often will have a 10-year capital plan that addresses anticipated future problems. Some agencies operate in this mode naturally and others are encouraged to do so by enforcement action. Typically an agency operating under a consent decree has less flexibility on how to solve its problems since the decree often adds constraints in schedule and scope. Consent decrees often reflect the fact that attorneys are attempting to solve engineering problems.
Fundamentally there are three key types of information about a collection system that a manager must have in hand to act proactively — inventory, condition and performance. Most cities have a well-developed inventory, which includes mapping, information on pipe size, material of construction, year of construction, etc. Inventory systems are commonly GIS-based. Similarly, most cities have made use of CCTV and manual inspections to determine the condition of the pipes.
What appears to separate the proactive agencies from reactive agencies is their approach to understanding system performance — the third rail of collection system management. Traditionally, reactive managers believed two rails were sufficient — condition and inventory. Proactive managers, however, have discovered and use the third rail of collection system management — performance.
In the document Optimization of Collection System Maintenance Frequencies and System Performance (http://www.epa.gov/npdes/pubs/optimization-finalreport.pdf), EPA discusses the concept of using performance indicators and suggests that matters such as complaints, basement flooding and overflows be considered as measurable performance indicators. Many agencies use this approach to determine how well their programs are working, but these measures are actually frequency of failure indicators, not true performance indicators.
In reality there is a long list of events that are invisibly and consistently changing the performance of a collection system. It is safe to say that no natural change in a sewer ever results in an improvement of performance. Every change, except man-made improvements, gradually causes deterioration in performance until visible signs of failure appear.
The invisible conditions are constantly working to deteriorate the system. These conditions will become visible when they combine to cause a failure — e.g. a worsening blockage combined with a rainfall of just the critical intensity.
Simply put, proactive managers are paying attention to the activities that are invisible, while reactive managers react to activities they can see. Some of the items that are invisible are matters of condition such as soil voids, debris and roots. However many of them are matters of performance and can be detected through sewer flow monitoring.
The challenge for collection system managers is to bring visibility to the occurrences that reduce capacity and increase flow before one or both can result in outright failure. Hydraulic models are popular because they remove much of the mystery by simulating and displaying what likely is happening in the sewer. A common characteristic of proactive cities and agencies is that they make use of flow monitoring data and hydraulic modeling to determine how well their system is performing well before failures are apparent.
What Can Be Learned From Flow Monitoring Data?
Flow monitors deliver two distinct types of information. The first and most commonly understood is the time series hydrographs of calculated flow rates. Hydrographs reveal information about the combined production of wastewater and /I in the service area upstream of the monitor. This data along with rain gauge data can be used to determine how leaky each part of the service area is and which basins are in need of rehabilitation.
The second and less commonly understood type of information is the scattergraph of depth and velocity data. Scattergraphs reveal both the hydraulic conditions in the pipe and how well the monitor itself is working. The use of scattergraphs in collection systems is relatively new and is dependent on reliable depth and velocity metering technology. ADS has pioneered the use and interpretation of scattergraphs in collection systems and several new intelligent features in ADS’ monitors and software are based on scattergraph knowledge. For more information on Scattergraphs and how to understand them, check out the information at www.adsenv.com/scattergraphs.
Here is an example of the performance information that can be learned from a scattergraph. This scattergraph shows that the 27-in. pipe operates normally up to depths of 5.5 in. and at greater depths moves into backwater. The iso-Q lines reveal the pipe is restricted to 8.9 mgd (50 percent of its 18-mgd carrying capacity). The pipe surcharges to a depth of 65 in. at which time a downstream overflow occurs indicated by the increase in velocity at the constant depth of 65 in. The downstream SSO discharges at a peak of 1.6 mgd.
This type of capacity information is one of the performance issues that typically lurk below the line of invisibility, yet it is a key contributor to sewer failures. It is common for an interceptor or trunk line following a winding river to have many turning manholes with a resulting capacity loss of 30 percent. It is also common to find various choke points in a collection system such as a railroad crossing or a junction structure that creates 30 percent capacity loss. It may be cost-effective to remove localized choke points but may not be cost-effective to restore capacity to the winding trunk line. The idea of removing choke points as part of a rehabilitation project is called “de-bottlenecking.”
Many managers believe that modeling is the best way to identify bottlenecks and capacity restrictions, but modeling by itself will identify only the bottlenecks that were designed into the system. A model cannot easily identify the capacity losses resulting from deterioration or poorly constructed structures. This type of capacity information is generated only through flow monitoring data. Skilled modelers seek as much flow monitoring data as possible to assure the model matches the actual system operation.
Some agencies collect flow data for many years and are never aware that this type of information exists in flow data. Cities that have been reactive typically have made their capital planning and management decision with little reliance on flow data and are unable to envision the value of flow data. Some will gather flow data only because they are forced to by enforcement action or to calibrate a model and never learn how to use it as a management tool.
Click here to view case studies of agencies that have incorporated the third rail of collection system management — PERFORMANCE.
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