April 26, 2011Manholes represent a rather complex form for assessing structural condition and “leakiness” potential. Factors such as the assembly of components with multiple construction materials and exposure mechanisms to rainfall and runoff make computational scoring and infiltration-and-inflow (I/I) quantification logic complicated. Manholes have the potential to be significant sources of extraneous I/I, but it is difficult converting visual inspections into I/I quantification.
In the early 1980s during the U.S. EPA’s Construction Grants program, communities could get grant funds to fix specific sewer defects if the utility could demonstrate the cost-effective benefit of fixing the defect and removing the I/I. What the wastewater industry and regulatory agencies eventually found was just fixing the cost-effective defects did not take into account groundwater migration. The result was that I/I reduction was not as successful in producing reduced system flows and related sanitary sewer overflows (SSO). The water migration dynamics were particularly evident in manholes where non-cost-effective defects could be leaking, sometimes significantly, after the initial cost-effective defect was fixed.
Despite the historical faults or underestimation of the fluid dynamics associated with assigning I/I to defects, the task today is more scientifically applied to the sanitary sewer evaluation and rehabilitation process. Although still a subjective process, the defect I/I assignment is more logical and defendable. This is possible from applying the results of a combination of manhole field and laboratory I/I tests; from the availability of equipment with much greater computation power both for field and office equipment; and from applying pair-wise comparison based I/I assignments whereby engineering experience and judgment are integrated to allow the relative defect I/I contribution differences to be scaled.
Developing a Defect I/I Quantification Approach
Anticipating the need to improve the sanitary sewer condition assessment process an American Society of Civil Engineers (ASCE) 2009 publication titled “Manhole Inspection and Rehabilitation” presented two suggested manhole I/I flow rating methods (ASCE 2009). One method showed rates for a one-year, 60-minute storm duration and the other method used a modified Rational Formula. The publication acknowledged that site conditions and the manhole cover design significantly influenced the potential I/I quantity.
While assisting a northeastern utility CH2M HILL decided to enhance its SCREAM Manhole Condition Assessment tool to assign and project I/I rates for standard manhole defects by building upon the ASCE methods.
Since the defect I/I rates and process were linked to ASCE’s wet weather condition and integrated with the utility’s I/I testing, the SCREAM tool was developed to project manhole I/I quantities for any specific storm event using local site conditions and field inspection data. The objective of the tool enhancement was to automatically compute the projected I/I quantity without major field or engineering manipulation or modification to the typical data and field crew inspection procedures.
Advanced Method I/I Project Principles
The eight I/I quantification and scoring principles listed in Table 1 were established to define the tool’s logic
and approach framework. The principles were comprehensive and set up the basis for extrapolating the interaction of specific storm event rainfall, runoff and groundwater movement over the 18 SCREAM Manhole components or locations. The components/locations were also divided into upper level (above the reducer/cone) and lower level (below the reducer/cone) manhole components (see Figure 1 on pg. 40).
There are nine SCREAM manhole components in the upper level compared to three ASCE components. Of these nine, the cover presents the most dynamic range of I/I possibilities. There are nine SCREAM components compared to ASCE’s five components in the lower level. The lower level components are assumed not to be effected by the exposure condition compared to the upper level components. Therefore, the upper level component projected I/I peak hour rates and volume are computed for the defined rainfall/runoff exposure conditions.
SCREAM used the I/I rates provided by ASCE for the defect severities Minor through Heavy to extrapolate a reasonable rate to the SCREAM codes. SCREAM selected a specific rate for the Sever I/I since ASCE provided a range of greater than or equal lower limit to the rate. SCREAM also calculated rates for the cover and cover seal. The cover and cover seal rates were calculated for various rainfall runoff and ponding conditions using orifice equations and work performed by the northeastern utility.
As stated in the initial principles, component defect I/I rates will vary according not only to the severity of the defect, but also the frequency, intensity and duration of the storm event. A one-hour period was used to define the peak rainfall and rate and 24 hours to define the volume of a storm and RDII. A series of tables were developed using National Oceanic and Atmospheric Administration (NOAA) rainfall tables that shows the rainfall and I/I rate relationship.
The upper level I/I quantification is more complex in that it considered four common mechanisms whereby the components could be exposed to rainfall/runoff. The four mechanisms were sheet-flow runoff; ponding with a limited tributary; ponding with unlimited tributary; and an elevated manhole. As would be expected, defects in the upper level would be strongly reactive to rainfall and runoff. A limited tributary meant the defects in the upper level would allow more runoff I/I into the manhole than there was being collected by the contributing service area and rainfall. Sheet-flow was flow limited to approximately 1/8-in. over the manhole.
The lower level components are less reactive to rainfall and runoff because they are deeper below the surface. The defects would be expected to react according to the Horton infiltration principle (1977). The possible exception to the Horton principle would be defects such as in the pipe connections, bench, and channel that are below the groundwater table. For instance, if groundwater is slightly above the pipe entering the manhole then the static head pressure would cause these lower elevation defects to leak more than similar defects above the groundwater level.
The northeastern utility’s storm event of interest provides an important criterion for calculating the quantity of I/I passing through a defect. It enables the calculation of the amount of runoff that would potentially influence a manhole through one of the four upper level exposure mechanisms described above. Similar to general storm water runoff calculations, the rainfall tributary area is one of the equation components to produce the runoff rate and volume. The same concept was used in the SCREAM Manhole tool to determine the potential volume of runoff the manhole upper level manhole components would be exposed. Therefore, field crew training was another important factor in accessing the influencing tributary runoff area. Field forms were developed for the field crews to capture either actual estimated tributary area length and width dimensions or, when using the database form, to use pre-loaded drop-down standard dimension choices. Other required data such as estimated ponding depth and manhole cover height relative to the ground allowed the tool to automatically determine the exposure mechanism and the runoff volume. The training and form design helped to minimize the potential for field crew decision inconsistencies.
I/I calculations were automatically generated by the tool for each manhole level and for the overall manhole. One calculation resulted in a peak hour I/I rate, which would subsequently be used by the SCREAM tool to score the manhole’s projected I/I. The other calculation resulted in the projected manhole I/I volume, which was based on the selected storm’s duration and volume.
I/I quantification and projection to the storm event of interest allows the manhole’s I/I peak flow rate impact on the downstream pipe to be calculated and converted to a numerical score. The peak hour I/I flow rate is converted to a percent of the pipe’s flow capacity occupied. This percent area occupied represents the same concept as would be applied to the cross sectional area occupied by sediment or other maintenance defect. The percent area occupied is converted to a numerical scale using hydraulic principles and engineering judgment. Scoring allows the manhole I/I quantification to be integrated with structural and maintenance defect scoring using the same SCREAM scoring scales (Rowe 2009). Therefore, manhole scores can be developed that represent both the physical and hydraulic condition of the structure.
The advanced I/I quantification and scoring process allows manhole I/I prioritization and rehabilitation either by the projected I/I score or the projected I/I rate. The SCREAM tool considers the peak hour I/I rate impact on the system’s capacity and therefore provides a more pertinent prioritization basis than just the volume of I/I. By projecting the I/I rate and volume utilities can link field inspection results with flow monitoring RDII flows.
Utilities that calibrate not only their manhole inspection results but also other inspection results with the basin’s flow monitoring RDII have a reliable basis to decide where to perform rehabilitation.
EDITOR’S NOTE: This article was based on a paper that was presented at the WEFTEC 2010 in New Orleans.
Reggie Rowe is CH2M HILL’s Conveyance & Storage Infrastructure Condition Assessment & Rehabilitation Global Technology Leader. He co-developed the CH2M HILL SCREAM pipe and manhole condition assessment scoring and I/I quantification tool. Rick Nelson specializes in projects related to buried infrastructure systems for municipalities, utilities, and industry. He currently serves as the Global Conveyance & Storage Service Team Leader (GSL) for CH2M HILL with regional and global infrastructure technology and business development responsibilities.
Table 1: I/I Quantification and Scoring Principles
- A manhole and its components are divided into two levels (elevations) to aid in the development of the I/I quantification principles; the upper level and lower level.
- For manhole components in the upper level (above the reducer/(cone), I/I through defects is calculated based on rainfall runoff over the manhole or on the potential ponding depth over the manhole. I/I is distributed among the observed defects for these wet weather conditions. The distribution will be in proportion to each defects potential I/I rate presented in the I/I rate tables that are included in the paper.
- Manhole defect components in the lower level (below the reducer/cone) will not be proportioned to the tributary or ponding volume but rather be considered subject to subsurface water migration. Observed defects in the lower level will be assigned I/I rates listed in the I/I rate tables that are included in the paper.
- Rainfall runoff varies according to the rainfall frequency/intensity/duration (FID) pattern selected for the study area.
- Manhole defect I/I will generally vary according to the rainfall FID.
- Manhole defect I/I in the upper level will be calculated based on manhole’s site configuration and the influencing surface tributary runoff area and rainfall FID. The exception will be when the manhole cover is subject to ponding conditions. For ponding conditions the I/I will be calculated based on cover type and condition using orifice equations where holes exist. The entire tributary runoff area is assumed to drain towards and into the manhole.
- Manhole I/I Scoring is based on the peak hour flow rate for the selected FID storm event using a SCS Type 2 rainfall distribution pattern. Manhole I/I Volume is based on the duration of the selected FID. The default duration will be 24 hours.
- The un-gasketed manhole cover seal will automatically be classified as a defect since it is not designed to be watertight. The un-gasketed cover seal I/I rate and volume contribution will be calculated similar to the cover using runoff and ponding principles.