Sewer condition assessment - GIS database without introducing processing errors
Procedure for Correcting GIS from Field Data
The largest difficulty discovered during the completion of this project was the inconsistencies
between the COP database and the real world. These inconsistencies include different manholes
found during the CCTV operation and not included in the database and large deviations in the
segment footage. The database footage is the result of as-built drawings. The team reviewed all
footage differences larger than 10 feet. This is what was considered a significant deviation.
The causes of the inconsistencies include as-built drawing errors, additional manholes
constructed but not as-built, relocations of pipe segments not as-built, and errors obtained during
the edge matching process.
The project team and the COP established the following procedural flow chart to handle new
features located during the CCTV investigation.
Rating System
URS established a rating system for the defects that are generally found in the COP sewer
system. The defect types and classifications are based upon a combination of PearPoint Flexidata
Observation Codes, COP Procedures, the UK Water Industry Manual of Sewer Condition
Classification, and prior experience of the project team with the COP Sewer System. The rating
system is shown in Table 1.
Table 1: Rating System
The scoring has been broken into two categories. These include debris/grit defects and pipe
defects. The pipe defects includes all defects except Debris, Grease and Obstruction. The pipe
defects are used to establish the CIP for rehabilitation/repair of the system. The grit/debris
defects have been used to analyze the capacity of the system, and to determine debris removal
costs and requirements.
Impact factors have also been assigned to each pipe segment. The impact factors are a
combination of a traffic factor, environmental factor, railroad factor, and age factor. The traffic factor is based on the pipe crossing a roadway with a certain 1999 Average Daily Traffic (ADT).
The environmental factor is based on the pipe segment crossing a wash, irrigation canal, or
drainage facility. The railroad factor is based on the pipe crossing a railroad track. The age factor
is based on the joint material used at the time of construction. The newer joint material and
construction method is more adept in keeping roots and groundwater out of the system. The total
impact factor is a combination of the impact factors and is multiplied by the raw pipe score to
determine and impact factor score. The impact factor score is used to determine the order that the
pipe segments will be rehabilitated/repaired. The impact factors are listed in Table 2.
Table 2: Impact Factors
The impact factors are automatically assigned to the pipe segments using GIS. The traffic
coverage was obtained from Maricopa Associations of Governments (MAG) in GIS format. The
railroads, washes, canals, and drainage ditches are geographic features. The year constructed was
obtained from the COP database.
