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Smooth Sailing Upstream
Sewer engineers need to be able to trace sewage flow through a network of pipes. This allows effective visualization of all sewers downstream or upstream from a given point, which can aid in identifying areas affected by a sewer problem and can help pinpoint the source of the problem itself. The challenge for the City of Fort Wayne was to design a sewer tracing application that was intuitive and accessible to all engineers, inexpensive (designed, built and supported by City staff), highly accurate, easily maintained, and would return a map and tabular results that could then be printed and distributed for analysis.
The City’s sewer pipes table is encoded with the upstream and downstream structures on each pipe record. This data structure forms a “logical network” and allows staff to discern pipe connectivity and direction of flow. City staff built a Perl script to take advantage of this logical network, and perform a sewer trace required by the engineers.
The script uses a “recursive algorithm” and takes a start structure as input. It then searches for the sewer pipe(s) for which that start structure becomes the upstream structure, thereby moving in the downstream direction. Once the pipe(s) is identified, it uses that pipe’s downstream structure as the new start structure, calling the subroutine again with the new start structure. This recursion continues in a “depth-first” search pattern until all downstream branches are traversed and the trace is complete. An upstream trace is accomplished by reversing the logic, and searching for pipes where the start structure becomes the downstream structure.
This tracing functionality was incorporated into the City’s GIS intranet, allowing any City employee to trace downstream or upstream sewers from a given start point. The pipes are displayed on a map that can be printed, along with a tabular list of pipes that can be exported for further analysis.
Results
The results allow engineers to determine problem areas affected by “pinch points” in the sewer system. These problem areas are where residents experience basement flooding and sewage backups into their homes. Combining sewage flow data from the City Utilities’ billing system with the sewer tracing results provides the amount of sewage flowing in those pipes. Subtracting current sewage flow from the total pipe capacity gives the engineer the amount of remaining capacity. When a sewer is near or over full capacity during peak flows, the engineer will plan for a sewer re-route or replacement project, thereby eliminating sewage backups.