GIS Analysis and Sewer Overflows: Achieving Baltimore’s Consent Decree

With more than 3,000 miles of pipeline, 116 pumping stations, and 60,000 manholes, Baltimore County’s vast sewage collection system handles roughly one billion gallons per year. In 2005, the Environmental Protection Agency and Maryland Department of the Environment brought forth a Consent Decree against Baltimore County, which outlines requirements to investigate and improve the sewer system’s operations over a 14-year timeframe to comply with the Clean Water Act. The Consent Decree’s overarching goal is eliminating sanitary sewer overflows.

The county and its consultant team (led by prime contractor Louis Berger) have implemented GIS technology in important ways to achieve the goals set forth by the Consent Decree and eliminate sewage overflows. EBA has been instrumental in identifying the best GIS tools and analytical methods to improve the project’s workflow and analyses.

What causes system overflows?

When food services facilities or residential households dump used cooking fat, oil, and grease (FOG) down the drain, problems ensue. These materials solidify in the sewer pipe downstream of the source and cause backups in the system.

Tree roots also often find their way through cracks and joints in sewer pipes. Over time, the roots block the passage of sewage and create backups or overflows.

Roots and FOG buildup cause most sewer blockages and subsequent overflows. Debris, structural pipe failure, vandalism, and inflow and infiltration during heavy rainfall can also cause overflows.

The role of GIS in eliminating overflows

All system overflows are reported to the county, including the address and, if available, the ID of the backed-up pipe segment. The overflow events are then mapped in GIS using the overflow address or by matching the pipe ID to the sewer segment. Even if only an address is available, spatial analysis can be used to select the closest sewer segment to the mapped GIS point and identify the clogged segment.

If roots have caused the overflow, the pipe segments are put on a special cleaning schedule to remove the future root growth and prevent additional clogging in the pipe. Inspections also determine if the pipe segment needs to be repaired or replaced.

If FOGs have caused the overflow, GIS is used to trace the sewer network upstream of the overflow and identify the source of the FOGs entering the system. The Environmental Health and Safety Department (EHS) provides quarterly violations and inspection data for food service facilities. This data is plotted in GIS and overlaid with the sewer network. Using the mapped data, the team can identify facilities that drain into the clogged pipe and may be culprits for dumping FOGs into the system.

If a food facility is found to be the source, EHS will conduct additional inspections of the facility to bring them back into compliance and eliminate FOG from entering the system.

Other times, tracing the network from the overflow in GIS reveals a source in a residential area where there aren’t any food facilities. In these cases, residents have likely dumped used cooking grease down the drain. The team will then post flyers in the residential area to educate residents about the proper disposal of cooking grease. GIS is also used to map the areas where flyers should be posted, based on the homes that drain into the clogged sewer.

GIS maps the road ahead

Since multiple years of overflow data are now available, the team can perform spatial analysis on all the mapped overflow data to identify areas of frequent overflow events. For example:

  • Kernel density analysis is run on the overflow event points to calculate the density of overflow around each event.
  • GIS analysis shows where overflow events are concentrated.
  • Areas of frequent overflows are analyzed to determine if additional cleaning, repair, or infrastructure replacement is necessary.

These are a just a few ways GIS is aiding in Consent Decree initiatives. GIS is also being used to automate work order creation, report work status, and provide easy access to non-GIS users in the field through web-enabled GIS data. EBA collaborates with both Baltimore County and the project team to scope ongoing project needs and communicate analytical results.

David Thaler, GISP, is a GIS Analyst for EBA Engineering, Inc., specializing in custom w


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David Thaler