The City of Portland and Portland Water District continuously monitor flow through local sewers and sewer overflow outlets with a network of meters throughout the sewer system.

Typically, reports of sewer overflows from those meters aren’t available until several weeks after a sewer overflow event has occurred – which makes those reports of limited value to anyone who’s curious if there’s sewage in Casco Bay right now.

However, because sewage overflows happen when it rains in Portland, and because we have a historical record of how much sewage overflowed for each rainstorm over the past year, we can make educated forecasts about when Portland’s sewers are likely to overflow, based on current weather reports.

In 2017, Portland’s sewers overflowed every time the city received over ½ an inch of rain within a 24-hour period. With that experience in mind, @CascoBayBot is programmed to tweet the warning that “sewer overflows into Casco Bay are likely, based on past experience,” when weather reports record over ½ an inch of rain.

In a number of cases, even smaller amounts of heavy rain over a short period of time overwhelmed the sewer pipes and triggered sewer overflows in specific neighborhoods. When moderate to heavy rainfall (over 0.1 inch per hour) is recorded at local weather stations, @CascoBayBot will also send out a warning.

Source code and technical documentation for this project is available at

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