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This brief paper is intended to give a description of the effects that flooding can
have sewer flows. The main focus is on how sewer overflows can occur during certain
storms and can contribute to increased pollution of the Mystic River and its
tributaries. The analysis will seek to be generic and not deal with any one specific
problem. Neither will it deal with trying to put numbers on flows or sewage
concentrations.
Storm waters have an influence on sanitary sewers in many different ways. In
some older cities like Cambridge, combined sewer systems will mean that by design
roof and street runoff will go directly into sewers without a separate storm drain
system. Combined sewer systems or CSOs will often have a large overflow pipe or
chamber near the MWRA trunk line (see Figure 1). The overflow pipe will have a
short connector pipe to the MWRA interceptor below, and all local flows will go into
this interceptor pipe for eventual passage to Deer Island and treatment, prior to
release into Massachusetts Bay.
If there is a heavy rain, the runoff will be so intense that the chamber will begin to
fill up. As the water rises, it flows over a wall or weir at the end of the chamber, and
the excess combined sewage now enters the stream or river. When this overflow
occurs, it is called a "sewer overflow," and it must be reported to local health
authorities in the affected cities or towns. Typically, the reporting will indicate the
time and number of hours the overflow occurred, but not the amount or concentration
of the sewage.