“Typical Year” CSO Modeling Fails for Alewife Brook

Please join us 6 pm on Thursday night, 12/15/22, at the Alewife Sewage Pollution Planning Meeting #2 by registering here.

At this meeting, Cambridge, Somerville, and the Massachusetts Water Resources Authority will talk about the new Future “Typical Year” sewage pollution modeling which will be used for new sewer system infrastructure planning and future performance assessment. The new “Typical Year” modeling will include Climate Change Projections.

Climate Change will bring an increase in rainfall, and an increase in the severity and number of storms. Increased rainfall means an increase in Combined Sewer Overflow (CSO) sewage pollution in the Alewife Brook. In fact, that increase appears to be exponential with an increase in rainfall. While Cambridge, Somerville, and MWRA should be commended for including Climate Change projections in the new “Typical Year” modeling – it’s not enough.

Why? Well, the “Typical Year” is basically an “Average” year.  But modest increases above the average rainfall produce much greater CSO sewage pollution discharges.  It is in the years when we experience “atypical” increases in rainfall that we experience greater amounts of CSO sewage pollution in the Alewife. 

Actual CSO Discharge volumes in the Alewife Brook have often been way above the Typical Year modeled values since 2015 when Alewife CSO projects ended. Data from MWRA reports which we wrote about here.

The graph above shows the annual “Typical Year” rainfall along with actual rainfall, as well as “Typical Year” CSO sewage discharges and actual CSO sewage discharges for Alewife Brook. This chart goes back to 2015, when the MWRA, Cambridge, and Somerville completed the last of their CSO projects. Note that Alewife CSO discharges increase dramatically with increase in rainfall; the actual annual average CSO discharges over this seven year period were 15.8 million gallons, while the modeled Typical Year value was only 6 million.

The new CSO modeling for Alewife needs to represent “atypical” years. By design, the new “Typical Year” model will not reflect the actual reality of Alewife CSO sewage pollution discharges unless it includes atypical year data.


Even with Climate Change Projections taken into consideration, a Future “Typical Year” based model would not represent the reality of sewage pollution in Alewife Brook.  Atypical years need to be included in the discharge and compliance analysis for the Long Term CSO Control Plan.

Untreated sewage pollution has harmful health impacts on the 5000 people who live in the Alewife’s 100-year flood plain. Therefore, an End to Untreated Alewife Sewage Pollution must be engineered for the Alewife Brook. It is not enough to include Climate Change projections in the “Typical Year” model. Gray and green infrastructure solutions must be used to end untreated sewage discharges. At the same time, the Great Cities of Cambridge and Somerville should continue the good work of separating their combined sewer systems. 

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