One of the most complicated overflow situations on Alewife Brook occurs during a storm of five or ten years frequency. Normally, this would mean five or six inches of rain over a two-day period, enough to raise the level of a stream to a level higher than the weir inside the sewer chamber. When this condition occurs, it is possible to get increased sewer backups into the local sewer pipes which feed into the main MWRA interceptor. With rain still coming down, there are now two inflow sources into the interceptor sewers : flow from local runoff and inflow from the stream or river. As the storm and runoff taper off, the dominant inflow into the trunkline sewer comes from either Alewife Brook or the Mystic River (Figure 4). This "brook inflow" will continue as long as the brook level is higher than the elevation of the top of the weir.

A pump station overflow can occur at the same time a major flood inflow occurs (see Figure 5). Floodwaters from the brook can flow backwards into the MWRA sewer through a CSO overflow pipe and then proceed to the downstream pump station, where the resulting high sewage content overflow will be redirected back into the brook -- from whence some of the stormwater originally came. The sewage concentration, however, will be much higher and will constitute a significant source of pollution to the stream and river system.

All of these complex flows will occur just before, during and after the cresting of the floodwater in Alewife Brook and Mystic River. In a typical ten-year flood for Alewife Brook, it takes about a day and a half for the flood elevation in the brook to fall below the level of the lowest weir, so "brook inflow" into the sewer system could occur continuously during this day and a half period. As with sewage, the amount of inflow would be limited by the downstream capacity of the MWRA sewer system.

Sewer overflows can also occur if there is an excess of sewage and inflow arriving at a location without a CSO structure -- such as a pump station or surcharging manholes. At a pump station, the equipment is set to process all the arriving flows, and if this flow exceeds downstream capacity, this excess is pumped up to the ground surface, and thence runs down to the brook or river by surface gravity flow.

This overflow process is quite complicated, and to my knowledge no municipality or state agency has modeled the flood flow of brook water into the sewer system. Neither is it evident that the FEMA flood studies have been able to include this inflow effect within the computer models for flooding used to date.