Typically, sewer pipes are about 1/6 to 1/10 full when there is no rain. About half of that "dry weather" flow is infiltration : regular leaks of ground water or other regular releases into the sewer pipes. During a storm, the flow into the pipes increased dramatically. The older combined sewer systems usually produce the highest stormwater flows. Even with separated sanitary and stormwater systems, there will be inflow into the sanitary sewer line. Water can get in though manhole leaks, various cross-connections, or simply from roof drainage which is improperly and possibly illegally tied into sanitary sewers.

As the inflow increases in the main sewer pipes, those pipes will fill up and at some point the capacity is reached. The most likely trigger to capacity backups in the sewer lines is the "bottleneck" -- similar to a traffic bottleneck which backs up traffic. Too much stormwater entering the sewer system can mean the capacity of the system to handle sewage and runoff is exceeded. The excess water must go somewhere, and it cannot be allowed to back up into peoples' basements. The typical "relief valve" to dump this excess water for the chamber to fill up and the excess water to flow over the weir and into the stream or river (Figure 2). When the storm begins to taper off, the runoff is less, and the main sewer pipe will now be able to handle this reduced flow of water. When the system is back to working within its capacity, the sewer overflows will stop. Dry weather is handled properly, and small storms. It is the large storms which overwhelm the sewer systems.

In past years, sewer overflows were seen to occur about once a week along Alewife Brook. In recent years, the overflows as reported appear to be only two or three a year. Maintenance of seer systems can reduce sewer overflows.

There are two types of sewer overflows depending upon the type of storm. One is a sudden flashy storm (such as an intense thunderstorm) which releases a short but heavy rainfall. The brook does not flood, but the runoff is very intense, with overflows as shown in Figure 2. With such intense/short storms, the sewer system will overflow, even through there is no evidence of any flooding in the brook.

The second type of overflow is caused by a steady rainfall which saturates the ground, causes extensive runoff over a wide area and results in flooding. For Alewife Brook, a good rule in such a lengthy rainfall is that every one inch of rain causes the brook to rise by a foot in flood elevation. This prolonged rain can cause saturated soils with increased inflow into the sewer pipes. This inflow can overload the sewer system and trigger a sewer overflow into the brook or river. The higher the flood waters are in the brook, the more difficult it is for sewer overflows to flow unimpeded into the receiving waters. (Figure 3)