ABC Stormwater Flooding Group - May 14th, 2019

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Three people attended tonight's meeting: Wayne Chouinard, myself, and a fellow from East Arlington named Derek. We didn't have representatives from Cambridge or Belmont, meaning there wasn't a quorum.

The three of us had a nice informal talk about the challenges of flash flooding -- think of a torrential downpour that drops a lot of water in a very short time. In a hilly area, these storms have the potential to turn roadways into rivers. Waterflow at the street edge can be wider than storm grates, and flows quickly enough that much of the water passes over the grates anyway. Water pressure inside the storm drain system can build up rapidly enough to dislodge manhole covers downhill. This has happened several times in the heights. Our roadway drainage systems were designed to handle 25-year storms, but the "traditional" 25-year storm happens more frequently.

Wayne tells us that the town is planning to install a number of infiltration trenches. These would be connected to catch basin outflows. The outflow would run into a length of perforated pipe, and the pipe would be buried in gravel. The design sounds similar to a septic system drainfield, but with a single pipe rather than a lattice. The idea is to store the initial rush of rainwater, and let it gradually discharge into the ground. Derek likened this to capacitance (i.e., the way a capacitor stores and releases electricity).

Finally, we discussed rainbarrels as a stormwater mitigation method. One could store the rainwater and use it for gardening. A barrel with a 1/8" hole near the bottom would also be effective -- it collects the initial flow from a gutter system, and allows it to slowly drain out. The lower flow rate is more easily absorbed by the ground.