A backflow preventer should keep street water out of your house, but what about the water at the rear of the house? In most bloomingdale row houses, the rear roof gutters are connected to downspouts that lead the water into your house`s sewage drain pipes. If you`re having a flood in the front of the house, the backflow preventer will close, leaving the roof water nowhere to go but your basement.
One proposed solution to this is to direct the roof water into a rain barrel, so that water collects in the barrel rather than in your basement. Good idea?? Well, only if the barrel is large enough. And large enough is pretty hard to define.
So let`s make some explicit assumptions. Let`s assume that a 20`x25` area of your roof drains into the rear of your house. Let`s assume a rain storm that unloads 3`` water. (The 9/2 storm was, according to DC Water, 2.79``) Your house may be bigger or smaller, and we certainly can expect occasional rains in excess of 3``. But I`m going to work with those figures, as an illustration.
Total roof area in square inches (20x12x25x12): 72,000
Total volume of water, in cubic inches (3 * 72,000): 216,000
Total volume of water, in gallons (216,000 / 231): 935
My conclusion is that typical rain barrels, 55 gal, 80 gal, will not help. They will fill up early in a storm, during the period when your bfp is open and storage is not needed. The barrel will be full and overflowing late in the storm, which is when you need the storage. Even a much more common 1 inch storm will dump 311 gallons onto this size of roof, and even that is far beyond the capacity of a typical rain barrel.
There are also above-ground cisterns that can hold hundreds of gallons. Some mid-sized ones, in the 400 gallon range, can fit through doors and be placed in a typical limited-access bloomingdale back yard. The larger cisterns, 1000 gallons and up, could only be installed in a limited-access back yard using a huge crane.
And one more thought. Rain barrels lead to standing water, which is the ideal mosquito habitat. See this.
So for me the bottom line is that typical 50-80 gallon barrels won`t help. A 400 gallon cistern is the scale that could help. It won`t collect all the water of an extreme event, but 400 gallons is significant. Unfortunately, cisterns large enough to be considered true solutions are simply out of reach, literally, for most of us with limited-access back yards.
To read more about rain barrels in DC, try this .