Here are some things to weigh when considering rainbarrels as part of a stormwater BMP. While rain-harvesting and storage are nice potential side benefits of the barrel, its future success relies on keeping the focus squarely on stormwater capture. If you look around the internet and review municipal rainbarrel programs that are being promoted, they are often discussing the 50- to 60-gallon barrel like it is a viable option.

It simply isn’t. Consisder these conservative assumptions:

Let’s say a typical residential roof is 1200 square feet (it is closer to 1600). And, let’s say that a minor storm passes through – on the order of 1/4″. That would create a runoff volume of approximately 187 gallons. Let’s assume that only 60 percent of that runoff made it to the downspout due to ponding and poor pitching of the roof. That would result in a runoff to the downspouts of approximately 112 gallons. If the homeowner had 2 downspouts and 2 50-gallon barrels, the system is are already overflowing. With 4 downspouts and 4 barrels, the system is more than half full after a very small storm – just one storm. Furthermore, the overflow checks that are typically designed into a rainbarrel are not able to discharge at a rate that can keep up with the runoff rate into the barrel. So, the notion that the barrel is safe from overflowing during a storm is on shaky ground.

Here comes the next issue. The homeowner wants to be able to store this rainwater for future use, but the weather is a little rainy and the need to irrigate is not currently there. A couple of days later, the next small storm comes through and your barrels that should be empty are starting off at 56 percent of capacity. The new 1/4″ storm overflows the barrels. And, this is if 4 barrels are installed – not typically the case.

Where does that leave us? If rainbarrels are to be considered as a viable BMP, then larger ones need to be used. Successful programs like those used in the cities of Pittsburgh and Washington, D.C., use barrels with 3 times the typical volume capacity at 150-gallons. This could provide some storage capacity for future use in more arid regions of the country. In the areas prone to heavier rainfall, use of a couple of larger barrels offers the ability to capture runoff from more substantial events, but still not likely act as a harvesting tool.

The long and short of it is this. Focus should primarily be on stormwater capture, especially of that first flush. Once that water is captured, it will likely need to be emptied onsite through controlled overflow to a drywell or dedicated infiltration area on the property. Homeowners need to be educated on these issues and keep tabs on capacity and maintenance.

A final point should be considered when designing rainbarrels for storwater capture. Is it properly constructed? I run across a lot of make-your-own barrel websites that are not always considering issues such as proper overflowing or covering of the barrel. The last thing a homeowner wants to deal with is a massive buildup of water right next to the foundation, or a mosquito population explosion in the sideyard.

While the DIY approach has some appeal, by and large the focus should be on proper design and engineering. If we can get away from poor planning and design, rainbarrels can be a great addition to the lot-level approach of managing stormwater.