Blog Post By Jona Elwell

When it Rains it Pours: What a Week of Rain Means to Washington, and our Urban Forest

July 21 was one of the rainiest days in Washington history. The National Weather Service recorded 4.64 inches of rain in D.C. as 11:25 p.m. on Saturday, more than double the previous record. The old record for July 21 was 2.56 inches set back in 1911.

It continued to rain for the majority of that week, disrupting Washingtonian’s commutes, roadways, houses, and even our mental as we questioned when ever we would put down the umbrella and see the sun again.

While many rejoiced when the sun came out last week and the humidity returned, the storm was written off as an anomaly. Unfortunately, that is not the case. WAMU’s Jacob Fensten explained, “Big storms are becoming more common, especially in the mid-Atlantic, so the terms 100-year or 500-year storm no longer apply. In a changing climate, the odds are quickly rising. In fact, in the D.C. region, a 100-year storm will be a 25-year storm by mid-century. In other words, big destructive storms will be four times as likely. More rain means more flooding.”

What’s a city to do?

D.C. is actually leading the way in terms of preparing for climate change induced storms. The D.C. neighborhood to our southwest, Bloomingdale, used to flood multiple times a year. D.C. Water alleviated part of that problem with a new stormwater runoff storage tunnel that can hold 3 million gallons. Better yet? There’s a much bigger fix under construction: a huge tunnel running all the way from Bloomingdale to RFK Stadium, which will be able to hold 90 million gallons.

Big, pricey, involved city projects are not the only way to combat rain, floods, and climate change though. Enter: trees!

The water flooding houses in Bloomingdale and pouring into the D.C. Water tunnels is not just rain water. It is an unfortunate mix of the city, referred to as stormwater. When rain falls on roads, driveways, parking lots, rooftops and other paved surfaces that do not allow water to soak into the ground, it picks up everything that was on those surfaces. This can include many different pollutants such as sediment, nitrogen, phosphorus, bacteria, oil and grease, trash, pesticides and metals. These pollutants come from a variety of sources, including pet waste, lawn fertilization, cars, construction sites, illegal dumping and spills, and pesticide application.

How to keep rain from falling on impervious city surfaces and flowing directly into our rivers and watersheds? One way is to intercept the water, which is exactly what a large canopy tree’s leaves would do.

Trees reduce runoff through rainfall interception by the tree canopy, by releasing water into the atmosphere through evapotranspiration, and by promoting infiltration of water through the soil and storage of water in the soil. The cumulative effect of tree canopy is to temporarily detain rainfall and gradually release it, regulating the flow of stormwater runoff downstream and thereby preventing some of the pollutants in rainfall and on urban surfaces from being transported to local waterways. Phew!

Our Science and Policy team put together this handy graphic explaining stormwater run off for their boat tour on the Anacostia River, one of the main beneficiaries of D.C.’s work to mitigate stormwater runoff.

Not to mention that trees provide additional water quality benefits through uptake of pollutants from the atmosphere, soil and groundwater. This tree benefits calculator we helped put together calculates the amount of stormwater your tree intercepts. A 10 inch wide red maple? That can intercept over 1,500 gallons of stormwater a year.

Want to learn more or do something to help mitigate stormwater? Plant a tree! There is a city wide program, RiverSmart Homes, that subsidizes trees you plant in your yard that help mitigate stormwater.

Here’s to hoping the rest of the summer brings sunshine to give our tunnels and trees a break!

The photo is from ABC7 reporter John Gonzalez’s twitter account and is  Accotink Creek  at Woodburn Road in Annandale.

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