Why DC floods. Yes, it’s climate change.

This is part one of a two-part series on urban flooding in DC.

Floods are the most frequently occurring natural disaster worldwide, accounting for 40% of all natural disasters. In the United States, flash floods are the leading cause of weather-related deaths; it is estimated that around 200 Americans die each year as a result of flash flooding.

In late September 2022, news channels and TikTok feeds were filled with coverage of Hurricane Ian, a Category 4 storm whose winds reached up to 155 mph. Florida residents with the resources and means to leave were faced with a decision: stay in their homes and hope the flooding would not reach their neighborhoods or flee to a safer location. As climate change increases the frequency and intensity of extreme weather events, more people will have to face this sometimes impossible decision.

The Washington region is no stranger to inundation, especially in recent years. On August 10, 2022 the District faced significant flooding. Most parts of the region saw at least one inch of rain, while others were bombarded with 2-4 inches of precipitation in just over an hour. This was just three years after several parts of the region received a month’s worth of rain in just an hour during the morning rush.

What’s happening now that’s different than before? Is frequent, intense flooding our new normal? Can something be done to help prevent these kinds of floods?

How we build communities can make flooding worse

Flash flooding and river flooding are the two most common types of inundation. River flooding occurs when excessive precipitation or snowmelt slowly overwhelms a body of water and causes significant runoff. Flash floods, on the other hand, occur when intense rainfall causes water levels to rise rapidly. According to the United States Geological Survey, a government agency known for its natural science expertise, “flash floods generally cause greater loss of life and river floods generally cause greater loss of property.”

Although these types of floods are often considered natural disasters, the destruction they cause is largely a result of human development. In undeveloped areas, rain is absorbed by grass and other vegetation. When these absorbent plants are covered with roads and buildings, rainwater is diverted to nearby bodies of water, which can become overwhelmed, leading to additional river and flash flooding.

Another form of inundation that occurs as a result of the way cities are built is urban, or internal, flooding. The Natural Resources Defense Council, which the New York Times considers as “one of the nation’s most powerful environmental groups”, explains urban flooding as inundation caused by rain that falls on impervious surfaces (like pavement) and overwhelms local stormwater drainage capacity. Without enough vegetation to absorb rainwater, it is instead diverted to sewage systems that can become overwhelmed by high levels of precipitation. When this happens, there is nowhere else for the water to go, and it begins to flood the streets.

Flooding in DC

River flooding occurs in the Washington region when there is heavy rain or significant snowmelt in the Potomac River watershed that then flows into DC. This can cause flooding up to days after the initial increase in water levels. DC is also vulnerable to coastal flooding, which happens when hurricanes or tropical storms cause water from the Chesapeake Bay or Atlantic Ocean to surge into the Potomac River. All the excess water overwhelms the river to the point of flooding.

Urban flooding in the Washington region is sadly not uncommon. Between 1984 and 2010, the impervious surface coverage in the area increased by 4%. Although this may not sound like a cause for concern, this figure equates to the construction of one million cubic meters of pavement and buildings every year for almost 30 years. This is one million cubic meters that was previously made up of vegetation able to absorb the majority of rainwater that now has to be diverted to the sewers.

The effects of this development on internal flooding are evident; prior to the most recent significant flooding events in DC, the Federal Triangle Flash Flood in June 2006 was the most significant urban flood. During this event rain overwhelmed the city’s storm drains and spilled into the streets, flooding federal buildings like the National Archives and causing $10 million of damage.

Most efforts to predict and prepare for inundation assume that the flooding will come from rivers or other bodies of water. This often leaves areas vulnerable to urban flooding out of maps that identify floodplains. Flood measures in DC followed this trend until the 2006 flood, when the government realized that urban floods also pose a significant risk. Now, the DC Flood Risk tool identifies Federal Triangle as a flood zone, as well as nearby museums such as the National Museum of Natural History and the National Gallery of Art. However, other floodplain maps are not being updated nearly as quickly, putting many people nationwide in the path of unexpected and damaging floods.

The role of climate change

The term ‘100-year flood’ is often used to describe the severity of a flood. In other words, there is a 1% chance that inundation will occur that year, although multiple 100-year floods can occur in a century. As the planet warms, glaciers melt and sea levels rise, which increases the risk of coastal flooding. In DC climate change models predict that sea level rise could be severe enough to cause areas that are now considered at risk of a 100-year flood to become vulnerable to 15-year floods by 2080. This means that the chance of inundation in any given year in these locations would increase from 1% to nearly 7%.

Warmer temperatures also increase the rate of evaporation, adding high levels of moisture to the atmosphere that must eventually be released. This will cause more extreme precipitation events that have the potential to cause “flashier” flash flooding than usual; extreme rainfall will be quicker and more intense, rapidly overwhelming sewage systems and causing even more destruction than past flash flood events.

It is believed to be likely that climate change is also increasing the frequency and intensity of severe river flooding events, although the number of moderate floods is decreasing. Higher temperatures dry the soil surrounding rivers out, which is helpful in preventing more moderate floods that require the surrounding soil to be wet in order to gain traction. However, even with dry soil near bodies of water, extreme flooding still occurs as the soil is not able to absorb such a high amount of water. These factors indicate that as the climate continues to change, most if not all flooding events will be severe.

What happens next?

Finally, while natural disasters can impact anyone, anywhere, the elderly, lower-income communities, and unhoused people face the greatest risk. These groups are least likely to have flood insurance, access to transportation to flee during an evacuation, and may not have the money or ability to relocate if their home is destroyed.

As the threat of inundation increases, flood mitigation measures must be improved upon and ensure high-risk communities will be provided sufficient consideration. Steps to mitigate floods can prevent loss of life and property, thereby reducing the overall impact of these extreme weather events. In Part II, I will explore a few mitigation strategies DC and the region have implemented already and measures that should be imposed moving forward to better prepare for future flooding events.