Why Does My Community Flood More Than it Used to?
Beyond being a nuisance, floods have a significant impact in our communities. Floods cause loss of life as well as damage to buildings and other structures. In 2017 alone, Pennsylvanians suffered nearly $7 million worth of property damage and two people lost their lives as a result of flooding.1
What exactly happens during a rain storm that would lead to a flood? It starts with understanding what options rain water has when it hits the ground. In a forest, up to fifteen inches of rain per hour can be absorbed into the soils – becoming groundwater that fills our aquifers and also provides water for trees and other plants to grow. In our developed communities, most of that rain water becomes runoff instead. There are very few places left for water to soak into the ground, so it rolls downhill off of our rooftops, over streets and sidewalks, until it reaches a low point like a stream or river.
The raindrops might also take the fast track downhill by collecting in a storm drain along the edge of a street or parking area. Inside the pipes below the storm drains, rainwater gathers in large quantities and picks up speed as it travels. Eventually the storm drain system ends at an outlet. In most Pennsylvania communities, that outlet is the nearest stream or river. And the water gushes out of the pipes with power and volume that those waterways are not able to handle without consequences. In a heavy rain storm, the water quickly overflows the banks and causes flash flooding.
Like many people, you might have noticed more and more talk about flooding in your community and across the United States. There are a number of different factors that are contributing to the increase in flooding. One thing that is contributing is the increased number of heavy rainfall events that we experience each year. Heavy downpours are increasing nationally, especially over the last three to five decades. The heaviest rainfall events have become heavier and more frequent, and the amount of rain falling on the heaviest rain days has also increased.2
We also have many more paved and developed surfaces in our communities than we used to. Homes, businesses, roadways, and parking lots all prevent water from soaking into the soils the way it would in a forest or meadow. Even our lawns prevent most rainwater from soaking into the grounds because they have been so compacted during development. Between 1990 and 2007, the amount of developed surfaces associated with new single-family homes were estimated to have increased about 34 percent across central Pennsylvania and the rest of the states surrounding the Chesapeake Bay. At that same time, the region’s population only increased by 18 percent. So our communities are adding these impervious surfaces faster than we are adding new people.3
Increased flooding can also be attributed to Pennsylvania’s aging communities. Many towns and cities have systems in place that are well over 100 years old. When they were designed and installed, people had no idea what volumes of stormwater people would be dealing with in modern day Pennsylvania. All of these factors add up to a flood of issues.
When local streets, parking areas, and home properties flood but they aren’t located right next to an overflowing stream or river – it could be because the storm drains are clogged with litter, mud, or yard waste and not allowing water in, or they are overflowing themselves from having to carry away more water than the pipes can handle. It could also be from poorly directed downspouts draining water off of rooftops into areas where the water has nowhere else to go.
When you see a flood, keep in mind that water isn’t the only thing carried into storm drains and into our local streams and rivers when it rains. The rainwater picks up litter and debris, automobile fluids and other hazardous chemicals, soils eroding off of disturbed areas, pet waste, and anything else that happens to be on the ground.
It’s important to play it safe around flood waters. All that water is carrying all sorts of potential hazards that it has picked up along the way. Children and adults should never consider flood waters as an opportunity for recreation. Just six inches of rushing water has enough power to knock over an adult too, making drowning a serious possibility. The power of flood waters is also strong enough that just 12 inches of water can carry away most small cars.4 Don’t take a risk and try to drive through flooded streets. You can’t see what potholes and other hazards are under the water that could damage your car.
All of the rushing water, and all of the pollutants it has picked up, will eventually travel to our streams, which flow to our rivers. Many of Pennsylvania’s communities rely on these waterways as their source for public drinking water, as a place for recreation and fishing, and as habitat to our diverse wildlife. Right now, Pennsylvania’s municipalities are working hard to improve stormwater management to reduce flooding and water pollution.
If you have additional questions about stormwater, or you are just interested in learning more, you can find a full series of videos and articles in the Penn State Extension Stormwater Basics series .
1 National Oceanic and Atmospheric Administration (NOAA). Storm Events Database
2 US Global Change Research Program. National Climate Assessment
3 Chesapeake Bay Program. Stormwater Runoff
4 National Weather Service – NOAA. Flood Safety Tips and Resources