Flooding, in short, happens when water overflows onto land. This can happen for many reasons:  

• Heavy rain may drop more water than the surrounding land can absorb; 
• A storm surge or wave may overtake the shore and impact nearby buildings and infrastructure; 
• Snow may melt too fast, draining into streets and roads;  
• Streams, rivers, or other bodies of water may overflow their banks; 
• Levees, dams, or other structures may break, allowing significant water to flood the surrounding area.  

The definition of flooding is wide and covers a range of conditions, from a few inches of rain accumulation to water that rises above the rooftops of houses. They can occur anywhere, but densely populated areas, areas near rivers and streams, low areas, and infrastructure like underpasses and parking garages are the most likely to experience flooding—especially flash or stormwater flooding. 

It is difficult to ignore the devastation that occurred from flooding because of the hurricanes that have struck the United States and Puerto Rico. Some areas are still feeling the impacts from hurricanes such as Harvey, Maria, Irma, and Ian which all made landfall within the last few years. The devastation caused by Hurricane Katrina remains in many people’s memories, with reconstruction continuing long after the storm had passed.  According to the CDC, flooding is the leading cause of death from hazards relating to storms. 

List of Costliest Atlantic Hurricanes

Another impact stormwater has on our lives is the damage to our roadways and bridges. Water will always find its way, and sometimes it finds one of our bridges in its path. This can cause bridges and culverts over streams and rivers to fracture, breaking apart from the surrounding land.  

Below is an image from before an emergency replacement our engineers worked on near Whiteville, N.C., after Hurricane Matthew in 2016. As you can see, the damage is extensive—no one would be safe driving over this! 

The keys for lowering the impact of this type of damage are prevention and a quick response to structural failure. The simplest (and often best) strategy is enforcing proper maintenance protocols. Engineers can monitor erosion of the surrounding soil, corrosion of foundations, and damage caused by debris. The small bridges and culverts over streams are at the biggest risk for this type of damage. Thankfully, most (if not all) states in the southeast have a low-impact bridge replacement program that replaces deficient structures before they can become a problem. 

However, mother nature does not care very much about good intentions and can take our defective bridges down before we get the chance to replace them. In this event, it is good to have a contingency plan.  

The culvert destroyed by Hurricane Matthew above is a notable example of a quick response by the state and an engineering team. When Matthew made landfall in early October of 2016, the state procured engineering services to replace it within two weeks. The engineering team conducted a review and noted a severe scour that allowed the culvert to settle and sever the roadway. In the design phase, the team determined the best solution was a replacement bridge, and construction was completed as early as possible.  

A fast response like the above example is the ideal for severe structural damage, but it doesn’t always go this smoothly. Sometimes, the situation is just out of anyone’s control. Dangerous site conditions after the fact can delay response times, making it difficult to ensure our roads and bridges are safely repaired. It might seem like an agonizingly slow process getting the bridge replaced on your fastest route to work, but everyone is truly working as fast as possible to get it up and running again! 

The stormwater runoff from a hurricane comes with significant risk to our environment as well. The damage caused by this is a literal “slippery slope.” When water runs off roadways and structures, it will accumulate pollutants such as insecticides, oils, trash, sediment, and human and animal waste. The contaminated stormwater can then flow into existing water sources such as rivers, lakes, and aquifers. These hazardous substances can pollute the water we drink and use daily, clog streams, spread disease, and destroy local ecosystems. That is a lot of damage all because of some extra rain. 

Flooding in the city of Franklin, Virginia after Hurricane Floyd

Even though engineers are constantly evolving the latest stormwater runoff practices, one of the largest contributors to stormwater pollution is our own daily activity. Thankfully, you do not have to be an engineer to help with the environmental impacts! The Environmental Protection Agency suggests taking these steps to reduce stormwater pollution: 

• Don’t dump waste into storm drains. 
• Keep yard clippings out of the street. 
• Dispose of household chemicals properly by following the directions on the package or by calling the local public works department for proper disposal guidelines. 
• Clean up oil spills and fix leaking automobiles. 
• Use drip pans to catch engine oil and other pollutants while repairing cars. 
• Recycle used motor oil. 
• Sweep driveways clean instead of hosing them down. 
• Wash your car at a commercial car wash, or at least wash your car on an unpaved surface so the ground can absorb the excess water. 
• Drain swimming pools and spas into a sanitary sewer outlet, never into a street. Check first with your local wastewater treatment plant before disposing of anything in the sewer. 

One of the primary ways we can mitigate flooding is through proper drainage. Engineers can easily design storm drainage systems that make the impacts of average rainfall and thunderstorms negligible, but hurricanes aren’t your average thunderstorm. Unfortunately, our existing drainage systems can become overwhelmed by hurricane rainfall. Without an unlimited budget to completely change the lay of the land of an area, we can’t eliminate the build-up of water in low-elevation areas and wetlands. 

Just because floodwater is dangerous doesn’t mean we don’t have the means to combat this force of nature, though. Here are a few strategies engineers implement to mitigate the risk of flooding: 

• Introduce permeable pavements that absorb water and reduce runoff. 
• Complete more complex drainage studies for problem areas. 
• Require pavement to follow land contours.  
• Regulate development in upland areas. 
• Enforce Low Impact Development techniques. 
• Plant more trees and vegetative buffers. 

An alternative to the common pavements we’ve come to know, permeable pavement is able to absorb rainwater, melting snow, and other sources of excess water. These are normally constructed from pervious asphalt or concrete and plastic grid pavers. In addition to mitigating flooding, permeable pavement can filter out pollutants and reduce the need for road salt and traditional drainage infrastructure.  

Urban drainage systems are integral to ensuring the safety of our urban environments, but their effectiveness is often hampered by issues native to the cities and townships where they are built. Well-designed drainage systems that accommodate for these challenges significantly reduce the damage brought on by excess rain and other stormwater hazards, but proper drainage studies need to be conducted to make these infrastructure designs a reality.  

Keeping water away from pavements is integral to the very foundation of structural design. Even so, drainage standards vary from state to state and municipality to municipality. By shaping pavement using crowns and slopes, runoff can be controlled, directing it to safe channels like swales and natural depressions that contour around the base of slopes.  

The costs of flood damage to productive farmlands, including the decline in water quality because of dangerous chemical runoff in stormwater, has led to multiple studies. When building upland, these issues can become even more pressing—upland development often results in higher erosion and can cause septic-system failures, affecting nearby bodies of water. This can also result in landslides if not properly mitigated. Furthermore, low-lying areas are of increased risk of flooding and flood damage. As storms become more powerful and frequent, municipalities have begun adopting new regulations, such as zoning ordinances, that enhance flood resilience and mitigate the erosion and environmental damage that building upland causes. 

According to the EPA, Low Impact Development (LID) “refers to systems and practices that use or mimic natural processes that result in the infiltration, evapotranspiration or use of stormwater in order to protect water quality and associated aquatic habitat.” In short, this is a green infrastructure technique that considers natural wet weather patterns and water flow in its design. LID utilizes native plants and soil types to capture water in green, ecologically friendly spaces, minimizing developmental impact on the environment and enhancing natural resilience. Plus, native plants often have the added benefit of not requiring as much chemical fertilizer or pesticides, as using native plants means that they’re growing in the environment they’ve adapted to over time.

According to the EPA, Low Impact Development (LID) “refers to systems and practices that use or mimic natural processes that result in the infiltration, evapotranspiration or use of stormwater in order to protect water quality and associated aquatic habitat.” In short, this is a green infrastructure technique that considers natural wet weather patterns and water flow in its design. LID utilizes native plants and soil types to capture water in green, ecologically friendly spaces, minimizing developmental impact on the environment and enhancing natural resilience. Plus, native plants often have the added benefit of not requiring as much chemical fertilizer or pesticides, as using native plants means that they’re growing in the environment they’ve adapted to over time.

The havoc inflicted by stormwater is scary, but it’s scarier knowing it’s a problem we can never fully eradicate. Nevertheless, our society has come a long way in mitigating the effects of stormwater, mitigation that will only continue to get better as our knowledge, technology, and safety measures progress. You can take pride in knowing that, even though stormwater is the most dangerous side effect of a hurricane, we can all pitch in to help each other with it. So, get out there and thank a civil engineer, wash your car on the grass, and pull that lawn chair out of your neighbor’s pool.