The Blocked Drain

Introduction

Has this ever happened to you? You are getting ready for work and you decide to brush your teeth. You turn on the water and notice the sink is backing up because the faucet is running too fast or the drain is partially clogged. Either one can create a problem, but if the faucet is running while something is also blocking the drain, the water rises much faster.

Now imagine that same thing happening across an entire city, with heavy rain coming down, rivers flooding, and storm drains failing to function. Scientists call this compound flooding, and new research suggests it will become an increasingly important part of flood risk as our climate changes. Welcome back to the podcast. I'm Dr. Mac, and this is The Climate Translation.

Where Is the Water Coming From?

When we say that a place flooded, we typically mean that there was too much water for an area to handle. But if you are a hydrologist, a meteorologist, or an engineer trying to understand what happened, there is another question you need to ask: Where did that water come from? That is important because not all floods work the same way.

Let's start with the most familiar type: pluvial flooding, which simply refers to flooding caused by rainfall. Heavy rain falls faster than the ground can absorb it or drainage systems can carry it away, causing water to collect in streets, parking lots, yards, and low-lying areas. You do not need to live near a river or the ocean for this to happen; a strong thunderstorm can drop several inches of rain in a short period and submerge an intersection that was dry an hour earlier. Then we have river flooding, or fluvial flooding, where rain falling across a watershed drains into streams and rivers until they overflow their banks into the surrounding floodplain. This type of flooding may occur hours or days after the rain ends as water travels downstream. Along the coast, strong winds from tropical storms and hurricanes generate storm surge, which, combined with high astronomical tides and rising global sea levels, pushes seawater onto land.

Historically, we evaluated these hazards individually, calculating how much rain a drainage system could handle, how high a river could rise, or how far inland storm surge could travel. But nature does not have to choose just one. When a tropical system approaches, heavy rainfall fills inland streams and swollen rivers carry water toward the coast, while storm winds and rising tides push seawater onshore. When these different water sources collide, compound flooding begins. A swollen river encounters elevated ocean water, slowing its discharge into bays or estuaries, while storm drains encounter seawater pushing back from the opposite direction. The faucet is running while the drain is closing, causing water to accumulate deeper and linger longer than individual hazard models would predict.

The Blocked Exit

If you want to see what compound flooding looks like in the real world, one of the clearest examples occurred during Hurricane Harvey in 2017. Most people remember Harvey because of Houston, where the storm stalled after making landfall as a Category 4 hurricane and dumped more than 50 inches of rain across southeastern Texas. But about 90 miles to the east, the community of Port Arthur was dealing with its own disaster.

Port Arthur is a city of about 55,000 people near the Texas-Louisiana border, bordered by Sabine Lake and crossed by rivers, bayous, and canals designed to move water away from developed areas. During Harvey, that drainage system was overwhelmed from multiple directions by extreme rainfall and elevated coastal water levels, making it difficult for runoff to escape. Researchers who later reconstructed the flood estimated that about 19 percent of the flooded area around Port Arthur resulted specifically from the interaction between rainfall and elevated coastal water, meaning that calculating the two hazards separately failed to capture the full extent of the flooding.

The researchers also modeled what a similar storm might look like in a warmer future with approximately 22 inches of additional sea-level rise and an 18 percent increase in rainfall. Under those conditions, the portion of the flooded area driven by compound interactions rose from 19 percent to roughly 33 percent. Climate change does not need to invent a new type of flood; it simply changes the environmental baseline by raising sea levels and increasing atmospheric moisture, making the physical interactions between coastal and inland waters far more severe.

When the Old Odds Stop Working

To understand how we plan for floods, we need to address one of the most misunderstood concepts in weather and climate science: the 100-year flood. A 100-year flood is not an event that happens once every century; rather, it represents a flood that has a one percent probability of occurring in any given year. You can experience multiple 100-year floods within a few years of each other, or even in consecutive years, because the term describes statistical probability rather than a fixed schedule.

We historically estimated those probabilities using past observations of rainfall rates and flood heights under the assumption of climate stationarity. But climate change alters those underlying conditions. When average sea level rises, storm surge reaches higher flood elevations much more easily, similar to lowering a basketball hoop so that previously missed shots begin going in. Simultaneously, a warmer atmosphere holds more water vapor, providing storms with the capacity to produce heavier rainfall totals. As both rainfall intensity and ocean water levels increase, the joint probability of these events overlapping and interacting rises significantly, turning what were once rare return-period events into more frequent occurrences.

Designed for One Flood at a Time

For engineers and city planners, adjusting to these shifting probabilities is becoming a central challenge. Cities rely on storm drains, culverts, canals, pumps, levees, and seawalls to manage water, but each system was historically engineered around individual hazards. A seawall may successfully block storm surge from entering a coastal neighborhood, but if heavy rain falls behind it and elevated seawater blocks the drainage outlets, the protective wall can trap stormwater inside the community.

Addressing compound flooding requires moving beyond isolated hazard design. Some communities are installing higher-capacity pumps, creating designated retention areas to temporarily store water, and restoring wetlands and natural floodplains to slow water movement through watersheds. Most importantly, engineers increasingly rely on integrated hydrodynamic models that simulate the complex interactions between rainfall, river discharge, tides, storm surge, and sea-level rise simultaneously. Compound flood dynamics extend upstream into estuaries and tidal rivers, reminding us that effective flood protection requires treating the entire watershed and coastline as one interconnected system.

Conclusion

Flooding has always been part of living with water, but as our climate changes, the volume of incoming water and the coastal conditions waiting to receive it are shifting as well. Understanding future flood risk requires looking beyond isolated hazards and recognizing the complex dynamics that occur when different waters meet.

I'm Dr. Mac. This has been The Climate Translation. If you have a question about the climate that you have been too afraid to ask, or if you have a differing opinion, I want to hear from you. I can use your viewpoints in a future episode. You can reach me at TheClimateTranslation@gmail.com. I'll see you next time.