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The Blocked Drain · The Climate Translation

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Has this ever happened to you, you're getting ready for work and you decide to brush your teeth.
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You turn on the water and you notice the sink is backing up.
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Maybe the faucet's running too fast or maybe the drain is partially clogged.
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Either one can create a problem.
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But if the faucet's running while something's also blocking the drain, you can notice the
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water rising much faster.
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Now imagine the same thing happening across an entire city with heavy rain coming down,
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rivers flooding and storm drains not functioning.
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Scientists call this compound flooding and new research out of Texas suggests it will
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become an increasingly important part of flood risk as the climate changes.
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Welcome back to the podcast on Dr. Mac and this is the Climate Translation.
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When we say a place is flooded, we typically mean there's too much water for a region
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or area to handle.
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But if you're a hydrologist, someone who studies how water moves on the planet, a meteorologist
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or an engineer trying to understand what happened, there's another question you need to ask.
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Where do the water come from?
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That's important because not all floods work the same way.
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Let's start with something familiar.
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Heavy rain falls faster than the ground can absorb it or our drainage systems can carry
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it away.
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So the water begins collecting in streets, parking lots, yards and low lying areas.
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Scientists call this pluvial flooding.
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Pluvial simply means related to rainfall.
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And importantly, you don't need to live anywhere near a river or an ocean for it to happen.
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You may have seen this, a thunderstorm produces several inches of rain and a short period
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of time and suddenly an intersection that was perfectly dry an hour ago is now underwater.
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Then we have river flooding or what scientists call fluvial flooding.
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In this case, rain falling across an entire watershed eventually makes its way to streams
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and rivers and those rivers rise until they overflow their banks and spread across the
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surrounding flood plain.
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This type of flooding may happen hours or days after the heaviest rain has ended because
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all that water needs time to work its way downstream.
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Then along the coast, you have another source of water altogether, the ocean.
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Strong winds associated with hurricanes and other coastal storms can push seawater towards
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the land, creating a storm surge.
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High astronomical tides can raise that water even further.
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And today the surge is occurring on top of sea levels that are already in many places higher
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than they were several decades ago.
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We've traditionally been pretty good about thinking about these hazards individually, how
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much rain can a drainage system handle, how high this is river normally rise, how far
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inland can a storm surge go.
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But nature has many paths to choose.
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Imagine a tropical system approaching the coast.
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Heavy rain begins falling inland, that water enters storm drains and streams and begins
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moving downhill.
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The rivers rise and carry more water towards the coast.
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Under normal conditions, much of that water eventually drains into an estuary, a bay or
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an ocean.
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But at the same time, winds from the storm are pushing seawater towards the shore.
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The tide is rising, the storm surge is building.
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Now, water is approaching the same community from multiple directions.
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That's where compound flooding begins.
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It isn't a case of two floods happening next to each other.
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It's different sources of water that are influencing one another.
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A swollen river reaches the coast and encounters high ocean water.
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Its ability to drain slows.
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Storm drains that normally empty towards the coast encounter water pushing back from the
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other direction.
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At the same time, rain is falling.
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The faucet is still running, but the drain is beginning to close.
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That interaction causes water to accumulate in places and reach depths or remain longer
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than we might expect if we calculated each hazard separately.
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And the different pieces don't have to peak at the same moment.
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A river can be rising after the storm surge has peaked.
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Soil can be saturated when another round of rain arrives.
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One event can leave conditions that make the next one worse.
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That's what makes compound flooding so difficult to predict for and a plan for.
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We're not just adding one source of water on another, we're dealing with an interconnected
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system.
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And nowhere is that connection more obvious than where rivers and drainage systems finally
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meet at the sea.
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If you want to see what compound flooding looks like in the real world, one of the best
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examples occurred during Hurricane Harvey in 2017.
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Most people probably remember Harvey because of what happened in Houston.
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The storm came ashore in the Texas Gulf Coast in late August.
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But after the landfall, Harvey slowed dramatically and spent days dumping extraordinary amounts
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of rain across Southeast Texas.
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Some locations received more than 50 inches of rain.
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Houston experienced catastrophic flooding, but about 90 miles to the east, another community
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was dealing with its own disaster.
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Port Arthur is a city of around 55,000 people near the Texas Louisiana border and it's surrounded
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by water.
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To the east is Sabine Lake.
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It's a large estuary connected to the Gulf of Mexico.
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The region is also crossed by rivers, bayous, canals, and extensive drainage systems that
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are designed to move water away from developed areas.
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During Harvey, that system was attacked from multiple directions.
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Rainfall across the city was extreme.
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At the same time, water levels and the coastal system were elevated.
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It's the exact type of interaction we just talked about.
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Water coming off the land had a hard time getting out.
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Years later, researchers at the University of Texas at Arlington reconstructed the flooding
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from Harvey and tried to separate those different influences.
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They modeled what would happen from rainfall alone, and then they looked at the effects
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of elevated coastal water and where the two interacted.
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And what they found was significant.
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Researchers estimated that about 19% of the flooded area around Port Arthur resulted
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specifically from that interaction between the different sources of water.
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In other words, simply calculating the rainfall flood and the coastal flood separately
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would not have told you what was going to happen.
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The interaction created additional flooding.
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And then researchers asked another question.
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What if we get a Harvey-like storm in the future, after climate change has warmed up the
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temperatures and increased sea levels, they modeled a scenario where about 22 inches of
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additional sea level rise occurred, along with about 18% increases in rainfall.
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Under those conditions, the portion of the flooded area associated with these interactions
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increased from 19% in Port Arthur to a third.
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Now those numbers aren't a prediction of exactly what's going to happen during the next
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hurricane Harvey.
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There is a scenario designed to help scientists understand how a system responds when conditions
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change.
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It's what makes studies like this useful.
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Climate change isn't going to invent a new type of flood.
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We're still going to have heavy rainfall, we're still going to have storm surges, we're
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still going to have high tides.
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What's going to change though is the environment in which those events occur.
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Raise the average level of the ocean and coastal flooding begins from a higher starting point,
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increase the amount of water available during extreme rainfall, and more water has to leave
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the land.
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But those changes together in a place like Port Arthur and the interaction becomes increasingly
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important.
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Harvey showed us what happens when several parts of the water system are overwhelmed at
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once.
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But there's something else to consider.
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Much of what we've built along our coast was designed using statistics that assumed
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the past could tell us what to expect in the future.
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And as we have discussed many times in the past, as climate changes, some of those old
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odds are changing with it.
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To understand how we plan for floods, we have to talk about one of the most misunderstood
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phrases in weather and climate science: the 100-year flood.
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It sounds like a flood that happens once every 100 years.
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But that's not what it means.
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A 100-year flood is an event that historically has had about a 1% chance of occurring in
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any given year.
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You can have two 100-year floods within a few years of each other.
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Theoretically you could have them in consecutive years.
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The number describes probability, not a schedule.
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But there's another assumption that's hidden in the calculation.
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We estimate those probabilities by looking at the past.
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In the historical record, how high is the water risen?
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How much rain has fallen?
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How frequently have extremes occurred?
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For a long time, engineers could reasonably assume that history gave us a guide to the
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future.
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But human-induced climate change has complicated the assumption.
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Just think about sea level.
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If average sea level rises, a storm no longer needs to push as much water towards the shore
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to reach a particular height.
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Imagine a basketball hoop that's slowly lowered.
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And now your shots are going in at a higher rate.
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You haven't suddenly become a better basketball player.
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Shots that were once missed can now go in easier.
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Sea level rise changes the threshold in the same way.
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And consider extreme rainfall.
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A warmer atmosphere can contain more water vapor, and that provides storms with the potential
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to produce heavier rainfall.
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So one side of the system can deliver more water, and one is working from a higher level.
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And now you have to calculate the probability of those things occurring at the same time.
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That's compound flooding, and it's challenging.
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Maybe an extreme rainfall event has historically been unusual.
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Maybe a particular coastal surge is also unusual.
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But what we're interested in is the joint possibility of the two overlapping and interacting.
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Researchers are finding that those probabilities are changing as our climate changes.
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Events that historically took a long time to return are becoming more frequent.
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But exactly how much this is changing depends on the location and how much future warming
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and sea level rise occurs.
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It creates a problem.
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A drainage system can be designed using rainfall statistics calculated decades ago.
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A bridge may have been built around the average of how high a river was supposed to rise.
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A coastal neighborhood may have been developed using a flood map based on records from
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a hundred years ago.
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None of those calculations were necessarily wrong when they were made, but the conditions
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have changed.
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And in some places climate change is changing the probability of reaching flood levels
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at a high rate.
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So what do we do if the odds are changing?
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For engineers and city planners, it's an important question.
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Cities are filled with systems designed to move water.
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Storm drains collect rainfall from streets, culverts carry water beneath roads, canals
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move it through developed areas, pumps remove water from places where gravity can't do the
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job, and levees and seawalls try to keep rivers and oceans out.
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But they each have their limits.
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A storm drain can only hold so much water.
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A pump can move only so many gallons per minute and a seawall protects only to a certain height.
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Historically many of those limits were calculated by looking at hazards.
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How much rain has this drainage system handled in the past?
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How high should this seawall be based on historical events?
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How much water has this river ever carried?
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Compound flooding forces engineers to ask different questions.
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What happens when those systems have to work together?
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Imagine a coastal neighborhood protected by a seawall.
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The wall may successfully keep the storm surge from entering the community, but heavy rain
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is still falling behind it.
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Normally storm drains would carry that water to the coast, but the water on the other
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side of the wall is elevated.
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Now the structure protecting the neighborhood from one type of flooding is complicating
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getting rid of another.
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Engineers can respond in many ways.
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Some communities are installing larger pumps or redesigning their drainage systems.
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Others are creating areas where water can collect rather than immediately trying to get
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rid of all of it.
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Wetlands and floodplains can provide additional space for water to slow its movement through
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a watershed.
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And sometimes the answer isn't a new structure at all.
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It's deciding where future development makes sense as our flood risk changes.
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But none of these choices begin with concrete pumps or wetlands.
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It begins with better information.
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Engineers need better models that show rainfall, rivers, tides, storm surge, drainage systems,
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and sea level, and how they interact in a changing climate that is warming.
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This is not just a problem for neighborhoods sitting directly beside the ocean.
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Research has shown that compound effects can extend upstream into rivers and estuaries,
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including places where people may not think of themselves as living on the coast.
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It's probably the biggest lesson from this research.
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We have built so much of our flood infrastructure by separating water into buckets or categories.
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There's a rain fall over here and river flooding over there and coastal flooding somewhere
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else.
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The categories are useful for understanding the individual pieces, but the physics of
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water involves gravity, pressure, and the multiple pathways available to it.
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And as climate change alters the amount of water entering those pathways, and the level
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of ocean waiting at the other end, planning for the next flood increasingly means understanding
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the entire system.
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Flooding has always been part of living with water, but as our climate changes the amount
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of water coming into the system and the conditions waiting for it when it arrives are changing
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too.
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That's why research like this is important.
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Understanding that future means looking beyond individual floods and recognizing what happens
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when the water meets.
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I'm Dr. Mac and this has been the Climate Translation.
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If you have a question about the climate that you've been too afraid to ask or if you
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have a differing opinion, I want to hear from you.
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I can use your viewpoints in future episodes.
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You can reach me at theclimatetranslation at gmail.com.
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I'll see you next time.