How intentionally flooding the right places will be key for keeping people safe from dangerous floods

Last week I had a post on Forbes.com in which I wrote about the benefits that arise from “misbehaving rivers of muddy water.”

The reference to “misbehaving rivers” was an extended metaphor about how we expect rivers to be predictable and to stay within their boundaries as if they were physical versions of the static blue lines we draw on maps to represent them.

Part of this extended metaphor is that rivers that don’t stay between the lines—i.e., rivers that flow out onto their floodplains—can produce benefits. This is true, in that river-floodplain connectivity can drive productive fisheries and supports flood recession agriculture, a version of farming that is similar to the very first cultivation practiced by people in river valleys thousands of years ago. ??

But while there certainly can be “good floods”, I didn’t know that I’d be posting this just days before the devastating floods that hit western North Carolina and eastern Tennessee as Hurricane Helene worked its way north (flooding is also hitting Nepal, with more than 100 dead).

So, here I want to elaborate further on floods and floodplains and emphasize that, while flooding in the right places can produce benefits, flooding in the wrong places can be devastating to people and property – and that health and safety for people remains the top priority for river and watershed management.

Further, a “good flood”—put simply as flooding in the right places—can also be a key part of river-management strategies designed to keep people safe.? In fact, figuring out how to allow more good floods will be crucial for keeping people safe in a changing climate with rising flood risk. ????

Assuming that rivers are predictable and static, like those blue lines on a map, can be very dangerous. Rivers have always flooded beyond their channels and moved across the landscape. For thousands of years, people adapted to that dynamism – avoiding the harms of flooding by moving out of the way, while taking advantage of the benefits that those floods left behind, including fertile soil, rivers with productive fisheries, and floodplain forests and wetlands full of wildlife.

But as Giulio Boccaletti (a former boss of mine) noted: “Once people?decided to stand still in a world of moving water, nothing was ever the same.”

Since then, we’ve increasingly focused on managing rivers to behave in predictable ways, using dams, levees and floodwalls to force them to stay between the lines. That approach has resulted in many benefits—cities with reduced frequency of flooding and extensive areas that can be farmed more predictably and productively—but its limitations have also become apparent.

Shifting land use, aging infrastructure, and now a changing climate have converged to drive rising flood damages in places like the United States (see image below, with average annual flood damages more than doubling in constant dollars between the beginning of the 20th century to the beginning of the 21st century).

These rising damages occurred during a time of massive investment in dams and levees.

In several river basins in the United States, including the Mississippi and the Sacramento, along with rivers in Europe (the Rhine and the Elbe), flood managers have realized that structures such as dams and levees alone cannot safely manage floods. In these places, flood managers have reconnected extensive areas of floodplains, giving “room for rivers” and providing relief valves – places where rivers can flood out onto their floodplains to reduce the risk of flooding for cities, towns and farmland.

In the Sacramento River basin, the reconnected floodplain of the Yolo Bypass is intentionally inundated during large floods, conveying more than 80% of the volume of floodwaters and leaving less than 20% to be managed by the leveed Sacramento River as it flows through its namesake state capital (see image below).

We can characterize what happens in the Yolo Bypass as a “good flood” for two reasons.? First, flooding in the Bypass is the single most important strategy for keeping the people of Sacramento—along with other communities and surrounding farmland—safe from flooding. To put it simply, good floods are key to reducing the risk of bad floods.

And second, the flooding in the Bypass creates a range of benefits for fish, wildlife, recreation and groundwater recharge.? These are the benefits of a muddy river that doesn’t stay between the lines that I referenced in the Forbes blog post.

While river systems such as the Sacramento and Rhine have seen the reconnection of floodplains, in much of the world, many rivers are still connected to extensive areas of floodplain, including in tropical areas of South America, Africa and Asia. These are also areas that are projected to see major increases in flood risk (see below; the areas in dark blue are those where a size of flood that today is relatively rare (a “1 in 100 event”) will happen relatively frequently (e.g., up to several times per decade)).

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As these areas develop plans to deal with rising flood risk, they can learn from the Sacramento and Rhine and work to integrate floodplains and “good floods” as much as possible to reduce the harm from bad floods.

This is not to say that these regions should rely only on floodplains. For many places, hybrid solutions will be necessary, using levees and floodwalls to reduce risk for populated areas, for example. But there is potentially much value in maximizing the resiliency and diverse benefits that come from strategically managed “good floods.”

Another caveat: the use of floodplains, and other nature-based solutions, may have limited utility for the types of floods we just saw in western North Carolina – flash floods and debris flows that rise quickly in rain-soaked mountainous regions (but then again, structures often have limited utility for managing flash floods as the region saw its dams and levees overwhelmed by the floodwaters). In places at risk from flash floods, zoning to reduce the number of people exposed to risk, alongside early warning systems, floodproofing of buildings and evacuation systems will all be critical.

Perhaps the most important thing we can do to reduce the number of devastating events like what we just witnessed in North Carolina (and Nepal, and Nigeria, and…) is to ensure that the world meets its targets to minimize further disruption from climate change.

Geologic and historic evidence shows that hurricanes have always been capable of generating disastrous flooding in mountainous areas, even those hundreds of miles from the coast (including in Asheville, North Carolina in 1916).

But the physics that underpins increased flood risk with climate change is simple: ?warmer air can hold more water, adding more “fuel” for flooding. While we haven’t yet seen analyses about the extent to which climate change exacerbated flooding from Helene (e.g., the type of analysis done by World Weather Attribution), the extreme rainfall associated with Helene is reminiscent of other recent storms, such as Hurricane Harvey which devasted Houston in 2017.? The World Weather Attribution group found that human-driven climate change made Harvey’s record rainfall approximately three times more likely and 15 percent more intense. ?The same research team found that a flood in Libya that caused two dam failures, killing at least 10,000 people, was “50 times more likely and up to 50% more intense” due to the current level of warming.

Floods continue to be the most damaging type of disaster worldwide, and regional projections have high confidence that climate change will exacerbate these risks.? Using floodplains and relying on “good floods” will be crucial to reducing future damages and dangers to people.