Every year floods make their way through
populated areas, costing lives and millions of dollars in damages, devastating communities, and
grinding local economies to a halt. If you’ve ever experienced one yourself, you know how powerless
it feels to be up against mother nature. And if you haven’t, be careful in thinking it can’t
happen to you. Nearly every major city across the world is susceptible to extreme rainfall
and has areas that are vulnerable to flood risk. Luckily, we’ve developed strategies and structures
over the years to reduce our vulnerability and mitigate our risk. We still can’t change how
much it rains (at least in the short term), but we’ve found lots of ways to manage that
water once it reaches the earth to limit the danger it poses to lives and property. Hey,
I’m Grady and this is Practical Engineering. On today’s episode, we’re talking about how large
scale flood control structures work on rivers. This video is sponsored by Curiosity Stream and Nebula. Get 26% off at the link in
the description. More on that later! We all know generally what a flood is:
too much water in one place at one time. But, I think there’s still uncertainty in how
floods actually occur. Part of the reason for that confusion, I think, is the huge variety
of scales we have when talking about flooding. Most river systems are dendritic. The
topography of the land and the long-term geologic processes mean that streams join and
concentrate the further you move downstream just like the branches of a tree. A watershed
is the entire area of land where precipitation collects and drains into a common outlet;
it’s a funnel. And as you move downstream, those funnels start to combine. The further
you go, the larger the watershed becomes as more and more streams contribute to the
drainage. So watersheds can be tiny or gigantic. Your front yard is a watershed to the gutter
on the street. If it happens to be raining hard directly on your house, the gutter will flood,
maybe even overtop the road onto the sidewalk. At the complete opposite end of the spectrum,
more than a million square miles (or three million square kilometers) make up the drainage area of
the Mississippi River in the U.S. A big rainstorm in one city is not going to make a dent in the
total flow of this river. But, if everywhere in the basin is having an unseasonably wet year, that
can add up into major flooding as all that water concentrates into a single waterway. This seems
simple, but it is a real conceptual challenge in understanding flooding, not to mention trying
to control it. Smaller watersheds only flood during single intense storm events, called
flash floods. Usually, this water is already long gone by the time the next storm comes. In
contrast, large watersheds flood in response to widespread and sustained wet weather. They aren’t
really affected by single storm events. Of course, in a dendritic system, there’s everything in
between which means a flood can be a local event affecting a few houses and streets for a
couple of hours during an intense thunderstorm or a months-long ordeal impacting huge
swaths of land and multiple communities. Riverine flooding is also a
challenge because it’s not linear. This is a generalized cross section through
a river. You have the main channel where most normal flows occur. Every unit of rise in
the river doesn’t equal that much extra width in inundation. Plus there’s not much
development within the banks of a river: maybe some low bridges and a few docks. But,
above the channel banks, things change. The slopes aren’t so steep and you end up with wide,
flat areas of land. And you know what we humans like to do with wide, flat areas near waterways -
we build stuff, like entire cities. That or use it as farm land. The problem is that, once a channel
overbanks, every unit of rise in the river equals much wider extents of inundation. You can see now
why this is called the floodplain. And looking at a cross sectional view, it’s easy to see one of
the most common structural solutions to flooding: levees. If overtopping the banks of the river
creates the problem, we can just make the banks of the river higher by building earthen embankments
or concrete walls. Levees protect developed areas by confining rivers within artificial banks.
That means areas outside the levees flood less frequently. It doesn’t mean they have zero flood
risk at all, since it’s always possible to have an extreme event that overwhelms the levees. For
earthen structures, overtopping of a levee can cause erosion and even failure (or breach) of the
berm. That can make the flooding even worse than it would have been otherwise, especially if people
weren’t evacuated from the area ahead of time. So, even though they are a pretty simple solution
to the problem of flooding, levees aren’t perfect. Sometimes getting that water out of the
channel is exactly what you want though. Another tried and true flood control technique is
diversion canals. These are human-made channels used to divert flood waters to undeveloped
areas where it won’t be as damaging. Often it’s not possible to widen an existing river
because there’s already too much development or for environmental reasons. So instead, we
create a separate channel to divert floodwater around developed areas and back into the natural
waterway downstream. In most cases there will be some kind of structure at the head of the
diversion channel to help control which route the water takes. For normal conditions, water
will flow through the natural river, but when a flood comes, most of that water will be diverted,
reducing the flood risk to the developed areas. But, it would be nice if all that water didn’t
make it into the river in the first place. That’s only possible with the other major
type of flood control infrastructure: dams. These are structures meant to impound or
store large volumes of water, creating reservoirs. Dams meant for flood control are kept
partially or completely empty so that, when a major flood event occurs, all that water
can be stored and released slowly over time. The theory here isn’t too complicated. We can’t change
the volume of water that comes from a flood, but with enough storage, we can change the time period
over which it gets released into the river. Big sloshes of water into this bucket come out slowly
over time. As long as the sloshes are far enough apart and the bucket is big enough, you almost
never see significant flooding out on the other side. But, not all dams are built specifically
for flood control. Many reservoirs are intended to stay as full as possible so the water can
be used for hydropower, supplying cities, or irrigation of crops. If a water supply reservoir
happens to be empty at the time of a big flood, it will work just like a flood control reservoir,
storing the water for later use. But, if the reservoir is already full, they have to open the
floodgates to let the water through. This can be frustrating for the residents downstream who may
have thought they had protection from the dam. In many cases, a dam can serve multiple purposes
at the same time. Different zones, called pools, are established for the different uses. One
pool is kept full to be used for hydropower or water supply and one is kept empty to be used for
storage in the event of a flood. Finding the right balance point between how much storage to keep
full versus empty is a complicated challenge that considers climate, weather, the maximum amount
of flow that can be released without damaging property downstream. Some dams vary the size of
these pools over the course of a year depending on the seasonality of flooding, and some even use
risk indicators like the depth of the snowpack within the watershed to dynamically adjust the
volume available to store a potential flood. I’ve been using the term “flood
control” throughout this video, but the truth is that term is falling out of
favor. Now if you ask an engineer or hydrologist, they’re more likely to talk about “flood risk
management.” Our ability to quote-unquote “control” mother nature is tenuous at best,
and the more we try, the more we realize this: even if expensive infrastructure is helpful in a
lot of circumstances, at best it is an incomplete strategy to reduce the impacts of flooding over
the long term. For one, flood control structures (especially levees) can protect some areas
while exacerbating flooding in other places. For two, overbanking flows are actually
beneficial in a lot of ways. Just like wildfires, flooding is a natural phenomenon that has positive
effects on the floodplain like improving habitat, ecology, soils, and groundwater recharge.
And for three, we are understanding more and more the true value of resiliency - that is
instead of reducing the probability of flooding, instead reducing the consequences. This
is normally accomplished with strategic development like reserving (or converting)
the floodplain for natural wetlands, parks, trails, and other purposes that aren’t as easily
damaged by flooding. In fact flood buyouts where high risk property is purchased and converted to
green space is often the most cost effective way to reduce flood damages in the long term (even
if not the most politically popular strategy). It’s not likely we’ll ever have the ability to
reduce the volume of rainfall during major storms, and in fact, many locations are already
experiencing more extreme rainfall events than they ever have due to climate change. But,
we will continue to develop strategies, both structural and non-, to reduce the risk
to lives and property posed by flooding. If you’re here at the end of this video,
I’m guessing you spend your valuable free time learning new things about the world. You
probably also don’t enjoy watching ads like this, which is great, because Nebula doesn’t have any.
Nebula is a streaming service built by and for independent creators like MinutePhysics,
Real Engineering, Wendover Productions, and a bunch of others. It’s a way for us
to try new ideas that might not work on advertiser-supported platforms like YouTube.
And, we’re super excited to be partnering with CuriosityStream, a service with thousands
of documentaries and non-fiction titles on pretty much every subject you can imagine.
CuriosityStream loves independent creators and wants to help us grow our platform, so they’re
offering free access to Nebula when you sign up at CuriosityStream.com/ practicalengineering.
There are a lot of streaming services right now, and if you add them all up it would be hundreds
of dollars a month. That’s why this bundle is such an awesome deal. For a short time, CuriosityStream
is taking 26% off an annual plan - that’s only $15 a year to get access to thousands of
awesome documentaries on CuriosityStream AND everything on Nebula as well. Watch how
castles were built or a deep dive into the railways of India on CuriosityStream or watch
Tom Scott pit other YouTubers against each other in his Nebula-exclusive gameshow, Money. It’s
a great way to support my channel and a whole host of your other favorite educational creators.
Plus it’s just a good deal. Do us both a favor and click that link in the description. Thank you
for watching, and let me know what you think!
