This is the Old River Control Structure, a relatively innocuous complex of floodgates
and levees off the Mississippi River in central Louisiana. It was built in the 1950s to solve a
serious problem. Typically rivers only converge; tributaries combine and coalesce as they move
downstream. But the Mississippi River is not a typical river. It actually has one place where it
diverges into a second channel, a distributary, named the Atchafalaya. And in the early 1950s,
more and more water from the Mississippi River was flowing not downstream to New Orleans in
the main channel, but instead cutting over and into this alternate channel.
The Army Corps of Engineers knew that if they didn’t act fast, a huge portion of America’s most
significant river might change its path entirely. So they built the Old River Control Structure,
which is basically a dam between the Mississippi and Atchafalaya Rivers with gates that control
how much water flows into each channel on the way to the Gulf of Mexico. It was certainly
an impressive feat, and now millions of people and billions of economic dollars
rely on the stability created by the project, the now static nature of the Mississippi River
that once meandered widely across the landscape. That’s why Dr. Jeff Masters
called it America’s Achilles’ Heel in his excellent 3-part blog on the structure.
You see, the Atchafalaya River is both a shorter distance to the gulf and steeper too. That means,
if the structure were to fail (and it nearly did during a flood in 1973),
a major portion of the mighty Mississippi would be completely diverted, grinding freight traffic
to a halt, robbing New Orleans and other populated areas of their water supply,
and likely creating an economic crisis that would make the Suez Canal obstruction seem
like a drop in the bucket. Mark Twain famously said that "ten thousand river commissions,
with all the mines of the world at their back, cannot tame that lawless stream,
cannot curb it or confine it, cannot say to it, Go here, or Go there, and make it obey;"
And engineers have spent the better part of the last 140 years trying to prove him wrong.
In my previous video on rivers, we talked about the natural processes that cause them
to shift and meander over time. Now I want to show you some examples of where humans try to
control mother nature’s rivers and why those attempts often fail or at least cause some
unanticipated consequences. We’ve teamed up with Emriver, maker of these awesome stream tables,
to show you how this works in real life. And we’re here on location at
their headquarters. I’m Grady, and this is Practical Engineering. On today’s episode,
we’re talking about the intersection between engineering and rivers.
One of the most disruptive things that humans do to rivers is build dams across them,
creating reservoirs that can be kept empty in anticipation of a flood or be used to store water
for irrigation and municipal supplies. But rivers don’t just move water. They move sediment as well,
and just like an impoundment across a river stores water, it also becomes a reservoir for the silt,
sand, and gravel that a river carries along. That’s pretty easy to see in this flume model of
a dam. Fast flowing water can carry more sediment suspended in it than slow water. The flow of water
rapidly slows as it enters the pool, allowing sediment to fall out of suspension. Over time,
the sediment in the reservoir builds and builds. This causes some major issues. First,
the reservoir loses capacity over time as it fills up with silt and sand,
making it less useful. Next, water leaving on the other side of the dam,
whether through a spillway or outlet works, is mostly sediment-free, giving it more capability
to cause erosion to the channel downstream. But there’s a third impact, maybe more important
than the other two, that happens well away from the reservoir itself. Can you guess what it is?
In the previous video of this series, we talked about the framework that engineers
and the scientists who study rivers (called fluvial geomorphologists) use to understand
the relationship between the flow of water and sediment in rivers. This diagram, called Lane’s
Balance, simplifies the behavior of rivers into four parameters: sediment volume, sediment size,
channel flow, and channel slope. You can see when we reduced the volume of sediment in a stream,
like we would by building a dam, Lane’s Balance tips out of equilibrium into an erosive condition.
In fact, according to Lane’s Balance, any time we change any of these four factors,
it has a consequence on the rest of the river as the other three factors adjust to bring the
stream back into equilibrium through erosion or deposition of sediments. And we humans make a
lot of changes to rivers. We want them to stay in one place to allow for transportation and
avoid encroaching on property; we want them to drain efficiently so that we don’t get floods;
we want them to be straight so that the land on either side has a clean border; we want to cross
over them with embankments, utilities, electrical lines, and bridges; we want to use them for power
and for water supply. Oh and rivers and streams also serve as critical habitat for wildlife that
we both depend on and want to preserve. All those goals are important and worthwhile,
but, as we’ll see (with the help of this awesome demonstration that can simulate river responses),
they often come at a cost. And sometimes that cost is borne by someone or someplace
much further upstream or downstream than from where the changes actually take place.
One of the classic examples of this is channel straightening. In cities, we often disentangle
streams to get water out faster, reduce the impacts of floods, and force the curvy lines
of natural rivers to be neater so that we can make better use of valuable space. I can show
it in the stream table by cutting a straight line that bypasses the river’s natural meanders.
The impact of straightening a river is a reduction in a channel’s length,
necessarily creating an increase in its slope. Water flows faster in a steeper channel,
making it more erosive, so the practical result of straightening a channel is that
it scours and cuts down over time. It’s easy to see the results in the model.
This is compounded by the fact that cities have lots of impermeable surfaces that send greater
volumes of runoff into streams and rivers. That’s why you often see channels covered in concrete in
urban areas - to protect against the erosion brought on by faster flows. And this works in
the short term. But, making channels straight, steep, and concrete-covered ruins the stream
or river as a habitat for fish, amphibians, birds, mammals and plants. It also has the
potential to exacerbate flooding downstream, because instead of floodwaters being stored
and released slowly from the floodplain, it all comes rushing as a torrent at once instead. And
it’s not just cities. Channels are straightened in rural areas too to reduce flooding impacts to
crops and make fields more contiguous and easy to farm. But over the long term, channelizing
streams reduces the influx of nutrients to the soils in the floodplain by reducing the
frequency of a stream coming out of its banks, slowly making the farmland less productive.
Stream restoration is big business right now as we have begun to recognize these long-term impacts
that straightening and deepening natural channels has and reap the consequences of the mistakes of
yesteryear. In the US alone, communities and governments spend billions of dollars
per year undoing the damage that channelization projects have caused. Even the most famous of the
concrete channels, the Los Angeles River, is in the process of being restored to something more
like its original state. The LA River Ecosystem Restoration project plans to improve 11 miles (18
km) of the well-known concrete behemoth featured in popular films like Grease and Dark Knight
Rises. The project will involve removing concrete structures to establish a soft-bottom channel,
daylighting streams that currently run in underground culverts, terracing banks with
native plants, and restoring the floodplain areas, giving the river space to overbank during floods.
Thanks to fluvial geomorphologists, projects like this are happening all around the world. But,
straightening channels isn’t the only way humans impact rivers and streams.
Another impactful place is at road crossings. Bridges are often supported on intermediate
piers or columns that extend up from a foundation in the river bed. Water flows faster around the
obstruction created by these piers, making them susceptible to erosion and scour.
Engineers have to estimate the magnitude of this scour to make sure the piers can handle
it. You don’t have to scour the internet very hard to find examples where bridges
met their demise because of the erosion that they brought on themselves. In fact,
the majority of bridges that fail in the United States don’t collapse
from structural problems or deterioration; they fail from scour and erosion of the river below.
But, it’s not just piers that create erosion. Both bridges and embankments equipped with culverts
often create a constriction in the channel as well. Bridge abutments encroach on the channel,
reducing the area through which water can flow, especially during a flood,
causing it to contract on the upstream side and expand on the downstream side. Changes
in the velocity of water flow lead to changes in how much sediment it can carry. Often you’ll see
impacts on both sides of an improperly designed bridge or culvert; Sediment accumulates on the
upstream side, just like for a dam, and the area downstream is eroded and scoured.
Modern roadway designs consider the impacts that bridges and culverts might have on a
stream to avoid disrupting the equilibrium of the sediment balance and reduce the negative effects
on habitat too. Usually that means bridges with wider spans so that the abutments don’t
intrude into the channel and culverts that are larger and set further down into the stream bed.
Just like bridges or culvert road crossings, dams slow down the flow of water upstream, allowing
sediment to fall out of suspension as we saw in the flume earlier in the video. The consequences
include sediment accumulation in the reservoir and potential erosion in the downstream channel,
but there’s one more consequence. All that silt, sand, and gravel that a dam robs from the river
has a natural destination: the delta. When a river terminates in an ocean, sea, estuary, or lake,
it normally deposits all that sediment. Let’s watch that process happen in the river table.
River deltas are incredibly important landscape features because they enable
agricultural production, provide habitat for essential species, and they feed the
sand engines to create beaches that act as a defensive buffer for coastal areas. Wind and
waves create nearly constant erosion along the coastlines, and if that erosion is not
balanced with a steady supply of sediment, beaches scour away, landscapes are claimed by the sea,
habitat is degraded, and coastal areas have less protection against storms.
And hopefully you’re seeing now why it’s so difficult,
and some might even say impossible, to control rivers. Because any change you
make upsets the dynamic equilibrium between water and sediment. And even if you armor
the areas subject to erosion and continually dredge out the areas subject to deposition,
there’s always a bigger flood around the corner ready to unravel it all over again.
So many human activities disrupt the natural equilibrium of streams and rivers, causing them
to either erode or aggrade, or both, and often the impacts extend far upstream or downstream. It’s
not just dams, bridges, and channel realignment projects either. We build levees and revetments,
dredge channels deeper, mine gravel from banks, clear cut watersheds, and more. Historically we
haven’t fully grasped the impacts those activities will have on the river in 10, 50, or 100 years.
In fact, the first iteration of the stream tables we’ve been filming were built by Emriver’s late
founder, Steve Gough (goff) in the 1980s. At the time, he was working with the state of Missouri
trying to teach miners, loggers, and farmers about the impacts they could have on rivers
by removing sediment or straightening channels. These people who had observed
the behavior of rivers their entire lives were understandably reluctant to accept new ideas. But,
seeing a model that could convey the complicated processes and responses of rivers was often enough
to convince those landowners to be better stewards of the environment. Huge thanks to Steve’s wife,
Katherine, and the whole team here at Emriver who continue his incredible legacy of using
physical models to shrink down the enormous scale of river systems and the lengthy time
scales over which they respond to changes down to something anyone can understand to
help people around the world learn more about the confluence of engineering and natural systems.
Historically, engineering and fluvial geomorphology have been entirely separate fields
of study, which means if you were an engineer and wanted to learn more about the impacts engineering
projects have on natural streams, you had to do some extracurricular learning. If you’ve done it,
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