Why Engineers Can't Control Rivers

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,   you know, teaching yourself math and science is a  hard thing to do, a problem that today’s sponsor,   Brilliant.org, takes on head first. There are a  lot of things you learn in school that you don’t   end up needing in life, but if you’re like me,  there are just as many things you didn’t learn   in school that you need to know to get ahead  in your career, reach your personal goals,   or just out of pure curiosity and the  delight of learning something new. Brilliant is a platform where you learn by  doing with thousand of lessons from math to AI,   data science, and engineering. I’ve gone  through a lot of these courses to freshen   up on topics that are new to me, but my absolute  favorite was the one on the physics of rockets   and tether launchers by my friend Brian at the  Real Engineering channel. It’s fun to hear about   the engineering challenges of structures  like this, but it’s a different experience   entirely to work through them yourself.  It just gives you a new appreciation for   the math and science, and a feeling that you  really understand the problems in a deep way. You should go try it yourself and see if you  agree at Brilliant.org/PracticalEngineering.   It’s totally free to try for an entire  month, and you can get through a lot   of courses in that time. But if you do  find yourself coming back like I did,   the first 500 people that click the link below  will get 20% off an annual premium subscription.   They even have an app so you can try this  on your phone. I really believe we should   never stop trying to learn new things,  and Brilliant makes that so easy to do,   and it supports the channel too. Thank you  for watching, and let me know what you think!

Channel: Practical Engineering

Views: 2,551,645

Keywords:

Id: vLZElIYHmAI

Length: 15min 52sec (952 seconds)

Published: Tue Apr 04 2023

Reddit Comments

[u/spredditer]

The unintended consequences of human activity on complex systems is something I think we can all appreciate.

[u/Khalku]

I love this channel so much. Always super interesting in a way that even stupid me can understand.

[u/skilledwarman]

This is such a great channel. Really in depth but digestible breakdowns of the content he explains. The Arceibo video and all the videos on different types of dams are especially good

[u/Thatsaclevername]

Over in East Helena there's a big hot-poured slag pile. Giant black piece of shit full of heavy metals from the old lead smelter days. This thing had hot slag poured on it as a way to dispose of said slag (simpler times) so for all intents and purposes it is one contiguous shit-metal rock.

Prickly Pear creek, which is at most 20 feet wide and 5 feet deep in it's deepest riffles, chewed through and undercut that slag pile so hard it required FURTHER cleanup efforts on the Superfund cleanup of the lead smelter.

"If you move a river, the river will come back. Do not move the river. It will fuck up everything you've built. You have to accept that if you're going to do land and stream restoration. The river finds a way" -my land and stream teacher in college lmao.

[u/Beans186]

Did he seriously just not mention Terminator 2?

[u/KamahlYrgybly]

Fascinating video. I learned stuff today.

[u/ThisIsNerveWracking]

Except the Chicago River.

[u/boondo]

laughs in chicago river

[u/Tinywampa]

That sediment table is an incredibly useful visual tool of a river flowing, especially when it showed a delta forming from the deposits.