From the 1940s to the 1960s, the US Army Corps of Engineers built a scale model of the entire Mississippi river basin; a huge part of the continental united states scaled down to the size of a large park. Their goal was to understand how flooding affected communities along the river, and how damage could be reduced through the use of manmade structures like levees. Now surrounded by Buddy Butts park outside Jackson Mississippi, you can still go visit the tiny engineers who live and work there. Not really. Actually the model was only used for a few years before it was decommissioned. It simply wasn’t feasible to continue operating such a massive facility, especially because in the 1970’s, another way to simulate river hydraulics started to become available. I’m Grady, and this is Practical Engineering. In today’s video, we’re talking about computational hydraulic river modeling.
Computational because it’s on a computer as opposed to a physical model like the one in Jackson. Hydraulic because we’re trying to understand how water moves through the system. And river model because we’re creating a representation of the real world. There are a bunch of software packages available that can perform hydraulic computations along rivers, but one specific model is ubiquitous in the water resources engineering industry, at least in the US: the River Analysis System, or RAS for the cool kids. The reason for its popularity is simple: it’s free. RAS is developed by the US Army Corps of Engineers, and luckily our federal government sees fit to release it without charge. RAS has grown from a rudimentary, command-line based program to a sophisticated software package capable of performing extremely complicated simulations. You can go on HEC’s website and download RAS for yourself, but why would you? No seriously, why do do we care about river hydraulics, and what are the questions that engineers try to answer with a model of a river system? Let’s use RAS to find out. And by the way, most of the demonstrations in this video are from example data that comes with the software.The places are real but the situations are not. My apologies in advance if you live in Munsie.
The number one use of hydraulic river models is to estimate flooding. For riverine flooding, engineers can follow a drop of water from inside a storm cloud to inside someone’s house. First you have to know how much rain falls and then how much of that rain makes its way into the creeks and rivers. That’s hydrologic analysis. Once you know how much water enters the river, you need a hydraulic model to find out how much the river rises as a result. This process is called floodplain mapping, and it helps us characterize which areas are at the highest risk of flooding so that we can be prepared if and when it happens. Another way we can use models is to evaluate projects. Putting a levy on a river may help flooding in one location but make it worse downstream. Models help to represent a real world situation without the cost and complexity of actually performing the test at full scale. Obviously we can’t create a flood just to see what happens, so models allow us to test out hydraulic structures like dams and levees to make sure they work as anticipated before spending millions of dollars on construction.
Another use for river models is simulating systems of reservoirs. When you have a number of dams along a river, all with different operating guidelines and all trying to accomplish different objectives like water supply, navigation, and flood control, things can get complicated in a hurry. A hydraulic river model can perform simulations of these complex scenarios to help with planning and optimization of operations. For example this model shows how two navigation dams on the Mississippi river manage the locks and gates during a flood situation. A model can also be predictive, helping operators anticipate conditions before they even happen.
Finally, we use hydraulic models to perform breach analyses for dams. If a dam fails, it can release a tremendous amount of water in a short period of time. In fact some of the worst engineering disasters in history have been dam failures. In a situation like this, emergency managers don’t have much time to react, so many dams are required to have emergency action plans. Part of these plans is mapping out the areas that might be affected in the event of a breach so that emergency officials can coordinate evacuations as quickly as possible. A dam breach is an extremely dynamic phenomenon, so engineers rely on sophisticated hydraulic models to predict the inundation zones. This model demonstrates what might happen in the event of a levee failure. Sorry Munsonians.
Speaking of catastrophic flood waves, another purpose for hydraulic models is fluvial geomorphology, or the study of how rivers form and evolve. Geomorphologists study streams in some of the most unlikely places, including Earth’s red neighbor, Mars. The Kasei Valles is one of the largest outflow channel systems on Mars, almost 20 times the width of the Grand Canyon, and several kilometers deep. Scientist believe that billions of years ago, liquid water may have flowed on the surface and in underground aquifers on Mars. As the planet cooled, the water on the surface froze, creating an impermeable cap to a deeper layer of pressurized liquid groundwater, just like the top to a bottle of soda. Scientists think that tectonic movement or volcanic activity may have melted the subsurface ice, "popping the top" on the cryosphere and releasing a massive flood wave of pressurized groundwater. These massive floods are what likely created many of Mars’s outflow channels like the Kasei Valles.
But what’s an earthbound geomorphologists to do when trying to learn about a river that’s 50 million kilometers away and hasn’t seen water since maybe several billion years ago? Make a hydraulic model, of course. Since I’m not a planetary geologist, and because RAS doesn’t have a way to change the gravitational acceleration, this model has about as much scientific rigor as a peanut butter and jelly sandwich. But let’s ignore that for a moment and just enjoy the technical beauty of being able to recreate the massive flood that may have created the Kasei Valles.
There are lot of other uses for hydraulic river models that can be simulated using RAS including bridge scour, pump stations, sediment transport, and even water quality analysis. If you want to learn more, you can download RAS for yourself. It’s probably a bit intimidating for a non-engineer, but the Army Corps has a lot of good example data you can play with, and they’ve done a great job with the user manual explaining all the features. And next time you see a river, you can always know that some civil engineer probably has a model of it on his computer, carefully constructed and ruthlessly flooded over and over again for your safety and maybe a few times just for fun. Thank you for watching, and let me know what you think!