The 100 Year Flood Is Not What You Think It Is (Maybe)

Hey, I'm Grady today in practical engineering we're talking about hydrology We all know the water cycle We've seen this classic image with the green earth and the blue water and the arrows and the smiley face on the Sun So you probably already know that the majority of the rain that falls from the sky ends up washing along the ground following gravity until it eventually finds a streamer channel, the more it rains the higher the creeks and rivers rise and if they rise Too much. You've got a flood now almost everyone agrees flooding is bad most years It's the number one natural disaster in the US by dollars of damage and flooding is particularly capricious So you can see why it's so crucial that we be able to characterize it to know it and understand it so we can try To get better at how we manage it in the future in the 1960s We started to recognize the fact that development was exacerbating flood risk by putting more and more expensive homes and businesses near rivers and creeks Everyone wants a waterfront view, right? so the federal government came up with a national flood insurance program to create a larger pool for flood risk and Incentivize management of the floodplain but what is the floodplain? Well, you can't let people go uninsured because the risks are too high But you also can't force every single person who owns real estate to pay flood insurance premiums Ultimately, you have to draw a line in the sand or on a map in this case that says as a Society how much flooding risk are we willing to tolerate and the answer ended up being the hundred-year flood an often misunderstood combination of statistical hydrology and societal risk tolerance two of my favorite topics by the way, but it's actually a pretty simple concept the hundred year flood is a particular magnitude of flow in a creek , and a hundred years is its return period The inverse of return period is annual exceedence probability So a hundred year flood has an annual exceedence probability of 1% let's say our hundreds of your storm is a flow of 10,000 cubic feet per second. That means in a given year. There's a 1% Probability that a flow of 10,000 CFS or greater will occur Using our return period is misleading because it makes storm sound cyclical It hints that the possibility that we could predict the future That somehow if a storm of a particular magnitude was to occur that we might have a period of security before it happened again or even that if a major flood had not occurred in quite some time that we might be more due for it to occur than if it Had happened recently You don't call Snake Eyes the 36 roll dice throw and you don't call a full house the 700 deal poker ham So if 100-year flood occurred last year, there's still a 1% chance that it could happen this year In fact many hydrologists has stopped using return period altogether in favor of just calling flood magnitudes by their annual exceedence probability Just to avoid this confusing notion that storms are on some kind of schedule So to answer our question from before the flood plain is the area of land that can reasonably be expected To have a 1% chance of flooding for a given year This has been mapped for the entire US and I'll put a link in the description where you can go on FEMA's website and look At the flood plain Maps for any area, but how are those maps developed? And how do we know where the flood plain is? Well, this is where the engineering comes in. Let's try a simple example first Take the height of all the men or women in your group of friends or office We know that height is normally distributed the variation fits this particular shape of distribution that is ubiquitous in the natural world To fit your data to a normal distribution The only thing you need to know is the average height and the standard deviation and these are both simple formulas The people whose heights you measured are sampled and the distribution helps you use the sample to characterize an entire population Once you fit your data You can make a reasonable guess at for example the percentage of the population that's over 7 feet tall Even if you don't have anyone in your sample that tall this works the same way with hydrology The US Geological Survey maintains thousands of stream gages across the u.s This is the gage on Williamson Creek in South Austin near my house he uses air pressure to measure the depth of a stream and then converts that to a flow You can log on to the USGS website and take a look at the data from any stream gage in the US once you have this data It's pretty simple to figure out the hundred-year flood even if you don't have a hundred years of data in my case I only have 25 You can take the peak flow from each year and fit that data to a probability distribution Just like we did with the height this way Even if we don't have a very long period of record we can make a reasonable guess at the magnitude for the lower probability floods But what if you don't have a stream gage in your watershed stream gages aren't that common and they're expensive to maintain? But rainfall gages are everywhere. You may even have one at your house that you check obsessively and brag to your colleagues about Well now we're really getting into the engineering part of it because somehow we have to use Rainfall data to come up with flows in a stream Now we usually only care about peak annual flow or what's the highest point the stream reached in a given year? There's a really simple way called the rational method of converting a rainfall intensity into a peak flow it's based on the idea that volumetric inflow to the watershed that's rainfall intensity times area is Equal to the outflow minus any water that soaks into the ground or evaporates or gets trapped in a puddle, etc And this is where you first start getting a sense that maybe engineering hydrology isn't the most exact of Sciences Because not only is the single C value supposed to encompass all of the factors that can affect the peak flow coming from the watershed for example vegetation slope soil type amount of impervious cover Etc. There's also this thing about the units that I love None of the imperial units for these values agree. But if you do the math the conversion factor is just about one So we ignore it if you just leave it off and we say close enough and what's hilarious to me is that's the perfect Acknowledgement of how imprecise engineering hydrology really is. This is not something you take out to the fifth decimal place if if You have a more complicated area with several watersheds, or maybe you just need to understand how flow varies over time You have to continue taking steps away from reality and tell you with an entire hydrologic model and this can include Developing a design storm, calculating how much runoff results from the rain? Routing the runoff over the ground and through any streams and reservoirs and we do this for each time step sometimes minute by minute using computer software hydrology is something that many engineers devote their entire careers to. water is Incredibly important to us and it shapes almost every facet of our lives and there's no rapid prototyping in civil engineering You get one chance to get the answer Right because infrastructure is really expensive and since rainfall and streamflow have such tremendous variability It's a really important job for the engineer to be able to characterize that So that we can make smart decisions about how he developed the world around us. I Hope you liked the video and if you did I would really appreciate you clicking the like button and subscribing to the channel that helps motivate me to keep making cool stuff if you want more details or you have a Question or a suggestion for a future topic. I'd love to hear from you in the comments below Thank you for watching and let me know what you think

Channel: Practical Engineering

Views: 2,205,736

Keywords: 100 year flood, water resources engineering, hydrology, flooding, hec-hms, austin flood, creek flooding, engineering hydrology, civil engineering

Id: EACkiMRT0pc

Length: 7min 45sec (465 seconds)

Published: Sun Mar 06 2016

Reddit Comments

[u/rnichaeljackson]

Very well done.

Working on a model right now actually!

I also TA'ed for a hydrology class and love how you talked about the uncertainty in hydrology. Was always slightly entertaining to see students go out to extreme decimals.

[u/Theman554]

I remember my water resource engineering professor telling us that to do calculations like these you take incorrect inputs plug them into incorrect equations, and you get an incorrect answer. But you hope that the incorrect answer is reasonably close enough to make an informed decision.

[u/Deranged40]

Glad to know that I'm not the only one who thoroughly enjoyed statistics in school, and still like to use it.

[u/uwnav]

Super interesting! I love videos like this. I know someone somewhere is thinking and working on this stuff, but it's great to actually get some information on it.

[u/BananApocalypse]

Those imperial units are hilarious. In metric we use Q=ciA/360 and when I saw that imperial had no correction factor it blew my mind.

[u/gmz_88]

I enjoyed the video and it looks like you have some other interesting videos on your channel. Subscribed.

[u/jesseaknight]

How is flood risk determined in coastal areas. The Tampa area, for example has very little elevation change (water rising 1 foot may expand a stream from a dozen feet to hundreds of feet, making flow models difficult. Tampa gets buckets of rain sometimes, and small floods are frequent, but the real danger comes from storm surge (wind and/or low pressure driven). Flood zones are a big deal hear, but do they calculate based on stream flow?

[u/thwllms]

Excellent work! As a water resources PE I've struggled to explain the often hand-wavey science of hydrology, even to other engineers. Now I'll just show them this video!

[u/havrancek]

so interesting! just, it´s a shame, we don´t have such huge net of measuring stations here in slovakia, it would be fun to predict flood models in this region, although we have a state institute for this purpose, but all i know is they can issue a warning to people