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
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