Do Droughts Make Floods Worse?
The answer isn’t as simple as you might think!
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One statistician famously said, “All models are wrong, but some are useful.” And even something as simple as the flow of water into the soil has so many complexities to keep track of. Like most answers to simple questions in engineering and in life: the answer is that it’s complicated.
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Just look up the effect that Ethopia’s poor agricultural practices had on erosion. The lack of proper land management caused soil compaction which then led to water just flowing down into tributaries and rivers: leading to dangerous floods that severely eroded the landscape.
I really enjoy your videos even without any background in engineering or desire to get into engineering there are lots of tidbits that can be applied to personal projects and things to take into consideration before designing or building diy projects that can really help take your project to the next level I’m slowly becoming a fan of over engineering my projects 😂
Could you make a video about the difficulties that permafrost bring with engineering?
Okay so I’m only a minute into the video but I noticed something really obvious that IDK if you address later but I just have to comment on it – the moister the grass is, the plumper it is, and the more difficult it is to form a seal with the ground. Can we confirm this is measuring the absorption of the water, and not just fluid leaking out when placed on an uneven surface? I would think it necessary to use bare soil for this experiment.
Though it’s true that bare soil is not a common natural condition.
Otherwise, perhaps this experiment could be repeated with a heavier vessel that will satisfactorily flatten the grass.
I’d like to see these experiments again, this time on an incline plane. A low infiltration factor should make more run off, when taken together with the rest of a drainage basin -> flood. Instead of direct from the hose try slow drops/spray from a sprinkler.
We found out, during our midwest drought this summer (which is still continuing), that 3″ of rain in 44 minutes will “outrun” the soil’s ability to absorb it, regardless of how thirsty it is! It did top up the swimming pool … and flooded the basement.
Decades ago while growing up on a dairy farm in the northeast US we had a ton of runoff and attendant erosion because after almost a month of nearly no rainfall (what fell was unmeasurably small amounts and very infrequently, not even reaching soil with any cover and rapidly evaporating) we had a rain that started as an absolute downpour. The period where the soil was hydrophobic was brief but it did mean runoff started in a few seconds and that had an outsized effect in that very hilly region.
3″!!! Gahhh!
That’s more than we’ve gotten in about 7 weeks!
The back yard is finally dead for the first time in 15-20 years.
I heard a lot about drought last year and it was fairly bad in my specific location last year …. but this year is way worse (I almost went hyperbolic and said “infinitely worse” … had to catch myself) and yet I’ve heard almost nothing.
2 for 1 swimming pools?
Practical Engineering inspires me.. My parents said if i get 50K followers They’d buy me a professional camera for recording..begging u guys , literally
Begging…
That tells others that they should dig a second swimming pool – outside.
I think one important thing to note in the viral cup demonstration is the fact that no bubbles were rising into the cup on the dry soil. The cups on the wet soil were letting in air from around the base which bubbled up into the cup and equalized the pressure above the water, whereas the cup on dry soil seemed to be forming a partial vacuum above the waterline inside the cup. This partial vacuum may have inhibited the water from seeping into the dry soil, slowing its absorption into the ground below.
Yes, I noticed the same thing, it may have even been the sole reason for the drastic outcome. That first cup had so much room for water to spread laterally and for the air to rush in and replace the water, it would be nowhere near representative of anything going on with soil infiltration. The other two are probably more reasonable, but still flawed by the pressure variables of the partial vacuum.
Agreed. The difference can be attributed to the different grass states. The live grass allowed more air to get up into the cup. This would have been better if either it was an open tube, as Grady showed or at least to cut the grass so the cup was sitting on the soil.
So all tests would need a better seal at the base and open top then it should function as intended?
@Heide Knight yes. Grady did it more or less properly, by driving it in and removing “leaking out the side” as an escape.
I was going to say the same thing.
A factor missing from the demos that may account for some of the differences is soil compaction. Soils in long term drought conditions get packed more tightly which I would imagine slows infiltration. It is intuitive to think about when you consider how much softer the same patch of ground gets when it is wet, there is more open space for the particles to move around in. Recently dried soils will retain some of those gaps until they settle down packing more tightly leaving less room for sudden rainfall.
I was expecting compaction to be the primary influence for reducing infiltration. This is the first I’ve heard of soil hydrophobicity and it should be interesting to look into.
I was waiting for the hydrophobic piece, but you are totally right about compaction!
Exactly, compaction and the soil types that are more prone to this definitely seems like it should’ve been mentioned. You can even see this effect in some of the stock footage in the video.
Compaction from the last rain drying out hard- some soil types tend to turn into a hard crust when they dry out, can’t imagine much water can penetrate soil that’s hard as plywood, quickly
pretty much any area that gets flash floods ‘regularly’
@MrMonotone Compaction is also a problem with wet soils too though it manifests differently there largely forcing water out of the ground however so I can sort of see why it was left out
I think it’s also important to highlight the difference between an areal flood and a flash flood – hydrophobic ground conditions in drought-impacted areas can lead to more flash floods as water slides across the ground surface instead of absorbing, while moist ground conditions can contribute to more areal floods.
Source?
@William Campbell try pouring water on a waterproof jacket, then one thats not waterproof but already soaks. you will see the principle in practice.
You are kind of right. When you are in a deep arid canyon that drains an area of severe local rainfall (perhaps miles away). The flood that washes you away is a flash flood.
@William CampbellJesus that’s obnoxious.
@William Campbellthe… Experiments on this video?
I think the biggest effect in the original demo is the fact that the edges of the cups are resting on the grass blades, leaving lots of spaces for water to leak out between the cup and the ground. Well-watered grass would be a lot thicker and springier than flat, dry, dead grass. It was the first thought I had when I first saw the demo and immediately dismissed it as useless.
That was a fun video. Watching the three cup infiltration demo at the beginning I wondered if some of the water in the glasses was not even making it into the soil because the cups were slightly suspended by the grass. I appreciate your garage demos to help visualize the effects.
I agree about the tremendous impact of soil types. I grew up in the Chicago area with a lot of clay soil. During droughts our yard turned into “concrete.” Now we live in southern NH with very sandy soils that water percolates through quickly. We have a small stream on our property and it is fun to watch the flow change during rain storms.
Cut a hole in the cup.
The flat dirt seals the edges, creating a vacuum.
That’s not how natural rain works so it’s a terrible test
That was my assumption as well. The large bubbles coming up from the grass in the middle shot kind of indicate that the seal from cup to ground wasn’t the same in all three runs of the experiment. Clearing a level patch of soil in each location and then setting the cups would be a better controlled test.
The air bubbles making it in clearly shows there is a lot of water seeping out under the edges of the middle and left cup, more on the far left than middle one.
On the right side there are 0 air bubbles, showing that that cup is well sealed against the edges.
That test does not at all showcase how the ground soaks in water, only how fast water runs out of a cup when you leave a large gap, small gap and no gap between the cup and the ground.
Because the right cup is so well sealed it is also holding a vacuum making the water in it have less pressure towards the ground, it will take far longer to empty than if it was an equally well sealed tube with a hole at the top.
The demonstration gets the point across but it isn’t close to actually doing what it is pretending to show.
There is also as this video showed no reason the middle cup should empty less fast than the left one, the middle cup would empty faster as there is less water in the ground but no seal due to existing moisture while the cup on the left is resting on soaked ground that should have little room for new water.
@Gianni Schicchihe says this in the video but just more politely
You folks beat me to it. The hydrophobic soil part does explain something I would see in monsoon season in Arizona (US).
I really like that ‘all models are wrong, but some are useful’
Any model will neglect some small effects, but they manage to show the larger picture.
and this rule works everywhere
In electronics there is rule that u shouldn’t use 100% of capacity of devices never, always use 20% stronger/faster solution then u think u should
I think the thing that helps reduce floods over dry ground is the cracks in the dry dirt and the fact that when fully damp dirt can’t absorb any water and so all the water goes to rivers and streams which makes them really dangerous
I like this too. Even something extremely precise, like working your way through thousands of Feynman Diagrams and averaging each contribution, is going to ignore an infinite number more of (decreasingly impactful) Feynman Diagrams.
So, in the end it’s “wrong” in that you can’t calculate the infinite number of contributions, but it is useful!
@Jovial Cupid9 I like the rule on networking of never using more than 60% of the actual total capacity, or bandwidth of a given link.
packed clay heavy soil becomes somewhat hydrophobic while dry. In areas where the rain only lasts for short amounts of time the rate of runoff outpaces the initial absorption rate. That’s why flash floods are a problem in places like southern Utah, our soil is rich in clay that has baked in the sun. The occasion where we get a long running slow rainfall is practically celebrated because it means the rain has a chance to start saturating soil before it turns to runoff.
This, the family farm is on top of 300ft of glatial till. Heavy, high refractory clay, mixed with boulders. Significant errosion whenever we get “normal” amounts of rain after a dry spell. Very annoying getting stuck in mud while there is bone dry powderized clay an inch over requiring a mask to work. Organic matter is worth it’s weight in gold.
Thanks to one of Nick Zentner’s uploaded Central Washington University Colloquium series last academic year I learn the geology is even more complex in terms of soil dynamics since areas which experience infrequent episodic flooding with high mineral load erosion like out west will actually experience mineral sedimentation/precipitation of primarily calcium carbonate which further binds the soil together until it effectively becomes a rock termed calcrete. That stuff can’t be dug through with soil machinery you need the kid of tools that break through rock since calcrete isn’t all that different from concrete or limestone
Oh and you also get biological crusts formed from bacteria fungi and algae which will form along surfaces out west now they are fairly fragile unlike calcrete but they will effect the hydrology too.
Great video! I grew up out in west Texas, dry as a bone most of the time, and the worst flash floods were almost always the first few in the autumn after a hot dry summer. I remember older folks around me pointing out how certain areas in the city were getting very dry (reasonably enough, all of them places with no automatic watering for lawns and so forth), and predicting that Wadley Avenue would flood (again). That street was infamous for it, but it was SO consistent that folks could just about judge the severity of the weather by asking if Wadley had flooded. I couldn’t tell you, all these decades later, whether those extra-dry areas were connected to Wadley Avenue, but it wouldn’t shock me to find that they were. But thunderstorms out west are frequently very intense at the start – gentle rains that go on for a day just aren’t all that common – and MOST of Midland’s infrastructure was built to handle such flash flooding, because it’s just that likely that a big cloudburst will dump half the year’s allotment of rain on the city, all in the space of thirty minutes!
Also I’d like to say – I appreciated that bit of humor. Bringing down a rainstorm at will would INDEED be a “ethically gray area,” haha!!
Out in AZ there’s the same picture, plus mountains for extra sources of flooding. Always funny noticing how short of a timeframe the rain comes down. Do the summer thunderstorms in Texas also typically dump their bucket of water just after sunset?
@Windsor MasonHere in East Texas, the thunderstorms dump whenever they dang well please. I feel like mid-afternoon is most common, though.
West Texas is probably more similar to your weather.