Rainfall flooding is becoming an issue | Hacker News Reader

Rainfall flooding is becoming an issue

slate.com

57 pointslxm50 comments

>Houston is trying to adapt. ... developers are now required to practice a kind of “leave no trace” policy when it comes to rainfall—once ... as a result, alongside Houston’s ubiquitous parking lots, many new projects feature grassy flood pits.

There's a strong case to be made here for more community parks & woodland trails. They serve as extremely good water containment areas

A comedic part about Houston flooding is that at least one large suburb on the west side was built within an overflow reservoir. The neighborhood is literally a water containment area.

Which one is that?

Canyon Gate, among others, in Barker Reservoir's flood zone. Typically excellent interactive NY Times article on it here https://www.nytimes.com/interactive/2018/03/22/us/houston-ha...

Thanks! I hadn't seen that before.

That's alarmingly common. It only takes one idiotic or corrupt planning team to flip decades of sensible flood planning. All this land set aside gets built on and everyone acts surprised when it's foot-deep in raw sewage.

Happens frequently in the UK. Too many fingers at every level of government happy to take a "donation" from a developer.

This is great if its designed in from day 1 but to retrofit into existing places with small fragmented sites and ownership is impossible - it would require considerable compulsory acquisition of land.

A lot of subdivisions here build around these as green spaces. They have jogging trails, planted trees, and some of em they also stock and you can fish within your neighborhood. It’s pretty neat.

Yes it’s becoming an issue because over the past century the pop of China has approximately tripled and people are living in flood zones.

Historically these flood plains have been both a boon and a curse. They bring nutrient rich soil for crops to grow in but also bring potential devastation, hence the dam building since the revolution.

It’s always been an “issue”.

I’m not saying climate change isn’t a thing. It is. I just hate opportunistic misinformation where things are framed in a particular way to spook people.

This is like saying malnutrition in underdeveloped countries is becoming an issue. It’s always existed.

I agree that floods in places where they haven't gone through centuries of natural selection of cities are not surprising.

What's surprising is that floods also killed hundreds of people in the old continent.

As someone living in Belgium, less than an hour away from various places that days ago experienced their worst floods ever and with friends who lost their homes… I wonder what the hell China has to do with those. Go on?

The article may talk about China but this is a general problem.

China also got floods 2 days ago. (Hi from Hotton ! ...with dry feet now)

How are the melons doing or is that just Hami?

Hi Hotton! I was in your area just a couple weeks ago. HMU for coffee if you're ever in Brussels or Mechelen. Email in my profile :)

It's because China isn't white. So people first look for explanations how it's about them not being proper white protestants and doing it all wrong.

Here in the Netherlands we always had this problem and we engineered around it. We were prepared for a once in a 100 year event (based on models on the 90'ties). Our current calculations however suggest this will happen once every couple of years now.

It is the reason we have 0 casualties thus far. But all those investments we made only made economic sense in the assumption it would solve the problem for a very long time..

It looks like to prevent property damage (and the emotional price of not feeling safe in your own house) we would need to spend that money again and again. Unclear if it will be enough, because the as a society we lost the ability to predict the climate changes . It's too chaotic now. We need more datapoints

I wonder much of this is caused by recent installments of agricultural field drainage tile?

Land price increases allow for more farmers to afford to "improve" their farmland by adding underground pipes that quickly wick standing water away into nearby creeks and rivers. What used to be evaporated or absorbed into soils is dumping straight into waterways.

You're asking if over 600mm of rain (i.e. more than half a year's worth) in a 24 hour period in Henan was caused by recent installments of agricultural field drainage? Or if that caused the jet stream to hold the storm over NRW to drop months worth in a 24 hour period?

1. https://weatherspark.com/y/127378/Average-Weather-in-Zhengzh...

2. https://weatherspark.com/y/58162/Average-Weather-in-Wupperta...

It should be relatively easy to calculate the max flood height and path of a area, with a given amount of rain on google maps. Anyone know such a tool?

If there was such a tool, and it was accurate, do you think insurance companies would have insured the parts of Germany that recently flooded?

I think you may be underestimating the complexity of both weather and climate modelling.

Where i live, i know the whole water from a defined area flows through in a little stream. I know the areas watersheds and i know how much poured down in extreme cases. Thus i can mark the area on a topographic map, i can calculate the area and multiply it with the max amount of downpour.

In my area thats 4490887(m^2) ×0,2(m^3 water) = 898 177.4 ^ m3 max rain water. Now all i want is a model of what areas would be reached on a stream if it received this much water.

I know that some here are grumpy because there houses may loose value like a sinking ship very soon. But knowing is better then not knowing. Always.

No fluid-simulations needed, just a basic Topographic map fill the pond.

In the US at least the USGS builds floodplain predictions for most of the major streams/rivers using this exact method. The problem is that those maps typically underestimate the amount of water that falls during these extreme events.

They are also sometimes based on assumptions about the land upon which the rain is falling - forests and meadows absorb way more water before runoff begins than concrete and asphalt do.

You're missing a key variable here: time.

If you dumped that amount of rain water in one second, you'd have a Tsunami. Dump it over a 24 hour period, it might flow out smoothly out of a little stream. Dump it 10% in an hour, 50% in the next hour, 10% the next hour, 30% over the next 12? Um, that's harder.

>"do you think insurance companies would have insured the parts of Germany that recently flooded"

Many insurance companies will be happy to write you a policy for something high risk but they will absolutely charge you more to compensate. It's all a numbers and probability game for them.

I wonder if the insurance companied would insure them anyway, because they might be legally obliged, and might not be allowed to "unfairly discriminate". I can easily imagine a setup where an insurer either averages out the risk profiles and premiums over a large administrative area to offer basically the same deal to every resident, or loses the license to operate in that area.

It’s called your government’s weather forecasting center or equivalent. They have all the data, tools and compute needed to calculate that with any accuracy, and they do.

SEPA here in Scotland produce detailed flood risk maps - I'd have thought most countries would have something similar:

https://map.sepa.org.uk/floodmap/map.htm

"Relatively easy" is brushing over an awful lot.

https://xkcd.com/793/

You can get a rough approximation, and that's been done at great effort for the entire US at the FEMA website:

https://msc.fema.gov/portal/home.

It's not a simple problem to solve. Some of the caveats are discussed in this Practical Engineering video:

https://www.youtube.com/watch?v=EACkiMRT0pc

Volume of outflow through a stream is equal to rainfall times watershed area. Seems intuitive, right? Except what about water that gets absorbed into the ground, or captured in a temporary holding pond, slowed by vegetation, or accelerated in a storm drain...I think Grady put it well when he pointed out that the silly Imperial units of outflow in cubic feet per second, rainfall in inches per hour, and watershed area in acres multiply together to 1.008, which is off by about 1% but close enough that civil engineers just throw away the constant entirely because it's such an imprecise approximation that the error is irrelevant.

In the US, FEMA maintains flood maps for areas of the USA that are prone to flooding. Your tax dollars at work! You can access them here: https://www.fema.gov/flood-maps

Local government in Japan publishes such maps.

Time to get a degree in civil engineering.

Cities usually don't have patches of green without concrete/paving of some kind to allow for proper drainage.

The West has money to build drainage networks. The most of Asia, doesn't.

Drainage networks take a lot of time and money to build. They are built for the best flood information at the time - but time and climate crisis has changed the inputs. Even if you have the money, it seems likely like a major risk that many areas will have shifted flood inputs and it will take more time than our current capacity to respond quickly. This is another reason to start addressing climate crisis more seriously ASAP.

Had this article mentioned a nation other than China, I would have give your opinion any value.

China has been putting so much money into its water management infrastructure (including dams, water re-routing) that most, if not all "western" nations put together are dwarfed. Go check your numbers before you spread more rumor, I will wait.

Just one sample (currently most ambitious and expensive engineering projects in human history): https://en.wikipedia.org/wiki/South%E2%80%93North_Water_Tran...

> Had this article mentioned a nation other than China, I would have give your opinion any value.

Yes, a very notable exception.

Most of China is catastrophically prone to floods, and would be unlivable without dams.

I don't think there is anybody not knowing that.

The "Most of Asia" here still stands.

Even the rich Japan, and Korea don't particularly care to build drainages in their 2nd tier cities.

The richest city in India, Mumbai, turns into Venice every mid-summer.

South-East Asia... no comments. Cities other than their capitals turn unto bogs for most of the summer.

I really wonder how people think valleys were created in the first place?

Does rather depend where you are - in lots of places glaciation has had far more effect on the landscape than running water.

I don't think most people spend time thinking about such things. They are likely more focused on finding/maintaining clean water, food, shelter, and health.

Looking at the comments here right now, it seems like "don't build in valleys" is the new "but humans didn't cause it" which was the new "climate change is a hoax."

The thing is, in the case of Germany at least, the reason people built there is because it hadn't flooded in those places in nearly a thousand years. That's why it's called a 500-year flood (due to the probability of it occurring.) They would have been washed away long ago if that weren't the case.

I didn't follow the weather running up to or the analysis following the flooding in Henan but in Germany the flooding was caused by a dense low pressure system hanging around too long and there seems to be consensus that this was caused by the disruption of the jet stream and increased temperatures.

We're not going to solve climate change by blaming people for living in the wrong place.

> We're not going to solve climate change by blaming people for living in the wrong place.

German here. We can and must blame politicians for handing out build permits in areas that once were flood beds for rivers that have been "straightened out".

Also, ffs we need to restore rivers to their natural beds again. The pictures of the flooded areas exactly show that the rivers took their millennia old paths back!

Yes, the Greens have been talking about it forever, but don't use it as a distraction from the fact that there were actually torrential rains and that most of these places were relatively safe without such torrential rains.

It doesn't have to be one xor the other.

I don't think it is about just not building in valleys.

There are two approaches to floods: one is to leave enough space for the river to meander, flood and soak into the ground. The other is to regulate the river - dig it deeper, more straight, build embankments, basically making it an effective drain pipe.

The second approach was popular in Europe because economy: it frees up valuable land (hence "building in valleys") and makes the river useful for things like transport.

But it has downsides: it destroys habitat and efficient drainage here means a flood elsewhere. And then when you have superefficient drainage everywhere, it becomes a problem for ground water levels.

So "don't build in valleys" is rather: don't over-regulate rivers to make buildable land.

You're still missing the point though. This wasn't a problem for most places before they started getting six months of rain in two days due to the disruption of the jet stream.

Becoming an issue?

I would suggest anybody who thinks this is a novel problem examine the history of north-central China. Every few centuries flooding is bad enough, and hydraulic infrastructure degrades enough that the Yellow river drastically changes its course.

Did you consider that China is just a recent example of flooding and the article is about flooding as such? The floods in Germany, Belgium and the Netherlands just happened last week.

You're just being inflammatory. And the European floods are the same problems as well.

In the USA, every other year there's some awful looking flooding on the Mississippi or the Missouri or the Ohio, because some heavy snowmelt, or heavy rain, and then a levee breaks, and the people that built in the floodplain get swamped out. It's a thing that happens, and has always happened, and will always happen, so long as human beings build cities in floodplains.

That's the problem. These floods didn't happen at the usual locations like Rhine and Moselle because they weren't caused by snowmelt but usual heavy rain. You are trivializing the problem, just like some trivialize climate change by saying that these also happened in the past. In both cases these things happened slower than now. The flood is one problem, the short time frame another and you are completely ignoring the latter.

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