Berlin Is Becoming a Sponge City [video] | Hacker News Reader

Berlin Is Becoming a Sponge City [video]

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214 pointsflohrian66 comments

These kinds of techniques are often claimed to be effective against flash floods, which makes sense since they slow down water transfer to rivers. Does anyone know any studies about how effective it could be against seriously heavy precipitation of the scale of Harvey or South Asia floods right now? In other words, how well does the approach scale?

Civil engineering has pretty well quantified the rate at which water is absorbed by certain soils, accounting for groundcover, trees, pavement, local detainment, etc. Berlin's sponge mission is baiscially distributed detainment, working to offset hardscaping (driveways and roofing). In other words, it's a well known and very predictable science, but we (we being US land development industry) has not cared. HOWEVER rain at the scale of Harvey cannot practically be planned for beyond "don't build on land that's lower than average"

Well, that's partly true. A lot of the flooding is due to the fact that the entirety of Texas doesn't care about hydrology, and all that water is ending up downstream, in Houston.

There'd still be flooding, and it would still be bad, but a certain degree of flooding was avoidable.

The other thought, I imagine, is that they felt the probability was low that they would see a flood of this scale even within their lifetime; that ended up being a poor bet. Even so, flood insurance is also cheaper when the perceived likelihood of a flood is low.

Since cities usually survive much more than one human lifetime, this is not good reasoning.

People build houses, cities don't.

As you almost certainly know, the city controls permissions for every building.

A city that gives planning permission for a development that is probably going to be destroyed with loss of life in the next hundred years is not serving people well.

Eh it is wholly true. Distributed detention schemes still have a finite volume of containment, and once the detention zones are saturated, those areas behave equally to pavement for each additional unit of water. 50 inches of water across the majority of Texas with severe storm surge cannot be practically prepared for beyond insurance, evacuation, and/or a boat. To say “it was avoidable” is a strong and expensive assertion requires some hard backup. Heres an introduction to runoff coefficient (disclaimer! exciting stuff!). Everything is based on a “design storm” with a specified duration and intensity, where those two variables lead to a sigmoid curve (as i recall) for runoff, or the greater the intensity and longer the duration, the less water that gets detained. http://www.brighthubengineering.com/hydraulics-civil-enginee...

Deregulation at work

I remember a demonstration from my childhood where water is released into 2 similar models. One model is dry, one model is wet. The water quickly overtakes the dry model and splashes on the other side (as in a flash flood). On the wet model, water takes longer to make it to the other side. Sort of how a wet sponge is far more absorbent than a dry sponge.

I know I didn't really answer your question, but these models were made to represent fields and swamps. So yes, I believe there are lots of studies on large scale effects.

I don't know of any studies but I would imagine a lot of variables come into play.

Soil Type & how deep of soil effects how much water the ground can soak up & how fast the water will run. All would be slower runoff than concrete.

Also the slope of the land would make a difference.

These are just the obvious ones. Bottlenecks, whether created on purpose or not would have a huge impact. Where I live we have large snowfall in the winter which creates ice jams & flash flooding in the spring.

I personally like the idea of this.

Every inch of rain absorbed makes a rather dramatic difference. Because, floods are moving a lot of water but they also contain a lot of water which is time shifted over a longer period of time.

Think of it like a building being evacuated though a stairwell. Being say 10 vs 11 floors can make a significant difference in average wait time. Except average weight time is equivalent to the height of flood waters.

My first thought is that it might be hard to prevent decay and leaks into the structures supporting these spongy surfaces. 80cm of water logged soil above a parking garage, for example.

The renderings they are showing for newer plans (at 1:27 of the video) seem to indicate a soil later running a long the wall of the building, in addition to the roof. As the result water would naturally travel down that wall and into the surrounding soil, preventing the buildup on the roof and negating need for a traditional drain.

That said, what would be required to make sure no barrier formed, say from the roots system or something else is really hard to say.

You'd also need some sort of lining that would prevent the moisture from being obsorbed by the building materials themselves.

Planting grass on top of parking garages isn't that unusual or new in Germany (though usually done for aesthetic reasons), so I assume this is a solved problem.

Sufficiently deep buildings and parking garages might also encounter ground water, so adding soil on top might not even add new problems to the engineering side.

Similarly, the rooftop plants are shown on rooftop designs that would already have to deal with stagnating water

Municipalities can (and commonly do) grant a reduction of fees for reducing sealed soil, so it's not only aesthetic reasons. https://de.wikipedia.org/wiki/Niederschlagswassergeb%C3%BChr

funny just watched this yesterday night. Was really surprised by it. When you live in Berlin it's not something you see that yet. But of course Berlin is a more green city than something of the size of New York just by being smaller.

The video only shows relatively small buildings - would that system work with high rise towers?

Not answering your main question, but the video shows relatively small buildings because that's what Berlin looks like[1]. High rise towers are not very common there.

[1] https://upload.wikimedia.org/wikipedia/commons/5/54/Berlin_P...

High towers have got the same land footprint, so yeah, it would work

Land footprint, yes, but they do have higher volume to surface ratios and that should play some role in heat absorption and dissipation I think.

What happens to the plants during the summer if there is too little rain? Do they also install watering systems? That would be expensive to maintain.

This is Berlin! Too little rain? I don't think so.

The problem you are talking about is different and can be solved with traditional means (i.e. sprinklers) if needed. But due the fact the prior rain water is present on site for a longer time instead of being drained off in sewers they may not need to run the sprinklers as often (another benefit!)

What generally happens to most wild plants during the summer? Most of them can survive a few dry spells.

Berlin was a sump before becoming a city, it still has ground water almost up to ground level in some areas. Has a bunch of pumping stations to lower ground water, too.

The grass areas will need occasional watering in the summer, but most of the plants shown look like robust native species that can survive summer without problems

The suitability and cost of the technique will vary by climate.

https://news.ycombinator.com/item?id=15128428

Interesting, but no new ideas. These rain water management ideas are being implemented for a couple of years now in The Netherlands when developing new real estate.

What's your point? They acknowledge taking inspiration from another city where they already implemented this 20 years ago, that these particular ways of coping with climate change are considered proven technology, and that the only thing preventing more wide-spread adoption is political will.

And The Netherlands probably got the idea from China: https://www.theguardian.com/sustainable-business/2015/oct/01...

Hopefully they steal good ideas from all over the place. The video mentions swales, which is a mainstay of permaculture, which has been tried out in various ways all over the world since the 70s.

Also of note, the water system built after the 50's as a result of the recurring floodings in the area [1]

[1] https://en.m.wikipedia.org/wiki/Delta_Works

It is such a joy to see a methodical and rational long term planning response to a problem.

Makes one wish there was a longterm planning organ in a democracy, who could veto on short term solutions.

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