How to Build a City That Doesn’t Flood? Turn It into a Sponge | Hacker News Reader

How to Build a City That Doesn’t Flood? Turn It into a Sponge

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169 pointsfern12101 comments

I think a great example of a good outcome when people give a hoot about stormwater (and other water removal options since it is an low lying coastal island!) and the environment is Hilton Head Island, SC. The town is aggressive or even militant about unnecessary tree removal.

The result is really interesting suburban infrastructure that reducing demand on the storm sewer. The Walmart parking lot isn't 5 acres of asphalt -- they left trees in place and have soil buffers with grass or other landscaping. Curbs are designed to direct water flow to those buffers.

I live in upstate NY, and even in that very different environment, those types of techniques as well as things like retaining pounds can really improve the situation with respect to flooding. Otherwise, the only way to remediate the impact of the increasing number of high volume thunderstorms is to dig up streets and build cisterns to sudden inflows.

While I think having more trees is good, I wonder if those trees and grass could lead to problems in the concrete and asphalt.

Winter takes care of that for the tree's. No worries, the surface will be potted and uneven in a year or two.

Depends on the tree and how the lots are built.

Won't somebody think of the concrete and asphalt!?

One of the rare comments that actually made me LOL

To be fair just paving over everything is a cheap and immediate solution. Lots of towns don't have the money for maintaining greenery.

"To be fair just dumping your waste into rivers is a cheap and immediate solution. Lots of companies don't have the money for safe disposal."

Generically:

To be fair just <something low cost, but high externalized cost> is a cheap and immediate solution. Lots of companies don't have the money for <high cost, low externalized cost>

Where high externalized cost is a long-term negative impact on a community, which ranges from undesirable to unethical depending on context.

"Kicking the can down the road" should be called out and minimized. The HN hivemind usually frowns on regulations, but I don't see another practical solution for protecting the community from environmental hazards?

Austin, TX taxes you for the amount impervious cover[1] that you have on your property. I'm not sure if the tax is large enough to really incentivize businesses to keep large portions of their property unpaved, but I do see many parking lots with trees and many areas set aside for storm water collection.

[1] http://austintexas.gov/faq/impervious-cover-what-impervious-...

San Francisco has a similar problem to the one described in Pittsburg. We use an old, single pipe, gravity based system that carries both storm water and sewage. When it rains heavily, the system backs up at various bottlenecks. Unfortunately, some of these are in heavily populated and residential areas, which get flooded with human waste. Even areas not at the bottlenecks may see more mild drainage backups.

Sadly, SF has done a pretty bad job managing this:

http://www.sfweekly.com/news/shit-storm-why-wont-sf-stop-flo...

SF's esponse to this problem has shaken my confidence in the city's governance. SF is going in the opposite direction from what is suggested in this article - we are paving over (with non-permeable surfacing) more than ever. The common practice of paving over your front year for extra parking is generally illegal, but like most quality of life issues in SF, enforcement is so rare that the city created an entitlement - and it has become the norm in some neighborhoods.

I know some people my chuckle that this particular issue would cause me to question SF's government, considering what is going on. But I think it's the straightforward nature of this - the clear engineering failure, the cause and effect between excessive paving and storm system overflow, the unenforced legislation to curtail it, the massive city budget and projects without adequate funding for critical infrastructure... it's the mundane failures that really drive home how badly SF's government is handling serious problems.

Amsterdam has this problem as well, it is now generally discouraged to pave your garden. Moreover projects using the "rain away tile" are popping up: http://rainaway.nl/

And also a the term "tegel-tax" (tile-tax) is introduced. The idea is to tax real estate based on the amount of non-permeable surface it has. More non-permeable surface resulting in a higher tax bill. Satellite & plane imaging combined with computational analysis would enable enforcement.

What’s wrong with parking on grass? Why do people feel a need to pave?

Parking on the grass kills the grass, and dead grass eventually turns into mud when it rains, and mud eventually gets your car stuck. People often use gravel to prevent this, which is permeable, but you need to add fresh gravel every year or so.

I have this childhood memory of parking spaces made of concrete tiles with holes in them, with grass growing through the holes. Now I finally know what they're for :)

Gravel is only permeable with permeable undersurface. This is usually not the case, because non-permeable plastics are used to prevent weed from growing in gravel area.

There are water permeable barriers that resist weeds too. It’s solvable.

Those surfaces are quite a bit more expensive than asphalt, so most companies won't put them in unless they are incentivized through something like a non-permeable surface tax.

True, it's up to the people involved. My point was more that it's possible

Thee are numerous drivable grass products specifically made for driving/parking on grass.

Really? I've looked for something like this in the past for my driveway, but come up empty handed. Are you able to share any of the product names with me? I would love to have something like this.

Apparently they're called "grass parking mats"

Here's an Australian product: https://www.grassreinforcement.com.au/

Here's another one: https://www.bunnings.com.au/cirtex-surepave-plastic-pavers_p...

It can also be called grass reinforcement mesh or grass pavers. Another cool approach is doing pavers with grass joints.

If you lay the gravel properly, (i.e. get it rolled when it's first laid), and give it a bit of cursory maintenance (raking it once a week), it should last a long time without you needing to add any more gravel, or at most adding a bag of gravel once a year.

Once it rains, that grass turns to mud and anyone without a 4x4 is hopelessly stuck.

You people must be terrible drivers or have a sufficiently liberal definition of "grass" to include most swamps.

More work to maintain and there’s a risk of getting stuck in the mud after a rain.

Indeed parking on grass is the best solution for this problem. We learned it at our city planning course decades ago already. You some amount of concrete but not 100% as of now.

Not to mention, having a grass lawn in the Bay Area is a major social taboo due to water conservationism. You get more props paving than planting in my neighborhood (as long as you pave expensively).

Pavement vs Lawn is a bit of a false dilemma, though. Nobody in SurfRider's "plant don't pave" campaign is advocating lawns. In fact, they advocate against lawns. There are low (or no) water, drought resistant, often native plants that will preserve the soil filter, which is crucial for a lot of reasons (storm overflows, sewage bottlenecks, pollution from runoff into the bay and ocean).

Also, pave-overs are illegal in San Francisco - the problem is that the law is generally unenforced. Enforcement is complaint driven, and there's a huge backlog.

http://www.sfgate.com/bayarea/article/Paved-over-S-F-yards-r...

Wait - you’re not allowed to park in your own driveway, even if not blocking the sidewalk? Am I reading this right? Enclosed garages only?

(I sort of understand not being able to convert lawn to concrete parking space, but this seems much more draconian).

A driveway is fine, but the rest of the front yard should be unpaved.

They advocate against lawns, and you can't park on typical "native" gardens (typically a lot of dirt, and some scrub brush), thus you are left with paving as the parking friendly eco-acceptable option. Or astroturf, but not sure how much better that drains than concrete.

Combined Sewer Overflows (CSOs) represent a big problem that is not technically difficult to address. The issue is only money and political will.

We have a great example in Maine of how well CSOs can be addressed and minimized in the twin-cities of Saco and Biddeford. These two municipalities face each other across the Saco River and have separate waste water systems. They both have historical issues with CSOs and have been working to address them.

Saco worked quickly, spent a ton of money, and raised a lot of fees to upgrade their sewer infrastructure to minimize CSOs into the Saco River. They are largely complete with that work and total CSOs has dropped to under 1 million gallons per year[0] estimated, and only 600k gallons in 2016[1] (highly dependent on rainfall).

Biddeford has been slower off the block, to be honest I'm not sure what they've accomplished although I know they've made progress. I believe a recent large Main Street reconstruction included CSO work. Their CSO discharge rate sits around 20-40 million gallons year year[1].

One big difference is that Saco charges an CSO impact fee on new sewer volume, currently $11.27 per rated gallon[2]. I'm pretty sure that Biddeford doesn't charge a CSO fee at all, at least it's not detailed on their wastewater permit application.

A community is a reflection of it's citizen values. Care about CSOs? Talk about them, build consensus, and raise money to deal with them. There will always be someone complaining about the municipal fees to build and live in a particular community, don't let those individuals dictate the narrative and thereby the world you live in.

It's easier to see your impact in a small community (like Saco, ME) as compared to a large community (like San Francisco, CA), I get that. Don't get discouraged, keep working.

[0]http://courier.mainelymediallc.com/news/2017-11-09/News/Saco...

[1]http://www.maine.gov/dep/water/cso/2016_status_report.pdf

[2]http://www.sacomaine.org/departments/water_resource_recovery...

> not technically difficult to address

While the concepts may not be difficult, the scale often is.

See the Milwaukee deep tunnel system, storing 521 million gallons in 28.5 miles of tunnel.

https://www.mmsd.com/what-we-do/wastewater-treatment/deep-tu...

But it does work: https://urbanmilwaukee.com/2014/06/12/murphys-law-the-remark...

https://en.wikipedia.org/wiki/Tunnel_and_Reservoir_Plan

Chicago's blows that away. 17.5 billion gallons of storage, 110 miles of tunnels. I think it's the largest current public works project in the US.

Yep, definitely. I brought up Milwaukee because I have a better understanding of it and its effects. As hard as 521 million gallons is to visualize, 17.5 billion gallons is. . . 34 times worse.

Totally, the scale can be outrageous. As you mentioned, Milwaukee's system is huge - but it works.

"Too Big, Will Fail" is a pretty common refrain from anti-infrastructure personalities, but when it comes to simple civil engineering projects it doesn't matter what the scale is if the mechanism is proven. Solving CSOs isn't the Hyperloop, it's just building a bathtub big enough in the right place and figuring out how to drain it appropriately when it fills up.

Milwaukee, Chicago - these are examples that prove that stormwater management is a solved problem. So let's apply that solution to places where the problem still exists.

Anyways, cool link, I didn't know about Milwaukee's system.

As a kid I thought of the Bay Area & San Francisco as progressive & environmentally friendly. When it comes to the cities themselves and not just which President they voted for, was it that way in decades past, or was this never the case?

Never.

In decades past they voted for Nixon.

Every demographic has ideological blind spots. For example, Republicans are generally for reduced government spending...except that when it comes to cars, they favor extremely expensive infrastructure.

The San Francisco board of supervisors and most of the voting public would almost certainly declare themselves to be progressive and environmentally friendly, and I don't think they are cynical or manipulative here. They really mean it, and believe they mean it.

The problem is where policy becomes enforcement. San Francisco, I believe, isn't good at translating intent into reality.

Pave overs are actually a good example. San Francisco has very specifically illegalized front yard pave-overs, and the board of supervisors voted to increase the fines. However, very little budget (in a massive, massive city budget for a city SF's size) is allocated to enforcement. They're also in a bind now, as they have allowed the practice to go on so long, with such light enforcement, that it's going to be difficult to roll it back.

Another experience I had was replacing my windows with energy efficient, double pane, insulated windows. The Bush administration (George W) created a tax credit for energy efficient home improvements. Gavin Newsom, SF's liberal mayor at the time, talked quite a bit about about the importance of "Green" San Francisco.

Gavin Newsom's San Francisco collected permit fees that more or less confiscated George W Bush's federal tax incentives to encourage energy efficiency. Savor that for a moment. Ironic, isn't it? But to me, this story, along with the pave-over debacle, kind of gets at how SF goes wrong on so many things.

Thanks. That makes it sound like this is all yet another victim of budget failures. Good intentions, but never enough money for basic programs, and a government always hungry for more revenue.

San Francisco also has the Front Yard Ambassador Program. This encourages homeowners to replace concrete in their front yard with plants. The City helps defray the costs. The goal of this program is to reduce runoff.

http://sfbos.org/supervisor-tang-front-yard-ambassadors-prog...

I have reservations about this program even though I hope it is a success.

The program provides partial funding and reduced permit fees if homeowners arrange for at least four houses on a block to remove the concrete. Citizens are encouraged to become “ambassadors” and knock on neighbors doors and try to convince them to remove it. If the citizen fails to get enough neighbors on board? No funding.

To me, this program takes an issue of permit enforcement of critical importance to the infrastructure and pushes it out to people with zero authority, and asks them to knock on doors, explain the law, and ask if they’ll please consider complying with permits.

The government gets to duck the awkward business of enforcing a law that turned into an entitlement, while pretending to do something ang getting a smiling feel good photo op (win win!)

Meanwhile, where I live, pave overs continue apace with no apparent enforcement at all. Houses flood with raw sewage. In one recent storm, a dog was electrocuted in the flooded garage. This area is one of the few parts of sf with a high percentage of children, including lower income children.

It turns out the government does need to establish and enforce sensible laws.

As is often the case - the truth is more complex.

That said: it’s often that the Bay is more socially progressive than fiscally or infrastructurally.

Plus they’re just shit at getting things done (often due to NIMBYism).

Hah, I had not thought to separate out social progressiveness from infrastructure progressiveness. That's a good point, they in no way have to go hand in hand.

The last time I saw something about these highly porous pavement alternatives, it was pointed out they tend to be very vulnerable to freeze-thaw damage and that this damage also lowers the porosity dramatically. Basically making these kinds of materials unsuitable for urban use outside of tropical- or near-tropical areas. Has this changed?

http://www.cement.org/cement-concrete-applications/paving/pe... "However, in climates prone to severe freeze-thaw cycles, some are hesitant to use pervious concrete pavements until it has been proven that pervious concrete can be made to resist freeze-thaw damage." Pittsburgh does go through freeze thaw cycle, so I assume there aren't much problems

https://www.unh.edu/unhsc/sites/unh.edu.unhsc/files/pubs_spe...

According to this, it also reduces the amount of salt needed 75% for deicing and the pavement doesn't seem to have been damaged from reading the abstract.

Well, if Pittsburgh is using it, it is terrible. We have so many potholes (and huge ones). It has become much worse over the last ~5 years. For several months we had a multi-foot hole blocked off by cones until the city could fix it. It essentially was in the middle of a two way road. It was terrible. I see more and more each winter.

That's not because of the cement but the geology of Pittsburgh itself. Limestone bedrock and the "underground river" contributes to potholes and other geologic anomalies. If anything, adding bioswales and having a way to put the water back into the ground would stop the potholes from happening in the first place.

Yes, but that still doesn't mitigate the rapid freeze/thaw effects on the concrete/asphalt that the City of Pittsburgh uses.

I think you are confusing sinkholes and potholes. The two aren't really related.

That sounds, from your description, more like a sink hole than a freeze-thaw induced crack? Hard to tell without seeing it.

Water flows through them, they don't really 'absorb' water. Of course there will be moments when it freezes when there's still water in them, but it's not equivalent to freezing a sponge. The parking lot on one of my buildings is paved with blocks like this (condition for getting the building permit), works great. Never slippery in winter, either. They weren't even more expensive than alternatives.

The main problem I see with them is people's perception about what porous pavements do. Great, water goes through them... and then what? The ground underneath the pavement is not some unlimited sinkhole for water. The ground table can and does fill up. Then it doesn't matter how porous your pavement is.

Trees, grass, wetlands, etc, do better in part because they are preventing the water from leaving. They pool water in small puddles, they absorb water, leaves have water sit on them, etc.

Or you can build a ginormous cistern beneath the city, as Tokyo did: https://gizmodo.com/tokyo-has-the-largest-underground-water-...

Which may not be large enough: http://www.todayonline.com/world/tokyo-preparing-floods-beyo...

There's a related story on Berlin being built into a sponge city. https://www.bloomberg.com/news/videos/2017-08-18/sponge-city...

I think Berlin's approach is better for colder climates. It just makes sure the water lands in soil and stays there. Each new construction site must do the same. So the roofs are green with soil and the road's water goes into the soil and the buildings have a layer of soil on top of underground garages.

In Brazil, there’s a capital called Curitiba that use rivers and parks with artificial lakes to control flooding with great success.

http://i2ud.org/2013/08/flood-management-in-curitiba-metorop...

anyone who hasn't watched a clip on Tokyo's massive drainage system should check this out https://www.youtube.com/watch?v=o85teh1vU_0

Super interesting. Essentially, they built an underground river which is connected to many of Tokyo's rivers (many kilometer underground tunnel) that diverts overflow water from the rivers to prevent catastrophic flooding.

It cost 3 billion dollars and took 13 years to build.

I would like to see how these held up during the tsunami.

Japan has spent a LOT of effort to mitigate floods. It does mean that all river banks anywhere near a city look the same, and very artificial.

Michigan has be fighting a similar fight for the past couple decades. Wetlands were disappearing quickly from development and they realized how important they were to environmental health, essentially turning the state into a sponge/filter. There is a law there now that removal of wetlands requires developers to build a new wetlands elsewhere. They still have 40% less wetlands than pre-european times.

Solutions mentioned are I think referred to as BMPs - best management practices [1]. Regulations exists and there are scientific software tools for tracking and planning [2].

[1] https://en.wikipedia.org/wiki/Best_management_practice_for_w...

[2] http://www.2nform.com/tools/

Dig up all the paved streets and replace with soil, with train tracks on top. Adapt vehicle wheels to train wheels and provide platforms on train wheels for the ones you don't adapt. Provide overhead lines to power vehicles adapted to use them.

You get 1. drainage, 2. reduction of heated urban micro-climates, 3. elimination of emissions, 4. "driving automation" in the sense that you don't really need to steer something on rails.

Are we planning to use manual labor to dig up the streets? Otherwise that sounds like a significant emissions problem in itself.

Depending on where you live, they may already be doing that every 2-3 years anyways.

I’m aware of resurfacing projects, but I’m not aware of total replacement projects.

>Dig up all the paved streets and replace with soil, with train tracks on top. Adapt vehicle wheels to train wheels and provide platforms on train wheels for the ones you don't adapt. Provide overhead lines to power vehicles adapted to use them.

Seriously?

Steel wheels on steel tracks are way more efficient than rubber on asphalt. There are many reasons this hasn't been done already.

How to build a City that doesent swarm with mosquitos? Design the sponge to kill live near the surface

Birds?

The techniques in the article will mitigate the effects of storm water. As was the case in Houston, urban flooding results from global weather events. No matter how Houston had been planned, all the water had to go somewhere and it was going to the low spots. The only counter-factual in which Houston would not have had some flooding is if there was no Houston.

Ancient cities in Mesopotamia flooded. Ancient Rome flooded. Cities flood because they are built on or near bodies of water and bodies of water flood. The reason cities are built on or near bodies of water: water is a necessary condition for any city. Water is also an economic engine: fisheries, agriculture, waste removal, transport, trade. This makes flooding a price of doing business.

It's not that mitigation is a bad idea. The bad idea is that a city will never flood. Though it is theoretically possible, it is practically unlikely.

One way to deal with flooding is to build highways in a manner where they become channels in such events. Kuala Lumpur, for example, has their SMART tunnel[1] which acts as a roadway normally but can be turned into a stormwater drainage tunnels.

Houston had a prime opportunity in 1996 to do something similar[2] when I-10 W was rebuilt- and engineers recommended building a conduit under I-10 to carry flood water from the Barker and Addicks reservoirs to the ship channel. Alas, that proposal was ignored so West Houston flooded during Harvey.

https://en.wikipedia.org/wiki/SMART_Tunnel

https://www.dallasnews.com/news/harvey/2017/09/05/houston-gr...

Mis-step after mis-step. Is anything now being done differently in TX? Or is the plan to forget about it as soon as possible and cling to NFIP for next time?

I live in Houston and there may be something in the works but if so I haven't heard about it yet. I sure hope someone is working on it...

Re: Kuala Lumpur

October 30, 2017: http://www.channelnewsasia.com/news/asiapacific/kuala-lumpur...

September 2, 2017: https://www.nst.com.my/news/nation/2017/09/275610/evening-do...

2014: http://www.themalaymailonline.com/malaysia/article/kl-shop-o...

One tunnel isn't going to solve all flooding problems in a huge city like KL. It's a step in the right direction, though.

It's more complicated than that.

> One way to deal with flooding is to build highways in a manner where they become channels in such events

That's exactly what Houston did. The problem with this is that it makes evacuations more difficult and risky - the main ways for people to leave the city during a hurricane or flood become the canals for draining the water!

Houston didn't. I live here. Our freeways fill up with water but don't channel it away.

Allowing real estate development in basins specifically designed to capture storm-water during flooding was the case in Houston.

The bad idea is building in a place that was specifically designed to flood.

https://projects.propublica.org/graphics/harvey-reservoirs

I enjoy reading Pro Publica. Their policy analysis in the linked article is interesting and reasonably accurate. The like the original article above, it describes flooding in terms of mitigation efforts. The dams above Houston manage stormwater but cannot prevent flooding.

It's easy to look at Port Arthur, TX and think "Yep, of course that's going to flood." It's hard to look at Houston and think the same thing. The difference is a matter of frequency and relative extent not possibility.

Bad policy makes the effects of floods more costly.But good policy does not prevent floods.

Some ideas along these lines (spongy waterfronts) for SF bay area cities at https://neighborland.com/resilientbay

Broad statement for the subject. I suspect even a sponge can be saturated.

Obviously, but that's not the test.

The test is whether the sponge can absorb rain as fast as it can fall for a long-enough time that other consequences of the rain become serious problems (e.g. deferred outdoor maintenance), while the sponge still isn't saturated.

It's mostly I think the limit of how fast the ground can absorb water after it has passed through the pavement. If the water rises through the pavement..

and imagine what happens when a freeze comes by and what it does to the sponges' life

The issue is one of rates not volume. They briefly mention Houston in passing, which is a perfect example. The water is all part of the same cycle and same watershed, but where it goes within that watershed and at what rate become critical. If you make 50% of an area impervious to absorbing water (i.e., you pave it) then the remaining 50% has to absorb much more water, and likely will become saturated. But first, the impervious nature of pavement will cause those low lying areas to flood. Because the water flows faster on the surface than it migrates through heavily packed soil, the result is a water build up that can not only flood the now saturated area, but will in fact flood the lowest areas period. Dirt has an absorption rate that is (somewhat counter-intuitively) actually negatively impacted by becoming excessively dry. This is why Phoenix, notable for rare but massive rain storms, has enormous flooding problems that they deal with mostly by creating buffer areas in the form of intentionally designed flood zones (e.g., parks with berms around them or underpasses).

Houston is designed like this as well (the Addicks and Barker reservoirs). But they are designed based on a given rate of rain over a given area. That assumption wasn't actually violated that badly by Harvey, it was within the margin of error. What was violated in the Houston flood model was the amount of hardscape and where the homes were. The assumptions about the amount of hardscape affects how much of a given rain will reach the reservoir (a lot never does to to evaporation or local use) as well as the rate at which that water reaches the reservoir. That whole system is designed to rate limit the flow of water into the core of the city of Houston. The model and assumptions were built when the core of the city was the main part of the city and the suburbs were much smaller. The water has to go somewhere, and it wants to go into the gulf of mexico...but the rivers and canals in the city core have a limited discharge rate into the gulf. Therefore the goal was based on creating a system that tolerated the hardscape based reduction of the cities 'sponge' and a greater reliance on the sponges of the suburbs. This is all not to mention that they actually built homes INSIDE the reservoirs that are a lot of what we talk about when we talk about the Houston flooding...those homes flooding was the system working as designed, the only people out of the loop were homeowners (slightly more true: and realtors, developers, and local officials who aren't hydrological engineers) [0]. They should have known better but they didn't, because people tend to not notice, understand, or take seriously these details of municipal planning. Then they are shocked when either it works as designed (as a net positive for society at potential costs to individuals) or when it fails because choices made from lack of knowledge have blithly broken a system that is designed for emergencies. Think of it this way, there were people in the Houston gov who knew exactly what was going to happen...do you think talking about a biblical level flood in Houston was going to be taken as a more powerful narrative than 'economic growth, jobs, housing, american dream' when deciding on zoning permits?

TL;DR any sponge can become saturated but the point is that you should not artificially reduce the size of your sponge. Cities know how big their sponge is, its the ground around them. They make predictions and plans on it...and then subdivisions and growth massively seal off parts of the sponge.

[0] https://projects.propublica.org/graphics/harvey-reservoirs

The problem isn't flooding, the problem is that people built the city in the wrong place.

I wouldn't say that. Cities were built next to rivers and seas because they provided ways of transport, water, and places to dump waste etc. You can't build a city inland without a lot of trouble. But rivers and oceans flood and have hurricanes..

Of course that's true but ships need water not buildings, I think we our civilization is old enough to know about flooding, we just don't care to address it until sometime happens. Can we not just transport the goods just a bit farther and start moving away from the flooded areas without having to waste money and manpower on sponges? Lol

Not in my country, there are rivers every 50 kilometres and canals between those.

I suspect it's cheaper to simply build a large enough drainage system initially. This is useful if you have already failed and are trying to cope with failure.

Still, many people are pushing it because you get more plants not because it's cost effective.

It depends if you have somewhere benign to drain all the water into. If there is a population downstream that might get annoyed if you dump lots of run-off on them, then making your land more sponge-like can be really useful. If "downstream" is the ocean a mile away, not so much.

There is also potentially a lot of benefit to having the water enter the soil and eventually make its way down into the aquifer instead of just shunting it "away".

There are certain ways of building swales[0] in an urban environment that can help absorb or slow the leeching of nasty chemicals in runoff into groundwater or communities downstream.

[0]http://nwrm.eu/measure/swales

Like the other guy pointed out, there isn't always an area to drain too. Also I'd say having more plants has other benefits, and so does draining water into the ground, which would lead to more ground water edit: and into an aquifer (like wanderr pointed out) instead of wasted into an ocean

Hmmm... this invites some unintended consequences, no?

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