Impervious Surface, Subsidence Risk, And Heat Islands: Or How To Deal With 18.5 M3 Rainwater Runoff

[Morakot]

Hello everyone,

I'd like to ask for your opinions and have a general discussion about impervious surfaces, i.e. covering the ground with impenetrable materials so that rainwater cannot seep into the groundwater. I live in an urban area in Thailand were impervious surfaces are the norm. I think, in line with many others who understand the issue, covering the ground en mass is a bad idea, because it increases the risk of subsidence, contributes to the creation of heat islands, increases the concentration of water pollution.

I am contemplating of having a rain garden or a natural runoff area for waterground infiltration. I have estimated my annual rainwater runoff from the building surfaces about 18.5m³. Currently I feed the rainwater onto a public road surface and some into a very old communal drainage system. This is the set up for most neighbours, who by and large have about 90-100% of their entire land surface areas covered with impervious surfaces. Most plots are raised through landfill in the usual way.

One of my questions is what kind of surface area would I require for adequate ground infiltration of about 1 to 2.5 m³ of rainwater per month? What else should be considered for this?

Many thanks!

[bbradsby]

Great subject. Non-impervious cover is the ideal for non-toxic runoff - not so much for car repair shops, dry cleaners... But since you are most likely talking about residential and/or light commercial occupancies, more porosity is better. Gardens being the ideal, of course. grass-crete blocks being best for residential driveways, pervious concrete next in line, and regular concrete coming in last.

Said that, you can only control the percolation rate of your applied topping or paving. The real limitation of the percolation rate - how fast water decends through the material - is usually the sub-strata on a particular site. In the alluvial river basins, such as Bangkok, there is a high clay content, which severely slows percolation rate. So even a non-impervious paving will quickly flood with water in a heavy rainfall. But at least you are reducing the run-off from the typical 100% runoff that normal paving creates. On a raised building pad common in LoS, percolation is rapid... but just down to the original clayey soil strata. Maybe that is good enough as it slows down runoff, stores water for a time, which decreases downstream flooding. Natural, planted waterways do the same thing, by slowing velocity & storing water so that the drainage system can handle the volume without overflowing the banks.

To design your system, analyze your substrata, best-guess, for what type of soil it is, Google soil percolation rates, and calc the liters/unit of area/unit of time and compare it to local rainfall rates... OR... just maximize the percolation rate of what you can actually control on your site via higo percolation rates of selected cover, dry wells, thirsty trees/plants with deep roots to penetrate clay strata...

As Urban areas grow, Thai style, we see most residential lots approaching 100% impervious cover (roofs + sitework paving) in just a few years after initial conversion from agricultural land use. Multiply this on an urban scale upstream of the old city, and you quickly begin to overwhelm the legacy drainage system. Then add subsidence and, viola, you get the increasingly devastating yearly floods in Bangkok, and cities across the world.

Thing is, the problems and solutions are not rocket science, unique nor new to Thailand.

[Morakot]

Thanks BB, very useful! Glad to read your enthusiastic and learned reply.

Yes this is for residential purposes and I'll do some soil analysis so I can better explore the options. Talking about grass-crete blocks, are they available in Thailand?

I also have created a poll here to see how other people manage their rainwater.

[bbradsby]

Grass crete is available even in upcountry village builders supply shops.

I'm a proponent, but one concern of non-impervious (also called "pervious', but that sounds a little bit, well, just Wrong somehow...) cover on compacted fill homesites - typical in LoS - is that water percolating down will tend to further compact only that area, causing differential settlement of the sitework and/or building foundations & footings. Not a good thing!

I would recommend total water management within two meters, minimum, of the face of buildings.

[Morakot]

Thanks BB for the input.

Subsidence --in terms of water (mis)management-- is caused by a decline of the groundwater level, which in turn is aggravated by non-impervious surfaces. If percolation causes sinking than I assume the groundworks were not prepared adequately: e.g. the landfill area has not settled properly, the ground below the foundation was not the compacted properly, etc.

What do you mean by "total water management within two meters"? If this means for example no percolation trench closer than 2m/ 3m to the structure that makes sense.

[bbradsby]

By water management, I mean making sure that water is not directed toward the edge of building and its foundation. there should be a noticeable slope away from the building perimeter, and ideally, gutters & downspouts/rainwater leaders that capture roof water and carry it well away.

[bbradsby]

with the typical filed sites for homes here, the building pad is not actually compacted in an engineering sense of the word, i.e., to 95% and can be tested as such. So with each rain, the pad compacts a little more. this is the problem with partial impervious cover and partial non-impervious cover on a building pad - it will result in differential settlement of the pad, and anything constructed on it.

If your foundation extends to undisturbed soil/strata (below the pad by definition), then your house should be fine as long as that strata is solid enough for your foundation loads. As for sitework paving & structures, maybe not so much. I suppose we've all seen garages become inaccessible to cars, front steps becoming comical, due to pad settlement by a meter or so over ten years.