Green Concrete?

[ricco67]

I saw this on CNN a few days ago, while I appreciate what they are trying to do, and it makes a certain amount of sense with rainwater, but I just can't help but wonder if this is a good idea given the climate.

Wouldn't this be better served in some of the warmer climates in parking lots or at least sidewalks? What about durability?

[Original Timmy Chan's]

I saw this on CNN a few days ago, while I appreciate what they are trying to do, and it makes a certain amount of sense with rainwater, but I just can't help but wonder if this is a good idea given the climate.

Wouldn't this be better served in some of the warmer climates in parking lots or at least sidewalks? What about durability?

A big problem with permeable pavement here in Houston is that the native soil (gumbo clays) that exist under the pavement are nearly as impermeable as regular concrete. That means that rainfall will run off the the pavement just as quick as if it was regular concrete pavement. The permeable pavement really doesn't help unless you replace the clay soil down to several feet deep with imported stone gravel that can provide some storage under the permeable pavement.

In addition, long-term maintenance is an issue. The voids in the permeable pavement will fill in with silt, so you have to have it vacuumed out on a regular basis. Not sure what kind of period (annually?), but it's something that has to be done, or else the pavement becomes impermeable. That's fine, as long as you have an agency (City of Houston or Harris County PID) that will assume the maintenance responsibility and get it done. If it's used for driveways and sidewalks in a neighborhood, are you ready to take the responsibility and expense of getting a special vacuum truck out on a regular basis to vacuum your driveway?

It's not a bad idea, but it definitely has some issues that come along with it.

[ricco67]

A big problem with permeable pavement here in Houston is that the native soil (gumbo clays) that exist under the pavement are nearly as impermeable as regular concrete. That means that rainfall will run off the the pavement just as quick as if it was regular concrete pavement. The permeable pavement really doesn't help unless you replace the clay soil down to several feet deep with imported stone gravel that can provide some storage under the permeable pavement.

In addition, long-term maintenance is an issue. The voids in the permeable pavement will fill in with silt, so you have to have it vacuumed out on a regular basis. Not sure what kind of period (annually?), but it's something that has to be done, or else the pavement becomes impermeable...

I didn't think about it being used here, but in areas where runoff is a major contributor of flooding.

Never thought about our soil in that way, but it is a good point.

Another reason a friend was thinking would be to use it for underwater storage tanks which seem to starting to get popular in several areas, but wouldn't a regular drain be just as effective?

[Original Timmy Chan's]

I didn't think about it being used here, but in areas where runoff is a major contributor of flooding.

Never thought about our soil in that way, but it is a good point.

Runoff is definitely a major contributor to flooding here. Runoff = Rainfall - Losses

Losses are mainly infiltration into the soil, plus some minor storage loss in depressions and water that sticks to vegetation and buildings. For an example, the 24-hr 100-yr storm in Houston is 13.5 inches of rain (in 24 hrs). Out of that rainfall, 11.7 inches (or 87%) will become runoff, and 1.8 inches are "lost" as infiltration/storage (for a typical single-family subdivision). So when we get a heavy rain, 87% of that rain ends up as runoff and goes into our ditches, storm sewers, and ultimately into our bayous. Runoff is a major contributor to flooding...I guess you could say lack of capacity would be another major contributor. If you don't have the capacity to carry off the runoff, it's going to become a flood.

In other parts of the country with sandier soils, they get much more infiltration into the ground, and therefore less runoff from a given amount of rain.

The fact that we get very heavy rains, and very little infiltration into the ground, leads directly to high rates and volumes of runoff. Those massive amounts of runoff in turn can lead to flooding when there is inadequate infrastructure...which is the case in most parts of Houston built before the mid-1980's.

Another reason a friend was thinking would be to use it for underwater storage tanks which seem to starting to get popular in several areas, but wouldn't a regular drain be just as effective?

If you're referring to underground detention storage, we have the same problems with infiltration into the ground...heavy gumbo clay just doesn't allow much to soak in...at least not very fast. I've seen several products that offer underground detention storage, with an unlined bottom to allow infiltration into the ground. While it's popular in other parts of the country where they have more porous soils, and don't get the intense rains we do, it just won't help with our major rains here. It will allow some infiltration, but not enough to get a benefit, storage-wise.

Low Impact Development is definitely having a hard time getting off the ground here...we have a lot of issues they don't deal with in other parts of the country. There are plenty of folks interested in finding solutions, but there are no easy answers here.