Pervious Concrete vs. Detention

[runoff]

Have any of you been running into the acceptance of pervious concrete being accepted as an alternative means for storm water runoff? Or a better way to ask this, are any of you seeing this as an enhancement to a storm detention area when you do not have space available that might be required due to the hydrology results?

 

[TDAA]

I was asked to look at the suitibility of use at one site (from a geotechnical perspective). I was told it would eliminate the pond from the design (site is currently paved). I am not sure if the design team is going to go forward with it.

 

[runoff]

That is funny. I am asked the same thing. The state is Tennessee and the soils are very good for this application. I have only done one parking area with it but it was a very small storage shed design. This is a shopping center and 2.55 acres of drainage. One option was underground detention but that is way more expensive. I think. I don't want to do it and it come back to bite us.

 

[LHA]

Pervious pavement alone can't replace a detention volume. All it can do is provide retention, and improve water quality for lower intensity storms.

To get detention during all design storms with pervious pavement, you need to provide underground storage under it.

Often, however, a reviewing agency will relax the requirement for a detention volume in exchange to the water quality benefits of increased retention/decreased impervious. Make sure that is what the reviewer is offering, because if the reviewer wants you to provide pervious pavement AND still prove detention of larger storms, you will not save anything.

Engineering is the practice of the art of science - Steve

 

[explortainment]

too bad the porous asphalt needs to be vacuumed otherwise it would last more than a few years. You should consider design life and maintenance.

 

[bltseattle]

Explortainment's points (clogging potential and life cycle costs) are important to consider to understand the benefit of permeable pavements over the design life.

So, what is the evidence about longevity and maintenance? Has anyone seen any studies/reports/documentation regarding the design life? Is the clogging potential an urban myth?

As with infiltration facilities, there are probably some historic instances of clogging that can be attributed to the construction practices or letting silt-laden runoff onto the permeable surface before the site is stabilized. The potential of clogging would be different on drive aisles than in parking stalls, and would likely vary depending on the frequency of use; consider the differences in overflow parking versus the most frequently used stalls.

 

[blueoak]

The clogging instances I have seen have been due to construction site problems. If you can stabilize the site so you don't get sediment on the concrete it seems to last longer. I looked at a parking lot at a park with light use during the weekend but full on the weekend and it seemed pretty good after about 6 years. The infiltration was at least half of that right at construction, but this was with no maintenance. The site was well vegetated so erosion onto the parking lot wasn't a problem. I always wondered about cold climates and deicing.

The problem I have seen and heard with clogging is that rehab is pretty much impossible after you clog it. We assumed a 25% of optimum infiltration rate with full pore space for volume.

I had trouble talking developers into it though because I didn't know if resurfacing is as easy as asphalt.

 

[runoff]

I spoke with the city official and they will accept the pervious concrete in conjunction with a detention pond or underground system. So, back to the drawing board. I guess I can lay the parking out with pervious in the parking areas and possibly add catch basins that would take residual flows to the basin. The basin is too small right now to take the entire flow from the site and the limited realistate also prevents me from expanding that pond. This is the reason for the alternative methods. I have a 6" pavement with 25% porosity and gravel base of 40% porosity and exfiltration rate calculated at 1.6 in/hour. According to my calculations this would allow for 70% available capacity after a 24 hour period. The city said they want the system designed to a 10 year event. So, that is what I will give them.

Note, the city I am dealing with does accept pervious an alternative means for first flush applications and detention. I guess they view as an improvement to water quality and solids removal. I do worry about the maintenance but I have stated very strongly the manufacturer's recommended cleaning and who is responsible.