Why Pave Green - Ask the Expert

I was talking with several engineers at Flood Control and the question was brought up, “Why are you not supposed to store water within the porous pavement layer?”

We all know that the goal is to store water within the gravel below the porous pavement layer, but we were not sure why you could not “fill it to the top”. I remember when you gave your presentation at Orco Block that you had mentioned that you should not store water within the porous pavement layer, but I cannot remember why. Could you please give me the reasoning behind this? And I might add, one of our engineers said that he will not accept “because I said so” as an answer.

This question was posed to me and I thought others might have an interest in my reply which I am passing on. If you have a response, I would be interested as well on this point as I have also been asked about calculated storage on top of the pavement as in traditional detention design as a strategy to reduce the depth of aggregate typically used for storage as well as structural design.

This has been an area that affects porous pavements more so than permeable pavements and would be an issue in freeze-thaw climates rather than Southern CA. Our industry recommends detention volume be calculated in the base and sub-base areas as the paver/setting bed area has limited void storage area and will be reduced in volume within the first five years due to sedimentation/particulates that will collect in the joints and void areas of the pavers. The other reason we would suggest a more conservative approach for storage and a form of redundancy by leaving this area out of the detention storage calculations.

Sedimentation is a cause for reduction in porous pavements and when water is present during a freezing condition will cause expansion within the pavement and eventual deterioration of an already compromised psi in this material. Permeable pavers have a psi of 8,000 and an absorption rate of less than 5% and are unaffected by freezing conditions. Another observation in your region, would be a heavy incidence of air born particulates and beach conditions with sand causing an accelerated rate of clogging in porous concrete that will reduce any storage planned in this product. Porous concrete cannot provide a documented procedure for sediment removal as can permeable pavers.

I look forward to your comments. I also would look forward to your help to do the right thing-   

On the move again and find myself at the Ohio Stormwater Conference in Mason, OH speaking to stormwater officials, water resource, watershed and water quality professionals who had gathered to discuss, what else? -  stormwater and appropriately the weather is storming. Pervious pavements were on display and the question of the day, clearly, was maintenance.

I am always amazed at the perception that permeable pavements require costly maintenance and some people believe that remedial maintenance, removal of sediment (schmutzdecke) needs to be performed every year. Unlike porous materials, permeable pavements are constructed with solid segmental pavers that will not clog, ever. The joints and voids are filled with open-graded aggregates that will act as a filter and will require periodic sweeping, blowing or vacuuming depending on the accumulation of sediment and debris build-up on the pavement surface. The frequency of normal sweeping for permeable pavements will be at the same rate as you currently provide for your pavements now.

How will I know when to address the task of remedial maintenance, you ask? I usually reply that when it is raining I would conduct a drive-by and see if the water is standing on the pavement and that is when the big trucks need to be called out. How often will that be? I would look to see this happen in about 15-20 years, but we really don’t know as I have not seen a completely clogged permeable pavement surface yet. We have academics who have simulated and studied this condition of sediment build -up, e.g. Dr. Wm James at the University of Guelph and Dr. Marty Wanielista at the Stormwater Academy at The University of Central Florida that have provided insight for infiltration reduction. We also have seen examples of sediment and sand extraction in localized applications, but not an entire parking lot. The key is that with vertical columns providing a path of infiltration, we find that with water and vacuum pressure, we can remove the clogged stacks. 

We will be discussing this point as well as actual demonstration at the Morton Arboretum in Lisle, Il on May 29, 2009. NCSU and LID Center have created a one day workshop dedicated to discussion of pervious pavement maintenance. Dr. Hunt has also invited Janet Attarian of CDOT, Mark Kinter of Elgin Sweeper

I look forward to your comments. I also would look forward to your help to do the right thing-   

What’s Up - Chuck?

I was in West Union, IA this week speaking to a small group of citizens regarding their desire to be a “green pilot” community that was being driven by a Vista volunteer, Michelle. I was struck by the similarity of their town as to other larger towns and cities regarding stormwater management and the need to become water management literate.

As I drove thru a state struck by a 500 year flood, I was amazed at the intensity and activity by the farmers to install field drainage systems. I also was amazed at the local citizens desire to better utilize storm drainage or LID techniques that will have a lasting impact on their town. They are entertaining geothermal energy to heat the downtown buildings as well as a snowmelt system under a new pervious pavement for the town’s streets. While this may not prove practical, the design team at Conservation Design Forum will continue to brainstorm LID concepts that will be an asset for this community. I find that the awareness of stormwater damage from flooding has affected this area and I would hope that others would learn to mange stormwater more effectively to reduce the possibility of flood damage to their towns and surrounding areas. Another example I spotted was the effort of a group of concerned citizens in Port Washington, NY to sponsor a website, www.howgreenismytown.org to provide accurate information and links for green materials and methods one of which is permeable pavements.

Perhaps others have creative ideas about how we can implement LID principles and would care to share. I have seen improvement in site planning and designs that employ permeable pavements to reduce runoff as well as the ability to store water under the pavers and as a result help mitigate peak flows. Research this site to learn more about this feature of PICP’s and I look forward to your comments. I also would look forward to your help to do the right thing-   

“The greener side of gray BMPs” – I read this article and arrived at a different conclusion than the author Jennifer Steffens. While I agree with her points on underground BMPs, it takes a system to achieve a sustainable community and part of that system may be bio-remediation or any number of post-structural BMPs to form a site specific treatment train that will vary from site to site, state to state you get the idea. The best form of stormwater management is trees and the worst form of stormwater management is man.

Doesn’t have to be that way, but only man can change his ways, mother Nature is sure to change but will be most unpredictable. I would suggest that one way in which we can change is to utilize a more ecological form of pavement that will reduce site runoff, capture and treat first-flush pollutants and promote underground storage that will allow for better land planning and use. Pervious pavements will provide a solution for this need to change and permeable pavements will provide an even better option for the wearing course of these infiltration type pavement systems.

In the coming months, I and several of my friends will try to provide an ongoing dialogue about pervious pavements, why they work, why we should look to move in this direction, what’s coming in product and system development and improvements to a method of pavement that is as old as the Appian Way built by the Romans in 312 BC. With this kind of history, maybe they knew something we have failed to maintain - we are only using the land for a short time and we need to give it back to others in a better way than which we found it.

Please join me and others as we try to do the right thing. Be seeing you and look forward to your thoughts and criticisms as to how we can improve this forum and make a difference.

SMUTZDECKE – this is not a bad batch of scrabble picks nor is it a Norwegian fish species. I first heard this term from Dr. Bill Hunt at an LID workshop in Elmhurst, IL and again in Tacoma, WA. I guessed at that time he was pretty much liking this term and I had best try to understand how to better use it. I again ran across the term in another article I was reading by another friend of mine John Doerfer and this led me to the Germanic explanation at Wikopedia:

Schmutzdecke (German, “dirt cover”, sometimes spelt schmutzedecke) is a complex biological layer formed on the surface of a slow sand filter. The schmutzdecke is the layer that provides the effective purification in potable water treatment, the underlying sand providing the support medium for this biological treatment layer.

Well there you have it, I’ve been calling this sediment build up in the joints and voids of permeable pavers something akin to potato chips or toenails and now I will have to give it its right name assuming I can pronounce it. However, as we have been building permeable pavements for many years we have seen this material less on open-graded systems than on sand systems.

The use of open-graded aggregates as the sub-base, base and setting beds will provide high flow rates and negate the need for geo-textiles in most designs and applications. The joint and void fill will also consist of #8 or #9 washed stone chip materials that have no fine particles. We also see that the accumulation of smutzdecke is greatly reduced especially in vehicular applications where tire traffic will typically self-clean the pavement and send the smutzdecke materials to the outer lane areas where they can easily be maintained with a street sweeper. If you are using permeable pavers in a pedestrian or plaza application or a residential setting, a leaf blower will work just fine.

Over time, as this smutzdecke material collects, infiltration rates will be reduced until such time that the openings are clogged completely. This will require remedial maintenance whereby water is introduced to plasticize the smutzdecke and aggregate chips that will be removed from the void areas by a vacuum device. The joint aggregates will stay in place as they are involved with load transfer and cannot be removed. I point this out for two reasons:
    •    Permeable pavers without voids may not be able to be cleaned in this manner and will require removal, cleaning and replacement.
    •    Porous materials cannot be remediated in this manner and will require removal and replacement.
These two items need to be assessed by the design team regarding their recommendation for a long-term solution to a simple problem.

When the removals have been made, new chips are swept into voids and infiltration rates will be reestablished.

As always, help me do the right thing- be seeing you!