Permeable Pavement for Oklahoma

Over the past few years I've fielded a number of questions about the use of permeable pavement in Oklahoma. Permeable pavements are an alternative to conventional concrete and asphalt paving that allow for infiltration of stormwater through the pavement and into a subsurface storage area or drain. Recently, there have been some significant installations, including Chicago's U.S. Cellular Field parking lot consisting of 6 acres of interlocking pavers. Its important to look into the advantages and limitations for its use on local sites.

Initially, I began by looking into the ratings for permeable pavement provided by the EPA. EPA showed permeable pavements to have a failure rate of about 75%1,2. That was not encouraging. I looked into the studies behind these findings and found that they were from 1992. Well, a lot has happened in the LID world since 1992, so what about more recent studies?

My inquiries took me to states like North Carolina and Georgia, where water quality regulation is intense, and to the EPA's LID Development Center in Portland, Oregon. As you may know, some advantage of permeable pavement include:

* The lifespan of a northern parking lot is typically 15 years for conventional pavements due to freeze/thaw stress, which is also common in Oklahoma. Porous asphalt or concrete parking lots can have a lifespan of more than 30 years because saturation during freeze/thaw is reduced3.

* Permeable pavement transforms areas that were a source of stormwater runoff to a system that can reduce or eliminate runoff that would have been generated from traditional paving. Studies show stormwater retention rates ranging from 25-100% depending upon storm intensity. http://www.thewaterchannel.tv/index.php?option=com_hwdvideoshare&task=viewvideo&Itemid=53&video_id=349

* Permeable pavement can reduce the amount of pollutants in stormwater runoff and can address suspended solids (high removal rates), acid rain (some types effective), phosphorous (moderate removal), nitrogen (moderate removal), and metals pollution (best in vegetated systems).

So what's not to like? There are some drawbacks and limitations to use including:

* Permeable pavement is not recommended, or in some states even allowed, where surface soils are silt or clay. That's most of northeastern Oklahoma. Generally, permeable pavement is only recommended in areas with sandy surface soil, like the western part of the State, or where stormwater has been pretreated.

* Even when properly sited, maintenance requirements are critical for the success of permeable pavement. Proper maintenance includes vacuum sweeping or high-pressure water washing at regular intervals and inspections after each storm. If you can't commit to maintenance, there are better options.

* Porous pavement has reduced strength compared to conventional materials and isn't appropriate for applications with high volumes, high speeds, or heavy loads.

So how might permeable pavement be used in Oklahoma? Walkways, driveways, alleyways, and overflow parking are some options. And consideration should be given to preventing sediment from reaching the pavement, especially during construction, which can trouble a pavement system before it's even in use.

Finally, we have a responsibility to put our best foot forward for LID as these topics are being introduced in local communities, to hit the ground with practices we know have positive reproducible results. Who wants to hear, "More of my tax dollars gone to waste." again? The future of LID is in our hands!

1 Schueler, T.R., P.A. Kumble and M.A. Heraty. 1992. A Current Assessment of Urban Best Management Practices. Metropolitan Washington Council of Governments, Washington, DC.

2 J. Galli. 1992. Analysis of Urban BMP Performance and Longevity In Prince George's County, Maryland. Prepared for Department of Environmental Resources, Prince George's County, MD.

3 Gunderson, J., Pervious Pavements: New Findings About Their Functionality and Performance in Cold Climates, Stormwater, September 2008.

Canopy Economics - The Power of Trees

During a news conference seven years ago this week, American Forests (americanforests.org) unveiled one my favorite studies - it showed that urban tree deficits were costing tax payers billions of dollars each year in air and water benefits. At that point, San Antonio, Texas, Charlotte, North Carolina and San Diego, California joined in as the first cities in the nation to take action to reverse the tree loss trend by incorporating tree cover data into their infrastructure database - considering trees on the same level as roads, bridges, and treatment plants.

The connection between tree canopy and stormwater may not be immediately clear. Trees reduce the volume of stormwater runoff by capturing some rain on their leaves and branches, which then evaporates back into the atmosphere. Other water infiltrates into the soil rather than running off the land, which must be managed. For example, San Antonio’s urban forest manages 974 million cubic feet of stormwater, valued at $624 million using a $0.64 per cubic foot value based on local engineering, construction, and land costs (Pape-Dawson Engineers).

The findings from American Forests showed that San Antonio had 27 percent tree canopy, seven percent open space, 64 percent impervious/bare urban land, and one percent water. However, if tree cover were increased from 27 to 35 percent citywide, 2.5 million pounds more air pollutants would be removed each year and stormwater runoff would be reduced by 103 million cubic feet during an average 2-year, 24-hour storm event. This service would be worth an additional $200 million in avoided stormwater facility construction.

"Flooding in San Antonio is an age-old issue," said Carol Haywood, a planner with San Antonio's neighborhood and urban design department. "Most folks think we need more concrete culverts to simply whisk the water away as fast as possible. We will use this green (infrastructure) data to model and demonstrate the ability of trees to perform a similar function without adding new concrete."

For its part, the City of Tulsa began a tree planting program in 2007, after that year's devastating ice storm. For comparison, that storm destroyed or damaged as many trees as Up With Trees had planted in its entire 30 year history. Since that time over 5,000 trees have been replanted, with a goal of 20,000 trees by 2010 through ReGreen Tulsa. The City, through its Tree Advisory Oversight Committee, seeks to increase tree canopy by 3% year over year through 2013. Overall, the American Forests recommend 35% canopy coverage in urban areas.

When municipalities are looking to control long-term stormwater infrastructure costs, it should be clear that canopy cover can be a key element. What if we designed for the 2-year 24-hour event with both concrete and canopy? How much less would taxpayers have to spend? And how much more we would enjoy our new public infrastructure.

Tulsa Raingardens: The Clyde Home

This week we're featuring a local raingarden designed and installed by Tony and Jen Clyde in midtown Tulsa. I've been talking with Tony about his raingarden plans for a couple of years now. Their garden was planted in the summer of 2008, so I was curious as to how it was coming along. The answer is, "Great!", as you can see from these photos.


Tony designed this raingarden to address runoff from one section of their home encompassing about 170 square feet of roof. The location sits off the southwest corner of the home, in full sun, and is just off a stone sitting area. To begin his garden design, Tony performed a permeability test at the proposed raingarden location and found he had loamy soil with good infiltration.



The roof section selected for this garden has two downspouts that were disconnected from the storm sewer and rerouted, as shown in these photos. The first shows the downspout connection. The second photo shows the inlet into the raingarden.


Tony then began to dig out the raingarden. He excavated about 1.5 cubic yards of dirt, which has since been used in other yard projects. The garden is kidney-shaped, measuring about 4.5' x 10', or 45 square feet, and is 6" deep.


As an overflow outlet, a rut left by a utility truck was purposefully not filled in. The rut is barely noticeable in the grassy yard but functions regardless, channelling water out of the raingarden and into the street (and storm sewer) during high rainfall events. The overflow is located in the bottom left corner of this photo. Every raingarden needs an overflow.


The garden was then planted with facultative wetland plants - that is, plants that can handle periodic flooding. For this garden, Tony selected swamp milkweed, swamp verbena, sawtooth sunflower, and leadplant. The plants were obtained from Pine Ridge Gardens in Arkansas, which specializes in native plant materials. The plants were shipped directly to his home and cost about $110. To finish it off, Tony selected a good cypress mulch.

When I visited, there were numerous different species of butterflies and bees present. Although the plants were not chosen to attract pollinators, it is certainly effective. The garden looks like a natural, but planned, part of the landscape.

I asked Tony about comments from neighbors. He said the most common have been regarding the garden's attractiveness. But he did tell about one visitor who remarked, "You know your pond doesn't hold water." Some things take a while to catch on. :)