During the heavy rainfall season, San Francisco faces some daunting challenges: Draining the water, keeping the roads from getting slippery, and containing and treating the runoff. Some storms are so severe that the city can't keep pace. That's when we see flooding in the Muni tunnels and sewage discharges into the bay.
But the solution -- or at least part of the solution -- could be as simple as changing the material that we use to pave our streets.
The city considered a wide variety of low-impact-design techniques for managing water at community meetings held in 2007 . Among the solutions was permeable pavement, a technique dating back centuries that fell out of favor during the fast-and-cheap highway booms of the last few decades.
As Miles Chaffee, President and founder of Milestone Imports explained to Streetsblog, the benefits of permeable paving are numerous. "It decreases impervious land coverage, provides a more stable load-bearing surface, and allows the water to go into the ground," he said. "It eliminates the need for detention ponds, which require additional space. And it takes off a lot of stress from the sewer systems when it's done correctly."
In addition, permeable paving can be made lighter in color, which reduces the urban heat island effect . It can be made of recycled materials, such as concrete and rubber, and by filtering the water, it removes pollutants. There are advantages for bicyclists as well: "It takes that film of water off the ground that makes it slippery," Chaffee said.
Municipalities around the country are starting to notice the benefits of permeable pavement. In Maryland, Montgomery County's "Rainscapes Reward" program  offers homeowners a subsidy of up to $1,200 for converting a driveway from impermeable asphalt.
The upcoming Cesar Chavez redesign will incorporate green street techniques as well , stretching from the 101 interchange "hairball" to Guerrero.
And San Francisco homeowners will be expected to pitch in: a new landscaping law requires that at least half of newly-constructed front lawns be water-permeable , whether through pavement, bioswales, or raingardens.
Cities like San Francisco, Chicago, and Portland can boast about early adoption of permeable pavement. The technique has long been favored in Europe (where some streets retain cobblestones dating back hundreds of years), but is still viewed with skepticism in the United States.
"They haven't quite gotten to where they're convinced that they can do streets that are low-maintenance," said Milestone's Chaffee.
Milestone imports porphyry, highly-durable paving stones that, when bonded by Propoxy , form a permeable pavement. In addition to its water management benefits, porphyry boasts another ecological advantage: reusability.
"When they have to dig up an asphalt street, they typically can't reuse that asphalt," Chaffee said. "Whereas with a permeable pavement, they dig up the cobbles ... fix whatever it is underneath, put everything back in -- including the exact same stones -- and reuse everything." Despite being called "cobbles," modern porphyry pavement isn't bumpy or roughly textured: it's ADA-compliant for wheelchairs, which means it's also easy on bikes.
Pavers like porphyry are just one of many solutions. A report by Curtis Hinman at Washington State University identified four successful surface types for constructing water-permeable pavement:
- Permeable asphalt -- Used for light traffic roads and parking
- Permeable concrete -- A well-regarded, generally-successful material
- Permeable pavers -- Best used for low-speed traffic
- Flexible plastic grid systems -- A grid filled with gravel or soil, sometimes planted with grass
Whichever method is used, it requires a thorough hydrological analysis prior to installation, since drainage and water flow can vary widely from one street to the next. It's wise to augment pavement with other ecological best practices  such as rain gardens, bioswales, and street trees, all of which absorb, retain, and help to clean storm water. Rain barrels and eco roofs also provide a significant boost to pavement-based water management techniques, which is why San Francisco's Public Utilities Commission offers subsidies and instructions  to residents interested in harvesting rainwater on their property.
Portland has also positioned itself at the forefront of innovative water management  techniques. "We can talk about all the multiple benefits that greenstreet facilities provide, but the bottom line is it saves taxpayers' dollars," said David Elkin, a landscape architect with the city's Bureau of Environmental Services. "Instead of just a patch or trench in somebody's street, we're going to leave behind a green, vegetated facility."
Chicago is taking steps to improve its urban watershed as well, with its much-admired "Green Alley" program . Featuring 5,000 alleyways comprising around 1,900 miles of pavement, the city's goal is to convert flood-prone asphalt into hydrologically sensitive conduits that include permeable surfaces, strategically-placed drainage pipes, and recycled pavement.
Of course, features such as these cost more than a simple asphalt surface. Porous concrete is estimated to cost  between $2 and $6.50 per square foot, interlocking pavers from $5 to $10. But according to Chaffee, the cost of installation is dwarfed by the alternative: deteriorating storm drains and overburdened water treatment plants that require expensive maintenance. That's consistent with findings in Chicago, where city officials observe that the high costs of improving alleyways is offset by decreased maintenance and operational costs .
Money is one obstacle to installing ecologically sound pavement. Time is another. The San Francisco PUC's water management charrettes were held three years ago, and feasibility studies continue to this day.
But perhaps the greatest obstacle is public awareness and confidence. Many people simple don't know of permeable pavement or its benefits. Chaffee believes that word will spread once more high-visibility projects appear in cities around the country. "There's going to be a tipping point," he told Streetsblog, "people are going to start seeing the benefits. It might start in neighborhoods first. And neighborhoods will push it forward through the city."