Paved with good intentions

How to keep rain going down, not sideways

Back when I asked my research assistant to work on the permeable pavement topic, he quickly pointed me to some right in the neighborhood.

When a raindrop or snowflake falls on an American town, what does it hit? In Providence, RI researchers found that only about 40% of that precipitation lands on grass, plants or gravel surfaces that allow it to seep through to underlying layers of soil, sand and rock.

The other 60% of drops and flakes land on asphalt or cement or rooftops, from which they flow mostly horizontally, becoming urban stormwater. The high amount of impervious surface in towns is a big contributor to urban drainage problems during storms. Flooding is the main threat, but once water starts traveling horizontally, it picks up oils and sediments and becomes a big source of local water pollution as well.

Climate change is increasing the intensity of storms, and city engineering departments are coming up with ways to protect local residents and infrastructure. We may see some of these adaptation measures just walking around our neighborhood.

Advantages of permeability

The purpose of permeable surfaces is to help more precipitation get down through the asphalt or cement surfaces and reach the gravel and soil. If we can get water to flow, not over but through concrete, there are lots of advantages.

• There’s less run-off, so less urban flooding, less erosion. Compared to asphalt or concrete, carefully installed pervious pavements can reduce the volume of stormwater by as much as 90 percent.

• The process can also filter contaminants out of the water as it seeps through the soil.

• Less urban flooding means less water flowing into the street sewer system. In towns where that sewer combines with the sanitary sewage system, the chances of a Combined Sewer Overflow (nasty and expensive) are reduced.

• Where storm drains have blockages and overflows, it can look better and cost less to reduce the run-off with permeable materials than to reconstruct the drain.

• To a small extent, water moving down through the soil rather than sideways into man-made drainage could actually reach and replenish a nearby aquifer, fighting the depletion of groundwater.

• When permeable pavement reduces flooding, it reduces the costs of flood repairs and cleanups, as well as lowering safety risks for drivers and pedestrians.

Water through concrete? Yes, it’s happening, and we can see some of the materials on sidewalks, driveways, playing surfaces, city and corporate properties, and housing developments. Increasingly they’re being used on urban streets.

What is the stuff?

It’s hard to imagine a surface that’s strong enough to support moving cars and trucks for years without cracking or crumbling, yet able to let water seep quickly down through it. But those materials are available and improving every year.

They come as pervious concrete and asphalt which can be laid down continuously, just like regular concrete and asphalt. And they come as pallet-sized slabs, interlocking pavers, or simple bricks between which water can descend.

Costs

Maintenance costs are said to be low. For surfaces like clay bricks, vacuuming debris from between them helps maintain the water flow. And replacing or leveling inexpensive aggregate stones underneath may be necessary in some places after a period of time.

Many of these pervious materials include recycled plastics The easy availability of lightweight plastics at a low cost helps make the manufacture of pallet-sized permeable surfaces cheaper. And their lightness can reduce shipping and transportation costs.

Another advantage of pervious pavement: it can be used for temporary purposes and then be recycled and reused. This makes the process affordable and reduces the negative impact of single-use plastic on the environment.

And hey, for anybody who plays or supervises sports, pervious materials on tennis, pickleball, or basketball courts or playgrounds make them available for use quickly after the rain stops.

One project that did a life-cycle analysis was a street in New Albany, Ohio. They concluded that the use of permeable clay brick pavers cost less than conventional asphalt and storm drains, after the expected cost of maintaining the asphalt and pipes in future years was included.

Limitations

• In areas where particles or oil can be tracked onto the permeable pavement surface, e.g. from adjacent gravel driveways, more frequent street vacuuming of those areas may be required.

• The permeable materials are less effective and durable on slopes of 5% or more.

• For interlocking pavers, the thin aggregate installed between paving blocks may need to be periodically replaced as it becomes loose or is pulled from between blocks.

• For bicycling surfaces, porous asphalt or concrete are safer than interlocking pavers, which may settle over time and feel bumpy to bikers.

These new surfaces are another way municipalities are working to protect residents from flooding and pollution due to climate change. The laid-brick pattern of pavers is easy to spot; pervious asphalt is not. But it sure is easy to see it if we walk around town in the rain watching for almost-dry pavement!

What we can do

If our driveway drains into the street, we can change to a permeable surface—and maybe urge our neighbors to do the same. But, as with many local initiatives to help our town adapt to climate damage, we are heavily dependent on City Hall. We can

• ask the Engineering or Public Works Departments what they know about permeable sidewalks and other surfaces, and whether they’re evaluating its use;

• ask to see any studies completed. Sometimes a previous administration did some useful spadework that got put on the shelf; and

• find a small area in town that holds an annoying level of water after a rain - maybe part of a street, or a sidewalk or sports court. If we take photos, we might help the residents nearby ask the town to make it a pilot project for pervious materials. Then nag City Hall about the idea.

And we should remind the kid who shovels our pervious driveway that the surface doesn’t help remove snow, only snowmelt.