John Snitzer
Poolesville Green column for the Monocacy Monocle, February 2024
What is pervious paving? Why might it be important? Is it the key to a brighter tomorrow?
Before Maryland was settled by Europeans, our land was mostly forested. Annual precipitation was about the same as now, 40 inches per year, but only about 10%, or four inches per year “ran off” in the sense of water flowing across a patio or running down a gutter. Roughly 90% of the rain and snow evaporated, or soaked into the ground, or was taken up by plants. Groundwater was more abundant, and streams ran higher in the summertime. Flooding was infrequent because of the ability of the landscape to absorb and process lots of water.
We have changed all that. As settlers transformed forests into fields and roads and towns, they increased stormwater runoff so that same 40 inches per year now produces 20 or 30% runoff or even more. In developed areas, modest rain now fills little creek banks full. Flooding is more intense and more frequent. Because infiltration is reduced, the late summer flow of our rivers is decreased by the lack of groundwater.
There are simple ways to attack this problem. Excessive runoff is caused in part by impervious surface—surfaces that shed water like roads, or concrete patios that do not soak up any rain at all and allow it to rush away. Pervious paving, which soaks up stormwater, can be a fine alternative to asphalt and concrete. Walkways, patios, and driveways can be constructed to be attractive paved spaces that are still water-cycle-friendly.
Let’s start by looking at traditional impervious paving — say, the road in front of your house. If you were to take a pickax or a backhoe and dig a hole in your street (don’t try this at home, kids), you would see that the top layer is a hard-wearing layer of asphalt or concrete. It is solid, with no pore space for water to move through it. Underneath that is a 8 to 24 inch layer of well-graded crushed stone or gravel. But this is specially engineered gravel: “well-graded” means that it has a mix of particle sizes, with big chunks and medium chunks and lots of tiny bits, or “fines”. The little bits fill in the open spaces between the big chunks, it is then densely packed, and has no pore space to allow water infiltration. After this compacted gravel, you would hit soil, but again it is densely compacted with little or no open pore space. You should sense a pattern here. The dense soil supports the compacted crushed stone which supports the solid paving layer on top. Very good at bearing heavy loads, but impenetrable. The water that used to filter down into the water table in the forest now runs off down the hillside and the flooding begins. Soil, nutrients, and pollutants are carried along, streams run muddy, and rivers start to overflow their banks.
Fortunately, this can be changed. Paving can be reimagined to be pervious, to absorb and even to store stormwater until it can soak into the soil. Starting from the middle layer, the gravel that supports paving does not have to be solid. Open graded gravel (vs. well-graded, above) has only big pieces of crushed stone, with no fines. When it is compacted, it can carry a heavy load but still has about 30% open space. That pore space not only allows water to move through, it also holds water and allows it to soak in after the storm has passed. This layer can support the paving on top because gravel spreads out the load of a pedestrian or a car tire or a truck.
Our load-bearing layer is now permeable, but what about the soil underneath? Undisturbed soils in our area will support 1500 to 2000 pounds per square foot and have extensive pore spaces. If the gravel layer is thick enough, the loads on top can be spread out so that uncompacted soil underneath is fine. It can retain its open structure so that water that makes it down through the gravel can soak into the soil over time.
Fixing the top layer is all that remains. Instead of a solid sheet of paving, we can use pavers with joints that allow stormwater to drain between the solid pieces. The surface remains solid—well suited to be a patio or a walkway. With a thicker layer of crushed stone and more robust pavers, pervious paving can support roadway loads.
Many materials work well as pavers. Block manufacturers make a wide range of styles of interlocking concrete pavers. Brick companies make pervious clay pavers. Granite cobbles or Belgian block, flagstone, river cobbles, and unconventional materials like cast iron tree grates can be embedded into crushed stone to make attractive functional patios and walkways. Recycled and mixed materials can be beautiful. The old concrete slab that was removed can be cut up into blocks and reused as part of a pervious pavement (the au courant term for reused concrete is “urbanite”). Worn concrete can look fabulous mixed with red clay pavers or rounded river rocks. In addition to allowing more creativity, these assemblages of materials are very functional.
My old landscaping company built two different pervious paving areas as part of a demonstration garden at the US Botanical Garden years ago. The garden was in place for two years and had over 600,000 visitors, many lifetimes worth of load for a backyard patio. When we took the project apart after two years, the pavers, the gravel underneath, and the soil below were all pristine. Unlike a poured concrete surface, pervious paving can be easily changed or moved as required. So, your new pervious walkway can be altered over time.
Montgomery County, through the Rainscapes program (https://www.montgomerycountymd.gov/water/rainscapes/index.html) , supports pervious paving projects by sharing some of the costs, particularly if it replaces impervious paving. The suppliers of stone and brick in our area have a wide range of materials and can give technical support. It is a technique that is suitable for reasonable scale DIY projects or can be installed by many landscaping and paving contractors.
As climate change leads to more extreme storms and flooding in our area, pervious paving is a way both to counter it, by reducing the loss of soil carbon in runoff, and to reduce its negative impacts by helping the soil to absorb and hold more water. So yes, pervious paving is the key to a brighter tomorrow, with greener, lusher landscapes, healthier watersheds, and nice outdoor spaces for you and your friends and family.