Greening the Driveway
As we have started to come to terms with the changing environment, many of us have become aware of the choices that we make.
For the past 20 years, Earth Wizards has designed, installed and maintained various environmentally friendly driveways so we’ll share our take with you.
Just as a refresh, there are two terms that you’ll often hear. Permeable and impermeable.
Permeable, or pervious, allows water to move through, whereas impermeable, or impervious, does not.
As the built landscape has moved to a predominantly impervious nature, that means less water reaches our aquifers underground. Not only does that affect our highly-valued drinking water supply, but the damage of runoff has become detrimental to property, livelihoods, habitat, soil erosion and a host of other ills.
What type of driveway is right for me to lessen my impact?
It might seem like a simple answer but of course it never is. Let’s cover a few options and then we’ll delve into what really might make the most sense for you.
This can be done as pavers (both concrete and clay), concrete and/or asphalt. It’s important that the underlayment (gravel) is designed with a uniform-sized angular aggregate.
Think of a tubful of angled marbles. The water collects in between the marbles until the drain can move the water through.
Concrete runners work great for a single car garage. Less pavement means less runoff. What to do with the non-pavement areas? Can be grass or a low ground cover that is suitable.
Here is a combination of permeable and non-permeable pavers
Often we consider rerouting runoff onto the property; ideally to a raingarden or bioswale. Routing rainwater can be done either through creating a graded swale in the surface or a channel drain/trench drain structure.
Raingardens should be designed to handle the amount of runoff it receives and pond (6-18”) for no more than 72 hours. A bioswale is a graded vegetated channel that doesn’t pond with any significance (less than a few inches)
This type of system allows for stormwater to be collected underground. Think of a bathtub underground. Similar to raingardens being designed to function properly, underground storage units also need to be designed appropriately to collect runoff and then allow it to infiltrate into the underlying subsoils.
By randomly putting a barrel with some rock (the old “french drain”) approach, the size of the system is limited and it’s important to know at what point it will fail and how water will move on/off your property when that happens.
What other considerations should be factored into my decision?
Subsoil type. Generally sandy soils are ideal for permeable, raingardens and underground systems. This isn’t to say that it can’t be done in less than appealing soils such as a more clayey type but it gets trickier in terms of designing appropriately.
One of the most often overlooked considerations is the carbon footprint. The amount of material coming off-site and being moved in. If there’s significant excavation, as with these options there’s more than a conventional approach, that means more equipment, more trucking and more disposal.
Additionally looking at the location source of the new materials. The ideal aggregate to use in permeable pavements is an angular granite base rock as the rock is more igneous base than sedimentary so it will hold up better to the introduction of large amounts of water. For the Mpls/St.Paul metropolitan area this means St. Cloud.
What else to consider?
Budget. Permeable pavements and underground storage will be more expensive than rerouting into a raingarden or bioswale.
Maintenance. Permeable pavements will require attention to areas that get clogged. Often overhanging trees or surrounding exposed soils will contribute to this issue. At some point, it may be necessary to remove the pavers, underlying choker course and replace that top layer of aggregate and pavers. So there’s some decent labor involve either that you hire out or do yourself.
With a raingarden or bioswale, the maintenance is similar to any garden area. If you choose to be a messy gardener because it’s better for pollinators less so than the Kew Gardens attendant.
Available space/mixed use. What amount of green space might I have to install a raingarden or bioswale? Can my driveway also be used as an outdoor patio rather than two separate hardscape areas?
Pollinators. It’s the talk of late and for good reason! A native planted garden will do so much more for pollinating habitat than permeable pavements.
Then there’s your aesthetic. Your preference with how you want your property to look and feel.
The last important thing to mention is combining approaches. An entire driveway need not be permeable. Maybe it’s a portion of the driveway to be made permeable, some portion downsized and a spot for water runoff to flow into a native planting.
So many choices and this is where it becomes fun!
Start with how you want your property to function, flow and feel. All these things can be incorporated into that ideal green driveway.
And then it’s become more than a driveway but a reflection of your style, your ethics and your home to be proud of.
Visit our gallery to start the ideation!