Gravel parking lots infiltrate stormwater, don’t they? Part 2 of the Vermont Green Infrastructure Initiative’s guide to stormwater misconceptions.
This is the second post in a series about stormwater misconceptions identified by Green Infrastructure Initiative Roundtable. We hope that these posts will foster greater understanding and integration of Green Stormwater Infrastructure practices and Low Impact Development principles throughout the Vermont landscape.
At various times, I have encountered people (and more than one website) who say that gravel parking areas and driveways reduce stormwater runoff by allowing water to move through them into the ground (i.e. infiltrate). At first glance, this appears to make sense. Often one can see air spaces between gravel pieces when looking at a newly placed gravel layer. However, these gravel structures can be impervious to infiltration, thereby generating stormwater runoff. Here’s how.
Typically, gravel parking areas and gravel driveways are constructed by first removing vegetation and the top 6 to 12 inches of native soil material, grading the cleared area, and then spreading layers of gravel. Depending on the nature of the remaining underlying soil, construction activities may include active compaction of the soil to provide support for the gravel and vehicle traffic above it. Regardless of whether site preparations include active soil compaction or not, the mere activity of typical construction equipment can compact the soil enough to cause runoff problems. As mentioned in a previous post, stormwater runoff can be generated from soils that have been compacted regardless of whether pavement is laid upon it or not.
To predict the magnitude to impacts from soil compaction and the potential for runoff, it helps to understand the make-up of underlying soil itself. In its natural undisturbed state, soils have varying abilities to allow water to infiltrate downward. The permeability of soil depends on its: composition (i.e. the relative amounts of particles of different size), structure (i.e. the arrangement of particles and empty spaces between them), amount of organic matter within it (which can hold on to water like a sponge), and slope of the landscape. Soil becomes more impervious (resistant to infiltration) as particle size decreases, structure contains less empty spaces, percent organic matter increases, and slope increases. In other words, a clay soil (made up of very small particle sizes) on a steep slope will produce more runoff than a sandy soil on flat ground. Soil compaction changes soil structure by rearranging and smooshing the particles together and greatly reducing the amount, size, and the connectedness of empty spaces through which water can flow (i.e. soil porosity).
Appearances can be deceiving. Newly constructed gravel parking areas may appear to infiltrate precipitation by virtue of the amount water storage available in the spaces between gravel particles. After a moderate rain, the rainwater gets contained within the gravel bed. However, over time, grading and adding more gravel to the surface compact the underlying subsoil. Vehicle traffic over time will deposit more sediment, further reducing any remaining porosity of the gravel structure. This results in a parking lot that is nearly as impervious as paved parking areas with no water storage capacity.
Because of the many types of soil, there are examples of gravel parking lots and driveways that do infiltrate stormwater. If constructed on sandy sub-soils, for example, soil compaction will not generally render a parking lot impervious. However, if constructed on more compactable sub-soils such as those composed of mostly clay and/or silt, gravel lots and driveways can be designed and constructed with an underdrain system that collects precipitation and conveys it to a GSI Best Management Practice that treat and infiltrate stormwater, such as bioretention. A more durable and sustainable option than a gravel surface is pervious pavement that is specifically designed to maintain its ability to infiltrate through required maintenance (vacuuming) and specific design standards.
Gravel parking areas and driveways are not unique in their susceptibility to soil compaction. Fortunately Low Impact Development principles that consider hydrology, soil science, natural resources management, engineering and landscape design provide us with options to minimize the impacts created by soil compaction and impervious surface construction. They include:
- Retaining vegetated areas to the maximum extent possible;
- Minimizing the size of impervious structures to fit the need;
- Employing soil restoration methods like tilling and amending to ensure re-establishment of native plant cover; and
- Managing stormwater on the site by selecting a set of appropriate stormwater solutions with Green Stormwater Infrastructure.