By Brian DeCelle
In its quest to remain on the cutting edge of the environmental movement, the Little City has now imposed a “stormwater fee” on non-organic surfaces. This fee, however, is suspect as it is applied, regardless of whether a given surface adds or subtracts from the stormwater system.
Before I get into specifics, it is important to know two basic factors affecting water infiltration in the city and much of Northern Virginia as a whole.
First, native soils in the Northern Virginia area tend to have high clay content which inhibits infiltration. Even when better-draining soil is native to a site, most construction will bring in “fill dirt” which adds clay content, rendering the soil relatively impervious. Infiltration rates are calculated by how much water is able to pass through the soil over a period of time. Undisturbed clay soils absorb just hundredths of an inch of water over an hour. Due to the equipment driving over it, clay soil at a newly built home will have an infiltration rate approaching zero, and it takes decades for that soil to return to its natural state.
Second, regardless of the soil’s infiltration rate, slopes have a major effect on how much water actually enters the ground. A well-draining soil may infiltrate as much as one inch per hour on paper, but that assumes that the water is staying on site. On hilly terrain, however, water very quickly finds its way to the lowest point, often the nearest storm drain.
Certainly vegetation slows the flow of water and helps absorption to a point. Grass will help break up the soil on the top layer of ground cover, and trees will break up soil even deeper, but anyone who has witnessed a major rain event in Falls Church can attest to the streams of water flowing through their yards into the street, where they eventually make their way to our streams and rivers.
That said, buildings and pavements certainly exacerbate the issue. Water flowing over concrete or off a roof will travel at a faster rate than water on grass, causing more water to enter the system at peak times. This is where cisterns, water gardens, rain barrels, and permeable pavers come into play.
The city has wisely taken these into account when assessing the fee, however the way in which these systems are assessed appears to have no quantifiable basis. Expected rain volumes are known quantities, and storage systems can be designed to contain the amount of water expected during even intense storms. A permeable paver driveway, for instance, should take in 100% of the water that falls directly on it, along with runoff from surrounding grass and walkways. If engineered to do so, it can even act as a storage area into which you may pipe your gutters, meaning that potentially no water from your driveway or roof would enter the city’s storm drains at peak times. Zero water entering the system from these surfaces means that the overall volume of runoff from your property would be significantly less than if your lot were simply a field of grass, which appears to be the city’s baseline.
Given that a large portion of the water that falls on grass flows into the storm drain, why does the city give a 40% discount on a permeable paver surface? If we are working with a fee, a surface that has a net positive impact on the storm system should be credited as such. Charging a stormwater fee for a permeable paver area is like charging someone with a solar roof for 60% of the power that they produce themselves when they are, in fact, giving back to the power grid.
Permeable pavers are hardly the only case where the city’s system of credits seems illogical. Cisterns and water gardens can provide the same sort of relief for the storm drains. Why would these be capped at 40%? Where did the city come up with a 10% discount for water barrels? I am no expert on them, but surely a city engineer could calculate how much water can be contained by 55 gallon drum. Trees have root systems which break up soil and increase its permeability. It makes sense to incentivize trees, but why would a new tree with fewer roots be credited while an established tree with a more extensive root system would not?
Knowing a number of City Council members, I have little doubt that they intended the stormwater assessment to be a “fee.” I can understand the wish to simplify the fee into a neat package as was presented in the June 5 News-Press. However, the true impact of a surface will vary widely based on soil types and how stormwater features are engineered. What was presented last week fails to come anywhere close to taking these features into account in a reasonable manner, and since the fee does not correlate to a property’s use of the storm system, it amounts to little more than a surface tax.
Brian DeCelle has been in the construction industry for 12 years and is a board member of the Greater Falls Church Chamber of Commerce.