This article was originally published in the August 2017 Princeton Echo.

Views of a rain garden designed by landscape architect Carolle Huber for a family on Cradle Rock Road.

You’re thinking about replacing that porous gravel driveway, prone to weeds in the summer and difficult to plow after a winter snowstorm, with a sleek new blacktop drive. Or that patch of lawn behind the house, you might imagine, could be an outdoor patio, with handsome pavers underfoot.

Before you call in the contractor you might first check with the township engineering department. Your home improvement could be the town’s “minor development.” And if it covers more than 400 square feet of ground (a 10-foot wide driveway leading 40 feet from the curb to the garage, for example), it would be subject to the municipal ordinance enacted in June that requires two gallons of stormwater retention for every square foot of added impervious surface.

In other words, that 400-square foot driveway or patio — classified as impervious along with wooden decks and other outside amenities — will require a retention system with a capacity of 800 gallons.

Environmentalists in town argue that stormwater runoff is the leading cause of water pollution, eclipsing “point source” pollution discharged from industrial sites. When a storm hits an urbanized area, runoff from aggregated lawns, driveways, patios, rooftops, and various other impervious surfaces picks up pesticides, fertilizer, and road salt as it gushes down the drain grate directly into our waterways.

Often the deluge (and the ordinance is based on the likelihood of a two-year storm event) overwhelms the storm water drainage system, flooding streets and basements.

If you think it’s not a problem in a relatively low density community like Princeton, you should think again, they argue. The Stony Brook Millstone Watershed Association, which assisted Princeton’s municipal staff in writing the new ordinance, reported that between 1995 and 2012 the impervious surface in the watershed area increased by 30 percent.
One advocate of the ordinance is Sophie Glovier, a 1987 Princeton alumna, the author of “Walk the Trails In and Around Princeton” (with a second edition recently published by the Princeton University Press), and also a member of the board of the Watershed Association. “I’ve been on the board for about seven years,” she says. Stormwater problems were not a major item of discussion at first. But “in the last five years or so everyone in town seems to have really noticed,” she says.

The presentation prepared by the Watershed Association showed the passenger tunnel under the railroad tracks in Princeton Junction flooded after Hurricane Irene. Just last year, after a late July deluge, streets in Princeton that had never previously flooded were flooded. One was tiny Branch Alley between Park Place and Willow Street, which — residents noted — had just been paved over by the town with, yes, impervious blacktop.

The advocates of the new ordinance believe in “green infrastructure,” designed to slow the flow of water to stormwater drains by instead retaining it onsite, allowing natural processes to reduce flooding, improve water quality, and recharge underground aquifers. The ordinance encourages the use of “green infrastructure” such as rain gardens, green roofs, and vegetated swales. Traditional water storage techniques such as an underground dry well are also permitted.

“Rain garden,” says Glovier, “is a fancy name for a depression in the soil” that you create in a low spot in your yard. To accommodate the 800 gallons of run-off from that hypothetical patio or driveway mentioned above, you would need a rain garden of about 110 square feet with a depth of about one foot — a single cubic foot holds roughly 7.5 gallons of water. (The Rutgers Agricultural Experiment Station estimates a cost of $5.5 per square foot of rain garden, though this does not include any outside labor or excavation equipment.)

If the soil isn’t already sufficiently absorbent to slowly drain the collected water after a storm you can improve it by digging a little deeper and adding a few inches of humus or other organic material. When it rains, the water will collect in the garden, filled with native plants that don’t mind the extra water but can also tolerate dryness.

Examples of rain gardens in Princeton can be seen at the Mountain Lakes House and at the new PNC Bank branch at the Princeton Shopping Center. The Watershed Association in Hopewell also has a demonstration rain garden.

Various homeowners in Princeton already have rain gardens in their yards, as part of eco-conscious landscaping. Carolle Huber, a Morristown landscape architect who designed a rain garden for a family on Cradle Rock Road in the former township portion of Princeton, will be one of the presenters at a discussion of the new ordinance on Wednesday, October 18, at 7 p.m. at the Public Library. Other presenters will be architect Kirsten Thoft and Watershed stormwater specialist Kory Kreiseder. The event is co-sponsored by the Princeton Environmental Commission and the Watershed Association.

Individual homeowners aren’t the only ones being asked to monitor their stormwater drainage. Larger commercial or residential development that add more than 5,000 square feet of impervious surface or disturb more than half an acre of land are considered major developments, a designation that triggers a host of engineering and system design requirements. For these developers the Princeton requirements exceed those of the state, which require stormwater management for projects that add a quarter-acre of impervious cover or disturb an acre of land.

More stormwater regulations could be looming. The recently passed ordinance pertains to new impervious surface, but much of Princeton’s developed land was constructed before the advent of stormwater management. A redeveloper tearing down and replacing an existing impervious footprint, built without considerations of storm runoff, currently does not have to account for stormwater impact.

“The Council and the Planning Board are interested in addressing redevelopment,” says Michael Pisauro, the policy director of the Watershed Association. “The Watershed’s position is if you don’t address the historic issue, you won’t have a reduction in stormwater runoff and improvement in water quality.”