Ross will require residents to curb stormwater runoff caused by residential building projects.

The Town Council approved design guidelines at a meeting Thursday. The rules apply to residential projects that increase the total area of impervious surfaces to reduce localized flooding.

The town’s municipal code already requires residents with permitted construction projects to mitigate increases in stormwater runoff. The new guidelines detail how to mitigate runoff from projects that add nonporous, hard surfaces.

“So when you introduce impervious surfaces, the rain can no longer sink into the ground,” Public Works Director Richard Simonitch said. “It collects on the hardscape and can runoff into the streets and into your neighbor’s rear yard.”

State law requires runoff be mitigated on projects that create over 2,500 square feet of impervious surfaces. Projects in Ross rarely trigger the state law, according to the staff report.

“The guidelines that we’re presenting tonight are for the smaller residential projects of less than 5 acres, which there aren’t really any guidelines that I know of that address the small watersheds,” Simonitch said.

The more impervious surfaces, the less water can seep into the ground and enter the watershed. Stormwater runoff can carry pollutants to the waterways and often increases local flooding and erosion.

The guidelines identify impermeable surfaces as non-native features that were not designed to allow water infiltration, such as asphalt, roofs, pools, spas, walkways, bricks and artificial lawns. Some non-native features are permeable, such as stone or gravel walkways, if they are placed directly on the ground with no cement grouting.

“These are very helpful guidelines because it’s clear what’s impervious and what isn’t and how you mitigate,” Vice Mayor Elizabeth Robbins said.

To mitigate increases in runoff, the guidelines state projects using permeable pavers or turf should feature a rock-filled chamber below to filter stormwater. Also, projects adding impermeable surfaces should route stormwater runoff into a bioretention basin.

Simonitch said bioretention basins are the only feasible way to reduce the runoff created by increased impervious surfaces on a property. He said evaporation and infiltration techniques in Ross Valley aren’t practical due to the mild climate and clay soils.

“So we have to have a way of retaining the water in a pond, sort of like a bathtub, and have it release slowly so that the runoff is mitigated,” Simonitch said.

Some bioretention basins can mitigate stormwater runoff for up to a 10-year storm event while also providing water-quality benefits. The size of the system is based on the square foot area of impervious surfaces before and after the project is completed, Simonitch said.

Councilmember Mathew Salter abstained from the vote after expressing concern about the cost to residents. He asked for estimates on a bioretention system. Simonitch said he believed systems constructed from rocks and bio-material should be relatively inexpensive.

“I do think it would be helpful, as you were talking about these projects, to ask the applicants, ‘how much did it cost you to comply with this?’” Salter said. “Because my fear of some of these ordinances we pass is we don’t know the unintended consequences or the cost and they can come back to haunt us.”