How to Handle Your Runoff Sensibly

Rainwater in our area is mostly sent to storm drains and channeled into the ocean. There are, however, other alternatives to this water-wasting, polluting system of handling storm runoff. Two that I like are rain gardens and infiltrators.

A relatively new concept in landscape design, the rain garden is a two- to three-foot depression, often 300-500 square feet in surface area, that gets filled with permeable materials and capped with deep-rooted native plants. Plant selection is crucial. They need to be able to withstand extremes of moisture (flooding and drought) as well as concentrations of sediment and nutrients, particularly nitrogen and phosphorus, common elements found in storm-water runoff. Good choices are plants from the edge of wetlands such as wildflowers, grasses, sedges, rushes, ferns, shrubs, and small trees. Those with deep fibrous roots tend to have a competitive advantage and provide the most cleaning and filtration benefits to the environment.

Rain gardens not only enhance infiltration but also, below ground, maintain or even augment soil permeability, provide moisture redistribution and aquifer recharge, and sustain diverse microbial populations involved in biofiltration. Above ground they add pleasing aesthetics, encourage wildlife and biodiversity, contribute to localized flood control, and reduce the need for irrigation.

The combination of the plants and porous planting media often reduces the amount of storm water and pollution reaching creeks, streams, and the ocean by 30 percent or more. The cost of creating a rain garden is small. The ensemble of benefits has led cities and regions across the country to adopt policies encouraging rain gardens.

Another option is the infiltrator chamber that can be configured to create an underground reservoir. Made of high-density polyethylene, often from recycled material, these arches, basically in the shape of an inverted U, are most often three feet across at the base and five feet in length. They interlock to form continuous drainage tunnels. It is advisable to place them on a gravel bed to aid percolation. Frequently, they run 50 feet in length with the capacity to handle large “surge” volumes of storm water and store it until it seeps into the ground beneath. Commonly buried about two feet deep, their ribbed configuration makes them structurally strong enough to take heavy loads like vehicular traffic.

Although not inexpensive to install, infiltrators cost less than permeable paving or drywells while accommodating much larger quantities of storm water. All strategies that retain and percolate storm water into the ground need to pay attention to the permeability of the soil. Infiltrators are often less costly than vegetated roofs or rainwater-collection systems, which are also strategies that keep storm water on-site.