How To Use Landscaping To Deal With Storm Water

How Landscape Drainage Works

Since these yard drainage ideas allow storm water to sink into the soil gradually, they help reduce flooding. Plus, they allow pollution, including oily residue from cars, to filter out naturally, so it doesn’t wind up in lakes or streams. “You can cure your wet-basement problem and do something for the environment at the same time,” says This Old House landscape contractor Roger Cook. Here are 3 drainage systems you can create in your landscape and yard:

1. Swales

Swales are depressions that follow the contour around the base of a slope (natural or created), channeling storm water from one place to another. They filter runoff along the way by allowing it to sink into the soil. Plants on a swale’s gently sloping banks—and sometimes down the center of the ­channel itself—take up much of this water. Fast-draining soil is also key. The ­addition of a ­perforated pipe laid in gravel underneath can help ­handle heavy water flow. A small swale might carry gutter water from a house to a dry well, while a more substantial one could run along the base of a hill above a low-­lying house to divert water around it. Jan Johnsen, a landscape designer in Mount Kisco, New York, ­often landscapes swales by lining them with river rock. Along the sides, she uses evergreen ferns, sedges, winterberry, grasses, and Siberian and Louisiana irises that thrive in moist conditions. Rugged prairie plants or other natives that are at home in fast-draining soil are another option.

Creating a Swale

You can build a small swale yourself, but for a long, wide one you’ll want to hire a pro with earth-moving equipment. Consult a landscape contractor or a civil engineer if you live near a bluff, have a septic tank nearby, or are on a slope that drops more than 1 foot over a horizontal distance of 20 feet. A swale should carry water to a place where it can be released safely, such as a garden bed with good drainage or a buried dry well; allowing it to be absorbed on-site, rather than flow into a storm drain, is important for protecting natural waterways. The sides of the swale should flare so they ­extend out three to four times more than they are tall, and the first 8 inches of soil should drain well. If your soil drains quickly (at least ½ inch per hour), it can just be loosened. If water sinks in half that fast, amend it with 40 percent compost. Where drainage is ­slower still, replace soil with 60 percent screened sand and 40 percent compost. If the swale itself can’t be made big enough to handle all the water, consider excavating another 6 to 8 inches, lining the trench with filter fabric, laying perforated pipe, then covering it with round ¾-inch gravel. Top the gravel with at least 8 inches of a compost-rich soil mix.

2. Rain Gardens

Shallow catch basins planted as flower beds, rain gardens allow water to pool during a downpour, then slowly percolate into the soil. Where a swale is mostly a travel route for water, a rain garden is a destination. A fast-draining soil mix encourages water to sink in and promotes lush plant growth. Runoff may flow into a rain garden from a swale or pipe, or may simply run in from a sloping yard.

Add a Drainage System Under Raised Decks

Backyard Drainage Solutions

Rain gardens are appropriate drainage spots for steeper slopes than swales can handle, but where the surface drops more than 3 feet over a 15-foot horizontal distance, you should get professional design help. Although a low spot in the yard might seem an ideal placement, if it stays soggy, it’s already saturated. Instead, pick an area that dries out quickly.

Calculations and Design

Designing a rain garden to handle all of the runoff from a roof or driveway entails careful calculations. But you can also learn by experimenting: Build one, watch what happens after a storm, and then enlarge it as needed. Locate a rain garden at least 10 feet from your house and at least four times that far from a septic system or steep slope.

Creating a Rain Garden

Though you can excavate a small (say, 5-by-10-foot) rain garden yourself, a landscaper with an earth-moving machine will get it done faster. Make sure machinery stays along the edge of the bed so it doesn’t compact the soil as it digs a wide ­depression about 2 feet deep with gently sloping sides. Mix in compost and sand, as needed, using the same proportions as for a swale. The end result should be a shallow basin with about 6 inches of “ponding depth,” or space for water to pool while it drains through 1 to 2 feet of amended soil. Plant the center of the area with species that tolerate wet conditions, such as native sedges and lady fern. Around these, put plants suited to occasional standing water, like redtwig dogwood. At the furthermost edges go plants that prefer ­drier soil, such as native evergreen and deciduous shrubs.

Anatomy of a Rain Garden

A: Water flows in from slope, paved area, or pipeB: Wide, shallow basin with flat, level bottom and gently sloped sides C: 12 to 24 inches of fast-drainingD: 2 inches of mulch E: 6 inches of ponding depth F: Berm or stones to stem or slow down overflowG: Perennial plants

3. Pervious Paving

Paving materials that incorporate small gaps allow ­water to seep through into quick-draining gravel layers underneath. This keeps the top surface dry, eliminates runoff, and lets water in the gravel layers gradually sink into the soil. There are three basic types: concrete pavers with voids in between to be filled with gravel or sand (with or without grass); porous concrete or asphalt made with little or no sand so there are built-in air pockets; and plastic grids that keep a surface layer of gravel or sand (with or without grass) from compacting, so water drains through.

Materials and Installation

Pervious paving is suitable for gentle inclines that slope no more than 1 foot over a horizontal distance of 20 feet. Though a small-scale paver or grid-system project like a pervious walkway or patio might be a DIY job, you’ll likely need professional design help for a driveway, where soil is heavy with clay or freezes deeply, or if the job is tied into a building permit. Pervious ­concrete (shown) and asphalt (which cost 20 percent more than their conventional counterparts) always require professional installation. Plastic-grid systems are the most DIY-friendly, requiring a gravel base layer of only 2 to 6 inches, compared with about a foot for the other systems. The base can be tamped with a plate compactor, which you can rent; the material is simply rolled out and pinned down along the edges; and spaces are filled with decorative gravel or sand, plus grass seed, if you like.

Paver Spacing

As more communities limit the amount of impervious surfaces (rooftops, conventional hardscape) allowed on residential lots, interest in pervious paving has skyrocketed. “It can literally be a trade-off between installing a pervious driveway and adding a room,” says designer Jan Johnsen. “Or a question of whether you can build that new patio at all.” In those cases, pervious paving is not only an attractive way to deal with runoff, it’s a double-duty enhancement, one that lets you keep on improving your home while you safeguard its natural environment. Shown: Some concrete pavers have larger spaces to hold sand and turf grass, which absorb some storm water near the surface.

Anatomy of a Quick-Draining Driveway

A: Concrete pavers with spaces in between for gravelB: Border paver (optional)C: Edge restraintD: 1½ inches of 1/8- to 3/8-inch crushed gravelE: 4 inches of ½- to 1-½-inch crushed stone extending 6 inches past drive to handle overflowF: 4 to 5 inches of 2- to 3-inch crushed stone, as neededG: Geotextile (optional)