When it rains buckets, a dry well or catchbasin helps to moderate extreme runoff from downspouts, driveways, and patios. It also reduces the water's velocity and energy, channels it away from the house, prevents erosion, and helps the soil absorb water.
A dry well is simply a wooden or plastic box buried in the ground under a downspout, with or without a bottom, and with a drainpipe to redistribute excess runoff. Dry-well boxes have a removable grate on top that allows runoff into the basin while making it easy to remove accumulated silt and debris. The grate also keeps animals from falling into the box.
Commercial catchbasins, made of heavy-duty plastic, are usually 9 or 12 inches square and 12 inches deep, with square grates and solid bottoms. Fittings on the side of the box adapt to a 3- to 4-inch-diameter flexible drainpipe to direct the water far from the house or erodible slopes. Catchbasins with grates (about $40) can be found at landscape or lumber supply outlets.
A better method is to build your own dry well. Homemade boxes can be larger than store-bought models (which often are overwhelmed during heavy rains) and can have open bottoms so water percolates into the soil during periods of light rain. Construct a 2-foot-square box from 2-by-6s or plastic wood. On one side of the box, cut a 3-inch-diameter hole 6 to 8 inches from the bottom; fit a 3-inch-diameter flexible drainpipe into the hole, fastening it with pipe sealant. Dig a trench, and run the pipe away from the house to a swale or low area where the water can drain. Line the bottom of the box with 2 to 3 inches of 1-inch-diameter stone or gravel. Custom-cut the slats for the grate from metal rebar or wood to fit the top of the box, spacing them 1 to 2 inches apart.
In the fall, clean dead leaves and other debris out of the bottom of the catchbasin or well so the outlet pipe doesn't clog.