Water runoff from roof needn't harm streams

Direct it onto lawns, gardens instead of street

April 21, 2002|By Dean Uhler

Have you ever considered the possibility that water runoff from your roof is contributing to degradation of local streams and that you might be able to do something about it?

The job of rain gutters is to collect rainwater from the roof and direct it away from the house as quickly as possible. Water from downspouts is often drained directly to the street or to a driveway that drains to the street. Once in the street, water runs rapidly to storm drains, which discharge directly to a local stream or creek.

The runoff from multiple homes produces a torrent that scours out streambeds and erodes banks.

It is important to direct water away from the foundation of a house, but it is generally not necessary to get it off the site. The soil next to the house is the critical place where water should not be discharged.

Problems occur when a large volume of water is concentrated within a couple of feet of a house's foundation. Water that collects that close to the house can rapidly soak into the backfill around the foundation, in essence dumping water into the hole that was dug to build the basement of the house.

Wet basement peril

In clay soils prevalent in much of Maryland, the only outlet for that water could be the utility trenches leading to the house. Water trapped around the foundation can seep through foundation walls to cause wet basements and crawlspaces, and it can cause foundations to shift.

Rainwater that discharges more than a couple of feet from the house does not adversely affect the house if the ground slopes at least slightly away from the house to prevent water from running back toward the house. The result is that the water benefits vegetation on the property.

If you have a yard with some absorptive surface, such as a lawn, garden or wooded area, consider directing rainwater from downspouts to those areas. This will allow the water to soak into the soil rather than run off to the street, which replenishes ground water and reduces surges in local streams.

50% runoff reduction

According to the Center for Watershed Protection, directing rainwater from roofs over an absorptive surface before it reaches an impervious one can decrease the annual runoff volume from residential developments by as much as 50 percent.

How large an area is required to handle the water from a roof depends on the size of the roof relative to the size of the lawn or garden and on the absorptive capacity of the soil.

A small yard might not be able to handle all of the water from a large roof, so in a hard rain the water could run off the yard to the adjoining walkway or alley. It's all right for some of the water to do that as long as your yard isn't washing away with it.

The Center for Watershed Protection's Web site has a good presentation on the impact of runoff on streams. To see it, go to www.stormwatercenter.net and click on "Slideshows," then "Impacts of Urbanization."

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