Hydrologic Modification to Urban Settings

What changes could be made by city planners to protect water quality? What would encourage developers to leave more natural areas? What would be the economic impact?

What do you see when you look at the landscape where you live? If you live in a town or city, then houses, pavement and parking lots have most likely replaced forests and fields, changing the view and the area’s hydrology–the way that water moves across the land.

While the act of developing an urban area can have dramatic short-term effects on water quality, the most enduring effect of urban hydrologic modification is the creation of a huge imbalance between pervious and impervious surfaces.

• Pervious surfaces allow water to soak in and include lawns and parks. These areas filter runoff, catching pollutants before they get into waterways.
• Impervious surfaces resist water and include asphalt, concrete and even building roofs. Water simply runs off these surfaces, moving toward the lowest point, as water always moves. In its journey to the lowest point, the water can pick up anything that is on the ground: trash, sediment, gas, oil, chemicals, paints, the list goes on and on.

Runoff in an urban setting normally doesn’t get sent to a wastewater treatment plant. The runoff either moves into storm sewers which carry it to streams or rivers, or it just keeps moving across surfaces, eventually making its own way back to a stream or river. Either way, it takes pollutants with it. Since it is impossible to track pollutants back to their source in this scenario, urban runoff is considered to be non-point source pollution.

What can be done? Probably the most important step is to increase the amount of water that soaks in to the soil. While it is important to keep heavily traveled roads as hard surfaces, the other impervious surfaces don’t have to stay that way.

• Install rain barrels to catch water from roof gutters.
• Use gravel, pervious asphalt or grass paving on low use areas like driveways and highway shoulders.
• Consider specially designed vegetated roofs instead of shingles for house tops.