Need to Reduce Impervious Cover to Prevent Flooding, Protect Water Quality

One of the most thought-provoking articles I have read lately is “The Need to Reduce Impervious Cover to Prevent Flooding and Protect Water Quality.” This brief, well-written article brings many flood-related issues into sharp focus. The Rhode Island Department of Environmental Management produced it. And the National Park Service helped fund it. Even though many of the recommendations would not fly politically in Texas, one might. The report is worth reading just to understand the factors that contribute watershed degradation and their relationship to each other.

Central Role of Impervious Cover In Variety of Issues

One main premise: As impervious cover rises above 10% there is almost always a measurable loss in water quality.

  • Between 10% and 25% impervious cover, these impacts increase, and pollution and flooding become evident.
  • 25%-plus impervious cover creates water quality impacts so severe that it may not be possible to restore water quality to pre-existing conditions.

The report claims that by keeping overall impervious cover below 10%, towns can ensure that land will be able to absorb and filter runoff from developed areas. This, they say, will also prevent excessive flooding, ecosystem impairment and contamination of water supplies.

A second major premise: Because water spends less time on site, infiltration declines dramatically. This can reduce groundwater in urban and suburban areas because there is not enough rainfall soaking into the ground.

The increased runoff that occurs during this process reduces groundwater recharge AND dramatically increases erosion.

Relationship Between Cover, Runoff, Other Measures

According to the EPA, under natural forested conditions, only about 10% of precipitation runs off the surface of a site. Another 50% soaks into the ground. And trees and other vegetation take up a surprising 40% and send it back into the atmosphere through the process of evapotranspiration. This protects the watershed and water quality.

But higher rates of runoff can impact and degrade them. With increasing development, both the rates of infiltration and evapotranspiration decrease as runoff increases. On average, runoff increases more than 5X between natural and fully developed conditions. But extremes can be much higher. See below.

Increases in Runoff Rates

Total runoff for a one-acre parking lot is about 16 times that produced by an undeveloped one-acre meadow.

The Need to Reduce Impervious Cover to Prevent Flooding and Protect Water Quality

Higher rates of runoff create several types of impacts:

  • Hydrological
  • Biological
  • Chemical
  • Physical
  • Health
Hydrological Impacts

The report sums up the hydrological impacts by saying, “Because the water is spending less time on site, infiltration declines dramatically. This is a particular concern in many urban and suburban regions, where groundwater has been reduced because there is not enough rainfall soaking into the ground. The increase in runoff that occurs during this process, combined with the loss of recharge to groundwater, has dramatic impacts on streams.”

Biological Impacts

Biotic integrity is the most sensitive indicator of impervious cover according to the report. “The decline of biological indicators is the first sign of stream degradation, and has been the most commonly studied result of increased impervious cover. As a result of a high percentage of impervious cover, naturally occurring aquatic insects, wetland plants, and amphibians decline and are gradually replaced by species that are adapted to pollution and flooding. … Impacts on overall biotic measurements were seen within a range of 3.6% to 15% impervious cover; the threshold for fish population health ranged from 3.6% to 12%, and macroinvertebrate health declined between a range of 8% to 15%.”

Chemical Impacts

“Impervious surfaces such as roads, driveways and parking lots collect a variety of chemical pollutants and hydrocarbons and discharge them to aquatic systems with every heavy rain.”

“The study found a strong correlation between water quality and percent impervious cover across a range of contaminants, including organic residue, nitrogen, phosphorus, dissolved chloride, and fecal coliform. In each case, as impervious surface increases so does the contaminant of concern.”

Physical Impacts

“Development of impervious cover in a watershed can happen so quickly that stream systems can’t adjust, resulting in erosion of stream banks and alteration of the stream bed, which tends to become straighter, deeper, and more U-shaped.”

“This…sends silt downstream, creating further damage.”

Natural portion of Ben’s Branch downstream from Woodridge Forest. Note heavy erosion and loss of trees. HCFCD spent most of 2019 and part of 2020 cleaning sediment out of the lower reaches of Ben’s Branch and will start another segment in January.
Health Impacts

“There is a strong correlation between increased impervious cover and increased risk to human health. A variety of chronic and acute illnesses are caused by microorganisms that either are swept into water bodies by increased runoff, or flourish because of increased nutrient pollution.”

“People can contract these illnesses through direct contact or through the consumption of tainted seafood.”

Smart-Growth Strategy Reduces Impervious Cover

So what are people to do. We need places to live. As population grows, so must developments.

The next sections of the report deal with strategies to control the growth of impervious cover. Most amount to fighting words in Texas, i.e., regulation and zoning. So here, I will only cover one strategy that the report discussed; it’s market based.

“Generally speaking, as density increases,” says the report, “the amount of impervious cover also increases. However, the overall pattern of development is also important.”

The next part of the report is counter-intuitive. It quotes the EPA, “…the large-lot zoning currently used to accommodate growth requires houses to be far apart, creating unnecessary impervious cover and encouraging more off-site impervious infrastructure, such as roads and parking lots.”

“Moreover, many of the surfaces remaining after large-lot development that are believed to be pervious actually behave like impervious surfaces. Research indicates that the volume of runoff from highly compacted lawns is almost as high as from paved surfaces.”

“The solution is to maintain the overall density [by] encouraging the use of more compact growth techniques that can reduce impervious cover on a per unit basis.”

“…by greatly reducing roads, utilities and other infrastructure costs, this approach can be profitable for developers while reducing house prices for consumers.”

Visually, the strategy looks something like this.

Food for thought as we turn over a new year. As land prices escalate, the market is driving new development in this direction anyway. Land now comprises 40% of the cost of a new home. A homebuilder told me it’s the single largest component of the cost of a new home.

But when I look at Scenario C, it raises a question. What’s the incentive to preserve the open space around the development?

Posted by Bob Rehak on 12/31/2020

1220 Days since Hurricane Harvey

The thoughts expressed in this post represent opinions on matters of public concern and safety. They are protected by the First Amendment of the US Constitution and the Anti-SLAPP Statute of the Great State of Texas.