Tag Archive for: impervious cover

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.

Growth of Impervious Cover Across Political Boundaries, Watersheds Complicates Flooding

In December, the New York Times published a story about a company called Descartes Labs, which had trained computers to scan satellite images to detect changes in impervious cover. Descartes found that Texas had 9 of the top 20 counties in the US when ranked by the growth of impervious cover. So I contacted them to learn more.

A Better Understanding of the Planet

Descartes positions itself as a data refinery for satellite imagery. They process images from the major NASA and ESA satellite constellations at scale, creating a digital data twin of the entire planet that monitors the whole earth, in near real-time. Their mission: to better understand the planet.

The red dots in the map below show land that is newly covered in concrete or rooftops. It is a form of “heat map” that shows the hottest areas for growth. Studying this map, we can learn several things:

  • Stories about the demise of suburbs and exurbs after the real estate crash in 2008 proved short-lived. From the numerous rings around major metropolitan centers, you can see that growth outside of major metropolitan areas continues.
  • Texas appears to have the greatest increase in density of any state.
  • Far more growth happened in the East than the West.
Growth of impervious cover in continental US from 2008 to 2018. Copyright ©2020 Descartes Labs. Used with permission.

A Closer Look at Texas

This next map, also courtesy of Descartes, zooms in on Texas and surrounding states. The yellow dots simply correspond to the names of metropolitan centers.

Growth around Houston seems fairly uniform, though my eye does detect slightly more weight to the north, west and south than the east. This will likely change in the next decade with the extension of the Grand Parkway toward the east.

Growth of impervious cover in Texas and surrounding states from 2008 to 2018. ©2020 Descartes Labs. Used with permission.

Factors Contributing to Flooding

Several factors contribute to Houston’s reputation for flooding:

  • Rapid population growth and corresponding growth of impervious cover, as Descarte showed. The impervious cover causes floodwaters to concentrate/accumulate faster.
  • Loss of wetlands And flood plain storage
  • Flat, poorly drained landscape
  • Gulf moisture that regularly brings hurricanes, tropical storms and torrential rains
  • Fierce dedication to individual freedom, property rights, and local authority (Hey, this IS TEXAS after all.)
  • Political fragmentation
  • Widely varying flood control regulations
  • Upstream development that overwhelms the capacity of downstream drainage channels

In the last century, Houston has exploded from a sleepy city of less than 200,000 to a 9-county metropolitan statistical area with a population of about 7 million covering more than 9 thousand square miles.

Nine counties comprise the Houston metropolitan statistical area, home to 7 million people. Source: Greater Houston Partnership.

Now, superimpose watersheds over those counties and you can see how difficult the flood control situation becomes.

Watersheds of the Houston MSA. The Upper San Jacinto River Basin (above Lake Houston) contains 13 major watersheds. But, there are many smaller watersheds within each larger one. Source: San Jacinto River Authority.

Solutions Will Require Cooperation

Rivers and streams cross political boundaries throughout this region. So solutions to flooding problems are, by definition, regional. Yet development regulations and guidelines are anything but.

Most regulations pay lip service to “no adverse impact” on downstream neighbors. But in many areas, the regulations may be based on ancient maps and antiquated data. Moreover, they may have little to no oversight or enforcement.

Retain Your Rain

The region’s growth depends on its reputation for quality of life. If we are to continue growing, we must work together to solve flooding problems.

If every developer did one simple thing, we could eliminate most of our flooding problems. Just be responsible for the rain that falls on your property. Detain it long enough to avoid adding to flood peaks.

It’s that simple and that difficult. Especially considering that Texans don’t like having other people tell them what to do.

Posted by Bob Rehak, with thanks to Descartes Labs

863 Days since Hurricane Harvey and 112 since Imelda

When Measured by Growth of Impervious Cover, Texas Has 9 of Top 20 Counties in U.S.

According to a recent New York Times article, nine of the 20 counties in the U.S. that have experienced the most development the last decade are in Texas. Prominent among them is Harris County. The article does not cite population growth. Rather, it relies on computer analysis of satellite imagery that detects the growth of impervious cover.

Analysis of Satellite Imagery Shows Land Newly Paved or Topped With Buildings

The Times cites the work of Santa-Fe-based Descartes Labs, which positions itself as a data refinery. The company trained a computer model to automatically identify newly impervious surfaces — land that appears paved or topped with buildings — in satellite imagery. It then produced dozens of paired images that show the effects of development. It also produced a map that shows where that development took place.

This Descarte map clearly shows the pattern in Texas. Each of the major cities looks like a bullet hole in the map with development splashing out ward…a ring of concrete.

Return of Suburban and Exurban Growth

The data suggests that the growth of suburbs and exurbs has returned. There was a brief hiatus of suburban development after the housing bust in 2008, which saw people returning to the inner city. But that trend appears to be over, according to this analysis.

I’m not sure if this should be a source of pride, alarm or both.

Texas Grows While Other Areas Lose Population

Many Rust Belt cities are experiencing population shrinkage. That presents another set of problems altogether. The Times article shows how several northern cities, including Detroit, are clearing thousands of dilapidated and abandoned homes. In the process, they are restoring pervious (natural) cover.

As luck would have it, another article in The NY Times the next day talked about a slowdown in U.S. population growth. Population grew at its slowest pace in decades in 2019. A decline in the number of new immigrants, fewer births and the graying of America accounted for the decline, which the Census Bureau estimated.

Given slow population growth on the national level, local growth in Texas and Houston must come from migration. I’m not talking about foreign immigration. I’m talking about one area attracting residents and businesses from another.

Texas Has Seven of Fifteen Fastest Growing Cities in U.S.

In marketing, if the market itself is not growing, the only way for a company to grow is to steal share from its competitors. And that is exactly what Texas seems to be doing. Markets such as New York and California are losing population while Texas gained more than 14% in the last decade. From 2010 to 2018, Texas had the largest population growth in America: 3,555,731.

Texas also had 7 of the top 15 fastest growing cities in the country between 2017 and 2018.

So clearly, from a marketing point of view, Texas must be seen as a desirable place to live by many people. We’re doing many things right.

Can Texas Meet the Challenge of Rapid Growth?

But in my 45 year career in marketing and advertising, I have seen many instances where companies had record growth one year only to have record losses later. It comes down to how you manage growth.

Can you deliver what you promise and keep product quality up as you grow?

Many areas can. Many areas can’t.

County officials face a conundrum: growing rapidly while maintaining quality of life. You want to attract growth, but you don’t want to be overwhelmed by it.

Montgomery Vs. Fort Bend Counties: Strategic Differences

Some compete for growth by relaxing regulations. For instance, this video from the East Montgomery County Improvement District boasts, “We don’t have rules that confine us.” The no-hassle upfront, anything-goes, follow-your-dream approach tempts many, especially those coming from other areas with onerous regulations.

Meanwhile, other fast-growing counties, such as Fort Bend, are adopting new flood plain regulations, designed to protect the quality of life they are selling.

Tougher Fort Bend County Regulations Went Into Effect New Year’s Day.

As of 1/1/2020, Fort Bend County adopted new Atlas 14 rainfall statistics and updated their drainage criteria manual accordingly to protect new homes AND existing downstream developments. Fort Bend is the fastest growing county in the region.

There you have it. Two opposite ends of the spectrum.

It will be interesting to see the outcomes that these two development strategies produce ten years from now.

Posted by Bob Rehak on 1/2/2020

856 Days since Hurricane Harvey and 104 since Imelda