Tag Archive for: Population growth

Five Watersheds Lose Population While Harris County Gains Slightly

Like damage, population is one of the “weighting factors” considered in the distribution of flood-mitigation dollars. Compared to 2017 estimates of watershed populations in Harris County, 2022 estimates show that five watersheds have lost population. But overall, the county has gained 155,254 people.

Data compiled from information provided by HCFCD in response to FOIA Requests. Alphabetical by watershed.

The map below shows the location of each of these watersheds.

Harris County Watersheds
Harris County Watersheds by Harris County Flood Control District

Reasons for Shifts Unclear

I had wondered whether the five major floods in Harris County between 2000 and 2020 (Allison, Tax Day, Memorial Day, Harvey and Imelda) would cause the most heavily flood-damaged areas to lose population. But that seems not to be the case.

The most heavily damaged watershed (Brays) lost the most population (in raw numbers). But the second most heavily damaged watershed (Greens) gained more than 16,000 people, the second largest gain.

I found no meaningful correlation between flood damage and population loss or growth. Nor do data suggest that flood-mitigation spending has much influence either. Contradictory examples abound. And statistical correlations rate as negligible to weak.

Five Watersheds Lost Population

Compared to the previous Census estimates from 2017:

  • Brays lost 5,822 people (0.8%)
  • Cedar lost 4,153 people (11.1%)
  • Hunting lost 1,112 people (1.4%)
  • Little Cypress lost 727 people (1.6%)
  • Vince lost 176 people (0.2%).

In percentages, Cedar Bayou lost the most population. Cedar was the sight of the Arkema disaster which blocked evacuation routes along Highway 90 during Harvey. The watershed currently has only about 37,000 people, ranking it 20th among the 23 watersheds in Harris County.

In five major storms between 2000 and today (Allison, Tax Day, Memorial Day, Harvey and Imelda), flood damage varied widely within those five watersheds:

  • Brays flooded 32,240 structures, the most of any watershed.
  • Cedar flooded 2,274.
  • Hunting flooded 15,763.
  • Little Cypress flooded 1,040.
  • Vince flooded 4,152.

18 Watersheds Gained Population

Eighteen other watersheds gained population and also had widely varying degrees of flood damage. These, too, showed little correlation.

Altogether, the county’s watersheds gained 155,254 people, despite 226,729 damaged structures during the five major storms.

Negligible Correlation of Flooding and Population Gain/Loss

The “population flees flooding” hypothesis didn’t hold much water.

Flood damage and “number of residents lost” correlated at only a 0.16 level – insignificant. Flood damage and “percent of residents lost” correlated at only 0.28, extremely weak.

A perfect correlation is 1.0. It indicates that for every unit of change in one variable, there is a corresponding unit of change in another variable. However, that was far from the case here. Variations seemed random.

If long-time Harris County residents are moving to higher ground, they may be replaced by newcomers unaware of flood risks.

The County has a strong draw: jobs. Also, family, friends, neighbors and support networks remain powerful attractions that keep most people anchored.

The new HCFCD data do not suggest why gains and losses occurred.

Top reasons for relocation typically include: greater safety, better schools, better housing, new jobs, and upgrading from apartments to homes.

LMI Population Trends

HCFCD also measures “LMI population.” LMI stands for Low-to-Moderate Income. The Census Bureau defines LMI as “families making less than the average for the region.”

Harris County has 42.6 % LMI residents. So 57.4% of residents make above the average for the region.

This is important because the LMI percentage plays a huge role in partnership grants from the US Department of Housing and Urban Development (HUD). Most, but not all, HUD grants go to areas with LMI populations higher than 70%. A notable exception is the pending $750 million HUD Harvey flood-mitigation grant. It only requires that 50% of the money benefits LMI households.

The latest HCFCD data shows that watersheds at both ends of the income spectrum lost LMI population. We still have the same eight LMI-majority watersheds we had in 2017. However, one deserves special mention.

Brays’ LMI population declined so significantly that it almost flipped from the “majority LMI” category to “majority upper income.” It went from 58% to 51% LMI.

Sixteen other watersheds gained LMI population. Some had huge flood losses; others had few.

The correlation between total flood damage and LMI Population Gain/Loss is .34, slightly higher than for the entire population but still considered “weak.”

Here are Harris County’s watersheds ranked in order of LMI population percentage.

As of 11/21/2022

My next post will discuss how the distribution of flood-mitigation funds relates to population changes and other factors. I will also discuss what the prospects for flood mitigation in the Lake Houston Area are during Lina Hidalgo’s second administration. Don’t miss it.

Posted by Bob Rehak on 11/21/22, based on data provided by HCFCD in response to a FOIA Request

1910 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.

World Population Tops 8 Billion: Will It Impact Impervious Cover?

On Tuesday, 11/15/22, the United Nations estimated that the Earth’s population topped 8 billion people. I promptly wondered about the impact on impervious cover, a notorious link to flooding. However, I discovered it’s not as simple as you might think.

Impervious cover directly links to flooding. But the growth of impervious cover (new homes, streets, parking lots, etc.) does not directly link to population. Two studies cited below found huge variations in the growth of impervious cover related to LOCATION and LIFESTYLE. It doesn’t all depend on population.

Increasing Rate of Population Growth

It took Earth 200 thousand years to reach 1 billion people in 1804. Since then, we’ve added 7 billion people in a little more than two centuries. The last billion took just 12 years!

UN World Population Milestones

Population in Billions12345678
Year Reached18041930196019741987199820102022
Years elapsed200,000+126301413111212
Source: Wikipedia

Clearly, growth has accelerated. Such numbers demand reflection. They prompt at least two questions: Are we living sustainably? And does the increase in impervious cover associated with population growth necessarily lead to a corresponding increase in flooding? I can’t answer the first. But based on these studies, I’ll answer the second with, “Not necessarily. It depends.”

2007 Study Shows Widely Varying Rates of Impervious Cover Worldwide

A 2007 study published in the journal Sensors estimated impervious surface area (ISA) in 100 counties. Called “Global Distribution and Density of Constructed Impervious Surfaces,” the authors included Christopher D. ElvidgeBenjamin T. TuttlePaul S. SuttonKimberly E. BaughAra T. Howard, Cristina MilesiBudhendra L. Bhaduri, and  Ramakrishna Nemani. Among other things, they examined the impacts of hydrological and ecological disturbances associated with the growth of impervious cover.

They note that:

  • ISA alters the character of watersheds by increasing the frequency and magnitude of surface runoff pulses.
  • Increased overland flow also alters the shape of stream channels, raising water temperatures, and sweeping urban pollutants into aquatic environments.
  • Hydrologic consequences of ISA include:
    • Increased flooding
    • Reductions in ground water recharge
    • Reductions in surface water quality.

So Who Has the Most Impervious Surface?

The three countries with the most ISA are China, the U.S. and India. But our population varies dramatically from the other two. With less than a third of the population, we have roughly four times more impervious cover. That makes our ISA per person roughly 4-5X higher.

Source: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3841857/. Note: China and the US are roughly 9.5 million sq km, but India is 3.3 million sq km.

So there’s not a direct correlation between population and impervious cover.

While noting that the world’s most developed nations also have the highest percentage of impervious cover, the study does not go much beyond that. It does not quantify the relative rates of flooding in each country studied. The main objective was simply to offer a framework and methodology for measuring impervious cover that other researchers could build on.

An Urban Planning Perspective

A second study reviewed the study above from an urban-planning perspective and led off with these two images.

Source: “Which Countries have the most pavement per person?”by Daniel Herriges in Strong Towns, 2019. Image used under a Creative Commons license.

As you probably already guessed, the area on the left has the most pavement per person, despite appearing to have less concrete.

The area on the right is in Germany, which has about one third of the paved surface per capita of the U.S. Both countries are comparably wealthy and both famed for their highways. This article digs deeper into planning issues associated with:

  • Distribution of impervious cover
  • Infrastructure maintenance costs
  • Urban planning strategies

Impervious Cover Related to Auto Culture

Daniel Herriges, the author, points out that impervious surfaces exist for three major reasons:

  • Buildings
  • Streets/Roads
  • Parking

He adds, “Two of those three have everything to do with cars. And on nearly every measure to do with car usage, well, America is #1, Baby.”

Using the EPA’s interactive EnviroAtlas, Herriges created heat maps of several major cities. They consistently revealed that the highest impervious surface per capita is in suburbs, not central cities.

He continues, “The paradox this data reveals is stark: New York City is dominated by brick and glass and concrete and steel. But NYC residents have just about the least amount of pavement to their name of any Americans. Meanwhile, our greenest places are in one sense the least ‘green,’ when you account for the parking lots and six-lane stroads that come with large grassy lawns.”

What Appears to Be Green Can Be Deceiving

Herriges concludes: “…what appears green can be deceiving.”

He argues to “Let cities be cities and rural be rural.” In productive places that generate wealth…we can afford to deal with stormwater through more sophisticated technological means: pipes, pumps, levees, as well as newer technologies like green roofs and permeable pavement.”

But he argues, “Places that produce comparatively lower revenue warrant a different approach, a more natural and low-tech one. It’s not that verdant suburbs are always bad: it’s that we should deal with drainage in those places by keeping our paved footprint to a minimum, and absorbing as much stormwater back into the ground as possible.”

Unfortunately, he doesn’t delve into the factors that drive suburban migration, such as school quality and crime rates. Nor does he hint at what to do with the auto-oriented suburbs and commuting culture we already have. Still, he’s a brilliant writer who offers much to think about.

If he proves one thing, it’s that population growth doesn’t automatically lead to more impervious cover per capita and increased flooding.

But is it possible to wean Americans off automobiles? It seems that’s an even bigger ask than preserving natural floodplains.

Posted by Bob Rehak on 11/16/22

1905 Days since Hurricane Harvey