Tag Archive for: impervious cover

Impervious Cover Percentage Raises Downstream Concerns

The Preserve at Woodridge based its detention basin calculations on 65% impervious cover. But photos taken on 11/26/22, a full year after they cleared the land, suggest the impervious-cover percentage may have been dramatically understated.

That affects the amount and speed of runoff. And that raises concerns for downstream residents along Ben’s Branch, many of whom have flooded in recent years, in part because of dense upstream developments like this one.

Looking straight down reveals little dirt between the densely packed rental homes and the concrete surrounding them.

Taken 11/26/22

I continue to be amazed at how the developer claims that one third of this dense, concrete bungle is NOT “impervious cover.” And lest you think I selectively cropped the photo above to exaggerate the percentage of concrete, the shot below shows virtually the entire development.

Taken 11/26/22. Area on right still does not have sidewalks.

Pushing the Limits

At my age, I don’t like the idea of carrying groceries blocks from my car to my house – which I would potentially have to do here.

Nevertheless, to give credit where credit is due, it appears that this developer has a flair for pushing limits. Just look at the development’s website. They offer “unmatched amenities” like vinyl flooring.

And some homes are 660 square feet. Much smaller and you would expect the residents to wear orange jumpsuits.

But still, this new concept in luxury living has its rewards:

  • No stairs to climb like in apartments.
  • An extra wall between you and your neighbor’s stereo.
  • On-street parking, just like Manhattan.
  • 147 parking spaces for 131 homes.
  • Plenty of nearby food-trucks.
  • A “Scream Park” and fireworks stand within walking distance.
  • No leaves to rake.
  • Your own toilet.

This is way better than life in a frat house. The stainless steel refrigerators are definitely a step up from Igloo coolers.

The only thing missing is a pet run that can accommodate a Chihuahua and Cocker Spaniel at the same time.

But seriously, this developer claims to have identified a niche between sleeping bags and starter homes. Perhaps the company will pioneer a new market and this will be the future of Montgomery County. To see their construction plans, click here.

Will Detention Basin Hold Enough?

I just hope their detention pond is big enough in case their impervious-cover calculations are off.

Preserve at Woodridge detention basin is built to pre-Atlas 14 rainfall rates. It appears partially fenced in so that residents can’t walk around it.

Montgomery County’s Subdivision Rules and Regulations specify that outfall ditches, such as the one in the photo above only need to carry a 25 year rain. (See page 9.) With that in mind, it seems that this detention pond would fill up quickly from ditch overflow in a 25-year rain and provide little detention benefit during 50- or 100-year rains. And that’s no joke.

Posted by Bob Rehak on 11/28/2022

1918 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

66% Impervious Cover? Really?

Because the Laurel Springs RV Resort was grandfathered under old drainage regulations, it got away with building a detention pond that was half the size required by current regulations.

But assuming the developers were just shrewd businessmen who legally and successfully exploited the system, did they follow the rest of the rules? Let’s look at two other things.

  • The percentage of impervious cover on the site
  • How the number of parking spots increased 25% without the impervious cover increasing.
Laurel Springs RV Resort as of 10/22/22. The contractor still has one more “pour” to complete the concrete in the far upper right of the image.

Were Impervious Cover Calculations Correct?

Detention-pond volume calculations begin with impervious cover (i.e., land covered by concrete plus the entire detention pond area). See below.

66% impervious cover
Laurel Springs RV Resort Detention Pond calculations from approved permit plans.

The total site covers 20.032 acres. The proposed impervious portion, they claim, covers 13.349 acres. That works out to 66.6%. So one third of the site should be grass, trees and other vegetation. But since the entire 5-acre detention pond counts as impervious, mathematically, the remainder of the site can have no more than about 60% concrete and still comply with the percentage they promised.

But just eyeballing that trapezoidal area in the photo above, it seems much more than 60% is covered with concrete.

If my eyeball assessment is correct, then the detention pond is even more undersized than I initially thought because the percentage of impervious cover has increased and with it the amount of runoff.

I wish the developer would show us the basis for those calculations.

Plans Show Increase in Density With No Increase In Impervious Cover

The developer’s permit allows 182 RV spaces, but the plans show 226 – about a 24% increase. However, the impervious cover shown on the plans before and after the permit approval did not change. That could also affect detention pond capacity requirements. And explain why the percentage of concrete appears higher than they claim.

Why Underestimate Impervious Cover?

Why would a developer underestimate the amount of impervious cover? Two reasons:

  1. It would make the detention pond smaller and thus allow the remaining property to produce more income.
  2. By claiming they’re providing more detention than required, they can get a discount on their drainage fees. See page 10.

I’m not alleging they did anything illegal. I’m just saying that much more than 60% of that trapezoid in the photo above appears to be concrete and I sure would like to see how they arrived at their figures. I requested the drainage analysis twice and never got it. That should tell you something.

Posted by Bob Rehak on 10/23/22

1881 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.

65% Impervious Cover? Get Out the Ruler!

In January of this year, when they kicked off construction at the Preserve at Woodridge, I posted about the Montgomery County development claiming 65% impervious cover. It’s time to get out the ruler. See pictures below taken on 10/22/22.

Looking SE across Woodridge Parkway toward Kingwood Park High School out of frame at the top.

I felt at the time that the 65% claim didn’t pass eyeball test. I feel even more strongly about that now.

The Preserve at Woodridge will boast 131 homelets on about 10 acres. Looking west toward St. Martha Catholic Church.
Some are just 4 feet apart. The wider row at the top will be divided into backyards.

The walkway going left to right through the bottom of the frame will be people’s front yards. Not quite southern mansions. But there’s plenty of room for a daffodil and a fire hose.

How Impervious Cover Can Contribute to Flooding

The higher the percentage of impervious cover, the less stormwater soaks into the ground. It runs off faster. And without sufficient detention pond capacity, flood peaks build higher.

That’s why I’m so concerned about the accuracy of the 65% estimate. The capacity of their detention pond was configured based on one third grass.

The lack of green space upstream is a growing issue downstream. Our drainage systems never anticipated this kind of density.

And don’t forget, this development also based its drainage calculations on pre-Atlas 14 rainfall rates.

Growth of Impervious Cover

USGS says one third of Harris County is now impervious cover. With more developments like this, the southern part of Montgomery County could one day surpass Harris County!

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 U.S. when ranked by the growth of impervious cover.

Areas with high rates of impervious cover, as determined by Descartes Labs. Black dots represent growth of impervious cover. Note the ring around Houston.

To put 65% impervious cover in perspective (assuming the developer’s estimate is accurate), nationwide only about 4.4% of the land in the U.S. has more than 40%. And usually only shopping malls and high-density apartment complexes have more than 65%.

Current drainage capacity rarely anticipates development like this. That’s why so much of Houston’s drainage infrastructure struggles to function properly in heavy rains. It’s also why in 2010, the City of Houston instituted a drainage fee based on the percentage of impervious cover. The purpose: to raise money to repair/upgrade antiquated drainage systems taxed by overdevelopment and to encourage developers to leave more green space.

Close inspection of this site shows that the developer did leave leave one row of pet-friendly trees along the northern side.

Somebody screwed up. They could have squeezed another row of homes in there.

Posted by Bob Rehak on 10/22/22

1880 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.

Water Flowing Uphill at Laurel Springs RV Resort

Quick, somebody call Ripley. Against all odds, water appears to be flowing uphill at the Laurel Springs RV Resort.

Note the pond to the left of the detention pond near the tree line. A sharp drop off exists below the bank of the detention pond, toward Edgewater Park on the left. Now see closeup below.

The only apparent source for water in the circle is the pond on the left. But that’s below the water in the circle.

Is this an optical delusion? Or could something else be going on here? Could water actually be leaking through the dike?

Location of Buried Pipe

Back in January, contractors dug a trench through the dike and released the contents of the resort’s detention pond into Edgewater Park.

stormwater runoff discharge
Silty stormwater being discharged into the wetlands of Harris County’s Edgewater Park below the RV resort. January 2022.

Within another day, contractors started burying pipe in the trench and covering it up…at this exact spot where we now apparently see water flowing uphill.

Contractors laying pipe under wall of detention pond to send stormwater into Edgewater Park
Contractors later laying pipe under wall of detention pond to continue sending stormwater into Edgewater Park.

TCEQ traced sediment from the detention pond 450 feet downhill into the county’s park. Harris County then issued a Cease and Desist warning. In it, the County threatened to sue the owners of the RV park.

Later, a contractor told me his company removed the pipe. But I couldn’t find a single nearby resident who saw them do it. Subsequently, I’ve noticed water apparently leaking from the detention pond into the park after several rains…at this same spot! See below.

May 2022. Exact location shown in photos above.

So one possible explanation for this violation of the law of gravity could be that the pipe remains buried in the dike and water leaks through it. But I just can’t believe a contractor would lie to me!

If the pipe remains in the dike, that would seem to violate the owner’s construction permit. It says, in big red letters, “Stormwater Runoff Shall NOT Cross Property Lines.” I can’t believe the developer would violate permit terms either!

It’s hard being a small business owner these days. Do you violate Newton’s Law of Gravity or City law? Given a choice, it might be cheaper to go with Newton. So all things considered, I guess they figured out a way to get water flowing uphill. I’ve heard the owners are marketing geniuses.

I’m sure vacationers would drive their RVs from all across North America to see water flow uphill. Imagine the postcard sales!

Mysterious Black Spots Reappear

But that’s not the only possible tourist attraction. You’ve heard of people getting blood from a stone. At the Laurel Springs RV Resort, oil appears to simply ooze from the ground. Move over Beverly Hillbillies!

Looking North from over Edgewater Park. Note dark areas in red circle and see magnified image below. Photo taken 8/11/22.
From a legal height, I shot straight down and photographed an oily sheen on the black spots.

This is the same location where I previously photographed contractors covering up black spots on several occasions. See one below.

Mysterious black spots in Laurel Springs RV Resort Detention Pond
Mysterious black spots in Laurel Springs RV Resort Detention Pond on March 10, 2022. Note bulldozer tracks.

Every time contractors cover them up, they reappear. The TCEQ could not determine the origin of the dilute sample they found after one coverup. And Railroad Commission logs show no pipelines or abandoned wells in the area. So it’s officially a mystery.

Never ones to pass up a marketing opportunity, the RV resort owners reportedly hope to hire Max Baer, Jr., the last surviving member from the classic Beverly Hillbillies sitcom from the 1960s and 70s. Rumor has it, they want the 83-year-old Baer, who played Jethro, to be the first person through the gates at the grand opening. He would reportedly pass out samples of the oily substance so that RV owners across America can live the dream and tell their grandchildren how they struck oil while vacationing in Texas.

Concrete Galore

In another feat of marketing genius, the developer convinced the City of Houston permitting people that one third of the property would NOT be impervious cover, i.e., concrete.

Developer claims only 2/3rds impervious cover. From Detention Plans.

The developer is not yet done with pouring concrete. But it appears as if it will cover a lot more than 2/3rds of the site.

Will one third really be pervious?

By the way, according Section 9.1.04.O of the City’s Infrastructure Design Manual, detention ponds count as 100% impervious cover regardless of whether they have wet or dry bottoms. So the pond doesn’t count toward the one-third – just those skinny slots between RV parking spaces. And does it look like they add up to seven acres!?

It looks like Harry Houdini would have a hard time squeezing between the narrower ones. Of course, Houdini died in the 1920s. So how will they divert attention from this one? The developer reportedly wants to hire David Blaine, reputed to be the world’s greatest living escape artist, to appear at the grand opening. Blaine will show anyone who complains about the tight parking spots how to squeeze into his/her RV.

I can’t wait for the star-studded grand opening when I get to see water flowing uphill, Jethro passing out oil, and City inspectors lining up for David Blaine’s autograph.

Posted by Bob Rehak on 8/11/2022

1808 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.

August Flood Digest: Brief Summaries of Nine Items in the News

Here’s a short digest of nine flood-related items in the news this month.

Fifth Anniversary of Harvey

This month marks the fifth anniversary of Hurricane Harvey. It’s hard to pinpoint an exact day for Harvey. The system moved off of Africa on August 13, 2017. It became a tropical storm on the 17th; moved into the Gulf on the 22nd; became a Cat 4 hurricane; and made landfall at Port Aransas on the 25th. The outer bands reached Harris County on the 26th.

Harvey dumped heavy rain over Houston for four days. It started moving back offshore on the 29th and 30th. Ninety percent of the river forecast points in southeast Texas reached flood stage; forty-six percent reached new record levels. Harvey dumped more rain than any storm in the history of North America. For more information, see the Hurricane Harvey tab on the Reports Page.

West Fork San Jacinto During Harvey. Looking NE toward Kingwood from the Townsend Park N Ride.

New SJRA Director From Lake Conroe

Most flooding in the Lake Houston Area during Harvey happened after the SJRA started releasing 79,000 cubic feet per second (CFS) from Lake Conroe to save homes there. Many Lake-Houston-Area residents blamed the absence of downstream representation on the SJRA board for what they saw as disregard for their property.

After touring the extensive damage by helicopter, Governor Abbott appointed two Lake-Houston-Area residents (Kaaren Cambio and Mark Micheletti) to the seven-person board. Cambio later resigned to avoid a conflict of interest when she joined Congressman Dan Crenshaw’s staff. Last month, the Governor appointed a Lake Conroe resident to fill her vacancy, Stephanie Johnson. That now leaves Micheletti as the lone downstream representative.

Lone Star Groundwater Conservation District Elections

Sometimes it seems that the main requirements for membership on the LSGCD board are half a brain, a willingness to kiss Simon Sequiera’s ring and indifference to science. Sequiera owns Quadvest, the largest private groundwater pumping company in Montgomery County. And excessive groundwater pumping in MoCo has been linked to subsidence and flooding. But concerned citizens will have a chance to take back the LSGCD board from a slate of directors backed by Sequiera. The deadline for applying is August 22. This page on the LSGCD site is all about the election and how to file if you are interested.

Edgewater Park

Harris County Precinct 3 is trying to jumpstart the development of Edgewater Park at 59 and the San Jacinto West Fork. The county has stated it is hiring a new consultant to re-design the park and that construction could begin 1 to 2 years from now. Quiddity Engineering will get the nod. The project will provide a boat launch, an additional park for the Humble/Kingwood Area, and a connection to the Spring Creek Greenway hike and bike trail. Quiddity’s contract will cover design, engineering, and other pre-construction expenses. Quiddity is the new name for Jones and Carter.

Houston Planning & Development Department News

The Planning and Development Department has a new initiative called Livable Places. The objective: create more housing options for Houstonians. The four options they visualize all increase housing density and impervious cover. I wrote them asking, “Won’t that increase flooding?” In essence, they said, “But it may help other places stay green.” True. But that’s not going to help flooding in the City much. Wasn’t our Drainage Fee designed to provide an incentive to REDUCE impervious cover. Oh well. These are different times. Can we get our drainage fees back now?

Flood Tunnels

Harris County Flood Control District (HCFCD) released Phase 2 of its $30 billion flood tunnel study last month – along with a recommendation to study the recommendations in more detail. The current plan for Phase 3 is to spend the next 4-6 months:

  • Working with the Army Corps to explore possible federal involvement
  • Scoping the Phase 3 study
  • Beginning procurement.

HCFCD hopes to start Phase 3 in early 2023. Said Scott Elmer, P.E. CFM and Assistant Director of Operations for HCFCD, “We expect it to take approximately 3 years to complete.” For the complete Phase 2 study, click here.

GLO HARP Program Deadline

The Texas General Land Office (GLO) announced that applications for its Homeowner Assistance and Reimbursement Program (HARP) will close at 5 p.m. on Aug. 31, 2022. Those include applications for repairs/rebuilds from Tropical Storm Imelda in 2019. To be eligible, you must submit applications by the deadline … unless funding runs out first. So hurry. 

The program includes repair or reconstruction of owner-occupied single-family homes and reimbursement up to $50,000 for certain out-of-pocket expenses incurred for reconstruction, rehabilitation, or mitigation. Repayment of SBA loans is also eligible for reimbursement.

The GLO has $71,604,000 to help residents of Harris, Chambers, Liberty, Jefferson, Montgomery, Orange, and San Jacinto counties. HARP is only available for a primary residences, not second homes. Interested homeowners should visit recovery.texas.gov/harp to apply online or download an application.

Harris County Attrition and Pay Reports

As reported in April, the loss of employees and managers in dozens of Harris County departments has created a brain drain that impacts delivery of county services. On Tuesday, 8/2/22, Commissioners considered two related reports. The first had to do with attrition. The second had to do with pay and benefits.

Commissioners did not discuss the first, but they did discuss the second at length. They also voted unanimously to have the Office of Management and Budget investigate pay disparities. Certain commissioners wanted to apply equity guidelines to low-paid employees and freeze pay for those making more. I didn’t hear the words “Pay for Performance” once during the discussion.

In the end, commissioners recommended having HR create a job architecture, pay structure, and new evaluations that would determine pay increases or freezes. More in future posts.

New Bond Package

Discussion of a new $1.2 billion bond package consumed the last 90 minutes of commissioners court this week. The County Administrator still cannot say where the money is actually needed. Commissioners Adrian Garcia and Rodney Ellis want to apply equity guidelines to this bond. And neither wants to say which projects they would spend the money on. Garcia even threatened in a previous meeting that Republican-leaning precincts would not get ANY of the money if their commissioners voted NO on the bond.

When Hidalgo suggested guidelines for distribution of the money, Garcia stomped out of the meeting. He later reluctantly agreed to a split that would give his precinct and Ellis’ $380 million each while Republican precincts would get only $220 million each.

During the debate, it came out that much of the money from the 2015 bond program still has not been spent. That raised the question, “Why do we need another bond?”

Bragging About Trickery on One Bond While Pitching Another

Also, Commissioner Rodney Ellis publicly bragged that he purposefully didn’t define “equity” in the 2018 flood bond. “It was side language,” he said. “It was not in the language that was on the ballot, but that was the side agreement we agreed to.”

Ellis later said, “Those poor neighborhoods are the ones who have gotten the short end of the process.” But the HCFCD July flood-bond update shows that Halls, Greens, White Oak, Brays and Hunting Bayou Watersheds have received $400 million out of the $1 billion spent to date from the flood bond. Twenty percent of the watersheds are getting 40% of the money. Short end?

I personally don’t plan to vote for another bond until I start seeing some benefit from the last two. Especially when there’s no guarantee how, where or on what the money will be spent. To me, this looks like a $1.2 billion dollar slush fund for Garcia and Ellis.

Posted by Bob Rehak on 8/6/22

1803 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.

65% Impervious Cover?

Only about 4.4% of the land in the U.S. has estimated impervious cover greater than 40%. Usually, high percentages of impervious cover are associated with shopping malls; large apartment complexes; manufacturing and warehouse districts; and densely populated urban neighborhoods. Now there’s a new entry in that category: the Preserve at Woodridge – single-family housing so close together that you can spread your arms and touch two homes.

If you like living close to neighbors, the Preserve at Woodridge will be for you. Guefen, the developer, claims 65% impervious cover. Assuming their calculations and claims are accurate, what does that look like?

We can now see. Builders have framed the first cluster of homes. I’ve posted before about how close together these homes would be. But until you see them, the proximity is hard to fathom. They certainly don’t pass the eyeball test for 65% impervious cover.

Pictures Dramatize Proximity of Homes

The good news is that these homes, some as large as 660 square feet, definitely have more space than a porta-potty or a cargo container. You’ll be able to vacuum the home in world-record time. And you’ll never have to wonder where you left your cell phone. It would be impossible to lose in a home this small.

The cluster of homes shown above is also shown in the schematic below.
Note 4.95-foot width between home in middle and one below it. See actual space below where ladder is.

You know things are tight when the developer measures the distance between homes down to the hundredth of a foot (1/12th of an inch).

Notice how the ladder leaning on one home is braced against the foundation of another. Those two homes have less than five feet of separation.

Regardless, the engineers claim the development has 65% impervious cover for the purposes of calculating detention pond volume. That means 35% would be pervious, i.e., grass. See below.

From developer’s drainage calculations, page 20.

But Where Do You Put the Lawnmower?

But with so much shade between the homes, can you really get grass to grow? And if you can, where do you put the lawnmower? The developer has only 34 garage spaces for 131 homes. Perhaps you can put the mower under your Murphy bed. Or in your gym bag.

More pads for the next batch of homes.
Status of Preserve-at-Woodridge construction as of 1/29/2022.

Guefen plans to rent, not sell these homes. I guess you could consider these a step up from apartment living. But the developer has not preserved much at the Preserve. They certainly won’t live up to the reputation of the Livable Forest.

This is going to feel more like high-density, inner-city living … without the public transportation. We’ll soon see if there really is a market for this concept in the Kingwood Area.

How Impervious Cover Can Contribute to Flooding

The higher the percentage of impervious cover, the less stormwater soaks into the ground. It runs off faster. And without sufficient detention pond capacity, flood peaks build higher.

Posted by Bob Rehak on 1/30/2022

1615 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.

Loophole Cuts RV Park Detention Pond Capacity Requirement in Half

When examining the floodwater detention pond capacity for the Kingwood Area’s first RV park at 1355 LAUREL SPRINGS LANE, it seemed impossibly low. The park will build a pond with 7.68 acre feet of stormwater detention. That’s more than the required 6.675 acre feet, but about half of what the City of Houston (COH) would require under new guidelines that went into effect within 90 days of the plan’s approval.

Because of that 90-day grandfathering loophole, the RV park will escape current City and County requirements for floodwater detention.

Current Regs Would Require 13 Acre-Feet of Detention Pond Capacity

Under today’s requirements, a 20.032-acre RV Park would require 13 acre-feet of detention pond capacity (20 acres x .65 = 13 acre feet). That’s because the COH 2021 infrastructure design manual makes properties larger than 20 acres follow HCFCD guidelines (see below).

New Harris County guidelines require a minimum rate of .65 acre-feet per acre.The RV Park is slightly larger than 20 acres.

This requirement is based on NOAA’s new, higher Atlas-14 rainfall probability tables adopted in the wake of Harvey.

COH 2021 Infrastructure Design Manual Defers to HCFCD Regs for Lots This Size

The City’s Infrastructure Design Manual imposed new criteria in July 2021 that would have required developments greater than 20 acres to follow the HCFCD standard for detention pond capacity. See screen capture below which bridges two pages.

Screen Capture excerpted from Pages 9-29 and 9-30 of COH 2021 Infrastructure Design Manual.

City’s 2020 Regulations Required Far Less Detention

However, the City’s 2021 standards also provided a 90-day grace period for plans initially submitted before July. That meant the City’s 2020 Infrastructure Design Manual applied. And those standards only required following HCFCD’s higher standards if the lot was larger than 50 acres – which this is not. It’s 20.032 acres. So the RV Park developer will get away with installing far less detention than current regs require.

Differences Between Old/New Versions of City/County Regs

Two primary differences exist between the City and County regs in 2020 and 2021.

The first is the basis for calculations. In both years:

  • The County bases required detention pond capacity on the total size of the site.
  • The City, except where it defers to HCFCD regs, bases detention pond capacity on the amount of impervious cover within the site.

The City also changed the minimum lot size required to follow HCFCD regulations between 2020 and 2021.

  • Sites larger than 50 acres had to follow HCFCD guidelines in 2020.
  • Sites larger than 20 acres had to follow HCFCD guidelines in 2021.

The 2020 requirements apply in this case because of when plans were first reviewed (January 2021). The 2020 Infrastructure Design Manual read as follows:

COH 2020 Infrastructure design manual requires 20- to 50-acre tracts to have detention volume (in acre feet) equal to half of the impervious cover.

Developer Applied Before Detention Pond Capacity Increased

Using that formula, the developer planned a detention pond that holds 7.68 acre feet of storm water detention, an acre foot more than required at the time the plans were first submitted, even though they were finally approved four months after the new regs went into effect. But that’s still far less than the 13 acre-feet required under today’s regulations.

Impervious Cover Calculations Need Further Review

I question whether the amount of impervious cover above is correct. That’s because the developer’s permit allows 182 RV spaces, but the plans show 226 – about a 24% increase. However, the impervious cover shown on the plans before and after the permit approval did not increase. That could also affect detention pond capacity requirements.

Timing, Impervious Cover Issues Raise Public Safety Questions

Assuming the impervious cover calculations above are correct, and that’s a big assumption, the developer is providing more than the minimum amount of required detention under 2020 requirements. But if they are not correct, the developer would be short.

And if the 2021 requirements applied, the developer would have to provide virtually twice as much detention.

A half-sized detention pond would require the RV Park to pump water into Lakewood Cove’s drainage system sooner and faster than with a larger pond. And under severe conditions, when Lakewood Cove’s drainage is already stressed, the extra water could over-stress it.

No Drainage Impact Analysis

I’ve requested a drainage impact analysis from the City for the RV Park on at least two occasions and have not received one. I therefore deduce that one does not exist. Such an analysis would quantify the impact on Lakewood Cove, Edgewater Park and surrounding roads.

Detail from most recent approved construction plan shows all water from detention pond going under Laurel Springs to Lakewood Cove storm sewer system toward homes below.
All the runoff from the 20 cleared acres will be funneled via a 24″ pipe toward the homes in the foreground.

Bottom Line: Park Will Provide Half of Today’s Detention Requirement

If this developer, LS RV Resort, LP, submitted plans today, it would have to provide almost twice as much detention pond capacity.

  • 2021 requirements call for .65 acre feet per acre times 20 total acres. That equals 13 acre feet of detention.
  • 2020 requirements call for .5 times the claimed 13.349 acres of impervious cover. That equals 6.675 acre feet of detention.

And we wonder how floods happen! Remember this the next time you see water rising toward your home.

I wonder if investors in the RV Park will be notified of the potential liability in a prospectus. Undersized detention ponds based on a similar grandfathering loophole in Montgomery County regulations became the central issue in lawsuits by hundreds of Elm Grove residents against Perry Homes, its subsidiaries and its contractors. The defendants recently settled those lawsuits.

Posted by Bob Rehak on 11/13/2021

1537 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.

USGS Says One Third of Harris County Now Impervious Cover

Did you know that one-third of the area in Harris County now has impervious cover? That Montgomery County had a 57.12% net increase of impervious surface area between 2001 and 2019? Or that 10% of land cover in the Lower 48 states changed during that same period? I discovered these and a multitude of other fascinating facts in a recently updated United States Geological Survey (USGS) website dedicated to monitoring changes in land cover, for example, from forested to developed.

When you live in an area for a long time, it’s easy to forget what happened two decades ago. And when you move to a new area, you just accept what is and don’t worry about what was.

But USGS gives you a quick and easy way to see and quantify changes in land use down to the county level. It’s useful in telling you where flood threats could develop over time and how fast they are developing.

About the USGS National Land Cover Database

USGS recently released updated land cover maps for the lower 48 United States. They show how the country’s landscapes have changed over an 18 year period in two- to three-year increments. It’s called the United States National Land Cover Database (NLCD). And it’s the fastest way to see how your county is changing.

Updates include 2001, 2004, 2006, 2008, 2011, 2013, 2016, and 2019.

Developed using Landsat imagery, NLCD classifies land cover into 16 groups with 30-meter resolution. The data includes both land-cover and urban imperviousness changes.

USGS claims 91 percent accuracy for the NLCD data. For more detail about how NLCD was developed see: Changes to the National Land Cover Database. More than nine billion pixels make up the land-cover dataset.

The USGS National Land Cover Database’s suite of GIS mapping products even includes a layer that defines the intensity of impervious surfaces across the United States. This information is used in runoff modeling, urban heat estimation, and a variety of other applications.

Mapping Land Cover Change in U.S. Over Time

Users can visualize land cover changes in the United States by accessing the the Enhanced Visualization and Analysis (EVA) tool. The online mapping tool was developed by USGS in partnership with the National Oceanic and Atmospheric Administration.

The tool allows users to select any county in the Lower-48 United States and generate a custom report on land cover change, developed areas, cropland change, and other factors.

Only one caution: the USGS site does not work with Apple’s Safari Browser. Mac users can use Firefox without problems, however. I have not tested other Mac browsers.

I ran two quick searches on Harris and Montgomery Counties by going to the EVA tool mentioned above. The findings astonished me.

Harris County Changes At a Glance

Between 2001 and 2019, in Harris County:

  • Almost one fifth of the land cover changed type (18.23%).
  • Developed portions of the county increased from 54.42% to 65.85% of the total acreage, a 20.99% percent net increase of developed area.
  • Forested parts of the county went from 10.64% to 6.29%, a percent net decrease of 40.92%.
  • The percent covered in wetlands went down from 8.28% to 7.02%, another percent net decrease of 15.24%.
  • The percentage of impervious surface increased from about a quarter to a third (26.28% to 33.39%), a percent net increase of 27.05%.
Screen showing development changes in Harris County with corresponding percentages of impervious cover. Green dots represent changes in land use. Clicking on icons in left column brings up different types of information.

MoCo Changes at a Glance

During the same period, in Montgomery County:

  • Even more land cover changed type (18.99%).
  • Developed portions of the county increased from 21.1% of the land area to 28.27%, a 33.97% net increase.
  • Impervious cover increased from 5.78% off the land area to 9.08%, a 57.12% increase.
  • Forested land decreased from 42.98% of the county to 38.96%, a 9.16% net decrease.
  • Wetlands decreased from 12.17% of the county to 11.35%, a 6.74% net decrease.
  • Agricultural land decreased from 12.28% to 10.31% of the county, a 16.04% net decrease.
Red areas represent areas in Montgomery County that changed land-cover type between 2001 and 2019.
Another screen showing areas in Montgomery County developed between 2001 and 2019.

Key Lesson

This database and GIS mapping system dramatize how quickly the region is growing and land use is changing.

Flood mitigation is or should be a two-pronged effort. We must fix problems that already exist downstream while hopefully preventing future problems from developing upstream. It’s not a just question of one county spending money to help prevent problems in another. It’s about surrounding counties protecting themselves. The outward expansion is relentless. People at the edge today will be downstream from someone else tomorrow.

There’s little anyone can do to change the FACT of development. But we can change the NATURE of development. If all new developments retained their own rain, no one would ever be doomed to the flood-mitigation treadmill of keeping up with ever-increasing amounts of upstream runoff.

Montgomery County already has a serious flooding problem of its own. Thousands of people flooded there during Harvey and Imelda.

Posted by Bob Rehak on 8/5/2021 based on USGS information

1437 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.

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.