Tag Archive for: runoff

Colony Ridge Drainage Reports Misrepresent Soil Types, Underestimate Runoff; Many Reports Missing

Drainage reports for the controversial Colony Ridge development in Liberty County misrepresent soil types in a way that underestimate runoff by as much as 6X to 9X. As a consequence, the massive development’s ditches and detention ponds are undersized. That contributes to downstream flooding. 

In addition, virtually all of the drainage reports supplied by the county in response to my FOIA (Freedom of Information Act) request were marked “preliminary” and many were missing. The Assistant County Attorney did not explain why. She said only that she had supplied all documents “responsive to” my request that the county had.

Let’s review soil types first.

USDA Findings Contradict LandPlan Engineering’s

U.S. Department of Agriculture (USDA) classifies soil into four groups (A, B, C, D) that represent rates of rainwater infiltration. Group A has the highest rate of infiltration and D has the lowest. Think gravelly sand vs. clays.

When USDA analyzed soils in the Colony Ridge area, it found less than 2% in Group A. However, virtually all  of LandPlan Engineering, PA reports used model inputs associated with soils in Group A. Hmmmm. Quite a contradiction. LandPlan is the engineering company for Colony Ridge that produced the drainage studies.

USDA says almost no Colony Ridge soils have the lowest rate of infiltration and LandPlan says almost all do.

Comparison of USDA Soil Survey and Landplan Engineering documents

Colony Ridge also has small percentages of soils in intermediate categories:

  • B = 2.3%
  • C = 1.2%

Finally, USDA shows some mixed soil types within Colony Ridge. For instance B/D or C/D. But a flood expert and professional engineer, who spoke on condition of anonymity, said that with mixed soil types, LandPlan should have classified them as Group D. “For all of the areas with B/D and C/D, you should assume that they are D because the soil is disturbed and probably compacted in some way.” So almost 95% of the soils should should be represented with a rate of infiltration equivalent to Group D.

Compacted soil on residential Colony Ridge lot. Note ponding water and damp soil at right. Note also the erosion under back fence next to ditch. Insufficient capacity of ditch contributed to erosion.

Soil Classification Consistently Off in One Direction

Liberty County supplied 39 drainage and construction documents in response to ReduceFlooding.com’s FOIA request. The soil classifications, as shown by the Curve Numbers in the reports all erred in one direction – the direction that favored the developer’s profits.

Almost 95% of the soils should be classified in the least porous group. But virtually all of the “curve numbers” reported by LandPlan Engineering are associated with the most porous group.

By classifying the soils as more porous than they actually are, the engineers could claim there was less runoff and therefore reduce the size of ditches. Likewise, they could reduce or eliminate detention ponds.

What Curve Numbers Mean

Curve Numbers (abbreviated as CN in drainage reports and construction docs) numerically represent the rate of rainwater infiltration. They correlate primarily to soil groups, but also land use and surface conditions. For instance, after soil is paved with concrete, the curve number goes up (indicating less infiltration).

Theoretically, CNs can range from 0 (100% rainfall infiltration) to 100 (totally impervious). In practice, however, the lowest CN is 30 and the maximum is 98, according to the USDA Natural Resources Conservation Service.

What Should Colony Ridge’s Real Curve Numbers Be?

TxDoT’s Online Hydraulic Design Manual shows curve numbers for residential developments (see Curve Number Loss Model section).

For 1/2 acre lots with average impervious cover of 25% (typical of Colony Ridge), USDA estimates the following Curve Numbers: 

  • Group A = 54
  • Group B = 70
  • Group C = 80
  • Group D = 85

LandPlan Engineering used Curve Numbers mostly associated with Group A. They should have used values mostly associated with Group D. See example below from the Drainage Report for Colony Ridge’s Bella Vista Subdivision Section 1.

Excerpt from Bella Vista Drainage Report. Note Curve Number for pre-existing conditions associated with Group A soils, i.e., those having the highest rate of infiltration.

USDA’s soil report for Bella Vista Section 1 shows that the soils are Group C (69%) and Group D (31%). According to USDA and the flood expert/engineer above, the Curve Number used to calculate detention requirements for the “developed condition” should have been closer to 85. But the Curve Number on which the detention is based is 56 (see below) – a number associated with Group A soils. Note: this is a subset of the larger report for Colony Ridge discussed above.

Bella Vista Section 1 shows post-development Curve Number of only 56, associated with the highest rate of infiltration.

Importance of Accurate Curve Numbers

While Group A can absorb .3 to .45 inches of rainfall per hour, Group D absorbs only 0.00 to 0.05 inches per hour. Had LandPlan used the correct values, they would have had to accommodate 6X to 9X more rainfall.


That would have required building larger ditches and detention ponds. But by using the Group A numbers, they could claim:

  • Floodwaters were soaking in.
  • Their roadside ditches could hold runoff. 
  • No, fewer, or smaller detention ponds were necessary.
Loss rate for each soil group represents the amount of rainfall infiltration per hour. Infiltration for Group A is at least 6-9X higher than Group D. Source: TXDoThttp://onlinemanuals.txdot.gov/txdotmanuals/hyd/hydrograph_method.htm

This suggests that LandPlan altered model inputs to achieve the desired output. The flood expert above called LandPlan’s Curve Numbers, “just plain wrong.” “Soils like that just don’t exist in this area,” he said.

Developer’s Environmental Consultant Confirms USDA’s Accuracy

One of the developer’s own environmental consulting firms confirmed the accuracy of the USDA’s and flood expert’s soil observations. Berg-Oliver developed a wetland assessment for the developer in 2014. “The United States Department of Agriculture (USDA) Web Soil Survey of Liberty County, Texas, was, for the most part, reasonably accurate in identifying the basic soil types on the property…” says the report. However, nobody in Liberty County, according to the documents supplied, questioned or even noticed the conflict between LandPlan, Berg-Oliver and USDA.

The Berg-Oliver report was NOT one of the documents supplied by Liberty County. I found it attached to an affidavit by the former Liberty County Engineer in a lawsuit between the ex-Mayor of Plum Grove and the developer of Colony Ridge.

Role in Downstream Flooding, FM1010 Washout, Erosion

Plum Grove residents report increases in the severity and frequency of flooding since Colony Ridge started clearing land. Water accumulates faster and peaks higher, they say, because of the loss of trees and wetlands. But the extra runoff that engineers have not accounted for in their calculations makes those problems even worse. That’s because Colony Ridge ditches and detention ponds can’t retain the extra runoff.

Mischaracterization of soil types likely also played a role in the washout of FM1010.

During Harvey, Colony Ridge drainage ditches discharged so much water into Rocky Branch that the stream then overtopped and destroyed FM1010. The blowout worsened during Imelda. No one has repaired it yet.

Finally, the “tractive” force (power) of rapidly moving water through undersized ditches accelerated erosion. Downstream, the eroded sediment built up and forms sediment dams that back water up, flooding additional homes in Plum Grove, or near the San Jacinto East Fork and Luce Bayou.

“Preliminary” Plans

My Freedom of Information Act (FOIA) request to Liberty County asked for ALL drainage analyses/surveys and construction plans for Colony Ridge subdivisions. However…

  • Virtually all of the plans that Liberty County supplied were marked “preliminary.” 
  • None was marked final or approved. 
  • Many were missing altogether. 
  • NOT ONE bore the signature, stamp, or comments of the Liberty County engineer or his agent, LJA Engineering. 

The 39 reports/surveys and plans are too large to post here; they comprise 1.5 gigabytes.

Liberty County has yet to clarify why so many of the plans are named “preliminary” or were missing. However, the Assistant County Attorney did verify that she supplied all Colony Ridge documents that pertained to my request.

Missing Documents

Here is a list of NINETEEN missing documents:

Missing Drainage Plans/Analyses (16)
  • Bella Vista – Section 2
  • Camino Real – All Four Sections
  • Grand San Jacinto – All Five Sections
  • Montebello – All Four Sections
  • Sante Fe – Sections 1 and 2
Missing Construction Plans (3)
  • Camino Real – Sections 1 and 2
  • Grand San Jacinto – Section 2

The problems in the 39 documents that Liberty County DID supply make one wonder what’s in the 19 they DID NOT supply.

Fallacy of Government Oversight

Not only are many documents missing, the ones Liberty County does have appear to be based on false assumptions about soil types.

I’m told by reputable engineers and floodplain administrators that this problem is common. Developers can always find engineers willing to sell favorable opinions – much like junkies know how to find doctors willing to write prescriptions for oxycodone.

Most people don’t have the expertise to evaluate reports like LandPlan’s. The hired guns know it and count on it. Cities and counties could hire engineers to thoroughly check these plans, but they don’t … for several reasons:

  • Awareness of this problem is low.
  • There’s no public pressure for counties to hire plan-checking engineers.
  • Developers make huge political contributions.
  • Floods often happen years after buildout of subdivisions.

By the time people flood, it’s too late. The damage has already been done. And the people responsible are often long gone.

Posted by Bob Rehak on 12/26/2020

1215 days since Hurricane Harvey and 464 since Imelda

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.

Study Suggests Large Cities Like Houston Can Intensify Rainfall and Runoff From Hurricanes

A November 2018 article appearing in the peer-reviewed scientific journal Nature found that urban growth can intensify both rainfall and runoff from hurricanes. Further, urban growth can increase the risk of flooding and shift the location of flooding. The article specifically studied the effects of Hurricane Harvey on Houston and found that urban growth increased the probability of such an extreme flood across the basin by 21X.

A sister publication, Scientific American, reviewed the article the same month and helped explain the findings in Nature.

The Nature study looks at two distinct effects of urbanization. The first is the impact of impervious surface on RUNOFF. The second is the impact of the urban landscape’s surface roughness on RAINFALL.

The Runoff Component

Numerous studies have looked at the relationship between percentage of impervious cover, runoff, and flooding – a well documented phenomenon. Impervious cover accelerates transport of rainfall from neighborhoods to rivers. That raises peak flows rather than spreading them out over time. Dr. William Dupre, professor emeritus from the University of Houston visualized the relationship this way.

Effect of Urbanization on Peak Stream Flows” by Dr. William Dupre, professor emeritus from the University of Houston.

Rainfall Component Much Less Studied

However, the effect of urban growth and a city’s surface topography on RAINFALL from hurricanes is much less studied. The authors say in Nature that, “Urbanization led to an amplification of the total rainfall along with a shift in the location of the maximum rainfall.” (Page 386).

“Much less is known regarding the urban effects on the organized tropical rainfall of a hurricane, in particular during one like hurricane Harvey, which stalled for several days.” They continue, “…experiments (with computer models) clearly show a large increase in rainfall arising from urbanization over the eastern part of the Houston area.”

The authors compared present and past urban landscapes and also modeled a scenario in which the entire region was cropland.

Mechanisms Responsible for Increase Rainfall

To understand the physical mechanisms responsible for the heavier rainfall, they analyzed the vertical convergence of winds and wind fields.

Kingwood Greens Evacuation During Harvey by Jay Muscat
Evacuation During Harvey. Photo courtesy of Jay Muscat.

“The enhanced rainfall … and the shift of rainfall … are tied to the storm system’s drag induced by large surface roughness,” say the authors.

Scientific American explains in more detail. Kerry Emanuel, an atmospheric scientist at Massachusetts Institute of Technology, who did not work on the study said, “We know cyclones are sensitive to characteristics of the surface—mountains, streams, marshland. This new twist is that cities have become big enough to tangibly alter the storm.” Said Gabriele Villarini, an environmental engineer at The University of Iowa and an author on the study, “We removed the urban areas from Houston and replaced them with cropland.”

“The presence of urban areas enhanced all the things you need to get heavy precipitation,” Villarini, one of the study’s authors says. “A stronger drag on the storm winds, associated with a larger surface roughness length” contributed to the increased rainfall.

Emanuel explained, “First, the artificial ruggedness of an urban area slows air down. Whenever air slows in a hurricane, he says, it gets shunted toward the center of the storm and up into the sky. That increases rainfall everywhere [in a metropolitan area].” He added, “A storm moves particularly slowly over downtown areas where buildings are tallest, but the winds bearing down from outside the city are still moving quickly. So, [the storm] is piling up on the city.”

Impact on and Implications for Houston

This increase in urban growth in flat terrain creates problems from a flood perspective, despite mitigation measures already in place.

Urbanization has increased the probability of an event like the flooding associated with Hurricane Harvey by about 21 times, say the authors in Nature on page 388.

The authors make several high-level recommendations.

  • Urban planning must take into account the compounded nature of the risk now recognized.
  • Flood mitigation strategies must recognize the effect of urbanization on hurricanes.
  • Weather and climate models must incorporate the effects of urbanization to increase forecast accuracy on local and regional levels.

“It is critical for the next generations of global climate models to be able to resolve the urban areas and their associated processes,” conclude the authors.

About the Authors and Models

The authors are:

  • Wei Zhang and Gabriele Villarini from the Department of Hydroscience & Engineering, The University of Iowa, Iowa City, IA
  • Gabriel A. Vecchi from the Department of Geosciences, Princeton University and the Princeton Environmental Institute, of Princeton, NJ
  • James A. Smith from the Department of Civil and Environmental Engineering, Princeton University, Princeton, NJ.

This presentation explains the Noah Model that the authors used to calculate air/ground interactions.

Local Questions Raised by Study

To date, the role of a city in altering rainfall during tropical cyclones has received very little attention. Houston has had the largest urban growth and the fifth-largest population growth in the United States in the period from 2001–2011. Much of that growth is now on the periphery of the city. The two fastest growing parts of the region are Fort Bend and Montgomery Counties.

As the city grows, we need mitigation measures that can offset the impact of that growth. That’s why the meeting of the Montgomery County Commissions on August 27th is so important. They will vote on whether to close a loophole that allows developers to avoid building onsite detention ponds. Closing that loophole is important. It will help protect hundreds of thousands of downstream residents as well as those in Montgomery County.

Also, the new NOAA Atlas-14 (rainfall measurements updated after Harvey) does not consider forward-looking urban growth effects. The precipitation frequency data in NOAA Atlas 14 was determined by a statistical analysis of historical rainfall, a key input for FEMA Flood Insurance Rate Map (FIRM) modeling. With all that uncertainty, we need to err on the side of caution in flood planning.

For more about Atlas 14, see this link.

Posted by Bob Rehak on 8/18/2019

618 Days after Hurricane Harvey

Even More Discoveries Demand Independent Investigation into Causes of Flooding Around Woodridge Village

On 8/28/2018, LJA Engineers’ project manager for hydrology and hydraulics, submitted a 59-page letter to the Montgomery County Engineer’s Department. The subject: Figure Four Partner’s proposed Woodridge Village development. It shows that the developer knew of the potential for downstream flooding, yet did not develop the site in a way that might have prevented or reduced flooding.

Specifically, the developer’s team failed to construct needed detention ponds in a timely manner. They could have helped offset the effects of clearcutting the southern section of land. Instead, the contractor continued clearcutting the northern section, filled in existing drainage, and sloped land toward Elm Grove BEFORE installing needed detention ponds.

The contractor also failed to repair a culvert running next to North Kingwood Forest. Engineers warned that the damaged culvert had to be replaced.

Finally the engineers may have mischaracterized the soil in modeling assumptions. They classified soil as sandy loam instead of clay. That could have skewed a key factor in runoff models by 2X to 3X.

Parts of Porter Also Flooded That Were Not in Any Recognized Flood Zone

LJA’s letter also shows that residents who flooded in Porter on the western edge of the new Woodridge development were NOT in either 100-year or 500-year flood zones. This supports the claims of Porter flood victims, such as Gretchen Dunlap-Smith. They say they never flooded before. They also claim that Rebel Contractors pushed dirt up against the western edge of the development while filling in natural drainage and wetlands. These actions likely constrained drainage on May 7th, before the contractor began installing storm sewers, drainage ditches and detention ponds in that area.

Flood Plain Maps Show What Developer’s Team Knew Before Permit Granted

Section 1.5 of LJA’s letter to Montgomery County states, “The project site is shown on FEMA Flood Insurance Rate Map (FIRM) panel 48339C0750H for Montgomery County, Texas and Incorporated Areas, revised August 18, 2014. The area just across the county boundary from the project site is shown on FIRM panel 48201C0305L for Harris County, Texas and Incorporated Areas, revised June 18, 2007.” On Page 51, the letter shows existing floodplains on the map below.

Page 51 from LJA letter to Montgomery County Engineer. The dark purple lines show the boundaries of the new development. The light purple and gray areas below the new development show the 100-year and 500-year flood plains in Elm Grove, North Kingwood Forest, Mills Branch and Woodstream Villages.
Drainage on the developer’s two tracts is sloped toward Taylor Gully, Elm Grove and North Kingwood Forest in red circle.

Clearcutting of the S2 detention pond area finished last November according to Nancy Vera of Elm Grove. However, only detention pond S1 and the flow-restricting box culvert next to Vera’s house had been substantially completed by May 7. Neither N1, nor the drainage ditch connecting it with N2 were excavated on May 7th; they still have not been excavated.

At the time of the May 7th flood, only detention pond S-1 had been installed. N-2 is on land owned by Montgomery County and was at least partially excavated in 2006, but none of the devices regulating flow into or out of it had been installed on May 7th.

LJA Engineering’s models assumed all the detention ponds are in and functioning, but we know they were not at the time of the flood. Instead of installing drainage first, the contractor focused on clearcutting and grading the northern section of land which exacerbated flooding on the southern section.

Drain Pipe Should Have Been Replaced

Page 3-1 of the LJA Letter mentions, “…an existing 36-inch-diameter x 290-foot HDPE culvert in Taylor Gully at the downstream end of the project. The upstream end of the culvert is within Montgomery County and the downstream end is within Harris County. Because of its poor structural condition, this culvert needs to be replaced.”

Intake end of the pipe referenced on page 3-1 of LJA Engineer’s letter to Montgomery County. Photo taken on May 12, 2019.

Judging by the poor condition of the pipe after the May 7th flood and the lack of disturbed soil around it, I feel it’s safe to say that it wasn’t replaced at the time of the flood.

Modeling May Have Included Faulty Assumption About Soil

Every time I re-read the letter to Montgomery County, new things jump out at me in light of new things I have learned. Today, I spotted another huge and potentially faulty assumption relating to runoff and flooding. The site description on page 1.1 states that the project site is “characterized by fine, sandy loam.” One of the oil industry’s leading geologists, however, characterized it as “mostly clay,” though he did say it became more sandy in natural drainage features, such as stream beds.

Major factors affecting the runoff coefficient for a watershed are land use, slope, and soil type. We know the contractor increased the runoff rate when it clearcut the forest and altered the slope of land. But I had not previously focused on how the engineers characterized the soil type, which affects water infiltration.

  • Sandy soils absorb more rain, generally reducing runoff.
  • Soils with more clay absorb less rain, generally increasing runoff.

In modeling runoff and flooding potential for Woodridge Village and downstream areas, LJA Engineers used the Army Corps’ Hydrologic Modeling System (HEC-HMS). Page 216 of the user guide for that program states that, “The sand percentage accounts for the effect of infiltration and surface runoff properties on hydrograph generation. Zero percent indicates essentially all-clay soils with characteristically low infiltration rates. Conversely, 100 percent indicates essentially all-sandy soils with characteristically high infiltration rates.”

BrighthubEngineering.com estimates infiltration rates in inches per hour for different types of soil. They show the rate for clay-based soils to average one-third to one-half the rate for sandy loam. That means…

The characterization of the soil could have skewed this component of LJA’s modeling by 2X to 3X. Certainly, that merits further investigation and verification of LJA Engineering’s results before contractors begin pouring concrete.

New Discoveries Argue for Independent Engineering Investigation

All of these observations argue for an independent investigation into the engineering of and construction practices on this site. They raise serious questions about the accuracy of LJA’s conclusions and whether their plans will protect downstream residents from future flooding.

Let’s pray that Montgomery County and the City of Houston commission a forensic investigation into the causes of this flooding. That’s the only way we’ll be able to prevent similar flooding in the future. By the time these issues work their way through the court system, contractors will have built homes and streets that could forever alter downstream flood potential. Harris County and the Federal government could be stuck with hundreds of buyouts costing tens of millions of dollars. A second opinion might save a lot of heartbreak, misery, and tax dollars. Better safe than sorry.

Posted by Bob Rehak on 6/2/2019

642 Days since Hurricane Harvey

The thoughts in this post represent my opinions on matters of public policy and safety. They are protected by the First Amendment of the US Constitution and the Anti-SLAPP statute of the great State of Texas.

Simple Demonstration Underscores How Clearcutting Contributes to Flooding

We all understand intellectually that vegetation helps reduce runoff. But I never fully appreciated how MUCH runoff it could prevent until I saw this video. Michael Jrab sent the link to me this morning. It shows a brilliantly simple, table-top experiment in a science class. The experiment dramatizes the value of vegetation and how clearcutting can contribute to flooding by accelerating the rate of runoff.

It takes only a minute or so to watch. Notice both the volume AND THE CLARITY of the water coming out.

Now contrast that with this shot of erosion in the clearcut area just north of Elm Grove. One can only wonder how fast the water moved through here.

Part of the 262 acres clearcut by Figure Four Partners, LTD, a subsidiary of PSWA and Perry Homes.

Posted by Bob Rehak on 5/13/2019

622 Days since Hurricane Harvey