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Questionable Assumptions by LJA Engineering May Have Compounded Elm Grove Flooding

Questionable assumptions about soil composition, rainfall patterns, wetlands and floodplain status for Perry Homes’ troubled Woodridge Village development may have compounded flooding in Elm Grove and North Kingwood Forest. Previously, I have focused on other more obvious issues, such as missing detention ponds and expected rainfall totals. However, Elm Grove resident Jeff Miller who has been studying the LJA Drainage Report, urged me to explore these additional issues.

Soil Type Closer to Clay than Sandy Loam

The LJA engineers doing the drainage analysis for Perry based their runoff calculations on a soil type called “sandy loam.” Different soil types absorb rainfall at radically different rates. According to 2011 data from Texas A & M Agrilife Extension, water infiltration rates for:

  • Sand = 2″ – 6″ per hour
  • Loam = .6″ – 2″ per hour
  • Clay = .2″ – .6″ per hour

Clay absorbs water very slowly, so rain turns into runoff quickly. Two photos below show what the Woodridge Village construction site looked like ONE WEEK AFTER a 2″ rain. It still had ponding water that did not soak in.

The northeastern portion of Woodridge Village looking southwest. Photo taken on 11/4/2019 one week after a 2″ rain. Note the ponding water that has not yet infiltrated.
A closer shot of the main portion of the north side of the site, also taken on 11/4/2019. Note the ponding water here, too. USGS characterized parts of this portion of the site as WETLANDS.
From the USGS National Wetlands Inventory. Note how the ponding water in the photo aligns with where the wetlands were.
Drone footage of southern section taken two days after May 7th flood. Courtesy of Jim Zura, Zura Productions.

In May, a retired local geologist from a major oil company estimated that the clay content in Woodridge soil was at least 50%, and could be as high as 80%. However, he could not commit to an exact figure.

Going by the A&M infiltration figures above and assuming an infiltration rate of 2″ per hour for a mixture of sand and loam, and contrasting that with the minimum infiltration rate for clay, .2″ per hour, you get a difference of 10X.

What does all this mean?

Based solely on soil type, the LJA Engineering report could err in its runoff calculations by as much as 10X.

Sections 1.1 and 2.1.3 of the LJA report discuss the runoff based on soil type. No matter how sophisticated the calculations, if you base them on the wrong soil type, the result will be inaccurate. “Garbage in, garbage out” as they say in the computer business.

Before clearcutting, there may have been more sandy loam in a thin surface layer. But contractors likely disturbed or buried that when they removed vegetation from the site and regraded the area.

If LJA wishes to challenge this, I will be happy to reprint their response verbatim. I would love to see their soil report.

But I would like to know how they explain the presence of ponding water throughout the entire northern section of Woodridge Village a full week after a two-inch rain. (Note that the northern section is the steepest and largest. It comprises 2/3rds of the Woodridge Village’s acreage.)

I took the aerial shots above at around 2PM on 11/4/2019. Here are the rainfall totals for the previous 7 days. Graph courtesy of Harris County Flood Warning System.

The Presence of Wetlands Should Have Been a Signal

The presence of wetlands should have been a signal to the developer, but the LJA report does not mention the word “wetlands” once.

Many residents who used to hike and bike this area before it was clearcut have told me that they could always find standing water there even in summer. Here’s an interesting article that explains why wetlands stay wet. The authors says, “Wetlands typically form in gently sloping or topographically convergent portions of a landscape, where surface and ground waters meet.” That certainly fits Woodridge Village.

As Miller says, “Drone footage; many photos; and the constant presence of water in the S-2 detention pond and the rectangular pond where N3 should be confirm that the ground is saturated. When the soil is totally wet, water will move over the surface.” And as LJA says in the intro to its report, “The project site naturally drains to Taylor Gully.”

And that’s exactly where so many people flooded when contractors accelerated the runoff through clearcutting and by not providing adequate detention.

Balanced Storm Assumption Rarely Accurate

Section 2.1 of the LJA report says that LJA models assumed a “balanced storm.” “This distribution is constructed such that the depth specified for any duration occurs during the central part of the storm (intensity position = 50%).” [Emphasis added.]

But as Jeff Johnson, the Montgomery County engineer, pointed out, using a “balanced storm” bases calculations on ideal assumptions. He also pointed out that only a small percentage of storms conform with ideal conditions. (Johnson made these remarks at a Montgomery County Commissioners Court meeting at which they discussed closing a loophole in flood regulations.)

According to the US Geological Survey, this graph represents a balanced storm.

In the ideal balanced storm, most of the accumulation happens in the middle of the storm.


But the May 7th and Imelda storms did not follow this pattern. The heavy rainfall was front-loaded in both.

More than half of the total rainfall during the May 7th event fell at the outset, not in the middle of the storm. Thus, it did not conform to the balanced storm model upon which LJA based its calculations.
During the 11 hours of rainfall from Imelda, more than 50% fell in the first three hours, and almost 80% in the first 4 hours. Thus, Imelda was also “front-heavy”.

LJA Assumed Woodridge Was Outside of 100-Year Floodplain

LJA also assumed (see section 1.5 of its report) that Woodridge Village was “outside of the 100-year floodplain.” For permitting purposes, that is technically true. LJA was going by accepted maps. But that area is not shown as flood plain only because FEMA did not model it. Note in the Montgomery County Flood Plain Map shown below how ALL FLOODPLAIN MAPPING STOPS AT THE COUNTY LINE.

That straight diagonal line you see is the Harris-Montgomery County Line.

Any engineer experienced in working with flood plains should know that physical boundaries do not stop abruptly at political boundaries. Any competent engineer should have questioned this.

Engineering Codes of Ethics Discourage Such Conduct

While LJA did what regulations required, they had a higher ethical obligation to protect people as licensed engineers. See the Code of Ethics of the NSPE – the National Society of Professional Engineers. It states:

“…engineers are expected to exhibit the highest standards of honesty and integrity. Engineering has a direct and vital impact on the quality of life for all people. Accordingly, the services provided by engineers require honesty, impartiality, fairness, and equity, and must be dedicated to the protection of the public health, safety, and welfare. Engineers must perform under a standard of professional behavior that requires adherence to the highest principles of ethical conduct.”

Under Fundamental Canons, the Society’s Code of Ethics also requires engineers to “Hold paramount the safety, health, and welfare of the public.”

Also see Professional Conduct and Ethics for Texas Engineers.

The latter states “In order to safeguard, life, health and property, to promote the public welfare, and to establish and maintain a high standard of integrity and practice, the rules relating to professional conduct in this title shall be binding on every person holding a license and on all firms authorized to offer or perform engineering services in Texas.” In this regard, LJA failed the people of Elm Grove and North Kingwood Forest dismally.

Overlooked Ethical Obligations Contribute to Dramatic Miscalculations

Ignoring the missing flood plain information, not mentioning wetlands, and mischaracterizing soil composition all contributed to dramatic miscalculations. Add those problems to ignoring new statistics that showed flood plain maps would need to be redrawn based on NOAA’s new Atlas 14, and that a 100-year storm would include 40% more rainfall.

Sometimes when you’re eager to make a project happen, optimism leads one to make “best-case” assumptions. But in my opinion, engineers should act on “worst-case” assumptions” to product public safety.

Sometimes, the cost of failure is simply unthinkable. This is one of them. Elm Grove flooding wasn’t as spectacular as a dam or a bridge failing, but it likely affected far more people.=

Webster and Spurlock law firms are currently trying to subpoena correspondence between LJA and Perry Homes and its subsidiaries. The Perry Homes gang is trying just as hard to stonewall production. It will be interesting to see what pressure, if any, they put on LJA to ignore these obvious problems…if the documents ever become public.

An even bigger ethical question: With such obvious problems, why did Montgomery County Commissioners and the City of Houston approve permits for this development?

Posted by Bob Rehak on 11/10/2019 with help from Jeff Miller and video from Jim Zura

803 Days after Hurricane Harvey and 51 after Imelda

The thoughts in this post represent my 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.