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.
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.
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.”
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.