I took this picture on May 26, 2021. It shows TxDOT construction of the new State Highway 99 (Grand Parkway) next to FM1485 in New Caney. The picture looks northeast toward Colony Ridge in Liberty County. The East Fork of the San Jacinto River flows under both bridges toward Lake Houston on the right.
How Much Rainfall Caused This?
Here is rainfall for the month of May as measured by the Harris County Flood Control District Gage at this location.
The gage upstream at FM2090 shows slightly more rain. It reported 14 inches for the month instead of 11, but it received exactly 2.28 inches on the same day this gage did. While 2+ inches in a day is substantial, few in this part of the world would consider it excessive – especially since it was spread out over 5 hours.
Likewise, according to Atlas-14 standards, the rain that fell in the week before would qualify as a 1- to 2-year rain – notable, but not historic.
Submerged 41 Times in 32 years
And after consulting Harris County Flood Control District records, I learned that FM1485 has gone under water 41 times since 1990 – an average of 1.32 times per year.
Rainfall data, road flooding frequency and the photo all suggest that a 1- to 2-year rain is enough to flood FM1485.
What Should a Roadway over a Major River Withstand?
Obviously TxDOT built a much higher road and a much wider, taller bridge for its new highway. The new one is approximately five times wider than the old one. Construction standards for major highways could account for that. But so could TxDOT’s experience with FM1485.
So What’s Going on Here?
Why did TxDOT make the new bridge so much wider and taller?
- Did TxDOT just get the engineering wrong on the old bridge?
- Did bridge standards change over time?
- Do state highways have higher standards than farm-to-market roads?
- Did Atlas-14 increase the risk?
- Did upstream development, such as Colony Ridge, alter the hydraulics of the watershed when the developer paved over wetlands and deforested thousands of acres while providing little detention-pond capacity?
- Did the mischaracterization of soil types in Colony Ridge lead to more runoff than anticipated?
- All of the above?
- Some of the above?
Jeff Lindner, Harris County’s meteorologist, cautions that, “Water surface elevations depend on many variables…rainfall patterns, intensity, soil conditions, water level in the river when the rain started, ect. It is usually difficult to compare events as no two are exactly alike. You really need a hydrological analysis of the location to determine the amount of run-off from that site into the river per an amount of rainfall.”
Good luck with that! More than six months after the Liberty County Attorney launched an investigation into Colony Ridge drainage reports, we still are waiting for answers.
Posted by Bob Rehak on 7/15/2021
1415 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.