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.
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).
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.
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.
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.
Texdot
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.
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.
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.