soil groups Archives - Reduce Flooding

Harris County residents are spending billions on flood mitigation, even as developers upstream claim they need no detention ponds. So are those developers’ claims valid? Maybe. Maybe not. A ReduceFlooding.com investigation into one found the developer’s engineer consistently mischaracterized soil types in a way that underestimated runoff from the property by 6X to 9X! Soil type is just one of the factors that that determine whether a development needs detention ponds, but it’s a big one.

Understand also that regulations vary widely from county to county and city to city. Some may not allow the type of study that the engineer above performed. But Liberty County does. And so does Montgomery County for developments under 650 acres.

To see whether a new development near you accurately reflects soil types in its drainage analysis , follow these steps.

High-Level Outline

You will need to:

Obtain the developer’s drainage analysis and construction plans.
Look up soils in the development via the USDA’s Web Soil Survey.
Compare the “Curve Numbers” used by engineers in Step 1 to Soil Groups in the development from Step 2.

Curve Numbers represent the rate of rainwater infiltration numerically. But USDA estimates infiltration by grouping soils alphabetically. So comparing them requires translation. Not to worry. Just look them up in the tables below.

Your Goal: To see if the developer accurately depicted the soils in his development.

Engineers can alter inputs to provide the desired outputs. And this is one of the main ways they can do it if they’re going to cheat on the volume of detention ponds necessary. See more below.

Background

Different soils absorb water at different rates. What doesn’t soak in runs off. And under developed conditions, it can run off quickly. That’s why developers build detention ponds. But detention ponds cost time and money. They also reduce the amount of salable land.

So developers and their engineers have a large incentive to avoid building detention ponds…if they can. By misrepresenting soils, unscrupulous engineers can make it look like more rainwater is soaking in, so they have less runoff to handle. Most people don’t have the expertise to challenge engineers and the engineers know it. So here’s one way to check their work before hiring your own consulting engineer.

Step One: Request and Review Plans

To obtain the developer’s plans, file a FOIA (Freedom of Information Act) or TPIA (Texas Public Information Act) request:

Find the County Engineer’s email address online.
Compose a short email.
Put FOIA or TPIA REQUEST in the title.
Specify the subdivision.
Ask for the Drainage Analysis AND Construction Plans for ALL parts of the development.
To speed up the process, request electronic delivery.

By law, the city or county has ten days to comply.

When you get the plans, check the construction docs to see if they have any detention ponds included.

Then look at the drainage analysis. It should contain a discussion about soils.

Also look for numbers on both the Drainage Analysis and Construction Plans preceded by “CN.” CN stands for Curve Number. That’s a numeric representation of the rate of infiltration for soil groups that engineers use in their runoff calculations. Note the curve numbers for future reference.

Step Two: Look Up Developer’s Soil Types Via USDA

USDA has surveyed soils of every county in Texas. They make it easy to see what kinds of soils are present in any development. USDA groups them via rate of infiltration, but assigns LETTERS to groups, not CURVE NUMBERS. (You will translate those in the next step.)

To determine the soils and their groupings:

Go to this link: https://websoilsurvey.sc.egov.usda.gov/App/WebSoilSurvey.aspx.
Click on AREA OF INTEREST (AOI) tab. Then NAVIGATE to the development in question.
Over the map, you’ll see the words AOI in two red buttons, one rectangular, the other a polygon. Pick one and click on it. The cursor will then turn into a plus sign.
DRAG the plus sign over the Area of Interest to DEFINE the AOI. A cross-hatched area should appear.
Click on the SOIL MAP tab. A white box will appear on the left showing all soils in the AOI, their acreage, and the percent of the AOI that they comprise. Contours will appear on the map showing soil-type locations.
Click on the SOIL DATA EXPLORER tab.
In the row under it, click the SOIL PROPERTIES AND QUALITIES tab.
From submenu, click on SOIL QUALITIES AND FEATURES.
From the next submenu, click on HYDROLOGIC SOIL GROUP.
Click VIEW DESCRIPTION to learn about differences between soil groups, then…
Click VIEW RATING. Your map should turn into colored groups. Underneath the map, a list of all the soils in the AOI will appear with their Group (A, B, C or D). Group A soils have the highest rate of infiltration; Group D the lowest. Most important, you can see the PERCENT of the AOI that each soil in various groups comprises.
For future reference, click PRINTABLE VERSION in the upper right. This lets you save your findings as a PDF or printout. The file will include: the MAP, a LEGEND, SOIL GROUPS, each group’s PERCENT OF THE AOI, and a SUMMARY of what the different groups mean.

Screen capture for newly developing part of Colony Ridge shows that only 3.2% of the soils should be classified lower than Rating Group D. See comment below about mixed groups **after** development. This represents **undeveloped** land.

After you do this once, the second time should take less than five minutes. Next…

Step 3: Compare USDA’s Soil List with Developer’s Curve Numbers

Now you need a way to compare the developer’s Curve Numbers with USDA’s soil groups. TXDoT does the “translation” for you in the two tables below taken from this page.

Table 4-17 shows infiltration rates by soil group in inches per hour. They range from a high of .45 inches to a low of 0.

Table 4-18 shows Curve Numbers for Development Type and Soil Group. Note how many houses per acre there are from the construction plans. Then look up the corresponding Curve Numbers under each Soil Group.

Create a weighted average of your findings. For soil groups A/D, B/D and C/D, use the curve numbers that correspond to D. That’s because, after development, soil will be compressed, reducing the rate of infiltration. AND note the last line: “Developing urban areas: Newly Graded.” Group D has a curve number of 94, close to the theoretical upper limit for runoff.

Step 4: Evaluate Your Findings

Did the developer use the right curve numbers for USDA soil types? In a 22,000 acre development in Liberty County with almost no detention, I found virtually all Curve Numbers associated with Group A soils (those having the highest rate of infiltration). But the vast majority of soils actually had the lowest rate of infiltration and the highest rate of runoff.

That meant that the development had 6-9X more runoff than the engineer’s runoff models showed.
Comparing TXDot Table 4-18 to Table 4-17

Curve numbers ALL erred in the direction that favored the developer’s profits.

If you find errors like that, demand explanations. Keep the system honest. Let people know you’re checking. Your home could be the next one to flood. In egregious cases, you may want to hire a consulting engineer to verify whether the rest of the analysis is valid and meets local regulations.

Posted by Bob Rehak on 12/29/2020

1218 Days after 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.

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.
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.

*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: TXDoT http://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

Tag Archive for: soil groups

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