Tag Archive for: Curve Numbers

When Developers Claim No Detention Ponds are Necessary…

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:

  1. Obtain the developer’s drainage analysis and construction plans.
  2. Look up soils in the development via the USDA’s Web Soil Survey.
  3. 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.


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:

  1. Go to this link: https://websoilsurvey.sc.egov.usda.gov/App/WebSoilSurvey.aspx.
  2. Click on AREA OF INTEREST (AOI) tab. Then NAVIGATE to the development in question.
  3. 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.
  4. DRAG the plus sign over the Area of Interest to DEFINE the AOI. A cross-hatched area should appear.
  5. 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.
  6. Click on the SOIL DATA EXPLORER tab.
  7. In the row under it, click the SOIL PROPERTIES AND QUALITIES tab.
  8. From submenu, click on SOIL QUALITIES AND FEATURES.
  9. From the next submenu, click on HYDROLOGIC SOIL GROUP.
  10. Click VIEW DESCRIPTION to learn about differences between soil groups, then…
  11. 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.
  12. 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.

From TXDoT

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.

Table from TXDoT

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.