Most of us have seen sinkholes. And most of us have probably given little thought to what causes them.

Retired engineer Chris Bloch, a local flood fighter, measured a 13-foot deep sinkhole near Taylor Gulley earlier this year that had become overgrown with vegetation. Working with the City of Houston and a local homeowner who first reported the problem, they found the sinkhole related to a nearby storm drain.

Other things can cause sinkholes, but in this area and in this part of the world, “storm drain gone bad” ranks high on the list of things to investigate.

Corrugated Pipe Commonly Used At Outfalls When Kingwood Built

According to Bloch, when Friendswood built Kingwood, they commonly used corrugated metal pipe (CMP) at storm sewer outfalls. CMP has an expected service life of approximately 35 years, says Bloch. And corrosion commonly causes failure at older outfalls constructed with such pipe.

For example, see the section of pipe below. This photo was taken on Ben’s Branch, not Taylor Gully, but it shows how the pipe rusted, bent, and crimped. Also look just to the right of the wooden posts, and you can even see a large hole in the pipe. It even tilts upward before it reaches the creek.

Taylor Gully Sinkhole One of Many In Area

Bloch frequently walks ditches looking for sinkholes to report to the City. He says he’s aware of at least five right now.

The 72-inch outfall to Taylor Gully serviced drainage area G03408-00-OUT which encompasses all of Greenriver Valley Drive and Mountain Bluff Lane as well as portions of Appalachian Trail, Natural Bridge and Echo Falls Drives. That area comprises 33.4 acres. See below.

The transition from a 72-inch concrete storm sewer to the 72-inch CMP outfall is at a manhole on the edge of the Taylor Gully right of way. See below.

Over the years, the CMP at the connection to the manhole failed. The earth above the failed pipe washed into the outfall and then downstream where it helped reduce the conveyance of Taylor Gully.

Repairs Started But Not Complete

The City of Houston has already repaired several storm sewers with failed corrugated metal outfalls. At least five additional storm sewer outfalls with sinkholes have been identified that have not yet been repaired.

Vegetation frequently hides the presence of these sinkholes. In addition to posing a danger to citizens walking along the banks of the drainage channels, the soil that falls into the sewer can be held up by the corrugations of the metal pipe reducing the flow capacity of the sewer, says Bloch.

Even without obstruction from soil falling into a sewer outfall, the rough surface of the corrugated metal pipe generates greater friction than smooth concrete. The friction slows the flow of storm water and reduces flow capacity which becomes critical during high intensity rains that cause street flooding.

Bloch somehow convinced the City of Houston Public Works Department to replace the damaged CMP at Taylor Gully with concrete pipe. See below.

“This upgrade in the outfall piping will significantly improve the flow capacity of the storm sewer system,” says Bloch.

Concrete Better for New Atlas-14 Rainfall Intensities

When Friendswood Development installed stormwater sewers in Kingwood, they met standards which applied at that time. With new Atlas-14 rainfall projections, we now understand that we should expect more rainfall. Several homes along Appalachian Trial suffered flood damage during Tropical Storm Imelda.

How to Report Sinkholes When You Find Them

As they say, it takes a village to reduce flooding. Now that you know the story behind sinkholes…

If you jog or walk along drainage ditches, exercise caution. Look out for developing sinkholes. And report them to 3-1-1 when you find them. They can pose dangers to children and even grown adults. To put that in perspective, Bloch (shown in the first photo) is more than 6 feet tall, but the sinkhole dwarfs him.

For More Information about Sinkholes

I highly recommend:

The US Geological Survey has an in-depth discussion of different types of sinkholes in different parts of the country and how each forms. This is geared toward students in science classes.

This NBC News story on YouTube has some spectacular examples of sinkholes in Florida along with easy to understand animations that show how they form.

Another 7 minute YouTube video from a series called Practical Engineering focuses on how sinkholes form. It contains both real world examples and table-top experiments that bring the processes to life.

Posted by Bob Rehak based on information and photos provided by Chris Bloch

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