Impervious Clay in Clear-Cut Area Accelerated Runoff Toward Elm Grove Before Detention Fully Developed

Hundreds of residents in Elm Grove and North Kingwood Forest, south of the new Woodridge Village development, flooded last week. Everyone says they never flooded before the contractor started clearcutting and grading the property. So what changed? Clearcutting removed all the plants and ponds that slowed the water’s velocity. It also exposed a highly impervious clay soil base, so slick when wet, that it offered very little resistance to flow. That accelerated stormwaters toward Elm Grove, where detention ponds had yet to be built and, according to residents, the developer had filled in natural drainage features.

Geologist Finds Impervious Clay

I asked a retired top geologist from one of the world’s largest oil companies to describe the soil composition. The answer? At least 50% clay. “Because clay grains are very small (<2 microns), plate shaped and tightly bonded, water does not easily or quickly move through or into clay-dominant sediments without the help of plant roots.” Thus, there’s much more runoff than absorption, especially after clearcutting and grading.

To check that assumption, he dug a hole and filled it with water from a bucket.

The water took 15 minutes to go down one-half inch.

Absorption rate indicates low permeability and high runoff rates. Note the ponding water halfway up the stake, still sitting on the surface from week-old rains.

Still Had Standing Water Eight Days after Rain

He continued. “The presence of many puddles of standing water from week-old rains indicate that clay-dominant sediment like I sampled is wide spread across the site as it is throughout our fluvial flood plain setting – except locally where sandy channel fills are also present.”

Standing water remained on the site, days after the last rain, indicating a high clay content.

What a Photo Can Tell: Decoding Erosion Patterns

I also asked him to analyze this photo below and tell me whether it changed his opinion of soil composition. The photo was taken directly north of the box culvert installed by the developer near Taylor Gulley. The area was several blocks from where he sampled soil.

Several days after this photo was taken the contractor excavated this area to form a retention pond that should have been there before the flood.

When the geologist saw the photo above, he said:

  1. The erosion itself indicates a high rate of water runoff and minimal absorption. 
  2. Steep edges imply cohesion typically associated with clay. Sand or less cohesive soils would slump.
  3. Standing water proves low percolation rate. Only clay rich sediments would hold water like that for more than a few hours.

What Contractor Should Have Known

The contractor developing the site had to know the soil was impervious. They had worked it for a year or more and had to see standing water on numerous occasions that reportedly caused delays. Still, they did several things that increased flood risk for downstream residents – before they completed site detention. For instance, they:

Basically, they increased the slope of land, reduced the friction that vegetation provides, and accelerated runoff toward an area that they knew could flood, across soil that they knew was impervious…before finishing the detention work.

Photo by Jeff Miller on 5/16/2019, more than a week after the Elm Grove flood, shows developer starting to excavate the detention pond near the portion of Elm Grove and North Kingwood Forest that Flooded.

Had all the detention been installed before the storm hit, Elm Grove and surrounding areas should not have flooded.

Hydrologist’s Claims at Odds with Performance

The hydrologist’s conclusion (see page 3 of the 59 page report) states that on-site detention should hold up to a hundred-year rain. But the Harris County meteorologist estimates that on the day Elm Grove flooded, the area received at most a 50-year rain. Maybe everything wasn’t working as planned after all. Maybe the developer should have changed its approach too construction. Developing detention sooner could have reduced flood risk.

Posted by Bob Rehak with help from Jeff Miller on 5/20/2019

629 Days since Hurricane Harvey