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Nature Never Forgets: Lessons of May 7th Flooding

A 1949 map of the area now called Kingwood reveals that the homes in Elm Grove that flooded on May 7 were built in something once called the Odom Lake Swamp. It turns out, “Nature never forgets.”

Area of interest for this discussion is circled in red.

I previously posted about this map in the context of how the West Fork has shifted over time. At the end of the post, I asked readers to write me if they found anything else interesting. One did. And it was a very interesting indeed.

He pointed to the area circled in red above. When I superimposed that over the present-day image below, my jaw dropped.

Same general area that was superimposed over old map.

It’s True. Nature Never Forgets

The area labelled Odom Lake Swamp matches very closely the outline of the May 7 flood in Elm Grove that damaged almost 200 homes.

I used the county line and the confluence of the East and West Forks to align the two images, then cropped this out of the center.
The reader who reported this came up with a slightly different map that shifts the label “Odom Lake Swamp” to the west side of Village Springs. I am not sure what alignment points he used, but his work and mine closely match.

At a Houston Geological Society seminar on flooding that I attended last year, I remember several speakers talking about this phenomenon, including former Harris County Judge Ed Emmett. During heavy rainfalls, water gravitates toward its original channels.

One expert, who talked to me on condition of anonymity because of the lawsuits swirling around this issue, explained it this way. “Where old channels, swamps, meanders, etc. were filled in, during major floods, water always seeks the lowest point. Even if you fill in an area, it usually is still the lowest point of a larger area (or watershed) and during large rain storms, the water finds its way there.”

Current Flood Map Echoes 1949 Map

He attributed the Elm Grove flooding to a combination of clear cutting without mitigation upstream, heavy rainfall, and lower elevation. In fact, FEMA’s National Flood Hazard Layer Viewer shows that this part of Elm Grove is in the 100-year (aqua) and 500-year flood plains.

2007 (most recent) flood plain map for Elm Grove, Mills Branch, Woodstream and Royal Brook subdivisions.

Implications

I’m not a lawyer and I don’t give legal advice, but it seems to me that this finding does little to change the legal lay of the land (no pun intended).

The residents south and east of Woodridge Village, despite being lower than surrounding areas, had never flooded before – even in Harvey.

They didn’t flood until the developer clearcut 268 acres, filled in natural streams, eliminated wetlands that act like natural detention ponds, and graded the property toward the area that flooded. All without constructing detention ponds until AFTER people flooded.

Had those ponds been in place, they should have held 13 inches of rain (a 100-year) rainfall. We didn’t get that much. The gage at US59 recorded 6.24 inches on May 7 over six hours. The heaviest rain fell during the noon hour when we got 3.64 inches.

From HarrisCountyFWS.org for May 7. The other official nearby gage at West Lake Houston Parkway received less rain that day.

Harris County meteorologist Jeff Lindner characterized the May 7th event as somewhere between a two and 50-year rain. Experiment with the different possibilities on the chart below.

NOAA Atlas 14 precipitation frequency chart.

Ten Lessons of May 7th

So aside from the Odom Lake Swamp being a historical curiosity, what can readers learn from this.

  1. Flooding doesn’t always come from the river. Streets can flood homes when the rainfall rate exceeds the capacity of storm drains.
  2. Before you buy a home, check historical maps. Learn whether the developer filled in lakes, ponds, swamps, or wetlands and then built your home on top of them. Remember: Nature never forgets!
  3. If the answer is yes, question how much you want the property. Use the knowledge to negotiate a discount with which you can purchase flood insurance. That’s the best way to discourage unsafe development practices.
  4. If you live downstream of an undeveloped area, be aware that floodplains are a shifting target. Just because you’re NOT in a floodplain today is no guarantee that you won’t be tomorrow. Upstream development can cause downstream flooding. So watch carefully.
  5. Pay no attention to anyone who says, “Oh, that area will never be developed.” The more worthless the land, the bigger the profit potential.
  6. If you live in this area, get flood insurance.
  7. If you buy a low-lying home, be prepared to have your life disrupted.
  8. Buyer beware.
  9. Pressure your elected representatives to turn areas such as Woodridge Village into park land. When it was wetlands, it protected the people downstream from flooding and provided recreation.
  10. Buying the property north of Elm Grove could have cost less than the damage to one home. (See appraisal below). Not buying the property was a costly decision.
Ironically, the Montgomery County appraisal district values the 60+ acres of land north of Elm Grove at about a quarter million dollars.

Posted by Bob Rehak on 6/19/2019

659 Days since Hurricane Harvey

Living Landscape: San Jacinto River Before Lake Houston and Now

Geologic change happens so slowly, most people won’t live or stay long enough in one place to perceive it. Then something happens to make you crank up the Wayback Machine and look more closely. Yesterday was one of those days for me. The Army Corps announced that it was going to begin dredging part of the West Fork mouth bar area.

That raised the question, “Which part?” That wasn’t announced. So I asked Tim Garfield, retired chief geologist for one of the world’s largest oil companies, what he would do. He felt it was important to re-establish the river’s natural channel. So I asked him where it was. (Spoiler alert: It’s between the mouth bar and Atascocita Point.) But in the process of figuring this out, I learned many more things about the mouth bar and a river I take for granted. I’ll save those for the end.

70 Years of Change on the River

Garfield led me to the Perry-Castañeda Map Collection of Texas Topographic Maps at the UT Library Online. He found this map from 1949 of Moonshine Hill. It’s exactly 70 years old! The 1949 date means we can see where the river was before the dam and lake were built in 1955.

The San Jacinto in 1949 before Lake Houston was impounded in 1955. For a higher resolution version of this map, click here. This map shows what geologists call “the relict channel.”

Kingwood, Atascocita and Huffman Before Settlement

This map shows areas that would eventually become Atascocita, Kingwood, and Huffman. It includes the area where the mouth bar has formed between Kings Point and Atascocita Point.

You can tell a lot by looking at this map. You can tell even more when you superimpose it over a satellite view of the area today in Photoshop. Suddenly, you see how the landscape has changed. In fact, it changed so much that I had problems aligning the two images.

Map Superimposed Over Satellite Image At Varying Opacities

However, the county line and 1960 are still in the same location. So I used those as reference points. Then I varied the opacity in the top layer (the old map) so that you could see more and more of the current landscape. At different percentages, you can see how various features have changed over time.

Here’s what the sequence looks like starting with 1949 and today. I started by cropping tighter on the area of interest, the West Fork where the Corps is dredging. I include several different opacity ratios because some changes become more apparent at one ratio than another.

100% opacity for 1949 map.
0% 1949 and 100% today.
60% 1949 and 40% today.
50% 1949 and 50% today.
33% 1949 and 67% today.
25% 1949 and 75% today.

Most Visible Changes

Starting from the left:

  • In the 33/67 image, notice how the river once meandered near US59 and how much further south it was.
  • In the 50/50 image, notice how much of the Romerica land was swamp in 1949…and still is.
  • In the 75/25 image, notice how much the river migrated north just north of Kings River estates.
  • In the 25/75 image, notice how much area the lake claimed.
  • In the 33/67 image, notice how far north the river has shifted under the West Lake Houston Parkway bridge.
  • In the 33/67 image, notice how Atascocita Point has grown past the relict channel.
  • In the 60/40 image, notice how the mouth bar grew at the confluence of a relict stream bed within the lake and the relict channel of the West Fork. You can also see this pretty clearly in the 25/75 image.
  • In the 25/75 image, notice how the relict West Fork channel used to hug Atascocita Shores.

Key Map

This image shows locations referenced above for those who may not be familiar with them.

Key to locations

Do you see other things that I did not? Please let me know through the contact form on this web site.

As the dredging program moves forward, these maps may also help inform dredging strategy. Stay tuned.

Posted by Bob Rehak on 6/13/2019

653 Days after Hurricane Harvey