In the rush to mitigate flooding after Hurricane Harvey, most people agreed that we should attack the areas with the worst flooding first. While almost everyone agreed with the “worst first” mantra, no one defined it – until after the flood bond passed.

Alternative Ways to Define “Worst First”

How would you define the “worst” flooding? The area with the:

• Most flooded homes?
• Most frequent flooding?
• Most damage in dollar terms?
• Largest population living in a 100-year floodplain?
• Most low-to-moderate income residents who can least afford to fix their homes?
• Largest minority populations?
• Highest Social Vulnerability Index (SVI)?
• Most damage to infrastructure (bridges, hospitals, grocery stores, schools, sewage treatment plants, etc.)?
• Highest water above the tops of banks in a widespread storm like Harvey?

You could make valid arguments for each. And people have – almost from the day the flood bond passed – as they fought for more funding for their watersheds.

Least Considered Alternative

But the one I have heard talked about the least is the last one: highest water above tops of banks. Surely, the depth of a flood must count for something. It affects the ability to evacuate and rescue people. It puts lives at risk. Can destroy infrastructure. Spread sewage. Increase erosion. Even cause rivers to migrate. And sweep whole apartment buildings into rivers.

Yesterday, as I reviewed a doctoral thesis from a student at the Colorado School of Mines, I saw a graph that compared the height of the Harvey flood at the San Jacinto West Fork and US59 to other rivers/streams in the region. The West Fork flood towered above the others. So it made me wonder. How did the flood at that location compare to other streams and bayous in Harris County?

How to Determine “Feet Above Flood Stage”

There’s an easy way to find out. The Harris County Flood Warning System contains all the pertinent information.

1. Go to the home page.
2. Click on a gage.
3. Click on the “For more information” link in the pop-up box.
4. On the new page (dedicated to historical information about that gage), click on the stream elevation tab.
5. Note the “flooding likely” elevation. This roughly coincides with the top of bank, though banks can vary slightly.
6. Scroll down to the bottom of the page and note the elevation of the flood at that location during Hurricane Harvey.
7. Subtract the “flooding likely” level from the Harvey level, to determine the feet above flood stage in Harvey.
8. Click on the drop-down box with the gage information to compare the same stats for additional gages.

North Harris County Had Highest Floods

This morning, I compiled the data for 33 gages. The chart below shows what I found. Of the gages I sampled, the four highest floods all occurred in the upper San Jacinto River Basin: West Fork at 59, Spring Creek at 45, Cypress Creek at Cypresswood, and West Fork at 99. Also note the preponderance of high water along Greens Bayou at numerous locations.

Limitations of Measure

“Feet above Flood Stage” by itself won’t tell you which area had the worst flooding during Harvey. It’s just one measure. You must consider other factors, too, such as those listed above.

But this chart sure makes it hard to ignore the fact that something is happening in north Harris County to exacerbate flooding. I’ve chronicled many of those things in the pages of this website: sand mining, development, rapid growth, impervious cover, loss of wetlands, lack of detention ponds, lax regulation/enforcement and more.

One must also acknowledge the role that topography plays in a chart, such as this. A narrow floodplain with steep banks can force water higher. A wider floodplain through a flat area allows water to spread out, lowering the height of floodwaters.

Variations in rainfall across an area can also skew results.

And finally, I didn’t click on every gage in the region. There are hundreds. You wouldn’t have been able to read the chart because the type would have been so small. So investigate on your own and let me know what you find.

Need for Active Discussion/Debate in Upcoming Elections

The Harris County Community Flood Resilience Task Force just voted to change the way “worst first” is calculated – again! The highest correlation between funding and all of the other factors I have evaluated is now with low-to-moderate-income population. It’s not with damaged structures, dollar damage, watershed size, or population density.

If you want to ensure that outlying areas get their fair share of flood mitigation dollars in the future, you need to demand them when you go to the polls this fall.

Posted by Bob Rehak on 3/29/2022

1673 Days since 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.