On August 24th, Dr. William R. Dupré , Professor Emeritus of Earth & Atmospheric Sciences from the University of Houston, gave a presentation at the Kingwood Community Center sponsored by the Houston Geological Society. The presentation is titled Flooding and Floodplains in the Houston Area: Past, Present, and Future. Professor Dupre’ has given ReduceFlooding.com permission to post his presentation. It consists of two parts. Together, they will help you understand how and why floodplains change over time.

The Basics of Flooding, Floodplains and Measurement

In Part 1, Dupré focuses on the basics of flooding, flood plains and measurement.  He begins by explaining:

• The difference between drainage basins, networks and watersheds
• How stream gages work
• How and where to find flood data (USGS, SJRA, Harris County Flood Warning System)
• How to compare hydrographs from different locations and assess your risk of flooding
• The difference between annual recurrence intervals and annual exceedance probabilities
• How to understand flood maps
• Assumptions behind flood plain calculations
• Different types of flooding (overbook, ponding, sheet flow, etc.)

At the end of Part 1, Dupré shows how some of these concepts apply to different watersheds within Harris County and discusses how flooding dangers differ in various parts of the County.

How and Why Floodplains Change Over Time

In Part 2, Dupré goes into greater detail about how floodplains change over time.  The four main reasons include:

• More data and a longer record
• Changing land use, (i.e., urbanization, prairie restoration, etc.)
• Structural changes (dams, levees, channelization and detention/retention basins)
• Changing climate

Part 2 concludes with a discussion of changing approaches to flood control and a brief discussion of the recently approved Harris County flood bond.

Do Sand Mines Play a Role?

Part 2 includes a discussion of sand mining under “Causes of Changes in Sediment and Sedimentation. Dupré talks about different types of sand mines and their impacts. While the professor and I disagree about the interpretation of several satellite images, we agree wholeheartedly about the need to locate pits outside of a ‘channel migration’ zone, as regulations  in Washington state and Arizona require.

The danger, he says, is that rivers can migrate to and through sand mine dikes. If this happens after abandonment of the mine, no one will me there to repair the dikes and the river will reroute itself through the pit, carrying stored sediment downstream.

Who Will Benefit from This Presentation

If you enjoy earth sciences, as I do, these presentations will feel like going back to college. If you’re simply a homeowner trying to figure out why you flooded, you’ll find lots of food for thought in these two presentations. If you’re debating whether to buy flood insurance, these presentations will make you a believer.

Key Messages

One of the key takeaways from Part 1 is that you should not think of the 100-year (1%) floodplain as a bright line where you’re safe on one of it and not safe on the other. Dupré calls that choice a “false binary.” He urges people to think of flood plains as ever shifting and flood plain boundaries as very fuzzy lines, much like the cone of uncertainty used for hurricane path prediction. The width of the line represents the margin of error behind the calculation of probabilities.

After reviewing Part 2, you should come away with a better appreciation for how gradual, almost unnoticeable changes in your environment can increase your flood risk.

Related Reading

The points Dr. Dupré  makes support the conclusions drawn in a report by the Bayou City Initiative titled “Houston A Year after Harvey: Where We Are and Where We Need to Be,” especially the section on the need to revise outdated flood maps.

Remember Flood Control Presentation At Community Center on 9/17

Matt Zeve, Director of Operations for Harris County Flood Control, will also discuss flood map revisions in his upcoming talk at the Kingwood Community Center on September 17 at 6:30. Don’t forget to mark your calendar.

Posted by Bob Rehak on September 11, 2018

378 Days since Hurricane Harvey and 17 years since 9/11