How Flat Terrain In Southeast Texas Increases Flood Risk

Flat terrain is one of the most under-appreciated aspects of flood risk. It complicates virtually every aspect of flood control.

Flat terrain increases flood risk primarily by reducing the speed of runoff. It causes water to pool rather than drain away. Also, lack of gradient lets floodwaters spread wider, increases sedimentation, backs water up into storm sewers, and creates unpredictable backwater effects.

Spreading Wider

In steep, mountainous areas, a one-foot rise in a river can be relatively contained; it won’t spread out much. But in flat areas, water spreads out easily. Even a slight rise in water level can submerge numerous properties. And that water may not drain away quickly. As water fills channels, it backs up water into storm sewers and may cause street flooding.

Increases in rainfall can also shift floodplain boundaries significantly, as we saw recently with the introduction of new draft flood maps based on Atlas 14. Homes once considered outside of floodplains are now within them.

Increasing Sedimentation

Flat terrain also makes rivers move slowly. This enables suspended sediment to settle and reduces a river’s capacity to convey stormwater, leading to more frequent flooding.

It can also lead to the creation of sand bars, especially where rivers meet standing bodies of water, such as Lake Houston. During Harvey, we saw “mouth bars” grow thousands of feet on the East and West Forks above Lake Houston.

Mouth bars are sand bars found at the mouths of rivers. Such blockages create partial sediment dams that back water up and promote even more deposition upstream. For instance, see below.

The Army Corps has since dredged the West Fork. However, while dredging can temporarily deepen a channel, it does nothing to increase the slope (gradient). Without a steeper slope, the river remains slow, and new sediment quickly refills the dredged areas.

That is why the Army Corps recommended a maintenance dredging program and why State Rep. Charles Cunningham’s Lake Houston Dredging District is so important.

Unpredictable “Stacking” Effects

In steep river systems, flood waves pass quickly. But in Southeast Texas, flood waves move slowly down rivers. Peaks linger and lengthen. This increases the probability that the peaks will synchronize, i.e., stack on top of each other.

That’s especially true in large storms, such as hurricanes, that may stall over an area for days and dump rain uniformly across the region. Stormwater peaks from different tributaries then stack on top of each other as they moves downstream.

Other Backwater Effects

The low gradient of streams in southeast Texas makes them extremely sensitive to a variety of backwater effects.

In flat river basins like the San Jacinto, Trinity and Brazos, mild slopes amplify “backwater effects.” Examples include

• Tributary confluences:
  • Where Spring Creek converges with the San Jacinto West Fork just upstream from the I-69 bridge, makes a classic example.
  • Stacking peaks created the highest flooding in Harris County during Harvey – 22 feet above flood stage!
• Reservoir backwater:
  • Lake Houston backs up the lower San Jacinto West and East Forks plus a number of creeks and bayous for miles upstream.
  • As peaks arriving from different streams arrive, they stack on top of each other.
  • During floods, even modest lake-level rises can push water far upstream into Montgomery County
• Tidal influence:
  • Near-coast systems can experience tidal backwater
  • Storm surge from the Gulf can reach miles inland, blocking inland rainfall from draining to the Gulf.
• Infrastructure bottlenecks:
  • Bridges, culverts, sand bars, levees, dikes, and new developments
  • Can constrict conveyance and back water up

Implications for Policy Making

In low gradient systems, many streams become hydraulically coupled during floods. So, basin-wide coordination is essential.

That’s why the SJRA’s Joint Reservoir Operations Study is so crucial and why fragmented governance increases flood risk. We need river-basin-wide flood control.

We must also be more sensitive to:

• Floodplain fill
• Setbacks from rivers for sand mines
• Preserving vegetation along streams to reduce erosion
• Controlling emissions from sand mines

Summary

To summarize, in flat coastal plains with low-gradients:

• Low slope = low velocity = poor drainage
• Water spreads laterally instead of moving quickly downstream, flooding many structures
• Stormwater from different tributaries has a higher chance of stacking up
• Sediment accumulates faster
• Infrastructure bottlenecks can have large spatial impacts.

Posted by Bob Rehak on 4/7/26

3143 Days since 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.