7/9/2026 – A young couple that is considering buying a home in my neighborhood asked me a simple question yesterday? “What is the current flood risk?” The home they were considering flooded during Hurricane Harvey. But that was an extreme event. And they wondered whether mitigation activities since then would prevent it from flooding again.

I tried to explain the various mitigation efforts since then and the current status of each. However, I quickly realized that the scale, complexity, relative impact and timing of the efforts make the task of estimating “current” flood risk virtually impossible. They are all at different stages. And few are complete.

What Goes into Calculating Flood Risk?

Flood risk is not static. Estimates change constantly. Predicting what will fall from the sky and where once every 100 or 500 years is only part of the battle. To name just a few things, we also need to understand:

• How the landscape is changing
• Where people are building
• How they are building, i.e., elevated structures vs. slab on grade
• What they are doing to offset increased runoff
• Soil types and rates of infiltration
• Changes in land-use patterns, i.e., forestry vs. retail development
• Reservoir operations
• Sediment accumulations, such as deltas, that can reduce channel conveyance
• Bridges that can create backwater effects
• Subsidence that can alter channel gradients
• Flood-mitigation efforts and their impacts
• Timing effects, such as the “stacking effect” at confluences, can erode margins of safety
• Changes in regulations in and across different parts of a watershed

Impacts of Uncertainty and Misunderstanding on Estimates

Understanding the impact of those factors on uncertainty may require a graduate degree in statistics. Every flood risk estimate contains uncertainty arising from:

• Rainfall estimates
• Future land development
• Climate variability
• Hydraulic model assumptions
• Terrain (LiDAR) accuracy
• Roughness coefficients
• Reservoir operations
• Future sedimentation
• Infrastructure changes

Then, layer on semantic confusion. To most people, a “100-year flood” sounds like one that happens once a century. But it actually has a 1% annual chance of occurring in any given year. Theoretically, you could have back-to-back-to-back 100-year floods, as we did in 2015, 2016 and 2017.

A 1% chance is no guarantee that a flood of that magnitude will only occur once every 100 years.

In fact, a 1% annual chance flood has a 26% chance (about 1 in 4) of occurring during the life of a 30-year mortgage. And that assumes nothing changes in the watershed. This often surprises people. Here’s how to calculate it.

And here’s how the probabilities break down for various return periods.

Annual Chance	Return Period	Chance Over 30 Years
10%	10-year flood	95.8%
4%	25-year flood	70.6%
2%	50-year flood	45.5%
1%	100-year flood	26.0%
0.2%	500-year flood	5.8%

Interaction of Factors Can Amplify Risk

Many of the issues above interact in ways that can amplify risk. For example:

• Rapid urbanization increases impervious cover and runoff
• Sedimentation and subsidence reduce channel conveyance
• Reservoir operations influence the timing and magnitude of downstream flows
• Political fragmentation can make watershed-wide coordination more difficult.

In combination, these factors mean that flood risk is not static. It evolves as the watershed, the built environment, regulations, and flood mitigation activities change…at different rates…across dozens of jurisdictions.

Complexity, Timing Make Current Risk Virtually Unknowable

The scale, complexity, interaction and timing of these factors makes accurate assessment of current flood risk virtually impossible eight years after data for the new flood maps was collected in 2018.

Since then, Montgomery County has been one of the fastest growing counties in America. Yet it didn’t update its floodplain regulations until 2025. And there’s no way to quantify how vigorously it has enforced its regulations.

During that time, Harris County and City of Houston have engaged in many mitigation activities, i.e., dredging. But since the dredging, sediment from sand mines and natural erosion has built back in somewhat. But how much?

There are no current surveys of water depth that I know of.

Advice for Homebuyers

Given all the uncertainty, experts recommend that homebuyers:

• Look beyond FEMA flood maps. They are valuable, but have limitations and may be outdated.
• Research the property’s actual flood history. Don’t rely solely on the seller’s disclosure. Talk with neighbors—they often know the area’s history better than anyone.
• Study the topography, local drainage, storm sewers, elevation, nearby creeks, and ditches
• Consider how the area is changing. Runoff can increase over time in a rapidly urbanizing watershed
• Don’t assume “never flooded” means “never will.” 
• Understand the residual risk behind flood-control infrastructure. Levees, reservoirs, detention basins, and drainage projects reduce risk in smaller floods, but increase it in larger ones by creating a false sense of security that encourages people to build in risky areas
• Evaluate flood insurance – even if it’s not required. 
• Think long term. Ask how conditions could change in 10, 20 or 30 years
• Be cautious with unusually attractive prices. Sometimes a lower purchase price reflects higher flood risk. That doesn’t automatically make it a bad purchase, but you should weigh the discount against higher insurance costs, potential repair costs, and resale challenges.
• Ask yourself “If this home flooded, could I recover financially and emotionally?” If the answer is no, that should weigh heavily in your decision to buy the property.

Posted by Bob Rehak on 7/9/2026

3236 Days since Hurricane Harvey