I reflect a bit on flooding situation in Houston, based on my previous post yesterday. Turns out, I have been through a lot of these in the past 25 years that I have been living here. So my personal experience alone is fairly lengthy in human life time. So let us see how many 100 year and 500 years event happened that I personally have experienced in the last 25 years!
First the definition:
A 100-year flood is an extreme flood event that has a 1% chance (1 in 100) of happening or being exceeded in any given year. A 500-year flood is an even rarer, more severe event that has a 0.2% chance (1 in 500) of occurring or being exceeded in any given year.
Key point in the definition: a 100 year flood is a 1% chance of happening in a given year! It is a statistical probability based on an entire given database. Here it is FEMA's US national database.
PS: It doesn't mean that the 100 year flood happens once in 100 year!!
I intend to prove that with my personal experience in living in Houston, TX alone!
It is about annual Risk!
FEMA maintains a national database of floods. It does have a very user friendly interface where you can type any address in the US and get the official assessment of risk. This is a public database and all information is public.
https://msc.fema.gov/portal/home
If you live in the US, feel free to type your address, as this is national free database.
I will use a sample but real address to explain this process. The address is
12110 Green Willow Falls Dr, Tomball, TX, 77375, USA
Here is the official legend:
| Category | Zone(s) | Annual Flood Risk | Description | Map Appearance |
|---|---|---|---|---|
| High Risk (SFHA) | A, AE, A1–A30, AO, AH | ≥1% (1-in-100 year flood) | Inland areas subject to inundation from 100-year flood | Shaded blue |
| V, VE | ≥1% (coastal + wave hazard) | Coastal areas exposed to storm surge and wave action | Shaded blue | |
| Floodway | Highest within SFHA | River channels + adjacent land needed to carry floodwaters | Red diagonal hatching overlay | |
| Moderate Risk | B / Shaded X | 0.2% (1-in-500 year flood) | Between 100-year and 500-year floodplain | Shaded orange |
| Low Risk | C / Unshaded X | <0.2% | Minimal flood hazard areas outside 500-year floodplain | Clear / unshaded |
| Undetermined | D | Unknown | Possible flood risk, but no detailed analysis completed | Not specifically defined |
If you look at the map above, the red diagonal area is the 'floodway'; in this case the Willow Creek. The Blue area is the overbank spillway, or the 100-yr flood zone, meaning each year, there is 1% or higher chance of this area getting flooded per year. The Orange area is between 100-yr and 500-yr flood zone, meaning there is a 0.2% or higher chance of this area getting flooded per year. Here is a simplified version of the same legend.
| Color on FEMA Map | Risk Level | Should You Build? 🚧 | Simple Interpretation |
|---|---|---|---|
| 🔵 Blue | High Risk | ❌ No | Regular flooding likely (100-year flood zone or worse) |
| 🔴 Red Hatch | Extreme Risk | ❌ Absolutely Not | Floodway — water must flow here during floods |
| 🟠 Orange | Moderate Risk | ⚠️ Be Careful | Less frequent, but still real flood risk (500-year) |
| ⚪ Clear / None | Low Risk | ✅ Generally Safer | Minimal flood risk, but not zero |
| 🟡 Undefined | Unknown | ⚠️ Proceed with caution | Risk not studied — uncertainty is a risk itself |
Let us closely look at the address I mentioned earlier in Tomball, and I will show you certain strange thing which is common when a subdivision development happens.
First, you see the following:
Do you see something funny? Or understand what I am getting at. Yes, usually subdivision built a highly higher earthed ground when the houses are built, which makes the area slightly higher than the surrounding. But it sort of magically follows the subdivision boundary. Regardless, would you be buying a property here in the Orange or 200 ft from the blue, even if it is deemed OKAY by this FEMA map?
Tell you what, let me share the flooding experience.
Note: these are all floods that I have personal memories, and I can not reading old news articles and listing them. I clearly remember these major events where almost all of greater Houston experience 500-yr floods. Meaning, in majority of Houston all of the Orange in the map was covered in water in each of the instances! Well not exactly everywhere all the time, but in most places.
2001 – Tropical Storm Allison
2005 – Hurricane Rita (evacuation crisis)
2008 – Hurricane Ike
2015 – Memorial Day Flood
2016 – Tax Day Flood + Memorial Day Flood
2017 – Hurricane Harvey
2019 – Tropical Storm Imelda
2020 – Beta
2021 – Nicholas
2024 – Hurricane Beryl
I count 11 event in the last 25 years. Among which 7 have been absolutely catastrophic! Now tell me how's that 500-year flood is working out for us? Let me list a few more details of those 7 events.
🟦 Tropical Storm Allison (June 2001)
🟦 Hurricane Ike (September 2008)
🟧 Memorial Day Flood (May 2015)
🟧 Tax Day Flood (April 2016)
🟧 Memorial Day Flood (May 2016)
🟥 Hurricane Harvey (August 2017)
One side note here: 30% of Houston homes had some kind of roof damage. It was in fact so severe, that after 2 years you can still see the Blue tarp on the roofs of the house, as the roofing and insurance industry had massive back logs!
🟨 Tropical Storm Imelda (September 2019)
So, I hope I can state where we live matters a lot and risks of flood changes accordingly. It is proven that rain storms, Hurricane and flooding are getting more severe every year and will continue to do so as the oceans progressively warm up due to Human Induced Climate Change.
When thinking about where to build or rent in flood-prone regions like Houston, FEMA map colors offer a simple but powerful guide. Areas shaded in blue or marked with red hatching are the highest risk—these are places where flooding is expected regularly or where floodwaters must flow, making them unsuitable for development if you want to avoid damage. Orange zones may appear safer but still carry meaningful flood risk over time, especially during extreme storms, so they require caution. Only the clear or unshaded areas represent relatively lower risk, though even these are not completely flood-free. In short, the closer a property is to blue or red on the map, the more likely it is that water will eventually find its way there—and building decisions should reflect that reality.
