What Is a 100-Year Flood?

Damage to a railroad bridge over the Salt River in Tempe, Arizona during the flood of 1891.

A “100-year flood” refers to a flood event that has a 1% chance of occurring in any given year. Contrary to what the term suggests, it doesn’t mean such a flood happens once every 100 years. Instead, it’s a statistical estimate based on historical data, indicating the probability of a flood of a certain magnitude happening in any single year.

For example, over a span of 1,000 years, a 100-year flood could occur about 10 times, but these events might happen in consecutive years or be spread out irregularly. This unpredictability underscores the importance of understanding flood risks, especially for properties near natural drainage areas.

Why Does This Matter for Arizona Homeowners

In Arizona, many homes are situated near natural washes—dry channels that can quickly fill with water during heavy rains. These washes are part of the state’s natural drainage system and can pose significant flood risks during intense storms.

If your property is near a wash, it’s essential to recognize that:

• • Flooding Can Occur Anytime: Even if a 100-year flood occurred recently, there’s still a 1% chance it could happen again this year.
  • Property Damage Risk: Floodwaters can cause extensive damage to homes, landscaping, and infrastructure.
  • Insurance Implications: Standard homeowners’ insurance policies typically don’t cover flood damage. Understanding your flood risk can help you decide if additional flood insurance is necessary.

Understanding Flood Zones and Maps

The Federal Emergency Management Agency (FEMA) provides flood maps that designate areas based on flood risk. Properties in high-risk zones, often referred to as Special Flood Hazard Areas (SFHAs), are areas where a 100-year flood is likely to occur.

Being informed about your property’s location relative to these zones can guide decisions on building, renovations, and purchasing appropriate insurance coverage.

Mitigating Flood Risks

To protect your property from potential flood damage, consider the following strategies:

• • Elevate Structures: If you’re building or renovating, consider raising structures above the base flood elevation to reduce flood impact.
  • Proper Drainage: Ensure your property is graded correctly and includes systems that divert stormwater safely away from buildings and foundations.
  • Maintain Drainage Systems: Regularly inspect and clean drainage culverts, storm pipes, and natural channels on or around your property. Preventing clogs or debris buildup is essential to avoid water backup or overflow during a heavy storm.
  • Work with Civil Engineers: For properties near washes or within flood-prone zones, consult with a civil engineer. They can help design effective flood mitigation solutions such as scour walls, berms, retention basins, or other drainage control methods tailored to your lot. These measures help manage water flow and reduce the chance of erosion or structural damage.
  • Landscaping Choices: Use flood-resistant landscaping materials and install features that promote water dispersion, such as gravel swales or dry creek beds.
  • Stay Informed: Check flood zone maps and stay alert during monsoon season or heavy rain forecasts. Knowing when and where flooding might occur is your first line of defense.

Conclusion

Understanding the concept of a 100-year flood is crucial for Arizona homeowners, especially those near natural washes. Recognizing the risks and taking proactive measures can help safeguard your property and provide peace of mind during heavy rain events.

Need Help with a Wash or Flood Zone on Your Lot?

If you’re planning to build on your lot and have concerns about a nearby wash or potential flood zone, give us a call. Our experienced team understands the challenges of building in these conditions and can help answer your questions, evaluate your site, and connect you with the right professionals—like civil engineers—to ensure your home is safe and up to code. We’re here to make the process easier and guide you every step of the way.

Let’s Build Together – Follow & Subscribe

If you have a project in mind and would like to discuss it, please get in touch — we’d love to hear from you. And don’t forget to subscribe and follow us below for more helpful tips and updates!

• Subscribe to the blog so you receive blog posts that are relevant to you.
• Follow us on social media to stay up to date on all of our useful blog content.
  • Facebook @VistaMontanaBuilders
  • Instagram @VistaMontanaBuilders