Flood Thresholds
What are flood thresholds and what do they mean? How do we determine them?
Screenshot from our dashboard during a flood event in the Allegheny Mountains of Virginia.
What Are Flood Thresholds - And Why Do They Matter?
When it comes to flood risk, every minute counts. That’s why flood thresholds are such a critical part of any flood monitoring and early warning system. These thresholds help communities and emergency managers know what kind of action to take and when, based on rising water levels in rivers, streams, and other waterways.
Why Do We Use Flood Thresholds?
Flood thresholds translate real-time water level data into meaningful alerts. Instead of just knowing that water is rising, decision-makers can understand how serious the situation is and respond accordingly. This clarity helps reduce confusion, improves response coordination, and ultimately protects lives and property.
Whether you're a city public works team or a rural emergency management agency, thresholds provide the clear guidance needed to act decisively before, during, and after a flood.
Flood Thresholds Are Not “One Size Fits All”
While the terminology around flood thresholds is relatively standard, the thresholds themselves are not. Every waterway behaves differently depending on topography, watershed characteristics, local infrastructure, and community needs.
For example, a minor rise in one stream might overtop a popular greenway or flood a parking lot, while the same rise in a nearby river might not impact anything at all. What matters most is how local conditions and human activity are affected—whether that means a submerged road, a threatened facility, or residents at risk.
That’s why flood thresholds must be tailored to each site. They’re defined not only by how water behaves, but also by what’s at stake nearby: roads, greenways, parking lots, buildings, critical installations, or residential areas.
The Four Common Flood Categories
At Green Stream, we follow the NWS standard of four common categories for flood levels:
— Action Stage: Water is rising. Conditions aren’t yet critical, but now is the time to prepare. Check systems, review response plans, and get ready.
— Minor Flooding: Water is beginning to overtop roads or low-lying areas. Property damage is minimal, but flooding is visibly occurring.
— Moderate Flooding: There is inundation of roads and structures near the water. Evacuations may be necessary, especially in flood-prone neighborhoods or facilities.
— Major Flooding: This level brings widespread dangerous flooding, significant evacuations, and high risk to life and property. Emergency responses are fully activated.
How to Set the Right Thresholds
When we install new stream gauges, we’ll work with you to set thresholds based on your needs. We’ll fine-tune the thresholds with you over time - ensuring your alerts reflect what really matters in your location.
Learn More
Want to learn more about how Green Stream’s stream gauges, precipitation gauges, and met stations can help increase flood resilience in your area? Contact us.
