How to Understand Flood Stages

Floods are the most common natural disaster, according to the Federal Emergency Management Agency (FEMA). Several factors determine your risk for experiencing a flood. Your home's proximity to rivers and streams is one major risk factor. Topography, rainfall, and water flow management measures also play a role. Even urban areas are at risk. Building and development replace land meant to absorb floodwaters with impervious surfaces, setting the stage for flash floods. The National Weather Service defines flood stage as the point when an increase in water surface level creates a hazardous situation for lives, property, or commerce.

Things You'll Need

• Computer with Internet access

Show More

Instructions

• 1

  Determine the river or stream that is closest to your home. The determination of a precise level of flood stage will vary by location. Therefore, in order to understand flood stages, you need to know the risk factors for your area.

• 2

  Locate your property on the FEMA flood zone map. The FEMA website has a convenient map viewer for locating your property and its location with the flood zones. Flood zones maps delineate areas based on flood risk. With this information, you can find out the flood stage that will impact your property.

• 3

  Use the Advanced Hydrologic Prediction Service (AHPS) of the National Weather Service to find out what flood stage you are in. The AHPS bases flood stage determination on readings from automated gauges set up along streams and other data sources such as weather observation stations.

• 4

  View real-time stream data at the U.S. Geological Survey (USGS). The USGS uses a rating system based on percentiles to determine flood stage. A percentile of 75 or more is considered above normal and approaching flood stage, the point when the stream level is equal to land level.

• 5

  Build a custom sequence using USGS to view water flow data over time. The site includes a series of parameters to build a table or graph depicting data. Data showing depth to water surface, for example, shows a pattern of water flow over time reaching flood stage.

• 6

  View data from the Great Flood of 1993 that occurred in the Upper Mississippi River Valley for a dramatic rendering of the Mississippi River reaching flood stage then exceeding it by 19 feet. A river at this level is said to be "flood stage is 19 feet."

• 7

  Learn about the data that the USGS collects and how it relates to flood stage. Stream gauges and standing staff gauges measure the elevation of the surface water. This information is then converted in computer modeling systems to stream flow. How quickly a river is flowing can help predict flood risk.