U.S. Geological Survey experts use a host of different high-tech devices to gather scientific data before, during and after a hurricane or tropical storm makes landfall. They also learn a great deal about flooding from a low-tech, but highly useful source of information: the debris left as stormwaters recede.

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These telltale signs of flooding are called high-water marks and they can consist of lines of seeds, leaves, grass, dirt and other debris deposited by floodwaters on things like stream banks, tree trunks, buildings, bridges, and other structures. As soon as conditions permit after a storm, teams of USGS scientists will go to flooded locations to find and measure these marks.

The physical evidence of flooding high-water marks provide is valuable information that can be combined with other flood data that USGS experts can use to reconstruct precisely where, at what depth, to what height, and in what volume floodwaters inundated a region. Right after a storm, the USGS’s early information from high-water marks can help emergency managers make informed flood management decisions.

In the weeks after a storm, USGS flood information can help the Federal Emergency Management Agency and the National Oceanic Atmospheric Administration to discern the difference between wind and water damage – important information for property owners and insurers. Over the long term, it can help emergency managers plan better for future floods; improve the computer models used by the National Weather Service to forecast flooding; and provide information used by FEMA to update the nationwide flood zone maps that underpin the National Flood Insurance Program.

High-water marks can easily be destroyed by rain or communities cleaning up after a storm so they must be flagged and measured as soon as possible. Because of this, the USGS can send out dozens of teams, often working seven days a week from dawn until dusk, to collect high-water mark data for a flood event before they disappear. A supplemental system for collecting high-water mark data is also used by the USGS and installed at most USGS streamgage locations. Called crest-stage gauges, these simple devices are tall metal pipes filled with a piece of wood with elevation measurements and ground cork. Holes in the bottom of the pipe allow water to rise inside the pipe and float the cork, which will stick to the wood and capture the highest point the floodwaters reached.

High-water mark data for current and past major flooding events can be seen on the USGS’s Flood Event Viewer. This online tool also includes data from USGS rapid-deployment gauges and storm-tide sensors and is available here.

*Editor’s note: The photo at the top of the story is from 2012 and shows USGS hydrologic technician Amy Simonson using GPS equipment to survey a high-water mark on Liberty Island, New York, caused by flooding from Hurricane Sandy. Photo by Michael Noll, USGS. LINK TO PHOTO

In case you missed them, here are links to the other posts in this series:

• Introduction to a Six-Part Series Highlighting USGS Hurricane Science
• Storm Tide Poses One of the Most Serious Hurricane Threats to People and Infrastructure
• Changes to the Coastline Can Affect Where, and How Severely, Flooding Occurs
• Hurricanes may pose a threat to people along the coasts and far inland
• Hurricanes can spread invasive species if they survive the ride
• Maps and imagery for hurricane response