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.
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. The physical evidence of flood levels that high-water marks provide is valuable information that can be combined with other flood data USGS experts can use to reconstruct precisely where, at what depth, to what height, and to what extent 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 after a storm, 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 containing a wooden or metal stick with a bit of granulated cork added to the bottom of the pipe. Holes in the bottom of the pipe allow water to rise inside the pipe and float the cork, which will leave a mark on the stick inside the pipe at 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.
