To learn more about USGS’ role providing science to decision makers before, during and after #Florence, visit the #USGS Hurricane Florence page at https://www.usgs.gov/florence
Working to keep vital flood information flowing
At least 80 U.S. Geological Survey scientists are in the field in the Carolinas and Virginia, working to ensure that vital information about river flooding continues to reach emergency managers, forecasters and others threatened by the catastrophic flooding that has been linked to 32 deaths.
Major flooding is underway Tuesday on several North and South Carolina rivers, including the Cape Fear, Northeast Cape Fear, Lumber, Waccamaw, PeeDee and Little PeeDee. While a few rivers on the Carolinas’ coastal plain reached their peak levels and began declining Monday, other rivers are still rising on the Carolinas’ coastal plain, and some may not peak until next week, said hydrologist John Shelton of the USGS South Atlantic Water Science Center.
USGS crews are evaluating many peaks of record, or instances when river and streamgages recorded the highest levels ever measured at those locations. At 14 streamgages the water levels were so high they are estimated to have only a one-in-500 probability of occurring in any given year. Two streamgages reached heights that have only a one-in-200 probability of occurring in a given year, and one recorded a one-in-100 probability peak.
The historic flooding that affects the entire state of North Carolina and eastern South Carolina is the result of the storm once known as Hurricane Florence, which dumped 36 inches of rain in the towns of Elizabeth and Swansboro, North Carolina. The storm moved westward into the Appalachian foothills and then took a northward turn, triggering tornadoes in Virginia late Monday. On Tuesday the remnant of a tropical cyclone was causing moderate flooding in portions of southern New York, and threatening to bring heavy rains to southern New England.
On Tuesday USGS streamgages at 70 sites in North Carolina were near or above flood stage, including 15 at levels classified by the National Weather Service as major flooding. In South Carolina 15 sites were near or above flood stage, including four classified as major flooding. In Virginia, 62 streamgages were near or above flood stage, with three recording major flooding, and in West Virginia 39 streamgages were near or above flood stage, with none at the level of major flooding.
Information from the USGS’ network of more than 475 river and streamgages in the Carolinas is essential to emergency managers and National Weather Service forecasters, who use it to track and forecast how rivers are responding to rainfall. River and streamgages generally show water levels and water flows, and some also show rainfall and wind data collected for the National Weather Service.
By Tuesday 18 of these gauges—13 in North Carolina and five in South Carolina—have been damaged or destroyed. That includes seven gauges that stopped transmitting data because their electronics were submerged by floodwaters or the sensors that send their readings to a satellite were underwater. USGS hydrologists said more streamgages are likely to be submerged, since some rivers are continuing to rise.
Because this information is vital to people making key decisions about public safety, USGS hydrographers have been in the field since Saturday, repairing or replacing damaged instruments and conducting streamflow measurements to confirm the gauges’ data. By Tuesday, 62 people were in the field in the Carolinas.
The USGS' stream and river monitoring network in the Carolinas has been supplemented with 21 rapid deployment gauges—these are streamgages specially designed for fast installation—set out by USGS scientists along Florence’s anticipated track before the storm made landfall. All of the network's instruments are designed to withstand powerful floods and are typically placed well above expected flood heights. But in some places, flooding rivers have risen higher than the gauges’ operational limits. The operational limit is the point at which a streamgage can no longer collect and transmit reliable data because high water has caused an equipment failure: electronics are inundated or the sensors that transmit readings to a satellite are underwater. Floodwaters may rise higher after that, but the gauge’s transmitted record won’t show it.
Since Saturday USGS hydrographers have been raising the heights of some gauges threatened with submergence. Where gauges are already submerged, if there is a site upstream or downstream that isn’t underwater and is likely to withstand heavy flooding, such as a bridge, they are installing temporary instruments called rapid deployment gauges there. When gauges are water damaged but accessible, they are repairing them. The goal is to get non-functioning gauges back in action within a day whenever possible, said USGS supervisory hydrologist Douglas A. Walters in Raleigh.
In the field, USGS hydrographers are also making on-site measurements of flow depth, width, and velocity and correlating it against the gauges’ water level readings, also known as river stages, to make estimates of streamflow. The relationship between river stage and streamflow plays a crucial role in river forecasts. After the storm has passed USGS hydrographers will reconstruct peak water levels in detail, using data recorded prior to the gage being submerged along with physical marks of flooding and other data.
At the USGS South Atlantic Water Science Center, which conducts water studies in Georgia and the Carolinas, flood experts are relaying up-to-date information from the USGS network to the National Weather Service, the Federal Emergency Management Agency, the U.S. Army Corps of Engineers, the affected states’ departments of transportation and the environment, local law enforcement, and the news media.
Before Florence made landfall as a large, slow-moving Category One hurricane on Thursday, September 14, USGS scientists in the Carolinas, Virginia, Maryland and West Virginia placed 160 storm-tide sensors and about 45 rapid deployment gauges in places that may be impacted by storm surge or floodwaters, but where the USGS does not have permanent streamgages. On September 15, USGS scientists installed eight storm sensors in Pennsylvania, anticipating that the storm may move in that direction, bringing heavy rainfall.
In Virginia, Florence-induced floodwaters are now moving downstream, with some rivers flooded to the equivalent of a 1-in-2 or a 1-in-25 chance of occurring in a given year. On Tuesday, 18 people were in the field making flood measurements in the Dan, Roanoke, New, and Shenandoah river basins.
As soon as flood waters start to recede, the next step will be to collect high-water marks in flooded areas. Floodwaters carry seeds and dirt can stick to buildings, bridges and trees. When floodwaters recede, they leave behind a line of debris that indicates the highest point the flood reached. These high water marks are fragile and easily destroyed, so observing, recording and surveying them is done as quickly as possible, and is labor-intensive.
After major floods, USGS often partners with the Federal Emergency Management Agency, the U.S. Army Corps of Engineers, the National Weather Service, providing these agencies and others with high-water mark information throughout the affected areas. The information can be used to steer emergency relief to the hardest-hit areas, and to improve future flood forecasting and federal flood hazard maps.
On Tuesday, scientists in South Carolina also began collecting the storm-surge sensors that were deployed before the storm made landfall. Storm surge is among the most dangerous natural hazards unleashed by hurricanes, with the capacity to destroy homes and businesses, wipe out roads, bridges, water and sewer systems, and profoundly alter coastal landscapes.
Many parts of the storm-battered shoreline were still inaccessible Tuesday, but crews are retrieving as many sensors as they can get to in a process that will begin in North Carolina Wednesday and is likely to continue for days. Next they will begin to analyze the data that the sensors recorded. The information will help define the depth and duration of the storm-surge. It will be used, along with aerial photography, to help assess storm damage, discern between wind and flood damage, and improve computer models used to forecast future floods.
The USGS studies the effects of hurricanes and tropical storms to better understand potential impacts on coastal and inland areas. USGS’s network of sensors in rivers, streams, tidal and coastal waters provide critical information for more accurately predicting and modeling storm effects like flooding. The knowledge gained by studying real-world storms can also contribute to safer, more cost-effective designs for buildings, bridges, roads and other structures, and inform public safety measures.
For more than 125 years, the USGS has monitored flow in selected streams and rivers across the U.S. The information is routinely used for water supply and management, monitoring floods and droughts, bridge and road design, determination of flood risk and for many recreational activities.
Access current flood and high flow conditions across the country by visiting the USGS WaterWatch website. Receive instant, customized updates about water conditions in your area via text message or email by signing up for USGS WaterAlert.