USGS Works to Predict and Track Isaias' Effects

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To learn more about USGS’s role in providing science to decision makers before, during and after Tropical Storm Isaias, visit https://www.usgs.gov/isaias

This story was updated August 3 to reflect changes in the coastal change and landfall forecasts for Isaias.

As Tropical Storm Isaias moves toward a forecast landfall along the Carolinas, U.S. Geological Survey scientists are working on sandy shorelines and at their computers to help gather valuable - and perishable - scientific information about the storm and its likely effects. 

Coastal change forecasters are predicting the potential for Isaias’ storm waves to erode sandy beaches from Florida to Virginia. Meanwhile, hydrologic experts finished installing more than 50 storm tide sensors and two rapid deployment gauges Sunday along a swath of the North and South Carolina coast, which is likely to be storm-washed. The coastal change forecast, storm-tide sensors and rapid deployment gauges provide essential information that local, state, and federal officials can use to help protect lives and property. 

Storm tides are among the most dangerous natural hazards unleashed by hurricanes. They are increases in ocean water levels caused by extreme storms, made up of a storm-generated surge combined with the effects of local tide cycles. They can destroy homes and businesses, wipe out roads, bridges, water and sewer systems, and profoundly alter coastal landscapes. On sandy shorelines, they can erode dunes, overwash these natural buffer zones, and even inundate areas behind them. On estuaries and tidal rivers, they can cause dangerous and destructive flooding.

Whenever a storm poses a significant threat to U.S. shores, USGS scientists with different types of expertise come together in a coordinated effort to predict and document coastal changes brought on by the powerful storms. In the case of Isaias, as with other storms, coastal change forecasts are issued by research oceanographers at the USGS St. Petersburg Coastal and Marine Science Center.

USGS coastal change forecasts can help local emergency management officials decide which areas to evacuate, where and when to close coastal roads, and where to position cleanup equipment in advance of the storm. The latest coastal change forecast for Isaias is at https://marine.usgs.gov/coastalchangehazardsportal/ui/alias/isaias2020. It uses information from the National Hurricane Center's surge forecasts and will be updated whenever the hurricane center's surge forecasts change.

The coastal change experts have predicted that storm waves kicked up by Isais are likely to cause erosion at the base of the dunes along about 20% of coastal beaches between Florida and Virginia. However, only about 3% of beaches in the region are likely to have waves overwashing the dunes. Inundation - the most severe type of beach damage - is not predicted anywhere in the region.

An image of the USGS Coastal Change Hazards portal showing risks of erosion along Atlantic Coast caused by TS Isaias.

This Coastal Change Storm Hazard Team map was created August 3, 2020 and shows forecast beach erosion (the strip of colored bars closest to the coast), overwash (middle strip) and inundation (outer strip) effects of Tropical Storm Isaias. Credit: USGS, Public domain.

“Predicted coastal change impacts from Isaias have increased in South Carolina and North Carolina as the forecast track of Isaias has narrowed in on a landfall in these states," said research oceanographer Kara Doran, leader of the USGS Coastal Change Hazards Storm Team based in St. Petersburg, Florida. “As the storm moves inland, storm surge and waves will increase ahead of landfall, and create the possibility of extensive beach and dune erosion." 

“It is important to remember that local conditions may vary,” Doran said. “Citizens should follow the evacuation advice of local emergency management authorities.”

The least severe level of storm damage is erosion at the base of the dunes, known as collision. In South Carolina, about 64% of beaches are predicted to erode at the dunes' base. Nearly 19% of North Carolina's beaches and 18% of Georgia's beaches are very likely to be eroded at their bases. None of Virginia's beaches are expected to erode.

Overwash is the middle range of potential storm damage. As waves and surge reach higher than the top of the dune, overwash can transport large amounts of sand across coastal environments, depositing sand inland and causing significant changes to the landscape.

A substantial amount of overwash is predicted in South Carolina, with about 13% of the state's sandy beaches very likely to be affected by overwash. In North Carolina, 2% of beaches are predicted to overwash and in Georgia, the amount is less than 1%.

The most severe category of beach damage is called inundation when seawater completely and continuously submerges beaches and dunes. None of the Southeast Atlantic beaches are predicted to be inundated by storm waves from Isaias.

The prediction is based on the USGS Coastal Change Forecast model, which has been in use since 2011 and is continually being improved. It starts with inputs from the National Hurricane Center's storm surge predictions and National Oceanic and Atmospheric Administration wave forecast models. The USGS model then adds detailed information about the forecasted landfall region's beach slope and dune height. It predicts how waves and surge will move up the beach and whether the protective dunes will be overtopped. The predictions define "very likely" effects as those with at least a 90% chance of taking place, based on the storm's forecast track and intensity.

Because the waves used in the USGS model are offshore in deep water, the model forecasts the storms' effects on coastal, seaward-facing sandy beaches, not estuarine shorelines. The USGS and its research partners are working on developing similar forecasting capacity for other types of shorelines.

To document Isaias’ effect in the Carolinas field crews from USGS's South Atlantic Water Science Center installed 12 storm-tide sensors in South Carolina and 40 storm-tide sensors in North Carolina. Two rapid deployment gauges were installed in New Bern and Swansboro, North Carolina, to monitor inland river flooding in real time. 

This rapid deployment gauge that was installed August 1 in Swansboro, NC to monitor potential flooding caused by Isaias.

This rapid deployment gauge was installed August 1 in Swansboro, North Carolina on the White Oak River to monitor water levels in order to track potential flooding caused by Isaias. Photo by Bryce McClenney, USGS. 

(Public domain.)

USGS storm-tide sensors collect data that help define the storm's surge's depth and duration and the time of its arrival and retreat. This information is used to fine-tune future storm surge and coastal change forecasts. It can also be used to guide recovery efforts, plan evacuation routes, identify areas hardest hit by storm surge, and improve structure designs to increase public safety.

The sensors installed to measure Isaias’ storm tide are housed in vented steel pipes a few inches wide and about a foot long. They are being installed on bridges, piers, and other structures that have a good chance of surviving the storm. Information on the storm-tide sensor deployment and the incoming data will be available on the USGS Flood Event Viewer https://stn.wim.usgs.gov/FEV/#2020HurricaneIsaias.

The two rapid deployment gauges installed in North Carolina will monitor critical locations that may be impacted by floodwaters, but where the USGS does not have permanent streamgages. These specialized gauges can be quickly installed to provide real-time information on water levels. They will augment a network of 283 permanent USGS streamgages in North Carolina that provide critical information to the National Weather Service, FEMA, and other USGS partners involved in issuing flood and evacuation warnings and coordinating emergency responses.

USGS offices from Florida through Maine are monitoring Isaias for the possibility of inland flooding. They are ready to measure floodwaters if needed. 

As the USGS continues to take all appropriate preparedness actions in response to Hurricane Isaias, those ­­­in the storm's projected path can visit www.ready.gov or www.listo.gov for tips on creating emergency plans and putting together an emergency supply kit.