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Published On: Thu, Sep 13th, 2018

Hurricane Florence nears US Coast as experts watch Isaac and Olivia

photo/ Gerd Altmann via pixabay

When a major hurricane threatens to make landfall in the United States or its territories, the U.S. Geological Survey mobilizes quickly to produce science on the go. USGS experts in flooding, storm surge and the impacts of coastal storms gather essential information and get it to flood and erosion forecasters, emergency managers and others who need it. The work starts well before the storm and continues for weeks or months afterward. Today and in the coming days, USGS scientists are working to provide key information that can be used to help protect lives and property from the Carolinas to Puerto Rico and Hawai’i.

September is considered the peak of the hurricane season, and the month is running true to form, with active storms right now in the Atlantic, the Caribbean and the Pacific.

The biggest mobilization so far has been for Hurricane Florence, which is expected to reach the North Carolina coast Friday as a dangerous Category Four hurricane with maximum sustained winds between 130 and 156 miles per hour.

USGS coastal hazards experts have forecast that about 75 percent of North Carolina’s beaches and 60 percent of South Carolina’s beaches are likely to erode up to the foot of their dunes in the early stages of the storm, with continuing, more extensive beach damage a strong possibility if the storm lingers in the area, as the National Hurricane Center has predicted it probably will. USGS flood experts based in science centers from Georgia to Delaware were in the field Tuesday, setting out instruments to measure the hurricane’s storm tides, its effects on rivers and streams, and potential inland and coastal flooding.

In the U.S. Virgin Islands and Puerto Rico, which are still recovering from the effects of last year’s hurricanes, similar preparations have been completed for a possible encounter with Tropical Storm Isaac, which has weakened from a Category One hurricane and has an uncertain path. And as Tropical Storm Olivia nears Hawai’i, USGS experts on storm-related hazards are standing by to conduct field work and research after the storm passes, to help minimize potential risks to lives and property from flooding, landslides and other hazards.

The far-flung response is being coordinated by the USGS Coastal Storm Response Team, which brings together experts in flooding, coastal erosion, storm tides, landslide forecasting, communications and state-of-the-science custom map making for emergency responders. The storm response team confers at least once a day when a major hurricane threatens any U.S. state or territory.

“The USGS had a very busy hurricane season in 2017, so our team is well prepared to respond to more than one hurricane at a time,” said Athena Clark, USGS Science Adviser for the Southeast Region and the bureau’s Çoastal Storm Team Leader. “Our biggest mobilization right now is for Hurricane Florence, which is a dangerous hurricane that poses a variety of hazards, including inland flooding and coastal flooding. And we are also taking all the necessary steps to track and respond to the impacts of Hurricane Isaac in the Caribbean and Hurricane Olivia in the Pacific.”

Crews Fanned Out to Get Ready for Florence

This USGS storm-tide sensor was installed Sept. 9 in Dare County, North Carolina, in preparation for Hurricane Florence.

This USGS storm-tide sensor was installed Sept. 9 in Dare County, North Carolina, in preparation for Hurricane Florence. Photo by Steve Harden, USGS (Public domain.)

To prepare for a major storm like Florence, USGS crews deploy in advance, placing specialized scientific equipment along the forecast path so that researchers, emergency managers and others can gain a full picture of the storm and its effects.

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. Hurricane rains can also cause flooding far from the coast, particularly when the storms are slow-moving, as the National Weather Service predicts that Florence will likely be. USGS water science centers are preparing today for possible inland and coastal flooding.

In the Carolinas, Virginia, West Virginia and Maryland, crews have installed at least 205 storm-tide sensors and rapid deployment gauges in places that may be impacted by storm surge or floodwaters, but where the USGS does not have permanent streamgages. The rapid deployment gauges can be quickly installed on bridges to provide real-time information on water levels. They will augment a network of several hundred existing streamgages in the region. The network provides information to the National Weather Service, the Federal Emergency Management Agency and other agencies involved in issuing flood and evacuation warnings and coordinating emergency responses.

Storm-tide sensors can measure the height, extent, and timing of the tide pushed ashore by the hurricane, and are housed in vented steel pipes a few inches wide and about a foot long. Designed for quick installation on bridges, piers, and other structures that have a good chance of surviving a hurricane, they collect water pressure readings that help define the depth and duration of a storm tide. That information helps officials assess storm damage, tell the difference between wind and flood damage, and improve computer models for forecasting future storms’ effects.

Storm-tides can powerfully affect beaches but the impact is hard to measure, because these landscapes often don’t have any permanent structures where the sensors can be mounted securely enough to ride out a strong storm. USGS scientists are using a labor-intensive but simple technique that worked well in an experiment during Hurricane Joaquin in 2017: they drove a row of long steel stakes deep into beach sand, mounted sensors on those, and then removed both the sensors and the stakes after the storm passed. Crews from Delaware and Virginia installed sensors Tuesday along a transect of rods driven 45 feet deep at Virginia’s Back Bay National Wildlife Refuge, not far from the Virginia-North Carolina state line.

To track inland flooding, the USGS gathers data from its network of streamgages, and from rapid deployment gauges. When flooding is occurring, USGS field crews make real-time streamflow measurements to verify the streamgages’ readings. After the storm passes, the crews quickly replace storm-damaged or lost gauges. During and right after hurricane flooding, these records help the Federal Emergency Management Agency target emergency relief to the hardest-hit areas.

USGS hydrologists are also preparing for possible inland flooding in the upper reaches of the Potomac River and storm surge effects in the Chesapeake Bay. USGS data shows the bay’s three major rivers, the Susquehanna, Potomac, and James are already well above normal due to the rains over the past weekend. More information on the real-time river flows can be found at https://waterwatch.usgs.gov/new/index.php?r=02&m=real.  The National Weather Service has issued a storm surge watch for the area just south of the bay, and USGS crews have deployed sensors in the southern Bay and the tidal Potomac.

The USGS Coastal Change Hazards Storm Team has forecast coast-wide levels of beach erosion, inundation and overwash due to Hurricane Florence’s expected path and landfall. Credit: USGS. Public domain.

The USGS Coastal Change Storm Hazards Team is predicting that Hurricane Florence is very likely to cause beach erosion at the base of sand dunes along almost three-quarters of the North Carolina coast and 60 percent of the South Carolina coast as it makes landfall. Storm surge is predicted to overwash, or reach the top of, about one-fourth of the North Carolina dunes and one-fifth of South Carolina dunes, say coastal change experts at the U.S. Geological Survey. Immediate damage in other states will likely be less.

However, the storm may cause continuing damage to Mid-Atlantic beaches and dunes if it moves very slowly after landfall, as forecasters expect.

The team, based in St. Petersburg, Florida, issues two different types of forecasts that, taken together, describe the range of effects that Florence may have on the coast. The first approach uses a computer model that analyzes coastwide beach impacts to predicts the probabilities of dune erosion and overwash for a wide area. The second approach, the Total Water Level and Coastal Change Forecast Viewer, is an experimental model that focuses on specific locations over longer periods, forecasting the timing and magnitude of elevated water levels at the shoreline.

“Our forecast is for Florence to cause a long-lasting coastal erosion process with more than one set of impacts to the Mid-Atlantic beaches,” said research oceanographer Kara Doran, leader of the USGS Coastal Change Hazards Storm Team based in St. Petersburg, Florida.

“As Florence makes landfall its storm surge will cause some erosion at the base of the dunes from Georgia through Virginia. In most places it is not likely to overtop that protective row of dunes and cause damage to the communities and natural areas behind them – at least not at first,” Doran said. “But if the storm lingers, and if high surge, higher than normal tides and strong waves persist over a period of days, the likelihood increases that the dunes could be overtopped and flooding could occur behind them.”

The team’s detailed forecasts can be used by emergency managers to guide decisions about where to evacuate coastal areas or position cleanup equipment for use after the storm. The team is also piloting a new forecasting tool called the Total Water Level and Coastal Change Forecast Viewer. Because it is experimental, this viewer is not meant as a guide to making important decisions, said research oceanographer and team leader Kara Doran.

Find information about the full range of USGS response to Hurricane Florence at https://www.usgs.gov/florence.

Chart shows high water at Duck, NC through Sept. 17

The Coastal Change Hazards Storm Team’s Total Water Level Viewer forecasts changing water levels at specific locations over time. Credit: USGS. Public domain.

Puerto Rico and Hawai’i Prepare for Isaac and Olivia

Forecasts call for Isaac, recently downgraded to a tropical storm, to pass by the U.S. Virgin Islands and Puerto Rico between Thursday night and Friday morning.  On Puerto Rico, crews deployed 22 storm-tide sensors along the island’s south and east coasts, where the strongest impacts are expected.

Direct hits from hurricanes are a rarity in Hawai’i, where the last major hurricane landfall was Hurricane Iniki, which devastated the island of Kauai in 1992. Last month Hurricane Lane, a Category Five storm, passed west of the islands, dumping heavy rains and causing some flooding. Olivia weakened as it neared the island of Maui Wednesday. Forecasters expect the storm’s rainfall to total 5 to 10 inches, with up to 15 inches possible in some spots.

Immediately after the worst of the storm has passed, USGS hydrologists will check inland streamgages to verify peak water levels, and measure high-water marks left by floodwaters on buildings, bridges and trees. In the days after the storm, this information helps emergency managers and insurers steer resources to the hardest hit areas. The crews will also calibrate and repair any streamgages damaged by the storm to ensure they continue providing valuable information.

Hurricane Florence photo/ NASA

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