Mangroves help shield coastal areas from storm surge

Butch D. Enerio in the Business Mirror (Philippines): Close to a thousand civil servants from different national and local offices, employees from private companies and students trooped to the shore of Taytay, El Salvador City in Misamis Oriental, on Thursday for their unified gesture to celebrate the World Environment Day (WED). They planted mangroves.

This year’s celebration, which is anchored on the theme, “Small Islands and Climate Change,” with the slogan—“Raise Your Voice, Not the Sea Level”—aims to promote worldwide awareness and action for the environment.

“It intends not only for people to realize their shared responsibility in taking care of mother earth, but for them to become agents for positive environmental change,” said Ruth M. Tawantawan, Department of Environment and Natural Resources (DENR) regional director for Northern Mindanao.

The Wednesday celebration was spearheaded by the DENR and the Philippine Information Agency in Northern Mindanao.

...The DENR said mangroves protect coastal communities from future typhoons and storm surges and other extreme weather events that may occur because it serves as buffer-shielding the coastal areas—as it also provides nursery grounds for fish, prawns and crabs that support fisheries production. Originally, the Philippines has 500,000 hectares of mangrove forest, but due to coastal development, land conversion and reclamation, only less than 100,000 hectares are visible in the country today....

Bamboo bridge and Mangroves at Bakhawan Eco-park and Research Centre,  shot by Paolobon140 , Wikimedia Commons, under the Creative Commons 3.0 license

Odds of storm waters overflowing Manhattan seawall up 20-fold

A press release from the American Geophysical Union: Maximum water levels in New York harbor during major storms have risen by nearly two and a half feet since the mid-1800s, making the chances of water overtopping the Manhattan seawall now at least 20 times greater than they were 170 years ago, according to a new study. Whereas sea-level rise, which is occurring globally, has raised water levels along New York harbor by nearly a foot and a half since the mid-19th century, the research shows that the maximum height of the city’s “once-in-10-years” storm tide has grown additionally by almost a foot in that same period.

The newly recognized storm-tide increase means that New York is at risk of more frequent and extensive flooding than was expected due to sea-level rise alone, said Stefan Talke, an assistant professor of civil and environmental engineering at Portland State University in Portland, Ore. He is lead author of the new study accepted for publication in Geophysical Research Letters, a journal of the American Geophysical Union. The research also confirms that the New York harbor storm tide produced by Hurricane Sandy was the largest since at least 1821.

Tide gauge data analyzed in the study show that a major, “10-year” storm hitting New York City today causes bigger storm tides and potentially more damage than the identical storm would have in the mid-1800s. Specifically, Talke explained, there’s a 10 percent chance today that, in any given year, a storm tide in New York harbor will reach a maximum height of nearly two meters (about six and a half feet), the so-called “10-year storm.” In the mid-19th century, however, that maximum height was about 1.7 meters (about 5.6 feet), or nearly a foot lower than it is today, according to tide gauge data going back to 1844, he noted.

“What we are finding is that the 10-year storm tide of your great-, great-grandparents is not the same as the 10-year storm tide of today,” Talke said.

... The study’s findings may indicate that “storm surges’ interaction with New York harbor has gotten larger so that in addition to sea level rise, the storm surges may have been enhanced,” said Chris Zervas, a scientist at NOAA’s Center for Operational Oceanographic Products and Services in Silver Spring, Md., who was not involved in the study. “For the latter part of the 1900s, [it shows] that the possibility of overtopping the seawall has increased quite a bit in addition” to sea-level rise, he added...

The Hugh L. Carey Tunnel (formerly known as the Brooklyn Battery Tunnel) on Oct. 30, 2012, during Hurricane Sandy, shot by Metropolitan Transportation Authority of the State of New York, Wikimedia Commons via Flickr, under the Creative Commons Attribution 2.0 Generic license 

New model now capable of street-level storm-tide predictions

David Malmquist in a press release from the Virginia Institute of Marine Science at William & Mary:   The water that surged into the intersection of New York City’s Canal and Hudson streets during Hurricane Sandy—to choose just one flood-ravaged locale—was ultimately driven ashore by forces swirling hundreds of miles out in the Atlantic.

That simple fact shows not only the scale and power of a tropical cyclone, but the difficulty of modeling and forecasting its potential for coastal flooding on the fine scale needed to most effectively prepare a response.

Sandy caused more than $50 billion in damage, left millions without electricity, and killed 72 people. © www.realscience.us Sandy caused more than $50 billion in damage, left millions without electricity, and killed 72 people. © www.realscience.us Now, a study led by Professor Harry Wang of William & Mary’s Virginia Institute of Marine Science demonstrates the ability to predict a hurricane’s storm tide at the level of individual neighborhoods and streets—a much finer scale than current operational methods.

The study, published in today’s issue of the Journal of Marine Science and Engineering, shows that with the right input, the team’s high-resolution computer model was able to simulate water levels to within 6-8 inches of those observed in New York City and surrounding areas during Hurricane Sandy’s approach and landfall in late October 2012. This includes sections of Manhattan where buildings and other infrastructure divert and channel floodwaters in exceptionally co
mplex ways.

“Storm-surge modeling is a tough problem,” says Wang. “People are interested in the possibility of flooding on a very fine scale, on the order of their house, office, or street.” But for a forecast model to work, he says, “We have to resolve the boundary conditions——data on tides and winds—very far away, out into the open ocean. And we have to have that information far enough beforehand to provide time for people and agencies to respond.”

Wang and his modeling team—fellow VIMS researchers Derek Loftis, Zhuo Liu, David Forrest, and Joseph Zhang—conducted their study by “hindcasting” Hurricane Sandy’s landfall along the U.S. Atlantic coast. In this technique, scientists initiate a computer model with data collected before a past event, and then test the model’s accuracy by comparing its output with observations recorded as the event unfolded....

Modeling Animation: Storm-tide flooding of the Battery in New York City through several tidal cycles during Hurricane Sandy as modeled by Professor Harry Wang and colleagues at VIMS. Values are in meters above sea level. Still from animation created by Dr. David Forrest.

CryoSat measures European storm surge

A press release from CryoSat at the European Space Agency: ESA’s CryoSat satellite measured the storm surge from the recent North Sea storms, as high waters passed through the Kattegat sea between Denmark and Sweden. During 5–6 December, a major storm passed through northern Europe causing flooding, blackouts, grounding flights and bringing road, rail and sea travel to a halt.

Since the storm coincided with a period of high tides in the North Sea, there were extremely high sea levels – a ‘storm surge’. In the UK, sea levels were at their highest since the 1953 North Sea Floods, while in Germany, parts of Hamburg were flooded.

On Friday night, CryoSat passed over Kattegat, providing an estimate of total water levels. The observations matched predictions, helping to confirm these models. The measurements were made by CryoSat’s radar altimeter that – although designed to measure sea-ice thickness – is providing outstanding results over sea and, especially, coastal areas.

Until recently, altimeter measurements of sea-level height could only be made over open oceans because of land interference closer to the coast. In the last few years, however, progress has been made in reducing these effects, also thanks to the new generation of radar altimeters being heralded by CryoSat. This has allowed scientists not only to map water levels closer to the coast, but also profile land surfaces and inland water targets such as small lakes, rivers and their intricate tributaries.

Altimeter measurements from space can be used to validate storm surge models as well as provide near-realtime information that can be incorporated into predictions. Under ESA’s Data User Element, the eSurge project is helping to optimise the use of altimetry and other types of satellite data to improve storm surge forecasting....

Sea-surface height over the Kattegat sea as measured by CryoSat on 6 December 2013. Image by ESA/NOC

New study proposes changes in New Orleans area levee systems

William G. Gilroy in Notre Dame News: Less may mean more when it comes to the levee systems designed to protect New Orleans from hurricanes. That’s the conclusion of a new study by a team of University of Notre Dame researchers led by Joannes Westerink, chair of the department of civil and environmental engineering and earth sciences and co-developer of the authoritative computer model for storm surge used by the U.S. Army Corps of Engineers, the Federal Emergency Management Agency (FEMA) and the state of Louisiana to determine water levels due to hurricane surge and to design levee heights and alignments.

The lower Mississippi River south of New Orleans protrudes into the Gulf of Mexico, and man-made levees line the west bank of the river for 55 kilometers of what is known as the Lower Plaquemines section. There are no levees on the east side of this stretch of the river. Westerink points out that, historically, sustained easterly winds from hurricanes have directed storm surge across Breton Sound into the Mississippi River and against its west bank levees.

“This study clearly shows that the man-built west bank levees on the lower Mississippi River enhance the capture of storm surge by the river,” Westerink said. “The surges are generated by the prevalent easterly winds that are common for regional hurricanes, but they spill into the river. These surges then propagate upriver, endangering New Orleans from the river side.”

As an alternative, the study shows that the lowering of man-made levees along the Lower Plaquemines river section to their natural state, to allow storm surge to partially pass across the Mississippi River, will decrease storm surge upriver toward New Orleans.

“By eliminating the 55 kilometers of man-made levees on the west bank of the river from Pointe a la Hache and Venice, the surges propagating in the river from Pointe a la Hache past New Orleans will be lowered by up to two meters,” Westerink said. “This would save billions of dollars in levee construction to protect communities upriver from Pointe a la Hache.”...

FEMA image of the 17th Street levee in New Orleans

More hurricane surges in the future

University of Copenhagen via EurekAlert: By examining the frequency of extreme storm surges in the past, previous research has shown that there was an increasing tendency for storm hurricane surges when the climate was warmer. But how much worse will it get as temperatures rise in the future? How many extreme storm surges like that from Hurricane Katrina, which hit the U.S. coast in 2005, will there be as a result of global warming? New research from the Niels Bohr Institute show that there will be a tenfold increase in frequency if the climate becomes two degrees Celcius warmer. The results are published in the scientific journal, Proceedings of the National Academy of Science, PNAS.

Tropical cyclones arise over warm ocean surfaces with strong evaporation and warming of the air. The typically form in the Atlantic Ocean and move towards the U.S. East Coast and the Gulf of Mexico. If you want to try to calculate the frequency of tropical cyclones in a future with a warmer global climate, researchers have developed various models. One is based on the regional sea temperatures, while another is based on differences between the regional sea temperatures and the average temperatures in the tropical oceans. There is considerable disagreement among researchers about which is best.

"Instead of choosing between the two methods, I have chosen to use temperatures from all around the world and combine them into a single model," explains climate scientist Aslak Grinsted, Centre for Ice and Climate at the Niels Bohr Institute at the University of Copenhagen.
               
He takes into account the individual statistical models and weights them according to how good they are at explaining past storm surges. In this way, he sees that the model reflects the known physical relationships, for example, how the El Niño phenomenon affects the formation of cyclones. The research was performed in collaboration with colleagues from China and England. The statistical models are used to predict the number of hurricane surges 100 years into the future. How much worse will it be per degree of global warming? How many 'Katrinas' will there be per decade?

Since 1923, there has been a 'Katrina' magnitude storm surge every 20 years. "We find that 0.4 degrees Celcius warming of the climate corresponds to a doubling of the frequency of extreme storm surges like the one following Hurricane Katrina. With the global warming we have had during the 20th century, we have already crossed the threshold where more than half of all 'Katrinas' are due to global warming," explains Aslak Grinsted….

NOAA image of a hurricane storm surge