Storm surge Totally Explained

Everything about Storm Flood totally explained

A storm surge or tidal surge is an offshore rise of water associated with a low pressure weather system, typically a tropical cyclone. Storm surge is caused primarily by high winds pushing on the ocean's surface. The wind causes the water to pile up higher than the ordinary sea level. Low pressure at the center of a weather system also has a small secondary effect, as can the bathymetry of the body of water. It is this combined effect of low pressure and persistent wind over a shallow water body which is the most common cause of storm surge flooding problems. The term "storm surge" in casual (non-scientific) use is storm tide; that is, it refers to the rise of water associated with the storm, plus tide, wave run-up, and freshwater flooding. When referencing storm surge height, it's important to clarify the usage, as well as the reference point. National Hurricane Center tropical cyclone reports reference storm surge as water height above predicted astronomical tide level, and storm tide as water height above NGVD-29.
In areas where there's a significant difference between low tide and high tide, storm surges are particularly damaging when they occur at the time of a high tide. In these cases, this increases the difficulty of predicting the magnitude of a storm surge since it requires weather forecasts to be accurate to within a few hours. Storm surges can be produced by extratropical cyclones, such as the "Halloween Storm" of 1991 and the Storm of the Century (1993), but the most extreme storm surge events occur as a result of tropical cyclones. Factors that determine the surge heights for landfalling tropical cyclones include the speed, intensity, size of the radius of maximum winds (RMW), radius of the wind fields, angle of the track relative to the coastline, the physical characteristics of the coastline and the bathymetry of the water offshore. The SLOSH (Sea, Lake, and Overland Surges from Hurricanes) model is used to simulate surge from tropical cyclones. The Galveston Hurricane of 1900, a Category 4 hurricane that struck Galveston, Texas, drove a devastating surge ashore; between 6,000 and 12,000 lives were lost, making it the deadliest natural disaster ever to impact the United States (Hebert, 1990). The second deadliest natural disaster in the U.S. was the storm surge from Lake Okeechobee in the 1928 Okeechobee Hurricane which swept across the Florida peninsula during the night of September 16. The lake surged over its southern bank, virtually wiping out the settlements on its south shore. The estimated death toll was over 2,500; many of the bodies were never found. Only two years earlier, a storm surge from the Great Miami Hurricane of September 1926 broke through the small earthen dike rimming the lake's western shore, killing 150 people at Moore Haven, Florida (Will, 1978). The storm surge that accompanied the New England Hurricane of 1938 killed as many as 700 people when it hit Long Island, New York and southeastern New England.

Mechanics

At least five processes can be involved in altering tide levels during storms. These include the pressure effect, the direct wind effect, the effect of the earth's rotation, the effect of waves, and the rainfall effect (Harris 1963). The pressure effects of a tropical cyclone will cause the water level in the open ocean to rise in regions of low pressure and fall in regions of high pressure. Wind stresses cause a phenomenon referred to as "wind set-up", which is the tendency for water levels to increase at the downwind shore, and to decrease at the upwind shore. This effect is inversely proportional to depth (Harris 1963). Wind set-up on an open coast will be driven into bays in the same way as the astronomical tide.
   Surge and wave heights on shore are affected by the configuration and bathymetry of the ocean bottom. A narrow shelf, or one that has a steep drop from the shoreline and subsequently produces deep water in close proximity to the shoreline tends to produce a lower surge, but a higher and more powerful wave. This situation is seen along the southeast coast of Florida. The edge of the Floridian Plateau, where the water depths reach 91 meters, lies just 3,000 m offshore of Palm Beach, Florida; just 7,000 m offshore, the depth plunges to over 180 m (Lane 1980). The 180 m depth contour followed southward from Palm Beach County lies more than 30,000 m to the east of the upper Keys.
   Conversely, coastlines such as those along the Gulf of Mexico coast from Texas to Florida, have long, gently sloping shelves and shallow water depths. On the Gulf side of Florida, the edge of the Floridian Plateau lies more than 160 km offshore of Marco Island in Collier County. Florida Bay, lying between the Florida Keys and the mainland, is also very shallow; depths typically vary between 0.3 and 2 meters (Lane 1981). These areas are subject to higher storm surges, but smaller waves.
   This difference is because in deeper water, a surge can be dispersed down and away from the hurricane. However, upon entering a shallow, gently sloping shelf, the surge can not be dispersed away, but is driven ashore by the wind stresses of the hurricane.
   Topography of the land surface is another important element in storm surge extent. Areas where the land lies less than a few meters above sea level are at particular risk from storm surge inundation.

Measuring surge

Surge can be measured directly at coastal tidal stations as the difference between the forecasted tide and the observed rise of water. This information can be viewed real-time on the NOAA Tides and Currents website, as long as the station is reporting.
   Another method of measuring surge was implemented by NHC starting in 2005, with a USGS team deploying pressure transducers along the coastline just ahead of an approaching tropical cyclone. This was first tested for Hurricane Rita. This method was validated against other surge measurements taken for Rita, and was subsequently used during Ernesto in 2006. These types of sensors can be placed in locations that will be submerged, and can accurately measure the height of water above them.
   After surge from a tropical cyclone has receded, teams of surveyors map high water marks (HWM) on land, in a rigorous and detailed process that includes photos and written descriptions of the marks. HWM denote the location and elevation of flood waters from a storm event. When HWM are analyzed, if the various components of the water height can be broken out so that the portion attributable to surge can be identified, then that mark can be classified as storm surge. Otherwise, it's classified as storm tide. HWM on land are referenced to a vertical datum (a reference coordinate system). During evaluation, HWM are divided into four categories based on the confidence in the mark; only HWM evaluated as "excellent" are used by NHC in post storm analysis of the surge.
   Two different measures are used for storm tide and storm surge measurements. Storm tide is measured using a geodetic vertical datum (NGVD 29 or NAVD88 ). Since storm surge is defined as the rise of water beyond what would be expected by the normal movement due to tides, storm surge is measured using tidal predictions, with the assumption that the tide prediction is well-known and only slowly varying in the region subject to the surge. Since tides are a localized phenomenon, storm surge can only be measured in relationship to a nearby tidal station. Tidal bench mark information at a station provides a translation from the geodetic vertical datum to mean sea level (MSL) at that location, then subtracting the tidal prediction yields a surge height above the normal water height. In the United States, one of the greatest recorded storm surges was generated by 2005's Hurricane Katrina, which produced a maximum storm surge on the order of 7.6 meters (25 ft) around St. Louis Bay, Mississippi, in the communities of Waveland, Bay St. Louis, Diamondhead and Pass Christian, with a storm surge height of in Pass Christian. Another record storm surge occurred in this same area from Hurricane Camille in August 1969, with the highest storm tide of record noted from a HWM as, also found in Pass Christian. The worst storm surge, in terms of loss of life, was the 1970 Bhola cyclone and in general the Bay of Bengal is particularly prone to tidal surges.

SLOSH

The National Hurricane Center forecasts storm surge using the SLOSH model, which stands for Sea, Lake and Overland Surges from Hurricanes. The model is accurate to within 20 percent. SLOSH inputs include the central pressure of a tropical cyclone, storm size, the cyclone's forward motion, its track, and maximum sustained winds. Local topography, bay and river orientation, depth of the sea bottom, astronomical tides, as well as other physical features are taken into account, in a predefined grid referred to as a SLOSH basin. Overlapping SLOSH basins are defined for the southern and eastern coastline of the continental U.S. Some storm simulations use more than one SLOSH basin; for instance, Katrina SLOSH model runs used both the Lake Ponchartrain / New Orleans basin, and the Mississippi Sound basin, for the northern Gulf of Mexico landfall. The final output from the model run will display the maximum envelope of water, or MEOW, that occurred at each location. To allow for track or forecast uncertainties, usually several model runs with varying input parameters are generated to create a map of MOMs, or Maximum of Maximums. And for hurricane evacuation studies, a family of storms with representative tracks for the region, and varying intensity, eye diameter, and speed, are modeled to produce worst-case water heights for any tropical cyclone occurrence. The results of these studies are typically generated from several thousand SLOSH runs. These studies have been completed by USACE, under contract to the Federal Emergency Management Agency, for several states and are available on their Hurricane Evacuation Studies (HES) website They include coastal county maps, shaded to identify the minimum SSHS category of hurricane that will result in flooding, in each area of the county.

Mitigation

Although meteorological surveys alert about hurricanes or severe storms, in the areas where the risk of coastal flooding is particularly high, there are specific storm surge warnings. These have been implemented, for instance, in Holland, Spain, the United States, and Great Britain.
A prophylactic method introduced after the North Sea Flood of 1953 is the construction of dams and floodgates (storm surge barriers). They are open and allow free passage but close when the land is under threat of a storm surge. Major storm surge barriers are the Oosterscheldekering and Maeslantkering in the Netherlands which are part of the Delta Works project, and the Thames Barrier protecting London.

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