Storm surge is an offshore rise of water associated with a low pressure A low pressure area, or "low", is a region where the atmospheric pressure at sea level is below that of surrounding locations. Low pressure systems form under areas of wind divergence which occur in upper levels of the troposphere. The formation process of a low pressure area is known as cyclogenesis. Within the field of atmospheric weather system, typically a tropical cyclone A tropical cyclone is a storm system characterized by a large low-pressure center and numerous thunderstorms that produce strong winds and heavy rain. Tropical cyclones feed on heat released when moist air rises, resulting in condensation of water vapor contained in the moist air. They are fueled by a different heat mechanism than other cyclonic. Storm surge is caused primarily by high winds Wind is the flow of gases on a large scale. On Earth, wind consists of the bulk movement of air. In outer space, solar wind is the movement of gases or charged particles from the sun through space, while planetary wind is the outgassing of light chemical elements from a planet's atmosphere into space. Winds are commonly classified by their spatial pushing on the ocean An ocean is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth's surface (~3.61 X 1014 m2) is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas's surface. The wind causes the water to pile up higher than the ordinary sea level Mean sea level is a measure of the average height of the ocean's surface (such as the halfway point between the mean high tide and the mean low tide); used as a standard in reckoning land elevation. MSL also plays an extremely important role in aviation, where standard sea level pressure is used as the measurement datum of altitude at flight. Low pressure at the center of a weather system also has a small secondary effect, as can the bathymetry Bathymetry is the study of underwater depth of lake or ocean floors. In other words, bathymetry is the underwater equivalent to hypsometry. The name comes from Greek βαθυς, deep, and μετρον, measure. Bathymetric charts are typically produced to support safety of surface or sub-surface navigation, and usually show seafloor relief or 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 is important to clarify the usage, as well as the reference point. National Hurricane Center The National Hurricane Center , located at Florida International University in Miami, Florida, is the division of National Weather Service's Tropical Prediction Center responsible for tracking and predicting the likely behavior of tropical depressions, tropical storms and hurricanes tropical cyclone reports reference storm surge as water height above predicted astronomical tide level, and storm tide as water height above NGVD-29 The Sea Level Datum of 1929 was the vertical control datum established for vertical control surveying in the United States of America by the General Adjustment of 1929. The datum was used to measure elevation or altitude above, and depression or depth below, mean sea level. Most casualties during a tropical cyclone occur during the storm surge.

In areas where there is a significant difference between low tide and high tide Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon and the Sun and the rotation of the Earth. Most places in the ocean usually experience two high tides and two low tides each day , but some locations experience only one high and one low tide each day (diurnal tide). The times, 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 Extratropical cyclones, sometimes called mid-latitude cyclones or wave cyclones, are a group of cyclones defined as synoptic scale low pressure weather systems that occur in the middle latitudes of the Earth having neither tropical nor polar characteristics, and are connected with fronts and horizontal gradients in temperature and dew point, such as the "Halloween Storm" of 1991 and the Storm of the Century (1993) A "disoriented area of low pressure" that formed in the Gulf of Mexico joined an arctic high pressure system in the Midwestern Great Plains, brought into the mid-latitudes by an unusually steep southward jet stream. These factors combined to produce unusually low temperatures across the eastern half of the United States, but the most extreme storm surge events typically occur as a result of tropical cyclones. There is a separate article on Extratropical Storm Surge Similar to tropical cyclones, extratropical storms cause an offshore rise of water. High winds pushing on the ocean's surface create storm surge by causing the water to pile up higher than the ordinary sea level. However, unlike most tropical cyclone storm surge, extratropical storms can cause higher water levels across a large area for longer. 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 Surge from Hurricanes is a computerized model developed by the Federal Emergency Management Agency (FEMA), United States Army Corps of Engineers (USACE), and the National Weather Service (NWS) to estimate storm surge depths resulting from historical, hypothetical, or predicted hurricanes by taking into account a storm's (Sea, Lake, and Overland Surges from Hurricanes) model is used to simulate surge from tropical cyclones.^[1] Additionally, there is an extratropical storm surge model that is used to predict those effects. ^[2]

The Galveston Hurricane of 1900 The Hurricane of 1900 made landfall on the city of Galveston in the U.S. state of Texas, on September 8, 1900. It had estimated winds of 135 miles per hour at landfall, making it a Category 4 storm on the Saffir-Simpson Hurricane Scale, a Category 4 hurricane that struck Galveston, Texas Galveston is a coastal city located on Galveston Island in the U.S. state of Texas. As of the 2005 U.S. Census estimate, the city had a total population of 57,466 within an area of 208 square miles (540 km2). Located within the Houston–Sugar Land–Baytown metropolitan area, the city is the seat and second-largest city of Galveston County in, drove a devastating surge ashore; between 6,000 and 12,000 lives were lost, making it the deadliest natural disaster A natural disaster is the effect of a natural hazard that affects the environment, and leads to financial, environmental and/or human losses. The resulting loss depends on the capacity of the population to support or resist the disaster, and their resilience. This understanding is concentrated in the formulation: "disasters occur when hazards ever to strike the United States.^[3] The second deadliest natural disaster in the United States ^ b. English is the de facto language of American government and the sole language spoken at home by 80% of Americans age five and older. Spanish is the second most commonly spoken language was the storm surge from Lake Okeechobee Lake Okeechobee locally referred to as The Lake or The Big O, is a freshwater lake in the U.S. state of Florida. It is the second-largest freshwater lake wholly within the continental United States (after Lake Michigan) and the largest in the southern United States. Okeechobee covers 730 square miles (1,890 km²), approximately half the size of in the 1928 Okeechobee Hurricane The Okeechobee hurricane, or San Felipe Segundo hurricane, was a deadly hurricane that struck the Leeward Islands, Puerto Rico, the Bahamas, and Florida in September of the 1928 Atlantic hurricane season. It was the second recorded hurricane to reach Category 5 status on the Saffir-Simpson Hurricane Scale in the Atlantic basin after the 1924 Cuba which swept across the Florida With an area of 65,758 square miles , it is ranked 22nd in size among the 50 U.S. states. Florida has the longest coastline in the Contiguous United States encompassing approximately 1,350 miles (2,170 km). The state has four large urban areas, a number of smaller industrial cities, and many small towns 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 recovered. Only two years earlier, a storm surge from the Great Miami Hurricane The 1926 Miami Hurricane was an intense hurricane that devastated Miami, Florida in September 1926. The storm also caused significant damage in the Florida Panhandle, the U.S. state of Alabama, and the Bahamas. The storm's enormous regional economic impact helped end the Florida land boom of the 1920s and pushed the region on an early start into of September 1926 broke through the small earthen dike rimming the lake's western shore, killing 150 people at Moore Haven, Florida Moore Haven is a city in Glades County, Florida, United States. The population was 1,635 at the 2000 census. As of 2004, the population recorded by the U.S. Census Bureau is 1,726. It is the county seat of Glades County. Moore Haven is located on the southwest shoreline of Lake Okeechobee^[4]. The storm surge that accompanied the New England Hurricane of 1938 The New England Hurricane of 1938 was the first major hurricane to strike New England since 1869. The storm formed near the coast of Africa in September of the 1938 Atlantic hurricane season, becoming a Category 5 hurricane on the Saffir-Simpson Hurricane Scale before making landfall as a Category 3 hurricane on Long Island on September 21. The killed as many as 700 people when it struck Long Island Long Island is an island located in southeastern New York, United States, just east of Manhattan. Stretching northeast into the Atlantic Ocean, Long Island contains four counties, two of which are boroughs of New York City, and two of which (Nassau and Suffolk) are mainly suburban or rural. In popular usage, the term "Long Island", New York New York City, the most populous city in the United States, is known for its status as a financial, cultural, transportation, and manufacturing center, and for its history as a gateway for immigration to the United States. According to the U.S. Department of Commerce, it is also a destination of choice for many foreign visitors. Both state and and southeastern New England In one of the earliest European settlements in North America, Pilgrims from England first settled in New England in 1620, to form Plymouth Colony. Ten years later, the Puritans settled north of Plymouth Colony in Boston, thus forming Massachusetts Bay Colony in 1630. In the late 18th century, the New England colonies would be among the first North.

The largest storm surge in the twentieth century was the North Sea flood of 1953 The North Sea flood of 1953 and the associated storm combined to create a major natural disaster which affected the coastlines of the Netherlands and England on the night of 31 January – 1 February 1953. Belgium, Denmark and France were also affected by flooding and storm damage, which killed a total of over 2,000 people in the UK The United Kingdom of Great Britain and Northern Ireland[note 7] is a sovereign state located off the northwestern coast of continental Europe. It is an island country, spanning an archipelago including Great Britain, the northeastern part of the island of Ireland, and many small islands. Northern Ireland is the only part of the UK with a land and The Netherlands The Netherlands (pronounced /ˈnɛðɚləndz/ ; Dutch: Nederland, pronounced [ˈneːdərlɑnt] ( listen)) is a constituent country of the Kingdom of the Netherlands, located in North-West Europe. It is a parliamentary democratic constitutional monarchy. The Netherlands borders the North Sea to the north and west, Belgium to the south, and Germany

Mechanics

At least five processes can be involved in altering tide Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon and the Sun and the rotation of the Earth. Most places in the ocean usually experience two high tides and two low tides each day , but some locations experience only one high and one low tide each day (diurnal tide). The times levels during storms A storm is any disturbed state of an astronomical body's atmosphere, especially affecting its surface, and strongly implying severe weather. It may be marked by strong wind, thunder and lightning (a thunderstorm), heavy precipitation, such as ice (ice storm), or wind transporting some substance through the atmosphere (as in a dust storm, snowstorm,: the pressure effect, the direct wind effect, the effect of the Earth's rotation, the effect of waves, and the rainfall effect.^[5]. The pressure effects of a tropical cyclone will cause the water level in the open ocean to rise in regions of low pressure atmospheric and fall in regions of high pressure. The rising water level will counteract the low atmospheric pressure such that the total pressure at some plane beneath the water surface remains constant. This effect is estimated at a 10 mm (0.39 in) increase in sea level for every millibar The bar is a unit of pressure equal to 100 kilo drop in atmospheric pressure.^[5]

Strong surface winds cause surface currents perpendicular to the wind direction, by an effect known as the Ekman Spiral The Ekman spiral refers to a structure of currents or winds near a horizontal boundary in which the flow direction rotates as one moves away from the boundary. It derives its name from the Swedish oceanographer Vagn Walfrid Ekman. The deflection of surface currents was first noticed by the Norwegian oceanographer Fridtjof Nansen during the Fram. 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. Intuitively, this is caused by the storm simply blowing the water towards one side of the basin in the direction of its winds. Because the Ekman Spiral effects spread vertically through the water, the effect is inversely proportional to depth. The pressure effect and the wind set-up on an open coast will be driven into bays in the same way as the astronomical tide.^[5]

The Earth's rotation causes the Coriolis effect In physics, the Coriolis effect is an apparent deflection of moving objects when they are viewed from a rotating reference frame. In a reference frame with clockwise rotation, the deflection is to the left of the motion of the object; in one with anti-clockwise rotation, the deflection is to the right. The mathematical expression for the Coriolis, which bends currents to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. When this bend brings the currents into more perpendicular contact with the shore it can amplify the surge, and when it bends the current away from the shore it has the effect of lessening the surge.^[5]

The effect of waves, while directly powered by the wind, is distinct from a storm's wind-powered currents. Powerful wind whips up large, strong waves in the direction of its movement.^[5] Although these surface waves are responsible for very little water transport in open water, they may be responsible for significant transport near the shore. When waves are breaking on a line more or less parallel to the beach they carry considerable water shoreward. As they break, the water particles moving toward the shore have considerable momentum and may run up a sloping beach to an elevation above the mean water line which may exceed twice the wave height before breaking.^[6]

The rainfall effect is experienced predominantly in estuaries An estuary is a partly enclosed coastal body of water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Hurricanes may dump as much as 12 in (300 mm) of rainfall in 24 hours over large areas, and higher rainfall densities in localized areas. As a result, watersheds A drainage basin is an extent or area of land where water from rain and melting snow or ice drains downhill into a body of water, such as a river, lake, reservoir, estuary, wetland, sea or ocean. The drainage basin includes both the streams and rivers that convey the water as well as the land surfaces from which water drains into those channels, can quickly surge water into the rivers A river is a natural watercourse, usually freshwater, flowing toward an ocean, a lake, a sea, or another river. In a few cases, a river simply flows into the ground or dries up completely before reaching another body of water. Small rivers may also be called by several other names, including stream, creek, brook, rivulet, and rill; there is no that drain them. This can increase the water level near the head of tidal estuaries as storm-driven waters surging in from the ocean meet rainfall flowing from the estuary.^[5]

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 well exemplified by the southeast coast of Florida With an area of 65,758 square miles , it is ranked 22nd in size among the 50 U.S. states. Florida has the longest coastline in the Contiguous United States encompassing approximately 1,350 miles (2,170 km). The state has four large urban areas, a number of smaller industrial cities, and many small towns. The edge of the Floridian Plateau, where the water depths reach 91 metres (299 ft), lies just 3,000 m (9,800 ft) offshore of Palm Beach, Florida The Town of Palm Beach is an affluent incorporated town in Palm Beach County, Florida, United States. The Intracoastal Waterway separates it from the neighboring cities of West Palm Beach and Lake Worth. As of 2000, Palm Beach had a year-round population of 10,468, with an estimated seasonal population of 30,000. As of 2004, the year-round; just 7,000 m (23,000 ft) offshore, the depth increases to over 180 m (590 ft).^[7] The 180 m (590 ft) depth contour followed southward from Palm Beach County Palm Beach County is the largest county in the state of Florida in area. As of 2007, the rapidly-growing county's estimated population was 1,751,236, making it the third most populous in the state and the twenty ninth most populous in the United States. Over 40 percent of the county's population lives in unincorporated areas near the Atlantic lies more than 30,000 m (98,000 ft) to the east of the upper Keys.

Conversely, coastlines along North America North America is the northern continent of the Americas, situated in the Earth's northern hemisphere and in the western hemisphere. It is bordered on the north by the Arctic Ocean, on the east by the North Atlantic Ocean, on the southeast by the Caribbean Sea, and on the west by the North Pacific Ocean; South America lies to the southeast such as those along the Gulf of Mexico coast from Texas to Florida, and Asia Asia is the world's largest and most populous continent, located primarily in the eastern and northern hemispheres. It covers 8.6% of the Earth's total surface area and with approximately 4 billion people, it hosts 60% of the world's current human population. During the 20th century Asia's population nearly quadrupled such as the Bay of Bengal, have long, gently sloping shelves and shallow water depths. On the Gulf side of Florida With an area of 65,758 square miles , it is ranked 22nd in size among the 50 U.S. states. Florida has the longest coastline in the Contiguous United States encompassing approximately 1,350 miles (2,170 km). The state has four large urban areas, a number of smaller industrial cities, and many small towns, the edge of the Floridian Plateau lies more than 160 kilometres (99 mi) offshore of Marco Island in Collier County. Florida Bay Florida Bay is the bay located between the southern end of the Florida mainland and the Florida Keys. Its area is variously stated to be 800 square miles (2,100 km2), or 850 square miles (2,200 km2), or 1,000 square miles (2,600 km2). Nearly all of Florida Bay is included in Everglades National Park. The southern edge, along the Florida Keys is in, lying between the Florida Keys The Florida Keys are a chain-like cluster of about 1700 islands in the southeast United States. They begin at the southeastern tip of the Florida peninsula, about 15 miles south of Miami, and extend in a gentle arc south-southwest and then westward to Key West, the westernmost of the inhabited islands, and on to the uninhabited Dry Tortugas. The and the mainland, is also very shallow; depths typically vary between 0.3 m (0.98 ft) and 2 m (6.6 ft).^[8] 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.^[5]

For a given topography and bathymetry the surge height is not solely effected by peak wind speed; the size of the storm also effects the peak surge. With any storm the piled up water has an exit path to the sides and this escape mechanism is reduced in proportion to the surge force (for the same peak wind speed) as the storm covers more area.

Measuring surge

Surge can be measured directly at coastal tidal stations as the difference between the forecast tide and the observed rise of water.^[9] This information can be viewed real-time on the NOAA Tides and Currents website, as long as the station is reporting.^[10]

Another method of measuring surge was implemented by NHC The National Hurricane Center , located at Florida International University in Miami, Florida, is the division of National Weather Service's Tropical Prediction Center responsible for tracking and predicting the likely behavior of tropical depressions, tropical storms and hurricanes 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.^[11] 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.^[12]

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 is 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.^[13]

Two different measures are used for storm tide and storm surge measurements. Storm tide is measured using a geodetic Geodesy , also named geodetics, a branch of earth sciences, is the scientific discipline that deals with the measurement and representation of the Earth, including its gravitational field, in a three-dimensional time-varying space. Geodesists also study geodynamical phenomena such as crustal motion, tides, and polar motion. For this they design vertical datum (NGVD 29 The Sea Level Datum of 1929 was the vertical control datum established for vertical control surveying in the United States of America by the General Adjustment of 1929. The datum was used to measure elevation or altitude above, and depression or depth below, mean sea level or NAVD 88). 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) Mean sea level is a measure of the average height of the ocean's surface (such as the halfway point between between the mean high tide and the mean low tide); used as a standard in reckoning land elevation at that location, then subtracting the tidal prediction yields a surge height above the normal water height.^[9][13]

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