What is Storm Surge?
According to the National Hurricane Center, storm surge is defined as “an abnormal rise of water generated by a storm, over and above the predicted astronomical tide”.
Although the severity of hurricanes are most commonly denominated by the maximum sustained wind speeds (i.e. the category grading system), storm surge is substantially the most dangerous aspect of hurricane events. Storm surge happens when water is pushed inland by powerful winds, at the speed of which the hurricane system itself is moving. The forward movement of a hurricane system is usually between 10-35 miles per hour.
How is Storm Surge Measured?
The most direct way of measuring the level of storm surge is by observing the difference in the forecasted tide and the actual rise in water levels. NOAA maintains 175 tide stations in areas generally sheltered from waves across the US coastline, which continuously monitor the tides. Sheltering the tide stations from waves allows for a more accurate reading of Stillwater levels. The main concern with storm surge readings from tide stations is their reliance on electrical power, which often gets cut during the peak of hurricane events. During Hurricane Rita in 2005, a new method was tested for the first time. The method involved deploying pressure sensors along the coast right before the hurricane event would arrive. The pressure transducers are attached to poles and other structures in areas that will be submerged, and the sensors will accurately measure the amount of water above them. They also provide accurate readings of the duration of the storm surge, as well as data pertaining to its arrival and retreat. Although these devices offer valuable data from storm events, researchers often face challenges related to recovering the devices when the storm has passed. Wave action may also provide skewed readings, resulting in overestimations of the surge levels.
Post surge, there are several ways to determine the level of storm surge that has occurred with some degree of accuracy, but they are not without their own difficulties. One of these methods includes mapping out high-water marks on land. In order to ensure collection of accurate data pertaining to the elevation and location of surge water, also known as vertical datum, researcher employ a rigorous and highly detailed process involving photos and descriptions of the marks in question. Researchers must act quick as these high-water marks perish within a finite timeframe, and such quick actions may be challenging if located in areas of utter destruction. Another challenge is the lack of ways to separate wave action data points from actual storm surge levels. Therefore, data can be scrambled as a low percentage of high-water marks accurately represents storm surge levels sans waves.
Since these datasets come with varying levels of confidence, the data is divided into 4 categories, whereas only the “excellent” high water marks are used by the National Hurricane Center in their post-storm analysis process.
It’s also important to note that storm surge only makes up part of the total rise in water levels. The total rise in water levels is defined as the sum of storm surge, tides, waves, and rainfall.
Why is Storm Surge Dangerous?
Storm surge is particularly dangerous for two reasons. First, a cubic yard of water weighs 1728 lbs., and even a single foot of storm surge is powerful enough to move vehicles and cause large scale damages to buildings and homes. Powerful hurricanes can cause storm surges exceeding one foot in areas located directly on the shoreline.
Second, storm surge often occurs rapidly and by surprise. Storm surge is known to appear as early as 24 hours prior to landfall, which can be difficult to plan for and react to.
An additional storm surge hazard (and for major flooding in general) is the increased potential for sewage overflow. Six months post Hurricane Sandy, 11 billion gallons of sewage spilled into rivers and bays in the eight states most severely hit by the hurricane. In some cases, raw sewage even spilled into city streets. To put in context, if Central Park was a pool, the aforementioned 11 billion gallons of sewage would make that pool 41 feet deep. Sewage overflows pose a multitude of potential health hazards and carries a high likelihood of causing infections and even sepsis for anyone who comes in direct contact with it.
What Determines The Level of Storm Surge?
As previously mentioned, the winds push the water that ultimately creates the storm surge. The stronger the winds, the more water that gets displaced and surges onto land.
Another determinant is the size of the storm itself. Larger storms will displace water from a larger area of the ocean and are more likely to generate storm surge across a wider area of the coastline, which helps to explain the massive damage on the East Coast resulting from Hurricane Sandy. Although Sandy was far from the most intense storm recorded in the Atlantic Basin, it was the largest measured by diameter with its tropical storm level wind field spanning 900 miles (1400 km). Hence its other nickname, Superstorm Sandy.
3-Forward speed of The Storm
A fast-moving storm will generally produce higher surge, while a slower moving storm is more likely to result in higher surge levels in bays and other enclosed bodies of water. Although most storms move between 10-35 miles per hour, it is important to note that Hurricane Dorian remained stationary above Grand Bahama island for more than 24 hours as a category 4/5 hurricane, causing extreme flooding and destruction. So should a storm stall over a region for an extended period of time, flooding may be tremendous.
4-Width and Slope of The Ocean Bottom
The steeper and narrower the continental shelf, the less storm surge is produced. The east coast of Florida has a relatively steep shelf, limiting the risk of catastrophic storm surge. Wide and gradual sloping coastlines are more susceptible to storm surge, such as the Gulf Coast due to its gradual deepening of the ocean floor.
5-Angle of approach to coastline
Should the storm hit a coastal area head on, the resulting storm surge is going will be substantially more severe than if the storm moves along the coastline.
6-Shape of the coastline
The final determinant of storm surge is the shape of the coastline itself. A concave (curved inward) shoreline will be much more vulnerable to storm surge than a convex (curved outward) one.
Worst Storm Surge Recorded
The highest storm surge ever recorded in the US occured during Hurricane Katrina when 27.8 feet were measured at Pass Christian, Mississippi. Pass Christian was also the site for the second highest storm surge ever recorded, which occurred during Hurricane Camille in 1969.
The highest storm surge ever recorded globally occurred in Australia in 1899 during the Bathurt Bay Cyclone, also known as Topical Cyclone Mahina. Although estimates vary due to the lack of modern technology for the purpose of recording and analyzing data, it is generally agreed upon that this extreme weather event caused a storm surge of approximately 30 ft, not including wave action and storm tide. Combined, total inundation was around 43 ft.
Storm Surge Mitigation
For public safety, ensuring timely warnings of incoming storm surge remains essential. However, prophylactic efforts such as storm surge barriers and dams are used in various places in the world, including the United States, United Kingdom, Netherlands and Russia. When no storms are inbound, these dams/barriers are open for passage, but can quickly close if extreme weather events are on the horizon. The Netherlands has constructed a series of dams in the southwestern part of the country in order to protect against storm surge from the North Sea, and to prevent widespread damage and loss of life. The largest structure in this project is the Oosterscheldekering, a 5.6 miles long storm surge barrier which took more than a decade to build. The dam has 65 pillars and 62 steel doors that each measure 138 feet wide. It is designed to last at for at least 200 years. The American Society of Civil Engineers declared it one of the modern seven wonders of the world.
For less extreme weather events with projected rise in water levels of 4 feet or less, there are other alternatives to provide a stable and effective barrier between storm surge and property, such as FlexaPRISM, an improved and best-in-class flood solution. For more information on how FlexaPRISM can protect your property, assets, and peace of mind, please read here and contact Coastal Innovations.