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Many think the most dangerous part of a tropical system is the wind or rain. However, the storm surge is often what causes the most destruction. Here we examine what storm surge is and what makes it so hazardous.
What Is Storm Surge?
Storm surge is a change in the water level that is due to the presence of a tropical system. Since storm surge is a difference between water levels, it does not require a reference level. All locations along the U.S. East and Gulf coasts are vulnerable to storm surge. Storm surge can penetrate well inland from the coastline. During Hurricane Ike, the surge moved nearly 30 miles inland in some locations across southeastern Texas and southwestern Louisiana.
The Saffir-Simpson Hurricane Scale only measures wind speed and does not include storm surge. Therefore, a stronger hurricane on the scale does not give a full description of the damage it could produce. Hurricane Katrina, a Category 3 storm at landfall in Louisiana, produced catastrophic damage with a 28-foot storm surge. Hurricane Ike, a Category 2 storm at landfall in Texas, also produced catastrophic damage with a 20-foot storm surge. At the same time, Category 4 Hurricane Charley only produced a 6 to 8-foot storm surge in Florida, while Category 1 Hurricane Irene had extensive damage in North Carolina with an 8 to 11-foot storm surge.
Meanwhile, storm tide is the combination of surge and tide, and it does require a reference point. For example, a 15-foot storm surge on top of a high tide that is 2 feet above mean sea level will produce a 17-foot storm tide.
What Causes Storm Surge?
Storm surge is primarily caused by the strong winds in a tropical system. The low pressure of the storm has minimal contribution to storm surge, so a lower pressure and therefore stronger storm does not necessarily mean higher storm surge.
The wind circulation around the eye of a tropical system blows on the ocean surface and produces a vertical circulation in the ocean. When there is deep water, there is nothing to disrupt this circulation, and there is very little indication of storm surge. As a tropical system approaches the coast, it interacts with shallower water and the vertical circulation in the ocean now becomes disrupted by the ocean bottom. The water can no longer go down, so it has nowhere else to go but up and inland.
Most often, storm surge occurs where winds are blowing onshore, which is usually in the northeastern quadrant of the storm. The highest surge tends to occur closer to the center of the eye, where winds are the strongest. Surge usually decreases on the northwestern side of the storm and farther away from the center.
What Is the Total Water Level?
In reality, storm surge only makes up a part of what causes water levels to rise along the coast during a tropical system. Here are the other factors that make up the total water level:
•Tides: Water levels rise and fall along the coast every day due to the gravitational pull of the moon and sun. In general, areas along the Gulf Coast, except for Florida, experience one high tide and one low tide per day. Elsewhere along the East Coast, there are two high tides and two low tides per day. Unfortunately, we cannot time the arrival of the storm within the tidal cycle, so it is always safer to assume high tide when making decisions.
•Waves: Breaking waves contribute to the water level rise through wave runup and wave setup. Wave runup occurs when a wave breaks, and the water is propelled onto the beach. Wave setup occurs when waves continually break onshore and the water from the runup piles up along the coast because it cannot get back to sea. The water level therefore rises as a tropical system approaches, especially since the waves become larger and more water is pushed onshore.
•Freshwater Input: Heavy rainfall ahead of a tropical system can cause river levels to rise well inland from the coast. Once all this water flows downriver and reaches the coast, local water levels, especially near deltas and in bays, will rise.
Total water levels are a combination of storm surge, tides, waves and freshwater input.
What Factors Influence Storm Surge?
There are many factors that contribute to the amount of storm surge a given storm produces at a given location.
•Central Pressure: Lower pressure will produce a higher surge, usually as winds are stronger compared to a storm with higher pressure. However, this is a minimal factor.
•Storm Intensity: Stronger winds will produce a higher surge.
•Size: A larger storm will produce a higher surge. First, the winds in a larger storm are pushing on a larger area of the ocean. Second, the strong winds in a larger storm will tend to affect an area longer than a smaller storm. Size is a key difference between the surge generated by storms like Katrina which was a large storm and Charley which was a smaller storm.
•Storm Forward Speed: On the open coast, a faster storm will produce a higher surge. However, a higher surge is produced in bays, sounds, and other enclosed bodies of water with a slower storm.
•Angle of Approach to Coast: A storm that moves onshore perpendicular to the coast is more likely to produce a higher storm surge than a storm that moves parallel to the coast or moves inland at an oblique angle.
•Shape of the Coastline: Storm surge will be higher when a hurricane makes landfall on a concave coastline (curved inward) as opposed to a convex coastline (curved outward).
•Width and Slope of the Ocean Bottom: Higher storm surge occurs with wide, gently sloping continental shelves, while lower storm surge occurs with narrow, steeply sloping shelves.
•Local Features: Storm surge is highly dependent on local features and barriers that will affect the flow of water. A good example is the coast of North Carolina, which has the complexities of such features as barrier islands, inlets, sounds, bays and rivers.
How Do We Observe and Measure Storm Surge?
There are three ways we can observe and measure storm surge, which all have their own pros and cons.
•Tide stations measure the variation in water level along the coast. They are available in real-time and are traditionally the most reliable way of measuring storm surge. However, there are a limited number of stations and can often fail at the height of an event due to loss of electrical power.
•High water marks are lines found on trees and structures marking the highest elevation of the water surface. Survey crews are deployed after a storm to locate and record reliable high water marks, usually through GPS methods. They are traditionally the best method for capturing the highest surge from an event. They are not available in real-time and need to be done as soon as possible after a storm, so it does not get destroyed. It can also be subjective.
•Pressure sensors from the USGS (U.S. Geological Survey) are temporary barometric pressure sensors that provide information about storm surge duration, times of arrival and retreat and maximum depths. They can provide timing information that cannot be obtained from high water marks and can be deployed ahead of the storm at the locations of the highest expected storm surge. However, the data is not available in real-time, can sometimes overestimate the effects of waves and could be difficult to recover the instruments after a storm.
When a tropical storm threatens your area, make sure to consider storm surge as it can sometimes be the difference between minimal and catastrophic damage.