Dec 2012

Storm Surge

The major hazards associated with hurricanes are:

  • storm surge and storm tide
  • heavy rainfall and inland flooding
  • high winds
  • rip currents
  • tornadoes

For coastal areas, storm surge and large waves produced by hurricanes pose the greatest threat to life and property.

Storm Surge is an abnormal rise of water generated by a storm’s winds. Storm surge can reach heights well over 20 ft. and can span hundreds of miles of coastline.

Storm Tide is the water level rise during a storm due to the combination of storm surge and the astronomical tide.

As shown throughout history, the destructive power of storm surge and large battering waves can result in loss of life, destruction of buildings, beach and dune erosion and road and bridge damage in coastal areas. Storm surge can travel several miles inland as shown during Hurricane Ike in 2008, when the surge moved inland nearly 30 miles in some locations in southeastern Texas and southwestern Louisiana. All locations along the U.S. East and Gulf coasts may be vulnerable to storm surge.

There have been many exceptions to the projected storm surge of the Saffir-Simpson Hurricane Wind Scale noted below:

Category Wind Speed Projected Storm Surge

1

74 to 95 mph

(120 to 153 km/h)

Possible storm surge 4 to 5 ft (1.2 to 1.5 m) above normal

2

96 to 110 mph

(154 to 177 km/h)

Storm surge of 6 to 8 ft (1.8 to 2.4 m) above normal

3

111 to 130 mph

(179 to 209 km/h)

Possible storm surge 9 to 12 ft (2.7 to 3.6 m) above normal

4

131 to 155 mph

(211 to 249 km/h)

Storm surge 13 to 18 ft (4 to 5.5 m) above normal

5

> 155 mph

(249 km/h)

Storm surge greater than 18 ft (5.5 m) above normal

Some notable exceptions include:

  • Hurricane Katrina, a Category 3 hurricane at landfall in Louisiana, produced catastrophic damage with a 28 ft. storm surge.
  • Hurricane Ike, a Category 2 at landfall in Texas, also produced catastrophic damage with a 20 ft. storm surge.
  • Hurricane Charley, a Category 4 hurricane at landfall in Florida, produced a storm surge of 6 to 8 ft.
  • Hurricane Irene, a Category 1 hurricane at landfall in North Carolina, produced extensive damage with an 8 to 11 ft. storm surge.
  • And most recently, Superstorm Sandy, downgraded below hurricane strength at landfall in New Jersey, produced devastating damage with up to 14 ft. storm surge.

Why are there so many exceptions? Because there are many factors that that can impact surge.

Storm surge is caused primarily by the strong winds in a hurricane or tropical storm. The maximum potential storm surge for a particular region/location also depends on a number of different factors including storm intensity, forward speed, size (radius of maximum winds-RMW), angle of approach to the coast, central pressure (minimal contribution in comparison to the wind), and the shape and characteristics of coastal features such as bays and estuaries.

  • Storm Intensity – stronger winds will produce a higher surge
  • 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.
  • Size – a larger storm will produce higher surge
  • Angle of Approach to Coast – the angle at which a storm approaches a coastline can affect how much surge is generated. 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.
  • Central Pressure – lower pressure will produce a higher surge
  • 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. Areas along the Gulf Coast, especially Louisiana and Mississippi, are particularly vulnerable to storm surge because the ocean floor gradually deepens offshore. Conversely, areas that have a steeper shelf such as the east coast of Florida typically encounter a storm surge that is not as high.
  • 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.

Let’s examine the above factors for Superstorm Sandy:

  • Storm Intensity – Sandy was once a Category 2 1urricane with peak winds of 110 mph before weakening to post-tropical status at landfall.
  • Storm Forward Speed – Sandy was a quick moving storm at near 30 mph at landfall.
  • Size – Sandy was the largest Atlantic hurricane on record (as measured by diameter, with winds spanning 1,100 miles (1,800 km)).
  • Angle of Approach to Coast – Sandy took a virtually unprecedented westward (perpendicular) turn, essentially a worst-case trajectory for pushing water from several directions into the densely populated region.
  • Central Pressure – lower pressure will produce a higher surge.
  • Shape of the Coastline – the concave shaped bay/harbor area (Sandy Hook, Raritan, New York Bay) of lower New York City and central New Jersey saw the greatest storm surge during Sandy.
  • Width and Slope of the Ocean Bottom – wide, gently sloping continental shelves a steeper shelf.
  • Local Features – areas inland of barrier islands were lesser affected; beaches and communities protected by properly constructed/maintained dunes fared much better than others that were not.

The Total Water Level = Storm Surge + Tides + Waves + Freshwater Input

Tidal ranges can reach approximately 5 ft. in the New York area as opposed to the much lower tide ranges down south. This was also a factor in the local flooding throughout coastal New York and New Jersey.

Freshwater Input takes into account heavy rainfall ahead of a hurricane, which can cause river levels to rise well inland from the coast. When this water flows downriver and reaches the coast, local water levels will rise, especially near deltas and in bays. Luckily, there were no sustained heavy rains just prior to Sandy in the northeast region.

In addition to the destructive power of storm surge, battering waves may increase damage to properties directly along the coast. Water weighs approximately 1,700 pounds per cubic yard. Repeated and extended pounding by frequent waves can destroy any building or structure not specifically designed to withstand such forces. The two work together to increase the impact inland because the surge makes it possible for waves to extend on land.

If your facility is built along the U.S. East or Gulf coasts, then it may be vulnerable to storm surge. If you would like an analysis of the level of preparedness and protection at your site, you may contact Risk Logic Inc. for a professional consultation. Risk Logic can also review plans and drawings for new construction and roofing systems to ensure they are properly designed with regard to windstorm resistance. These are services offered to all locations under contract in Risk Logic’s property loss control program.