Storms

I A. Expected rather than unusual event

1.Cause most erosion.

2. Two types: Extra-tropical and Tropical

II A. NC protrudes into Atlantic Ocean -receives more storms than neighbors, one every three years

1. Statistics misleading

2. Worst one along NC -- 1761

3. Hurricane Fran most recent catastrophic event

4. Season lasts June 1- Nov 30

5. Max activity- late August until early October

6. Origins: Caribbean, Atlantic Ocean south of FL,

Gulf of Mexico, and off Cape Verde Islands

7. Speed of storm generally 12-15 mph

8. Damage occurs in several ways, winds > 200 mph,

storm surge, and flooding due to rain and surge

III A. Hurricane Classification based on wind speed, and barometric pressure

1. Saffir and Simpson devised scale of five categories, # 1 mimimal (74-95 mph) to # 5 catastrophic (>155 mph).

2. Andrew and others had wind speeds > 200 mph

IV A. Extra-tropical (ETs), winter storms equally and maybe more important over the long term.

1. Form in mid-latitudes, move across the coast into the ocean and travel north, generally go unnoticed

2. Significant due to duration and size

3. Winds blow from northeast (gales) & impact beach

4. Can produce very large steep waves and surge

5. Along Outer Banks 850 storms/25 years

IV B. Rating Winter Storms

1. Perception differs based upon experience and damage in area.

2. Damage not always related solely to atmospheric factors

3. Dolan and Davis devised scheme similar to Saffir-Simpson Scale

4. Based on relative wave power index, H²D = height x duration (hr) for waves greater than 5 ft.

5. Devised 5 classes: # 1, Weak, 6ft waves, lasts 8 hrs, 50 % of storms; # 5, Extreme, 23ft waves, lasts 4 days, 0.6 % of storms

a. Return frequency ranges from 3 days for # 1 to 67 years for #5, Class 2 once a month

V A. Storm Surge

1. Most dangerous aspect of storm

2. Definition - mound or ridge of water rising above normal sea level, produced by the push of the winds associated with the storm

3. Why worry? Elevated water levels allows storm waves to break well inland of normal surf zone

4. More than 3/4 of deaths and most of damage due to surge/flooding

5. Surge levels: Camille (1969) 20-30', Hugo (1989) 22', Hazel (1954) 17', Fran (1996) 12', Bertha (1996) 5', Dennis (1999) 2'

VII A. What causes surge to develop?

1. Develops in deep water and hardly noticed

2. Low pressure causes a small percentage of rise

3. Simultaneous with pressure drop, winds are swirling water counterclockwise, may extend hundreds of feet below surface

4. As storm approaches shallow water and coast water piles up as it begins to touch bottom

5. Other factors can make storm surge worse such as embayed coastline or very shallow shelf