make summary for each lucture

Storm Surge

Hurricanes affect the elevation of the sea in two important ways:

1. Low pressure in the core of the storm pulls water up. About 1 cm of sea level rise for each millibar of surface pressure fall; the most intense hurricanes on record have pressure drops of around a hundred millibars, pulling the sea surface by about a meter [low pressure is often not the main cause of surge].

2. The stress that the wind exerts on the water can cause a sudden rise of the sea level near the time of maximum wind [this is the main driver of storm surge].

Storm Surge: One of the most deadly phenomena accompanying hurricanes is the storm surge – a sudden rise of sea level.

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Storm Surge

Wind blowing across the surface of a partially filled box piles up water on its downwind side.

[Ref: Divine Wind, 2005]

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Storm Surge

A simple formula for change in elevation of the water at the downwind side of the tank:

h = 0.00035*V2L/H

h is the elevation change in feet, V is the wind speed in mph, L is the length of box in miles, H is the average water depth in feet

Water rise is proportional to the square of wind speed; water rises with the fetch over which wind is blowing; the smaller the average water depth, the greater is the rise.

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Storm Surge

In real situation, water trying to pile up against a straight beach can flow off to the sides, and this reduces the surge.

Surges can be large in narrow bays from which water has nowhere to escape.

Storm surge can increase significantly due to resonance. [In the box example switch off the wind, wait for the water to slosh back to the upwind and turn on the wind as soon as water starts to move back to the downwind side].

Such resonance effect, hard to achieve in the ocean, can significantly increase surge when a hurricane moves over a shallow lake [e.g. Florida’s Lake Okeechobee].

A resonant sloshing of lakes or bays is called a seiche.

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Storm Surge

Water levels can rise and fall dramatically several times within few hours owing to resonance between the storm surge and oscillations that can occur in shallow water.

A second rise in water level can be larger than the first one and is sometimes referred to as a resurgence.

Hurricane Carol (1954) inflicted significant damage to Cape Cod, Massachusetts, as a result of a resurgence.

Sea level often rises few hours or sometimes days before the storm surge itself arrives. These sea level increases are called forerunners.

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Storm Surge

[Ref: Leatherman and Williams, 2008]

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Storm Surge

[Ref: Leatherman and Williams, 2008]

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Storm Surge For storms approaching the coast head on, the surge will be larger if:

1. storm has lower central pressure

2. onshore component of wind speed is larger

3. eye is larger (however there is not much effect if the eye > 30 miles)

4. storm approaching the coast faster

5. seafloor shoals gradually

6. coastline is part of a bay that is as narrow as or narrower than storm’s eye

Astronomical tide can significantly affect surge -- a strike at high tide could be devastating (such as a hurricane striking eastern Maine at high tide).

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Storm Surge

[Ref: Leatherman and Williams, 2008]

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Storm Surge

[Ref: Leatherman and Williams, 2008]

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Storm Surge

[Ref: Leatherman and Williams, 2008]

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