Why was the flooding from Hurricane Sandy so bad?

Flooding from Hurricane Sandy caused billions of dollars of damage to property.  Low-lying coastal areas were devastated.  Tunnels and part of the NYC subway system flooded.  Barrier islands got hammered, airport runways flooded, and the list goes on and on.

Here are some photos of the flooding along the eastern seaboard:

New Jersey

NYC subway

Atlantic City, NJ

Delaware

Hoboken, NJ

Brooklyn, NY

New York

Rodanthe, North Carolina

Staten Island, NY

New York, flooded airport runway

Ground Zero Site, NYC

Maryland

Subway, Hoboken, NJ

New Jersey

Why was the flooding so bad?

Six factors combined to make flooding pretty much as bad as it could be.  They are:

1. Sea level rise
2. Full moon and high tide
3. Hurricane low pressure
4. Hurricane-force winds and associated storm surge 
5. Low-lying coastal areas
6. Shallow sloping shoreline

First: Sea Level Rise

A report in National Geographic summarizes observations about sea level rise along the east coast of the United States.  It states that sea level rise is occurring nearly twice as fast along the east coast as the global average.  You can read more about that by clicking this link:

http://news.nationalgeographic.com/news/2012/06/120625-sea-level-rise-east-coast-us-science-nature-climate-change/

Sea level has been rising between Cape Hatteras, NC, and Boston, MA at the rate of 2.0-3.8 mm/year between 1950 and 2009.  If we go with a middle value of 3 mm/year, then sea level has risen about 7 inches since 1950.  That may not sound like a lot, but it becomes significant when you start looking at flood conditions.  All indications are that the rate of sea level rise is increasing as global warming progresses.

Second: High Tide

People directly affected by weather and flooding from Hurricane Sandy wouldn't have seen this, but there was a full moon on 10/29/2012.  The height of ocean tides are affected by the relative positions of the Earth, Sun, and Moon.  High tides are highest and low tides are lowest when the Earth, Sun, and Moon all line up in the same plane.  This happens when we have a full moon and a new moon.  Unfortunately, it was a full moon on 10/29, the same night Hurricane Sandy came ashore.  This means that the tides that night were already higher than normal.

Third: Low Air Pressure

A hurricane is a low pressure system.  This means that in the eye of the storm in particular and the whole storm in general has lower air pressure than high pressure systems have.  In order to understand this part of the equation you need to imagine the entire height of the atmosphere above your head.  It extends upward 100s of miles, but most of the mass of the atmosphere is in the few miles directly overhead.

The weight of the atmosphere directly overhead produces the air pressure we experience.  Interestingly, high pressure pushes down on water, causing tides to be lower than they would otherwise be.  And, vice versa, low air pressure allows tides to be higher than they would otherwise be.  How much of a difference?  A change in 1mb (millibar) of air pressure relates to up to 1cm of tidal height when high pressure pushes down on the water surface.  When air pressure is low, however, it may allow tides to be a bit higher, but it does not by itself drive tides significantly higher than predicted.

Average sea level air pressure is about 1013mb.  The air pressure in the middle of Hurricane Sandy was 946mb when it came ashore.  This ties the lowest air pressure for a hurricane making landfall this far north.  That last one was in 1938!

This means that air pressure did not mitigate tidal heights.

Fourth: Storm Surge

Storm surge is the biggest factor in coastal flooding associated with hurricanes.  The height and effect of storm surge is determined by several factors: storm intensity, tidal height, angle of waves to shorelines, presence of bays and inlets, slope of the shoreline, etc.

Here's what happens.  As a hurricane approaches shore the effects of tides are felt first.  So the first significant effects are felt as tides rise, often well above normal because of the amount of water being pushed by the storm.  Then, waves produced by the storm start coming ashore.  These tend to increase in size as time goes on.  This is because wave size is determined mainly by two factors: the strength of wind and fetch (the distance wind blows across water).

Waves produced by hurricanes can be huge because both wind velocity and fetch are massive.  Hurricane Sandy, for example, was over 1000 miles across.  And though windspeed didn't get high enough to reach more than category 1 status, the wind it produced blew over vast expanses of ocean.

So once the tide was in and Sandy came ashore, wave after wave piled up on the shore with no way for the water to get back offshore, so it was pushed farther and farther inland.  This is the water that flooded subways, tunnels, airports, etc., etc.

There are some good animations that demonstrate the combined effects of tide and storm surge.  You can view the by clicking these links:

This link shows the action of storm surge along shores with a shallow slope:
http://www.nhc.noaa.gov/surge/animations/surgea.swf
This link shows the action of storm surge along shores with a steep slope:
http://www.nhc.noaa.gov/surge/animations/surgeb.swf

Fifth: Low-Lying Areas

The coastal flooding was particularly bad because the NJ, NYC area is low-lying.  This means that there was not much there to slow or stop the high storm tide (regular tide + storm surge) that Hurricane Sandy produced.

Sixth: Shallow sloping seafloor and narrow passages between landmasses

This image of the greater NYC area shows that this highly populated area is clustered on islands and land masses separated from each other by narrow waterways.  This means that when the storm tide (tide  + surge) pushed into these areas, water stacked up and spilled more readily onto land.  This had to contribute significantly to the flooding as well.

Wrapping up

So when you combine sea level rise, high tide, low air pressure, storm surge, and local geography with a storm the size of Sandy, that's a recipe for disaster!