Why did Sandy leave so much sand in Seaside Heights and even NYC?

Sand in Seaside Heights, New Jersey on 10/31/2012
Photo by Mario Tama/Getty Images published here

Coastal storms move sand in several directions: on-shore, off-shore, and sometimes, along-shore. I thought it would be worth reviewing some dynamics of sand movement during big storms in light of the inevitable decisions about moving around sand post-Hurricane Sandy. This topic is worthy of an in-depth discussion that would occupy a significant fraction of a coastal geomorphology course, so this is just a shallow overview. (Plus, I live in central Illinois, not exactly full of beaches where I could get first-hand experience!)
    While this is a particular and immediate case study, sand movement is an issue at many coastal cities. For example, project SANDAG is a $28 million regional sand replenshment project in the San Diego area; the Army Corps of Engineers proposes to spend $251 million on sand projects for Encinitas, Solana Beach, and San Clemente.

Schematic of sand movement

    Beaches are both formed and destroyed by wave action in the normal (note that word "normal") course of geologic processes. Waves move sand particles either in suspension or by saltation--bouncing along the bottom. These particles come from the erosion of rocks offshore, or from the erosion of sediments along the coast.  Sometimes the sediment is transported long distances--the white sand along Florida's "Emerald Coast" comes from the erosion of quartz in the Appalachian Mountains. Some waves, referred to as "constructive" waves move sand up onto a beach; "destructive waves" move it from the beach, forming a  slopes that disappear some distance below the surf. Wind often moves the sand on beaches into dunes.  Offshore, a common coastal landform is a barrier island, a long linear sandy island or series of islands that can extend over 100 km along a shoreline. Long Island is an island formed on glacial moraines, but has elements of barrier island geology where barrier islands of sand divide the lagoons south of it from the Atlantic.  The true barrier islands are Coney Island, Long Beach Barrier Island, Jones Beach Island, Fire Island, and Westhampton Island. In New Jersey, the famous beach spots of Seaside Heights, Atlantic City and Wildwood are on barrier islands. During the storm, water covered many of the barrier islands. As bulldozers and bucket loaders tried to clear the streets from sand brought inland, they were piling up like snow after a winter snowstorm, in some places there were piles up to 20' high.
      Beaches change all the time in response to the seasons and to individual storms. Change is the norm. During storms, waves attack parts of the beach system, moving sand from high-energy sites to lower energy environments. This process can take sand toward the shore, by the process known as "overwash" that appeared in the reports about Hurricane Sandy, or can take sand away from the shore, transporting it offshore and out, sometimes into sand bars. This process tends to cause storm waves to break further offshore and decreases the wave action that actually reaches the beach. As sea level rises in the future, beachs will migrate landward, somewhat like a tractor tread rolling over itself.  The transport of sand into New York City or Seaside Heights (photo above) is an example of the transport of sand inward, and a portent of things to come if the number or intensity of hurricanes increase due to global warming.
    Prior to human intervention, cycles of storms would work on beaches, repeating this process over and over on many scales, resulting in the beaches that we had in the early part of the 20th century.  As we humans have constructed infrastructure, often far from the beaches, the supply of sand to the coasts has been greatly altered, often reduced. Thus, the normal beach building processes have, for many decades, been altered by humans. Beaches are now, at great expense, often only replenished as sand is carted from one place to another in an ongoing intervention with where "Mother Nature" wants to put it. Fittingly, the first beach replenishment project in the U.S. was at Coney Island in 1922-1923.

Here's a link to a before and after image of the Mantoloking Bridge and damage near it where the hurricane cut a new connection across the barrier island connecting the Atlantic Ocean and the Jones Tide Pond. This bridge, built in 2005, cost nearly $24 million, with an additional $5 million for design and purchase of rights-of-way. It has been labelled as "unstable" now.