I had been warned about the swirling currents of Hell Gate, but it was the profusion of lights from ships, shore and navigational aids that overwhelmed me as I entered Lower New York Harbor after sunset in search of an overnight anchorage. Most of the harbor is wide open to the wind and swells coming in from the Atlantic, but Sandy Hook, a narrow point of land that extends north into the harbor, offers sufficient protection for the southeast corner of the bay. I turned my bow south and looked for the Sandy Hook lighthouse.
Set against the surrounding Jersey Shore, Sandy Hook is an oddity. It is a long and narrow barrier spit, nearly six miles long and one mile wide. I looked at the chart and wondered why the lighthouse was in the middle of the spit and not on the end.
As it turns out, when it was first built in 1764, the Sandy Hook lighthouse actually was on the northern end of the spit. However, as the sand around it shifted over time due to longshore drift, the lighthouse moved miles inland.
Longshore Drift is a current that travels parallel to then shoreline. At Sandy Hook, it carries sand from the inland end of then spit and deposits it on the shore, extending the spit long past where the lighthouse stands.
Only two things need to be present to create the energy that drives the current: waves and gravity. The wind generates waves, which approach the shore and are pushed up the beach at the angle of wind direction. When the waves retreat, gravity pulls the water straight down the beach slope. The waves approach and retreat at differing angles, creating a net flow sideways along the shore. Erosion from the waves traps sand in the current and moves it along with the water until the current reaches the end of the land and loses energy. Any sand particles that were suspended in the water are deposited there.
That is how Sandy Point came to be. After some time, the barrier spit was long enough to become a navigational concern for ships, so a lighthouse was built near the end of
it. Then as the years passed, longshore drift continued to carry sand along the shore and deposit it at the end of the spit, extending it 1.5 miles farther into the bay.
This Sea Science Article originally appeared in SAIL Magazine December 2012.