The Letting of Georgica Pond, 2008
Since pre-colonial times, Georgica Pond in East Hampton, NY has been occasionally “let”, or in other words, temporarily opened to the sea. This coastal pond is normally separated from the ocean by a beach about 180 feet wide. Rainwater runs into the Pond over the land surface and filters in from the ground. A little of it will seep away under the beach to the ocean, but, because the Pond has no other outlet, the water level rises. The Pond is let by digging a narrow trench across the beach that allows pond water to flow out, and sea water to flow in. The water level in the Pond drops and the Pond becomes temporarily saline. Soon, however, ocean waves and the natural shift of sand isolate the Pond once again from the sea.
The letting of the Pond on Wednesday, 23 April, 2008 was unique. It was the first time this event was scientifically monitored in order to document how the new inlet opened. Measurements were made to measure how the inlet grew in size, how fast the pond water ran through the inlet, the amount of sand that was shifted by the letting and the changes in both water level and salinity in the Pond. Observations were made over the course of several days until the inlet closed naturally.
The water level in the Pond was about 4.8 feet higher than the ocean when the letting began. An incipient channel about 20 feet wide was dug across the beach with a backhoe by an experienced operator. Some 800 cubic yards of sand were displaced and the entire operation took about two hours. Pond water immediately rushed out through the new inlet at a speed as high as 7 knots (or about 8 mph, at the pace of a marathon runner). At such a rapid speed, the water was tumbling through a channel only three or four feet deep. Steep, permanent waves stood up on the water’s surface and a standing wall of water, called a hydraulic jump, formed at the mouth of the inlet where the discharge emptied out to the ocean. (This type of water flow is technically referred to as “supercritical”. It is water’s equivalent to “supersonic”.) The sand at the bottom of the channel was fluidized in some places, like quicksand.
During the first hour, the cut widened from 20 feet to 50 feet by the progressive, sudden collapse of steep sections of bank along its length. During this time the width of the inlet grew at a rate of about eight inches every minute. Its rate of growth then slowed to about one foot every three minutes. Five hours after it was opened the cut was 100 feet wide and water was still rushing through it at a speed of 6 knots. The cut eventually stabilized at a width of 130 feet before it stabilized.
Water ran out of the Pond for almost two days before seawater was able to get in. Forty-two hours after the inlet was opened, the salinity in the Pond jumped suddenly from 7 ppt to 19 ppt and small, irregular tidal oscillations in the Pond’s water level began to be seen. The Pond’s salinity reached 26 ppt about four days after the inlet was first created.
Over the course of five days, the water level in the pond fell a total of 1.6 feet. One hundred and forty million gallons of water drained into the ocean. The rush of water removed some 5,000 cubic yards of sand from the beach, enough to have filled over 400 dump trucks. This sand piled up in a large, submerged “ebb shoal” at the mouth of the inlet and sand was gradually swept to the west and back into the western beaches by waves.
By April 28, the inlet was blocked with sand and the salinity in the Pond had already started to go back down. The water level in the Pond started to creep up an inch or two by April 29 as rainwater and groundwater started to refill the Pond. On May 2, eight and a half days after the letting, salinity had dropped to 13 ppt and continued to fall slowly as more and more freshwater drained into the Pond. By mid-July, 2008 the water level in the Pond was back up to being 4.8 feet above sea level, as it was right before the letting.
Rarely do scientists and engineers have the opportunity to observe the birth of an inlet at such a large scale. Not only do these measurements help to better understand how the letting process occurs in Georgica Pond but they also will help with forecasting the incipient stage of other inlets such as those that might be opened naturally by storms at other locations along the coast.
This project was done with the cooperation of Diane McNally, Clerk, and the Trustees of the Town of East Hampton as well as with assistance of Larry Penny, Department of Natural Resources, Town of East Hampton. The work was accomplished by Nicholas Kraus and Sophie Munger of the U. S. Army Corps of Engineers; Mr. Rory MacNish from Cornell Cooperative Extension; Tim Miller, George Walbridge Surveyors PC; Frank Buonaiuto, Hunter College; Henry Bokuniewicz, Michael Slattery, Ruth Coffey, Zak Duval, Henry Feldman, Dan Hoffman and Charlie Flagg of Stony Brook University. Chris Schubert of the U. S. Geological Survey made the tidal measurement. Aerial photography was taken by Aram Terchunian of First Coastal, Inc. Funding was supplied by the U. S. Army Corps of Engineers Research and Development Center, Coastal and Hydraulics Laboratory.