.....Groins are man-made structures designed to trap sand as it is moved down the beach by the longshore drift. As the longshore drift current approaches the groin, it is forced to slow down and change direction. This chance in velocity causes sand suspended in the current to be deposited on the up-drift side of the groin. As the current then continues around the groin, it becomes turbulent and actually contributes to erosion on the down-drift side of the groin.
	(click on image for higher resolution)
	......Although groins trap sand, the erosion they cause makes them detrimental to a beach environment. Homeowners who build groins on their beachfront property in attempt to protect their beach can actually cause erosion of their downdrift neighbor's property. Some states have even outlawed the construction of groins due to the damage they cause to the beach. ..... Below are some example of groins at work on Folly Beach. This particular groin is the second to last one before you reach the Lighthouse Inlet, on the northern end of the island. Notice the levels of the sand on the up-drift and down-drift sides of the groin. In the first two the orange marker represents the same position in each photo. The third photo is looking seaward down the same groin. The longshore current is moving from left to right.
Downdrift side of groin (click on image for higher resolution)
Updrift side of groin (click on image for higher resolution)
(click on image for higher resolution)