This is Surf Beach off the coast of Lompoc, California. It is closed off to beachgoers from March 1st to September 30th every year in conjuction with the breeding season of the snowy plover.
The snowy plover is a coastal western bird that once numbered in the thousands and now it is estimated only 1500 or so remain. This is one of the few places that they still have left to breed due to habitat loss.
The use of only warning signs to stay off the dunes to save the birds was not effective and later on led to this fence and locked gate.