As Hurricane Erin churns offshore, coastal communities across Long Island are feeling its effects — with rough surf, high tides and growing concerns over flooding and beach erosion.

Powerful waves pounded the shoreline at Cooper’s Beach in Southampton on Wednesday, drawing crowds of onlookers. Flags and posted signs made it clear that swimming is strictly prohibited.

“Little rough for swimming or going in the water right now, I’d say,” resident John Allan said, as waves crashed behind him.

In response to the potentially life-threatening rip currents and risk of coastal damage, the Town of Southampton has declared a state of emergency. Officials are particularly concerned about the storm’s potential to erode protective dunes and cause localized flooding.

“The waves rushed up close to some of the houses near the beach,” said Richard Adler, of Southampton. “And one could imagine the waves rushing over some of the dunes.”

Those fears aren’t unfounded. On Dune Road in Hampton Bays, floodwaters have already started to pool. At Sundays on the Bay, a popular waterfront restaurant, staff say they’re seeing a noticeable drop in business.

“I definitely see an effect happening here,” said employee Sarah Montalvo. “The restaurant has slowed down a little bit, staff haven’t been getting as much money.” She added with a smile, “Erin, you better be quick and get out.”

As the storm continues its path past Long Island, local officials say they’ll be closely monitoring the situation, especially the impact on beaches along the South Shore. In some areas, crews have already begun piling sand to reinforce dunes and protect vulnerable properties.