Typically, beaches and dunes undergo a seasonal transformation from a “summer” beach to a "winter" beach. A summer beach has a wide, well-developed berm often with a vegetated dune where American beach grass grows seaward onto the berm. A winter beach is lower, may not have a berm, and often shows signs of loss of beach grass.
Winter Beach: As storms and wave heights (along with a general change in wave and wind direction) increase during the fall and winter months, beach berms and sometimes the dunes erode in response, lowering the beach as sand is pulled offshore from the upper portions of the beach and deposited in protective offshore sandbars. The result is typically a flatter, more concave beach shape. The sandbars that form offshore in winter help protect the beach by causing waves to break farther offshore.
Summer Beach: In the late spring and early summer months, smaller, calmer waves dominate, and sand slowly returns to the beach and berm, and the beach and dunes typically recover, as long as sediment is not lost offshore. The key to this equilibrium is the berm, which is the part of the beach that changes most during the seasonal cycle.
(Above: Seasonal changes in the beach at Kinney Shores in Saco.)