Chesapeake Bay Beach Structure

New Point Comfort Lighthouse

Chesapeake Bay Beach Structure New Point Comfort Virginia

By John Quensen

A fundamental property of beaches is that they constantly change due to the action of wind and water. Waves create beaches by washing up sand from offshore deposits. Larger, high-energy waves that occur in winter and during storms can erode the beach by carrying the sand back offshore. The sand on the beach also can be transported inland by the wind and shaped into dunes. The different zones of the beach are defined by the action of wind, waves, and tides.

Chesapeake Bay Beach Structure Diagram

The FORESHORE or intertidal-terrace is the portion of the beach between the low-tide and high-tide marks; it slopes toward the water.

The BACKSHORE or berm portion of the beach extends landward from the high-tide mark; is essentially level or may slope slightly away from the water.

LONGSHORE BARS are off-shore ridges of sand (sand bars) that roughly parallel the beach.

DUNES are formed when wind carries sand further inland from the berm.

Chesapeake Bay Sand Texture While waves move across the surface of the water, the water within the wave moves up and down in a near circular orbit. These orbits decrease in size with increasing depth, but they can resuspend off-shore sand into the water column as long as the water is not too deep. As the wave reaches the beach it falls forward or breaks, carrying the suspended sand with it up the foreshore. In the case of smaller, low energy waves generally typical of summer, most of the water sinks into the beach as the wave retreats, leaving sand behind on the foreshore. In this way the beach is built up. During falling tides, you can often see small ridges of sand left at the upper edge of the waves travel up the foreshore (lap marks).

New Point Comfort Chesapeake Bay Erosion Large high-energy waves more typical of winter and storms have a completely different effect. As they break and crash onto the beach, the turbulence resuspends the beach sand. Because the volume of water in the wave is so large, most of it runs back out over the beach (the back wash) before it has a chance to sink into the beach. In doing so, large waves carry away more sand than they bring ashore, and the beach erodes as a result. Erosion from a storm often results in a sharp drop off or step between the berm and the foreshore.