Beaches

Beach features. The area of sand or gravel (more rarely silt) that covers the shoreline from the low‐water edge to a well‐defined upper elevation, such as a bluff or vegetated surface, is called a beach. The side of the beach facing the ocean is the beach face and is steepest because it experiences the greatest amount of erosion by wave action. Offshore beyond the beach face is the gently sloping marine terrace, a platform that may occasionally be seen at low tide. It is composed of sediment deposited by retreating waves or may be a bedrock surface that has been eroded by the waves. The landward edge of the beach is marked by the berm, the edge of a platform of sediment deposited by higher waves during severe storms. Berms can be quite coarse grained and contain abundant shell debris.

Beach deposition. The movement of sediment parallel to the shore by wave action is called longshore drift. A wave that washes across a beach face at an angle carries sand at that angle until it has lost all its energy; at that point, the water returns to the sea by running straight down the face of the beach into the surf zone. This process constantly moves sand across the beach face. The sand is carried the same way by the next wave, and moves across the face in a series of arcs. Called beach drift, this zig‐zag pattern can transport sand and pebbles hundreds of meters a day along the beach.

The majority of the sediment in the beach environment is carried by the longshore current in the surf zone. The friction and erosion between the breaking waves and the bottom loosens and suspends sediment particles, which are then transported long distances in the current. The sediment is eventually deposited as fingerlike features called spits or baymouth bars, which can block an open bay from the ocean. A tombolo, a bar of sediment that connects an island to the mainland, forming a small peninsula, can also be formed.

Beach composition. Most beaches are composed of quartz sand, the majority of which is river sediment deposited in the ocean and reworked by ocean currents. Because of their high densities, black streaks and layers of heavy metallic minerals (magnetite, ilmenite) are also concentrated on beaches. Beach materials can also be limestone or basalt grains.

Seasonal changes. Wave action in the summer months tends to bring up sand from deeper water and builds wider beaches. Winter wave action generated by stormier weather erodes the sand and reduces the width of the beach. The sediment is carried out to sea and deposited as an underwater sandbar, which is then eroded by the next summer's waves to rebuild the beach.

Beach engineering. Engineering efforts designed to protect beaches and harbors interfere with sand drift and the natural development of beaches and coastlines. Breakwaters, built parallel to the shoreline to provide quiet water for pleasure boating, can result in extreme sedimentation that actually closes off the area. Jetties are walls that are built on both sides of a harbor and extend into the ocean to protect the harbor from excessive sedimentation and destructive waves. In most cases one jetty will trap the sand, resulting in “starving” the beach behind the other jetty, which begins to recede from erosion. Groins are series of walls built perpendicular to the coast to widen beaches that are losing sand to longshore drift. The natural beach environment represents an equilibrium between sand, wind, and waves. Human attempts to modify beach dynamics result in sedimentation patterns that generally frustrate the designers.