5.2: Anatomy of a Beach

Though beaches vary widely in their sediments, they generally exhibit a similar appearance—a common structure—with recognizable features. Identification of these different parts helps us appreciate the nature of beaches and better understand the processes that shape them. (Our discussion here generally follows Pilkey et al. 2004.)

When you enter from a parking lot or a cliff, you first encounter the back of the beach, a typically flat portion known as the backshore. At some locations (such as Oceano Dunes near Pismo Beach, California), you may walk over sand dunes prior to entering the backshore. The backshore is generally the most stable portion of the beach because it is only occasionally inundated by waves.

The backshore may also contain accumulations of sand in the form of berms, which represent shelves of sand left by waves in a receding tide. Multiple berms may be present on a backshore, especially in the days following the highest tides (e.g., spring tides). During periods of high surf or seasonally extreme tides—popularly known as king tides—the backshore may be completely submerged on some beaches. Much of the debris found on the upper portion of the backshore is deposited during periods of high surf or unusually high tides. Large swaths of sand may be removed from the backshore during these times as well. Otherwise, if the sand on the backshore is untouched by waves or tides, winds may create accumulations of sand as beach dunes.

The lower portion of the beach is called the foreshore. The part that slopes toward the sea—alternately submerged and exposed during a tidal cycle and subject to daily wave action—is called the beach face. The backshore berm, or clifflike beach scarp, if present, marks the upper boundary of the foreshore, while the lower boundary of the foreshore ends at the location of sea level during low tide. Note that the width of the foreshore changes dynamically as a result of the action of waves and tides. The foreshore may grow during periods of high-energy waves and spring tides (when tidal range is greatest). Waves and high water levels push the berm toward the backshore. During periods of low-energy waves and neap tides (when tidal range is lowest), the foreshore shrinks as the berm migrates seaward.

A common feature on the upper boundary of the foreshore is a line of seaweed and debris known as the wrack line (pronounced “rack”). Kelp wrack is common on Atlantic and Pacific coast beaches where kelp grows nearby. Ocean currents drag sargassum weed from the Sargasso Sea and deposit it (often in great quantities) on Florida beaches. In truth, any kind of dislodged seaweed and marine debris may be deposited on the beach face as the tide recedes. The location of the wrack line on any given day indicates the height of the highest tide at the beach. At times when the height of the highest tide is lower each succeeding day, you may observe several wrack lines, each corresponding to the height of the highest tide of the previous days.

Beach cusps, a series of crescent- or scallop-shaped deposits of sand parallel to the beach, are another common feature on the upper beach face. Scientists remain uncertain as to the mechanisms that form beach cusps. A type of shore-parallel wave called an edge wave may play some role. However they form, beach cusps are fascinating to observe and a prominent feature on a beach face.

On some beaches the lower portion of the beach face may be flat and exposed during low tide. This is the low-tide terrace. I like to think of it as the ocean’s patio, albeit one with holes, bumps, and streams. The low-tide terrace represents the shallowest portion of the seafloor. If you’re standing on the low-tide terrace, you can truly say that you are walking on the seafloor.

Sandbars often mark the seaward side of a low-tide terrace. Sandbars represent a “reservoir” of sand that forms when high-energy waves move sand offshore. Under low-energy conditions, the sandbar may be pushed toward the beach and build up the foreshore.

As the waves and tide recede and expose the low-tide terrace, water that has penetrated the sand will flow back into the ocean. Because the foreshore is sloped, the water may flow out of the sand as little streams. As these streams of water flow, they move sand, and, much like a river, they erode the sand and leave tell-tale signs of their flow. The tree-like, or dendritic, patterns left by water seeping out of the sand at low tide are called rills. Look for them the next time you’re at the beach at low tide.

Often the low-tide terrace is separated from the foreshore by a runnel, a strip of shallow water parallel to the beach. A runnel most often appears in the summer, when small waves push sand toward the beach. Small fish, sea stars, conch shells, and other kinds of marine life may get trapped in these temporary pools. Look, but don’t take. A living creature plays a valuable role in the ocean ecosystem, something it can’t do sitting on a shelf collecting dust in your home.

Directly offshore of the low-tide terrace, you will encounter the area that many people travel to the beach to experience—the surf zone. It extends from the point where the waves begin to break to the edge of the beach. A current may be present here which flows parallel to the beach between the surf zone and the foreshore (see below).

The region below the low-tide terrace and beyond is called the offshore. The term also refers to a direction—away from the shore. Winds blowing from the land toward the ocean—such as occurs during Santa Ana wind conditions in Southern California—are classified as offshore winds.

Finally, the region of the ocean from the low-tide terrace to the edge of the continental shelf is called the coastal ocean (or neritic zone). The coastal ocean includes all the waters above the continental shelf (discussed in Chapter 7). Beyond that, we’re in the open ocean, the deep blue sea, largely removed from any terrestrial influences.