Flat coast

A flat coast is a shoreline where the land descends gradually into the sea. These coastal environments are typically composed of loose materials, such as sand and gravel.

They can form in two primary ways. One method involves the sea advancing into terrain that already has a gentle slope. Another method occurs through the abrasion of loose rock by moving water. These coasts are dynamic systems where wind and waves constantly redistribute material. The sea often washes pebbles and sand away from one area and deposits them in another.

The sequence of landforms found on these coasts is known as a littoral series. This series begins in the shoreface, which is a permanently flooded shallow water region.

Within this shoreface, waves transport and deposit material on the seabed to create a bar. A longshore bar is an elongated ridge of sand or gravel that runs parallel to the shore. The sides of these bars usually fall away gently. Between a sandbar and the shore, a basin forms called a runnel or a swale.

In some tidal flats, drainage troughs also run parallel to the coast.

Defining the specific boundaries of a flat coast can be difficult because different authors use different terms. The shoreface is often described as an underwater platform subject to constant water movement. Its landward boundary is generally considered the average low-water mark. Some researchers define the beach as the area extending from the low-water mark to the highest high-water mark. This zone is only periodically flooded by waves or storm surges. Other experts refer to the area between mean low and high tides as the intertidal zone or foreshore. They call the area above the high-water mark the supratidal zone or backshore.

On the beach platform, you will often find a specific ridge called a berm. A berm is a bank of sand or gravel that runs parallel to the shoreline. It is usually only a few tens of centimeters high. Berms are formed when breaking waves throw material beyond the average sea level. The coarse-grained material is too heavy to be carried back by the backwash.

Because of this, the material remains behind to build the ridge. The size and location of a berm change with the seasons. For instance, winter berms created by storm surges are often higher and more prominent than summer berms.

Storms and normal wave action have opposite effects on the shape of a flat coast. During a storm surge, heavy erosion occurs and the beach profile becomes steeper.

However, normal wave action tends to build the beach up over time. This can create a series of parallel berms located one behind the other. This process causes a gradual increase in height and can move the shoreline seaward. A notable example of this is Skagen Odde in Denmark. Located on the northern tip of Vendsyssel, this headland is still growing today as new berms are added.

The landward edge of a flat coast is often defined by a belt of dunes. These dunes mark the farthest point inland that a storm surge can reach.

Wind plays a major role here by transporting fine grains of sand inland over the dunes. To prevent wind erosion, certain plants like sea buckthorn or beach grass colonize the dunes. These plants help stabilize the sand. In areas where humans want to stop erosion, they may use coastal defenses. These include groynes, stone walls, or concrete tetrapods that act as breakwaters.

Understanding flat coasts involves studying the complex relationship between hydrology and geomorphology. The movement of water shapes the seabed through the creation of bars and runnels. Simultaneously, the wind shapes the land through the creation of dunes and the movement of fine sand. These processes show how energy from the ocean and the atmosphere constantly reshapes the Earth's surface. By observing the littoral series, scientists can track how material moves through the coastal system.