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What Causes Baymouth Bars?

A gleaming white beach near a bustling harbor may seem like a static coastal landscape of recreation and industry, but the angled approach of breaking waves off the ocean belies this fiction. Beaches are living landforms, built and destroyed ceaselessly by the movement of water. The fingerlike projections of sand and other sediments called spits or bars may be thought of as offshore extensions of a beach. One of the most significant formations is the baymouth bar, a landform humans often strive to prevent.

Longshore Currents
- Longshore current conveys sediments along coastlines.
  
  Longshore currents are waves sloughing into a shore at oblique angles. Such movements of water tend to transport sediments along beaches in a process known as littoral drift. The sediments themselves, like sand, are mostly derived from rivers dropping particle loads at their mouths in the sea; the sand grains are then washed ashore by the ocean waves, building beaches.
Formation of Spits
- When sediment-laden longshore currents encounter a shift in the contour of the shoreline, they may release some of their load and deposit it. In the case of a bay, which may mark the estuary of a river or another natural indentation of coastline, the currents lose some of their focused energy and dissipate as the shore falls back. The deposition of the littoral-drift materials creates a spit, a narrow extension of a coastal beach advancing across the mouth of the bay. Often the heads of spits are curled because of the action of currents roiled around them.
Baymouth Bar
- A spit becomes a baymouth bar if it advances entirely across the bay and contacts the other side of the mouth. This cuts the bay off from the ocean by an exposed belt of accumulated sand, mud and silt. Behind the bar, the bay, no longer exposed to active ocean waves, becomes a lagoon and may begin to accumulate sediment, transforming into a shallow salt marsh. If contoured like a crescent moon, as often happens, a baymouth bar may be called a lunate bar.
Breaching and Preventing
- A bar may be naturally breached, or prevented by the installation of jetties.
  
  A baymouth bar may be breached in a number of ways. In the case of a cut-off estuary lagoon, the incoming river, strengthened because of heavy rains or some other high-level input to its channel, might cut through the bar and resume the link to the sea. Particularly powerful storm waves or tidal currents could perform the same action. Humans construct long jetties at the mouths of bays to prevent the formation of spits and baymouth bars, which can render a harbor unusable because of the resulting sediment build-up. These jetties steer the waves seaward, performing a similar function as the groin structures built to reduce wave erosion on swimming beaches.

Nature