During what season, and under what ocean conditions do deposition of beach sands usually occur?

Answer 1

Beaches are dynamic landforms altered by wind and waves in a continual process of creation and erosion. Seasonal cycles of sand deposition and loss dramatically affect the appearance of beaches from summer to winter. Wide and gently sloping in summer, they become steep-fronted and narrow in winter, and cam vanish overnight, stripped of sand by violent storm waves. Most of the sand removed from winter beaches is deposited in offshore sandbars and is returned to the beach during the mild summer months by gentle swells that push the sand to the exposed shore. River sediments are the source of 80 to 90 per cent of beach sand; some beaches are built to great widths by sediments washed to the sea by episodic floods, gradually eroding until the next major flood replenishes the sand.

Beach formation begins as eroded continental material--sand, gravel, and cobble fragments--is washed to sea by streams and rivers. Two separate processes result in the deposit of this sand and sediment on the shore. Most sediment is suspended in sea water and transported along the coast by the longshore current, a stream of water flowing parallel to the beach that is created by the action of waves breaking at an angle to shore. Longshore transport can deliver up to a million cubic yards of sediment annually to a single beach. In the second process, sand deposited onshore by the longshore current is then oscillated by waves breaking onto and receding from the beach. This continual onshore-offshore movement gradually pushes the sand along the beach edge. both the longshore transport of sediment along the coast and the movement of sand by waves along the foreshore are a part of the process called littoral drift.

The California coastline has been divided into geographic segments called littoral cells, that incorporate a complete cycle of beach sediment supply, sand transport by the longshore current, and eventual permanent loss of sand from the littoral cell. The five types of littoral cells along the California coast are each characterized by a different littoral process determined by the geographic features unique to the cell type. One type of cell is defined by a long stretch of coastline that begins at a headland and terminates in a submarine canyon, such as at Mugu Canyon in Venture County and La Jolla Canyon in San Diego County; another cell type consists of a large river delta bounded on either side by rocky headlands, such as at Humboldt Bay; a third type of littoral cell is defined by a crescent-shaped by downcoast of a promontory, like Half Moon Bay in San Mateo County; and a fourth type of cell consists of a rocky headland downcoast of a beach where waves break in a line parallel to the shore, as at Ten Mile Beach in Mendocino County. Finally, lagoons and closed bays with restricted tidal flow create a fifth type of littoral cell, such as Bolinas Bay in Marin.

Apart from littoral cell type, there are characteristic differences between Northern and Southern California beaches, depending upon the directions of prevailing wind and upon local coastal geology. Along California's north coast, cove or pocket beaches are common where the granitic and basaltic rock that composes the sea cliffs has been sculpted by prevailing northwesterly winds and battered by high energy waves over millions of years. In Southern California, beaches often consist of long ribbons of sand interrupted by widely separated rocky points. The bluffs of easily eroded shales and sandstones that edge the coast here continuously crumble away, creating on even coastline over time.

Answer 2

Deposition of sand occurs when waters wash the sand from the mountains. The spring time when snows melt and rains fall is typically the season when most erosion occurs. The sand is deposited in high energy environments along the shore. Finer grained sediments tend to settle in the low energy environments in the deeper marine environments.

Answer 3

Loren is absolutely right (and got a thumbs up). Winter storms wash sediment off shore, summer re-establishes the sand on the beach.