9.12.2: Waves and Beaches

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Waves

Waves have oscillating current directions every few seconds. The flow in both directions is equal in deep water, but not necessarily near shore.

More information on waves and their interactions with shorelines

Wave Ripples

Wave ripples are different from current ripples because that they experience transport in both directions over the time scale of seconds.

Wave ripples can be recognized in rocks by their symmetric shape (if flow in each direction is the same speed) and most importantly, the presence of cross-laminations dipping in two directions. This is the truly distinctive feature and can be present even if the ripples are not very symmetric. At low flow, the boundary layer doesn’t have enough speed or momentum to remove the crest of the ripple and the grains that are moved are deposited right on the upper part of the lee slope. Thus, crests are sharp. At higher flow, the crests erode and deposition occurs farther down the lee slope. Thus, high flow ripples have rounded crests.

In shallow water, currents from the waves can be strong enough to flatten out the ripples, but they are not consistent enough in one direction to form dunes; the flow switches directions too frequently for dunes to build up. Thus, where flow speeds are too fast to form ripples, the sedimentary surface tends to be planar or broadly scalloped as the waves are focused into certain areas. This produces a flat lamination (not upper planar lamination) where waves are in very shallow water relative to their height, e.g. from the breaker zone towards the shore.

KEY POINT FOR WAVES: Bi-directional flow every few seconds

Beaches

Waves create beaches with the constant washing back and forth of sediment.