Sunday, September 12, 2010

Wave ripples on an eroding beach

I shot these photos in 2003, at Sea Rim State Park in east Texas, close to the border with Louisiana, a relatively remote and beautiful state park along the Gulf coast that suffered a lot of damage during both Hurricane Rita in 2005 and Hurricane Ike in 2008. On that chilly November day the light was great and the variety of shapes and patterns created by wave ripples and exposed during low tide was amazing.

Wave ripples are more symmetric than current ripples. Needless to say, wave ripples originate thanks to the back-and-forth movement of sand by waves, whereas current ripples form under unidirectional flows (like rivers and turbidity currents). Wave ripples are also more regular than current ripples, extend for much longer distances laterally, and often terminate - or continue - in 'Y'-shaped junctions. For the same wavelength, they are also taller; the L/H ratio of most wave ripples is between 4 and 10, in contrast with current ripples that have an L/H value of ~20.

Perfectly symmetrical ripples form under bidirectional currents that are perfectly symmetrical themselves; but this tends to be the exception rather than the rule, as shoaling waves create a net shore-directed movement of the water. The resulting ripples are asymmetric, with the steeper side facing the coast, but still more symmetric and more regular than pure unidirectional ripples. Weak tidal currents can cause the asymmetry as well. The photo below shows wave ripples with a significant asymmetry that makes them difficult (if not impossible) to distinguish from current ripples.

This set is also asymmetric, but to a lesser degree:

Sea Rim State Park is at a location along the Gulf coast where the beach is eroding and the coastline is retreating; it is a typical example of a transgressive coast. The erosion is the results from both sea-level rise and from lack of longshore sediment transport strong enough to nourish the beach with sand. The evidence for the transgression is quite obvious: banks of well-consolidated muds that were originally deposited in the lagoon behind the coastal barrier are being eroded by the advancing waves:

More on wave ripples at Olelog.
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