Las incidencias cataclismáticas de las bravezas en la evolución de la costa de Chile central

Abstract

The coast of Northern and Middle Chile suffers incidental surges or "bravezas", wich are meteorological surges. They have as cause the swells originated in the Pacific's storms, without important tide influence, as evidenced by the tide wave analysis and sinoptic interpretation. For the last great meteorological surge ocurred in 1968, the forecast gaves nearly stationary fronts over 2.000 n.m. S-SW from the coast. Perhaps the convergence of these frontal systems is the critical situation or threshold of surge incidence.

The waves of the surge swells arrive into low holocenic coastal platform. Sometimes, their action can to attack the low levels of the Upper Pleistocene, showing a marine morphogenetic recurrence. The 1968 "bravezas" gaves the opportunity for appling an evolutionary model in a cliff with crystalline, jointed rocks, wich dip into the sea. The boulder removal on the joint plane produces an evolution by parallel retreat, wheter there is an unimpeded basal removal. The dip angle of the joints is greater than the angle of friction and the critical slope is equal than the dip angle.

The geomorphological changes produced, are more stable on reefs and cliffs than on beaches. In the back-shore there is a tendency to recover the ancient profile. Every surge is a break of equilibrium and the seasonal conditions tend to restore themselves, but some forms are more permanentas the abrasion cliff of the marginal dune, realted to a new transverse concave profile including the back and foreshore together.

This experience shows that in the aperiodic geomorphological mapping it does not must be distinguished as taxons the erosion beach from the accumulation beach, because they are transitional forms.

Finally, it is possible to elaborate process response models of "braveza" coastal changes for danger forecasting. By using the frecuency and threshold notions, danger forecasting maps can be made.