Groundwater dynamics on macrotidal gravel beaches of Tierra del Fuego.

FEDERICO IGNACIO ISLA; BUJALESKY, GG

Journal of Coastal Research

CERF

Lugar: Palm Beach, USA; Año: 2005 vol. 21 p. 66 - 72

0749-0208

On bimodal microtidal beaches, there are grain-size segregations and the relationship between mean size and fore- shore slope is not clear. Dealing with macrotidal beaches composed of gravel and sand, the dynamics vary in response to the tide: reflective conditions at high tide, and a dissipative behaviour during low tide. These dynamic changes are related to grain-size segregations: the upper beach is dominated by gravel (or gravel with coarse sand) and the low- tide terrace is exclusively composed of fine sand. Along the Atlantic coast of Tierra del Fuego, nineteen beaches analysed vary in mean grain size between 24 and 4 phi units. The slope of the foreshore (reflective portion) is linearly related to grain size. The slope of the low-tide terrace (dissipative portion) has no relationship to grain size (fine to very fine sand). These segregations cause temporal changes in the composition of the groundwater sceping at the low- tide terrace. Test experiments performed at the beach permitted to recognise the relationships between coastal and ground water. Salt groundwater similar to the sea (31 practical salinity units; psu) seeps at the initial stages of the dropping tide. Seeping groundwater salinity diminishes progressively, and at low tide it arrives to a minimum of 10 psu. Tidal effects are therefore conditioning (chemically and physically) the water exchange between the beach and the nearshore areas.