Resumo

MARCOMINI, Silvia C  e  LOPEZ, Rubén A. Alteración de la dinámica costera por efecto de la explotación de arena de playa, partidos de General Alvarado y Lobería, provincia de Buenos Aires. Rev. Asoc. Argent. Sedimentol. [online]. 1999, vol.6, n.1-2, pp.1-18. ISSN 1853-6360.

The effects on the dynamics, morphology and texture of the beaches between Miramar and Nutria Mansa creek produced by beach mining activities in a sand pit located 2 km southwards from Centinela del Mar were analyzed. Also the beach recovery since the end of the sand exploitation in July 1997 was evaluated for mining area and for the case studied: Mar del Sur. The mining activity affected an area of 200.000 m2, with an extension of 1.800 m along the coast, with a sand volume estimated in 150.000 m3. The sand was mainly extracted from the berms and foreshore. A rapid increased in the coastal retreat has been detected for all the localities studied since 1984 in coincidence with the beginning of the mining activities on the beach. Otherwise the backward movements rates of the coastline decrease with the proxioity to the mining site. A decrease in mean grain-size and an increase in standard deviation was observed from the comparisson of grain-size analysis performed in 1956 and 1998. It indicated an increase of the erosive power of littoral currents during this period. An increase in the positive skewness was also detected in the foreshore during the same period. The morphologic studies, the grain size analysis and field observations showed that the sand mining generated an erosive wave which was moving northward in the direction of the littoral currents. The sand exploitation from the beach produced a decrease in the littoral current saturation favoring the degradation of the beach sand deposits and increasing cliff retreat, mainly of those carved over dunes. Several morphologic evidences indicated active erosion: - Wave cut platforms with no sand cover where ancient beaches were described. - Active scarp formation in present foredunes. - Isolated dune relicts on the beach. - Displacement of the intertidal beach to the seafront housing area. - Roads cut by active cliffs. The beach recovery induced by the sand mining after closure of the mining activities was studied considering the case type at Mar del Sur. An accumulative sedimentary budget, was recorded in the exploitation area for all the profiles, after a year of the interruption of the mining activities on the beach along 900 m. The budgets obtained from northeast to southwest are: profile 1: none, profile 2: 25,94 m2/m, profile 3: 41,69 m2/m, profile 4: 21,00 m2/m, profile 5: 18,59 m2/m. A net accumulation of 23.300 m3 was estimated. An annual negative budget of -554 m3 at Mar del Sur beach was detected after the interruption of the sand mining activities in July 1997. An accumulative sector located in the middle of the bay was observed with a positive budgets of 3.176 m3 as well as an erosive zone at the north with a negative budget of -3261 m3. Near the groin a small erosive zone was observed with a negative sedimentary balance of -486m3. The total budget of the bay is neutral (-68 m3) if the erosive effect of the groin is discarded. The comparison of the annual budget obtained by other authors, the morphologic features of the coast and the accumulation budgets registered in the middle of the bay may imply that the beach erosion was attenuated after the closure of explotation.

Palavras-chave : Beach sand mining; Sedimentary dynamics; Coastal erosion.

        · resumo em Espanhol     · texto em Espanhol     · Espanhol ( pdf )