Dune fields have always drawn the interest of many disciplines (sedimentology, petrography, biology) due to their naturalistic and economic significance. Recently, also coastal engineering focused on dunes because they are correctly perceived as a primary factor to withstand erosion processes. During periods of decreasing sediment supply, they actually work as a reservoir of sand, which is released to feed the nearshore. Therefore, the artificial reconstruction of frontal dunes is considered a viable option to restore a natural, more stable configuration of a beach system. The increasing utilization of this technique encouraged the need for an in-depth comprehension of dunes in terms of grain size variability. The aim of this study is to define the grain size characteristics of a dune field along the coast of Pisa (Tuscany, Italy) to evaluate possible correlations with the evolution of the River Arno, which is the main sediment source of the area. In addition, a section of an eroded frontal dune, showing a continuous series of layers constituting its inner structure, has been systematically sampled to evaluate grain size differences within the very dune. The study area is comprised within the Migliarino – San Rossore – Massaciuccoli Regional Park and is about 23 km long. The landward limit is defined by the last recognizable coastal dune ridge system (generally 1-to-3 km away from the coastline), which formed about 2100 years ago. The dunes were mapped by the classification of frontal (those continuously subjected to erosion and accretion processes), semi-mobile (those seldom subjected to erosion and accretion processes), and steady (those no more subjected to processes that led to their formation). Sampling took place along five transects normal to the coastline. The transects, not equally distributed in the study area, were traced out along significant positions, i.e. updrift and downdrift river’s mouth and defense structures. Samples were collected from the beachface, the berms, and from the lee-side, the stossside, and the crest of both frontal and semi-mobile dunes; the steady dunes were sampled on the crest and on the interdune areas. Grain size analyses showed a definite decrease of sediment grain size towards the ancient dunes. In particular, a distinct drop occurs at the transition between active and inactive dunes (from 0.33 to 0.18 mm). Besides, steady dune crest samples are characterized by coarser grain size than interdune samples. The grain size drop occurring at the transition between active and inactive dunes, related to the XVIII century, is possibly linked to a variation of the sediment discharge of the River Arno. The resulting data may provide useful indications to improve and optimize artificial dune reconstruction.
Grain size characterization of modern and ancient dunes within the Migliarino – San Rossore – Massaciuccoli Regional Park (Pisa, Italy)
Bertoni D.;SARTI, GIOVANNI
2011-01-01
Abstract
Dune fields have always drawn the interest of many disciplines (sedimentology, petrography, biology) due to their naturalistic and economic significance. Recently, also coastal engineering focused on dunes because they are correctly perceived as a primary factor to withstand erosion processes. During periods of decreasing sediment supply, they actually work as a reservoir of sand, which is released to feed the nearshore. Therefore, the artificial reconstruction of frontal dunes is considered a viable option to restore a natural, more stable configuration of a beach system. The increasing utilization of this technique encouraged the need for an in-depth comprehension of dunes in terms of grain size variability. The aim of this study is to define the grain size characteristics of a dune field along the coast of Pisa (Tuscany, Italy) to evaluate possible correlations with the evolution of the River Arno, which is the main sediment source of the area. In addition, a section of an eroded frontal dune, showing a continuous series of layers constituting its inner structure, has been systematically sampled to evaluate grain size differences within the very dune. The study area is comprised within the Migliarino – San Rossore – Massaciuccoli Regional Park and is about 23 km long. The landward limit is defined by the last recognizable coastal dune ridge system (generally 1-to-3 km away from the coastline), which formed about 2100 years ago. The dunes were mapped by the classification of frontal (those continuously subjected to erosion and accretion processes), semi-mobile (those seldom subjected to erosion and accretion processes), and steady (those no more subjected to processes that led to their formation). Sampling took place along five transects normal to the coastline. The transects, not equally distributed in the study area, were traced out along significant positions, i.e. updrift and downdrift river’s mouth and defense structures. Samples were collected from the beachface, the berms, and from the lee-side, the stossside, and the crest of both frontal and semi-mobile dunes; the steady dunes were sampled on the crest and on the interdune areas. Grain size analyses showed a definite decrease of sediment grain size towards the ancient dunes. In particular, a distinct drop occurs at the transition between active and inactive dunes (from 0.33 to 0.18 mm). Besides, steady dune crest samples are characterized by coarser grain size than interdune samples. The grain size drop occurring at the transition between active and inactive dunes, related to the XVIII century, is possibly linked to a variation of the sediment discharge of the River Arno. The resulting data may provide useful indications to improve and optimize artificial dune reconstruction.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
