Short- and medium-term response to storms on three Mediterranean coarse-grained beaches

Abstract

The storm response of three Italian coarse-grained beaches was investigated to better understand the morphodynamics of coarse-clastic beaches in a microtidal context. Two of the studied sites are located on the eastern side of the country (Portonovo and Sirolo) and the third one (Marina di Pisa) is on the western side. Portonovo and Sirolo are mixed sand and gravel beaches where the storms approach from two main directions, SE and NE. Marina di Pisa is a coarse-grained, gravel-dominated beach, exposed to storms driven by SW winds. Gravel nourishments were undertaken in recent years on the three sites. Beach topography was monitored measuring the same network of cross sections at a monthly (i.e. short-term) to seasonal frequency (i.e. medium-term). Geomorphic changes were examined before and after storm occurrences by means of profile analyses and shoreline position evaluations. The beach orientation and the influence of hard structures are the main factors controlling the transport and accumulation of significant amount of sediments and the consequent high variability of beach morphology over the medium-term. For Marina di Pisa, storms tend to accumulate material towards the upper part of the beach with no shoreline rotation and no chance to recover the initial configuration. Sirolo and Portonovo showed a similar behaviour that is more typical of pocket beaches. Both beaches show shoreline rotation after storms in a clockwise or counter-clockwise direction according to the incoming wave direction. The wider and longer beach at Sirolo allows the accumulation of a thin layer of sediment during storms, rather than at Portonovo where, given its longshore and landward boundaries, the beach material tends to accumulate in greater thickness. After storms, Sirolo and especially Portonovo can quickly recover the initial beach configuration, as soon as another storm of comparable energy approaches from the opposite direction of the previous one. Large morphological variations after the storm on mixed sand and gravel beaches do not necessarily mean a slower recovery of surface topography and shoreline position. Considering that all the three beaches were recently nourished with gravel, it emerged that the differences between the nourishment and the native material, in terms of size and composition, seem to have an important influence on the dynamics of the sediment stock. Considering that recent studies have remarked the high abrasion rate of gravel, further understanding of the evolution of nourishment material with time is needed. The peculiar behaviour of gravel material artificially added to an originally sandy beach suggests the need to modify the widely used classification of Jennings and Shulmeister (2002) adding a fourth additional beach typology, which could represent human-altered beaches

The Role of Detailed Geomorphic Variability in the Vulnerability Assessment of Potential Oil Spill Events on Mixed Sand and Gravel Beaches: The Cases of Two Adriatic Sites

“…Given its predominant gravelly fraction, Portonovo is constantly affected by rapid burial (Figures 2B1,2), which can be led not only by severe storms as already documented by Grottoli et al (2017), who analyzed the storm response of the beach with a typical wave climate for the area. The high dynamicity of Portonovo was also experienced with low energy conditions which generated 0.5 m of burial due to the formation of the fair-weather berm in the intertidal zone (Grottoli et al, 2019).…”

“…Due to its orientation (NW-SE), the beach is largely exposed to incident storm waves coming both from the SE and NE, but SE waves, due to the smaller accommodation space of zones 3 and 4 (Figure 3), can pile up larger sediments (pebbles and cobbles) in storm berms from 1 to 3 m high (Figures 3B,D,F). In sites like Portonovo (Figures 3, 4) the beach limits are crucial, not only in confining the water circulation in the case of an oil spill (González et al, 2009) but, primarily, for increasing the chances of significant burial in case of severe storms (i.e., Hs of 3.5-5 m, an approximate energy of 600-800 m 2 h and at least 30 h of storm conditions; Grottoli et al, 2017). The strong downdrift coarsening of sediments in accordance with the storm direction was already experienced by Carr et al (1970) in Chesil Beach (United Kingdom).…”

“…The gravel fraction usually occupies the swash zone, with granules and fine pebbles normally found on the fairweather berm and in the swash zone and cobbles and boulders usually found on the step zone. The beachface typically slopes at 0.2 (11 • ), whereas the seabed seaward of the step is approximately 0.01 (0.5 • ), as typically on the northern part of Adriatic seabed (Grottoli et al, 2017). According to the Jennings and Shulmeister (2002) classification of gravel beaches, Portonovo is a mixed sand and gravel beach (MSG) since a complete intermixing of sandy and gravelly sediments occurs ( Figure 1D).…”

“…The southernmost edge of the beach is also limited by hard structures (Figure 1E). The beach is N-S orientated, with the beachface typically sloping at 0.16 (9 • ) whereas the seabed seaward of the step is approximately 0.01 (0.5 • ; Grottoli et al, 2017). As in Portonovo, the only sediment source for Sirolo is represented by the limestone cliff behind the beach: small rockfalls occur during major storms or after heavy rainfall.…”

A Wireless Sensor Network Framework for Real-Time Monitoring of Height and Volume Variations on Sandy Beaches and Dunes

“…In particular, sand composition [26,27] and bioclasts [28,29] have been used as natural tracers; fluorescent [30,31] and radioactive [32] tracers have been used as artificial tracing techniques. Another method to infer sediment transport is frequent topographic surveying [33,34]. The most intense modifications to the beach profile are achieved during storms, as high-energy waves deeply rework the subaerial and underwater portions of the beach.…”