Sediment transport and morphodynamics of mixed beaches: case studies of two Mediterranean sites

Sediment transport and morphodynamics of two Mediterranean mixed beaches have been investigated. Portonovo was the study site chosen for the Adriatic coast while Marina di Pisa was the one chosen for the west side of Italy; both sites are characterised by a microtidal regime. Little is known about mixed beaches in microtidal regime since the majority of these environments is located on the ocean shores at high latitudes where the tidal regime is meso- or macrotidal. The work have mainly focused on pebble transport under low and moderate wave conditions in order to establish how size and shape of particles can affect their transport. The investigation was possible by means of radio tracer technique (RFID Radio Frequency IDentification). The remarkable asset of the method is to univocally gather displacement information for each single tracer. The technique works both on the submerged and the emerged beach and it also allowed to monitor pebble transport in the long term. Three tracer experiments were carried out with RFID technique: one at Marina di Pisa and two experiments at Portonovo. Tracer experiments had to be supported by a strong substrate of morphodynamics data which were suited on the basis of constant field data gathering: at Portonovo, beach sampling was undertaken for one year and topographic surveys were repeated for two years consecutively. The work deals also with nourishment practices since the two sites are replenished beaches. Under low energy conditions gravel and pebbles need just a small quantity of energy to be destabilized: in both experiments (Marina di Pisa and the first Portonovo experiment) swash action provided that energy, considering that the run-up levels exceeded the maximum tracer elevation for almost the entire duration of the experiments. Once that threshold is reached, marked pebbles can be displaced away from the injection point even though wave motion and swash processes are at minimum. Discs can cover greater distances than spheres but are less dynamic. Once lifted and shifted by swash flows, the discs can travel long paths, reaching a stable location characterized by feeble forces under low wave energy. On the other hand the threshold to initiate movement for spheres is lower so it is more difficult for them to find more stable position on beach profile. "Big"-sized pebbles (-5.5 to -6.5 phi) are less dynamic compared to the finer classes ("Medium", -5 to -5.5 phi; "Small", -4.5 to -5 phi). Nevertheless the “Big”-sized spheres resulted slight more dynamic than discs of the same size. Very low and steady energy conditions facilitate pebble cross-shore and offshore movement rather than longshore. A slight increase in wave height produces a predominant longshore transport characterized by non-negligible displacements. Portonovo beach seemed to be a close system regarding at least pebble transport: the central sector of the beach is a transfer zone for pebble motion while the most part of tracers was always found at beach edges. Weak storms combined to swash grazing are able to move pebbles and cobbles alongshore with great displacements (displacements 2 months after the injection: mean 190 m; max 445 m; min 15 m). Beach rotation is a common phenomenon in Portonovo; the system seems to act like a pocket beach: erosion and accretion areas change in accordance to the most frequent direction of each storm. The central area of the beach represents the pivotal point for beach rotation. Despite the high grain size heterogeneity, at Portonovo the sediment pattern is the result of the last storm direction: evident downdrift coarsening of sediments in response to storms normally occurs. Periods of fair weather (with at least very weak storms) develop a striped pattern of surface sediments. Stripes of different grain size run parallel to the shoreline: the swash zone and the lower part of the backshore increase their sediment size becoming gravelly (granules or fine pebbles) and better sorted, while scattered and non continuous stripes of sand cover the landward and the upper part of the backshore. The fill material for nourishment purposes should fit as best as possible the native sediment. At Marina di Pisa the material used for beach refill was sufficiently good: covering the natural sandy backshore with pebbles and cobbles did not prevent users from going to the beach. Deeper studies on coarse sediment abrasion rate are needed for better assessment on replenishment material and to better estimate contingent loss of volume in the refill material. At Portonovo beach, the material provided by local authorities for nourishment projects is quite compatible with the native one. In order to have an even more compatible material the use of spherical pebbles is suggested, to take advantage of their higher dynamicity relative to the discs. A size comprised between -4.5 and -5.5 phi (24 to 48 mm) should be preferred. Further investigations focusing on particle shape are needed to identify the possible primary factors that control pebble movement. Actual measurements of swash velocities for different shapes should be obtained in order to improve threshold velocity formulae, which currently do not involve any shape parameter for particles. It is believed that shape can be a discriminating factor for coarse and very coarse pebble transport (from 16 to 64 mm according to Udden-Wentworth grain size scale) at least under low energy conditions.