Impressive abrasion rates of marked pebbles on a coarse-clastic beach within a 13-month timespan

In this paper the abrasion rate on a coarse-clastic beach was evaluated by calculating the volume loss recorded on indigenous pebbles within a 13-month timespan. The experiment was carried out at Marina di Pisa (Italy) on an artificial beach that was built to counteract the erosion processes affecting this sector of the coast. A total of 240 marble pebbles (120 rounded and 120 angular) were marked using the RFID technology and injected on the beach. The volume loss measured after consecutive recovery campaigns was progressively increasing, reaching the maximum value after 13 months (61% overall). The average volume loss is consistent between rounded and angular pebbles at any time (59.3% and 64.2% after 13 months respectively), meaning that the roundness is not a primary control factor on abrasion rate. The pebbles that did not reach such abrasion rates after 8 and 10 months (volume loss less than 20%) were found at heights equal or greater than 2 m above mean sea level, on the crest of the storm berm that formed during the strongest storms. This implies that the highest wearing is achieved in the lower portion of the backshore, which is also the area that underwent major topographic modifications. Here, sea water action might also exert chemical influence on the pebbles, adding to the mechanical abrasion. The main result of this research, indicating an impressive volume loss on beach pebbles in a short timespan, could be of key importance for coastal managers. The optimization of coarse sediment beach nourishments is also relevant, taking into right consideration that the volume loss due to sediment abrasion might exceed 50% of the original fill volume just after 1 year in the most dynamic portion of the beach

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

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

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

Exploring the Relationship between River Discharge and Coastal Erosion: An Integrated Approach Applied to the Pisa Coastal Plain (Italy)

Coastal erosion coupled with human-induced pressure has severely affected the coastal areas of the Mediterranean region in the past and continues to do so with increasing intensity today. In this context, the Pisa coastal plain shows a long history of erosion, which started at the beginning of the nineteenth century. In this work, shoreline positions derived from historical maps as well as airborne and DGPS (Differential Global Positioning System) surveys were analyzed in a GIS (Geographic Information System) environment to identify the main changes that have occurred in the last 142 years. These analyses were compared with 100 years of discharge data measured at the S. Giovanni alla Vena gauge to identify a possible correlation between the two sets of information. Finally, Sentinel-2 and Landsat images were studied to identify the dispersion of sediments transported by the Arno River. In particular, we found a minimum of fluvial discharge in the years 1954, 1978, and 2012 corresponding to a peak of erosion, while the reduced erosion rate and the fluvial discharge increased in the years 1928–1944, 1954–1975, and after 2012. The qualitative anticorrelation between discharge and erosion is particularly true if we take into account flood events with a value of discharge greater than 700 m3/s, which are those able to transport suspended sand. The remote sensing analyses of Sentinel-2 images acquired during the floods of 6 February 2019 and 3 December 2019, under the most typical wind and sea state conditions for this area (wind coming from SW and storms coming from W/SW and SW) show that during these events a consistent amount of sediment was transported by the river. However, the majority of these sediments are not deposited along the coastline but are dispersed offshore. Grain-size analyses on the transported sediment show that plumes are formed by coarse-to-medium sand, suitable for coastal nourishment, but the reconstructed sediment dispersion lines show that some sectors of the coastline are constantly in the shade. These areas are the most affected by erosion

The Relationship between Nearshore Wave Conditions and Coarse Clastic Beach Dynamics

Beaches composed of pebble to boulder-sized material are a common feature of rocky coastlines globally and provide effective protection against wave attack. However difficulties with the proximal deployment of wave gauges and measuring the transport of coarse clasts in the field has limited research in this area. The overarching aim of this thesis is to use contemporary ‘off-the-shelf’ technology in a series of interlinked, field-based experiments to improve quantitative understanding of relationship between coarse clastic beach dynamics and the nearshore wave conditions. In the first of the empirical chapters (Chapter 3), indigenous clasts implanted with motion loggers provided a unique insight into the threshold-entrainment within the littoral zone on: (1) an exposed macro-tidal field site in Abbotsham, UK and; (2) a fetch-limited system in Flathead Lake, USA. The threshold-entrainment was positively related to the power of waves that prevailed over the hour-long experiment and accurately predicted with the equations of Lorang (2000). The relationship between clast mass and power of individual incident waves linked to each entrainment event was more complex. There was the propensity of large clasts to occasionally become entrained by exceedingly weak waves. The processes behind this result remain unclear. Comprehensive experiments measuring variability in the concurrent wave conditions along the inner shore platform and offshore are presented in Chapter 4, along with daily longshore clast displacement and depth of activation measurements across the adjacent coarse clastic beach at Abbotsham. The longshore transport rate varies in concert with the longshore component of wave power, which is dependent on interactions between incident waves and shore platform morphology. These findings are used in Chapter 5 to model the (positive) longshore flux of clastic material towards the economically-valuable natural barrier, which has diminished in recent years. It was found that a disproportionately large fraction of positive transport was generated by storm events, the effectiveness of which was moderated by water depth. Net annual positive transport was predicted to increase with sea level rise and storminess. However, the increased influx was small in comparison to the volume of the barriers itself, and therefore unlikely to halt, or ameliorate future degradation. The experiments presented in this thesis provide new insight into nearshore wave transformations and the relationship between wave properties and clast transport at a time when sea level rise and the potential increase in storminess is threatening coastlines globally

Seventh International Symposium "Monitoring of Mediterranean Coastal Areas. Problems and Measurement Techniques"

The Seventh International Symposium Monitoring of Mediterranean Coastal Areas. Problems and Measurement Techniques (Livorno, Italy June 19-20-21, 2018) was organized by the CNR-IBIMET in collaboration with the City of Livorno, the LEM Foundation, the FCS Foundation and the Compagnia dei Portuali di Livorno, with the patronage by Accademia Nazionale dei Lincei, Accademia dei Georgofili, The University of Florence, the Tuscany Region and the North Tyrrhenian Sea Port System Authority. The Symposium, in which scholars from all over the Mediterranean basin participated, was an opportunity to illustrate new proposals and to promote actions to protect the Mediterranean coastal marine environment. In particular, the event was characterized by the search for technical and instrumental solutions to problems related to: energy production in the coastal area, morphology and evolution of coastlines, flora and fauna of the littoral system, management and integrated coastal protection, coastline geography, human influence on coastal landscape

Morphological and longshore sediment transport processes on mixed beaches

Mixed beaches, with sediment sizes ranging over three orders of magnitude, are an increasingly important coastal defence on the heavily populated coasts of SE England and N France. Yet longshore transport rates and volumes, important in understanding beach sustainability, remain understudied for such beaches. This thesis addresses the knowledge gap via field investigations of beach profile evolution, active layer measurements and tracer pebble scattering patterns on two macrotidal mixed beaches at Cayeux-sur-Mer (France) and Birling Gap (UK), eastern English Channel. The beach topography data at both sites enabled observation of how reactive the beach profile is to hydrodynamic conditions. Each beach, in different environmental conditions, was found to have a profile that responded extremely quickly to changing hydrodynamics as a result of the combined effects of the High Water Level (HWL) and wave height. The most significant topographical changes are associated primarily with variations in the across-shore position and height of the berm. The research contributes new baseline data to help refine the currently limited understanding of the relationship between depth of disturbance and wave height, wave period, wave direction and the degree of mixture on mixed beaches. Consistent patterns of pebble behaviour were identified during each tidal cycle at different locations on the beach profile. The distance travelled varied with location on the beach profile, with pebbles from the upper beach tending to travel further than pebbles placed on lower parts of the beach. Upper beach pebbles generally showed a clear displacement seaward across-shore whereas lower and middle beach pebbles were affected by shorter across-shore displacements. These displacements are explained by variations in the hydrodynamic conditions, swash flows, groundwater flow, beach slope and grain size. Finally, wave conditions and water level directly influenced the Longshore Sediment Transport (LST) rates. From these results, applying the energy flux approach, a drift coefficient (K) of 0.04 was derived for both sites

Proceedings of Seventh International Symposium "Monitoring of Mediterranean Coastal Areas. Problems and Measurement Techniques"

The Seventh International Symposium Monitoring of Mediterranean Coastal Areas. Problems and Measurement Techniques (Livorno, Italy June 19-20-21, 2018) was organized by the CNR-IBIMET in collaboration with the City of Livorno, the LEM Foundation, the FCS Foundation and the Compagnia dei Portuali di Livorno, with the patronage by Accademia Nazionale dei Lincei, Accademia dei Georgofili, The University of Florence, the Tuscany Region and the North Tyrrhenian Sea Port System Authority. The Symposium, in which scholars from all over the Mediterranean basin participated, was an opportunity to illustrate new proposals and to promote actions to protect the Mediterranean coastal marine environment. In particular, the event was characterized by the search for technical and instrumental solutions to problems related to: energy production in the coastal area, morphology and evolution of coastlines, flora and fauna of the littoral system, management and integrated coastal protection, coastline geography, human influence on coastal landscape

Mixed sand and gravel beaches: accurate measurement of active layer depth to develop predictive models of sediment transport

This thesis addresses the need to calibrate longshore sediment transport equations for use on heavily managed mixed sediment beaches, which are becoming increasingly common but are still relatively understudied. This is achieved through the use of active layer measurements, tracer pebble experiments, and monitoring of morphodynamics over a total period of 16 months. It is unique in investigating a complex, artificially replenished, groyned mixed sand and gravel beach in this way. The groynes have a significant impact on beach morphology and sediment transport at Eastoke. Beach levels were monitored using repeated profiles at three main locations within a groyne cell, allowing for the effect of these structures on beach profile shape and response to changing wave climates to be assessed. This provides a more accurate representation of morphodynamics than would otherwise be available from semi-annual profile surveys. The beach response to hydrodynamic conditions was shown to be rapid, and not always indicative of drift direction. Sediment sorting was observed, but was not strongly correlated with wave conditions. A vast dataset of active layer measurements indicated significant variability, which has not been made clear by previous studies of active layer depth. The key finding of this thesis is that active layer depth as a percentage of wave height can be predicted from significant wave height using the equation ln(y) = -0.797x + 3.565 (R2 = 0.578), and this value can then be converted into a depth measurement. The prediction can be applied at a daily scale to the combined upper and mid beach as an estimate of the average active layer depth. Further field sites should be investigated to determine whether this equation can be applied to other mixed sediment beaches. Short- and medium-term pebble transport patterns were observed using passive integrated transponder (PIT) tagged pebbles, deployed in stages throughout the research. Sediment transport volumes were estimated based on field data, and hydrodynamic data used to calculate a range of drift coefficients (k) for the commonly used CERC transport formula. Low detection rates ultimately limit the confidence which can be applied to final calculations of longshore transport volumes and values of k, but this study provides insight into the complexities associated with studying a site like this, and suggestions for improvements which could be made to future research