A straightforward conceptual method is proposed to quantitatively assess the seasonal-scale tendency of retreatment or advancement on microtidal beaches by using the backshore/foreshore length ratio. This method is based on measuring the cross-shore profile of a beach when it passes through the “transitional state” that separates the high-from the low-energy season, period during which the morphological characteristics of the beach tend to its equilibrium profile. In order to obtain real measurements of backshore ( B ) and foreshore ( F ), the definition of the limits bounding these two important components in subaerial beaches is reviewed and discussed. The approach based on the measurement of the B/F length ratio assumes that foreshore and backshore have equivalent lengths in beaches that approximate to their state of morphodynamic equilibrium ( B/F ~ 1). A backshore length exceeding the foreshore length is indicative of a state of beach recession, with a B/F length ratio > 1. When the foreshore length is greater than the backshore length, the shoreline is advancing or, alternatively, it is developing in a state of morphological confinement, i.e. due to the presence of a sea cliff, with a B/F < 1. This practical method is then tested against 36 sand and gravel microtidal beach profiles measured along the coasts of Basilicata, in southern Italy. The various “beach states” are summarised into seven classes (I-VII), each identified from specific value intervals of the B/F length ratio.
Field analyses carried on modern beaches are usually retained crucial in detecting their morphodynamic status, in terms of equilibrium and disequilibrium [
In either sand and gravel beaches, the subaerial sector represents the zone where the signatures of the recent hydrodynamic activity of the sea can be easily recognised by detecting a series of distinctive components, in- cluding the backshore and the foreshore length. The linear extent of these sub-environments reflects faithfully the hydrodynamics that has affected a beach in recent times: backshore and foreshore express the quantity of sediment that is moving on a beach in relation to the natural forces that dominate that beach [
In the last fifty years, a number of different conceptual models contributed to delineate and differentiate sub- environments in modern microtidal subaerial beaches, distinguishing internal zones, usually dominated by the wind action except during extreme storms, and external zones of a beach, where the wave dynamics prevails.
However, although the models showing sub-environment partitioning of the subaerial part of a beach are nu- merous and useful, the distinction between backshore and foreshore still remains confusing and the boundary between these two important sub-environments is often arbitrarily chosen and placed without any physical evi- dence.
In this work, a conceptual model is applied to the study of the coastal dynamics in order to assess quantitatively the rate of retreatment/advancement of microtidal beaches by using a backshore/foreshore (B/F) length ratio. This method can be practically applied to detect short-term beach variations, measured once a year during the transitional or “sweep period” of a beach, possibly during the spring or fall (April-May in the North Temperate Zone, and October-November in the South Temperate Zones). During this interval, a beach previously subjected to high-energy storms, tends to re-establish the sediments lost, changing its bi-dimensional shape to- wards an equilibrium profile [
The importance of the linear dimension of the backshore and foreshore measured in different coastal settings, where the effects induced by the tidal range are negligible at the coast, is then discussed and referred to the their state of preservation at the time of the measurement.
The beach examples considered in this study are from the Tyrrhenian and Ionian shorelines of the Basilicata, in southern Italy. Along these microtidal coasts, a number of different beach types have been measured and studied in order to document and compare backshore and foreshore zones with different features.
In a cross-shore profile, the beach is conventionally divided into zones on the basis of morphological, hydrodynamic and sedimentary processes [
sector coincides with the area mostly used for recreational activities that extends no farther seaward than the shoreline, having a width that varies with changing water level. This segment of the beach is usually divided into two main sub-environments: backshore and foreshore. The part of the beach which is subject to variations in water line depending on the correspondent tidal range (micro-, meso- or macrotidal), is termed intertidal zone. (2) The subaqueous sector of a beach is that zone lying constantly under the water line and is consequently sub- ject to the influence of the water motions (fair-weather and storm waves, alongshore currents and tides). This sector is generally thought as nearshore. The nearshore comprises shallow water environments in which waves strictly interplay with the sea-floor and clastic sediments. The nearshore includes the shoreface directly attached to the subaerial beach and where waves break, whereas deeper environments are termed offshore (
According to the most common and used definitions (e.g., [
The landward limit of the beach is generally marked by a change in the sediment grain size and in the mor- phology. This sub-environment of the subaerial beach can include aeolian dunes. Sediments are very well sorted, having a dominant grain size from medium to fine sand with uni-modal distributions [
The foreshore is the sector of the beach that lies in contact with the water line and, therefore, it is directly influenced from the wave hydrodynamics [
Generally, this sector of the beach includes the swash zone (
The recognition of the limit that separates the backshore (B) from the foreshore (F) is here considered crucial in order to identify the real linear dimension of a beach profile, whose physical components are, in turn, directly dependent on the state of equilibrium or disequilibrium during periods of shore advancement or retreatment [
In order to identify a beach shape that well approximates to the reality, beach profiles can be surveyed by us- ing a level and graduated stadia rod along profiles set up normal to the shore [
The profile of the subaerial beach that is measured using these techniques extends from the base of the swash berm (coincident with the mean low tide level and, therefore, lying below the water line) landwards up to the back dune zone [
Usually, repeated profile surveys should be acquired at constant time interval of a few months (e.g., every three-four months during each season or, more simply, one during the summer and one during the winter) in or- der to determine patterns and rates of changes on a beach [
In beaches affected by a state of general (long-term) advancement or retreating, the sweep zone is asymmetrical, resulting in different linear extension of the physical components of the subaerial beach. Therefore, the representative changing pattern of a subaerial beach can be assessed if a single profile measurement is acquired in proximity of such transitional state. The backshore and foreshore lengths, if properly detected and measured, can be then used as proxy in order to recognise short-term patterns in a changing microtidal beach.
Despite its apparent simplicity, the boundary dividing backshore and foreshore in a subaerial beach is in practice a challenge to identify; indeed, it was differently defined in the past literature according to various types of subdivisions or schemes.
Starting from the 50 s, most of the morphology-based models placed the B/F boundary at the limit of the high water swash [
Bird [
sedimentological evidences, Walker and Plint [
However, the B/F boundary placed at the first berm, which is produced by the wave erosion to the beach and coincident with the medium high tide level [
Taking into account sedimentological, morphological and hydrodynamic features which are easily detectable in a microtidal subaerial beach, a revised beach partitioning and relative limits is thus proposed.
Based on the most common definitions, the foreshore represents the external part of the subaerial beach which is seasonally subject to the wave run-up (i.e., the maximum vertical extent of wave uprush on a beach [
From this point landwards, the backshore develops. This zone is affected very rarely by waves or can be occasionally inundated by long waves with enough inertial energy to reach this part of the beach. Therefore, since this infrequent condition represents an indirect effect of the wave motion, the backshore can be considered as dominantly subjected to the wind action. In sand beach, this zone usually shows fine and very well sorted sediments, associated with small wind ripples and dunes. Depending on the local geomorphologic setting, a dune field can be characterised by high forms (up to 10 - 15 m ), usually covered by typical vegetations. The landward limit of the backshore corresponds with the first alluvial sediments in wide beaches or with the base of the sea cliff in confined beaches [
The physical signature marking the limit of maximum wave run-up can be preserved on a beach for weeks or months later after the storm event (
The boundary separating backshore and foreshore in a subaerial beach acts as a “balance point” in the distribution of sediment masses (
This condition changes if backshore or foreshore undergo substantial sediment mass variations: e.g., during beach accretion, foreshore length increases as swells accumulate additional sediments (
Consequently, beaches characterised by a morphodynamic equilibrium condition have backshore and foreshore with lengths roughly tending to the equivalence (B/F ≈ 1;
Therefore, the beach equilibrium (B/F ≈ 1) represents a morphodynamic state to which a beach profile tends after periods of repeated storms (winter) or, on the contrary, in absence of them (summer). These intermediate conditions represent “transitional states” that record decreases (beach retreat) or increases (beach accretion) of sediment mass. Consequently, relevant variations in sediment volumes result in backshore and foreshore length changes.
The retreating or advancing ratio can thus be quantitatively assessed on the base of the B/F value measured in a beach cross-shore profile and acquired during the time span when a given beach is reaching its own equilibrium (transitional state). This method has been applied and verified on the 36 beach types chosen along the Tyrrhenian and Ionian coasts of Basilicata. For synthesis reasons, only seven of these beaches are illustrated and discussed in the present work, whereas data deriving from the other examples are reported in
Basilicata lies in the microtidal central Mediterranean (
The NW-SE-trending Tyrrhenian coast of Basilicata is ~20 km long (
The NE-SW-trending Ionian coast of Basilicata is ~40 km long (
The beach examples documented in this paper were acquired in coastal zones where the ubiquitous presence of the human activity was less invasive in the last decades, allowing the preservation of the main original features of the beaches considered. Among the 36 sites measured (see
Beach types of western Basilicata (
Coast | Beach | Total profile length | Profile orientation | Starting point coordinates | Backshore (B) length | Foreshore (F) length | B/F length ratio |
---|---|---|---|---|---|---|---|
Tyrrhen Iancoast | Acquafredda | 45.34 | N56˚E | 40˚02'18"N - 15˚39'55"E | 24.00 | 21.34 | 1.12 |
Cersuta | 18.00 | N50˚E | 40˚00'32"N - 15˚20'46"E | 8.61 | 9.39 | 0.92 | |
Spiaggia Nera | 44.31 | N75˚E | 39˚58'60"N - 15˚43'22"E | 20.87 | 23.44 | 0.89 | |
Acquafredda 1 | 44.00 | N233˚E | 40˚02'12"N - 15˚40'02"E | 10.00 | 34.00 | 0.29 | |
Fiumicello | 66.60 | N257˚E | 39˚59'52"N - 15˚41'59"E | 30.00 | 36.60 | 0.82 | |
Foce Noce 1 | 85.00 | N249˚E | 39˚55'51"N - 15˚45'11"E | 45.00 | 40.00 | 1.13 | |
Foce Noce 2 | 130.00 | N251˚E | 39˚55'33"N - 15˚45'19"E | 90.00 | 40.00 | 2.25 | |
Praia a Mare | 98.00 | N242˚E | 39˚53'45"N - 15˚46'46"E | 55.00 | 43.00 | 1.28 | |
Ion Iancoast | Metaponto | 54.40 | N121˚E | 40˚22'48"N - 16˚51'18"E | 43.76 | 10.64 | 4.11 |
Lido Quarantotto | 55.97 | N129˚E | 40˚19'17"N - 16˚48'14"E | 42.30 | 13.67 | 3.09 | |
Lido Terzo Cavone | 163.95 | N107˚E | 40˚14'46"N - 16˚44'46"E | 108.35 | 55.60 | 1.95 | |
Policoro | 82.45 | N123˚E | 40˚10'14"N - 16˚42'13"E | 52.45 | 30.00 | 1.75 | |
Marina di Ginosa 1 | 13.00 | N100˚E | 40˚ 2' 23"N - 16˚50'66"E | 9.00 | 4.00 | 2.25 | |
Metaponto Lido 1 | 37.00 | N116˚E | 40˚21'17"N - 16˚49'59"E | 32.00 | 5.00 | 6.40 | |
Metaponto Lido 2 | 51.90 | N119˚E | 40˚21'16"N - 16˚49'59"E | 47.00 | 4.90 | 9.59 | |
Argonauti Nord 1 | 61.50 | N122˚E | 40˚20'34"N - 16˚49'23"E | 55.00 | 6.50 | 8.46 | |
Argonauti Nord 2 | 59.40 | N125˚E | 40˚20'34"N - 16˚49'22"E | 50.00 | 9.40 | 5.32 | |
Argonauti Sud 1 | 24.50 | N121˚E | 40˚20'03"N - 16˚48'58"E | 12.00 | 12.50 | 0.96 | |
Argonauti Sud 2 | 31.00 | N118˚E | 40˚20'01"N - 16˚49'01"E | 16.00 | 15.00 | 1.07 | |
Lido Quarantotto 1 | 70.90 | N120˚E | 40˚19'10"N - 16˚48'16"E | 56.00 | 14.90 | 3.76 | |
Lido Quarantotto 2 | 85.80 | N122˚E | 40˚18'580"N - 16˚47'52"E | 71.00 | 14.80 | 4.80 | |
Terzo Cavone S 1 | 29.30 | N110˚E | 40˚17'08"N - 16˚46'32"E | 8.00 | 21.30 | 0.38 | |
Terzo Cavone S 2 | 66.90 | N114˚E | 40˚16'18"N - 16˚46'00"E | 46.00 | 20.90 | 2.20 | |
Marina di Pisticci 1 | 79.00 | N120˚E | 40˚18'10"N - 16˚47'11"E | 56.00 | 23.00 | 2.43 | |
Terzo Cavone S 3 | 57.00 | N111˚E | 40˚16'08"N - 16˚45'55"E | 32.00 | 25.00 | 1.28 | |
Scanzano 1 | 75.20 | N104˚E | 40˚14'43"N - 16˚44'56"E | 50.00 | 25.20 | 1.98 | |
Scanzano 2 | 94.90 | N103˚E | 40˚14'29"N - 16˚44'50"E | 69.00 | 25.90 | 2.66 | |
Scanzano 3 | 61.00 | N107˚E | 40˚14'18"N - 16˚44'45"E | 34.00 | 27.00 | 1.26 | |
Agri Sud 1 | 44.00 | N120˚E | 40˚11'52"N - 16˚43'35"E | 15.00 | 29.00 | 0.52 | |
Agri Sud 2 | 45.00 | N122˚E | 40˚11'41"N - 16˚43'25"E | 14.00 | 31.00 | 0.45 | |
Lido Policoro 1 | 101.00 | N123˚E | 40˚11'02"N - 16˚42'53"E | 70.00 | 31.00 | 2.26 | |
Lido Policoro 2 | 100.00 | N121˚E | 40˚10'44"N - 16˚42'34"E | 65.00 | 35.00 | 1.86 | |
Oasi WWF 1 | 101.50 | N117˚E | 40˚10'14"N - 16˚42'08"E | 63.00 | 38.50 | 1.64 | |
Oasi WWF 2 | 94.00 | N108˚E | 40˚10'08"N - 16˚42'03"E | 57.00 | 37.00 | 1.54 | |
Nova Siri 1 | 135.00 | N134˚E | 40˚07'48"N - 16˚39'34"E | 90.00 | 45.00 | 2.00 | |
Nova Siri 2 | 133.00 | N130˚E | 40˚07'32"N - 16˚39'12"E | 82.00 | 51.00 | 1.61 |
The three beaches selected along the Tyrrhenian coast of Basilicata represent sites where the low anthropogenic impact has partially preserved the primary morpho-sedimentological features of the beach systems. These “poc- ket beaches” (Acquafredda, Cersuta and Spiaggia Nera in
The Acquafredda beach is a ~250 m long, pocket beach, with a NW-SE-trending shoreline enclosed between promontories (
In plan view, the Cersuta beach is a triangular-shaped, pocket beach with a NW-SE-trending ~119 m long shoreline (
profile of the subaerial beach (profile Cs in
The NNW-SSE-trending, ~202 m long Spiaggia Nera system is another pocket beach where the primary morpho- sedimentary features are still preserved (
Along the Ionian coast of Basilicata, the four beaches reported in this paper (
The SW-NE-trending Metaponto beach (
The Lido Quarantotto Beach is located ~10 km southwards the previous example (
a gently-inclined, sand beach with a 40 - 50 m of cross-shore profile length (
10 km southwards the previous site, the Lido Terzo Cavone beach occurs (
The Policoro beach is located ~3 km N the Sinni River mouth (
The subdivision between backshore and foreshore detected in the studied subaerial beaches based on their genetic significance and bi-dimensional length, allows assessing quantitatively the state of equilibrium or disequilibrium at the time of the profile measurement. Advancing, receding and stable beaches are thus identifiable into seven classes of beach states obtained by their respective values, or value intervals, of the B/F length ratio (Ta- ble 2). Accordingly, B/F < 0.5 indicates “strongly advancing” beaches that may correspond to the “Class I”; B/F comprised between 0.5 and 0.8 indicates “moderately advancing” beaches (“Class II”), and B/F comprised between 0.8 and 1 “slightly advancing” beaches (“Class III”). “Stable” beaches with B/F = 1 correspond to the “Class IV”. Analogously, values of B/F comprised between 1 and 2 indicate “slightly receding” beaches of the “Class V”, 2 < B/F < 3 corresponds to “moderately receding” beaches of the “Class VI” and, finally, B/F > 3 designates “strongly receding” beaches of the “Class VII” (
As documented by a number of studies (e.g., [
Class | Beach State | B/F Length Ratio | ||
---|---|---|---|---|
Advancing | I | Strongly Advancing | <0.5 | |
II | Moderately Advancing | 0.5 - 0.8 | ||
III | Slightly Advancing | 0.8 - 1 | ||
IV | Stable (Neither Advancing Nor Receding) | 1 | ||
Receding | ||||
V | Slightly Receding | 1 - 2 | ||
VI | Moderately Receding | 2 - 3 | ||
VII | Strongly Receding | >3 |
basins behind the coasts (e.g., [
The beach examples analysed in the present study pertain to two very different coastlines, including sand, gravel and mixed beaches. All these beaches lie in states of retreatment, advancement or stability depending on highly varying local conditions of sediment supply, morphological confinement and exposure to dominant winds and waves.
The Tyrrhenian beaches are confined or semi-confined systems (
The Ionian examples allow verifying the B/F ratio in different settings. Previous studies have documented as the retreating tendency of the Ionian beaches varies greatly, from strongly-receding beaches in the north-eastern sector (Metaponto) to slightly-receding beaches in the south-western sector (Terzo Cavone) [
The morphological profile observed in the studied beaches from the Tyrrhenian and Ionian coastlines of Basilicata results from a series of dynamic processes that shape the littoral, leaving the signature of their energy, duration and direction [
The morpho-bathymetric profile of a shoreface changes continuously in time, because primarily subjected to the wave action during either fair-weather and storm episodes [
Temporal changes in external controls, including (i) amount of sediment discharged from the rivers, (ii) wave climate and (iii) persistence of along shore currents, alter the geometry of the equilibrium profile (e.g., [
The subaerial sector of a beach can be consistently considered as that part of a beach that is primarily subject to the effect of a short-term modification of the state of equilibrium. This modification occurs as: 1) a morphological change of the profile shape; 2) a redistribution of sediments along the profile; and 3) a variation of the length of its components (backshore and foreshore). Therefore, the seasonal-scale measurements of a beach profile allow identifying short-term trends in a changing coastline.
However, it is well known that a one-year-long beach evolution occurs through a series of “stages” (expressed by different topographic profiles) which endpoints are represented by the winter and summer profiles (see again
Thus, a beach profile which is measured in the middle of this continuum that cyclically (even not regularly) moves between two end-members, can be theoretically considered as a close approximation of the equilibrium profile. A subaerial profile acquisition during this transitional state represents an “instantaneous picture” on the state of preservation of the physical components of a beach which is balancing on a dynamic equilibrium.
This trend is a short-term variations of a longer-term tendency calculated over tens of years (e.g., temporary beach accretion could result from a high-frequency (seasonal) state of shoreline advancement over a period of general retreating).
As shown in
This plot can also be used to estimate long-term retreatment/advancement patterns for beaches with known B/F ratio. The example in (b) shows a y-axis-directed deviation from the trend line that indicates as all the meas- urements, whose data points are dispersed in the top-left corner of the diagram, belong to a long-term retreating shoreline. Such a tendency implies, for instance, that beaches showing B/F < 1 undergo momentary states of accretion during a longer-term recession (αT > αC) (see
Retreating beaches are positioned around the top-left corner of the diagram, where data indicate long backshores associated with small foreshores. Beaches with foreshore longer than backshore are placed in the bottom of the plot. These two areas point out: 1) shoreline erosion and consequent retreating beaches and 2) advancing beaches or cliff-confined beaches, respectively (
In the present example, the trend line is displaced with respect the correlation line traced between equivalent x, y values (dotted line in
In this paper the bi-dimensional extension of the subaerial part of microtidal, wave-dominated beaches are con- sidered as proxy in the short-term evaluation of their state of morphodynamic equilibrium. During the passage from the high-energy to the lower-energy season, a beach crosses a “transitional state”, represented by a quasi- equilibrium profile whose shape lies between the two endpoints of the winter and summer profiles. This intermediate state reflects more faithfully the mean dynamics of a beach that results in the dimension of its physical components forming the subaerial sector. Thus, backshore and foreshore, if properly detected and bi-dimen- sionally measured, can be consistently related to the state of beach retreatment or advancement, at the moment of the profile acquirement.
This very quick and low-cost method thus requires only one profile measurement per year, which has to be strategically acquired during the late spring or fall, when a beach is expected to lie in its “transitional state”.
The B/F length ratio calculated in 36 sand, gravel and mixed, microtidal, wave-dominated beaches (7 of which were focused in this paper) indicates that foreshore and backshore have equal lengths along stable shorelines. Backshore length exceeding foreshore length was instead detected in retreating beaches. Systems with opposite conditions (foreshore greater than backshore) indicate advancing beaches or confined (pocket) systems. These states, summarised into seven classes, identify numerically ranges of advancing, retreating and stable beach conditions.
Financial support for this research was provided by Ministero dell’Università e della Ricerca Scientifica (ex 60% Grant 2010-2011 to S. G. Longhitano). An early version of this manuscript benefited of the constructive comments by Dr. William A. Birkemeier and Dr. Robert J. Thompson. The author would like to thank Prof. Giu- seppe dell’Olio for the stimulating discussion on the meaning of the B/F plots and the MSc students of the Coastal Dynamics class at the Basilicata University, for their enthusiastic help during beach profile measurements.