1. Introduction
Coastal sand dunes represent an important resource for coastal areas, not only because they represent a crucial sediment supply for the dune-beach system [
1], but also because they act as a first line of coastal defence against sea intrusion, attenuating the impact of storms and storm surges and preventing salt water leakage into the aquifer [
2,
3]. They also represent a unique habitat for specialised species, both animal and vegetal, constituting an irreplaceable ecosystem. For all these reasons, coastal managers must prioritise the preservation of coastal sand, given their environmental and economic value.
According to many authors, coastal sand dunes are aeolian morphological elements modelled by the dynamic of several forcing factors, mainly marine and aeolian forces and the vegetation conditions [
4,
5,
6,
7,
8,
9]. On the one hand, the sea action determines beach morphological characteristics and provides sand supply to nourish the dune [
10,
11], but, on the other hand, it can also be a strong cause of erosion when storms occur. The connection between dune morphology and sea action is very strong, as dunes are the “foremost elements at the edge of the backshore, reflecting the short-, medium- and long-term surfzone-beach-dune processes operating on any particular beach” [
12,
13,
14]. The wind, especially blowing onshore, represents the builder force that transports sand from the beach to the dune, where the decrease of energy and the presence of specialised vegetation trapping the sand, increase dune mass: this implicates that the aeolian climate (direction, frequency, speeds, etc.) of a region is one of the most influencing factors in dune morphology and evolution. Like the sea action, wind can also have an erosive effect; it can remove the sand by blowing on a specific part of the dune field where the vegetation is deteriorating or has been removed or excavated within the dune body. This process represents a major damaging factor because it tends to feed itself: as the wind digs into the dune, blowing into an increasingly narrower space, it accelerates and intensifies its erosion potential [
15]. According to Hesp [
16], this dynamic creates “blowouts”, (saucer-, cup- or through-shaped) depressions or hollows with a derived depositional lobe formed by wind erosion [
17,
18]. Just like the previous two forcing factors, vegetation has a multi-level influence on the dune’s morphology, since geomorphology and vegetation dynamics are naturally interrelated and affect each other considerably [
19]. Always acting as a stabilising factor right from the early stages of dune life, vegetation can positively affect soil consolidation and can slow down the wind’s speed, thereby reducing its sediment transport capability. The process usually follows well-defined steps: a very specialised vegetation colonises the dune habitat, which is characterised by extreme conditions regarding several environmental parameters (i.e., temperature, sand soil, salty air spray, salty water). The vegetation is usually organised by phyto-sociological successions, starting from pioneer species which allow the germination of many other vegetal forms (annual, perennial, grasses, bushes, trees), thus modifying the soil and consolidating it by means of roots and gradually enriching it with nutrients. The vegetation complex effect on morphology can depend on several secondary factors, such as plant species, growing rate, density, distribution or plant height [
20]. According to Hesp [
6], plant density is probably the most influencing factor because of its effect on wind speed and sand deposition. A high plant density decreases the degree of near-surface flow penetration, while drag increases [
5].
Apart from all these natural factors, human activities are an important component, especially in the last century, which is characterised by an increased human pressure on the coast; erosion, fragmentation, dismantlement, as well as interference in the biosphere, are only some of the human-induced perturbations. In recent years, coastal managers have tried to attenuate the influence of human pressure, conceiving and applying different restoration techniques aiming to reactivate original morphological and ecological integrity [
21]. These techniques principally employ two kind of approaches that are always integrated [
22]: (i) construction of “hard” structures, such as semipermeable wooden fences [
23,
24,
25], conceived to improve the deposition of wind-blown sand, to reduce tourist trampling and to protect the original dune species; (ii) re-vegetation efforts that have become very popular in recent years, due to the pivotal role that vegetation plays in this environment. Re-vegetation favours the consolidation of dune’s loose sediments thanks to the roots’ action; it dissipates storm wave energy and improves the capability of trapping the sand transported by wind, thus constantly incrementing dune growth. All these efforts have been recently defined as “dune gardening”, a term referring to modern coastal management which aims to maximise biodiversity and preserve priority species, resulting in a preservation of the dune status which mainly follows human wish lists rather than natural evolution, with little knowledge on dunes’ life stages in a wider and millennial context [
26]. Despite the lack of a wider and long-term vision, especially in the Mediterranean context where dunes and coastal environments have always been subordinated to human will [
27], recent remote sensing techniques can help to understand in detail the physical influence and the effectiveness of such restoration projects on dune environments.
Recent studies focused on the dynamic interaction between the beach and the dune system when restoration projects are in place [
28,
29], but due to the lack of high-resolution and large-scale morphological data across the entire beach-dune system, this interaction is still poorly understood [
30]. Furthermore, the sustainability and efficiency of such nature-based solutions requires a multidisciplinary approach combined with a long-term monitoring and a quantification of the dune dynamism with high-resolution data [
29]. With regards especially to vegetation replanting intervention, according to Sigren [
31], there is a lack of quantitative studies and a scarce knowledge of the impact of plants on protecting the dune from erosion.
In recent decades, the development of many high-accuracy remote sensing (RS) techniques has introduced several advantages: they now allow the acquisition of much more accurate data, with a much higher survey frequency. This can be fundamental to dune systems study, because it allows researcher to perform multi-scale analyses, that are crucial to understanding the complex connection between dynamic factors influencing dune fields [
32]. Among RS techniques, the most appropriate for usage in dune environment are: (i) Laser Scanning (LS), airborne or terrestrial, especially for high-resolution monitoring of linear-shaped morphologies [
33,
34]; (ii) Structure from Motion (SfM) photogrammetry, usually from Unmanned Aerial Vehicles (UAV) [
35,
36,
37,
38,
39,
40]. Sediment distribution related to vegetation types on coastal dunes can be investigated combining airborne multispectral and LiDAR dataset [
41], but low cost UAVs have several advantages, being usually light-weight devices, that are easily transported and very fast performing. This increases the surveys’ frequency and allows conditions for building high resolution orthomosaics, as well as point clouds and digital surface models with a resolution of few centimetres. Further advancements in the analysis of satellite images (i.e., Sentinel-2 programme) can now be applied for low resolution phenology studies of coastal dunes [
42], whereas huge potential is given by UAVs when combined with multispectral data for plant species discrimination at high resolution [
36,
43]. Attempts to distinguish among dune vegetation communities using UAV equipped with multispectral cameras (red, green and near-infrareds bands) have been performed by [
44] and allowed to produce vegetation maps at 0.15 m resolution by means of pixel-oriented and object-oriented algorithms.
For this study, a UAV monitoring program was conceived in order to survey the influence of a restoration project, undertaken by the local municipality, on the spatial vegetation cover and the geomorphic evolution of a residual coastal dune field. The restoration project aimed to stop the dune erosional trend and to reactivate the natural dynamics of the dune system by improving its natural resilience in a local context of strong and long-lasting human intervention and destabilisation of the coastal environment. The project included wooden fences and raised footpaths, and replantation of endemic species in the foredune in order to reduce the erosion that is mainly due to trampling by beach users on the seaward side of the dune. The aim of the paper is to test if low cost and high-resolution drone-derived products (RGB orthophotos and Digital Surface Models), combined with semi-automatic Geographic Information System (GIS) tools, are suitable and rapid methods to identify and quantify the spatial vegetation cover variation and the geomorphic evolution of the dune through time. The low-cost survey techniques and rapid GIS analyses used in this paper are believed to be replicable by local managers to easily track the overall effectiveness of restoration projects when expensive multispectral cameras or LiDAR data are not available.
2. Study Area
The area of study is located in Punta Marina, a renowned Italian seaside town along the North Adriatic Sea coast, within the Ravenna municipality. The dune field is a residual of a much more continuous dune ridge, which human activities connected to tourism have largely damaged and fragmented during the last 50 years by [
45,
46]. The Ravenna coastal belt is about 50 km long and is dominated by intermediate to dissipative, mild-slope beaches [
47]; the majority of these beaches are oriented NNW-SSE, according to the dominant currents and wind direction, and the beaches show an erosive trend, that is connected not only to human activities and land use, but also to strong alongshore drift, typically from South to North in this part of Adriatic sea [
48,
49].
In spring/summer, the aeolian climate is dominated (in terms of energy) by SE and NE wind [
50] (“Scirocco” and “Bora”, respectively), while in the autumn/winter season winds blow more frequently from W-WNW [
51]. Marine condition are dominated by SE and NE currents, with a low-energy wave climate [
52], while the tidal regime is semidiurnal and diurnal having comparable amplitude [
53,
54], from 0.3–0.4 m (neap tide) to 0.8–0.9 m (spring tide).
The investigated dune field is located along a 500 m section of the urbanised Ravenna coast, which is the only area where building is forbidden by law and economic exploitation of the beach is off-limits in an area also known as “public beaches”. In this segment of the coast, beaches are largely protected by breakwaters and groins. The beach has an average width of about 70–80 m, and periodically it receives nourishments to counteract beach erosion [
55].
From a morphological point of view, the dune ridge is constituted by two segments (a northern and a southern one), separated by a large access pathway to the beach. The whole dune is constrained on both lateral sides by walkways and, on the backside, by a dense pinewood, grown above the paleo-dune ridges. The natural morphology of this dune is also influenced on the seaward side by sand mounds (“winter dunes” or “artificial dunes”) artificially built by beach scraping and sand nourishment, accumulating sand taken from adjacent zones [
56,
57], with the aim of protecting touristic structures (beach huts, also locally called “bagni”) during the winter season. The dune presents a “barchanoidal” shape, influenced by the artificial dune accumulation that is carried out every winter [
58]. Despite all these artificial constrictions, the dune ridge preserves its natural dynamics, exceeding in some parts an elevation of 5.80 m above m.s.l.
With regard to vegetation, the typical zonation of the north Adriatic coastal dunes, sea to land sequence, firstly consists of annual pioneer species,
Salsola kali–Cakiletum maritimae (EUNIS Code B1.12), growing closest to the shoreline. Then
Agropyretum, Echinophoro spinosae–Elymetum farcti (EUNIS Code B1.3), which includes the so-called psammophilous vegetation, from coastal sandy and fine-pebbly dunes throughout the Mediterranean region [
59], as well as the
Echinophoro spinosae–Ammophiletum australis (EUNIS Code B1.3) class; together these two classes usually occupy the largest part of the dune surface and represent the semi-stable vegetation of the dune. Moving landward, according to Pignatti (1952), the grey dune vegetation is made by “perennial dry short-grasslands, whose structure is mainly determined by a thick carpet of cryptogams, among which there are therophytes, hemicryptophytes and chamaephytes”, namely the
Tortulo–Scabiosetum class (EUNIS Code B1.4); the Juniperus communis and the
Hippophaetum fluviatilis association (EUNIS Code B1.63), which represents the bush strip on the stable dune. Lastly, a coastal pine woods (EUNIS Code B1.7) closes the sequence [
60].
In May 2015, local authorities conceived a plan for the dune restoration to reduce the increasing erosion and contain the geomorphic and vegetation deterioration. In 2015, before the implementation of dune restoration actions, the erosion was mainly due to trampling by beach users as accelerated by wind action, and it occurred primarily on the seaward side of the dune, as well as in several blowouts located in the northern section. To improve sand trapping [
61,
62,
63], the restoration project included a 1-m elevated wooden pathway crossing the dune system and a wooden fence, built in front of the dune foot. In 2016, 5500 native dune plants were planted to accelerate growth of the incipient dune and strengthen stability of the whole system. The principal species chosen were
Agropyron junceum and
Ammophila arenaria, renowned to have a strong effect on the wind flow thanks to its high and dense canopies [
64] and
Euphorbia paralias, that is the species with the highest capability in sand trapping and dune stabilization [
49]. The surveys were limited to the zone within the red line in
Figure 1, with an extension of about 31,568 m
2. This specific area of interest was chosen in order to delimit the morphological analysis exclusively to the dune’s domain area, to exclude the pinewood, which would have strongly influenced both classification and morphological analysis.
6. Conclusions
The UAV monitoring campaign turned out to be suitably accurate to study coastal dune dynamics, especially for two reasons: firstly, the practicality of these devices, united to their lightness, affordability and operating velocity, in comparison to traditional methods; and secondly, the synergy between high-resolution morphological data and spectral RGB data represents a huge advantage when conducting 2D analyses of vegetation dynamics and its relationship to topography.
The limit of photogrammetric surveys in this kind of environment is represented by the complexity and, at a certain level, the impossibility of a complete separation of the bare morphological components (i.e., ground elevation data) from the vegetation, or any other structure or artificial object situated on the soil surface. This factor strongly complicates detailed morphological considerations, and high-resolution topological data can be assumed as realistic within a limited distance from the sea, where vegetation spreading and growth have little or no influence.
Semi-automatic classification algorithms, able to work on RGB spectral data, represent a rapid and useful way to monitor the vegetation cover when related to morphological data. The centimetric resolution of UAV data allowed elaborations that until today were much more approximate, mainly due to the use of spectral data from satellites that, in the best case (i.e., if the satellite data are recent), have a metric resolution. The availability of small high resolution sensors, like RGB cameras or multi-spectral cameras, that UAV can transport is increasing, opening a new approach for high spatial resolution studies: species identification, health conditions of single plants, interaction between transport of loose sediment underneath single plants or within a highly vegetated area, could be just a few future developments, which will highly improve the knowledge of the multi-scale dynamics and interdisciplinary processes of coastal environments. Further developments should involve the refinement of the survey methodology, aimed to reduce the influence of environmental light conditions on the spectral information assessment, as well as considering the seasonal factors that controls vegetation phenology.