Dune restoration projects that integrate vegetation efforts with natural, sustainable, and soft solutions have become increasingly popular in coastal communities. Eco-restored dunes have high aesthetic value and are likely to be more resilient to near-surface wind effects and/or minor storms events. However, environmental-friendly restoration strategies still lack scientific insight from systematic research and the role of eco-restoration in dune erosion resistance and resilience has not yet been rigorously investigated. The question is: how can we optimize eco-friendly solutions in dune ecosystem restoration to maximize the resistance and resilience of coastal dunes against erosion from wind, wave, and storm surge attack? The present paper proposes an innovative colloidal silica-based consolidation technique for coastal sand dunes. A special focus is placed on the description of the experience gained from two pilot field studies started in Salento coastal area, in the south of Italy. The preliminary observations clearly show that mineral colloidal silica increases the mechanical strength of non-cohesive sediments allowing the adopted solution to reduce the volume of dune erosion and the dune scarp retreat rate, thus improving the resistance and resilience of the dune system. Besides furthering our knowledge of coastal erosion, the results of these and future studies will be of value to coastal managers and policy makers responsible for dune restoration projects.
Coastal sand dunes are known for their eco-functions and aesthetic qualities providing a unique habitat with high value of biodiversity for flora and fauna. Furthermore, coastal dunes are recognized for their protective capabilities as they act as natural barrier against flooding due to storm surges and wave attack (Tomasicchio et al., 2011a; Tomasicchio et al., 2011b, D’Alessandro et al., 2012; D’Alessandro and Tomasicchio, 2016). Unfortunately, over the years, many coastal dunes have been damaged by natural forces, human activities or a combination of both. In Europe, it has been estimated about 70% of dune systems loss during last century mainly due to strong anthropogenic pressure and urbanization processes (Brown and McLachlan, 2010). In particular, in Italy, coastal dunes are among the most threatened environments; the existing portions of dune habitat are subjected, in particular during summer, to intense trampling and degradation due to uncontrolled access of tourists. Problems with disruption of vegetation cover commonly arise where pedestrian and vehicles beach access is poorly managed (Dahm et al., 2005).
Thus, it is evident that there is an urgent need to preserve such as valuable eco-systems from erosion and, for this reason, the coastal community is paying attention to find new environmental-friendly solutions for dune restoration.
The society for ecological restoration (SER) defines restoration as “the process of assisting the recovery of an ecosystem that has been degraded, damaged or destroyed. It is an intentional activity that initiates or accelerates ecosystem recovery with respect to its health (functional processes), integrity (species composition and community structure), and sustainability (resistance to disturbance and resilience)” (Martínez et al., 2013).
The most recent contribution in literature refer to the resilience of vegetated coastal dunes (Figlus et al., 2014; Ayat and Kobayashi, 2015; Silva et al., 2016; Ajedegba et al., 2109) and to hybrid coastal protection structures (Wamsley et al., 2010; Spalding et al., 2014; West, 2014; Figlus et al., 2015). However, substantial knowledge gaps still exist in understanding the role of efficient vegetative restoration techniques and/or natural, sustainable and soft solutions in dune resistance and resilience.
The EUropean Maritime and Environmental (EUMER) laboratory at University of Salento, in collaboration with local partners, coordinates the current research project titled “Sperimentazione di tecnologie innovative per il consolidamento di dune costiere – Experimental investigation on innovative techniques for coastal dune consolidation (INNO-DUNECOST)” which focuses on the proposal and verification of an innovative and environmental-friendly technique for consolidation of sand coastal dunes. The proposed solution aims to reduce the (i) near-surface wind effect, (ii) impact of more frequent minor storms and (iii) events that continue to erode away the base of coastal dunes; it makes use of a mineral colloidal silica-based grout. The mineral grout injected into the sand dunes is similar to the material naturally present locally, both from a mineral and a chemical point of view, that is why the colloidal silica is like a “liquid sand”.
The present paper describes the recent experience gained from two pilot field studies started in Salento coastal area, in the south of Italy, and tries to suggest future trajectories of research on this topic.
Materials and methods
Coastal dunes are landforms that occur when there is a sufficient supply of sand-sized material (within the size range 0.2-2.0 mm) and strong enough wind to promote sediment transport and, often, some type of an obstacle, vegetation being the most common, that allows the blown sand to accumulate.
The beach-dune system is a naturally dynamic environment and its morphology is determined by a variety of factors ranging from climate variability, relative sea level, wind and wave energy, and vegetation (Sloss et al., 2012). At the most basic level, dunes can be divided into those that form from the direct supply of sediment from the beach face (primary dunes – Figure 1), and those that form from the subsequent modification of primary dunes (secondary dunes). Primary dunes are composed of sand blown directly from the beach face (active beach), whereas secondary dunes develop following the subsequent modification of primary dunes. Primary dunes are the dunes closest to the shoreline, dynamically linked to beach processes and significantly influenced by wave action; they constitute the subject of the present work.
Vegetation is a crucial element in the evolution of dune landscapes. It is necessary to trap the sand in order to make for dune growing, but also to stabilize the ground. The pioneer species, by this action, will facilitate the establishment of other species, increasing biodiversity richness (flora and fauna).
The main driver for coastal sand dune erosion is the near-surface wind vector (speed, direction) consistent with standard formulations of aeolian sediment transport models (e.g. Bagnold, 1941; Kawamura, 1951). Grains are readily detached from each other and their shape, size and density aid movement by the wind. Sand drift resulting from wind erosion can cause considerable economic and environmental damage.
Wind velocity is generally reduced by plant cover, encouraging deposition and trapping of wind borne sand. However, it may also accelerate locally in gaps between plants, especially those having a clumpy form.
Beyond the wind velocity reduction and the deposition of wind-blown sand that can be obtained by vegetation and/or traditional techniques (e.g. dune-forming fences), an alternative solution to reduce the wind-induced erosion process at dune systems is to consolidate their composing material stabilizing the surface sediment by increasing its mechanical strength. Specifically, the injection of a mineral colloidal silica-based grout, into the most exposed face of the dune composed of the native material, will allow (i) to increase the mechanical strength of non-cohesive sediments significantly reducing the volume of dune erosion and dune scarp retreat rate and (ii) the coastal dune to get back to its natural dynamics, thus improving the resistance and resilience of the system.
Innovative eco-friendly dune consolidation technique
In the present work, an innovative mineral grout colloidal silica-based consolidation technique for coastal sand dunes is proposed and tested/verified. It represents an environmental friendly solution, long-lasting and extremely easy to apply.
The colloidal silica-based grout is a non-toxic transparent mineral suspension of nanometric particles of silicon dioxide (SiO2) like “liquid sand”; it is a harmless substance for the environment with different uses in research and engineering projects (Kakavand and Dabiri, 2018). The colloidal silica-based grout presents low values of viscosity which allow
s to easily penetrating inside very fine sand and silty ground. Due to its polymeric characteristic, the mixture resulting by the injection of colloidal silica into the sand can contribute to increase considerably the mechanical/geotechnical behavior of the native/loose sand and to waterproof partially the treated area. The sand-colloidal silica mixture is characterized by lower values of hydraulic conductivity than the original/natural sand; this result does not compromise the drainage efficiency and the permeability properties of the dune system that is slightly altered due to the porosity decreasing. As presented in the following section, the addition of an accelerator (water with 10% Sodium Chloride - NaCl) increases the consolidation rate of the sand-colloidal silica mixture.
The field technical operations consist of different phases: irrigation, perforation, and injection.
In order to evaluate the efficiency/suitability of the innovative proposed technique, a monitoring activity is conducted by surveys of the dune ante and post operam. Specifically, temporal elevation and volumetric changes in sand are quantified using remotely piloted aerial system (RPAS) (or “drones”) photogrammetry, ground terrestrial laser scanning (TLS) of different dune components, measurements using satellite positioning with GPS or DGPS tools and conventional survey methods (e.g. simple optical measures using rods and measuring tapes; field leveling using optical or laser theodolite and levelers). The complements of such research techniques include the analysis of topographical maps, aerial and field photos, and videos.
Monitoring of the dune will illuminate the role of the consolidation process in volume accretion and erosion after wind and/or minor storms events further contributing to coastal dune restoration and management.
Once we better understand the role of the consolidation in coastal dune resistance and resilience, we may be able to optimize the protective capabilities of restored dunes.
Moreover, the intervention is designed according to the need to safeguard the environmental biodiversity, in order to protect the entire habitat. In this path, all precautions will be taken to preserve the existing vegetation, with particular care for the most valuable elements, avoiding the injections into the portions where the radical vegetative contribution is developed.
Testing of sand-colloidal silica mixture
Laboratory tests have been performed at EUMER in order to evaluate the influence of an accelerator additive (NaCl based) on the consolidation behavior of sand-colloidal silica mixture.
With the aim to evaluate if the natural sand salinity is sufficient to activate the consolidation process of the sand-colloidal silica mixture without any additive accelerator, a comparison between two samples of sand-colloidal silica mixture, respectively with and without additive accelerator, has been performed (Figure 2). The laboratory tests showed that the natural sand salinity is not sufficient to completely activate the consolidation process of sand-colloidal silica mixture. In particular, the entire volume of the sample of sand with injected colloidal silica-based grout, in presence of the accelerator, resulted successfully consolidated, whereas, without accelerator, only a partial consolidated volume has been observed (Figure 3). The addition of accelerator evidently increased the rate of consolidation of the sand-colloidal silica mixture.
The two field sites
The Salento coast is blessed with an amazing variety of coastal dunes. Over the past several million years sand has accumulated on the continental shelf and it has been moved landwards as a result of sea-level rise and wave and wind action. Two field sites, located at Otranto and Porto Cesareo, along the Adriatic and Ionian Sea, respectively, have been selected for the application of the proposed innovative eco-friendly technique (Figure 4).
The field site at Otranto (Figure 5a) is located at the Alimini beach, between two sites called “I due Mori” and “Fico d’India”. The considered stretch of coast, selected mainly because of its exposure to strong winds, extends for approximately 25 meters and it is characterized by dunes with good state of vegetation, and sand with a nominal diameter, Dn50, equal to 0.25 mm. The more frequent winds blow from N, NNW, and S. The extreme events, with intensity comprised in the range between 7 and 22 kts (3.60, and 11.32 m/s, respectively), come from N, and S, with percentage occurrences up to 38% during the winter (Lisi et al., 2011).
The field site at Porto Cesareo (Figure 5b) is located in a natural reserve, between two sites called “Serra degli Angeli” and “Palude del Conte”. The considered stretch of coast extends for approximately 25 meters and it is characterized by dunes with good state of vegetation, and sand with a nominal diameter, Dn50, equal to 0.50 mm. The more frequent winds blow from NNW. The extreme events, with intensity comprised in the range between 7 and 22 kts (3.60, and 11.32 m/s, respectively), come from NNW, SE, SSE, and S, with percentage occurrences up to 20% during the winter (Valentini et al., 2019). Wind data are available at https://windfinder.com.
Pilot field study
The pilot field study carried out at Otranto solely is described, as it is representative of both cases. The set-up of the intervention site has been fast and did not produce any impact on the natural environment (Figure 6).
Before starting the experimental testing, the existing dried out elements of the plants have been removed also in order to facilitate the monitoring activity (after the installation of fixed points).
The intervention consisted of different operational phases: irrigation, perforation and injection (Figure 7). During the irrigation phase, mineral colloidal silica-based grout has been sprayed on the exposed dune face for an average thickness of 7 cm allowing to create an armor layer in order to make easier the subsequent injection phase (Figure 7b). A fill valve tube, with a 16 mm diameter, has been adopted for the injection of the mineral colloidal silica-based grout inside the dune. The tube has been temporary inserted vertically by hand (perforation phase) for a length of 0.50 m within a mesh 0.30 x 0.30 m; a few hours after the injection phase, it has been lifted up (Figure 7c). The equipment used to perform the injection phase consisted of: (i) high-speed mixer, (ii) mix stirrer, (iii) piston injector, and (iv) flow and pressure regulator.
Visual observations of the dune before and after the intervention make evident a slight improvement of vegetation growth as shown by the appearance of green and tender branches on the top of the dune and an average deposit from 10 to 30 cm of sand on the exposed dune face (Figure 8).
Figure 9 shows the eco-restored dune after the consolidation process. As a result, the proposed intervention gave an aesthetic value also increasing the dune resistance and resilience to near-surface wind effects and/or minor storms events.
The results from the monitoring activity, still in progress, will give further insight on the efficiency of the proposed technique to optimize eco-friendly solutions in dune ecosystem restoration to maximize the resistance and resilience of coastal dunes against erosion from wind, wave, and storm surge attack.
Effective dune management and restoration could play pivotal roles in mitigating the erosion process globally associated with natural and human forcing (population and development growth, rising sea level, and increasing vulnerability to more frequent and intense wind/wave storms). The research activity described in the present paper aims to contribute to improve the knowledge regarding efficient natural, sustainable and soft eco-restoration techniques in dune resistance and resilience.
Main results from preliminary observations gained performing two pilot field studies in Salento coastal area, in the south of Italy, are listed below:
eco-restored dunes by the proposed innovative mineral grout colloidal silica-based consolidation technique have high aesthetic value and are likely to be more resistant and resilient to near-surface wind effects and/or minor storms events;
colloidal silica-based mixture increases the mechanical strength of non-cohesive sediments allowing the adopted solution to reduce the volume of dune erosion and the dune scarp retreat rate;
visual observations of the dune before and after the intervention make clear a slight improvement of vegetation growth as shown by the appearance of green and tender branches on the top of the dune and an average deposit from 10 to 30 cm of sand on the exposed dune face;
as an advantage respect to more traditional solutions for dune restoration (e.g. dune-forming fences), the proposed intervention, with zero impact on the environment, does not increase the dune size footprint avoiding further obstacles; this aspect can be relevant especially if the restoration intervention may take place on a beach area important for tourism and recreational purposes;
after these mentioned satisfactory preliminary observations, additional physical model tests will be conducted at the wave flume of the EUMER laboratory at the University of Salento aiming to (i) investigate the behavior of the consolidated dune under regular/irregular wave action and also (ii) test the influence of soil properties (cemented sand bulk density and particle size distribution), on the mechanical strength related to hydrodynamic loadings (e.g. bed shear stress testing). These experiments will give guidelines for dune restoration, bioengineering, and management, ultimately providing tools to preserve dune ecosystems and increasing the resilience of both urban and natural coastal communities.
The authors gratefully acknowledge the public administration of Otranto city and the “Area Marina Protetta” at Porto Cesareo for their helpful assistance in organizing the pilot field studies.
The present work has been supported by the Regione Puglia through the grant to the budget of the project titled “Sperimentazione di tecnologie innovative per il consolidamento di dune costiere (INNO-DUNECOST)”, POR Puglia FESR FSE 2014-2020 - Sub-Azione 1.4.B, Contract n. RM5UKM2.