The issues of hydro-sedimentary processes in lagoons, estuaries, and enclosed bays are important in terms of coastal management, because they are generally characterized by a strong human presence that results in the release of contaminants and large quantities of nutrients into the ecosystem [1
]. There are many methodologies to study the nutrients in ecosystems; one of them, possibly the most used, is the three-dimensional (3D) numerical model.
The use of numerical models as a tool to anticipate the effects of human impacts or climate change effects on marine ecosystems has increased in the last decade. They permit to simulate hydrodynamic process, tides, currents, wave action, sediment transport, pollutant dispersal or biological connectivity. A large list of models is available, like CSIRO [2
], Delft3D-FLOW [3
], MARS3D [5
], MIKE 21/3 [6
]; MIKE3 [7
], MOHIDWater [8
], 3D-MOHID [9
], NHWAVE [10
], REF-DIF1 [11
], ROM [12
], SELFE [14
], SCHISM [16
], SHYFEM [18
], SHORECIRC [20
], SWAN [21
] or TELEMAC [22
]. Some of these models are mainly applied to shallow coastal waters. A synthesis of concepts and recommendations for model selection, mainly applied to coastal lagoons modelling, is presented by [23
In the Mar Menor lagoon, ROMS has been used to evaluate the water renewal conditions, and a time series spanning a period longer than one year (between 2010 and 2012) provides a record of the seasonal variability of the lagoon hydrodynamics, which was used to validate the hydrodynamic model implemented [24
]. SHYFEM has been used to compare the water exchange and mixing in 10 Mediterranean lagoons (including the Mar Menor) in terms of water exchange and mixing behavior. The authors found that the exchange with the open sea highly influenced the transport time scales, although the wind can also enhance the exchange mechanisms in lagoons with more than one inlet [18
]. The SHYFEM has been more specifically used for the Mar Menor lagoon under different dredging scenarios of the inlets, covering different dredging depths and extensions. From this study, it can be concluded that the impacts of dredging the channels on the hydrodynamics of a coastal lagoon increase with the magnitude of the dredging activities. In this sense, there is a threshold for the magnitude of these activities beyond which important environmental effects can be expected [25
]. Additionally, the SHYFEM has been used to model environmental parameters such as salinity and temperature [26
] as well as the connectivity between the Mar Menor lagoon and the Mediterranean using a Lagrangian model [27
], so as for predicting the effects of climate change [29
The 3D numerical models need precise and updated initial conditions (Xo, Yo, Zo) to run them [1
]. In all the cases, but mainly for applications in coastal zones, the applicability and usefulness of the results are highly dependent on the quality and spatial resolution of the bathymetric information and model grid setup, which determines not only the water column depth, but also information on bed roughness and the incorporation of specific structures and features that affect water and sediment transport dynamics. Furthermore, although the numerical models are very reliable, bottom conditions are dynamic and can change due to natural dynamics, human activities, disasters induced by cut-off lows (CoLs), such as flooding [30
], and changing of the coastline [31
]. Therefore, it is crucial to regularly update the information and to improve both the accuracy of bathymetric measurements and the interpolation techniques to build high-resolution grids [32
One of the approaches to get extensive bathymetric information in inland and coastal water masses with an increasing spatial resolution is developing different tools based in remote sensing techniques [37
In several places around the world, the CoLs contribute to increases in precipitation and in extreme rainfall events [42
]. In Spain, this atmospheric phenomenon has been known traditionally as the “gota fría”, and also recently (due to the influence of meteorologists) as “dana”; both refer to CoLs. In recent years, CoLs have become more frequent in South East (SE) Spain and they are modifying the morphology of the coastline as well as the depth, seagrass, and bottom of aquatic ecosystems, particularly of the Mar Menor lagoon. An upgrade was designed to update the depth data and thus maintain the accuracy of the 3D numerical models, using the best techniques already available [43
] or in development [44
During the last semester of 2019, there were two CoLs over the Spanish Mediterranean coast that caused environmental disasters and human life loss.
Remote sensing has been used to monitor these environmental disasters [45
]. Besides, remote sensing can also show the initial conditions in unaffected areas where the water is clear and the energy from the sun reaches the bottom. The energy in the water reaches the optical sensor placed above the water. The relationship between the water-leaving reflectance just above the surface and the water-leaving reflectance just below the surface is constant [46
]. If the water is clear, the energy arrives from the bottom through the water column [49
], in this case it is possible to determinate the water depth. The use of high-resolution satellite imagery over variable bottom types has allowed the determination of water depth with a new algorithm, which uses the relation between the neperian algorithms of two bands. The influence of the bottom is determined by a simple model, in which the bottom optical properties and the water optical properties are included [50
]. It is noteworthy that the first studies on water depth by remote sensing in the 1970s were by simple regression methods with one band or multiple regression methods using two bands [52
]. The second stage was to assess the influence of different types of bottom [55
]. On the other hand, the water depth using passive optical sensors were compared with depth water obtained from LiDAR data [56
]. According to the potential of the remote sensing, the images from different sensors have been used to determinate the bathymetric maps: Landsat 5 [57
], Landsat 7 [59
], Landsat 8 [61
], ASTER [63
], IKONOS [64
], Worldview 2 [65
], SPOT-4 [66
], GeoEye-1 [67
], and Pleiades [68
]. In previous works carried out in the Mar Menor lagoon, this approach using remote sensing has been used to calculate the extinction coefficient of light (K
). Then, after demonstrating its spatial heterogeneity, it was used to determine the concentration of chlorophyll a
in the water column in shallow areas where the reflectance from the benthic meadows interferes with the signal of the water column [69
]. Therefore, the calibration of the model requires field information provided by sounding equipment installed in ships, which cannot measure depths of less than 0.6 m [67
]. In the last few years this limitation has been overcome due to the use of an Unmanned Surface Vehicle (USV) [64
], which provides data from depths of just a few centimeters. The images from new sensors in satellites [70
], planes, and new platforms (such as drones) have provided new and better tools for the management of coastal areas since the 1980s [72
]. All remote sensing advances, tools, and techniques have been evaluated so that they can be adapted to new scenarios and to water management necessities [73
]. However, the management of each water body has specific problems that have to be analyzed in terms of the local conditions in order to get the best results in the least possible time. Often, it is very difficult to provide solutions because there is not enough information, above all when studying areas such as the one that concerns us, with great human and environmental pressure. Good management needs a fast and efficient methodology as the basis for prediction models in near real-time, such as the monitoring Mar Menor geoportal in near real-time. In any case, the first step is to get good and precise initial data to feed the prediction models. Therefore, the aim of the present work is the use of high and very high-resolution spatial data, as well as time series, to assess the sand movement and volume of shallow water in natural channels that connect the Mar Menor lagoon and the Mediterranean Sea.
The results show that multispectral optical imagery with very high spatial (i.e., 0.5 m) and temporal (in this study, 22 images) resolution is suitable for the tracking of water-level fluctuations of the Mar Menor golas and shallows at a fine scale. The results are a function of the different steps of the project; these are summarized as follows:
IMIDA06 USV has been developed by IPH and IMIDA and has provided a good bathymetry map (for 13 March 2017), with a spatial resolution of 50 cm and an error equal to ±3 cm (Figure 9
The NDWI values allow to evaluate the change of the water body and golas, as well as the annual movement of sediments and land area (Figure 10
, Appendix A
At the same time, the Water Depth Extraction Model [90
] allows to obtain a bathymetric map from different satellite images and the parameters of this model (m and n). These parameters are not constant and depend on the water clarity (Figure 11
The relationship between both methods showed a good fit. On one hand, the slope of the regression is close to 1 (0.84 in the case of low transparency and 1.02 with clear waters), indicating the accuracy of the satellite image estimations and field data. The RMSE was analyzed at three different dates (Figure 12
): one, five, and ten days of difference between the field data and the image. Firstly, for 2019, the RMSE2019
is 0.179 m. Secondly, the RMSE2017
is 0.186 m. Finally, in 2019, the image precedes the CoLs and the field data are subsequent to it; RMSE2019CoLs
= 0.311 m. Besides, the slope of the line is reasonably close to 1: 0.92, 0.84, and 1.02. The RMSE values represent the accuracy of the measurements and dispersion of the data, while the slope is the true measure of accuracy.
The bathymetric map obtained from the Pleaides satellite images of 11 November 2017 (Figure 13
) allowed to calculate the exchange water between Mar Menor and the Mediterranean Sea. The exchanged volume increased the water level at Mar Menor due to the low atmospheric pressures registered from 15 October 2012 to 28 November 2019 (Figure 14
The bathymetry, the barometric pressure, and the wind of the Mar Menor are responsible for the currents and therefore for the distribution of sediments in the lagoon. Therefore, an updated and high-resolution bathymetric characterization is especially important in order to know its evolution and its dynamics at different spatio-temporal scales. In addition, an accurate bathymetry is decisive to know the total volume of water in the lagoon and for hydrodynamic calculations, relevant to the determination of the water exchanges between Mar Menor and the Mediterranean Sea, through the three active golas. Mar Menor is a coastal lagoon with a high degree of hydrodynamic confinement and, as such, is the final recipient of all the transport processes that take place in the large (1350 km2
) watershed of Campo de Cartagena. The soil uses in the coastal area has undergone significant transformations in the last decade, which have contributed to the acceleration of these transport processes and, consequently, the bottom of the lagoon have experienced an increasing accumulation of sediments (Figure 15
) between 2008 and 2009 and between 2016 and 2017 (Table 4
and Table 5
Mar Menor underwent a generalized silting up (an average of +18 cm) between 2008 and 2016, mainly the result of sediment contributions due to the very frequent extreme precipitation events [47
]. Very intense rainfall events with a high return period occurred in the Campo of Cartagena in the last decade. As examples, the event of 27–28 September 2009 corresponded to a 200-year return period, where the CA12 rain gauge (located in La Palma) recorded 268 mm in 30 h. Then, during the event registered between 17 and 19 December 2016 in Campo of Cartagena (with a 500-year return period), the rainfall exceeded 50 mm in one hour in the Torre Pacheco (TP42) and San Javier (TP22) rain gauges [47
]. Finally, the CoLs event named dana
of 12–15 September 2019 generated maximum hourly rainfall intensities of 70.4 mm and 60.6 mm at the Torre Pacheco and Pozo Estrecho rain gauges, respectively. The maximum daily rainfall of 217.8 mm recorded by the Torre Pacheco rain gauge (12 September 2019 12:00 h to 13 September 2019 12:00 h) corresponded to a 500-year return period and generated severe flash floods in the Campo of Cartagena area. The estimations of return periods were based on [48
] by applying the SQRT-ETmax cumulative distribution function.
The deployment of an IMIDA06 USV is an example of how the free disposition of research findings can stimulate scientific advances, with the contribution of anyone, anywhere in the world. The images and their distribution are low-cost, which has allowed the development of new prototypes that, in turn, will be surpassed by novel work. Now it is necessary to take advantage of the developments from the recent years and make them useful to the community. In this sense, the IMIDA06 USV, as a source of field data, has allowed the development of a satellite image tracking system that is able to provide valuable data that helps solve environmental, social, and economic problems.
The Mar Menor lagoon and its area of influence is a complex and dynamic environmental, economic, and social system [94
]; therefore, distinct types of information are necessary for its good management. The use of 3D numerical models represents a solution to obtain data and predict situations in the short term. However, model inputs condition the results and, therefore, special attention should be paid to their accuracy and precision; e.g., the initial conditionals (Xo, Yo, Zo) have to be precise enough. Remote sensing provides information on large coastal water bodies, such as the Mar Menor lagoon, besides providing good precision for Xo and Yo. However, knowing the depth, even in the shallowest waters, requires transparent waters that allow the sunlight to reach and be reflected by the bottom.
The initial idea was to use an unmanned aerial vehicle or unscrewed aerial vehicle (UAV), but in this area the use of a UAV is prohibited due to military activities. However, it is possible to use high spatial resolution satellite images to obtain a real-time bathymetry with an acceptable error that can be applied in different fields, such as scientific research, technology, and management.
At the same time, the idea was to select several places where very high spatial resolution bathymetric data could be obtained, using an aquatic drone, and then relate this data to the high spatial resolution images from satellites.
During this work, different water situations were induced by CoLs. This allowed producing distinct equations according to the transparency of the water, and a good adjustment with an acceptable RMSE was obtained. The methodology was improved by introducing a new step, since after calculating the relationship between LnBlue and LnGreen, the analysis of the ratio values is necessary for choosing the appropriate algorithm.
The changes in the depth of the lagoon depend mainly on the sediment loads mainly transported by runoff during flash-flood events, which deposit soil and other particles on the bottom. This contribution can reach 1863 t yr−1
]. Contributions from the Mediterranean or exchanges through the inlets have not been studied in depth, but there is evidence that they can be significant throughout the recent history of the Mar Menor lagoon, especially during extreme storm events [96
]. Sedimentation rates have changed during the evolution of the Mar Menor lagoon linked to human uses in the drainage basin, and have increased with time, from 30 mm/century before the Middle Age to more than 30 cm/century in the 20th century, particularly due to deforestation and agricultural practices [97
]. To the best of our knowledge, no work had been done that allowed to know the recent changes and the spatial variability of the sedimentation rates in the lagoon.
In the present work, the lowest RMSE value obtained was 0.179 m and increased over time; in 2017 it was 0.186 m and after the CoLs of 2019 there was a random movement and deposition of sand according to the RMSE value.
In the Encañizada gola the deposition of sand meant that the channel, through which water enters and leaves the Mar Menor lagoon, has gradually closed over time (Figure 16
). In the Marchamalo and Estacio golas the deposition of sand meant that the channels were shallower compared to the last bathymetry carried out by the UPCT, in 2011 (Figure 17
It was demonstrated that multispectral images are enough to obtain a precise bathymetry in shallow water. In this case, the shallow water is defined by the fact that the sunlight reaches the bottom of the lagoon. It was possible because, as shown in the bathymetric map, the maximum depth in the Mar Menor lagoon is 7 m; however, a depth of 4 m is presented in the photic zone, and this is the depth of the lagoon that it has been determined using optical sensors from different platforms: satellite, aircraft, or UAV.
The hyper-high spatial resolution of the sensor allows to obtain enough spatial detail, while the high resolution provides a temporal vision to observe details of the influence of the tides. Since the acquisition of the images is always at the same time and in the same place, the variation in the amount of water can only be due to movement of sand or movement of water.
The generated maps provide much information for the management of such a complicated area, but the most important thing is not only the information itself, but that it can be generated in quasi real time, allowing stakeholders to have accurate data in a short time for decision making. The depth model allows the use of images, obtained by aerial drones, of very high spatial resolution (18 cm) or the ones that the stakeholder prefers, since the resolution depends on the drone flight height; it can even reach 3 cm. As the gola is a small area, the stakeholder could have the bathymetric map a few hours after the flight. The low cost of obtaining spatial information from an area with great dynamism between water and land will allow the development of predictive models of soil movements and of increases or decreases in the amount of water in the gola. These models will provide the stakeholders enough information to be able to take decisions about the area with the aim of enhancing its conservation and the use of its resources. If the studies had been limited to the Encañizadas gola alone (Supplementary Materials S2
), the sight of the importance of the other golas and the movement of water through them, between the Mediterranean Sea and the Mar Menor lagoon, would not have been known. Therefore, increasing knowledge of the movement of the sand in all the golas is fundamental for the area, in both economic and environmental terms (Figure A1
and Figure A3
Once again, remote sensing was shown to be a tool, perhaps the most effective and cheapest one, for obtaining precise data. It may be used in 3D numerical models and in the management of the aquatic environment. It is necessary to remember that the Mar Menor lagoon is a dynamic ecosystem and the quantities of water and sand in each location within it are changing over time. Thus, although the bathymetric map generated throughout this work is the most up-to-date, it will be necessary to validate it from time to time due to the dynamic nature of the lagoon.
The results obtained show that the sedimentation processes occurring in the golas have produced a considerable reduction in their storage capacity, according the bathymetry. Although no significant changes have been detected in the current water renewal time in the lagoon compared to those estimated in the 1980s [25
], the salinity has shown a tendency to approach the lower values of Mediterranean salinity rather than an increase [26
]. It would be important to know how the observed changes can affect the capacity for water exchange between the Mar Menor lagoon and the Mediterranean Sea. In addition, climate change will negatively affect the frequency and intensity of natural hazards, such as extreme rainfall events in the area. Therefore, more frequent and severe flash flood events are expected for the contributing basins of the Mar Menor lagoon, which would affect their hydrodynamic [29
] and sedimentation rates.
The bathymetries obtained from high-resolution satellite images, such as Pleiades, are demonstrated as very useful tools for complementing the bathymetries obtained with echo sounders in areas of shallow (<2 m) and clear water (Figure A2
). Consequently, the precision of the calculation of the water volume of the Mar Menor lagoon has improved over time with the incorporation of new, much more precise technologies, such as interferometric echo sounders.