The quantified size of both positive and negative surface areas appears in the upper rows of Table 1. The negative surface area is wider than the positive surface area. Nevertheless, some general observations can be made. The result of this comparison is not high-resolution, due to the low accuracy of the bathymetry from 2017. The comparison of these two bathymetries ( Figure 9A) covers only the area of the bathymetry from September 2017, which is smaller than the other two. It is important to note that the areas of lower slope displaying erosive features have moved toward the southeast margin if compared with the previous bathymetry. On the other hand, the erosive features of the deeper area are also evident ( Figure 6D). These indicate a sedimentary bypassing of sandy materials from the fluvial area to the estuarine mouth.
Other dune fields also extend into the northeast area, bordering the escarpment ( Figure 6C). Between this ridge and the escarpment, fields of bedforms (3D dunes) have developed. This southernmost part of the crest is formed by attached individual cones separated by depressions ( Figure 6B), whereas the northern part is formed by individual cones. The ridge formed by the conical elevations observed in 2017 was much more evident in 2018. It is significant that the runnel separating this bulge field from the escarpment, observed in 2017, has nearly completely disappeared in this new bathymetry. The surface of these bulges reveals an irregular, rough appearance ( Figure 6A), which suggests the presence of erosional processes. The good resolution of this new bathymetry allows us to observe how the bulge field on the southwest margin described in the previous bathymetry changes geometry and dimensions. This bathymetry allowed a better visualization of the floor features ( Figure 5). The bathymetry performed in 2018 not only had a better fit of data between the lines obtaining a better resolution, but also covered a wider area than that performed in 2017. From a tectonic point of view, it is astable zone with no vertical movement of tectonic origin, having been described in this area since at least the Holocene period. The Iberian Pyrite Belt is one of the largest sulfide ore deposits in the world and has been exploited since at least 3000 BC. The drainage basin excises the materials of the Iberian Pyrite Belt, which forms part of the South Portuguese Zone, the southernmost unit of the Variscan Iberian Massif. This river rises in the Huelva mountain range, and its course, which is approximately 100 km long, crosses the province until it flows into the estuary. The Tinto estuary is the eastern branch of this estuarine system and receives contributions from the Tinto river. Both estuaries have a common marine sector but function as separate estuaries into the central and fluvial domains. This estuary consists of the shared estuaries of the Tinto and Odiel rivers ( Figure 1). In the central sector of the Huelva coast is the estuarine system known as the Ria de Huelva. These changes in dynamicsled to erosion and deposition in various areas of the bed. The data in this work document the changes in the dynamics of the estuary in relation to these vertical movements. The evolution of the topographic position of the floor was also analyzed by comparing different records of the estuarine bed in the margins of the stockpile. A crest formed by cones and a bulge area could be interpreted as injection structures. This manuscript describes several surficial features using multibeam echosound. Part of the sediment was injected into the marginal areas, where the load pressure is minor, causing significant vertical movement in the floor of the estuarine channel. The weight of that enormous amount of waste produced active subsidence in the underlying sediment. This waste was stockpiled in a stack 25 m high, covering 1200 ha of what was once a salt marsh. For more than 40 years, the industrial complex which developed near Huelva (in SW Spain) produced a huge amount of phosphogypsum as a waste product of manufacturing fertilizers.
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