Extremely Shallow-water Morphobathymetric Surveys: The Valle Fattibello (Comacchio, Italy) Test Case
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Marine Geophysical Researches (2005) 26:97–107 DOI 10.1007/s11001-005-3710-0
Extremely shallow-water morphobathymetric surveys: The Valle Fattibello (Comacchio, Italy) test case Luca Gasperini Istituto di Scienze Marine (ISMAR) Sezione di Geologia Marina – Bologna, Via Gobetti 101, 40129, Bologna, Italy (Tel: +39-051-6398901; Fax: +39-051-6398940; E-mail: [email protected]) Received 20 January 2005; accepted 4 May 2005
Key words: bottom classification, bottom reflectivity, coastal lagoons, sediment distribution, ultra-shallow bathymetry, Valli di Comacchio
Abstract A morphobathymetric survey has been carried out of a coastal lagoon south of the modern Po delta (Valle Fattibello, Comacchio). Acquisition of bathymetric data in such an extremely shallow water environment (more than 80% of the area lies between 0 and )1 m) faces technical difficulties that led to the development of ‘ad hoc’ solutions. Methods to obtain morphobathymetric and reflectivity maps of the lagoon are presented, including descriptions of data acquisition and processing. ‘Ground-truthing’ of the reflectivity data, provided by analysis of sediment samples, allowed description of the sediment distribution within the lagoon, and the imaging of sedimentary features. Combined analysis of morphobathymetry and reflectivity maps identified arcuate, sand-rich features not in equilibrium with the present-day ‘low energy’ regime of the lagoon. These features might constitute the substratum over which the lagoon system has been formed, not completely overprinted by anthropogenic and biogeochemical processes at the lagoon bottom.The system we have developed could be used on board remote-controlled or autonomous vehicles that could perform rapid, low-cost surveys and collect data useful for quantitative estimates of sedimentary dynamics. Rapidity and low-cost are important conditions for carrying out periodical surveys, and taking a series of ‘snapshots’ of the lagoon bottom. This would allow predictive geological models on sediment budgets of these complex environments, characterized by multiple sediment sources and sinks that are difficult to quantify.
Introduction Coastal lagoons, due to their natural and economic importance, require periodical environmental monitoring because they are prone to ecological crises often related to anthropogenic impact. The study of these ecosystems involves a combination of investigative approaches, such as biology, physical and chemical oceanography, and geology. The starting point of each of these studies is the knowledge, at different levels of accuracy, of the morphology of the sediment-water interface, i.e., the bathymetry, which is also important for describing the status of the system and monitoring its changes in time caused by sedimentary processes. The main characteristic of coastal lagoons is the extremely shallow bathymetry and the variability of bottom types, controlled by geological/ biological factors, such as mean grain-size of the sediments, erosional or diagenetic levels, presence of bioconstructional
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