Spatial and Temporal Changes of Tidal Inlet Using Object-Based Image Analysis of Multibeam Echosounder Measurements: A Case from the Lagoon of Venice, Italy

Lukasz Janowski,Aleksandra Kruss,Agnieszka Kubowicz-Grajewska,Emanuela Molinaroli,Jaroslaw Tegowski,Fantina Madricardo,Stefano Fogarin

doi:10.3390/rs12132117

Abstract

Scientific exploration of seabed substrata has significantly progressed in the last few years. Hydroacoustic methods of seafloor investigation, including multibeam echosounder measurements, allow us to map large areas of the seabed with unprecedented precision. Through time-series of hydroacoustic measurements, it was possible to determine areas with distinct characteristics in the inlets of the Lagoon of Venice, Italy. Their temporal variability was investigated. Monitoring the changes was particularly relevant, considering the presence at the channel inlets of mobile barriers of the Experimental Electromechanical Module (MoSE) project installed to protect the historical city of Venice from flooding. The detection of temporal and spatial changes was performed by comparing seafloor maps created using object-based image analysis and supervised classifiers. The analysis included extraction of 25 multibeam echosounder bathymetry and backscatter features. Their importance was estimated using an objective approach with two feature selection methods. Moreover, the study investigated how the accuracy of classification could be affected by the scale of object-based segmentation. The application of the classification method at the proper scale allowed us to observe habitat changes in the tidal inlet of the Venice Lagoon, showing that the sediment substrates located in the Chioggia inlet were subjected to very dynamic changes. In general, during the study period, the area was enriched in mixed and muddy sediments and was depleted in sandy deposits. This study presents a unique methodological approach to predictive seabed sediment composition mapping and change detection in a very shallow marine environment. A consistent, repeatable, logical site-specific workflow was designed, whose main assumptions could be applied to other seabed mapping case studies in both shallow and deep marine environments, all over the world.

Highlights

Precise maps of the seabed are critical for the observation and exploration of marine environments, giving the possibility to describe the physical, biological, and geological aspects of the ocean
A bathymetric comparison between the surveys suggested a similar arrangement of geomorphological structures and progressive deepening and extension of scour holes
This study concerned the importance of research approaches and methodical tools that could be used for marine environment assessment of various scales

Summary

Introduction

Precise maps of the seabed are critical for the observation and exploration of marine environments, giving the possibility to describe the physical, biological, and geological aspects of the ocean. They are crucial for sustainable management of coastal areas, and for marine spatial planning and benthic habitat mapping, forming a basis for designating the Marine Protected Areas (MPAs) [1]. The development of standardized mapping methods is required to establish good environmental status (GES) of Descriptor D6, Seafloor Integrity, of the European Union Marine Strategy Framework Directive (MSFD; D6 Commission Decision (EU) 2017/848 of 17 May 2017). The authorities of some countries are aware of seafloor exploration, establishing national mapping programs, such as RITMARE, MAREANO, MAREMAP, AusSeabed, and INFOMAR [3,4,5]

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