The world’s most famous lagoon not only has historical significance for the city of Venice (Figure 1), but is a fascinating, ever-changing natural environment. The lagoon consists of approximately 8% dry land, 12% open water and 80% tidal mudflats and salt marshes. This environment is home to complex ecological and morphological systems, which are influenced by tidal cycles, as well as freshwater and sediment inflow from river systems. Monitoring is becoming increasingly important because of changing sea levels and increasing anthropogenic influence, such as the MOSE flood protection project, which allows connection to the Adriatic Sea to be closed, as well as dredging of the shipping channels. High-resolution bathymetry surveys are critical to the investigations and inspections that monitor the changing environment of the Venetian Lagoon. The data must be gathered quickly and cost-effectively, because of the requirement for large-scale repetition of the measurements. The water depth in relevant areas is often as shallow as 1 m. It has therefore been proposed to use a shallow water wide swath multibeam with a large coverage, with the possibility of mobile installation so local boats with shallow draft can be deployed. A pilot study of the locale involving research and monitoring of the lagoon has been conducted by the ISMAR-CNR research institute, in collaboration Kongsberg Geoacoustics.
The field study area
A number of representative areas in the Venetian Lagoon, which has a total surface of 550 km2, have been examined. The lagoon has three inlets to the Adriatic Sea. The average depth is 0.8 m and typical features are natural navigation channels with depths of about 20 meters, tidal channels and streams of several meters depth, and tidal flats and salt marshes. The lagoon originated about 6000 years ago, during the last marine transgression. It has previously experienced severe morphological changes in short periods, for example, the proportion of salt marsh has decreased by more than 50%, from 68 km2 to 32 km2 in the years 1927 to 2002. Major transformation at the inlets (MOSE project) are currently underway, so it has become necessary to detect sudden changes quickly and reliably to counteract negative trends in the short term. This is done by repeat measurements in ‘hot-spot’ areas, which show very high erosion and sedimentation rates, where changes can be detected most easily. Figure 2 illustrates areas in which test measurements were made. The Scanello channel and adjacent salt marshes are used as an example in this article. The area has been subject to previous research in which, among other things, the hydrodynamics of the channel could be related to sediment erosion and deposition. The Scanello Channel is a natural tidal channel with water depths of 0.5 m – 7 m so the hydrographic environment is challenging due to low water depth and complex hydrodynamic processes. The tidal range is about 1 m, whilst tidal velocities of 0.2 ms-1 and 1 ms-1 can be observed. The water sound profile shows strong temporal changes that are caused by salt and fresh water mixing and sun heating (see Figure 3).
Methodology
In order to carry out a high-resolution bathymetry survey with full coverage the Kongsberg Geoacoustics shallow water multibeam GeoSwath Plus was chosen. The system applies the phase measurement bathymetric sonar principle to acquire high resolution bathymetry and co-registered geo-referenced side scan data with a beam angle of 240 ° x 0.75 ° (in the used 500 kHz version). A measurement of the water surface to nadir is thus given, without tilting the transducer. The coverage depends on the quality of the backscattered signal and reaches extreme shallow water values ??over 12 times water depth. The peripheral sensors used were a Hemisphere V101 GPS compass, registering position data with differential GPS correction and heading data, a TSS DMS05 motion sensor measuring roll, pitch and heave motion as well as a Valeport MiniSVS sound velocity probe. The motion sensor is combined with the compact 500 kHz transducers in the transducer mount on a pole, on the starboard side of the vessel of opportunity with a draft of c. 0.5 m. At the upper end of the pole the GPS compass was installed so that the sonar system including peripheral sensors were all combined on the rigid pole, which facilitates the installation and calibration of the system and avoids relative movements between the sensors as well as vibrations. Additionally a Valeport MiniSVP probe was used to regularly record sound velocity profiles. The data was acquired with the system’s own GS+ software. Further processing was done with CARIS HIPS / SIPS and for data visualization both, GeoSwath + and QPS/Fledermaus software was used. System calibration was performed with the help of patch test data recorded locally.
Data results and discussion
Figure 4 shows the bathymetry and side scan data of Scanello Channel. A coverage of up to 15 times the water depth was reached in areas of less than 1 m depth below the transducer. The data also show that the channel slopes can be imaged up to the water surface. The channel reaches as a branch of the main channel into a salt marsh. Following a 200 m long section to the east bend of the canal to the north to get to 300 m split into two arms ending in shallow mudflats. In correspondence with the cut bank a typical depression of 7 m can be observed (Figure 5). The point bar has a characteristically smaller slope. The erosion-deposition pattern is characteristic of a meandering tidal channel and sediment waves can be observed with a wavelength of several meters, which increases with greater depth. The lack of sediment waves in the deepest part indicates a high rate of flow in these areas. A further depression can be observ
ed in the confluence area of ??the channel bifurcation. The side scan data partly reflects the morphological features. In flat the different grey levels indicate changes in the seabed type. The data is used to classify different seabed types and establish their relationships with hydrodynamic parameters such as flow rates as well as different marine habitats, such as native oyster banks that are important to local fisheries.
Conclusions
The study in the Venetian Lagoon has shown that it is possible to perform full coverage high resolution bathymetry and side scan surveys in extremely shallow water areas in order to image relevant morphological structures from the deep seafloor up to the water surface. The measurements have sufficient repeatability to document changes in the structures and the GeoSwath Plus portable system can be easily and quickly installed on available boats. In extremely shallow water, an overlap of 12-15 times the water depth can be achieved, allowing for efficient measurements, which in turn supports the requirement for regular repeat surveys.
Figure 1: St. Mark’s Square with the Campanile and the bathymetry of the busy strait between the Piazzetta and the island of San Giorgio Maggiore.
Figure 2: The Venetian Lagoon is connected to the Adriatic Sea through three inlets. In the study, a number of areas were examined that are characterised by their specific erosion and deposition patterns. The data from Scanello channel near the island of Burano is considered as an example in this article.
Figure 3: Tidal data and sound velocity profiles for three locations in Scannelo channel. The survey was carried out between 14:00 and 15:00 h, during which the sound velocity profiles were measured. The tidal data was computed using a local high resolution tidal model. Even over distances of 400 meters in a 1 hour period relevant tidal height variations and strong differences in water sound profile of several m / s can be observed.
Figure 4: Bathymetry and side scan data from Scanello channel. The shallow water multibeam GeoSwath Plus provides complete coverage of the depth data up to the surface without tilting the transducer. In extremely shallow water coverage of 15 times water depth is achieved in a single transect, so that the whole surface was covered with only a few lines in less than 60 minutes. The geo-referenced side scan data was collected at the same time and can be used to distinguish seabed types.
Figure 5: Die Bathymetry of Scanello channel in 3D view shows the characteristic morphology of meandering tidal channels.
In a box
Authors:
1. Martin Gutowski (Kongsberg Geoacoustics, Great Yarmouth, UK)
2. Fantina Madricardo (Istituto di Scienze Marine – Consiglio Nazionale delle Ricerche, ISMAR-CNR, Venedig, Italy)
3. Federica Foglini (Istituto di Scienze Marine – Consiglio Nazionale delle Ricerche, ISMAR-CNR, Bologna, Italy)
For further information contact Dr Martin Gutowskimartin.gutowski@kongsberg.com