Combined hydroacoustic and optical mapping for marine munition dumpsite characterisation and DMM mass calculations

After the Second World War, large quantities of conventional munition were dumped in the German Baltic Sea. Most of the munition is concentrated in coastal munition dumpsites in water depths between 10 and 25 meters (Kampmeier et al., 2020). The toxic explosives they contain and their degradation products pose a threat to the local marine environment, as shown by Beck et al. (2021), Strehse, et al. (2020) and Schuster et al. (2021).

Despite these alarming news, the precise number of discarded munitions material (DMM) and its role in complex marine ecosystems is not yet fully explored. For the development of future remediation plans, the number, types and locations of the munitions needs to be known and an ecological and geological characterisation of the sites performed. Multibeam echosounding in combination with photo mapping are key-methods not only for spatial munition mass calculations, but also for detailed marine habitat mapping. High-resolution 400 - 700 kHz MBES data provide bathymetric maps with decimetre resolution on which DMM hotspots can be identified. AUV-based photo mapping can subsequently be used at certain locations to produce highly detailed photo mosaics and digital elevation models  in millimetre resolution. The combination of both mapping methods is used to compare two DMM hotspots within the German Baltic Sea, Kolberger Heide in Kiel Bay and Pelzerhaken in Luebeck Bay. We will show prominent differences in terms of type and number of munitions, corrosion and deposition status and local habitat. The goal is to transfer such details to the larger-scale MBES data in order to provide dumpsite- habitat characterisations and make DMM mass calculations within the Baltic Sea more robust.