Operational observations methods during offshore sand mining — case study in Tallinn Bay, the southern Gulf of Finland

Abstract

Offshore sand mining is increasing activity nowadays. Environmental impact of sand mining appears usually through the transport of suspended particulate matter (SPM) away from the actual mining area. Satellite remote sensing is efficient tool for the operational monitoring of SPM distribution during harbor dredging, but problematic in the case of sand mining, as SPM remains mainly below the water surface. We used satellite remote sensing, in-situ measurements of optical properties of seawater and combined wave, hydrodynamic and sediment transport numerical modeling for assessment of the area affected by sand mining in Tallinn Bay, the southern Gulf of Finland. Sand mining took place from October 2008 to April 2009 with short breaks that were random in time. Vertical profiles of spectral attenuation and absorption coefficients by spectrometer AC Spectra, underwater light field and albedo by radiometer Ramses-ACC-VIS were measured in situ. In satellite remote sensing MODIS images with 250 m spatial resolution were used for the qualitative estimation of the surface area that was affected by sand mining. Nested 2D hydrodynamic model and wave model SWAN with 400 m spatial resolution at mining site gave input fields of currents and bottom shear stresses to the Lagrangian type particle transport model. While in-situ measurements and satellite remote sensing give snapshot about the SPM distribution numerical modeling enables to have dynamics of the ongoing process. In-situ measurements showed that the concentrations of SPM were the highest at the mining operation. The thickness of the elevated SPM layer was about 6 m. Satellite remote sensing showed minor or no signal of elevated SPM concentrations on the water surface in comparison to surrounding area. Model result show clearly that eastward transport of SPM prevailed during the sand mining activities. The SPM covered larger area during autumn than during winter and spring. This can be attributed to the stronger winds that forced higher waves and stronger currents. Wave activity is responsible for keeping the SPM in suspension, which favors the transport of SPM away from the mining site. In the environmental point of view, the most affected area remains in the radius of 1 km from the mining site. In conclusion, the use of satellite remote sensing and in-situ measurements can be misleading when considering environmental impact assessment caused by SPM.