WP2: Detecting and modelling of the coastal currents and water transport

To quantify a possible dispersal of contaminants in seawater and therefore the environmental impact, detailed knowledge of the current conditions and the sediment dynamic in the study area is essential. These factors also need to be considered in their temporal variability on a daily (tides, storm events), weekly (lows, inflow events) and annual (seasonality) basis. There are existing current models for both the North and the Baltic Sea that can be applied on larger scales.

Detailed studies were carried out along the German Baltic coast, for example to assess the ecological consequences of cold water influx into the Greifswald bodden or the environmental impact of a planned permanent Fehmarn Belt crossing. During the BMBF-funded project PACE, the hydro dynamics and suspended matter dynamics in the tidal flats of the North Sea were successfully investigated. Over the last couple of years a modelling system for the German coastal waters was developed at the IOW, which allows realistic and high-resolution simulations on a local scale (e.g. during the large Baltic Sea inflow event in December 2014). This modelling system is currently being expanded within the framework of the BMBF-funded project MOSSCO by implementing additional components like sediment and swell modules.

However, the possible dispersal of contaminants (in this case from old weapons and munition or their by-products) is still a challenge for the model. One aspect of WP3 is to investigate how the contaminants are being transported: 1) in solution or 2) through accumulation at suspended matter (sand, silt, organic particles). In the first case, the transport of the contaminants is strongly influenced by the residual transport and thus an estuarine circulation (distal transport in surface water masses and coast-directed transport in near-bottom currents). Especially these near-bottom currents are a challenge during the delaboration of the weapons, since possible contaminants could be released and transported to the nearby beaches. Although contaminants have a high transport range in solution, the absolute concentration is diluted over time. In the second case, the interaction of currents and waves is very important, since only the wave-induced soil shear stress transports sand and silt particles from the sediment into the water column.

There are different re-distribution patterns depending on the suspended matter fraction. Larger suspended matter fractions (e.g. sand) sink faster and thus are only re-distributed locally. Higher transport ranges can be observed If the contaminants are adsorbed to silt particles, since these particles can remain inside the water column for several days. In addition, the concentration of the contaminants shows only little dilution over time if adsorbed onto suspended matter and transported through the water column. Therefore, quantifying the transport of chemical contaminants requires a deep understanding of suspended matter transport and dynamics.

The figure (see above) shows an example of results of the physical model GETM and a passive tracer continuously released in the bottom water in the restricted area “Kolberger Heide” (red dotted line) and simulating the release of compounds typically found in explosives. The compounds can be transported eastwards along the coast (February 2006) or may penetrate westwards into the Kiel Fjord (September 2006). The arrows indicate the average wind direction at the two periods.

Aims and responsibilities

To investigate the dissemination of the contaminants a combination of both modelling and field measurements is used. One long-term (3-6 months) profiling current sensor is deployed in each study area and the physical properties of the water column are measured (temperature, salinity, turbidity). The data collected from these measurements will be implemented into high-resolution current models (Purikani et al., 2015; Gräwe et al., 2013) of the study area to simulate the current conditions and therefore the dissemination of chemical contaminants and their potential impact on different habitats.

Based on the maps generated from WP1, the morphological changes are identified and monitored, which can then be used to validate the morphological reconstruction produced by the model and in addition to evaluate the predictive value of the results. Furthermore, the model is used to assist in the planning of sampling stations (water and sediment) within WP3 and WP4 to investigate the dissemination of chemical contaminants during the disarming of the weapons.

The model is also used to predict the consequences of tidal influences on the dissemination of the  contaminants, which will be important during the delaboration process especially in the North Sea. Here, the release of particles from a source point (in UDEMM the drift of contaminants) can be simulated using a 3D particle tracking tool that was advanced at the IOW (Gräwe and Wolff, 2010) and also enables forward and backward simulations.

Especially the backward simulations can be used to gain new insights into the transport of contaminants through the water column and can also help to identify the source of the contamination. Two forward model calculations for a period of 1 and 2 years  respectively are planned prior to the delaboiration to facilitate an optimized sampling strategy within WP3 and WP4.

 

Purkiani, K., J. Becherer, G. Flöser, U. Gräwe, V. Mohrholz, H. M. Schuttelaars and H. Burchard (2015). Numerical analysis of stratification and destratification processes in a tidally energetic inlet with an ebb tidal delta. J. Geophys. Res. Oceans 120: 225-243, doi: 10.1002/2014JC010325

Gräwe, U., R. Friedland and H. Burchard (2013). The future of the western Baltic Sea: two possible scenarios. Ocean dyn. 63: 901-921, doi:10.1007/s10236-013-0634-0

Gräwe, U, Wolff, J-O, Ribbe, J,  Impact of Climate Variability on an East Australian Bay Estuarine, Coastal and Shelf Science, Vol. 86(2), 247-257, doi:10.1016/j.ecss.2009.11.020