Modeling of the environmental state of inner Oslofjord considering emission permits and population development

Summary

In this report, the objective has been to model the environmental state of the fjord within Drøbak. NIVA Fjord model has been used. To study the whole fjord, the fjord has been divided into different basins. This has made it possible to capture to a greater extent where in the fjord negative effects due to discharge will be greatest. NIVA Fjord Model (NFM) is a multidisciplinary model, which describes both the physical, chemical and biological conditions in fjord basins. This model has been used to simulate the environmental conditions in the Inner Oslofjord. The fjord within Drøbak has been divided into nine different basins. Each basin is treated as a box where only one value for each parameter is calculated, and this value represents the entire horizontal area covered by the box. Each box is divided into many layers, where the vertical resolution is 2 m in the surface and gradually increases to about 10 m at 100 meters depth. The water exchange between the basins is calculated from horizontal pressure gradients. The area outside Drøbak represents the outer basin, where conditions are specified in the model input data. The model is validated against observations from three periods, 1978-1979, 1995-1998 and 2015-2018. In the late 1990s, several measures were implemented that reduced the discharge of phosphorus, while there was still a large discharge of nitrogen. During the next 20 year, the population of Oslo has increased by approx. 200000, which in turn has increased the supply of phosphorus. During the same period extensive measures have been implemented to reduce the supply of nitrogen. (...)