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WATER QUALITY ASSESSMENT / 25 Feb 2019

3D hydrodynamic modelling enables better decisions in water quality assessment

3D hydrodynamic modelling is a tool that can be used to find out the effect of effluents or water construction works on water quality in lakes and coastal areas before construction.

Did you know that Pöyry has utilized 3D hydrodynamic models in over 20 project locations in Finland and the Baltic, including metal mine effluents, elevated road construction, paper and pulp industry releases, and thermal releases?

From flow to water quality

In a 3D hydrodynamic model the target water body is divided into computational cells both in horizontal and vertical dimensions, which enables an accurate description of complex surface water bodies, such as coastal areas with archipelago and inland waterways. To compute water flow, also boundary conditions, such as weather data, in- and outflowing river discharges, and water level, salinity and temperature at open model boundaries are needed. As a result, the flow computation produces a 3-dimensional time-dependent flow field of the target area.

The aim of a typical surface water modelling study is to estimate the future water quality in the modelled area. This is done by using the flow field produced by the flow model to compute the transport and spreading of substance in the water. Different processes taking place in the water, such as settling and sedimentation, and oxygen consumption by various reactions are computed simultaneously with the transport. When the processes depend on external conditions (for example, algae growth depends on water temperature) the required information is often already available from the flow computation.

3D models can be applied to variety of problems

In a model, any initial data or parameter can be modified. For example, a new waste water release can be added to the area. Also, by modifying the model grid, the effect of structures, such as piers and elevated roads can be estimated before construction. As an analysis tool, the model can be utilized to find out the sphere of influence of an existing discharge or the contribution of an existing discharge to the total water quality of the area.

A typical application of a 3D hydrodynamic model is an assessment of a new or modified discharge to the state of the target water body, performed as a part of an environmental impact assessment process or water permit application. 3D models are required especially in locations where the water flow is not self-evident, such as on large lakes and in coastal areas. Another problem, well suited for 3D modelling, is the spreading of saline waste waters in fresh water lakes.

The setup, calibration and verification of a 3D hydrodynamic model may take more time than the same process for a simpler model. However, additional time and costs can often be justified by more verified and reliable model results.

From model to impact assessment

Surface water modelling is often performed as part of a larger environmental impact assessment project. The modelling always starts from the careful specification of the modelling requirements and scenarios.

Not everything can be reliably simulated, and therefore the quantitative model results often require further assessment, to relate the model results to the state of the modelled water body and ecosystem. The interpretation is typically performed by a limnologist or other suitable expert.

Sustainable decisions require reliable information

Sustainable management of water bodies requires the correct information. But is a 3D hydrodynamic modelling the correct approach? The setup, calibration and verification of a 3D hydrodynamic model may take more time and money than using a simpler model. Complex problems, however, require tools that can take all relevant aspects of the problem into account. Additional time and costs used in modelling can often be justified by better understanding of the relevant processes taking place in the target area and increased understanding of relevant uncertainties, leading to more informed decision making.

Hannu Lauri
Environmental Consultant