Modelling of water and material transport in River Storån to Lake Bolmen

Abstract: The change of color of water from yellow to brown termed as brownification process is mainly due to leaching of humus and iron concentrations. In recent years, this problem is increasing in lake Bolmen, a major source of drinking water in Skåne county, Sweden. This process has been a threat to aquatic life and increasing the water treatment cost too. Storån river being the main tributary of the lake has a significant contribution to this browning process. This study deals with the hydraulic model and water quality model in the river system, modelled in HEC-RAS software to visualize the variation of brownification in Storån, for which color concentration was taken as a parameter to measure. Daily varying discharge and watercolor concentration were taken as input for the HEC-RAS model for the simulation. The 67 km reach of Storån river starting from downstream of Flatten Lake to Bolmen lake was modelled and multiple sources of color concentration from major tributaries were simulated with time-varying discharge data series. For model setup, a Digital Elevation Model was created from bathymetric data from 18 measured cross-section and LiDAR data were taken and then merged and developed in Arc GIS and RAS mapper tools. For hydraulic analysis, daily discharge data from 2004-2019 were taken from SMHI. In the water quality model, color data from 2012-2016 were taken from SLU and analyzed. The actual transport of materials was mimicked by tracer analysis with multiple sources. The transport mechanism was solved through Advection Diffusion Equation. The hydraulic model was calibrated through velocity and elevation comparison of computed and observed values during the field survey. On the other hand, the water quality model was calibrated by steady analysis with some color data taken from SLU. Result shows that there is an increasing trend of the brownification in river Storån. From year 1985 to 2019, the color concentration has been increased by more than 1.5 times (from 159 mg Pt./l in 1985 to 265 mg Pt./l in 2019). The reason behind this could be the change in the land use pattern, climate change and seasonal variation in the catchment. From the model simulation, it was found that the watercolor concentration in the river is higher in a wet climate, like in autumn and winter, rather than in the drier season like in spring and summer. The possible reason for this is that high runoff draws more materials from catchments for the brownification process. Among all tributaries, it was found that catchment for the Lillån- Havridaån (R7) has significant effect in the color variation. The study would have been better if there were more measurement stations as well as a high frequency of measurements in all tributaries. Keywords: Brownification, Water Color, Dissolved Organic Matter, Discharge, hydraulic analysis, Tracer Analysis, Dispersion coefficient, Advection and Diffusion, Bolmen Lake, Storån River