| dc.description.abstract | Stormwater is of growing concern with increased heavy rainfall and runoff surfaces from urbanisation. The challenges of stormwater has two aspects; risk of flooding and risk of pollution. Road runoff in particular, can at times transport a cocktail with hydrocarbons, heavy metals and other pollutants which could be detrimental to the environment of receiving water bodies.
One way to manage these challenges is by sending water through stormwater detention tanks. In Sandnes, Norway, the road runoff from a new highway is lead through an underground modular settling system (MSS), which serves the purpose of both water detention and pollutant removal by sedimentation. Established guidelines are in place for the function of water detention, but design for optimal treatment is based on a "Best Available Technology" principle.
The treatment performance of stormwater detention tanks is highly influenced by hydraulic characteristics, such as hydraulic residence time, mixing and short-circuiting. Longitudinal mixing and short-circuiting may reduce the treatment performance. In this thesis, a tracer study is applied to characterize these hydraulic processes.
Four analysis methods were selected from literary findings: (1) Visual inspection of tracer curves, (2) Method of moments (MOM) technique (3) Tank-in-series (TIS) and laminar convection flow (LCF) modelling and (4) Volume-based residence time analysis for variable flow. From the analysis, the hydraulic parameters of flow regime, mean residence time, dispersion, mixing scale and tracer mass recovery found that the hydraulic behavior of the MSS involved moderate amounts of deadzones, relatively high mixing and possibly multiple flow paths. The findings suggest the design of the MSS can be further improved to optimise hydraulic behavior for particle settling. The results should be treated with caution, however, as a statistical analysis was not possible due to practical limitations for replication tests. In addition, outflow rate had to be estimated by calculations due to instrumental errors for flow measurements. Additional studies should be done to validate the findings of this thesis, by the use of proper outflow measurements. Future tracer studies should be paired with an analysis of particle removal to better understand how hydraulic behavior and removal efficiency relates to each other and how they are influenced by different design configurations. | |
